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Expanding the synthesis space of 3D nano- and micro-architected lattice materials

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EXPANDING THE SYNTHESIS SPACE OF 3D NANO- and MICRO-
ARCHITECTED LATTICE MATERIALS

by

Alina Rochelle Garcia Taormina

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(MATERIALS SCIENCE)

May 2022

Copyright 2021 Alina Rochelle Garcia Taormina
ii

To my family, friends, and mentors for their endless support

iii

Acknowledgements
First, I would like to thank my advisor, Professor Andrea Hodge, for her continued
support, guidance, and mentorship throughout my Ph.D. journey. She afforded me the
opportunity to work in her research group and has taught me much about strength and resilience
in addition to how to be a great engineer and researcher. I am grateful for the time and effort she
invested in both my professional and personal growth.
I want to thank members of the Hodge Materials Research Group for your support,
advice, and meaningful discussions along the way. I am grateful for all the fun times we spent
together as a group that allowed us to recharge and bond as a group. Throughout my time at USC
I have been surrounded by incredible colleagues and amazing friends. Thank you, Dr. Joel
Bahena, Dr. Chelsea Appleget, Dr. Theresa Juarez, Dr. Sebastián Riaño, Dr. Nathan Heckman,
Dr. Angelica Saenz-Trevizo, Dr. Leonardo Velasco, Daniel Goodelman, Adie Alwen, Karina
Hemmendinger, Danielle White, and Roya Ermagan. It was wonderful to work with you all and I
wish you the best in all your future endeavors.
I would also like to thank the Core Center of Excellence in Nano Imaging (CNI) at USC,
John Curulli, Lucas Jordao, and especially Dr. Matthew Mecklenburg for all your assistance
throughout the years. I would also like to acknowledge the staff and faculty at who helped me
navigate my time here at USC, especially Dr. Lessa Grunenfelder, Kim Klotz, Andy Chen, Tracy
Charles, and Natalie Guevara, among many others. Thank you to Professor Balakrishna,
Professor Branicio, and Professor Ravichandran for agreeing to serve on my dissertation
committee.
iv

Funding for this project was provided in part by the Air Force Office of Scientific
Research (AFOSR) under grant FA9550-14-1-0352 as well as a fellowship award through the
National Defense Science and Engineering Graduate (NDSEG) Fellowship Program, sponsored
by the Air Force Research Laboratory (AFRL), the Office of Naval Research (ONR) and the
Army Research Office (ARO). I would also like to acknowledge funding through the National
Science Foundation under grant OISE-1460006 and OISE-2106597; it afforded me the
opportunity to participate in international research experiences at the Karlsruhe Institute of
Technology and learn new techniques and skills to further my research. I would also like to
thank the USC Graduate School for the Provost Fellowship.
This work would also not have been possible without my distinguished collaborators
including Chantal Kupiers and Dr. Ruth Schwaiger at the Karlsruhe Institute of Technology and
Forschungszentrum Jülich GmbH, as well as Dr. James Oakdale and Dr. Juergen Biener at
Lawrence Livermore National Laboratory. Also, a big thank you to Dr. Chris Buser at Oak Crest
Institute of Science for the microtome and TEM work.
Thank you to all of my other mentors along the way, including peers I have met in my
courses, those I’ve met through the Society of Hispanic Professional Engineers, and those I have
met while interning at Northrop Grumman. I am also thankful to Dr. Emily Jarvis, Dr. Omar Es-
Said and Dr. Bouvier-Brown at Loyola Marymount University who provided me the opportunity
to partake in research as an undergraduate student and motivate me to pursue a graduate degree. I
would also like to thank Rob Beeston at Northrop Grumman for the opportunity to participate in
an immersive internship at the company, and for showing me how my background in materials
science translates to work in the microelectronics realm for space systems.
v

An enormous thank you to my family, friends, and partner for your continued support
throughout my Ph.D. journey. Without you all, I would not be where I am today, and I cannot
thank you enough for your unwavering support and guidance. Thank you for being my sounding
board, my outlet, and my foundation; you’ve all grounded me throughout this process.
vi

Table of Contents
Dedication ....................................................................................................................................... ii
Acknowledgements ........................................................................................................................ iii
List of Tables ............................................................................................................................... viii
List of Figures ................................................................................................................................ ix
List of Abbreviations ................................................................................................................... xiv
Abstract ......................................................................................................................................... xv
Chapter 1: Introduction ................................................................................................................... 1
Chapter 2: Background ................................................................................................................... 4
2.1. Cellular Materials ................................................................................................................ 4
2.2. 3D Nano- and Micro-lattice Materials ................................................................................ 6
2.3. Polymer-based Fabrication Techniques ............................................................................ 10
2.4. Deposition Techniques for Coatings Nano- and Micro-lattice Materials ......................... 18
2.5. Mechanical Properties and Functionalities of Architected Lattice Structures .................. 24
Chapter 3: Experimental Methods ................................................................................................ 31
3.1 Two-Photon Polymerization Direct Laser Writing ............................................................ 31
3.2 Magnetron Sputtering ........................................................................................................ 33
3.3 Characterization Techniques .............................................................................................. 36
3.4 Micro-tensile Testing ......................................................................................................... 40
Chapter 4: Current State of Coated Nano- and Micro-lattice Materials ....................................... 42
4.1. Coating Materials Workspace ........................................................................................... 42
4.2. Geometry Workspace: Coating Non-truss Based Lattices ................................................ 47
4.3. Coating Characterization ................................................................................................... 48
4.4. Properties and Functionalities of Coated Nano- and Micro-lattice Structures ................. 54
4.5. Conclusions and Outlook .................................................................................................. 64
Chapter 5: Development of Core-shell Composite Micro-lattice Structures via
Magnetron Sputtering ................................................................................................................... 66
5.1. Introduction ....................................................................................................................... 67
5.2. Experimental Methods ...................................................................................................... 70
5.3. Results and Discussion ...................................................................................................... 72
5.4. Conclusion ........................................................................................................................ 84
Chapter 6: Tensile Behavior of Stitched Nano-Lattice Structures Fabricated via Direct
Laser Writing ................................................................................................................................ 86
6.1. Introduction ....................................................................................................................... 86
6.2. Experimental Methods ...................................................................................................... 88
6.3. Results and Discussion ...................................................................................................... 90
vii

6.4. Conclusion ........................................................................................................................ 96
6.5. Supplementary Material .................................................................................................... 96
Chapter 7: Conclusions and Future Work ..................................................................................... 99
7.1. Conclusions ....................................................................................................................... 99
7.2. Future Work .................................................................................................................... 102
References ................................................................................................................................... 104
Appendix A: Summary of Core-Shell Composite Tetrahedral Truss Micro-lattice
Samples ....................................................................................................................................... 120
Appendix B: Fabrication of Macroscopic Lattice Structures via Digital Light Processing
and Magnetron Sputtering ........................................................................................................... 126

viii

List of Tables
Table 1: Summary of commonly utilized light-based fabrication techniques, their
corresponding resolution, and advantages/disadvantages ............................................................. 11
Table 2: Main techniques for coating 3D nano- and micro-lattice structures with
corresponding deposition materials and resulting uniformity ...................................................... 18
Table 3: Functionality and applications of various core-shell composite and hollow-tube
nano- and micro-lattice systems.................................................................................................... 61
Table 4: Summary of sputtering parameters for ICM and planar cathode configurations ........... 71
Table 5: Summary of average maximum achieved load before failure ........................................ 95
Table 6: Relative density and calculated effective density of log-pile gage sections ................... 98
Table 7: Calculated compressive yield strength and experimental tensile yield strength
values of samples containing no stitch .......................................................................................... 98
Table 8: Planar Cathode Sputtered Samples ............................................................................... 121
Table 9: Inverted Cylindrical Magnetron Sputtered Samples ..................................................... 123
Table 10: Experimental Planar Cathode Sputtering Temperature Profiles ................................. 128
Table 11: Summary of Coated DLP Printed Structures via Planar Magnetron Sputtering ........ 129

ix

List of Figures
Figure 1: Ashby plot representing yield strength versus density for common commercially
available engineering materials. Adapted from Schaedler et al. [55]. ............................................ 5
Figure 2: Representation of the cellular architecture of example a) open-cell and b) closed-
cell stochastic materials, as well as c) periodic cellular solids. Adapted from a,b) [57] and
c) [58-60]. ....................................................................................................................................... 6
Figure 3: Schematic outlining the three factors for developing coated 3D nano- and
micro-lattice structures: coating material selection, geometry [6, 62], and feature size. ............... 7
Figure 4: Representative schematic outlining the different classifications of periodic lattice
structures: solid beam, core-shell composite, and hollow-tube. Lattice configurations were
generated using nTopology Element Pro software. ........................................................................ 9
Figure 5: Schematic representations of commonly utilized light-based fabrication
techniques for the development of three-dimensional micro- and nano-lattice structures: a)
self-propagating polymer waveguide, b) digital light processing, c) projection micro-
stereolithography, and d) two-photon polymerization-direct laser writing. a & c)
Schematics inspired by [83] and [35], respectively. ..................................................................... 12
Figure 6: Different views (top, perspective, side) of a polymer micro-lattice fabricated
with three UV beams of light [84]. ............................................................................................... 13
Figure 7: Schematic of unit cell geometries that can be fabricated using self-
propagating polymer waveguides: a) 4-fold symmetry, b) 3-fold symmetry, c) six-fold
symmetry [93]. .............................................................................................................................. 14
Figure 8: Scanning electron micrograph of a) polymer pentamode mechanical metamaterial
fabricated by dip-in direct laser writing. The inset shows a schematic of the pentamode unit
cell (adapted from Kadic et al) [112].

Scanning electron micrograph of

b-d) 3-D kagome,
cuboctahedron, and tetrakaidecahedron topologies, with the insets showing CAD models of
the respective unit cells (adapted from Meza et al) [41]. .............................................................. 17
Figure 9: Illustration depicting the three main deposition techniques leveraged for
developing core-shell and hollow-tube lattice structures: a) atomic layer deposition, b)
electroless plating, and c) magnetron sputtering. b) Based on schematic from [22]. ................... 19
Figure 10: (a) SEM micrograph of hollow TiN nanolattice developed via TPP-DLW and
ALD. Scale bar is 20 μm. (b) SEM micrograph of hollow nanolattice, with the inset showing
the cross-section of a hollow TiN strut. Scale bar is 5 μm (inset is 1 μm) [23]. (c) SEM
micrograph of alumina coated octet-truss nanolattice fabricated via TPP-DLW with
corresponding (d) high-magnification SEM micrograph of alumina octet-truss nanolattice,
where the inset shows an isolated hollow strut [8]. ...................................................................... 21
Figure 11: Scanning electron micrographs of sputtered a) Zr56Ni22Al22 [27], b) Au [25], c)
Cu60Zr40 [29], and d) CoCrFeNiAl0.3 [18] nano- and micro-lattice structures highlighting a
x

coating thickness gradient both around individual struts and from top to bottom of the
structure. Arrows indicate thinner (green) and thicker (orange) regions to visually
demonstrate the gradient around individual struts. ....................................................................... 24
Figure 12: Absolute compressive strength versus density material-property chart of a wide
range of nano-, micro‐, and macro-lattices as well as stochastic nanoporous and commercial
bulk materials. Symbol shapes relate to the constituent material, and colors indicate the
length scale of structure feature (filled in relates to feature diameter, and lines indicate wall
thickness, if any). Points enclosed by the black oval refer to a hollow NiP octet micro-lattice
material with a relative density of 0.3%, and points enclosed by the light blue oval represent
a hollow NiP octahedral type nanolattice with similar relative density. Adapted from Bauer
et al. [4]. ........................................................................................................................................ 27
Figure 13: Mechanical data and still frames from the compression test on a (a-e) 10 nm thin-
walled alumina nanolattice depicting the ductile deformation, local shell buckling, and
recovery of the structure after compression and (f-j) a 50 nm thick-walled alumina
nanolattice demonstrating catastrophic brittle failure and no recovery [8]. ................................. 28
Figure 14: a) Photopolymerization chemical reaction of the two-photon polymerization
process. b) Jablonski energy diagram depicting an electron which can become excited from
the ground state, which subsequently relaxes and undergoes a spin-flip transition entering an
excited triplet state. Adapted from Maruo et al. and LaFratta et al.[81, 158] ............................... 32
Figure 15: Schematic of the direct laser writing process in 3D-space inside a resist layer.
The inset shows the voxel at the focal position, where two-photon absorption only occurs
within the voxel [162]. .................................................................................................................. 33
Figure 16: General schematic showing the process of magnetron sputtering: an inert gas
(Ar) is introduced into a vacuum chamber and ionized by the negatively biased target,
where the Ar
+
ions are attracted to and strike a target causing the ejection of target atoms
that coat a substrate. In this example, the substrate is a 3-D micro-lattice structure. ................... 34
Figure 17: Schematic comparison of planar and inverted cylindrical cathode target
geometries and deposition pathways. ........................................................................................... 35
Figure 18: Schematic of the hollow cathode sputtering process. .................................................. 36
Figure 19: Schematic of several types of signals generated by the interaction between a
primary electron beam in a scanning electron microscope with the corresponding available
regions from which the electrons can be detected [174]. .............................................................. 38
Figure 20: a) Image of a milled metal/polymer core-shell composite micro-lattice structure
exposing the coating cross section. b) SEM micrograph of a strut cross section exposed
with Ga-based FIB milling highlighting the material redeposition caused during sectioning
[19]. ............................................................................................................................................... 40
Figure 21: Overview of the various materials deposited on 3D nano- and micro-lattice
structures and the corresponding coating techniques utilized. ...................................................... 43
xi

Figure 22: Comprehensive chart categorizing geometry and material combinations of core-
shell and hollow-tube nano- and micro-lattice structures fabricated to date, where structures
denoted with an asterisk have been mechanically tested. Representative images of each
lattice geometry are provided [6, 10, 18-21, 27, 38, 40, 41, 179]................................................. 46
Figure 23: Chart outlining prominent characterization techniques utilized to analyze the
microstructure and composition of deposited coatings on nano- and micro-lattices (left), as
well as the resultant coating thickness values and uniformity (right). Note that asterisks denote
less commonly employed techniques for characterizing coated nano- and micro-lattice
structures. ...................................................................................................................................... 48
Figure 24: Representative scanning electron micrographs depicting the uniformity of
common deposition techniques on various nano- and micro-lattice geometries [8, 14, 27]. ....... 53
Figure 25: Various examples of mechanical behaviors observed by coated nano- and micro-
lattice structures including: a) increased modulus [18], b) exceptional recoverability [8], and
c) enhanced yield strength under compression [19], as well as d) increased flaw tolerance
under tension [45]. ........................................................................................................................ 56
Figure 26: Functionality and application examples of coated nano- and micro-lattice
structures including: a) cell growth [11], b) battery electrodes [14], c) supercapacitors [42],
and d) water splitting [47]. ............................................................................................................ 62
Figure 27: Cu–Al/polymer composite micro-lattice structure across four orders of
magnitude in length scale: from the width of the tetrahedral-truss structure (≈70 μm) down
to the Cu-Al 2wt% coating thickness (≈200-10 nm). ................................................................... 69
Figure 28: Schematic of a) inverted cylindrical magnetron and b,c) planar sputtering
configurations used in this work, where the line-of-sight for each respective set-up is
detailed. The colored arrows represent the different sputtering arrangements, where green
arrows correlated to samples coated using the ICM (360° line-of-sight), yellow are coated
using the planar cathode (90° line-of-sight), and orange denote samples tilted to ±30° while
coated with the planar cathode. ..................................................................................................... 72
Figure 29: a) X-Ray diffraction scans of Cu-2wt% Al films for each sputtering
configuration and corresponding deposition parameters. Green scans correlated to samples
coated in the 360° ICM configuration, where yellow and orange scans represent 90° and
±30° planar set-ups, respectively. Corresponding top surface SEM images of as-deposited
films with a sputtering rate of 0.48 nm/sec in the b) ±30°, c) 90°, and d) 360°
configuration, respectively. ........................................................................................................... 75
Figure 30: a) Representative SEM micrograph of Cu-2wt% Al coated micro-lattice structure.
b) SEM image of a tetrahedral-truss structure milled using PFIB with a corresponding
micrograph of a strut cross-section. c) Representative TEM micrograph of a microtome
prepared cross-section and a complementary cross-sectional image of an analogous strut to
that represented in b). The topside coating thickness of the designated strut depicted in b)
and c) for each sputtering configuration are presented in table d). The PFIB and microtome
xii

thickness measurements obtained from the top-side strut regions are outlined with red lines
in b) and c), respectively. .............................................................................................................. 77
Figure 31: a) Schematic of tetrahedral-truss cross-section with struts of interest outlined in
black. Areas in red indicate the region of measurements taken for obtaining the average
coating thickness of the “outer” and “inner” struts. Measurements were taken on TEM
micrographs from the microtome cross-sections for each sputtering configuration/set of
parameters and presented in table b). ............................................................................................ 80
Figure 32: a) TEM micrographs of representative strut cross-sections prepared via
microtome showing varying levels of coating coverage around the strut. The red dotted lines
highlight the number of sides covered, where (1) indicates a coating on the topside of the
strut, (2) refers to coating covering on the top of the strut and one side wall, and (3) indicates
coating coverage on the topside, as well as on two side walls of the strut. b) Bar chart
representing the distribution of coating coverage for each set of microtome prepared
samples, where green bars correlate to the samples coated in the ICM configuration, and
yellow and orange represent the planar coated 90° and ±30° configurations, respectively. A
total of twelve analogous struts in each coated sample were surveyed. ....................................... 82
Figure 33: SEM micrographs highlighting the side coating quality and morphology of the
top struts sputtered in the a-c) 360° (green), d,e) 90° (yellow), and f,g) ±30° (orange)
configurations. The inset of each show higher resolution micrographs featuring the transition
from the top of the strut to the side. region imaged is highlighted by the white box, and inset
micrographs show higher resolution images of the transition from the top of the strut to the
side wall. ....................................................................................................................................... 84
Figure 34: Photographs of (a) a custom micromechanical testing apparatus and b) 1 N load
cell with representative DLW I‐beam tensile samples printed on the glass side. c) A
top‐view schematic showing the slot and key method for gripping tensile specimens: a
mechanical grip in which samples are slotted into a lock piece and pulled in tension until
fracture. ......................................................................................................................................... 90
Figure 35: Representative cross‐sectional SEM micrographs highlighting the relative density
of the a) low‐ and b) high‐density samples, respectively. Representative micrographs of the
log‐pile features with an accompanying table of the measured feature dimensions for the c)
low‐ and d) high‐density samples, respectively. ........................................................................... 91
Figure 36: Representative SEM micrographs of low‐density tensile specimens: a) no stitch
samples, b) samples printed with a stitch line, and c) those with a stitch line and a 2 μm
overlap. The yellow arrows indicate regions of stitch‐induced defects. Micrographs d–f)
show the corresponding gage sections, where the yellow dashed line highlights the location
of the stitch line. Inset c) highlights a representative region of axial shear stress denoted by
the yellow bracket. ........................................................................................................................ 93
Figure 37: a) Representative stress–strain curves of uniaxial tension tests for low‐ and
high‐density log‐pile I‐beam samples, where the stress values are normalized by the
effective density of the architected gage. Green curves show continuously (C) printed
xiii

pillars, orange curves show samples printed with a stitch (S), and blue curves show samples
printed with a stitch line overlap (SO). b) Representative SEM micrographs of fracture
surfaces of the low‐density samples for each type of stitch protocol. .......................................... 94
Figure 38: Representative SEM micrographs of high-density tensile specimens, where a)
show the gage section of a sample with a stitch line and 2 μm overlap. The yellow arrow
indicates the stitch induced defect. b) Shows the grip/gage interface, where the yellow
bracket highlights the observed axial shear stress. ....................................................................... 97
Figure 39: CAD models depicting the various macroscopic cubic lattice structures designed
with varying complexity. ............................................................................................................ 126
Figure 40: Schematic showing the deposition configuration used to coat cubic lattice
structures printed via DLP. ......................................................................................................... 127
Figure 41: (a) Representative optical micrograph of a DLP printed cubic lattice coated in
Inconel 600, where (b-e) show representative SEM micrographs of the coating quality on
the front face of the structure. ..................................................................................................... 128

xiv

List of Abbreviations
ALD Atomic layer deposition
AM Additive manufacturing
APT Atom probe tomography
BSE Backscattered electrons
CAD Computer-aided design
CVD Chemical vapor deposition
DC Direct current
DiLL Dip-in laser lithography
DLP Digital light processing
DMD Digital micromirror device
EDS Energy-dispersive X-ray spectroscopy
ERDA Elastic recoil detection analysis
FIB Focused ion beam
HEA High-entropy alloy
ICM Inverted cylindrical magnetron
LED Light emitting diode
NIR Near-infrared
PE-CVD Plasma enhanced chemical vapor deposition
PFIB Plasma focused ion beam
PµSL Projection micro-stereolithography
PVD Physical vapor deposition
RF Radio frequency
SAED Selected area electron diffraction
SE Secondary electron
SEM Scanning electron microscopy
SPPW Self-propagating polymer waveguides
TEM Transmission electron microscopy
TPMS Triply periodic minimal surfaces
TPP-DLW
Two-photon polymerization direct laser
writing
UV Ultraviolet
XRD X-ray diffraction

xv

Abstract
Over the last two decades, architected lattice materials have garnered increasing attention
due to their ability to achieve unique combinations of properties and functionalities linked to
their carefully controlled topologies. Recent advances in additive manufacturing (AM), such as
two-photon polymerization direct laser writing (TPP-DLW), have allowed for the fabrication of
novel 3D architected lattice materials comprised of nano and microscale resolution, enabling
researchers to investigate previously unexplored phenomena and property spaces. However, both
the scalability and available materials working space of such additively manufactured nano- and
micro-lattice structures remain crucial challenges. Presently, there exists a small selection of
materials that can be reliably printed with sufficiently fine features and complex topologies. Such
materials are mainly restricted to polymer-based systems, greatly narrowing the achievable
functionality of these emerging lattice materials.
Thus, advancements towards both developing a scalable solution to fabricate larger high-
resolution lattice structures comprised of nanoscale feature, as well as alternative synthesis
approaches, such as the subsequent deposition of coatings on the printed polymer structures,
present remarkable areas of research. As such, this dissertation discusses both the development
of coated micro-lattice materials via magnetron sputtering, as it offers an expansive materials
workspace, and the scaling the scaling-up of nano-architected lattice structures fabricated via
TPP-DLW via stitching methods. The studies described in this dissertation provide a foundation
for expanding the synthesis space of 3D nano- and micro-architected lattice materials.
Specifically, these works address two critical aspects within the emerging field of architected
lattice materials, including: (1) the development of scaling methods and testing methodologies to
investigate the effect of stitching on the integrity and mechanical behavior of TPP-DLW
xvi

fabricated structures under tensile load and (2) the assessment of fundamental sputtering
deposition parameters and influences for the generation of novel coated nano- and micro-lattice
systems. Improvements in the fabrication and coating capabilities of such materials are crucial
for the development and expansion of advanced materials with designed architectures.

