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Inorganic/organic hybrid materials and grafted coatings via vapor phase deposition processes

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Inorganic/organic hybrid materials and grafted coatings via vapor phase deposition processes

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Inorganic/Organic Hybrid Materials and
Grafted Coatings via Vapor Phase
Deposition Processes
by
Mark M. De Luna

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree of
DOCTOR OF PHILOSOPHY
in
CHEMICAL ENGINEERING
December 2019
2
Committee Members

Dr. Malancha Gupta (Chair)
Dr. Jayakanth Ravichandran
Dr. Aiichiro Nakano

3
Executive Summary

This dissertation details the fabrication of grafted coatings and inorganic/organic hybrid
materials deposited via initiated chemical vapor deposition (iCVD) and DC magnetron sputtering.
Chapter 1 introduces vapor phase deposition techniques, in particular iCVD and DC magnetron
sputtering. Background on the use of low vapor pressure liquids in both the iCVD and DC
magnetron sputtering processes is discussed. Lastly, the utility and current fabrication methods of
hybrid materials are introduced. Chapter 2 focuses on modification of the iCVD process by using
a photoinitiator to covalently attach functional polymer coatings onto Parylene C. The grafted
coatings are characterized using X-ray photoelectron spectroscopy and contact angle goniometry.
Chapter 3 studies the effects of viscosity and surface tension of gold and silver sputtered onto low
vapor pressure liquids. A morphological phase diagram is given to guide the selection of the
required liquid properties for a desired morphology. Chapter 4 demonstrates the facile fabrication
of inorganic/organic hybrid materials by combining DC magnetron sputtering and iCVD onto low
vapor pressure liquids. Using the various metal-liquid and polymer-liquid interactions, the
fabrication of several hybrid materials is demonstrated. Chapter 5 discusses the conclusions
derived from this work and how this work can impact and guide future hybrid reactor designs.
4
Acknowledgements
Many people have helped me throughout my academic journey, and this work would not
have been possible without the support I have received in all aspects of my life. I would like to
thank all those that have helped me throughout my academic career. Most importantly, I would
like to thank my advisor Dr. Malancha Gupta for giving me the opportunity to learn and grow in
her lab. This work would not have been possible without her input and direction as an advisor and
as an invaluable mentor. I would like to thank Dr. Jayakanth Ravichandran and Dr. Aiichiro
Nakano for serving on my dissertation committee. Thank you to the entire Gupta Lab for the help
and support all throughout this work. The Gupta Lab made this work possible through their
continual input and enjoyable company.
I would like to thank my mom, Patricia De Luna, and my dad, Mark A. De Luna, for their
unconditional love and support. I want to thank them for giving me all the resources I needed to
succeed and more. None of this would have been possible without them supporting me and guiding
me throughout my entire academic journey. I would like to thank my sister, Monique, for always
being there for me to talk and to hangout. I would like to thank the love of my life, Ashley, for her
continued support throughout my life, for encouraging me when I needed it most, and for making
our journey together an amazing one.
5
Table of Contents

List of Figures and Tables ............................................................................................................ 7
Chapter 1: Introduction ................................................................................................................ 9
1.1 Vapor Phase Deposition Processes ................................................................................................... 10
1.2 Initiated Chemical Vapor Deposition ................................................................................................ 11
1.2 Grafting via iCVD ............................................................................................................................. 12
1.3 iCVD onto Liquid Substrates ............................................................................................................ 13
1.4 Sputter Deposition onto Liquid Substrates ........................................................................................ 16
1.5 Inorganic/Organic Hybrid Materials ................................................................................................. 17
Chapter 2: Solventless Grafting of Functional Polymer Coatings onto Parylene C ............. 19
2.1 Abstract ............................................................................................................................................. 20
2.2 Introduction ....................................................................................................................................... 20
2.3 Experimental ..................................................................................................................................... 23
2.3 Results & Discussion ........................................................................................................................ 26
2.5 Conclusion ......................................................................................................................................... 33
2.6 Acknowledgements ........................................................................................................................... 34
Chapter 3: Effects of Surface Tension and Viscosity on Gold and Silver Sputtered onto
Liquid Substrates ......................................................................................................................... 35
3.1 Abstract ............................................................................................................................................. 36
3.2 Introduction ....................................................................................................................................... 36
3.3 Experimental ..................................................................................................................................... 39
3.4 Results & Discussion ........................................................................................................................ 40
3.5 Conclusion ......................................................................................................................................... 50
3.6 Acknowledgements ........................................................................................................................... 50
Chapter 4: Synthesis of Inorganic/Organic Hybrid Materials via Vapor Deposition onto
Liquid Surfaces ............................................................................................................................ 51
4.1 Abstract ............................................................................................................................................. 52
4.2 Introduction ....................................................................................................................................... 52
4.2 Experimental ..................................................................................................................................... 53
4.3 Decorated Polymer Nanoparticles ..................................................................................................... 57
4.4 Encapsulated Nanoparticle Dispersions ............................................................................................ 62
4.5 Gels with Embedded Nanoparticles .................................................................................................. 64
4.6 Conclusion ......................................................................................................................................... 66
6
4.7 Acknowledgments ............................................................................................................................. 67
Chapter 5: Conclusions & Future Work ................................................................................... 68
5.1 Conclusions ....................................................................................................................................... 69
5.2 Future Work ...................................................................................................................................... 71
References .................................................................................................................................... 74