1

Chapter 1: Introduction
Architected materials are widespread in nature, allowing for unique properties to be
attained. For example, the spicules in Euplectella aspergillum sea sponges, known as Venus’
flower basket, achieve strength and toughness values greater than those of their constituent
ceramics by leveraging elements of architectural hierarchy [1]. Within the last decade,
technological advancements in polymer-based AM, such as TPP-DLW, have enabled the
physical realization of 3D architected lattice materials with features down to the nanometer scale
[2-7], propelling researchers on their quest for uncovering novel mechanical and multifunctional
materials. Leveraging design principles linked to hierarchical architecture and material size-
effects, 3D nano- and micro-lattice materials have drawn increasing attention as ultra-
lightweight, mechanically robust materials have been highly coveted for decades. Nano- and
micro-lattices materials are comprised of three-dimensional unit-cells repeated in space with
features such as strut diameter, coating wall thickness, and unit-cell sizes spanning tens of
nanometers to micrometers. These man-made materials have been shown to exhibit remarkable
mechanical properties including high recoverability and specific strengths [5, 8-10], and have the
potential to offer revolutionary advances in a variety of applications and functionalities ranging
from battery electrodes to biomimetic scaffolds [11-14].
Despite technological advances, two present challenges currently limit the synthesis
space of 3D nano- and micro-lattice structures. Presently, a narrow selection of materials exist
that can be reliably 3D printed with adequate feature resolution, and fabricated structures are
confined to small print volumes. To date, few studies have addressed print volume restrictions to
progress towards fabricating larger surface areas with nano- and micro-lattice strut members for
emerging engineering applications [15-17]. In this dissertation, scaling techniques are explored
2

to evaluate the feasibility of “stitching” together multiple printed volumes to form larger
structures. The influence of various types of stitch interfaces and the relative density of the
structures on the tensile behavior of the of nano-architected lattice structures are explored.
Moreover, current AM techniques for the development of nano- and micro-architected
lattice are mainly restricted to polymer-based systems, which narrows the functionalities and
mechanical robustness of these architected lattice materials [2, 3]. Resolution and print material
availability, among other factors, currently hinder the direct printing of 3D nano- and micro-
lattice materials using metal-based AM techniques. Thus, to address such materials workspace
limitations, alternative synthesis approaches such as the combination of polymer-based AM and
coating deposition techniques, have been explored to expand the functionality space of these
advanced materials. As a result, several novel nano- and micro-lattice systems have been
developed through means of various polymer AM methods and the subsequent coated via a
variety of deposition techniques [6-14, 18-49]. Specifically, by leveraging magnetron sputtering,
as it allows for the deposition of a wide range of ceramics, single element metals, and alloy, it
enables researchers to further bridge the gap between metal and polymer AM technologies to
ultimately develop engineering relevant metal architected structures at the microscale. However,
achieving uniform sputtered coatings on complex 3D structures remains a prominent challenge.
Further insight into the effect of processing parameters and synthesis techniques of architected
lattice materials should be studied to broaden the functionalities and improve the properties of
these advanced materials.
The present study will focus on addressing both sputter coating uniformity and TPP-
DLW scaling challenges to further expand the synthesis space of 3D nano- and micro-lattice
materials. Various magnetron sputtering conditions and configurations will be employed to
3

assess the fundamental processing parameters that influence coating uniformity and coverage as
well inform further optimization for the development of metal/polymer composite architected
lattice materials. Micromechanical testing methods will also be leveraged to evaluate the
influence of stitching techniques and print parameters on scaled-up nano-architected structures.
Ultimately, the results from the studies performed in this dissertation will expand the design and
synthesis space of nano- and micro-lattice materials by elucidating the influence of fundamental
processing parameters for the fabrication of stitched lattice structures and the development of
sputter coated 3D nano- and micro-lattice materials.

4

Chapter 2: Background
Portions of this chapter (Sections 2.2-2.5) have been adapted from Garcia-Taormina, A. R.,
Alwen, A., Schwaiger, R., & Hodge, A. M. "A review of coated nano-and micro-lattice
materials." Journal of Materials Research (2021): 1-21.
2.1. Cellular Materials
Cellular materials with structural features ranging from nanometers to millimeters are
ubiquitous in nature. For example, bamboo, wood, cancellous bone, and sea sponges are porous
biological materials that are lightweight yet mechanically robust [50]. By mimicking the
structural hierarchy observed in natural materials, the introduction of pores in bulk materials has
given rise to cellular materials that exhibit high stiffness-to-weight ratios, great energy
absorption, and mechanical damping [10, 36, 50-52]. These properties have allowed cellular
materials to be successfully implemented in a wide range of structural and functional
applications including heat exchangers, filtration, catalytic supports, vibration damping, and
biomimetic scaffolds [53-56].
Weight reduction without sacrifice of mechanical performance is a prominent focus in
most industries. While foams have demonstrated the ability to sustain large compressive strains,
there remains potential for designing and developing cellular materials that reach previously
inaccessible regions of the compressive strength vs. density material property space, commonly
referred to as the “white space.” Figure 1 shows an Ashby plot highlighting the strength-to-
density performance of several common engineering materials. A major goal of the current
cellular solids and research is to generate novel low-density materials that exhibit mechanical
properties that can populate this unoccupied region of the strength-density material property
chart.
5

Figure 1: Ashby plot representing yield strength versus density for common commercially
available engineering materials. Adapted from Schaedler et al. [55].

2.1.1 Types of Cellular Solids
Fundamentally, a cellular solid is comprised of a network of ligaments, struts, plates, or
shells that constitute a unit-cell which is then tessellated in two or three dimensions to compose
the overall material. They are categorized by their pore distribution as stochastic or periodic,
which corresponds to either a random or repeating unit-cell, respectively. As highlighted in
Figure 2a,b, stochastic materials, commonly referred to as foams, can be further categorized as
“open-cell” or “closed-cell,” where open-cell foams are characterized by a lower density and
higher permeability. Examples of periodic cellular solids include honeycomb structures or
truss/lattice frameworks, as represented in Figure 2c. Each type of cellular materials can be
developed from a wide range of polymers, metals or alloys, and ceramics, where the distinct
mechanical properties depend on the relative density and properties of the constituent material.
6

Figure 2: Representation of the cellular architecture of example a) open-cell and b) closed-cell
stochastic materials, as well as c) periodic cellular solids. Adapted from a,b) [57] and c) [58-60].
2.2. 3D Nano- and Micro-lattice Materials
3D nano- and micro-lattices constitute a subset of cellular solids that are comprised of a
three-dimensional periodic array of unit cells with nano- and microscale constituent dimensions.
This dissertation will primarily discuss truss-based nano- and micro-lattices structures with strut
diameters <1000 nm, or between 1 μm and 500 μm, respectively. Recent advances in AM
7

techniques, including the reduction of achievable feature sizes and the expansion of printable
material systems, have enabled a nearly limitless design space in which new materials can be
realized [4, 23, 41, 61]. Figure 3 outlines the various design elements that affect the behavior and
function of nano- and micro-lattice structures and serves as a guide for what will be covered
throughout this dissertation, namely coating material, and deposition methods in conjunction
with geometry and feature size. Leveraging these design elements, this emerging class of
materials have been shown to exhibit remarkable mechanical properties including high
recoverability and specific strengths [5, 8-10], and have the potential to offer revolutionary
advances in a variety of applications and functionalities ranging from battery electrodes to
biomimetic scaffolds [11-14].

Figure 3: Schematic outlining the three factors for developing coated 3D nano- and micro-lattice
structures: coating material selection, geometry [6, 62], and feature size.

8

Nano- and micro-lattice structures derived from metallic and ceramic systems have been
long sought after for their anticipated mechanical robustness and unique behaviors. However, the
current state of AM technologies restricts the direct printing of lattice structures with nanometer
to micron resolution to primarily polymer-based techniques, which will be further discussed in
Section 2.3. It is important to note that the direct printing of metal-based lattice structures with
strut diameters on the order of hundreds of microns have been fabricated in materials such as
316L stainless steel [63-65], AlSi10Mg [66], NiTi [67], and Ti-6Al-4V [68-70]. Despite this,
there are several challenges that hinder the advancement of metal-based AM approaches for the
development of nano- and micro-lattice materials, including resolution, undesired porosity,
surface roughness, and microstructural control [71-73]. As such, a frequent method for
generating metallic, ceramic, and composite 3D nano- and micro-lattice structures is the multi-
step process of: (i) fabricating a polymeric scaffold, (ii) depositing a coating, and if desired, (iii)
removing the base polymer through means of chemical etching, plasma etching, or thermal
decomposition [14, 23, 31, 36, 40]. The deposition of ceramic and metallic coatings on polymer
templates offers a flexible and feasible approach for bridging the resolution and structural
complexity gap between current AM technologies.
2.2.1 Nano- and Micro-lattice Classifications
In general, nano- and micro-lattice materials can be divided into three main lattice
classifications: (i) solid-beam, (ii) core-shell composite, and (iii) hollow-tube structures. Figure 4
schematically defines each class, where (i) “solid-beam” refers to the base printed scaffold.
Subsequent post-processing of the solid-beam polymeric structure, such as pyrolysis, has been
leveraged to develop more mechanically robust and functionally tailored nano-lattice materials
such as pyrolytic carbon [5, 7, 74] and silicon oxycarbide [75, 76] nano-lattices. However, such
9

solid-beam systems are not generally coated, except for in one recent study by Bauer et al [7].
The development of (ii) “core-shell composite” and (iii) “hollow-tube” lattice structures are
solely unique to the added deposition of coating materials to the base solid-beam structure. To
date, a variety of multi-material composites as well as hollow-tube metallic and ceramic systems
have been realized [6-14, 18-49], expanding the functionality and property space of nano- and
micro-lattice materials.

Figure 4: Representative schematic outlining the different classifications of periodic lattice
structures: solid beam, core-shell composite, and hollow-tube. Lattice configurations were
generated using nTopology Element Pro software.
Lattices classified as core-shell composite and hollow-tube structures derive their
functions and mechanical properties from structural characteristics including geometry, coating
composition, and feature size (Figure 3). Owing to the precision and resolution offered by
polymer-based AM techniques, achievable geometries range from first order truss structures, like
octet- and tetrahedral-truss, to higher ordered hierarchical structures such as cubo-octet and
octahedra-of-octets [6, 35]. More recently, periodic topologies beyond truss lattices, such as
shell-based geometries drawn from triply periodic minimal surfaces (TPMS), including Schwarz
10

P, Schwarz D, and gyroid, have been explored to reduce stress concentrations commonly
observed at the nodes of truss-based structures [40, 46, 77-79].
In addition to architectural factors, the properties of coated 3D nano- and micro-lattice
materials are also governed by the nature of the coating material and corresponding features.
Mechanical size-effects imparted by the thickness of the coating material drive the property
space for such materials. A number of “thin-walled” metallic and ceramic systems have been
deposited on polymer-based nano- and micro-lattice scaffolds resulting in increased strength [20,
38], enhanced flaw tolerance [45], and unique recoverability [8, 9]. The overall synergy between
lattice classification, geometry, coating material and scale, allow for the tailoring of the
functionality and behavior of these fine-featured materials. To better understand the
extraordinary potential of these advanced engineered materials, it is important to highlight the
fabrication methods for the base polymer scaffolds and their respective challenges.
2.3. Polymer-based Fabrication Techniques
Advances in AM technologies rooted in photopolymerization phenomena have enabled
the integration of geometric complexity and allowed for the fabrication of complex 3D lattice
structures with feature sizes down to 100 nm [4, 80]. Four of the most common light-based
fabrication approaches, outlined in Table 1, leverage photosensitive material to produce a range
of lattice geometries with overall structure dimensions on the order of microns to centimeters.
Self-propagating polymer-waveguides, digital light processing, projection micro-
stereolithography, and two-photon polymerization direct laser writing are all established
photocuring techniques that offer different advantages in achievable resolution, print speed,
freedom of design, and print volume/scale.

11

Table 1: Summary of commonly utilized light-based fabrication techniques, their corresponding
resolution, and advantages/disadvantages
Various extrusion-based methods such as direct ink writing, 3D ink extrusion, and
aerosol jet printing have been utilized to print non-polymeric material systems such as TiO2 [86],
Ag [87], nanoporous Au [88], nanoporous Cu [89], graphene assemblies [90], and high-entropy
alloys [91]. However, such approaches are limited to non-complex filament-based topologies
like log-pile structures and will not be discussed. Thus, this section will overview the four most
successfully applied fabrication techniques (Figure 5) for development of complex polymer-
based nano- and micro-lattice structures.

Technique
Min.
Resolution/
Voxel Size
Advantage Disadvantage Refs.

Two-photo
polymerization
100 nm
High-
resolution;
precision
Scalability;
print speed
[6, 26, 81]

Projection micro-
stereolithography
< 5 µm
Fabrication
speed;
material
availability
Overcuring;
surface finish
[10, 82]

Self-propagating
polymer
waveguides
≈ 10 µm
Rapid
formation;
scalable
Non-
complex
geometry;
microscale
resolution
[61, 83, 84]

Digital light
processing
≈ 10 µm
Low cost;
fabrication
speed
Surface
finish;
microscale
resolution
[20, 85]
12

Figure 5: Schematic representations of commonly utilized light-based fabrication techniques for
the development of three-dimensional micro- and nano-lattice structures: a) self-propagating
polymer waveguide, b) digital light processing, c) projection micro-stereolithography, and d)
two-photon polymerization-direct laser writing. a & c) Schematics inspired by [83] and [35],
respectively.

2.3.1 Self-Propagating Polymer Waveguides
Self-propagating polymer waveguides (SPPW) is a high-throughput fabrication process
capable of generating scalable polymer micro-lattice structures in a matter of seconds to minutes.
As depicted in Figure 5a, an angled source of collimated ultraviolet (UV) light is guided through
13

a quartz mask into a volume of photosensitive monomer, solidifying upon exposure and forming
a 3D array of self-propagating waveguides along the path of light [83, 92]. The resulting material
(Figure 6) is an open-cellular micro-truss polymer material with an interconnected array of self-
propagating polymer waveguides. Feature resolution and build volume are dependent on the
diameter of the mask apertures and the propagation distance of the UV light, respectively. Micro-
lattice structures can be resolved with minimum feature sizes on the order of tens to hundreds of
microns.

Figure 6: Different views (top, perspective, side) of a polymer micro-lattice fabricated with three
UV beams of light [84].

14

The geometry of the cellular lattice is controlled by the incident angle of the UV light as
well as the pattern of the circular apertures on the mask [83, 92]. Truss and shell-like
architectures with 3-fold, 4-fold, and 6-fold symmetry [84], as well as truncated conical [40] and
octahedral-like [33, 36] unit cells are achievable. Select examples of attainable unit cell
geometries are schematically depicted in Figure 7. However, despite the feasibility of this
process, SPPW is limited by the attainable angles of incidence, which greatly restricts the
obtainable lattice topologies and structural complexity, excluding topologies like octet-truss,
tetrahedral-truss, 3D kagome, and tetrakaidekahedron.

Figure 7: Schematic of unit cell geometries that can be fabricated using self-propagating polymer
waveguides: a) 4-fold symmetry, b) 3-fold symmetry, c) six-fold symmetry [93].
2.3.2 Digital Light Processing
Digital light processing (DLP) is a freeform fabrication technique that utilizes a digital
micromirror device (DMD) to generate 3D structures (Figure 5b). Like SPPW, this process
leverages photopolymerization principles to develop 3D micro-lattice structures, however, DLP
processes offer greater geometric flexibility as a result of the layer-by-layer print approach.
Desired truss geometries are first designed using computer-aided design (CAD) software, which
are then virtually sliced into layers and sent to the DLP printer. Figure 3b outlines the general
printing process, where light is emitted from a projector or light emitting diode (LED) towards a
15

DMD that redirects the light pattern into a volume of photosensitive resin for each respective
layer. Upon exposure to the incident light, the polymerization reaction is initiated, and the layer
solidifies. The XY feature resolution is limited by the pixel size of the projector and can be as
high-resolution as 10 μm [85]. Exposure time, light intensity, and the concentration of
photoabsorber and photoinitiator in the resin all dictate resolution in the Z direction, where the
cure depth is often limited to a minimum of 10 microns [10, 94, 95]. The DLP 3D-printing
approach can be utilized with a variety of photocurable resins and preceramic polymers [96, 97].
2.3.3 Projection Micro-stereolithography
Projection micro-stereolithography (PµSL) is a fabrication process that utilizes a UV
LED, and DMD as a dynamic mask to fabricate complex 3D structures in a layer-by-layer
fashion (Figure 5c) [10, 98]. For each layer, a bitmap image generated from two dimensional
slices of a 3D model are projected onto the surface of a volume of liquid photosensitive resin.
Upon illumination, the projected pattern is cured via photopolymerization reactions and
subsequently lowered into the resin bath. A newly established layer of resin forms on top of the
solidified layer, where the successive process is continued until the desired 3D structure is
fabricated. Analogous to DLP, the resultant layer thickness, on the order of 10 to 100 microns, is
governed by a number of factors including: exposure time, light intensity, and chemical
composition of resin [10, 95, 99, 100]. The achievable feature size, on the order of a few
microns, is dependent on the resolution of the projector as well as the various
projection/reduction optics present [4, 101]. Within a matter of hours, micro-lattice structures on
the scale of centimeters can be resolved. In addition to fabrication speed and scalability, P SL
offers the ability to fabricate both first and second order truss geometries [10] in a variety of
resins. Without forgoing geometric complexity, P SL can produce 3D micro-lattice structures
16

derived from resins blends of inorganic nanoparticles [102, 103] and preceramic polymers [104-
106].
2.3.4 Two-Photon Polymerization Direct Laser Writing
Sub-micron resolution is achievable through multi-photon lithography processes such as
two-photon polymerization direct laser writing (TPP-DLW), which offers the highest polymer-
based AM resolution to date [4]. These processes enable the fabrication of intricate and arbitrary
3D nano- and micro-lattice structures [81] (Figure 8a-d). As highlighted in Figure 5d, a near-
infrared (NIR) femtosecond laser with a wavelength of 780-820 nm is tightly focused through a
high numerical aperture objective inside a small volume of negative-tone photoresist, where the
maximum absorption occurs at the focal point or voxel [4]. The absorption reaction depends on
the square of the light intensity, and polymerization is initiated at the focal point by the
simultaneous absorption of two NIR photons. The resulting elliptical voxel with resolution
typically ≥ 150 nm is then scanned in three dimensions, initiating crosslinking along the written
laser path. A few studies have elected for coupling TPP-DLW with positive-tone photoresists,
which when in contact with the written laser path becomes soluble and removed, resulting in a
3D lattice mold patterned into the volume photoresist [13, 107]. This route allows for subsequent
electroplating of metallic material into the mold for the development of a select few metal-based
solid-beam lattice structures.
Various TPP-DLW configurations can be employed impacting the resulting writing
speed, print volume and resolution. Dip-in laser lithography (DiLL) is the most leveraged
arrangement for the development of nano-lattice structures as it allows for the highest achievable
resolution using TPP-DLW [4, 108]. As depicted in Figure 3d, the laser is focused directly into a
drop of photoresist which is suspended upside down on a substrate. As such, a voxel size on the
order of 100 nm can be resolved due to the incident laser not having to travel through a
17

transparent substrate as in conventional lithography configurations. Use of a piezoelectric xyz-
stage enables nanometer precision granting greater complexity and resolution in structures
unattainable with SPPW, DLP, and P SL [4]. While TPP-DLW technology offers major
advantages in resolution and precision, a notable limitation of the technique is the overall
achievable build volume, which is notably restricted to a 300 m x 300 m x 300 m writing
region while employing the piezoelectric stage. Scaling-up methods such as stitching of several
print volumes need to be further investigated to fully exploit the benefits that sub-micron features
can have on the functionality of TPP printed materials [109-111]. A more detailed discussion on
the recent advances for scaling up TPP-DLW lattice structures is provided in Chapter 6.

Figure 8: Scanning electron micrograph of a) polymer pentamode mechanical metamaterial
fabricated by dip-in direct laser writing. The inset shows a schematic of the pentamode unit cell
(adapted from Kadic et al) [112].

Scanning electron micrograph of

b-d) 3-D kagome,
cuboctahedron, and tetrakaidecahedron topologies, with the insets showing CAD models of the
respective unit cells (adapted from Meza et al) [41].

18

2.4. Deposition Techniques for Coatings Nano- and Micro-lattice Materials

While technological advances in polymeric light-based AM techniques allow for rapid
development, topological complexity, and state-of-the-art resolution, restrictions to polymer-
based print-materials limit the overall mechanical robustness of the fabricated structures and
consequently inhibit the synthesis space of such advanced materials. Post-processing techniques,
such as the deposition of ceramic and metallic coatings, can be leveraged to overcome this
constraint and enable new fundamental studies of coated nano- and micro-lattice structures. In
general, coating material selection is driven by the desired functionality, where ceramic coatings
are typically elected for high-strength applications [6-8] and metals are often leveraged for
deformation and energy absorption capabilities [9, 22, 31]. While new AM methods and
techniques are being developed, the versatile multi-material fabrication approach outlined here
currently presents the most flexible and accessible route for developing mechanically robust
lattice materials at the nano and microscale.
Table 2: Main techniques for coating 3D nano- and micro-lattice structures with corresponding
deposition materials and resulting uniformity
Technique Deposition Materials Uniformity
Atomic layer deposition Mainly oxides and nitrides Conformal
Plating techniques
Limited to single elements and some
binary alloy systems
Relatively conformal
Magnetron Sputtering
Nearly unlimited selection of single
element, alloy, and ceramic systems
Varies

The most common deposition techniques employed for generating coated lattice
structures include atomic layer deposition (ALD), plating techniques such as electroless plating
and electrodeposition, and magnetron sputtering as shown in Figure 9. While each method bears
19

a tradeoff between available deposition materials and achievable coating uniformity, as
highlighted in Table 2, utilization of these techniques has led to the production of multifunctional
ceramic-polymer and metal-polymer core-shell composites, as well as hollow-tube structures that
exhibit unique combinations of mechanical properties [8-10, 35, 45, 46]. This section serves to
elucidate the feasibility and flexibility of ALD, plating techniques, and magnetron sputtering for
the development of coated nano- and micro-lattice structures. Further discussion on the various
core-shell composite and hollow-tube nano- and micro-lattice systems fabricated to date and
their characterization can be found in Chapter 4.

Figure 9: Illustration depicting the three main deposition techniques leveraged for developing
core-shell and hollow-tube lattice structures: a) atomic layer deposition, b) electroless plating,
and c) magnetron sputtering. b) Based on schematic from [22].

20

2.4.1 Atomic Layer Deposition and Plasma Enhanced Chemical Vapor Deposition
ALD has been extensively utilized to deposit ceramic coatings on nano- and micro-
architected lattice structures [6-8, 10-12, 23, 24, 34, 37-39, 41, 44, 46, 48]. The chemical vapor
deposition (CVD) technique leverages a set of sequential chemical reactions of two or more
gaseous precursors to synthesize films one atomic layer at a time, and thus allows for precise
control of coating uniformity on 3D lattice materials [113, 114]. Figure 9a outlines an example
of the pulse/purge ALD process for Al2O3 on a complex lattice structure, where initial pulsing of
the first gaseous precursor trimethylaluminum, Al(CH3)3, is introduced into the reaction chamber
and bonds to active hydroxyl (OH
-
) sites on the surface of the structure. Methane gas, CH4, is
generated as a byproduct and is purged from the system. Subsequently, water vapor, the second
gaseous reactant, is pulsed into the chamber to react with remaining CH3 molecules, producing
additional methane byproduct and hydroxyl reactive sites. The gaseous reactants in each
sequential pulse undergo a self-limiting reaction, meaning the reaction is terminated when the
reactive sites on the surface are completely depleted. The cycle is continually repeated to build
up the coating layer-by-layer, where the amount of deposited material per cycle is governed by
the flux and concentration of the reactant at the substrate surface [115]. Layer growth is typically
on the order of 1 Å/cycle, resulting in both amorphous and nanocrystalline oxide or nitride
coatings with thicknesses ranging from 5 nm to 200 nm on nano- and micro-lattice scaffolds [8,
10]. Examples of ALD coated structures are highlighted in Figure 10.
Additional CVD processes, such as plasma enhanced chemical vapor deposition (PE-
CVD), have also been explored for the deposition of ceramic coatings on architected lattice
structures [116]. In PE-CVD, plasma is utilized to help encourage the reaction between two or
more gases on the surface of a substrate. This allows for the reaction to take place at
21

temperatures lower than conventional CVD processes, which helps mitigate any melting or
warping of the polymer structure during deposition [117]. For example, Al2O3 deposition via
ALD on micro-lattice scaffolds have reached deposition temperatures as high as 250°C [34],
while deposition of SiN on polymer micro-lattice scaffolds via PE-CVD reach close to 40°C
[116]. While CVD methods have allowed for the deposition of thin uniform coatings, there are a
number of disadvantages that limit the feasibility of ALD and PE-CVD for the development of
architected composite lattice materials. These limitations include a slow growth rate (on the
order of angstroms/cycle) and a restricted material selection, which is mainly limited to metal-
oxide and -nitride coatings, ultimately eliminating common engineering materials such as Ti and
Al alloys [118].

Figure 10: (a) SEM micrograph of hollow TiN nanolattice developed via TPP-DLW and ALD.
Scale bar is 20 μm. (b) SEM micrograph of hollow nanolattice, with the inset showing the cross-
section of a hollow TiN strut. Scale bar is 5 μm (inset is 1 μm) [23]. (c) SEM micrograph of
alumina coated octet-truss nanolattice fabricated via TPP-DLW with corresponding (d) high-
magnification SEM micrograph of alumina octet-truss nanolattice, where the inset shows an
isolated hollow strut [8].

22

2.4.2 Plating Techniques
Plating techniques, namely electroless plating, have been employed for generating
metal/polymer core-shell composite and hollow metallic micro-lattice structures [10, 14, 22, 30,
31, 33, 35-37, 40, 42, 43, 49]. The electroless plating process is an autocatalytic method that
relies on the chemical reduction of aqueous metal ions without use of an external current [119].
Single element and binary-alloy surface-coatings are deposited by immersion in a solution which
supplies its own internal electric current, where various film properties, including microstructure,
composition and thickness are controlled by adjusting plating parameters such as constituent
solution concentrations, and immersion time [120]. Figure 9b summarizes an example of the
electroless deposition process for NiB coatings on a polymer scaffold, where the base polymer
surface first goes through a series of surface activation steps, such as palladium catalyzation or
chemical etching, to ensure the surface is catalytic [22, 30, 33, 42]. In some instances,
electrodeposition is employed to deposit coatings on architected structures, where unlike
electroless plating, this process requires an electrical current (direct or pulsed current) to initiate
deposition, as well as a conductive metal surface to deposit onto. A number of studies have
electroplated or sputtered thin metallic seed layers onto the polymeric scaffold in order to
metallize the surface of the lattice structure prior to electrodeposition [14, 33, 121-123]. Plating
thicknesses achieved on micro-lattice scaffolds range from approximately 10 nm to 25 μm [22,
33]. While such techniques allow for the deposition of more commonly used materials like Cu
and Ni, the nature of the deposition method confines coating materials to primarily single
elements and a limited selection of binary-alloy systems [124].
2.4.3 Planar Magnetron Sputtering
More current studies have explored physical vapor deposition techniques (PVD),
specifically radio frequency (RF) and direct current (DC) magnetron sputtering, as means to
23

synthesize new combinations of core-shell composites and hollow lattice materials [9, 11, 12, 18-
20, 25-29]. Unlike ALD and plating techniques, sputtering techniques allow for the deposition of
a wide range of single element, alloys, and ceramic materials. Figure 9c illustrates the planar
magnetron sputtering process, where an inert gas, most often Ar, is pumped into a vacuum
chamber and subsequently ionized as result of an applied negative bias to the target (cathode)
[125]. The magnetron affixed behind the target causes secondary electron circulation on a
confined path at the target surface, creating a high-density plasma around the cathode [126, 127].
Consequently, high-energy positively charged ions bombard the target with enough momentum
and energy to eject target material atoms. The ejected atoms move across the vacuum chamber
and deposit onto a substrate or lattice structure, generally at ground potential [128]. Magnetron
sputtering is explained in further detail in Section 3.2.
Studies have shown that sputtering can achieve both nano-grained and amorphous
metallic coatings on nano- and micro-lattice structures [9, 25-28]. While the coating thicknesses
in these studies have spanned ≈10 nm to 800 nm [20, 27, 28], sputtering allows for films to be
deposited from a few monolayers up to 100+ μm [129-131], although they typically do not
exceed 25 μm [132, 133]. The deposited coatings on nano- and micro-lattice scaffolds have
ranged from single elements [25, 26] to Ni-based super alloys [19] and high-entropy alloys [9,
18, 20]. To date, the evaluation of sputtered coatings on architected polymer scaffolds have
revealed large coating gradients irrespective of deposition material. Figure 11 highlights
examples of sputter coated nano- and micro-lattice structures that display regions of non-
conformal coating thicknesses, where orange arrow and green arrows highlight regions of thicker
and thinner coatings, respectively. The observed thickness gradient is realized from the top of the
structure to the bottom, as most clearly depicted in Figure 20a, as well as around individual struts
24

throughout. While the variations in deposited coatings may lead to an inaccurate evaluation of
properties and deformation behavior, the flexibility of sputtering allows for the realization of a
much wider selection of architected material systems. Optimization of sputter coating uniformity
will ultimately allow for the accurate mechanical assessment and synthesis of new systems of
advanced architected materials. An in-depth discussion on sputter coating uniformity can be
found in Section 4.3.2 and Chapter 5.

Figure 11: Scanning electron micrographs of sputtered a) Zr56Ni22Al22 [27], b) Au [25], c)
Cu60Zr40 [29], and d) CoCrFeNiAl0.3 [18] nano- and micro-lattice structures highlighting a
coating thickness gradient both around individual struts and from top to bottom of the structure.
Arrows indicate thinner (green) and thicker (orange) regions to visually demonstrate the gradient
around individual struts.

2.5. Mechanical Properties and Functionalities of Architected Lattice Structures
Three-dimensional architected materials with nano- and microscale features have been
shown to exhibit unprecedented properties, such as high specific strength, high-recoverability,
25

and unique deformation behavior [9, 10, 19, 38, 116, 134]. Such materials have been referred to
as “mechanical metamaterials”, which combine influences from topological design and material
size effects at the micro- and nanoscale to achieve properties ranging from ultrahigh strength to
recoverability in intrinsically brittle materials like ceramics. Generally, the mechanical properties
of open-cell lattice materials are governed by the relative density, constituent material properties,
nodal connectivity or topology of the cellular structure, and the length scale, which influences
any size-dependent strengthening. The reliability and durability of these materials are primarily
determined by their strength and recoverability. This section serves to provide a general
summary of the various mechanical behaviors and functionalities achieved by nano- and micro-
architected lattice materials.
2.5.1 Mechanical Behavior

The specific strength of architected lattice materials is influenced by relative density,
topology, and material size effects. In general, for structures with relative densities ≤ 1%, the
topology seems to have a greater impact on the strength of the than the feature sizes. This trend
is observed in the material-property chart shown in Figure 12, where the compressive
strength/Young’s modulus versus relative density of an expansive set of hollow-tube and core-
shell composite macro-, micro-, and nano-lattice structures are plotted [4]. The impact of
introducing designed architectures into materials is evident when comparing rigidly designed
hollow-tube nickel based micro-lattice materials (example outlined in black oval) to lattices with
non-rigid architectures (example outlined in blue oval), where the hollow Ni-based rigid lattice
(𝜌 ≈ 0.3%) [10] exhibits a strength roughly 20 times higher than the hollow non-rigid Ni-based
lattice [135] of similar relative density.
26

Strengthening from material size effects have also been observed when the length scale
of the material is decreased to the nanoscale [136-138]. This “smaller is stronger” phenomenon
has been observed in architected materials that contain strut diameters on the nanoscale, and in
ceramic hollow-tube nano- and micro-lattice structures, where the wall thickness of the thin
coating is in the nanoscale regime. For example, it is observed that hollow-tube alumina octet
nano-lattices with wall thicknesses ranging from 5 to 50 nm attain strength-to-density ratios
similar to certain aluminum alloys [4, 8]. However, the “smaller is stronger” phenomenon is not
always the case. For instance, Montemayor et al. tested Au hollow-tube octahedral nano-lattices
with wall thicknesses of 200 nm, 327 nm, and 635 nm under uniaxial compression that showed a
yield stress increase of 120% between the 200 and 327 nm tube wall thicknesses, and
approximately 13% as the thickness increased from 327 nm to 635 nm [25].

27

Figure 12: Absolute compressive strength versus density material-property chart of a wide range
of nano-, micro‐, and macro-lattices as well as stochastic nanoporous and commercial bulk
materials. Symbol shapes relate to the constituent material, and colors indicate the length scale of
structure feature (filled in relates to feature diameter, and lines indicate wall thickness, if any).
Points enclosed by the black oval refer to a hollow NiP octet micro-lattice material with a
relative density of 0.3%, and points enclosed by the light blue oval represent a hollow NiP
octahedral type nanolattice with similar relative density. Adapted from Bauer et al. [4].
28

Studies on architected materials have also highlighted a trade-off between strength and
recoverability. Schaedler et al. demonstrated that hollow-tube Ni-P octahedral micro-lattices
displayed complete recovery post compression to a strain >50%, but with the strength only
reaching about 10 kPa [36]. Similarly, Meza et al. showed that hollow-tube alumina octet nano-
lattices with a wall thickness of 10 nm only achieved a strength of 1.0 MPa, but showed full
recovery to approximately 40% strain. As the nano-lattice wall thickness was increased to 20, 40,
and 60 nm, the achieved strength increased from 4 to 30 MPa; however, the stronger nano-
lattices experienced brittle fracture and collapsed at compressive strains greater than 5-10%
(Figure 13) [8]. The strength-recoverability trade-off is also observed in architected core-shell
composite lattice materials. Surjadi et al. fabricated polymer micro-lattice structures coated in an
800 nm thick high-entropy-alloy (HEA) coating. These metal-polymer composite micro-lattices
exhibited a strength of 6.9 MPa under compression, but fractured at a strain of only 6.5%,
suggesting that at strains >7%, the structures would not experience any recovery [20]. However,
in a more recent study by Zhang et al., novel octet HEA/polymer nano-lattice composites were
developed that exhibit a combination of high specific strength (11.6 MPa) and recoverability
under compression to strains >50%, overcoming the strength-recoverability trade-off [9].

Figure 13: Mechanical data and still frames from the compression test on a (a-e) 10 nm thin-
walled alumina nanolattice depicting the ductile deformation, local shell buckling, and recovery
of the structure after compression and (f-j) a 50 nm thick-walled alumina nanolattice
demonstrating catastrophic brittle failure and no recovery [8].

29

Recoverability has been demonstrated in both bending- and stretching-dominated
structures, where a transition from brittle failure to a recoverable deformation via shell buckling
of thin-walled struts is realized below a certain wall-thickness/strut diameter threshold. However,
shell buckling recovery mechanisms in architected materials require that the relative density of
the structure be very low and that wall thickness be thin and have a large radii of curvature in
order to promote recovery of the structure [4]. Non-rigid lattices have been shown to exhibit
beam buckling dominated failure, such as in the case of structures with octahedral-like unit cells,
where nodal reinforcement and the beam slenderness ratio (length/radius) dictate the post-failure
recoverability [83]. As a result of local damage, the post-yield properties of the material are
predominantly lower than the pre-tested material [31, 36].
2.5.2 General Applications of Architected Materials
The most explored application of nano- and micro-architected lattice structures is in
photonic crystals for wave guiding applications [139-143]. The periodic structure of optical band
gap materials brings about photonic band gaps, which is a range of electromagnetic waves
(certain frequencies and corresponding wavelengths) which cannot propagate inside the
structure. Photonic crystals with nanoscale features allow for the generation of unique optical
properties such as negative refraction [139, 144], which can be leveraged to develop lenses for
subwavelength imaging [145, 146]. Similarly, periodic phononic crystals, which are designed to
prohibit wave propagation of certain ranges of frequencies have been fabricated and can tailor
ultrasonic wave propagation [147]. Such materials can be employed for vibration damping,
acoustic waveguiding, and acoustic filters.
TPP-DLW printing techniques have been more recently utilized in developing auxetic
materials. These are materials that exhibit a negative Poisson’s ratio, which means they stretch
30

perpendicularly to an applied tensile stress and shrink along the horizontal direction upon
vertical compression. 3D auxetic lattice materials have also been shown to exhibit acoustic
absorption that is an order of magnitude better than conventional foams [148]. Additionally,
there is potential for fabricating lattice micro-structures similar to auxetic materials with tunable
thermal expansions for micromirror arrays in space applications [149].
Architected materials exhibit other unconventional properties useful for thermal
management, energy storage and biomedical applications. Lattice materials have been employed
as heat pipe structures for active cooling, which exhibit advanced thermal properties when
compared to conventional foams [150, 151], and the incorporation of microscale architecture has
also allowed for enhanced heat transfer rates in heat exchangers [152, 153]. Furthermore,
microfabrication techniques have been employed to develop nano- and micro-architected lattice
structures in electronics as supercapacitors and battery electrodes [14]. In addition to applications
in electronics and thermal management, architected materials have been demonstrated to play a
significant role in the biomedical field, such as scaffolds for cell culturing and tissue engineering
[12, 154, 155], as well as arterial and esophageal stents [156, 157]. Continuing research on the
emerging class of architected lattice structures will generate novel functional materials for new
applications. An extended discussion on the properties and functionalities of coated nano- and
micro-lattices can be found in Chapter 4.

31

Chapter 3: Experimental Methods
The following section includes an overview of the fabrication techniques used to generate
nano- and micro-lattice polymer structures and the deposition configurations for this study.
Characterization methods used for imaging the fabricated structures and assessing the coating
microstructure, morphology, and coating uniformity of core-shell composite micro-lattice
structures. Additionally, the following section provides an overview of the mechanical
characterization technique used to study the mechanical behavior of stitched TPP-DLW nano-
architected structures.
3.1 Two-Photon Polymerization Direct Laser Writing

TPP-DLW is a microfabrication technique that employs two-photon polymerization, a
process that enables printing of nanoscale features, the highest achieved polymer AM resolution
to date [4]. The photopolymerization reaction outlined in Figure 14a highlights the process,
which is first started by the simultaneous absorption of two NIR photons. Full initiation of the
reaction occurs through the generation of free radicals produced from the cleavage of C=C bonds
present within in the resin monomers and oligomers (Figure 14b) [81, 158]. The crosslinking
chain reaction then propagates until termination takes place, as a result of either the self-reaction
of radicals by combination and/or disproportionation, the reaction of a radical with an initiator
derived radical (primary radical termination) or a radical with another species (inhibition) [159].
This nonlinear absorption process is made possible through use of a femtosecond laser which is
tightly focused inside a small volume of photosensitive polymer resist. In the localized focal area
or voxel, the laser intensity is sufficiently high, and polymerization is initiated. Figure 15
illustrates this process, where 3D structures are realized by tracing of the voxel in three
dimensions.
32

Figure 14: a) Photopolymerization chemical reaction of the two-photon polymerization process.
b) Jablonski energy diagram depicting an electron which can become excited from the ground
state, which subsequently relaxes and undergoes a spin-flip transition entering an excited triplet
state. Adapted from Maruo et al. and LaFratta et al.[81, 158]
A Nanoscribe Photonic Professional GT laser lithography system was used to fabricate
samples in this work. The system can utilize a DiLL write-mode in which a drop of Nanoscribe’s
epoxy based proprietary resin, IP-Dip, is placed on a fused silica slide and then mounted upside
down on a piezoelectric stage, which has a restricted stage range of 300 µm in the X, Y and Z
directions. The architected structures are printed on the surface of the inverted glass slide and are
then developed in a propylene-glycolmethyl-ether-acetate solution followed by isopropyl alcohol
to remove uncured resist. Lastly, the slide is subjected to super critical CO2 drying to mitigate
any warping or collapse of the structural features [160, 161].
33

Figure 15: Schematic of the direct laser writing process in 3D-space inside a resist layer. The
inset shows the voxel at the focal position, where two-photon absorption only occurs within the
voxel [162].

3.2 Magnetron Sputtering
Magnetron sputtering is a versatile PVD technique that leverages ionized gas particles to
coat a substrate. The flexibility of the deposition parameters and available coating materials
makes sputtering an ideal candidate for this study. Figure 16 shows a representative schematic of
the planar magnetron sputtering process, where an inert gas, most often Ar, is pumped into a
vacuum chamber and subsequently ionized as result of an applied bias to the target (cathode)
[125]. The magnetron affixed behind the target causes secondary electron circulation on a
confined path at the target surface, creating a high-density plasma around the cathode [163]. The
positively charged ionized gas is attracted to the negatively biased target and bombards it,
causing the ejection of target material atoms resulting from the momentum and impact of the
incident ions. The ejected atoms move across the vacuum chamber and deposit on a substrate
(anode) generally at ground potential [164].
34

Figure 16: General schematic showing the process of magnetron sputtering: an inert gas (Ar) is
introduced into a vacuum chamber and ionized by the negatively biased target, where the Ar
+
ions are attracted to and strike a target causing the ejection of target atoms that coat a substrate.
In this example, the substrate is a 3-D micro-lattice structure.

There are several controllable parameters during sputtering that influence the overall
deposition rate, temperature, and uniformity, as well as the resulting coating microstructure.
These include the pressure of inert gas that is flowed into the system, the voltage, current and
power applied to induce target polarization, the distance between the target and substrate, the
configuration of the cathode(s) in the chamber, as well as substrate rotation during deposition
[18, 19, 132, 165, 166]. Additionally, the type of power supplied to the target, either direct
current (DC) or radio frequency (RF) power, can be varied. DC power allows for sputtering at
low pressures, generally <5 mTorr, leading to higher deposition rates at lower supplied powers
and improved coating uniformity when compared to RF sputtering [167, 168]. Magnetron
sputtering is a line-of-sight limited deposition method, meaning that areas of a substrate not
directly in line with the target are shadowed from the source material. This effect yields a
thickness variation in the deposited coating; thus, sputtering is most effective for the deposition
of films on flat substrates.
35

Figure 17: Schematic comparison of planar and inverted cylindrical cathode target geometries
and deposition pathways.
Different cathode geometries and configurations exist in the sputtering regime — namely
planar or inverted cylindrical magnetron (ICM) (Figure 17) [169, 170]. Deposition with a planar
cathode results in unidirectional deposition, while the ICM cathode provides an unprecedented
360° line-of-sight. Figure 18 highlights the ICM sputtering process, which makes use of a hollow
cylindrical target. It is important to note that that the principles of the sputtering process in the
ICM configuration is analogous to planar sputtering, however deposition is not limited to one
direction. ICM sputtering has also demonstrated the ability to ionize larger fractions of sputtered
material and increase ion bombardment during film deposition due to the tubular nature of the
target geometry. The magnetic configuration also traps a greater number of electrons compared
to conventional planar sputtering, which allows it to achieve a higher-density plasma during
deposition [171, 172].
36

Figure 18: Schematic of the hollow cathode sputtering process.
3.3 Characterization Techniques
Multiple characterization techniques are leveraged in this dissertation to investigate the
features of as-printed nano- and micro-lattice structures, as well as characterize the
microstructure and morphology of any coatings deposited on the polymer scaffolds. These
various techniques are discussed in the following section.
3.3.1 X-ray Diffraction
In this work, X-ray diffraction (XRD) will be used to identify the crystallographic
orientation (texture) and phase of the metal films developed via magnetron sputtering. XRD is a
nondestructive characterization technique that utilizes X-rays, which upon scattering give rise to
diffraction peaks that correlate to the spacing of lattice planes in the sample. The produced
spectrum reflects the scattered intensity as a function of 2θ, where the resulting direction of the
37

scattered x-rays is determined by the wavelength of the incident wave and the atomic
arrangement of the sample. The diffraction from the film is described by Bragg’s law:
𝑛𝜆 = 2𝑑 𝑠𝑖𝑛𝜃
where n is a positive integer, λ is the incident wavelength, d is the interplanar spacing of the
lattice, and 𝜃 is the incidence angle of the X-ray beam. This relationship is satisfied when there is
constructive interference between the incident wave and sample atoms.
XRD is performed on powders and flat surfaces, therefore the direct coatings deposited
on 3D polymer structures cannot be characterized. Thus, thin film coatings deposited on glass
substrates (Corning) will be used to identify the crystallographic orientation of the deposited
metal coatings. These measurements will be carried out using a Rigaku Ultima-IV X-Ray
diffraction instrument.
3.3.2 Scanning Electron Microscopy and Electron Dispersive Spectroscopy
Scanning electron microscopy (SEM) is a technique used for examining, imaging, and
analyzing the microstructure and chemical composition of a material. In SEM, a focused electron
beam generated by a field emission gun scans over the surface of a sample producing signals of
secondary and backscattered electrons, as well as characteristic X-rays, which are collected by a
detector. The underlying principles of SEM depend on a signal produced by the interactions
between an electron beam and the specimen, as depicted in Figure 19 [173, 174]. Such
interactions can be either elastic or inelastic, where the former is distinguished by a negligible
amount of energy loss.
38

Figure 19: Schematic of several types of signals generated by the interaction between a primary
electron beam in a scanning electron microscope with the corresponding available regions from
which the electrons can be detected [174].
When the primary electron beam strikes the sample surface, the most common signal produced is
a secondary electron (SE) emission signal. The incident electron beam causes the ionization of
surface atoms, effectively emitting loosely bound SEs. Due to their low energy values (≈3-5eV),
the SEs are attracted towards the detector and provide topographic contrast in the SEM, most
useful for visualizing surface texture and roughness [174].
Higher energy electrons (>50 eV) that are elastically scattered from the sample are
referred to as backscattered electrons (BSE). The larger energy that is possessed by these
electrons comes as a result of the large specimen area from which they originate, resulting in a
lower achievable resolution when compared to SEs (Figure 19). These electrons are leveraged in
39

characterization techniques such as electron backscattered diffraction, which is utilized to obtain
grain orientation/texture information of polycrystalline materials [175, 176].
X-ray signals can also be collected as a result of the interaction between the incident
electron beam and the sample. These signals are produced when a non-valence electron is
disturbed and displaced as a result of an inelastic collision with an incident electron [174].
Following this type of ionization event characteristic X-rays of the atoms are emitted, where the
associated energies can be identified and quantified. Characteristic X-rays are leveraged in
techniques such as energy-dispersive X-ray spectroscopy (EDS), which provides elemental
composition information of a specimen.
3.3.3 Plasma Focused Ion Beam
Plasma focused ion beam (PFIB) is used as a sectioning method for exposing the cross-
section of micro-architected lattice structures allowing for coating thickness and uniformity
characterization, as shown in Figure 20. PFIB systems utilize a beam of Xe
+
ions to bombard and
effectively mill/erode the specimen, and as a result emit secondary electrons from the surface of
the structure which are measured by a detector for imaging. Juarez et al. highlighted that the Ga
+

FIB process on micro-architected lattice structures results in a substantial amount of redeposition
(Figure 20b) [19] which can lead to an inaccurate characterization of the coating thickness
measurements and uniformity determinations. Compared to conventional Ga-based FIB, Xe-
based PFIB results in a much higher milling rate, and has been shown to produce less surface
damage [177].