7
List of Figures and Tables
FIGURE 1-1. EFFECT OF MONOMER SOLUBILITY ON THE RESULTING POLYMER MORPHOLOGY. 14

FIGURE 2-1. SCHEMATIC OF THE PROCESS TO DEPOSIT GRAFTED COATINGS. BENZOPHENONE (BP) IS USED AS
THE PHOTOINITIATOR TO CREATE RADICALS ON THE SURFACE OF PARYLENE C AND EGDA IS GRAFTED
FROM THESE SURFACE RADICALS. TBPO INITIATES POLYMERIZATION OF MONOMER (M) FROM THE
UNREACTED VINYL BONDS OF THE PEGDA SURFACE. 27

TABLE 2-1. XPS SURVEY SPECTRA OF THE GPEGDA SAMPLES (E1-E6) AND GPVP SAMPLES (V1-V6) VERSUS
THE ICVD REFERENCE FILMS.
A
SAMPLES WERE WASHED WITH METHANOL AND DI WATER. 28

TABLE 2-2. CONTACT ANGLE MEASUREMENTS FOR THE GPVP SAMPLES (V7-V12) BEFORE AND AFTER
DURABILITY TESTING.
A
SAMPLES WERE SONICATED IN METHANOL FOR FIVE 1 HOUR ROUNDS.
B
SAMPLES
WERE SOAKED IN 1X PBS SOLUTIONS AT 37 ºC FOR 30 DAYS. 31

FIGURE 2-2. REPRESENTATIVE STATIC SESSILE DROPLET AND CAPTIVE BUBBLE GONIOMETER IMAGES FOR
PARYLENE C, GPEGDA SAMPLES, AND GPVP SAMPLES. 31

TABLE 2-3. XPS SURVEY SPECTRA FOR THE GPVP SAMPLES (V1-V6) BEFORE AND AFTER DURABILITY TESTING.
A
WASHED WITH METHANOL AND DI WATER.
B
SONICATED IN METHANOL FOR FIVE 1 HOUR ROUNDS.
C
SOAKED IN 1X PBS SOLUTION AT 37 ºC FOR 30 DAYS. 32

FIGURE 2-3. STATIC SESSILE DROP AND CAPTIVE BUBBLE MEASUREMENTS OF GPONBMA SAMPLES BEFORE
AND AFTER 1 HOUR UV LIGHT EXPOSURE. 33

FIGURE 3-1. (A) CAMERA IMAGES, (B) MICROSCOPE IMAGES, (C) TEM IMAGES OF THE BULK, AND PARTICLE
SIZE COUNT (INSET) OF GOLD SPUTTERED ONTO 48 CP (LEFT COLUMN) AND 96 CP (RIGHT COLUMN)
SILICONE OILS. 42

FIGURE 3-2. (A) CAMERA IMAGES, (B) MICROSCOPE IMAGES, (C) TEM IMAGES OF THE SURFACE, (D) TEM
IMAGES OF THE BULK, AND PARTICLE SIZE COUNT (INSET) OF GOLD SPUTTERED ONTO 339 (LEFT COLUMN)
AND 485 CP (RIGHT COLUMN) SILICONE OILS. 44

FIGURE 3-3. (A) CAMERA IMAGES, (B) MICROSCOPE IMAGES, (C) TEM IMAGES OF THE SURFACE, AND (D) TEM
IMAGES OF THE BULK OF SPUTTERED GOLD ONTO SQUALENE (FIRST COLUMN), 400 MW PEG (SECOND
COLUMN), AND GLYCEROL (THIRD COLUMN). 46

FIGURE 3-4. (A) CAMERA IMAGES, (B) MICROSCOPE IMAGES, AND (C) TEM IMAGES OF THE BULK OF SPUTTERED
SILVER ONTO 48 CP (FIRST COLUMN), 96 CP (SECOND COLUMN), 339 CP (THIRD COLUMN) AND 485 CP
(FOURTH COLUMN) SILICONE OIL SYSTEMS. 48

FIGURE 3-5. (A) CAMERA IMAGES, (B) MICROSCOPE IMAGES, (C) TEM IMAGES OF THE SURFACE, AND (D) TEM
IMAGES OF THE BULK OF SPUTTERED SILVER ONTO SQUALENE (FIRST COLUMN), 400 MW PEG (SECOND
COLUMN), AND GLYCEROL (THIRD COLUMN). 49

8
FIGURE 3-6. MORPHOLOGICAL PHASE DIAGRAM OF THE RESULTING STRUCTURES. X’S REPRESENT THE DATA
POINTS IN THIS STUDY. 50

FIGURE 4-1. (A) SCHEMATIC REPRESENTATION OF THE FABRICATION PROCESS TO DECORATE POLYMER
NANOPARTICLES WITH METAL NANOPARTICLES, (B) A TEM MICROGRAPH OF A PHEMA NANOPARTICLE
DECORATED WITH GOLD NANOPARTICLES, AND (C) A TEM MICROGRAPH OF A P4VP NANOPARTICLE
DECORATED WITH GOLD NANOPARTICLES. 59