40

Figure 20: a) Image of a milled metal/polymer core-shell composite micro-lattice structure
exposing the coating cross section. b) SEM micrograph of a strut cross section exposed with Ga-
based FIB milling highlighting the material redeposition caused during sectioning [19].
3.3.4 Microtome
In addition to PFIB, microtome cross-section is employed to characterize coating
thickness and uniformity of sputter coated micro-lattice structures. The technique is commonly
utilized as a biological sectioning technique for electron microscopy sample preparation. Sputter
coated micro-lattice structures were heat polymerized and embedded in an epoxy resin (Epon)
and gently removed from the glass slide by thermal cycling in liquid nitrogen. The samples were
then re-embedded in epoxy resin to encase the structures from both sides. Once fully embedded
in epoxy, the samples were sectioned 100 nm thin with a diamond knife on an ultramicrotome,
floated on water and picked up on formvar-filmed 2x1 mm slot grids. The grids were then
imaged in a transmission electron microscope. This sectioning technique allows for accurate
coating thickness measurements to be obtained due to the embedding process.
3.4 Micro-tensile Testing
Micro-tensile tests on stitched TPP-DLW printed specimens were carried out on a
custom-built micro-tensile testing machine (Section 6.2, Figure 34). The tensile apparatus
consists of a coarse actuator (ThorLabs DRV001), a piezo actuator (ThorLabs PAZ015), a 2D
alignment stage, a 1 N load cell (Phidgets CZL639HD), a camera (Pixelink PL‐B782F), and a
41

light source. The actuators are controlled by a custom MATLAB code, which allows for
movement in the forward or reverse direction for compression or tensile loading, respectively.
The testing stages available for the instrument are interchangeable and can be designed for
specific testing configurations including tensile or compression tests, and three or four-point
bending.

42

Chapter 4: Current State of Coated Nano- and Micro-lattice Materials
The following work is published as an invited review article titled A Review of Coated
Nano- and Micro-lattice Materials and is published in the Journal of Materials Research in
pages 1-21 (DOI: 10.1557/s43578-021-00178-6).
As discussed in Section 2, the emerging class of nano- and micro- lattice materials, which
are primarily fabricated from polymers, can be further functionalized through the deposition of
ceramic and metallic coatings yielding unprecedented mechanical behaviors and functionalities.
By leveraging an expansive material workspace, architectural hierarchy, and materials size
effects, these structures have been shown to achieve high strength, low density, and exceptional
recoverability with promise in applications ranging from cell and tissue growth to energy
storage. The following sections in Chapter 4 serve to address the current development space of
coated nano- and micro-lattice structures and highlight important considerations, such as coating
material workspace and achievable uniformity, that influence their behavior and functionality. In
discussing the various coated nano- and micro-lattice systems fabricated to date and present
challenges, this section offers foundational insight on generating new advanced coated lattice
systems.
4.1. Coating Materials Workspace
As outlined in Section 2.4, the available coating material working space for core-shell
composite and hollow-tube nano- and micro-lattice structures is governed by the deposition
technique. Figure 21 outlines the various coating compositions and corresponding techniques
leveraged to date for both nano- and micro-lattice fabrication. Additionally, lattice classification
is indicated in the figure, where boxes designated with solid and dashed outlines correlate to
core-shell composite and hollow-tube systems, respectively. Less common coating methods such
43

as dip coatings, traditional chemical vapor deposition techniques, and electron beam evaporation
are also included.

Figure 21: Overview of the various materials deposited on 3D nano- and micro-lattice structures
and the corresponding coating techniques utilized.

44

Electroless plating and electrodeposition were the first coating techniques used to
develop coated lattice structures. Studies utilizing these methods focused on the generation of
Ni-based lattice structures including pure Ni, NiP, NiB, as well as other elementary metallic
systems for energy dissipation and elevated strength applications [10, 22, 30-33, 37, 43, 49, 122,
123, 178]. Following this, ALD was leveraged to deposit various oxide and nitride ceramic
coatings [6-8, 11, 12, 23, 34, 37-39, 41, 44-46, 48]. Jang et al. [23] and Maloney et al. [37] were
of the first to employ this technique, developing hollow-tube TiN and SiO2 nano- and micro-
lattice systems for investigation into the mechanical implications of the added ceramic coating
material. Following this work, several publications authored by Bauer et al. [7, 34, 38, 44] and
Meza et al. [6, 8, 24, 41] explored the mechanics of various Al2O3 coated core-shell composite
and hollow-tube nano- and micro-lattice structures derived via ALD and two-photon
polymerization direct laser writing.
Given the compositional limitations of both ALD and plating methods, the studies
presented to date have already explored most of the material working space for these techniques.
Appropriately, an increasing number of recent studies have elected for generating coated nano-
and micro-lattices through means of physical vapor deposition due to the expanded availability
of deposition materials. Employing DC and/or RF sputtering has allowed for the deposition of
single element metallic coatings like Au [14, 25, 26] and Al [19], high entropy alloy systems [9,
18, 20], metallic glasses [27-29] , metal-oxides [11], and complex engineering alloys like Inconel
600 and Ti-6Al-4V [19]. However, the development of magnetron sputtering as a deposition
technique for nano- and micro-lattice structures is still in the early stages and offers great
potential in developing novel architected materials.
45

In conjunction with broadening the coating materials working space, researchers also
look to build upon the lattice geometry space of nano- and micro-lattice structures. Figure 22, to
the best of the authors’ knowledge, offers a comprehensive chart detailing all combinations of
lattice configurations and coating compositions used to date to fabricate coated nano- and micro-
lattice systems. Structures are categorized by geometry and further broken-down by lattice
classification, where shaded and unshaded symbols correlate to core-shell and hollow-tube
structures, respectively. Representative images for each geometry are provided as reference.
Systems which have been mechanically tested, either under tension or compression, are denoted
with an asterisk. As the list of coating materials continues to grow and become greater
diversified, there exists a vast opportunity for new coating material/lattice geometry
combinations to be realized, potentially reaching new mechanical and functionality spaces.
46

Figure 22: Comprehensive chart categorizing geometry and material combinations of core-shell
and hollow-tube nano- and micro-lattice structures fabricated to date, where structures denoted
with an asterisk have been mechanically tested. Representative images of each lattice geometry
are provided [6, 10, 18-21, 27, 38, 40, 41, 179].

47

4.2. Geometry Workspace: Coating Non-truss Based Lattices
To date, the deposition of coatings on nano- and micro-lattice structures has primarily
concentrated on truss-based architectures [5-26, 29-35, 37-39, 41, 43-45], while coating studies
on other lattice topologies have been limited [36, 42]. Progress in the fabrication of shell- and
plate-based lattice structures with nano- and micro-scale resolution [36, 42, 54, 60] has expanded
the available architectures for coating studies. Recently, coatings on shell-based lattice structures
comprised of non-intersecting thin-walled surfaces and a periodic network of interconnected
porosity have been leveraged [36, 42]. These smooth, curved, open-cell architectures have
allowed for the deposition of uniform coatings through gaseous-state chemical processes like
ALD [42, 58, 59], and relatively uniform coatings via solution-based methods like
electrodeposition [36]. However, deposition through means of a solid precursor, such as
sputtering, presents a challenge as physical vapor deposition methods are largely line-of-sight
dependent. The aspect ratio and volume fraction of the shell walls can potentially lead to a higher
degree of blocking within the lattice structure [120] compared to truss-based lattices. In the case
of plate-based nano- and micro-lattices, the mechanical properties of polymeric [60] and
pyrolytic carbon [54] structures have recently been explored; however, to the best of the authors’
knowledge, there have been no coating studies as of yet. The closed-cell nature of plate-based
lattice designs imposes a challenge in implementing the deposition of ceramic and metallic
coatings as they will be predominantly restricted to the outer surfaces of the lattice structure [54].
Coating studies for further expanding the mechanical property and functionality spaces of nano-
and micro-lattice structures will continue to develop as the fabrication of new and more complex
nano- and micro-lattice topologies are realized.

48

4.3. Coating Characterization
Despite the wide range of material systems and geometries fabricated to date, a discrete
set of microscopy techniques are employed for characterizing the coating microstructure,
composition, thickness and uniformity on nano- and micro-lattice materials. Frequently utilized
characterization methods are outlined in Figure 23, where less common techniques are
highlighted with an asterisk. The flowchart is divided into two sections: one focused on
analyzing coating composition and microstructure, and the other on coating thickness and
uniformity. As is highlighted in the figure, certain techniques are broadly employed for
characterizing multiple facets of nano- and micro-lattice coating materials, while other methods
are specifically leveraged for the determination of composition, microstructure, or coating
thickness.

Figure 23: Chart outlining prominent characterization techniques utilized to analyze the
microstructure and composition of deposited coatings on nano- and micro-lattices (left), as well
as the resultant coating thickness values and uniformity (right). Note that asterisks denote less
commonly employed techniques for characterizing coated nano- and micro-lattice structures.

49

4.3.1 Composition and Microstructure
The characterization of coating composition and microstructure will first be explored as
outlined on the left-hand side of Figure 23. Several studies couple scanning electron microscopy
(SEM) with energy dispersive x-ray spectroscopy (EDS) to quantify the elemental distribution
and composition of the deposited coatings. Publications leveraging ALD for coating nano- and
micro-lattice structures indicate chemically uniform thin films on the surface of the polymeric
scaffolds [8, 23] as the nature of the deposition method allows for high degrees of compositional
control [113]. However, the composition of coatings deposited via wet chemistry approaches like
electroless plating greatly depend on constituent solution concentrations in the submersion bath
[120], resulting in a more variable composition control. EDS measurements on various NiP
plated micro-lattice structures highlight the elemental variations via electroless plating, with
compositions spanning 2.6 wt% P [43] to 24.7 wt% P [30]. As for sputtering, variations in thin-
film composition relative to the target material have been reported as a result of preferential
sputtering substrate bias, and substrate temperature [180-183], however sputter coatings
deposited on nano- and micro-architected scaffolds exhibit stoichiometric compositions in close
agreement with the target material [9, 11, 12, 18-20, 25, 26]. EDS measurements from sputter
coated nano and micro-lattice structures ranging from Inconel 600 and Ti-6Al-4V [19] to Zr-Ni-
Al metallic glass [27, 28] and CoCrFeNiAl high entropy alloy (HEA) [18] were all in close
agreement with the sputtering target utilized in each respective study.
Limitations in EDS detector sensitivity can result in misidentification or quantification of
low atomic number elements (Z<11) such as C, N, and B [184]. Therefore, other techniques can
be leveraged for accurate compositional analysis; for example, Mieszala et al. utilized elastic
recoil detection analysis (ERDA) measurements to quantify the B content in NiB coatings
deposited on micro-lattice scaffolds via electroless plating [22]. X-ray diffraction (XRD) has also
50

been exercised for determining coating composition [35], but is more often included for phase
and crystallographic orientation identification of the coatings [9, 19, 21, 22, 28, 30, 33, 43]. A
number of publications utilizing electroless plating of NiP and NiB coatings report broad peaks
detected in the XRD scans [22, 30, 33, 43]. Such a characteristic either indicates that the coatings
are amorphous in their as-deposited state or as in the case of Torrents et al., suggests the
presence of nanocrystalline grains (~7 nm) and internal lattice distortion due to the presence of P
in solid solution [33]. Similarly, as-deposited amorphous coatings on polymeric scaffolds have
been realized by means of magnetron sputtering. For example, both Thompson et al. [28] and
Liontas et al. [27] sputtered Zr54Ni28Al18 metallic glass coatings on octahedron nano-lattice
structures resulting in an amorphous structure. Nonetheless, a number of studies exploring
sputter deposition on nano- and micro-lattice structures report strong texturing and distinct
phases [9, 18, 19]. At times, metastable phases can occur in sputter deposited coatings on lattice
structures as a result of the rapid condensing of high-energy atoms on a the surface of a low
temperature substrate [185]. This phenomena is highlighted in the work of Juarez et al. [19],
where XRD data of sputtered Ti-6Al-4V indicates the unexpected presence of β-phase Ti with no
α-phase peaks detected despite the β transus of Ti-6Al-4V occurring at approximately 980°C
[186].
Several publications have further characterized the microstructure of deposited coatings
using transmission electron microscopy (TEM) and selected area electron diffraction (SAED)
techniques. Both qualitative and quantitative information such as grain morphology and grain
size as well as crystal structure can be extracted from TEM micrographs and indexed electron
diffraction patterns, respectively. For instance, in addition to XRD and EDS, Gao et al. utilized
high resolution TEM to characterize the microstructure of a sputtered CrFeCoNiAl0.3 HEA
51

coating and was able to observe the presence of small nanocrystals throughout the film, as well
as determine the grain size ranged from 5 to 20 nm [18]. SAED patterns further confirmed the
nanocrystalline nature of the coating. Additionally, 3D atom-probe tomography (APT), a 3D
imaging and chemical composition analysis technique, was used to verify homogeneous
elemental distribution and phase structure throughout the HEA coating [18].
4.3.2 Coating Thickness and Uniformity
A vast majority of studies do not include detailed information regarding characterization
steps for coating thickness measurements; however, of those which do, a number leverage
similar methods to those discussed for characterizing composition and microstructure. As
outlined on the right side of Figure 23, two main paths are taken to determine coating thickness
on nano- and micro-lattice structures. The first is an indirect method that analyzes the deposition
on a flat substrate and extrapolates nominal coating thickness values and deposition rates for the
3D lattice structure. In this case, SEM is typically employed for determining deposition rates for
both plating and sputtering methods. Cross-sectional micrographs provide film thickness
measurements with resolution down to ≈10 nm, allowing for deposition rates to be calculated
depending on the overall deposition time. Less frequently utilized methods, such as
spectroscopic ellipsometry [8] and X-ray reflectivity [22] have also been applied for coating
thickness determination on flat substrates.
The direct route for determining coating thickness is to examine the deposited coating on
the 3D structure rather than a flat substrate [9, 19, 27, 28, 32, 40, 49]. As covered in Section 3.3,
focused ion beam (FIB), which typically utilizes a beam of Ga
+
ions to bombard and effectively
mill/erode a specimen, is often employed for sectioning sacrificial struts to both etch away the
base-polymer scaffold to produce hollow-tube structures as well as to provide exposed regions
for coating thickness evaluations [6, 8, 12, 19, 23, 25-29, 41, 45, 48]. However, a study by Juarez
52

et al. highlighted that the Ga
+
FIB process on metallic coated micro-lattice structures can result
in a substantial amount of coating redeposition, leading to inaccurate coating thickness
measurements [19]. To mitigate artifacts from FIB the researchers leveraged microtome, a
precise sectioning technique commonly used for biological sample preparation for electron
microscopy [187, 188]. Once embedded in epoxy, use of a sharp diamond blade allows for thin
enough sections to be obtained for evaluation in TEM, where unimpaired coating thickness
values were clearly resolved [19]. Less common analysis techniques used in studies for assessing
coating thickness directly on structures include X-ray tomography and nano CT scans, in which
3D volumes are generated from 2-D X-ray image slices [32, 40, 49].
Moreover, the aforementioned techniques are also employed for determining coating
uniformity. Figure 24 outlines representative examples of the coating uniformity achieved by
each of the main deposition techniques; ALD deposited coatings exhibit excellent conformality,
plated coatings are typically reported as relatively uniform, and sputter deposited coatings
generally result in large coating gradients throughout the entirety of the structure as well as
around individual struts. However, many publications report achieving “conformal” or “uniform”
coatings on a wide range of nano- and micro-lattice topologies without offering comprehensive
microscopical evidence or analysis throughout the structure [18, 22, 30, 36, 42]. A number of
these studies offer accounts of uniformity predicated on a sole micrograph of an isolated strut
within the entirety of the structure. For example, Xue et al. offered one SEM image of a 5 μm
thick NiP electrodeposited coating with no indication of characterization region within the
structure [42].
53

Figure 24: Representative scanning electron micrographs depicting the uniformity of common
deposition techniques on various nano- and micro-lattice geometries [8, 14, 27].

Despite the increasing number of studies exploring deposition routes to develop both
core-shell and hollow-tube nano- and micro-architected lattice structures, only a select few
publications provide thorough evaluations of overall coating uniformity [19, 26-28]. Such works
have revealed strong coating thickness gradients throughout plated and sputter coated structures.
For example, using X-ray tomography, Han et al. highlighted uneven wall thicknesses
throughout electrodeposited NiP shell-based micro-lattice structures, and further confirmed a ±
20% variation in wall thickness via SEM measurements [40]. Furthermore, the momentum-
54

driven directional nature of magnetron sputtering poses a prominent challenge for achieving
uniform coatings on fine featured lattice materials. More recent studies point out prominent
thickness variations achieved within sputter coated nano- and micro-lattice structures [19, 27,
28]. Liontas et al. reported coating thickness measurements on Zr54Ni28Al18 coated octahedron
nano-lattices ranging from approximately 45 nm to 900 nm after deposition for 240 min, while
Juarez et al. showed about an 80% variation in thickness measurements from the top to the
bottom of a sputtered Al tetrahedral lattice structure [19, 27]. Both studies provided thickness
uniformity measurements and microscopical evidence spanning the entirety of the structure, as
well as around individual struts at various locations throughout the structure. It has also been
noted that unit-cell size and geometry can also impact the degree of sputter coating uniformity
[19, 26]. For example, Montemayor et al. highlighted coating variations in Au sputtered octet
nano-lattice structures exhibiting a unit cell volume decrease from 15 μm x 15 μm x 15μm to 10
μm x 10 μm x 10 μm; where similar trends were observed in 3D kagome nano-lattices as well
[26]. Overall, these works warrant additional studies that investigate improved sputter coating
and plating uniformity on nano- and micro-architected lattice structures.
4.4. Properties and Functionalities of Coated Nano- and Micro-lattice Structures
As more research has emerged on coated nano- and micro-lattice materials, it has been
observed that the properties and functionalities of such materials are governed by contributions
from both topological design and constituent material properties, especially at the nanoscale [4,
62, 80]. The deposition of metallic and ceramic coatings on polymeric 3D nano- and micro-
lattice scaffolds has resulted in highly controlled and rationally designed novel lattice systems
with uncapped potential in a variety of property spaces. This section serves to summarize the
55

mechanical behaviors and functionalities achieved by coated nano- and micro-lattice structures
and provide an overview of the current state of research in the field.
4.4.1 Mechanical Behavior
A prevalent aim in materials research is the development of advanced and multifunctional
materials exhibiting exceptional mechanical performance. The recent advent of coating 3D
polymeric nano- and micro-lattices to generate core-shell composite and hollow-tube structures
has brought researchers closer to developing novel material systems that occupy previously
uninhabited property spaces. As outlined in Figure 25, the addition of metallic and ceramic
coatings on nano- and micro-architected scaffolds has resulted in: (a) increased stiffness, (b)
enhanced recoverability, (c) higher yield strengths, and (d) improved flaw tolerance. Given the
vast combinations of architectures, feature sizes, and constituent coating materials, the full
potential of coated nano- and micro-lattice structures is actively the subject of many research
endeavors. However, there are several works have extensively documented the mechanical
properties of both coated and uncoated systems [4, 62, 80, 189-194]; therefore, this section will
served as a summarized discussion of coated systems only. In general, coated nano- and micro-
lattice materials derive their superior mechanical performance through the synergistic effects of
topological design and size-dependent material properties. Due to experimental testing
limitations, the majority of mechanical tests have been conducted in compression, with only a
handful of publications having investigated the tensile properties of coated nano- and micro-
lattice structures [35, 44, 45, 48]. Thus, the scope of this section is limited to the compressive
response of these advanced materials. It is important to note that the mechanical properties of the
systems should not be directly compared due to the vast range of topologies, coating thicknesses
and compositions, and structural feature sizes explored.
56

Figure 25: Various examples of mechanical behaviors observed by coated nano- and micro-
lattice structures including: a) increased modulus [18], b) exceptional recoverability [8], and c)
enhanced yield strength under compression [19], as well as d) increased flaw tolerance under
tension [45].
Altering unit-cell geometry and architecture has been shown to effectively influence the
mechanical behavior of 3D lattice-based materials due to variations in structural rigidity and
nodal connectivity [41]. As outlined in Section 2.2, lattice architectures can be categorized as
either truss-, shell-, or plate-based, where the first can contain various orders of hierarchy for
increased mechanical performance. Hierarchically designed materials can induce improved
buckling resistance and recoverability with the introduction of structural features on multiple
length scales [6, 35]. Nodes are commonly characterized as initial points of failure in truss-based
lattice structures due to high stress concentrations located in those regions [49]. However, the
57

mitigation of failure at the nodes as a result of shape optimization and nodal strengthening has
been observed following the sputter deposition of Inconel 600 on tetrahedral-truss structures
[19]. The authors highlighted webbing of the metallic coating around nodal components,
inducing a shift in fracture behavior from nodal failure to fracture along the struts [19]. Nodal
failure has also been suppressed by varying lattice topologies [40, 46, 74, 78, 79]. Shell-based
lattices derived from triply periodic minimal surfaces yield smooth shell interconnected nodes
throughout the entirety of the structure, eliminating the presence of nodal stress concentrations
[78]. However, only a minimal enhancement in strength is achieved in these structures due to
deformation being dominated by local buckling [195]. The mechanical performance of shell-
based structures can be further improved with topology optimization of the shell shape and size,
as well as through the deposition of coating materials [40, 46].
Beyond architectural factors, the mechanical behavior of coated lattice structures is also
determined by its constituent materials [4, 62, 80]. Work by Juarez et al. highlighted the
influence of the coating material’s properties on the compressive performance of core-shell
composite micro-lattices [19]. A comparison of Ti-6Al-4V and Inconel 600 coated tetrahedral
truss-structures with the same coating thickness (≈35 nm) and unit cell dimensions revealed an
increase in specific stiffness for the Ti-6Al-4V coated lattice and an enhancement of specific
strength for the Inconel 600 coated micro-lattices, respectively [19]. Material size effects derived
from the intrinsic (i.e. microstructural) and extrinsic (i.e. dimensional) features of the constituent
coating and solid-beam lattice materials have also been shown to enhance the overall mechanical
behavior of core-shell composite and hollow-tube nano-lattices, such as increased toughness
[196] and Weibull strengthening [23] in brittle materials. Reducing coating thickness results in a
complementary reduction in maximum intrinsic flaw size, which ultimately decreases the
58

probability of finding a critical flaw within the material and leads to enhanced flaw tolerance [23,
45, 196] and induced plasticity [197, 198]. It should be noted that nanocrystalline metallic
coatings have been shown to follow a “smaller is weaker” trend regarding coating thickness.
Such behavior has been observed in numerous metal coated core-shell composite and hollow-
tube studies, where a decrease in strength is reported as coating thicknesses are reduced [9, 19,
22, 25, 27, 28, 30, 35].
Size-dependent phenomena can be leveraged to tune the compressive behavior of thin-
walled hollow-tube structures and create ductile and recoverable metamaterials. “Thin” shelled
hollow-tube structures generally undergo ductile-like shell buckling accompanied by strain
bursts occurring in a layer-by-layer fashion resulting in enhanced recoverability [8, 27, 28]. This
has been observed in studies by Meza et al., where ALD coated octet-truss ceramic nano-lattices
exhibited 98% recoverability upon compression to 50% strain [6, 8], and an 80% recovery of its
original height after 85% strain [8]. This shell-buckling failure mechanism has also been
observed in sputter coated hollow 3D nano-architected metallic glass structures. Uniaxial
compression experiments conducted by Liontas et al. on Zr54Ni28Al18 nano-lattices highlight a
transition from brittle failure to deformable layer-by-layer collapse in structures with median
wall thicknesses of ≤ 38nm [27]. The compressive behavior of hollow-tube metallic glass lattice
structures has also been investigated under extreme environments [28, 29]. Thompson et al.
explored the strength and deformability of Zr54Ni28Al18 nano-lattices under radiation
environments and indicate an average yield strength increase of 35.7% and enhanced
deformation [28]. Moreover, Lee et al. highlighted a change in deformation mode from brittle
failure to elastic bucking as the wall thickness of Cu60Zr40 metallic glass nano-lattices was
decreased from 120 nm to 20 nm at cryogenic temperatures (130K) [29]. Leveraging material
59