FIGURE 4-2. (A) SCHEMATIC REPRESENTATION OF THE FABRICATION PROCESS TO DECORATE POLYMER
NANOPARTICLES WITH METAL NANOPARTICLES VIA CONDENSING, (B) A TEM MICROGRAPH OF P4VP
NANOPARTICLES DECORATED WITH GOLD NANOPARTICLES, AND (C) A MAGNIFIED VIEW OF A
REPRESENTATIVE NANOPARTICLE. 62

FIGURE 4-3. (A) SCHEMATIC DIAGRAM OF THE PROCESS TO ENCAPSULATE DISPERSED GOLD NANOPARTICLES
WITHIN A POLYMER SHELL AND (B) THE RESULTING ENCAPSULATED MARBLE. (C) SCHEMATIC DIAGRAM
OF THE PROCESS WITH THE STEPS REVERSED AND (D) THE RESULTING ENCAPSULATED MARBLE. (E) TEM
MICROGRAPH OF THE EXTRACTED LIQUID FROM THE COATED MARBLE IN 3B AND THE LIQUID BLOTTED ON
CHROMATOGRAPHY PAPER (INSET). (F) TEM MICROGRAPH OF THE EXTRACTED LIQUID FROM THE COATED
MARBLE IN 3D AND THE LIQUID BLOTTED ON CHROMATOGRAPHY PAPER (INSET). 64

FIGURE 4-4. (A) SCHEMATIC OF THE PROCESS TO EMBED GOLD NANOPARTICLES WITHIN GEL BEADS, (B)
STEREOSCOPE (LEFT) AND CONTACT ANGLE (RIGHT) IMAGES OF THE RESULTING GEL BEAD, AND (C)
STEREOSCOPE (LEFT) AND CONTACT ANGLE (RIGHT) IMAGES OF THE GEL BEAD THAT IS FORMED WHEN
THE GOLD IS SPUTTERED AFTER THE FORMATION OF THE GEL BEAD. 66

FIGURE 5-1. ARGON PLASMA GENERATED WITHIN AN ICVD REACTOR. THE PLASMA IS GENERATED BY SENDING
A 50 W, 13.56 MHZ RF SIGNAL THROUGH THE FILAMENT ARRAY USING AN EXTERNAL RF POWER SOURCE
AND MATCHING NETWORK. 72

FIGURE 5-2. PROPOSED HYBRID REACTOR SET UP. FILAMENT 1 IS CONNECTED TO AN EXTERNAL DC POWER
SUPPLY AND RESISTIVELY HEATED TO ENABLE ICVD. FILAMENT 2 IS CONNECTED TO AN EXTERNAL RF
POWER SUPPLY AND MATCHING NETWORK TO GENERATE AND CONTROL THE PLASMA. THE PLASMA IS
LOCALIZED NEAR THE METAL TARGET USING A MAGNETRON TO PERFORM RF MAGNETRON SPUTTERING.
73

9

Chapter 1: Introduction

10
1.1 Vapor Phase Deposition Processes
There are many vapor phase deposition techniques that exist, and many are widely used in
both academia and industry. Vapor phase deposition processes have garnered much attention due
to their solventless nature and versatility. Organic coatings can be deposited using a group of
methods that fall under the category of chemical vapor deposition (CVD).
1,2
While a complete
review of all CVD processes is outside the scope of this dissertation, CVD of polymers is important
to note.
CVD can be used to deposit a wide variety of functional polymers. In the context of this
work, CVD is a vacuum process in which a monomer species is vaporized and subsequently
polymerized by an initiating species into a functional polymer coating. CVD processes can differ
in the way the polymerization process is initiated and the types of polymer coatings they can
deposit. For example, initiated chemical vapor deposition (iCVD) is initiated by the thermal
initiator di-tert butyl peroxide (TBPO) which is cleaved by a heated filament array into free-
radicals to initiate free-radical polymerization from a monomer with a vinyl bond.
3–5
In plasma
enhanced chemical vapor deposition (PECVD), a plasma is generated between two electrodes
using either a RF or DC power source which ionizes an inert or reactive gas to initiate
polymerization on a reactive surface.
1
In oxidative chemical vapor deposition (oCVD), an
monomer species is oxidized to yield a monomeric cation which can dimerize with another
monomer leading to polymerization of the monomer species.
5
In particular oCVD can deposit
conductive coatings, such as poly(3,4-ethylenedioxythiophene) (PEDOT), which can be used for
applications in electronics.
5
While each CVD process has its utility, iCVD is the focus of this work
because of its ability to deposit biocompatible, hydrophilic, hydrophobic, or photo-responsive
coatings on a wide variety of substrates.
1,3,6–8

11
1.2 Initiated Chemical Vapor Deposition
The initiated chemical vapor deposition (iCVD) process is a solventless, free-radical
polymerization process that is used to deposit a wide array of functional polymer thin films such
as the hydrophobic poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA),
7
hydrophilic poly(4-
vinylpyridine) (P4VP),
9
and the biocompatible poly(2-hydroxyethyl methacrylate) (PHEMA).
8