size-effects from coatings, even under extreme environments, can bring about ductile-like
behavior and recoverability in intrinsically brittle bulk materials.
Despite the shift in compressive failure in many coated systems, several studies have
highlighted a trade-off between strength and recoverability in both core-shell composite and
hollow-tube nano- and micro-lattice systems [8, 35, 36, 189]. For example, Schaedler et al.
demonstrated that 100 nm thick hollow-tube Ni-P octahedral micro-lattices displayed complete
recovery post compression to a strain > 50%, but achieved yield strength values only reaching
≈10 kPa [36]. Similarly, Meza et al. showed that as hollow-tube Al2O3 octet nano-lattice wall
thicknesses are increased from 10 nm to 60 nm the resulting yield strength increased from 1 MPa
to 30 MPa [8]. However, the thicker shells experienced brittle fracture and collapse upon a
compressive strain of ≈5-10% [8]. This strength-recoverability trade-off has similarly been
observed in core-shell composite systems. Surjadi et al. fabricated polymer micro-lattice
structures coated in an ≈800 nm thick CrMnFeCoNi high-entropy-alloy (HEA) coating, which
exhibited a strength of 6.9 MPa under compression, but fractured at a strain of only 6.5%,
suggesting no degree of recovery would be obtained at strains >7% [20]. However, the
HEA/polymer composite structure achieved higher strength values than other core-shell
composite micro-lattice systems, including polymer/NiB [22], NiP [43], and Si3N4 [21],
demonstrating the potential of HEA materials for the development of high strength-to-weight
ratio composite lattice structures. Taking this one step further, a recent study by Zhang et al.
demonstrated novel octet-truss polymer/HEA nano-lattice composites with 14 nm to 50 nm
coatings that exhibit a combination of high specific strength (up to11.6 MPa) and recoverability
under compression to strains >50%, ultimately overcoming the commonly observed strength-
recoverability trade-off [9]. Core-shell composites are seemingly most effective in achieving
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combinations of high strength and recoverability owing to a ductile polymeric core for structure
recovery after large deformations and the strong metallic coatings for improved strength.
It is important to note that imperfections in constituent coatings can significantly
influence the mechanical properties and behaviors of nano- and micro-lattice structures [6, 8, 24,
32]. The strength increase in core-shell composite and hollow-tube systems is often not entirely
reflected in their achieved effective strength as a result of sensitivity to structural imperfections.
For example, ALD coatings on polymer surfaces can result in the formation of lower density
films attributed to gas-phase reactants diffusing into the substrate and creating nanosized flaws
[199]. Moreover, variations in sputter coating thickness can consequently lead to an inaccurate
evaluation of properties and deformation behavior, thus further research needs to be conducted to
achieve conformal coatings via PVD methods on polymeric lattice scaffolds. Such improvements
have the potential to unlock novel mechanical metamaterial systems.
Three-dimensional architected materials with micro- and nanoscale features have been
shown to exhibit unprecedented properties, such as high specific strength, specific modulus,
absorption, and deformation behavior.[9, 10, 19, 38, 116, 134] Generally, the mechanical
properties of open-cell materials are governed by the relative density, constituent material
properties, nodal connectivity or topology of the cellular structure, and the length scale, which
influences any size-dependent strengthening. The reliability and durability of these materials are
primarily determined by their strength and recoverability. While ceramic and metallic hollow
lattice materials have been extensively studied, the mechanical properties of polymer core
composite metamaterials remain largely unexplored. New material property spaces can be
reached through the synthesis of architected material composites that leverage the ductile
61

polymer core for structure recovery after large deformations and the strong sputter-deposited
metallic coating for improved strength.
4.4.2 Functionality and Applications
Unlike mechanical behavior, functionality studies have been much more limited for
coated nano- and micro-lattice structures. A small selection of publications highlight various
non-mechanical functionalities for cell growth [11, 12] (Figure 26a), battery technology [13, 14]
(Figure 26b), supercapacitors [42] (Figure 26c), and catalytic water splitting [47] (Figure 26d).
Table 3 provides an overview of the limited functional and application-based publications, all of
which will be covered in this section. With advancements in innovative fabrication technology
and flexibility in deposition techniques, researchers were able to study an array of functions
linked to rationally designed nano- and micro-lattice materials.
Table 3: Functionality and applications of various core-shell composite and hollow-tube nano-
and micro-lattice systems
Geometry
Coating
Composition
Coating
Thickness
Hollow or
Composite
Functionality and
Application
Octahedral Au 5 µm Hollow Li-O2 battery cathode
Octet Si 250 nm Composite Li-O2 battery anode
Octet NiP + rGO 5 µm Composite
Quasi-solid
supercapacitor
Octet NiP + NFNS 2.7 µm Composite Water splitting
Tetrakaidecahedral TiO2 18 nm Composite Bone tissue growth
TiO2 + Ti 268 nm
TiO2 + W 268 nm
TiO2 18 nm Hollow
Tetrakaidecahedral TiO2 20 nm Composite
Osteogenic cell growth
and functionality
TiO2 + Ti 140 nm
TiO2 + SiO2 140 nm

62

Figure 26: Functionality and application examples of coated nano- and micro-lattice structures
including: a) cell growth [11], b) battery electrodes [14], c) supercapacitors [42], and d) water
splitting [47].
At the outset of self-propagating polymer waveguide technology, Xu et al. investigated
the feasibility of leveraging micro-lattice structures as a positive electrode for Li-O2 batteries
[14]. Hollow-tube structures comprised of ≈5 m thick electrodeposited Au were subjected to
63

cycling events to determine the galvanostatic discharge behavior of the micro-lattice as well as
analyze morphological changes upon discharging [14]. While the discharge rate achieved in this
work was significantly lower than a previous study on nanoporous Au foils [200], this work
established a basis for studying fundamental electrochemistry and discharge product morphology
on 3D architected electrodes, as well as provided insight on surface composition influences and
design constraints for developing other lattice-architected Li-O2 electrodes. Conversely, Xia et al.
explored Si coated Cu nano-lattices for use as a Li-ion battery anode [13]. Octet-truss Si/Cu
core-shell structures achieved an ≈250% lithiation induced volume expansion of the Si coating
with minimal overall electrode expansion, and remarkably no observed cracking [13]. Further
finite element modeling of the core-shell system indicated that topological factors and plasticity
mechanisms contribute significantly to damage prevention of nano-lattice electrodes.
In more recent studies, the metallization of polymeric lattice scaffolds has been leveraged
to examine the viability of metal-based micro-lattice structures for energy storage and catalytic
functionalities. For example, Xue et al. combined digital light processing techniques with
electroplating methods to produce an innovative 3D micro-architected metallic composite
supercapacitor [42]. The hierarchically designed quasi-solid supercapacitor comprised of an
outer porous reduced graphene oxide layer deposited onto a Ni/polymer octet-truss lattice
revealed areal capacitance, rate capability, and lifespan values comparable to the state-of-the-art
carbon-based supercapacitor devices [201-205], laying the groundwork for energy storage
devices derived from 3D architected materials. Moreover, owing to their enhanced mechanical
performance, designed architectural features and simplistic fabrication, Su et al. explored the
catalytic response of octet-truss nickel–iron-(oxo) hydroxides nanosheet/NiP/polymer electrodes
for water splitting capabilities [47]. The core-shell composite truss exhibited promising oxygen
64

and hydrogen evolution reaction catalytic performance as well as long-term catalytic stability
attributed to the larger surface are of 3D architected structures [47]. Such work highlights the
potential of exploiting hierarchically designed 3D printed electrodes as water-splitting catalysts.
Core-shell composite micro-lattice materials have also been fabricated to study cell
behavior and viability for bone implant development [11, 12]. A major challenge in the field is
creating biocompatible materials with increased moduli [206] and features comparable to the size
of osteoblast cells [207]. However, the newfound ability to achieve lattice architectures on the
order of nanometers to a few microns has allowed researchers to investigate the effect of pore
size and distribution, as well as improved stiffness on cell response and growth, and provides
advances in achieving optimal bone remodeling. Maggi et al. developed three core-shell
composite tetrakaidecahedral micro-lattice systems that exhibit relative density values close to
that of trabecular bone: (a) 20 nm TiO2/polymer, (b) 20 nm TiO2/120 nm SiO2/polymer, (c) 20
nm TiO2/120 nm Ti/polymer [11]. Nanomechanical compression tests revealed the moduli of the
materials ranged from ≈2-9 MPa, respectively [11]. Osteogenic cells were grown on the
structures in a microenvironment similar to that of natural bone resulting in significant cell
attachment to the coated lattices and also aided the growth of chemical species commonly found
in natural bone [11]. Such work suggests that fine featured lattice materials can effectively serve
as scaffolds for cell growth and creation, leading to the development of improved bone implants.
4.5. Conclusions and Outlook
The deposition of metallic and ceramic coatings on these finely architected lattice
structures has resulted in the emergence of new material functionalities and mechanical
behaviors unattainable by their bulk counterparts. Coating methods such as atomic layer
deposition and plating techniques have been extensively employed and have generated highly
65

robust lattice structures, however, the materials working space of these techniques is confined
due to chemical reaction limitations. Accordingly, there has been a recent uptrend in developing
core-shell composite and hollow-tube nano- and micro-lattice structures using physical vapor
deposition techniques like magnetron sputtering. Owing to their flexibility and vast material
selection, sputter coating methods have allowed researchers around the world to further explore
this emerging class of materials. Until progress in direct metal- and ceramic-based additive
manufacturing allows for resolution, integrity, and intricacy at the same magnitude of light-based
polymer techniques, the deposition of coatings on polymer nano- and micro-lattice scaffolds
offers a viable route for developing mechanically robust and functionally unique advanced lattice
materials.

66

Chapter 5: Development of Core-shell Composite Micro-lattice Structures via
Magnetron Sputtering
The background and experimental techniques discussed to this point were used to
evaluate various magnetron sputtering parameters, configurations, and characterization methods
for the development of core-shell composite micro-lattice structures. As previously discussed,
magnetron sputtering has gained increased attention as a versatile deposition method for
generating novel core-shell composite nano- and micro-lattice materials. Sputtering offers an
expansive materials workspace consisting of a wide range of ceramics, single element metals,
and alloy systems; although, achieving uniform coatings on such architected structures remains a
challenge. Thus, in this work, a foundational assessment of various sputtering configurations,
cathode geometries, and deposition parameters were carried out to investigate their implications
on the coating thickness and uniformity of 3D micro-lattice scaffolds. Specifically, tetrahedral-
truss structures fabricated via direct laser writing are coated by leveraging planar and inverted
cylindrical magnetron cathodes at select deposition rates, sputtering powers, and Ar working
pressures. Both plasma focused ion beam and microtome sectioning techniques were employed
to evaluate the cross-section of individual struts and assess overall uniformity. Overall, this study
highlights the influence of key sputtering factors for the design and development of core-shell
composite nano- and micro-lattice materials and provides a pathway for future sputter coating
optimization on these complex structures.

67

5.1. Introduction
For the past decade, the emerging class of 3D nano- and micro-lattice core-shell
composite materials have garnered increasing interest as they have facilitated exceptional
advances in material property and functionality spaces [208]. Recent progress in high-resolution
polymer-based additive manufacturing (AM) techniques, such as two-photon polymerization
direct laser writing (TPP-DLW), and various coating deposition methods has enabled the
development of novel complex core-shell composite nano- and micro-lattice materials [7, 9, 13,
18-22, 34, 38, 39, 42-44, 46, 208-210]. By exploiting design elements related to nanoscale size-
effects and hierarchical architecture, these fine-featured composite materials have demonstrated
unique mechanical properties including high specific strength and recoverability [9, 210], as well
as functionality in a broad range of applications ranging from lithium-ion battery technology [13]
and energy storage [42] to catalysis [47] and tissue/cell engineering [11, 12].
Core-shell composite nano- and micro-lattice systems are generally generated through a
multi-step process of: (i) fabricating a solid-beam lattice scaffold, which are commonly polymer-
based via TPP-DLW, and (ii) subsequently depositing a metallic or ceramic thin film coating via
techniques like atomic layer deposition (ALD), electroless plating, and magnetron sputtering
[208]. It should be noted that advances have also been made in non-polymer-based AM
technologies allowing for the development of solid-beam metallic and ceramic nano- and micro-
lattices [71, 211, 212]. However, such structures are still under development and currently
include a narrow selection of material systems such as Ag [211] and Cu [107], can also exhibit
undesired surface roughness and porosity [71-73, 211], or contain strut diameters on the order of
hundreds of microns [64-69]. Thus, the fabrication of nano- and micro-architected core-shell
composite lattices through means of thin film coating deposition on nano- and micro-architected
68

polymeric scaffolds offers a flexible route for generating mechanically robust and functionally
tailored materials from a vast material working space.
Magnetron sputtering, a physical vapor deposition (PVD) coating technique, has emerged
as a versatile method for generating novel core-shell composite nano- and micro-lattice
structures [9, 11, 12, 18-20, 209, 210]. Unlike ALD and plating techniques, which both rely on
specific chemical reactions that ultimately constrain the materials working space to mainly
oxides and nitrides, and some binary alloys [114, 124], respectively, magnetron sputtering
affords the ability to deposit nearly any metal alloy, single element, and ceramic system. Sputter
coated core-shell composite nano- and micro-lattices have been generated in a variety of
materials such as Inconel 600 [19], Ti-6Al-4V [19], and high entropy alloy systems [9, 18, 20].
However, it has been observed that sputtered coatings on complex lattice scaffolds yield large
coating thickness and uniformity gradients as consequence of the momentum-driven line-of-sight
nature of the deposition technique [19, 27, 28, 208]. In general, studies employing magnetron
sputtering to generate core-shell composite nano- and micro-lattice materials have been limited
to set-ups leveraging planar cathodes and singular sputtering conditions, which result in
unidirectional deposition, a confined line-of-sight, and overall lack of sputtering parameter
information. In some previous works, sample rotation has been integrated in planar sputtering
assemblies with an aim to induce greater homogeneity in the coating distribution on intricate
lattice structures with strut widths smaller than 1 µm [9, 18, 19]. Though it is important to note
that no direct correlation has been made relating the effect of rotation axis and rotation frequency
on the coating uniformity. Additional sputtering configurations such as inverted cylindrical
magnetron (ICM) and oblique angle deposition can also be leveraged to introduce greater
degrees of line-of-sight [213, 214], although to date, there are no underlying studies to date
69

surveying the line-of-sight implications of such techniques for complex nano- and micro-lattice
materials. Thus, there is a need to fundamentally examine the effects of varying both cathode
geometry and sputtering parameters to understand the challenges in achieving uniform sputtered
coatings on complex lattice structures for the generation of core-shell composites.
In this study, three distinct sputtering configurations leveraging stationary planar and
ICM cathodes at various deposition parameters were employed to assess the impact of line-of-
sight and deposition conditions on the coating uniformity of Cu-2 wt.% Al/polymer core-shell
composite micro-lattice structures (Figure 27). The aim of this study is to understand at a
fundamental level the influence of sputtering conditions and configurations on the conformality
of sputtered coatings on complex lattice scaffolds, and to develop a basis for optimizing such
coatings in the future. This work provides an innovative approach to understand the effect of
cathode geometry, line-of-sight, and deposition conditions on the quality and uniformity of
sputter coated 3D nano- and micro-lattice materials.
Figure 27: Cu–Al/polymer composite micro-lattice structure across four orders of magnitude in
length scale: from the width of the tetrahedral-truss structure (≈70 μm) down to the Cu-Al 2wt%
coating thickness (≈200-10 nm).

70

5.2. Experimental Methods
Tetrahedral-truss micro-lattice structures with a relative density of ≈0.18 were printed via
two-photon polymerization direct laser writing using a Nanoscribe GmbH Photonic Professional
GT. The lattice structures were printed on fused silica glass slides in the dip-in configuration
using IP-Dip photoresist (Nanoscribe GmbH). Following the print writing process, the structures
were developed for 20 minutes in a propylene glycol methyl ether acetate immersion bath
followed by a 5-minute cleaning in isopropyl alcohol. The samples were then dried in a critical
point dryer (Leica). Further details on the TPP-DLW print process can be found in work by
Valdevit and Bauer [2]. The lattice structures were designed with an overall width of ≈70 µm
and overall height of ≈35 µm, with a strut length and width of ≈10 µm and ≈1.5 µm,
respectively.
The tetrahedral scaffolds were sputter coated in Cu-2 wt.% Al in a vacuum chamber
under various coating deposition parameters and configurations using either an ICM or planar
cathode (see Figure 28 and Table 4). Samples sputtered in the ICM were positioned at a working
distance equal to the radius of the inverted cylindrical target (Kurt J. Lesker Company, 99.99%)
of ≈4.8 cm. Planar cathode sputtered samples were positioned at an equivalent working distance
of ≈4.8 cm away from a Cu-2 wt.% Al cylindrical target (Plasmaterials, 99.99%) with a 7.6 cm
diameter. Samples were sputtered at a base vacuum pressure of at least 5.5 x 10
-6
Torr with argon
as the working gas. All sputtering parameters reached deposition temperatures of less than 200°C
to mitigate excessive heating of the polymer scaffolds. In addition to the micro-lattice structures,
1 µm thick films were deposited on flat glass substrates (Corning) at each configuration for
coating material characterization.

71

Table 4: Summary of sputtering parameters for ICM and planar cathode configurations
Configuration
Sputtering
Power
[W]
Ar Pressure
[mTorr]
Sputtering Rate
[nm s
-1
]
EDS Composition
[wt.%]
360° 100W 3 0.48 Cu: 97.13; Al: 2.87

100W 6 0.50

200W 3 1.10

90° 23W 3 0.48 Cu: 97.06; Al: 2.94

46W 3 0.95

±30° 30W 3 0.48

60W 3 0.99

After sputter deposition, the resultant core-shell composite micro-lattice structures were
characterized by SEM (Thermo Fisher Helios G4 PFIB UXe DualBeam, 10kV) and energy
dispersive spectroscopy (EDS). EDS scans were performed directly on the structures using an
Oxford Ultim Max 170 Silicon Drift Detector and AZtec software for elemental determination
and confirmation of coating compositions for both the ICM and planar targets (summarized in
Table 4). XRD using a Rigaku Ultima-IV diffractometer was employed for characterizing
crystallographic structure of the deposited 1 µm thick metal films. XRD scans were conducted
over a 2θ range of 30° to 110° at a rate of 1° min-
1
.
The coating thickness and uniformity of the composite micro-lattice structures were
characterized through a combination of PFIB (Thermo Fisher Helios G4 PFIB UXe DualBeam,
30 kV, 1-4 nA) equipped with a xenon plasma FIB column and SEM, as well as microtome
(Leica Ultracut EM UC6) and TEM. PFIB was used to mill away regions of the coated
tetrahedral truss structures to expose the cross-section from which the topside coating thickness
of a designated strut was measured via SEM. A total of ten measurements of coating thickness
were performed on the topside of the designated strut. Average topside coating thickness values
were determined for all seven sets of samples sectioned via PFIB. Microtome cross-sections
were then additionally obtained for a select set of samples. For the microtome cross-sectioning,
72

the samples were heat polymerized and embedded in an epoxy resin (Epon) and gently removed
from the glass slide by thermal cycling in liquid nitrogen. The samples were then re-embedded in
epoxy resin to encase the structures from both sides. Once fully embedded in epoxy, the samples
were sectioned 100 nm thin with a diamond knife on an ultramicrotome, floated on water and
picked up on formvar-filmed 2x1 mm slot grids. The grids were imaged at 80 kV in a Zeiss
EM10 transmission electron microscope (Carl Zeiss Inc.) and images were recorded with an
Erlangshen CCD camera (Gatan, Inc.). Measurements of the topside coating thickness (ten per
strut cross-section) were obtained using TEM images and analyzed with ImageJ software.
Figure 28: Schematic of a) inverted cylindrical magnetron and b,c) planar sputtering
configurations used in this work, where the line-of-sight for each respective set-up is detailed.
The colored arrows represent the different sputtering arrangements, where green arrows
correlated to samples coated using the ICM (360° line-of-sight), yellow are coated using the
planar cathode (90° line-of-sight), and orange denote samples tilted to ±30° while coated with
the planar cathode.
5.3. Results and Discussion
5.3.1 Film Synthesis and Microstructure
Tetrahedral-truss micro-lattice structures with a strut width of ≈ 1.5 µm were printed via
TPP-DLW and subsequently coated with Cu-2 wt.% Al under various magnetron sputtering
configurations and conditions, utilizing either an ICM or planar cathode (Figure 28). A total of
seven sets of samples (21 total coated structures) were sputtered with a nominal coating
73

thickness of 200 nm, which is defined as the predicted thickness on a flat substrate based on the
sputtering rate. Samples sputtered inside the ICM, denoted as 360° line-of-sight, were affixed
perpendicular to the axial direction (Figure 28a) to ensure maximum exposure to the target
surface. Due to the ICM geometry, samples were sputtered at a fixed working distance equal to
the inner radius of the hollow cylindrical target. As depicted in Figure 28b-c, samples sputtered
with a planar cathode were affixed in two orientations with respect to the target surface: parallel,
which is denoted as the 90° configuration, and tilted to ±30° for an increased line-of-sight. In the
90° and ±30° arrangements, the structures were positioned at an equivalent working distance to
ICM. The colored arrows represent the different sputtering configurations, where green
correlates to samples coated with the ICM, and yellow and orange with the planar cathode under
the 90° and ±30° assemblies, respectively. Please note that the same color guideline is utilized in
all figures. All sputtering parameters, including input power (W), Ar working pressure (mTorr),
deposition rate (nm s
-1
), and compositional details of the coatings for each of the configurations
can be found in Table 3. It is important to note that the baseline condition for each configuration
was chosen to equate the deposition rate (0.48 nm s
-1
) along with the working distance and Ar
working pressure, such that the sole effect of cathode geometry and line-of-sight on coating
thickness and uniformity could be assessed. From the baseline condition, the input power was
then doubled to determine if an increase in deposition rate (from 0.48 to ≈1 nm s
-1
) impacted the
overall coating distribution. Lastly, the effect of increased Ar working pressure in the ICM
configuration was assessed to understand the influence of gas scattering on the resulting coating
uniformity. The chosen deposition parameters can serve as a baseline for future modifications
and optimization of coatings on 3D nano- and micro-lattice materials.
74