The solventless nature and mild substrate temperatures of the process allows for the coating of a
wide array of substrates such as electrospun fibers,
10
micropillar arrays,
11
and microfluidic
channels.
12
In a typical iCVD reaction, both monomer and initiator are flown into the cylindrical
reactor simultaneously, where the initiator is thermally cleaved into free-radicals by a heated
filament array (200-260 ºC) to initiate free-radical polymerization.
4,13
The monomer and the
radicals are adsorbed onto the substrate where polymerization continues and the thin film grows
in thickness until the reaction is terminated. The typical reactor pressures are between 20-1000
mTorr, with the operating pressure depending on the partial pressure (Pm) and saturation pressure
(Psat) of the monomer. The saturation pressure is important because if the reactor conditions result
in a Pm/Psat > 1, then the monomer will condense leading to polymer droplet formation.
14
The
monomer saturation pressure is determined by the Clausius-Clapeyron equation, given by:
𝑃
"#$
= 𝐴∗exp (−
∆𝐻
𝑅𝑇
)
Where A is the preexponential factor, ∆𝐻 is the enthalpy of vaporization, 𝑅 is the universal gas
constant and 𝑇 is the stage temperature. Typical stage temperature values range from 10 – 60 ºC,
which allows the iCVD process to coat a wide variety of substrates, such as plastics and sensitive
electronics.
5,15
While the initial experiments had included the deposition of acrylates
13
, the iCVD
process has been expanded to include other monomers containing vinyl bonds.
16
One drawback of
the iCVD process is that the coatings are only physisorbed to the underlying substrate through
12
weak chemical interactions. As a result, efforts have been made to chemically alter the surface of
the substrate to use iCVD to covalently attach functional polymer coatings to the underlying
substrate.
17–21

1.2 Grafting via iCVD
Since the iCVD process has shown promise for many long-term applications, such was
water filtration
10
and microfluidic synthesis
12
, much work has been done to improve the durability
of coatings deposited via iCVD. In order to accomplish grafted polymer coatings, a radical must
be present on the surface of the substrate. Solution phase techniques
22
have involved irradiating a
type-II photoinitiator, benzophenone, with ultraviolet light to create a high energy di-radical which
abstracts a hydrogen atom from the surface.
23
Martin et al. used benzophenone in the iCVD process
to create covalently attach an antimicrobial coating, poly(2-diethylamino) ethyl acrylate, onto
Nylon fibers.
17
In the grafting initiated chemical vapor deposition (gCVD) process, benzophenone
was heated to 121 ºC, flown into the reactor in the vapor phase, and deposited onto the substrate.
After ultraviolet light irradiation, a radical was formed on the surface of the substrate, which
allowed for free-radical polymerization without the standard thermal initiator used in the iCVD
process. Martin et al. also showed that the coatings remained intact after multiple sonication rounds
in methanol, which evidenced that the coating was grafted to the Nylon fibers. Sedransk et al.
showed that the gCVD process can be used to deposited the biocompatible polymer, poly(vinyl
pyrrolidone), for a more durable scleral lens coating.
24
Their coatings achieved the desired
thicknesses initially, but the durability and functionality was shown to be a function of initiator
dosing time, ultraviolet light irradiation, and monomer dosing time. Other more generalizable
grafting methods have been developed. For example, Ye et al. used a hybrid grafting technique by
depositing the copolymer, poly(dimethylaminomethyl styrene-co-ethylene glycol diacrylate), and
13
then ceasing the flow of the cross-linker without breaking vacuum to have a
poly(dimethylaminomethyl styrene) top layer.
19
However, the hybrid grafting technique does not
covalently attached the cross-linked layer to the underlying substrate. Sojoudi et al. showed that
high energy methyl radicals can be generated in the iCVD chamber by using a filament temperature
of 310 ºC and these radicals can abstract a hydrogen atom from a pretreated surface.
20
However,
this process was limited to pretreated surfaces with hydroxyl moieties. While all of these works
have successfully grafted functional polymer coatings onto various substrates, some of the
generalizable studies acknowledged the limited grafting density that was achieved, and limited
functionality that was realized in the final polymer coating.
17,22,24

1.3 iCVD onto Liquid Substrates
Our group was the first to demonstrate that low vapor pressure liquids can be used as
substrates in the iCVD process. Haller et al. studied the deposition of poly(1H,1H,2H,2H-
perfluorodecyl acrylate) (PPFDA) and poly(2-hydroxyethyl methacrylate) (PHEMA) onto a
droplet of the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate
([bmim][PF6]).
25
In the study, it was shown that polymerization could either occur at the vapor-
liquid interface or within the bulk of the liquid. PFDA monomer is not soluble in [bmim][PF6] and
therefore polymerization occurred at the vapor-liquid interface leading to the formation of a film
whereas HEMA monomer is soluble within [bmim][PF6] and therefore polymerization occurred
within the bulk IL at all reaction conditions tested. In subsequent studies, the importance of the
monomer solubility on the resulting polymer because if a monomer is not soluble in the liquid then
polymerization at the vapor-liquid interface will occur and can result in smooth films, particles, or
microstructured films (Figure 1-1). Polymerization within the bulk will occur if the monomer is
soluble within the liquid, resulting in gels or heterogenous films (Figure 1-1). Solubility is not the
14
only important parameter in determining the resulting morphology, but rather the surface tensions
of the polymer and liquid are also important.

Figure 1-1. Effect of monomer solubility on the resulting polymer morphology.