An initial investigation of the coating crystal structure and surface morphology was
conducted on 1 µm thick Cu-2 wt.% Al films sputtered onto flat glass substrates (Corning).
Figure 29 presents X-ray diffraction (XRD) spectra and complementary scanning electron
microscopy (SEM) images of the as-deposited coatings from the various configurations and
parameters. It can be observed from the XRD patterns highlighted in Figure 29a, that in all
sputtering configurations and under all deposition parameters the Cu-Al films show a strong
texture along the (111) growth direction. Sputtered Cu commonly results in a (111) preferred
orientation as it possesses the lowest surface energy [40-42]. However, all ICM sputtered films
(green scans) indicated mixed crystalline orientations with the additional presence of (200),
(220), and (311) diffraction peaks that become more pronounced with an increase in Ar gas
pressure and input power. The presence of more randomized orientations in the ICM system may
be attributed to an increase in the range of deposited atom angles of incidence resulting from the
cathode geometry, and enhanced adatom mobility due to increased energy delivered to the
surface by the sputtered species [38, 43]. For further comparison, Figure 29b-d includes
representative top-surface SEM micrographs for the Cu-Al films deposited at the three different
sputtering configurations at the same deposition rate (0.48 nm s
-1
). The SEM images reveal
varying surface morphologies, where films sputtered using the planar cathode (Figure 29b,c)
feature regions of rounded nanocrystallites. However, Figure 29b also shows characteristic
elongated growth regions are present in the ±30° configuration due to the introduction of an
oblique angle of deposition with respect to the target surface, where growth is along the same
direction of the incident sputtered atoms [39, 44]. Figure 29d highlights a less dense structure
with no distinct shape or size which envelop distinct larger circular nanocrystallites formed
during deposition with the ICM. The effect of sputtering configuration and deposition parameters
75

on thickness, uniformity and coating quality on the complex lattice structures are addressed in
the next sections.
1.
Figure 29: a) X-Ray diffraction scans of Cu-2wt% Al films for each sputtering configuration and
corresponding deposition parameters. Green scans correlated to samples coated in the 360° ICM
configuration, where yellow and orange scans represent 90° and ±30° planar set-ups,
respectively. Corresponding top surface SEM images of as-deposited films with a sputtering rate
of 0.48 nm/sec in the b) ±30°, c) 90°, and d) 360° configuration, respectively.
5.3.2 Core-shell Composite Thickness Evaluation
To evaluate the thickness of Cu-2 wt.% Al coatings on tetrahedral-truss micro-lattice
scaffolds, various imaging and cross-sectioning techniques were employed (Figure 30). Figure
30a highlights a representative Cu-2 wt.% Al/polymer core-shell composite micro-lattice with
corresponding xenon-plasma focused ion beam (Xe
+
-PFIB) (Figure 30b) and microtome (Figure
30c). Scanning electron and transmission electron microscopy (TEM) were used, respectively, to
image and quantify the topside coating thickness of a designated strut in row 2 of the structure
76

(see insets of Figure 30b,c). Thickness measurements were obtained in the topside region of the
struts as emphasized by red dashed lines. This area was selected for all samples as it exhibited
the cleanest and clearest region via both sectioning methods. Previous studies have employed
Ga
+
-based FIB to expose regions of coated nano- and micro-lattice structures [8, 19, 27-29, 208,
215]; however, it has been demonstrated to yield inaccurate and obscured thickness
quantifications due to damage [19]. Thus, as highlighted in Figure 30b, emerging Xe
+
-PFIB
technology was exercised in this study with the aim of mitigating FIB damage [216-218]. A
select set of samples were additionally evaluated using microtome sectioning as such technique
has been established as an accurate and effective method for evaluating sputter coating thickness
variations on coated micro-lattice structures [19]. A representative TEM micrograph of a
microtome cross-section is highlighted in Figure 30c, where the coated lattice structure is
embedded in epoxy such that the metal coating is preserved upon sectioning. For each sectioning
technique, average topside coating thickness values under each sputtering configuration and
deposition parameter are reported in the summary table outlined in Figure 30d. Please note that
the objective of this work was to assess and isolate the direct impact of sputtering deposition
conditions and line-of-sight without the implementation of rotation.

77

Figure 30: a) Representative SEM micrograph of Cu-2wt% Al coated micro-lattice structure. b)
SEM image of a tetrahedral-truss structure milled using PFIB with a corresponding micrograph
of a strut cross-section. c) Representative TEM micrograph of a microtome prepared cross-
section and a complementary cross-sectional image of an analogous strut to that represented in
b). The topside coating thickness of the designated strut depicted in b) and c) for each sputtering
configuration are presented in table d). The PFIB and microtome thickness measurements
obtained from the top-side strut regions are outlined with red lines in b) and c), respectively.
Upon review of PFIB prepared cross-sections, regions of redeposition along the
underside and sidewall regions of the designated strut were noted (inset of Figure 30b); no
evidence of redeposition or damage to the topside region of the struts along row 2 was observed.
Based on the PFIB measurements for samples coated at similar deposition rates (≈0.48 nm s
-1
),
the ICM sample at the lowest Ar pressure (3 mTorr) achieved the greatest topside coating
78

thickness along the designated strut. This suggests that the greater degree of line-of-sight
rendered by the ICM cathode geometry at lower deposition rates and Ar pressures allows for
improved coating thickness to be achieved. Conversely, at a sputtering rate of ≈1.0 nm s
-1
, the
90° configuration yielded the highest topside coating thickness. It was also noted that an increase
in deposition rate from ≈0.48

to ≈1.0 nm s
-1
resulted in an increase in coating thickness for both
the 90° and ±30° planar configurations, while the ICM configuration demonstrated a decrease in
the topside coating thickness. Such observations within the ICM were confirmed by
measurements obtained from microtome cross-sections, which showed close agreement with
PFIB measurements. Typically, irrespective of cathode geometry, the coating thickness achieved
on flat substrates increases with an increase in deposition rate, which is dependent on many
parameters including the Ar gas pressure, sputtering power, and substrate-to-target distance [215,
219]. However, based on the aforementioned observations, it is evident that a more complex
relationship exists between sputtering parameters and configurations on the coating thickness of
intricate lattice structures due to intrinsic blocking from strut members and redeposition effects
[19].
In order to assess the additional factors affecting deposition on the micro-lattice
structures, it is important to understand the impact of deposition parameters and cathode
geometry beyond deposition rate. Variations in Ar pressure and sputtering power, as well as
cathode geometry greatly influence the plasma density within the sputtering system, and in turn
the degree of collisions during deposition and directionality of the depositing atoms [220-223].
Fundamentally, the tubular nature of the ICM cathode promotes more collisions amongst trapped
electrons with neutrals and ionized species and inherently produces higher plasma densities
compared to planar configurations [222, 224]. The subsequent increase in Ar pressure (from 3 to
79

6 mTorr) and sputtering power (from 100 to 200 W) within the ICM may have resulted in
additional degrees of scattering of the depositing flux due to a corresponding increase in ion and
electron density [223, 225], ultimately decreasing the proportion of incident sputtered metals
atoms penetrating the lattice structure. This suggests that deposition at lower sputtering powers
and working pressures within the ICM can promote less variation in coating wall thickness on
nano- and micro-lattice scaffolds. In contrast, under the planar cathode configurations (90° and
±30°), an increase in sputtering power appears to have induced a favorable degree of coating
penetration. As such, the results indicate that the collisional process of the sputtered atoms under
various cathode geometries can have a large influence on the growth of coatings on nano- and
micro-lattice scaffolds.
To further investigate the influence of sputtering parameters and configurations on
coating thickness variations, measurements on additional struts throughout the tetrahedral-truss
structure were taken using the microtome cross-sections (Figure 31). As illustrated in Figure 31a,
topside thickness values were obtained on a set of “outer struts” and “inner struts” spanning from
row 1 to row 4 of the structure to assess the variation in wall thickness that occurs on the outer
sides of the structure, as well as from the top to the bottom. The struts of interest are outlined in
black, where red areas designate regions upon which thickness measurements were obtained.
Average topside coating values for the outer and inner struts of each microtome prepared sample
are listed in Figure 31b. Regarding the outer struts, samples sputtered in ICM configuration at
the lowest power and working pressure achieved the highest average coating thickness (≈165
nm). It should be noted that the ±30° configuration resulted in the lowest average outer coating,
indicating that potential blocking of the depositing atoms by the substrate holder may have
obstructed the side coverage. As for the inner struts, the ICM and 90° planar samples coated at
80

equivalent sputtering rates (≈0.48 nm s
-1
) and gas pressure (3 mTorr) both achieved the highest
average topside coating thicknesses of ≈78 nm. Therefore, both the ICM configuration at 100W
and 3 mTorr, and the 90° planar at 23 W and 3 mTorr produced improved coating penetration
during deposition. Such results suggest that comparable topside coating coverage of the inner
struts can be achieved leveraging both planar and ICM cathode geometries. Moreover, the results
further indicate that greater coating penetration on nano- and micro-lattice structures can be
achieved during ICM sputtering at slower deposition rates and low working pressures as a result
of a lower degree of collisions.

Figure 31: a) Schematic of tetrahedral-truss cross-section with struts of interest outlined in black.
Areas in red indicate the region of measurements taken for obtaining the average coating
thickness of the “outer” and “inner” struts. Measurements were taken on TEM micrographs from
the microtome cross-sections for each sputtering configuration/set of parameters and presented in
table b).

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5.3.3 Core-shell Composite Thickness Evaluation
Beyond coating thickness, the coating uniformity on nano- and micro-lattice structures is
a key factor in generating functionally tailored core-shell composite materials, as large variations
in coating coverage can lead to inaccurate and non-uniform evaluations of properties and
behavior. In this work, the coating distribution on individual struts was surveyed for all samples
prepared via microtome (Figure 32). On each structure, twelve analogous struts were inspected
to obtain an overview of the achieved sputter coating uniformity relative to one another. As
illustrated in Figure 32a, three distinct characteristic coating coverages were observed, where
struts were classified as “1-3” based on the number of sides coated. A rating of “1” denotes a
coating on the topside of the strut, “2” refers to coating coverage on the topside region and one
sidewall, and “3” indicates coating on the topside region and on both sidewalls of the strut. A bar
chart highlighting the coating coverage distribution for each sputtering configuration and
condition is presented in Figure 32b. Overall, the ICM sputtered samples achieved the greatest
coating uniformity, exhibiting the largest number of struts coated on three sides. Samples coated
under the planar cathode in the 90° configuration resulted in the least uniformity, where half of
the struts only exhibited a topside coating and no struts achieved deposition on three sides. The
implementation of the ±30° tilt yielded an increase in the overall planar coating coverage as
demonstrated by a larger proportion of struts being coated on two and three sides. It was also
observed that structures coated in the ICM configuration showed some variation in uniformity as
a result of Ar gas pressure. Samples coated at a lower pressure (3 mTorr) achieved analogous
coating distributions, while an increase in gas pressure (6 mTorr) resulted in a slight drop in
uniformity. The results highlight the influence of line-of-sight and cathode geometry on coating
uniformity and indicates a strong dependence of Ar pressure in ICM cathode configurations. It is
important to note that upon review of all strut cross-sections, irrespective of configuration, no
82

evidence of coating coverage was observed along the underside of the struts. Accordingly, as
previously mentioned, rotation mechanisms can be implemented to induce greater coating
uniformity under planar cathode configurations [9, 18-20, 209], however, the influence of
rotation within an ICM has yet to be explored.
Figure 32: a) TEM micrographs of representative strut cross-sections prepared via microtome
showing varying levels of coating coverage around the strut. The red dotted lines highlight the
number of sides covered, where (1) indicates a coating on the topside of the strut, (2) refers to
coating covering on the top of the strut and one side wall, and (3) indicates coating coverage on
the topside, as well as on two side walls of the strut. b) Bar chart representing the distribution of
coating coverage for each set of microtome prepared samples, where green bars correlate to the
samples coated in the ICM configuration, and yellow and orange represent the planar coated 90°
and ±30° configurations, respectively. A total of twelve analogous struts in each coated sample
were surveyed.
Further insight into the effect of line-of-sight and cathode geometry on the resulting
uniformity was obtained through examination of the as-deposited coating quality and surface
morphology on the micro-lattice scaffolds (Figure 33). Representative scanning electron
83

micrographs show the coating quality of the topside and sidewall of the specified strut outlined
by the white box under all sputtering configurations and parameters presented in this work. As
highlighted by the insets, a characteristic transition from a smooth topside coating to a rough,
directionally oriented sidewall morphology was observed in all samples coated using the planar
cathode, and to a lesser extent, in the samples coated within the ICM. The directionally oriented
growth along the sidewall of the 90° and ±30° samples is indicative of atomic shadowing due to
the region being orthogonally oriented to the planar cathode surface [220, 226]. It was also noted
that samples coated in the 90° planar configuration showed a distinct divot in the sidewall
coating further indicating that the sidewall regions experience low exposure to the deposition
flux under planar cathode configurations, which ultimately can result in large uniformity
gradients. A more homogeneous morphology was achieved in the ICM coated samples as result
of the increased angles of exposure to the sidewalls offered by the cathode geometry. An
increase in both the Ar gas pressure (from 3 to 6 mTorr) and sputtering power (from 100 to 200
W) within the ICM resulted in comparable degrees of surface roughness on the topside and the
sidewall of the selected strut, leading to mitigation of the morphological transition observed in
the planar samples. As such, the enhanced homogeneity in the coating morphology via the ICM
configuration can be correlated to an increase in sidewall deposition coverage and thus resultant
uniformity.
84

Figure 33: SEM micrographs highlighting the side coating quality and morphology of the top
struts sputtered in the a-c) 360° (green), d,e) 90° (yellow), and f,g) ±30° (orange) configurations.
The inset of each show higher resolution micrographs featuring the transition from the top of the
strut to the side. region imaged is highlighted by the white box, and inset micrographs show
higher resolution images of the transition from the top of the strut to the side wall.
5.4. Conclusion
Understanding the fundamental interplay between sputter deposition configurations and
parameters on coating thickness and uniformity for complex lattice structures is imperative for
further development of novel core-shell composite nano- and micro-lattice materials. In this
study, Cu-Al/polymer core-shell composite tetrahedral-truss micro-lattices were generated
leveraging both planar and inverted cylindrical magnetron cathodes under various sputtering
conditions. This work examined both the coating thickness and uniformity. Cross-section
85

evaluations revealed that coating thickness is largely dependent on the sputtering power and Ar
gas pressure depending on cathode geometry, where the planar (90° and ±30°) and ICM (360°)
configurations exhibited variable degrees of coating penetration. Samples coated within the ICM
achieved improved uniformity and coating coverage when compared to planar deposition set-
ups. Overall, the ICM appeared to achieve the best overall coating coverage at lower sputtering
powers and gas pressures without the aid of rotation. However, optimization studies would
require the investigation of additional factors directly influencing ICM coating uniformity
including substrate rotation, plasma conditions, and particle energy during deposition. Thus, a
foundational understanding of sputter deposition on nano- and micro-lattice materials will enable
more accurate design and synthesis of novel architected core-shell composites and provide a
pathway for achieving optimized uniformity for applied functionalities and properties.

86

Chapter 6: Tensile Behavior of Stitched Nano-Lattice Structures Fabricated
via Direct Laser Writing

The work in this chapter has been published as a journal article titled Scaling-up of
Nano-Architected Micro-Structures: A Mechanical Assessment and is published in the journal
Advanced Engineering Materials in Volume 21, Issue 11 (DOI: 10.1002/adem.201900687).
In addition to the materials workspace and coating uniformity challenges covered in
Chapters 4 and 5, the scalability of nano- and micro-architected materials generated by ultra-high
resolution 3D printing techniques such as two-photon polymerization direct laser writing (TPP-
DLW) remains a crucial challenge as larger high-resolution samples require stitching smaller
blocks of the structure of interest together. The study presented in Chapter 6 explores scaling
techniques and testing methodologies to investigate the effect of stitching on the integrity and
mechanical behavior of stitched TPP-DLW log-pile I-beam specimens with relative densities of
21.5 and 54.7% under tensile load. Micro-tensile tests revealed that the higher-density log-pile
samples exhibit brittle behavior with fracture loads at least four times higher than those of the
lower-density samples. The location of sample failure depended on the type of stitch introduced
in the sample, as well as on the relative sample density. Overall, this study highlights the
importance of stitching techniques and relative density for the design of nano- and micro-
architected lattice materials.
6.1. Introduction

There has been a considerable amount of interest in two-photon polymerization
processes, such as direct laser writing, as they enable the fabrication of nearly any topology or
complex structure with sub-micron resolution [227-229]. For example, TPP-DLW can be used
for generating nano- and micro-architected lattice materials with notable mechanical properties
[8, 38] and unparalleled optical behavior [230]. Accordingly, such TPP-DLW architected
87

materials offer advances in a wide range of applications such as photonics [231-233], cell culture
and tissue engineering [12, 154, 155], energy absorption [22], and lightweight structural
components [234]. A number of previous studies have further functionalized DLW printed
materials through modification of the printing resin [235, 236] or by deposition of ceramic and
metallic coatings onto the scaffolds [18, 19, 23, 27, 237]. These materials can achieve new
combinations of strength, density, toughness, and stiffness when compared to conventional
monolithic materials [10, 194, 238]. However, in all these TPP studies the absence of scalability
has remained a prominent challenge.
Indeed, several authors have highlighted the inability to scale-up print volume as a
critical issue in TPP fabrication [15, 192, 239-241]. Consequently, recent studies have explored
stitching techniques to produce millimeter-sized porous materials comprised of microscale
building blocks with nanoscale features [110, 111, 242, 243]. While these works demonstrated
that stitching provides a viable approach to expand the overall print volume, they also revealed
structural defects and other non-uniformities at the stitch boundaries, which may impact the
macroscopic behavior of the material. To date, limitations in print volume have also restricted
the evaluation of TPP-DLW architected structures to predominantly compression tests [6, 38,
134, 238, 243, 244]. Therefore, both scaling techniques and testing methodologies need to be
developed to assess stitching and its effect on structural integrity, which to the best of the
authors’ knowledge, have yet to be explored.
In this study, DLW I-beam micro-tensile specimens with log-pile architectures were
printed with two different relative densities (21.5% and 54.7%) using three different stitch
protocols: no stitch (C), a stitch line (S), and a stitch line with a 2 μm overlap (SO). The effect of
88

the presence and nature of the stitch interface on the mechanical integrity of the DLW log-pile
samples was then tested in tension using a custom micro-tensile machine.
6.2. Experimental Methods
A Nanoscribe GmbH Photonic Professional GT in the dip‐in configuration [237] was
used to fabricate 50 × 50 μm
2
, 100 μm tall log‐pile I‐beam specimens. The piezo scan mode
limits the maximum achievable print range of any structure to a 300 μm × 300 μm × 300 μm
cube. The writing laser, a FemtoFiber pro near‐infrared (NIR) laser supplied by TOPTICA
operating at 780 nm with a 100 fs pulse duration and 80 MHz repetition rate, was focused
through a Zeiss plan‐apochromat 63 × 1.4NA Oil DIC M27 objective, resulting in elliptical
voxels with ≈3.5:1 aspect ratio. Two sets of log‐pile tensile test specimens with integrated grips
were printed: “low‐density” samples with a log‐pile gage section programmed with a pitch size
of 2 μm and a Z spacing of 750 nm and “high‐density” samples with a pitch size of 1 μm and a Z
spacing of 500 nm. Note that the gage section is defined as the entire architected area between
the grips. SEM was used to assess the dimensions of the representative “as‐printed” specimens.
Prior to imaging, samples were coated with a thin layer of Pt using a Cressington 108 Manual
Sputter Coater. All SEM imaging was performed on a FEI Nova NanoSEM 450 at 5 kV.
The specimens were fabricated by applying a drop of IP‐Dip, Nanoscribe's commercial
acrylate‐based photoresist, to a 25 × 25 × 0.7 mm
3
glass slide (obtained from Nanoscribe). Prior
to printing, the glass slides were functionalized with alkoxysilane compounds to promote better
adhesion to the polymer samples. Detailed functionalization steps can be referenced in the
Supporting Information. The I‐beam tensile specimens were printed under three varying stitching
conditions: a group of samples were printed continuously with no stitch (C), a second set was
printed with a 5 min rest period in the middle of the gage section with no overlap (S), and the last
set was printed with a 5 min rest period and a 2 μm overlap in the middle of the gage section
89

(SO). For stitched samples, the lower half of the structure was printed directly onto the glass
slide, the print process was then paused for 5 min, and then the upper half was printed. No
overlap is defined as restarting the printing process after the rest period directly on the structure
surface, whereas the overlap (SO) protocol involves restarting the printing process following the
rest period 2 μm within the already polymerized lower half.
Uniaxial tensile tests were carried out on a custom‐built micromechanical testing
apparatus (Figure 34), comparable with the mechanical testing apparatuses used by Slaby et al.
[245] and Balk et al. [246] The machine, shown in Figure 34a, consists of a coarse actuator
(ThorLabs DRV001), piezo actuator (ThorLabs PAZ015), 2D alignment stage, 1N load cell
(Phidgets CZL639HD), camera (Pixelink PL‐B782F), and light source. The glass slide with
printed tensile specimens was directly attached to the load cell (Figure 34b). Control of the
testing stages and data acquisition were carried out through a custom MATLAB script. Tensile
tests were carried out using a slot and key method, as shown in Figure 34c. An optical camera
above the sample stage was used for aligning and slotting the tensile specimens into a
corresponding fully dense fabricated lock piece, as well for imaging the samples during testing.
Once the I‐beam specimen was slotted, the tensile tests were performed by moving the lock piece
at a speed of 0.1 μm s
−1
.

90

Figure 34: Photographs of (a) a custom micromechanical testing apparatus and b) 1 N load cell
with representative DLW I‐beam tensile samples printed on the glass side. c) A top‐view
schematic showing the slot and key method for gripping tensile specimens: a mechanical grip in
which samples are slotted into a lock piece and pulled in tension until fracture.

6.3. Results and Discussion
This study focuses on several factors that may impact the scaling feasibility, such as
sample relative density and stitching protocol. Figure 35 shows the gage sections of both low‐
and high‐density samples, as well as the as‐printed feature dimensions. Cross‐sectional scanning
electron microscopy (SEM) micrographs, outlined in purple, show the characteristic log‐pile
architectures of the low‐ (78.5% porosity) and high‐density (45.3% porosity) samples in Figure
35a and b, respectively. The 1) width and 2) length of the elliptical voxels for both sets of
samples were determined to be 250 and 850 nm, respectively (Figure 35c,d). The low‐density
samples were printed with an 3) XY spacing of 1.75 μm and a 4) Z spacing of 650 nm, whereas
the high‐density samples were printed with an XY spacing of 850 nm and Z spacing of 450 nm.
A comparison of the as‐designed to the as‐printed feature dimensions indicates ≈12–16%
material shrinkage, consistent with the previously reported values for Nanoscribe's photoresist,
IP‐Dip [110, 111, 242]. The effects of this shrinkage on the materials' functionality, however,
can be widespread, and its impact on the mechanical integrity of the architected materials is still
unknown. Representative SEM micrographs of the as‐printed stitch protocols C, S, and SO of the
low‐density samples (Figure 36) show several types of defects, such as warping and disruptions
91

in continuity at the stitch interface that are similar to previously reported findings [110, 111, 242,
243].

Figure 35: Representative cross‐sectional SEM micrographs highlighting the relative density of
the a) low‐ and b) high‐density samples, respectively. Representative micrographs of the log‐pile
features with an accompanying table of the measured feature dimensions for the c) low‐ and d)
high‐density samples, respectively.