Haller et al. investigated the effect of surface tension by studying the deposition of various
polymers onto liquids with varying surface tensions, and the data showed that the spreading
coefficient (𝑆) dictates the final polymer morphology at the vapor-liquid interface.
26
The spreading
coefficient of low surface energy materials is defined as:
𝑆 = 𝛾
56
∗(1+𝑐𝑜𝑠𝜃
=
)−2𝛾
?6

where 𝛾
56
is the surface tension of the liquid, 𝜃
=
is the advancing contact angle of the liquid on
the polymer, and 𝛾
?6
is the surface tension of the polymer.
27
If a polymer-liquid system has a
positive spreading coefficient, then it is energetically favorable for the polymer to spread over the
liquid surface which results in a polymer film. If a polymer-liquid system has a negative spreading
coefficient, then it is energetically favorable for the polymer to aggregate over the liquid surface
which results in polymer particles. In addition to surface tension and solubility, the liquid viscosity
also plays a critical role in determining the polymer morphology because the diffusivity of
polymer chains on the liquid surface is inversely proportional to the liquid viscosity.
28–30

15
Linear polymers that are insoluble in the liquid substrate have a negative spreading
coefficient form polymer particles.
26,31,32
These polymer particles can either remain at the vapor-
liquid interface or submerge into the bulk liquid which is determined by the Gibbs free-energy of
particle detachment from the vapor-liquid interface (∆𝐺):
∆𝐺 = −𝜋𝑟
C
𝛾
56
(1−𝑐𝑜𝑠𝜃)
C

where 𝑟 is the radius of the particle, 𝛾
56
is the liquid-vapor surface tension, and 𝜃 is the equilibrium
contact angle of the liquid on the polymer.
33
If the contact angle of the liquid on the polymer is
nonzero then the polymer nanoparticle will remain at the vapor-liquid interface whereas if the
contact angle is zero (complete wetting) then the free energy required for particle detachment from
the vapor-liquid interface is negligible and the polymer particle will submerge into the bulk.
31

Frank-Finney et al. showed that polymer particles that remain at the vapor-liquid interface
undergo a two-stage growth mechanism where the particles nucleate, and newly deposited chains
aggregate to the existing particles, allowing the particles to grow.
32
Polymer particles that remain
at the vapor-liquid interface allow for more size control through deposition time and rate because
deposited polymer chains aggregate with existing polymer particles. In addition, since the liquid
substrate viscosity is inversely proportional to polymer chain diffusion and aggregation, the
viscosity of the liquid has an effect on the resulting polymer particle size.
In contrast, phenomena at the vapor-liquid interface has less of an impact on submerging
polymer particle systems. Poly(4-vinylpyridine) (P4VP) has a negative spreading coefficient
(polymer particle formation) with silicone oil, and silicone oil has a 0º contact angle on each
polymer film (particles submerge). Haller et al. showed that polymer particles formed and
submerged into the bulk liquid using deposition conditions where the monomer partial pressure
was less than the saturation pressure (Pm/Psat < 1) by depositing P4VP onto uncured
16
polydimethylsiloxane (PDMS), curing the PDMS, and taking a cross-sectional SEM.
31
Karandikar
and Gupta showed that polymer particles with a larger average diameter can be formed via
condensation of monomer during deposition on liquids.
14
By increasing the monomer partial
pressure above its saturation pressure (Pm/Psat > 1), monomer droplets condense on the liquid
surface and submerge within the bulk liquid where the droplets undergo coalescence and are
subsequently polymerized. The coalescence results in a heterogenous polymer particle size
distribution and a larger average particle size as compared to polymer depositions at Pm/Psat < 1.
Coalescence could be inhibited by increasing the viscosity of the silicone oil substrate thereby
reducing average particle size and the polydispersity index.
Polymer-liquid systems with a positive spreading coefficient will form smooth dense
films.
25,26,34,35
In particular, PPFDA is a low surface energy polymer (13.6 mN/m) that will form a
film on all low vapor pressure liquids tested in previous studies, from low surface tension silicone
oil (22.1 mN/m) to high surface tension glycerol (63.4 mN/m).
26
Bradley and Gupta showed that
the a cross-linked PPFDA film can be used to encapsulated ionic liquids using the concept of liquid
marbles.
36
Additionally, in a separate study, Bradley and Gupta showed that PPFDA can be used
as a capping polymer in the formation of microstructured films.
37
Other interesting morphologies
arise from using iCVD to deposit polymers onto low vapor pressure liquids if the monomer is
soluble in the liquid, such as gels
38,39
and heterogenous films.
40
These works of iCVD onto liquids
directly influenced our interest in studying the effects of liquid viscosity and surface tension on
high surface energy materials, such as metals deposited via sputtering.
1.4 Sputter Deposition onto Liquid Substrates
Sputtering has the ability to deposit a wide range of materials efficiently and effectively.
41

In particular, DC magnetron sputtering is advantageous because of its ability to confine the target
17
atoms to the substrate and allow for efficient deposition without degrading the substrate.
42
Previous
works have studied magnetron sputtering on both solid and liquid surfaces, with the latter being
of greater interest.
43
For example, Borra et al. sputtered silver onto 1-ethyl-3-methylimidazolium
ethylsulphate in order to fabricate a high reflectivity surface and found that sputtering silver
directly onto the IL surface led to the formation of a film consisting of colloidal particles with
diameters in the tens of nanometers.
44
However, sputtering a thin layer of chromium followed by
silver led to a smooth film because of the higher nucleation density of chromium. Torimoto and
coworkers found that sputtering gold onto IL surfaces resulted in the formation of dispersed gold
nanoparticles due the electrostatic interaction between the IL and the gold.
45
The nanoparticle size
was found to be dependent on the type of IL used, while the nanoparticle concentration was
dependent on the duration of the sputtering process. Other types of low vapor pressure liquids,
such as silicone oils and castor oil, have also been used as a substrate in inorganic vapor deposition
processes where both thin films and nanoparticles are formed.
46–48
For example, Xie et al.
described a two-stage growth model for the deposition of gold and silver onto silicone oil surfaces
with the first stage involving nucleation and growth of molecules and the second involving
diffusion along the liquid surface.
48
Wender et al. sputtered gold onto biocompatible castor oil and
found that they could increase the size of the resulting nanoparticles by increasing the sputtering
voltage.
49
However, beyond the influence of electrostatic interactions, little work has been done to
understand the effects of the viscosity and surface tension of the liquid substrate. In this work, we
show the effects of the liquid viscosity and surface tension on the resulting morphology.
50

1.5 Inorganic/Organic Hybrid Materials
The development of new synthetic routes for the production of hybrid materials composed
of inorganic and organic components is important for the fabrication of materials with enhanced
18
properties. Hybrid materials have been shown to have a variety of applications in optics,
51

sensors,
52
electronics,
53,54
and catalysis.
55
For example, noble metal decorated polymer
nanoparticles have shown great promise because of their catalytic ability,
56
biocompatibility,
57
and
high surface area-to-volume ratio.
58
Polymer-metal hybrid microcapsules are useful for
applications in drug delivery because of the ability to direct the capsules contents to the desired
location.
59,60
Additionally, hybrid gels are useful for because of their electronic properties and self-
healing capabilities
61
or optical temperature sensing capabilities.
62
However, the synthesis
methods to create these polymer-metal hybrid materials often suffer from long reaction times, use
dangerous precursors, and create environmentally damaging by-products. Despite the fabrication
bottlenecks of these methods, the robustness and applicability of polymer-metal hybrid materials
accentuates the need for an efficient, environmentally friendly, and generalizable fabrication
method.

19

Chapter 2: Solventless Grafting of Functional Polymer
Coatings onto Parylene C

M. M. De Luna, B. Chen, L. C. Bradley, R. Bhandia, M. Gupta. “Solventless grafting of functional
polymer coatings onto Parylene C.” Journal of Vacuum Science & Technology A, 2016, 34,
041403.

20
2.1 Abstract
In this work, we studied the use of vapor phase deposition to covalently attach functional
polymer coatings onto Parylene C. Parylene C is important for several biomedical applications due
to its inertness and biocompatibility, however the surface properties are not ideal. We modified
the surface properties of Parylene C using a stepwise procedure in which a photoinitiator was first
used to covalently attach a cross-linked anchoring layer to the Parylene C surface and then a
thermal initiator was used to polymerize functional monomers onto the cross-linked anchoring
layer. This process has several benefits because no solvents are used during the polymerization
process. The generality of this procedure was demonstrated by depositing poly(vinyl pyrrolidone)
and poly(ortho-nitrobenzyl methacryate) as the functional polymers. Durability testing showed
no loss in functionality or change in the elemental composition of the coating after sonication in
methanol or long-term soaking in PBS solution. This process can be used to covalently attach a
range of functionalities to Parylene C for potential use in biomedical applications.
2.2 Introduction
The integration of Parylene C into biomedical devices has been a subject of great interest
due its biocompatibility,
63
dielectric properties,
64
solvent resistance,
65
and chemical stability.
64,66,67

These attractive properties have led to the wide use of Parylene C coatings on a variety of
biomedical devices such as shunts,
68
metal stents,
69
and implantable neural prosthesis.
70,71
While
Parylene C exhibits desirable properties, its hydrophobicity promotes the adsorption of cells and
proteins, limiting its anti-fouling ability.
17,24,63,72
For this reason, multiple studies have investigated
the surface modification of Parylene C to improve its hydrophillicity.
22,72

There are many ways to modify the surfaces of polymer substrates with polymer coatings.
For example, spincoating has been used to apply poly(4-methyl-1-pentane) onto microporous
21
polysulfone,
73
poly(methyl methacrylate) has been dropcasted onto poly(dimethylsiloxane)
micropillers,
11
and poly(2-vinyl pyridine) has been polymerized in solution phase on a silicon
wafer and then transferred to polyester membranes.
74
These techniques create coatings that are
physisorbed to their polymer substrate, which can lead to leeching or degradation.
63
For practical
applications, it is necessary to covalently attach polymer coatings to Parylene C. For example,
solution phase grafting was used to covalently attach poly(2-methacryloyloxethyl
phosphorylcholine) to Parylene C to provide enhanced lubrication to an implantable biomedical
device,
72
and solution phase photo-grafting was used to covalently attached poly(acrylamide) to
Parylene C to provide improved hydrophilicity.
22