In general, the root cause of these defects is the non‐uniform material shrinkage, which is
typically concentrated at interfaces, such as at the substrate–sample interface and at the grip/gage
transition, where the material properties, like density or modulus, change abruptly. Figure 36a–c
shows the overall sample morphology of the low‐density tensile specimens for the C, S, and SO
protocols, whereas the higher magnification SEM micrographs in Figure 36d–f highlight the
differences at the stitch interface region. Please note that Figure 36a,d does not contain a stitch
interface and thus serve as a reference. Additionally, there is no observable difference at the
stitch boundary between samples with the stitch line (Figure 37e) and those with the overlap
(Figure 36f) as the only difference between the two protocols is the overlap, where the writing
92

path starts at 2 μm into the already printed lower half of the specimen. All samples, irrespective
of relative density or stitch type, show the expected regions of axial shear stress at the interfaces
between the grip and gage sections due to non‐uniform material shrinkage. An example of this
type of shrinkage is highlighted in Figure 36c (additionally, see Figure 38 in Supplementary
Material). Other researchers, such as Liu et al. [110], observed similar axial shear stress in
analogous regions along the interface of a printed log‐pile structure and in a fully dense “top
cap,” which is comparable to the grip pieces printed in the present work. In addition, we also
observe non‐uniform shrinkage at the stitch line (marked by yellow arrows) as evidenced by the
sudden noticeable change in gage width (Figure 36e,f). This is a surprising observation as the
structures (and thus the density) above and below the stitch line are nominally the same. We
hypothesize that non‐uniform shrinkage at the stitch line is due to proximity effects [243, 247,
248], which leads to reduced oxygen inhibitor concentration within the photoresist at the writing
front, which, in turn, increases the degree of conversion [249] and ultimately reduces the extent
of shrinkage observed for a given laser power [250]. The abrupt pause and subsequent 5 min rest
period of the stitched specimens allow the oxygen inhibitor concentration to return to its (higher)
equilibrium value. Upon continuing the writing process, this results in a lower degree of
conversion and consequently greater degree of shrinkage. Nonetheless, it is clear that the regions
of shrinkage‐induced defects are exacerbated by the stitching process, which, in turn, could
impact the viability of stitching for the scaling‐up of DLW‐architected structures [110, 111, 242,
251].
93

Figure 36: Representative SEM micrographs of low‐density tensile specimens: a) no stitch
samples, b) samples printed with a stitch line, and c) those with a stitch line and a 2 μm overlap.
The yellow arrows indicate regions of stitch‐induced defects. Micrographs d–f) show the
corresponding gage sections, where the yellow dashed line highlights the location of the stitch
line. Inset c) highlights a representative region of axial shear stress denoted by the yellow
bracket.

Micro‐tensile tests were performed to assess the impact of these defects on the
mechanical performance. To our knowledge, no previous tensile tests have been reported on such
types of samples. Figure 37 shows the corresponding stress–strain curves normalized by the
effective density (ref. Table 6 in Supplementary Material) for low‐ (21.5%) and high‐density
(54.7%) samples, as well as representative images of the samples at failure. A minimum of two
samples at each density and stitch type have been measured. The corresponding average
maximum tensile load achieved for each type of specimen is shown in Table 5. The data show
that the high‐density samples failed at loads at least four times higher than the low‐density
samples, and of these, those with a stitch line overlap reached the highest average load of
48.0 mN before failure. In contrast, the low‐density samples fractured at much lower loads,
where the highest load of 10.8 mN was achieved in samples printed with no stitch. The observed
trend with a varying relative density is similar to that of open‐cell polymeric foams, where it has
94

been observed that higher‐relative density foams achieve higher maximum tensile stresses [252,
253]. However, as highlighted in Figure 37a, some ductility was observed in low‐density
samples, whereas the high‐density samples exhibited brittle‐like behavior. Polymeric foams are
often brittle in tension [253-256] but have also been shown to undergo ductile failure [252]
depending on the cell geometry and properties of the base material. To date, these materials have
been mainly tested under compression and have been shown to follow the expected strength and
stiffness scaling laws depending on their respective architectures [4, 6, 257]. The only study to
date exploring tensile characterization of DLW micro‐architected structures reported that the
strength of the structure was ≈50% lower in tension than in compression [44], which is consistent
with the results found in this study (see Table 6 in Supplementary Material).

Figure 37: a) Representative stress–strain curves of uniaxial tension tests for low‐ and
high‐density log‐pile I‐beam samples, where the stress values are normalized by the effective
density of the architected gage. Green curves show continuously (C) printed pillars, orange
curves show samples printed with a stitch (S), and blue curves show samples printed with a stitch
line overlap (SO). b) Representative SEM micrographs of fracture surfaces of the low‐density
samples for each type of stitch protocol.

95

Table 5: Summary of average maximum achieved load before failure

Stitch type Max load achieved [mN]
Low-density C 10.8 ± 0.9
S 3.6 ± 0.0
SO 5.4 ± 0.2
High-density C 41.9 ± 1.1
S 41.4 ± 2.6
SO 48.0 ± 1.0

To further understand the implications of stitching on the feasibility of scaling‐up, the
samples were characterized postmortem. As determined from camera images (not shown), all C
printed samples failed in the grip region, whereas all S printed samples failed at the stitch
interface in the midsection of the gage, irrespective of relative density. To further understand the
effect of stitching on sample failure, Figure 37b highlights the representative SEM micrographs
of the low‐density fracture surfaces because these samples have been affected most by the
stitching protocols. In contrast to the fracture morphology of C samples, where the fracture cuts
through multiple layers, a clean fracture along the stitch plane was observed in S printed
samples. The failure along the stitch line in S samples could be attributed to the stitching process
and the rest period, which is expected to locally reduce the degree of polymerization at the stitch
boundary due to increased oxygen inhibition [258, 259]. Similarly, it can be expected that the
stitch line overlap (SO) protocol prevents failure at the stitch interface and enhances the strength
of the structures due to the additional laser raster in the stitch region, which should locally
increase the degree of crosslinking in the polymer [234, 257]. This was indeed observed for the
high‐density SO samples, which achieved the highest failure load among all samples. However,
low‐density SO samples show evidence of interplanar fracture, indicating that the stitch plane is
no longer the weakest point of the sample. The fracture near the stitch line in these samples can
be attributed to the stress concentration within the stitch plane caused by non‐uniform shrinkage
and pre‐existing axial stress in this region (see Figure 36). The discrepancy in failure location
96

and mechanical performance among low‐ and high‐density samples reinforces that relative
density plays a significant role in the tensile behavior of stitched architected materials and needs
further study.
6.4. Conclusion
The effect of stitching, a critical component of the scalability of TPP techniques, on the
mechanical behavior of printed log‐pile structures with two different densities and three different
stitching protocols was investigated by performing micro‐tensile tests. These tests revealed that
the high‐relative‐density structures exhibited brittle behavior, whereas the low‐relative‐density
samples demonstrated a more ductile mechanical response. Additionally, the relative density
affected the location of failure and the maximum strength achieved by the samples, where the
high‐density samples failed at higher loads than the low‐density samples. Overall, this study
provides valuable insights into overcoming the scaling limitations of DLW‐architected materials
and lays the foundation for future studies on the role of stitching protocol and relative density.
6.5. Supplementary Material
6.5.1 Functionalization of Glass Slides
A well-known method for treating glass substrates is by way of self-assembled
monolayers (SAMs) via silanes, which was leveraged in the present study [260]. Both (3-
mercaptopropyl)trimethoxysilane (MPTMS) (Gelest) and 3-[Tris(trimethylsiloxy)silyl]propyl
methacrylate (TSPMA) (Sigma Aldrich) are chemically reactive alkoxysilane compounds and
contain acrylate and thiol functional groups, respectively, which can create covalent attachments
to the glass slides via SAMs. Similarly, a few studies have incorporated the initial step of
functionalizing glass slides prior to printing to enhance adhesion between the fabricated 3D
structure and substrate [154, 236]. The glass slides were first placed in a petri dish and treated
97

with a solution of 3 parts concentrated H2SO4 (Sigma Aldrich)/one part 30% H2O2 (Sigma
Aldrich). After 2hr, the glass slides were removed and thoroughly rinsed with de-ionized water
followed by methanol, and lastly heated at 110°C for 1hr to dry. Subsequently, the slides were
further functionalized with either MPTMS or 3-[Tris(trimethylsiloxy)silyl]propyl methacrylate.
Slides functionalized with TSPMA were submerged in 40 mL of ethanol (reagent grade)
containing 2% acetic acid and 1 mL of 3-[Tris(trimethylsiloxy)silyl]propyl methacrylate.
MPTMS functionalized slides were submerged in 40 mL of toluene and treated with 1 mL (3-
mercaptopropyl)trimethoxysilane. Both mixtures were allowed to sit in a sealed container
overnight. Subsequently, the TSPMA and MPTMS slides were washed thoroughly in ethanol
and toluene, respectively. Lastly, the slides were placed into a 100°C oven to ensure the silane
treatments were fully cured.
6.5.2 Defects in High-Density Structures

Figure 38: Representative SEM micrographs of high-density tensile specimens, where a) show
the gage section of a sample with a stitch line and 2 μm overlap. The yellow arrow indicates the
stitch induced defect. b) Shows the grip/gage interface, where the yellow bracket highlights the
observed axial shear stress.

98

6.6.3 Calculated Effective Density of Gage Sections
Table 6: Relative density and calculated effective density of log-pile gage sections

Relative Density
[%]
Effective Density
[kg m
-3
]
Low-Density 21.5% 275.2
High-Density 54.7% 700.2
Density values of the log-pile gage sections were calculated by using a programmed laser path
and the bulk density value of 1280 kg/m
3
for IP-Dip.[38]
6.5.4 Comparison of Calculated Bending-Dominated Yield Strength Under Compression to
Experimental Tensile Yield Strength
Table 7: Calculated compressive yield strength and experimental tensile yield strength values of
samples containing no stitch

Calculated Compressive Yield
Strength
[MPa]
Experimental Tensile Yield
Strength
[MPa]
Low-Density 7.7 4.6 ± 0.4
High-Density 31.2 18.0 ± 0.5
The compressive yield strength, σy, of bending- dominated cellular structures scales with
relative density according to:
σy ≈ 0.3𝜌 ̅ 3/2
σys Equation (S1)
where 𝜌 ̅ and 𝜎 𝑦𝑠
are the relative density and yield strength of the parent solid, respectively.
Additionally, the yield strength of IP-Dip, the parent material used in this study, was determined
using Equation (S1) and values from Juarez et al [19]. The average experimental tensile strength
of the two sets of samples were determined from those containing no-stitch. The low- and high-
density tensile strength was determined to be approximately 59.7% and 57.7% of the calculated
compressive strength, respectively.

99

Chapter 7: Conclusions and Future Work
7.1. Conclusions
Nano- and micro-architected lattices are an emerging class of materials with promise in a
wide range of property and functionality spaces. However, to truly expand the overall workspace
of these advanced materials, further evaluation of both the scalability and material selection
shortcomings present in current high-resolution additive manufacturing technologies, such as
two photon polymerization direct laser writing (TPP-DLW), is needed. Thus, the work presented
in Chapters 4-6 explored: 1) the impact of sputtering conditions and configurations on the
coating uniformity for the development of core-shell composite micro-lattice structures, and 2)
the influence of stitched boundaries on the mechanical performance of TPP-DLW nano-lattice
structures for scaling-up implications. The work presented in this dissertation provides
contributions towards expanding the fabrication space of 3D nano- and micro-architected lattice
materials.
In Chapter 4, it was shown that magnetron sputtering is a viable deposition technique for
the development of new metallic hollow-tube and core-shell composite nano- and micro-lattice
materials. To date, a variety of lattice geometries and coating material combinations have been
developed as sputtering of such complex structures is an emerging fabrication route, however,
due to the momentum-driven line-of-sight nature of the deposition technique, large coating
thickness gradients exist throughout the coated structures. Thus, to advance towards a more
fundamental understanding of the key factors influencing sputter coating uniformity, an
assessment of various deposition conditions, cathode geometries, and configurations for the
development of core-shell composite micro-lattice structures was carried out in Chapter 5.
Specifically, both planar (90°, ±30°) and inverted cylindrical magnetron (360°) sputtering
techniques were employed under various deposition powers and deposition rates, as well as
100

substrate configurations to evaluate the impact of cathode geometry and line-of-sight on the
coating coverage of tetrahedral truss micro-lattice structures. The coating quality and
morphology/microstructure of the as-deposited Cu-2 wt.% Al coatings were characterized using
XRD, SEM, and EDS, revealing that the coatings all exhibited strong [111] texturing, but the
ICM coatings showed evidence of a more randomized texture with more pronounced [220] and
[311] peaks. Additionally, the elemental compositions of the deposited alloys confirm that the
stoichiometry of the sputtering targets were preserved.
The coating thickness and uniformity were further evaluated by sectioning the as-coated
micro-lattice structures via plasma focused ion beam (PFIB) and microtome. PFIB milling was
initially implemented to section the structures and determine their coating coverage relative to
one another. However, redeposition of the milled material led to inaccurate sidewall and
underside coating thickness evaluation, and thus only topside thickness measurements were able
to be obtained. Thus, microtome was employed to assess the unaffected cross-section of the
coated structure. The cross-sections revealed that the topside coating thickness of samples coated
at the same deposition rate (≈0.48 nm sec
-1
) but different configurations (90°, ±30°, 360°) was
thickest for samples coated in the ICM. Additionally, it was observed that samples coated within
the ICM resulted in the greatest coating uniformity, as compared to those coated under planar
cathode configurations. It should also be noted that the there was no evidence of underside
coating along the struts amongst all samples coated in this study. Such results indicate that
cathode geometries that implement increased line-of-sight can be leveraged to introduce greater
degrees of coating coverage when depositing on nano- and micro-lattice structures. However,
further studies should be conducted to further optimize the coating coverage achieved by ICM
sputtering. Further discussion of potential future steps can be found in Section 7.2.
101

Additional work has been performed in this dissertation to assess the feasibility of
stitching methodologies for scaling-up architected lattice materials, and to understand the
mechanical impact that stitching defects/non-uniformities may have on the integrity of such
structures. This work aimed to overcome the limitations polymer-based TPP-DLW fabrication
methodologies by establishing alternative scaling to expand the working space of 3D nano- and
micro-architected materials. The current print volume of structures fabricated via TPP-DLW is
restricted by the maximum stage travel using a piezo motor. Thus, to yield larger structures,
multiple printed areas are stitched together, which introduces a stitch interface or region of
potential weakness in the structure. The tensile strength and mechanical behavior of this interface
was investigated leveraging testing methodology and a custom-built apparatus designed for this
work. I-beam micro-tensile samples were printed using TPP-DLW with a designed log-pile
architecture gage section with varying relative densities – referred to as “low-density” (≈21.5%)
and “high-density” (54.7%). The low- and high-density specimens were printed under various
print protocols which introduced different stitch boundaries. The first set of samples were printed
continuously (C), such that no stitch was present in the structure, the second set printed
implemented a stitch interface (S) in the center of the gage section through a 5-minute rest
period, and the third set of samples introduced a stitch overlap (SO) at the center of the gage
section with a 5-minute rest period and 2 µm overlap.
A slot and key method was employed to mechanically test the samples such that the I-
beam samples were gripped by slotting the sample into a complimentary lock-piece and pulling
the sample in tension. It was observed that low-density samples exhibited additional ductility
prior to fracture and failed a loads 4x less than the high-density samples. Additionally, samples
that were printed continuously withstood the highest loads before failure amongst the low-
102

density samples, and those printed with a stitch overlap (SO) failed at the highest loads in the
high-density set of samples. Regardless of relative density, the stitch line (S) samples regularly
failed along the stitch interface, and continuously (C) printed samples at the grip. This work
seeks to build a foundation for scaling-up architected lattice materials with nano- and microscale
features such that processing protocols can be built up to mitigate stitching effects.
7.2. Future Work

The studies presented in this dissertation have addressed multiple challenges in the
fabrication space of nano- and micro-lattice materials, but work remains to optimize and explain
the key driving parameters for achieving both mechanically robust scaled-up structures and
enhanced sputtering coating uniformity. Promising future routes building off this work include:
(1) expanding the topologies explored for stitching feasibility and print processing parameter
optimization for scaling-up, and (2) to further isolate the contributions of sputtering parameters
and configurations in ICM deposition on nano- and micro-lattice structures.
For the mechanical assessment of scaled-up structures, further studies implementing dog-
bone specimens that more closely resemble a standard tensile sample would potentially mitigate
the overwhelming failure observed at the grip/gage interface. The I-beam geometry of the
structures tested in this dissertation is not ideal for tensile testing due to the stress concentrations
at the grip/gage interface. Additionally, investigating the influence of various topologies and
corresponding relative densities would provide a greater picture as to the impact of beam
slenderness and rigidity on the robustness of the stitch interface. Leveraging dog-bone specimens
and exploring more complex gage section topologies would provide further foundational
information to build upon our understanding of the challenges associated with scaling-up TPP-
103

DLW nano- and micro-lattice structures, as well as aid in the development and optimization of
stitching print protocols.
As for sputtering coating optimization, future studies are needed to investigate additional
factors directly influencing ICM coating uniformity on nano- and micro-lattice structure. Such
additional influences include accounting for implemented substrate rotation, plasma conditions,
and particle energy during deposition. To date, sputter coatings on architected lattice structures
have resulted in uniformity gradients that make the mechanical performance of such materials
difficult to accurately assess. A foundational understanding of the key elements effecting sputter
deposition on nano- and micro-lattice materials will enable more accurate design and synthesis
of novel architected core-shell composites and provide a pathway for achieving optimized
functionalities and properties. The overarching goal is to optimize the coating uniformity via
magnetron sputtering and extend it to new material systems to improve the mechanical
performance of these advanced architected lattice materials. The proposed future work will
further elucidate the controlling parameters within ICM sputtering that yield optimal coating
coverage for improved mechanical behavior and functionality.
The studies detailed in this dissertation have contributed towards fundamental knowledge
for expanding the fabrication space of nano- and micro-lattice materials. Through continued
exploration into the effects of magnetron sputtering parameters, varied lattice topologies and
print processing parameters, the scalability and materials workspace challenges in TPP-DLW
fabrication can be circumvented, and previously unexplored lattice materials can be studied.

104

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120

Appendix A: Summary of Core-Shell Composite Tetrahedral Truss Micro-
lattice Samples

This appendix contains tables summarizing all sputtered films and coated Nanoscribe
printed micro-architected lattice structures throughout the course of this dissertation. The
following tables outline structures coated via planar (Table 8) and inverted cylindrical (Table 9)
magnetron sputtering. Note, all tetrahedral truss samples coated in this dissertation were of the
same dimensions (a strut length of 10 µm and strut diameter of 1.5 µm). All substrate
information, as well as deposition conditions and configuration are provided. Samples are
labeled according to the following:
Coating Material = Cu-Al 2wt %, Cu-Al 6wt%, Inconel 600 (INC), Ti-6Al-4V (Ti64)
Coating Configuration = Planar (P), Hollow Cathode (HC), Oblique Angle (±30°)
Substrate Type = Silicon Wafer (Si), Glass, Flat Ip-Dip (Ip-Dip), Flat Ip-S (Ip-S)
Type of Coating = Sputtering Rate (SR), 1 µm Thick Film (Film), On-structures
(Samples, Slide #)
Substrate Orientation in Hollow Cathode = Perpendicular (PERP), Parallel (PARA)
Deposition Conditions = Sputtering Power (W), Ar Pressure (mTorr)
Rotation = Center Rotation (CENTER ROT), Off-Axis Rotation (OFF-AXIS ROT)

121

Table 8: Planar Cathode Sputtered Samples
Sample Name Substrate
Coating
Material
Set-Up
Power
(W)
Ar
Pressure
(mTorr)
Thickness
(nm)
Sputtering
Rate (nm/s)
INC_SR_WD2.5_1
POS 1
Si wafer Inconel 600
3" gun, top, 2.5”
distance
16 4 - 0.13
INC_SR_WD2.5_1
POS 2
Glass Inconel 600
3" gun, top, 2.5”
distance
16 4 - 0.15
INC_WD2.5_2
POS 1
Glass Inconel 600
3" gun, top, 2.5”
distance
16 4 150 0.13
INC_WD2.5_2
POS 2
Si wafer Inconel 600
3" gun, top, 2.5”
distance
16 4 150 0.15
Ti64_WD1.875_1 SR
POS 1
Glass Ti-6Al-4V
3" gun, top,
1.875” distance
95 4.5 500 0.28
Ti64_WD1.875_2 SR Glass Ti-6Al-4V
3" gun, top,
1.875” distance
150 3 1327 0.75
Ti64
SR1, P, POS 1
WD 5.5"
Glass Ti-6Al-4V
3" gun, top, 5.5”
distance
65 2 176 0.065
Ti64
Glass1, P, POS 2
WD 5.5"
Glass Ti-6Al-4V
3" gun, top, 5.5”
distance
65 2 176 0.065
Cu-Al 2wt%
Si, P, POS 1
WD 5.5"
Si wafer Cu-Al 2wt%
3" gun, top, 5.5”
distance
65 2 3557 0.25
Cu-Al 2wt%
SR, P, POS 2
WD 5.5"
Si wafer Cu-Al 2wt%
3" gun, top, 5.5”
distance
65 2 3557 0.25
Cu-Al 2wt%
SR1, P, WD1.875"
18W, 3mTorr
Glass Cu-Al 2wt%
3" gun, top,
1.875” distance
18 3 269 0.33
Cu-Al 2wt%
SR1, P, WD1.875"
200W, 3mTorr
Glass Cu-Al 2wt%
3" gun, top,
1.875” distance
200 3 2622 3.64
Cu-Al 2wt%
SR, P, Si
25W, 3mTorr
Si wafer Cu-Al 2wt%
3" gun, top,
1.875” distance
25 3 300 0.50
Cu-Al 2wt%
SR, P, Si
48W, 3mTorr
Si wafer Cu-Al 2wt%
3" gun, top,
1.875” distance
48 3 517 0.96
122

Cu-Al 2wt%
SR, P, Si
24W, 3mTorr
Si wafer Cu-Al 2wt%
3" gun, top,
1.875” distance
24 3 298 0.50
Cu-Al 2wt%
SR, P, Si
22W, 3mTorr
Si wafer Cu-Al 2wt%
3" gun, top,
1.875” distance
22 3 274 0.46
Cu-Al 2wt%
SR, P, Si
23W, 3mTorr
Si wafer Cu-Al 2wt%
3" gun, top,
1.875” distance
23 3 287 0.48
Cu-Al 2wt%
SR, P, Si
23W, 3mTorr
Si wafer Cu-Al 2wt%
3" gun, top,
1.875” distance
23 3 167 0.40
Cu-Al 2wt%
P FILM
23W, 3mTorr
Glass Cu-Al 2wt%
3" gun, top,
1.875” distance
23 3 1000 0.48
Cu-Al 2wt%
SR, P, Si
46W, 3mTorr
Si wafer Cu-Al 2wt%
3" gun, top,
1.875” distance
46 3 452 0.95
Cu-Al 2wt%
P FILM
46W, 3mTorr
Glass Cu-Al 2wt%
3" gun, top,
1.875” distance
46 3 1000 0.95
Cu-Al 2wt%
SR, P, Si, ±30
25W, 3mTorr
Si wafer Cu-Al 2wt%
3" gun, top,
1.875” distance,
±30° tilt
25 3 167 0.40
Cu-Al 2wt%
SR, P, Si, ±30
33W, 3mTorr
Si wafer Cu-Al 2wt%
3" gun, top,
1.875” distance,
±30° tilt
33 3 229 0.55
Cu-Al 2wt%
SR, P, Si, ±30
30W, 3mTorr
Si wafer Cu-Al 2wt%
3" gun, top,
1.875” distance,
±30° tilt
30 3 201 0.48
Cu-Al 2wt%
P FILM, ±30
30W, 3mTorr
Glass Cu-Al 2wt%
3" gun, top,
1.875” distance,
±30° tilt
30 3 1000 0.48
Cu-Al 2wt%
SR, P, Si, ±30
60W, 3mTorr
Si wafer Cu-Al 2wt%
3" gun, top,
1.875” distance,
±30° tilt
60 3 238 0.99
Cu-Al 2wt%
P FILM, ±30
60W, 3mTorr
Glass Cu-Al 2wt%
3" gun, top,
1.875” distance,
±30° tilt
60 3 1000 0.99
10/25/19 Tetrahedral
Prac Silica-Ip-Dip
SLIDE 1
Tetrahedral
Structures
Ti-6Al-4V
3" gun, top,
1.875” distance,
rotation
150 3 65 0.75
10/25/19 Tetrahedral
Prac Silica-Ip-Dip
SLIDE 2
Tetrahedral
Structures
Ti-6Al-4V
3" gun, top,
1.875” distance,
rotation
150 3 216 0.75
123