Since solution-phase coating techniques have inherent limitations related to surface tension
effects, solvent compatibility issues, and solvent leeching, the development of dry methods to
covalently attach coatings is necessary. The use of initiated chemical vapor deposition (iCVD) is
promising because it is an all-dry, vacuum process that could be used to deposit conformal polymer
coatings with a wide range of functionalities. For example, iCVD has been used to conformally
apply ultrathin zwitterionic coatings to polyamide membranes to prevent biofouling
10
and has been
used to apply poly(1-H,1H,2H,2H-perfluorodecyl acrylate-co-ethylene glycol diacrylate) onto
PDMS microfluidic channels to control droplets.
12
In the iCVD process, a thermal initiator and a
monomer are introduced into a vacuum chamber and the initiator is broken into free radicals by a
heated filament array. These radicals, along with the monomer molecules, are adsorbed to the
surface of the substrate and polymerization occurs via a free-radical mechanism. The iCVD
coatings are physisorbed to their substrates. A modified version of iCVD called photoinitiated
chemical vapor deposition (piCVD) has been used to covalently attach polymers to other polymer
substrates. For example, a previous study by Martin et. al. showed that (dimethalamino)methyl
22
styrene (DMAMAS) and (diethylamino)ethyl acrylate (DEAEA) could be grafted onto nylon
fabric allowing the nylon to retain its conformality while adding antimicrobial functionality.
17
In
the piCVD method, a photoinitiator such as benzophenone is used instead of a thermal initiator.
17,24

Ultraviolet (UV) light causes benzophenone to create a diradical that abstracts a hydrogen atom
from the surface of the polymer substrate.
22,23,75
The resulting radical on the surface then initiates
free radical polymerization of the monomer.
17,24,75
Although piCVD can achieve grafting, the
grafting densities can be low and the exposure to benzophenone and UV radiation throughout the
polymerization process can result in undesirable side-reactions or the destruction of sensitive
functionalities which ultimately limits the chemistries that can be deposited using piCVD.
17,24

In order to modify the surface of Parylene C with robust functional coatings that have high
surface coverage, we developed a procedure that involves a combination of piCVD and
conventional iCVD. The coatings are composed of a grafted cross-linked anchoring bottom layer
and a functional top layer. The cross-linked anchoring layer is made using benzophenone as the
photoinitiator in order to covalently attach the coatings to the surface of Parylene C, whereas the
functional top coating is made using a less reactive thermal initiator, tert-butyl peroxide, to
preserve surface functionality. We demonstrate the versatility of our method by depositing
coatings with top layers consisting of a biocompatible polymer poly(vinyl pyrrolidone) (PVP) and
a photo-responsive polymer poly(ortho-nitrobenzyl methacrylate) (PoNBMA). PVP was chosen
because it is hydrophilic, biocompatible,
24,76,77
and the amide functionality provides a nitrogen
environment to easily differentiate it from other components during X-ray photoelectron
spectroscopy (XPS) analysis. PVP is also of interest for biomedical applications because of its
antimicrobial and anti-fouling properties,
24,76,78
as well as its potential as a drug delivery agent.
79

PoNBMA was chosen for its potential as a photo-responsive coating.
80
We show that both top
23
layers retain their functionalities with PVP remaining hydrophilic and PoNBMA retaining its
photo-responsiveness by switching from a hydrophobic to a hydrophilic state after UV light
exposure. Although we demonstrate our procedure on Parylene C surfaces, our method can easily
be extended to any surface containing labile hydrogen atoms that can be abstracted by
benzophenone radicals. Although we demonstrate the grafting of two functional polymers, this
process can be extended to graft other functional polymers that can deposited via iCVD, including
thermo-responsive poly(N-isopropyl acrylamide),
3
hydrophilic poly(2-hydroxyethyl
methacrylate),
8
and hydrophobic poly(1H,1H,2H,2H-perfluorodecyl acrylate).
7