5/14/20
Prac Ip-Dip
SLIDE 2
Tetrahedral
Structures
Ti-6Al-4V
3" gun, top, 5.5”
distance
65 2 200 0.065
5/14/20
P Samples FLAT 23W,
3mTorr
SLIDE 8
Tetrahedral
Structures
Cu-Al 2wt%
3" gun, top,
1.875” distance
23 3 200 0.48
5/14/20
P Samples FLAT 46W,
3mTorr
SLIDE 9
Tetrahedral
Structures
Cu-Al 2wt%
3" gun, top,
1.875” distance
46 3 200 0.95
5/14/20
P Samples, ±30
30W, 3mTorr
SLIDE 10
Tetrahedral
Structures
Cu-Al 2wt%
3" gun, top,
1.875” distance,
±30° tilt
30 3 200 0.48
5/14/20
P Samples, ±30
60W, 3mTorr
SLIDE 11
Tetrahedral
Structures
Cu-Al 2wt%
3" gun, top,
1.875” distance,
±30° tilt
60 3 200 0.99

Table 9: Inverted Cylindrical Magnetron Sputtered Samples
Sample Name Substrate
Coating
Material
Set-Up
Power
(W)
Ar
Pressure
(mTorr)
Thickness
(nm)
Sputtering
Rate (nm/s)
Cu-Al 2wt%
HC SR
2mTorr, 0.2kW
Glass Cu-Al 2wt% ICM, parallel 200 2 1200 1.02
Cu-Al 2wt%
HC SR
4mTorr, 0.1kW
Glass Cu-Al 2wt% ICM, parallel 100 4 246 0.41
Cu-Al 2wt%
HC SR
4mTorr, 0.2kW
Glass Cu-Al 2wt% ICM, parallel 200 4 613 1.02
Cu-Al 2wt%
HC SR PERP2
4mTorr, 0.1kW
Glass Cu-Al 2wt%
ICM,
perpendicular
100 4 - -
Cu-Al 2wt%
HC SR PERP2
4mTorr, 0.2kW
Glass Cu-Al 2wt%
ICM,
perpendicular
200 4 890 1.34
Cu-Al 2wt%
HC SR PERP3
4mTorr, 0.2kW
Glass Cu-Al 2wt%
ICM,
perpendicular
200 4 710 1.20
Cu-Al 2wt%
HC FILM PERP
4mTorr, 0.2kW
Glass Cu-Al 2wt%
ICM,
perpendicular
200 4 1000 1.20
Cu-Al 2wt%
HC SR PERP
200W, 6mTorr
Glass Cu-Al 2wt%
ICM,
perpendicular
200 6 650 1.09
124

Cu-Al 2wt%
HC SR PERP
200W, 3mTorr
Glass Cu-Al 2wt%
ICM,
perpendicular
200 3 658 1.10
Cu-Al 2wt%
HC SR PERP
100W, 3mTorr
Glass Cu-Al 2wt%
ICM,
perpendicular
100 3 343 0.48
Cu-Al 2wt%
HC SR PERP
100W, 6mTorr
Glass Cu-Al 2wt%
ICM,
perpendicular
100 6 362 0.50
Cu-Al 2wt%
HC FILM PERP
100W, 3mTorr
Glass Cu-Al 2wt%
ICM,
perpendicular
100 3 1000 0.48
Cu-Al 2wt%
HC FILM PERP
200W, 3mTorr
Glass Cu-Al 2wt%
ICM,
perpendicular
200 3 1000 1.10
Cu-Al 2wt%
HC FILM PERP
100W, 6mTorr
Glass Cu-Al 2wt%
ICM,
perpendicular
100 6 1000 0.50
Cu-Al 2wt%
HC PERP Ip-Dip
FLAT
200W, 3mTorr, ROT
Ip-Dip Cu-Al 2wt%
ICM,
perpendicular,
center rotation
200 3 500 1.10
Cu-Al 2wt%
HC PERP Ip-Dip
FLAT
200W, 3mTorr, ROT
Ip-Dip Cu-Al 2wt%
ICM,
perpendicular,
center rotation
200 3 1000 1.10
Cu-Al 2wt%
HC PERP Ip-S FLAT
200W, 3mTorr, ROT
Ip-S Cu-Al 2wt%
ICM,
perpendicular,
center rotation
200 3 500 1.10
Cu-Al 2wt%
HC PERP Ip-S FLAT
200W, 3mTorr, ROT
Ip-S Cu-Al 2wt%
ICM,
perpendicular,
center rotation
200 3 1000 1.10
Cu-Al 2wt%
HC PERP, SR 1
200W, 3mTorr
OFF-AXIS ROT
Glass Cu-Al 2wt%
ICM,
perpendicular, off-
axis rotation,
≈1.1” distance
200 3 521 0.87
Cu-Al 2wt%
HC PERP, SR 2
200W, 3mTorr
OFF-AXIS ROT
Glass Cu-Al 2wt%
ICM,
perpendicular, off-
axis rotation,
≈1.1” distance
200 3 469 0.92
Cu-Al 6wt%; HC
PERP, SR
200W, 3mTorr
CENTER ROT
Glass Cu-Al 6wt%
ICM,
perpendicular
200 3 560 0.93
Cu-Al 6wt%
HC PERP, SR
200W, 3mTorr
OFF-AXIS ROT
Glass Cu-Al 6wt%
ICM,
perpendicular, off-
axis rotation,
≈1.1” distance
200 3 601 1.00
Cu-Al 6wt%
HC PERP Ip-Dip
FLAT; 200W
3mTorr, ROT
Ip-Dip Cu-Al 6wt%
ICM,
perpendicular,
center rotation
200 3 500 0.93
125

Cu-Al 6wt%
HC PERP Ip-Dip
FLAT; 200W
3mTorr, ROT
Ip-Dip Cu-Al 6wt%
ICM,
perpendicular,
center rotation
200 3 1000 0.93
Cu-Al 6wt%
HC PERP Ip-S FLAT
200W, 3mTorr, ROT
Ip-S Cu-Al 6wt%
ICM,
perpendicular,
center rotation
200 3 500 0.93
Cu-Al 6wt%
HC PERP Ip-S FLAT
200W, 3mTorr, ROT
Ip-S Cu-Al 6wt%
ICM,
perpendicular,
center rotation
200 3 1000 0.93
5/14/20
High Laser; Ip-Dip
SLIDE 3
Tetrahedral
Structures
Cu-Al 2wt%
+ W
ICM,
perpendicular
200 4 450 1.20
5/14/20
HC Samples PERP
100W, 3mTorr
SLIDE 6
Tetrahedral
Structures
Cu-Al 2wt%
ICM,
perpendicular
100 3 200 0.48
5/14/20
HC Samples PERP
100W, 3mTorr
SLIDE 4
Tetrahedral
Structures
Cu-Al 2wt%
ICM,
perpendicular
100 3 200 0.48
5/14/20
HC Samples PERP
200W, 3mTorr
SLIDE 5
Tetrahedral
Structures
Cu-Al 2wt%
ICM,
perpendicular
200 3 200 1.10
5/14/20
HC Samples PERP
200W, 3mTorr
SLIDE 7
Tetrahedral
Structures
Cu-Al 2wt%
ICM,
perpendicular
100 6 200 0.50
5/14/20
HC Samples PERP
100W, 3mTorr
SLIDE 12
Tetrahedral
Structures
Cu-Al 2wt%
ICM,
perpendicular
100 3 200 0.48
Cu-Al 2wt%
HC PERP STRUC
200W, 3mTorr
OFF-AXIS ROT
Tetrahedral
Structures
Cu-Al 2wt%
ICM,
perpendicular, off-
axis rotation,
≈1.1” distance
200 3 200 0.92
Cu-Al 2wt%
HC PERP STRUC
200W, 3mTorr
CENTER ROT
Tetrahedral
Structures
Cu-Al 2wt%
ICM,
perpendicular,
center rotation
200 3 200 1.10

126

Appendix B: Fabrication of Macroscopic Lattice Structures via Digital Light
Processing and Magnetron Sputtering

In this work, macroscopic cubic lattice structures of varying complexity were coated
using magnetron sputtering. Cubic structures comprised of 1x1, 2x2, and 5x5 unit cells were
fabricated along with full density cubes leveraging digital light processing (DLP). As depicted in
Figure 36, the simple cubic structures were designed to be affixed atop a triangular prism to
allow for all faces of the structure to be exposed to the coating material during deposition. As
covered in Chapter 2, Section 2.3, DLP is a top-down 3D printing technique comparable to
stereolithography, but rather than a laser, the technology utilizes a digital micromirror device
(DMD). The fabrication process leverages photopolymerization principles to develop three-
dimensional parts. An Autodesk Ember 3D printer was be used in conjunction with Autodesk’s
acrylate based PR48 resin to fabricate the lattice structures with strut lengths on the order of
hundreds of microns. After printing, the lattice structures were coated in Inconel 600, Ti-6Al-4V,
and Al6061 utilizing planar magnetron sputtering and a rotating substrate holder (Figure 37).
Sputtering rates were determined on 3D printed rods with a ≈300 μm width.
Figure 39: CAD models depicting the various macroscopic cubic lattice structures designed with
varying complexity.

127

Figure 40: Schematic showing the deposition configuration used to coat cubic lattice structures
printed via DLP.
Figure 38 shows a representative Inconel 600 coated 5x5 cubic lattice, where higher
resolution SEM images are highlighted in Figures 38b-e. To mitigate any temperature-induced
warping, preliminary temperature readings were acquired by using a wafer thermocouple. The
temperature measurments outlined in Table 9 were obtained for both Inconel 600 and Ti-6Al-4V
targets. A comprehensive list of samples coated to date are summarized in Table 10.

128

Figure 41: (a) Representative optical micrograph of a DLP printed cubic lattice coated in Inconel
600, where (b-e) show representative SEM micrographs of the coating quality on the front face
of the structure.

Table 10: Experimental Planar Cathode Sputtering Temperature Profiles
Target Material Target Diameter (cm) Working Distance (cm) Power (W) Temperature (°C)
Inconel 600 7.62 6.35 16 111
Inconel 600 7.62 6.35 20 131
Inconel 600 7.62 6.35 30 167
Ti-6Al-4V 7.62 10.16 346 228
Ti-6Al-4V 7.62 13.97 346 156

129

Table 11: Summary of Coated DLP Printed Structures via Planar Magnetron Sputtering
Sample Name Substrate
Coating
Material
Set-Up
Power
(W)
Ar
Pressure
(mTorr)
Nominal
Thickness
(nm)
Sputtering
Rate (nm/s)
R1 Rod Inconel 600
(2) 1.3" guns,
top/bottom, 5.5”
equidistant,
rotation
65 5 - -
R2 Rod Inconel 600
(2) 1.3" guns,
top/bottom, 5.5”
equidistant,
rotation
65 5 490 0.07
R3 Rod Inconel 600
(2) 1.3" guns,
top/bottom, 5.5”
equidistant,
rotation
65 5 490 0.07
R4 Rod Inconel 600
(2) 1.3" guns,
top/bottom, 5.5”
equidistant,
rotation
130 5 - -
R5 Rod Inconel 600
(2) 1.3" guns,
top/bottom, 5.5”
equidistant,
rotation
65 5 250 0.07
R6 Rod Ti-6Al-4V
(2) 1.3" guns,
top/bottom, 5.5”
equidistant,
rotation
65 15 - -
R7 Rod Inconel 600
(2) 1.3" guns,
top/bottom, 5.5”
equidistant,
rotation
65 5 - -
R8 Rod Inconel 600
1.3" gun, top, 5.5”
distance, rotation
65 5 434 0.048
R9 Rod
Ag(70at%)/A
u(30at%)
1.3" gun, top, 5.5”
distance, rotation
65 5 - 0.07
R10 Rod Ti-6Al-4V
1.3" gun, top, 5.5”
distance, rotation
65 5 200 0.028
R11/R12_2 Rod Ti-6Al-4V
3" gun, top, 5.5”
distance, rotation
346 5 - 0.13
R12 Rod Al6061
1.3" gun, top, 5.5”
distance, rotation
65 5 230 0.032
R13 Rod Ti-6Al-4V
3" gun, top, 4”
distance, rotation
346 5 297 0.16
I4_1
Cube,
4mm, 5x5
Inconel 600
(2) 1.3" guns,
top/bottom, 5.5”
equidistant,
rotation
65 5 1000 0.07
130

I4_2
Cube,
4mm, 5x5
Inconel 600
(2) 1.3" guns,
top/bottom, 5.5”
equidistant,
rotation
65 5 500 0.07
I5_1
Cube,
5mm, 5x5
Inconel 600
(2) 1.3" guns,
top/bottom, 5.5”
equidistant,
rotation
65 5 1000 0.07
I_4_1
Cube,
4mm, 5x5
Inconel 600
(2) 1.3" guns,
top/bottom, 5.5”
equidistant,
rotation
65 5 200 0.07
I_4_2
Cube,
4mm, 5x5
Inconel 600
(2) 1.3" guns,
top/bottom, 5.5”
equidistant,
rotation
65 5 200 0.07
I_4_3
Cube,
4mm, 5x5
Inconel 600
1.3" gun, top, 5.5”
distance, rotation
65 5 284 0.048
I_4_4
Cube,
4mm, 5x5
Inconel 600
1.3" gun, top, 5.5”
distance, rotation
65 5 284 0.048
I_4_5
Cube,
4mm, 5x5
Inconel 600
1.3" gun, top, 5.5”
distance, rotation
65 5 284 0.048
I_4_6
Cube,
4mm, 5x5
Inconel 600
1.3" gun, top, 5.5”
distance, rotation
65 5 284 0.048
I_4_7
Cube,
4mm, 5x5
Inconel 600
1.3” gun, bottom,
5.5” distance,
rotation
65 5 284 0.048
I_4_8
Cube,
4mm, 5x5
Inconel 600
1.3" gun, top, 5.5”
distance, rotation
65 5 284 0.048
I_4_10
Cube,
4mm, 5x5
Inconel 600
1.3" gun, top, 5.5”
distance, rotation
65 5 284 0.048
I_4_11
Cube,
4mm, 5x5
Inconel 600
1.3" gun, top, 5.5”
distance, rotation
65 5 284 0.048
I_4_12
Cube,
4mm, 5x5
Inconel 600
1.3" gun, top, 5.5”
distance, rotation
65 5 284 0.048
I_4_13
Cube,
4mm, 5x5
Inconel 600
1.3" gun, top, 5.5”
distance, rotation
65 5 500 0.048
I_4_14
Cube,
4mm, 5x5
Inconel 600
1.3" gun, top, 5.5”
distance, rotation
65 5 500 0.048
131

I_4_15
Cube,
4mm, 5x5
Inconel 600
1.3" gun, top, 5.5”
distance, rotation
65 5 500 0.048
Ti_4_1
Cube,
4mm, 5x5
Ti-6Al-4V
1.3" gun, top, 5.5”
distance, rotation
65 5 200 0.028
Ti_4_3
Cube,
4mm, 5x5
Ti-6Al-4V
1.3" gun, top, 5.5”
distance, rotation
65 5 200 0.028
Ti_4_4
Cube,
4mm, 5x5
Ti-6Al-4V
1.3" gun, top, 5.5”
distance, rotation
65 5 500 0.028
Ti_4_6
Cube,
4mm, 5x5
Ti-6Al-4V
1.3" gun, top, 5.5”
distance, rotation
65 5 500 0.028
3"Ti_FDWD4_1
Cube,
3mm, Full
Density
Ti-6Al-4V
3" gun, top, 4”
distance, rotation
346 5 200 0.16
3"Ti_FDWD4_2
Cube,
3mm, Full
Density
Ti-6Al-4V
3" gun, top, 4”
distance, rotation
346 5 200 0.16
3"Ti_1x1WD4_1
Cube,
3mm, 1x1
Ti-6Al-4V
3" gun, top, 4”
distance, rotation
346 5 200 0.16
3"Ti_1x1WD4_2
Cube,
3mm, 1x1
Ti-6Al-4V
3" gun, top, 4”
distance, rotation
346 5 200 0.16
3"Ti_2x2WD4_1
Cube,
3mm, 2x2
Ti-6Al-4V
3" gun, top, 4”
distance, rotation
346 5 200 0.16
3"Ti_2x2WD4_2
Cube,
3mm, 2x2
Ti-6Al-4V
3" gun, top, 4”
distance, rotation
346 5 200 0.16
3"Ti_4WD4_1
Cube,
4mm, 5x5
Ti-6Al-4V
3" gun, top, 4”
distance, rotation
346 5 200 0.16
3"Ti_4WD4_2
Cube,
4mm, 5x5
Ti-6Al-4V
3" gun, top, 4”
distance, rotation
346 5 200 0.16
3"Ti_FD5.5_1
Cube,
3mm, Full
Density
Ti-6Al-4V
3" gun, top, 5.5”
distance, rotation
346 5 200 0.13
3"Ti_FDWD5.5_2
Cube,
3mm, Full
Density
Ti-6Al-4V
3" gun, top, 5.5”
distance, rotation
346 5 200 0.13
132

3"Ti_1x1WD5.5_1
Cube,
3mm, 1x1
Ti-6Al-4V
3" gun, top, 5.5”
distance, rotation
346 5 200 0.13
3"Ti_1x1WD5.5_2
Cube,
3mm, 1x1
Ti-6Al-4V
3" gun, top, 5.5”
distance, rotation
346 5 200 0.13
3"Ti_2x2WD5.5_1
Cube,
3mm, 2x2
Ti-6Al-4V
3" gun, top, 5.5”
distance, rotation
346 5 200 0.13
3"Ti_2x2WD5.5_2
Cube,
3mm, 2x2
Ti-6Al-4V
3" gun, top, 5.5”
distance, rotation
346 5 200 0.13
3"Ti_4_2
Cube,
4mm, 5x5
Ti-6Al-4V
3" gun, top, 5.5”
distance, rotation
346 5 200 0.13
3"Ti_4_3
Cube,
4mm, 5x5
Ti-6Al-4V
3" gun, top, 5.5”
distance, rotation
346 5 200 0.13
KIT_Ti_4_1
Cube,
4mm, 5x5
Ti-6Al-4V
KIT Chamber, 2”
gun, bottom,
10.6” distance, 2
guns, rotation
75 50 sccm 200 0.086
KIT_Ti_4_2
Cube,
4mm, 5x5
Ti-6Al-4V
KIT Chamber, 2”
gun, bottom,
10.6” distance, 2
guns, rotation
75 50 sccm 200 0.086
KIT_Ti_4_3
Cube,
4mm, 5x5
Ti-6Al-4V
KIT Chamber, 2”
gun, bottom,
10.6” distance, 2
guns, rotation
75 50 sccm 500 0.086
KIT_Ti_4_4
Cube,
4mm, 5x5
Ti-6Al-4V
KIT Chamber, 2”
gun, bottom,
10.6” distance, 2
guns, rotation
75 50 sccm 500 0.086
KIT_Ti_4_5
Cube,
4mm, 5x5
Ti-6Al-4V
KIT Chamber, 2”
gun, bottom,
10.6” distance, 2
guns, rotation
75 50 sccm 500 0.086
KIT_Ti_4_6
Cube,
4mm, 5x5
Ti-6Al-4V
KIT Chamber, 2”
gun, bottom,
10.6” distance, 2
guns, rotation
75 50 sccm 500 0.086
Al6061_4_1
Cube,
4mm, 5x5
Al6061
1.3" gun, top, 5.5”
distance, rotation
65 5 200 0.032
Al6061_4_2
Cube,
4mm, 5x5
Al6061
1.3" gun, top, 5.5”
distance, rotation
65 5 200 0.032
Al6061_4_3
Cube,
4mm, 5x5
Al6061
1.3" gun, top, 5.5”
distance, rotation
65 5 200 0.032
133

Al6061_4_4
Cube,
4mm, 5x5
Al6061
1.3" gun, top, 5.5”
distance, rotation
65 5 500 0.032
Al6061_4_5
Cube,
4mm, 5x5
Al6061
1.3" gun, top, 5.5”
distance, rotation
65 5 500 0.032
Al6061_4_6
Cube,
4mm, 5x5
Al6061
1.3" gun, top, 5.5”
distance, rotation
65 5 500 0.032

Abstract (if available)

Abstract Over the last two decades, architected lattice materials have garnered increasing attention due to their ability to achieve unique combinations of properties and functionalities linked to their carefully controlled topologies. Recent advances in additive manufacturing (AM), such as two-photon polymerization direct laser writing (TPP-DLW), have allowed for the fabrication of novel 3D architected lattice materials comprised of nano and microscale resolution, enabling researchers to investigate previously unexplored phenomena and property spaces. However, both the scalability and available materials working space of such additively manufactured nano- and micro-lattice structures remain crucial challenges. Presently, there exists a small selection of materials that can be reliably printed with sufficiently fine features and complex topologies. Such materials are mainly restricted to polymer-based systems, greatly narrowing the achievable functionality of these emerging lattice materials. ❧ Thus, advancements towards both developing a scalable solution to fabricate larger high-resolution lattice structures comprised of nanoscale feature, as well as alternative synthesis approaches, such as the subsequent deposition of coatings on the printed polymer structures, present remarkable areas of research. As such, this dissertation discusses both the development of coated micro-lattice materials via magnetron sputtering, as it offers an expansive materials workspace, and the scaling the scaling-up of nano-architected lattice structures fabricated via TPP-DLW via stitching methods. The studies described in this dissertation provide a foundation for expanding the synthesis space of 3D nano- and micro-architected lattice materials. Specifically, these works address two critical aspects within the emerging field of architected lattice materials, including: (1) the development of scaling methods and testing methodologies to investigate the effect of stitching on the integrity and mechanical behavior of TPP-DLW fabricated structures under tensile load and (2) the assessment of fundamental sputtering deposition parameters and influences for the generation of novel coated nano- and micro-lattice systems. Improvements in the fabrication and coating capabilities of such materials are crucial for the development and expansion of advanced materials with designed architectures.

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Asset Metadata

Creator Garcia Taormina, Alina Rochelle (author)

Core Title Expanding the synthesis space of 3D nano- and micro-architected lattice materials

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Materials Science

Degree Conferral Date 2021-12

Publication Date 12/17/2021

Defense Date 11/01/2021

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag architected materials,coatings,lattice materials,magnetron sputtering,OAI-PMH Harvest

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Hodge, Andrea (committee chair), Branicio, Paulo (committee member), Ravichandran, Jayakanth (committee member), Renuka Balakrishna, Ananya (committee member)

Creator Email agarciat@usc.edu,alina.taormina@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-oUC18652761

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