2.3 Experimental
Benzophenone (BP) (99%, Sigma-Aldrich), di-tert-butyl peroxide (TBPO) (98%, Sigma-
Aldrich), 1-vinyl-2-pyrrolidone (VP) (97%, Sigma-Aldrich), ethylene glycol diacrylate (EGDA)
(90%, Sigma-Aldrich), ortho-nitrobenzyl methacrylate (oNBMA) (95%, Polysciences, Inc.) and
methanol (Macron, absolute) were used without further purification. The coatings were deposited
in a vacuum chamber (GVD Corporation, 250 mm diameter, 48 mm height) equipped with a
nichrome filament array (80% Ni, 20% Cr, Omega Engineering). Reactor pressure was maintained
using a throttle valve (MKS 153D) and measured using a capacitance manometer (MKS Baratron
622A01TDE). BP was heated to 70 ºC, EGDA was heated to 35 ºC, and VP was heated to 60 ºC.
The substrate temperature was controlled using a 4 cm x 4 cm x 0.47 cm thermoelectric cooler
(TEC) (Custom Thermoelectric). The stage temperature was kept constant at 42 ºC throughout the
deposition using a recirculating chiller to prevent deposition onto the rest of the stage. Ultraviolet
light irradiation was accomplished using a 250 MW Hg-lamp (UV-technik). The lamp was placed
6 cm above the samples to provide a light intensity of 70 mW cm
-2
at 254 nm through the quartz
glass as measured by an optical power meter (PM100A, Thorlabs). The coatings were deposited
24
onto reference silicon wafers and silicon wafers with a layer of 5 µm thick Parylene C. The
Parylene C was deposited using the Gorham process in a Parylene deposition chamber (PDS 2010,
Specialty Coating Systems, Inc.). The dimer, di-chloro-di-para-xylylene (Specialty Coating
Systems, Inc.), was vaporized at 135 ºC and 35 mTorr and pyrolyzed into precursors at 690 ºC.
The precursors then passed into the deposition chamber at room temperature where polymerization
occurred on the silicon wafer substrate to form a film of poly(monochloro-p-xylylene). The total
deposition time was 2 hours to achieve a thickness of 5 µm. Fourier Transform Infrared (FTIR)
Spectroscopy (Thermo-Nicolet iS10) was conducted on Parylene C samples before and after
exposure to 20 minutes of UV light within the reactor under vacuum. The FTIR demonstrated that
no significant chemical changes occurred to the Parylene C.
To create the cross-linked anchoring layer, benzophenone was first flown in at a reactor
base pressure of 30 mTorr and a substrate temperature of 40 ºC. After 30 min, the flow of
benzophenone was ceased and UV light irradiation was started. After a UV pretreatment time of
2 minutes, the cross-linker EGDA was flown in at 0.2 sccm, the TEC temperature was lowered
to 32 ºC, and the reactor pressure was set to 60 mTorr. After 8 minutes, the functional top layer
was created by turning off the UV light and flowing the monomer (VP or oNBMA) along with
the thermal initiator TPBO flow at 0.25 sccm and 0.5 sccm, respectively. The reactor pressure
was set to 120 mTorr and the filament array was heated to 240 ºC. EGDA flow was gradually
decreased over 2 minutes to prevent pressure deviations. The reaction then proceeded for an
additional 18 minutes with the desired monomer (VP or oNBMA) flow and TPBO flow. To
fabricate the grafted PVP coatings with no cross-linked anchoring layer (gPVP-noXL), the BP
deposition and UV pretreatment time were conducted as described above. After the UV
pretreatment time, VP was flown in at a rate of 0.25 sccm at a reactor pressure of 60 mTorr with
25
the UV light on for 30 minutes. A quartz crystal microbalance with a 6 MHz gold-plated crystal
was used to estimate the adsorption of BP onto the Parylene C. BP was flown into the reactor
and the measurements were performed at the reactor base pressure (30 mTorr) and the
temperature of the QCM was kept at 40 ºC. BP flow was ceased after 60 minutes.
To test the chemical composition and functionality of the cross-linked anchoring layer, the
cross-linked anchoring layer was deposited onto 4 samples per deposition over 3 depositions for a
total of 12 samples (E1-E12). These samples were then removed from the reactor and analyzed.
To test the chemical composition and functionality of the grafted-PVP samples, the entire process
(the cross-linked anchoring layer plus the functional top layer) was deposited onto 4 samples per
deposition over 3 depositions for a total of 12 samples (V1-V12). Samples E1-E6 and V1-V6 were
used for XPS analysis and samples E7-E12 and V7-V12 were used for contact angle goniometry.
Three samples from each set (E1-3, V1-3, E7-9, and V7-9) were used for methanol sonication.
Three samples from each set (E4-6, V4-6, E10-12, and V10-12) were used for the 1X PBS soak
test. For the oNBMA samples, 2 samples were prepared per deposition over 2 depositions (O1-
O4). Samples O1 and O2 were used for XPS analysis and samples O3 and O4 were used for contact
angle goniometry. All samples were washed with methanol and deionized (DI) water for 30
seconds each and dried with compressed air. These washed coatings were allowed to dry for at
least 1 hour under vacuum before analysis was performed.
The chemical composition of the samples was analyzed using X-ray photoelectron
spectroscopy (Kratos Axis Ultra DLD) with a monochromatic Al Ka X-ray source. Survey spectra
were taken from 0 to 900 eV with a step size of 1 eV a total of five times. Each sample (E1-6 and
V1-6) was scanned in two different spots. Static, advancing, and receding contact angles were
measured in triplicate using a goniometer (Ramé-Hart Model 290-FI) by depositing a 7 µL droplet
26
of DI water onto different locations on each sample (E7-12 and V7-12). Advancing and receding
contact angles were measured using the tilting-base method where the recorded angles where those
right before the droplet rolled off or at 90º tilt. Captive bubble was performed using a DI water
bath in a quartz glass container (ramé-hart) by dispensing a 7 µL air bubble using an automated
dispenser. Each sample was measured in three different positions. For durability testing, samples
E1-3, V1-3, E7-9, and V7-9 under went five 1 hour rounds of sonication in methanol and samples
E4-6, V4-6, E10-12, and V10-12 were incubated in 1X phosphate buffered saline (PBS) solution
at 37 ºC for 30 days. After each test, the samples were removed, rinsed with DI water, and dried
under vacuum for at least 1 hour before analysis. Averages and standard deviations are reported
from each sample set.
2.3 Results & Discussion
In order to obtain a high coverage of functional polymer on Parylene C using vapor phase
deposition, we developed a stepwise process to create a layered polymer coating consisting of a
photoinitiated cross-linked anchoring bottom layer and a functional top layer with the desired
surface chemistry (Figure 2-1). The cross-linked anchoring layer of grafted-PEGDA (gPEGDA)
was formed by first adsorbing the photoinitiator onto the Parylene C surface. Benzophenone was
chosen because it is a type-II photoinitiator that has previously been used in the vapor deposition
process to successfully graft functional polymers onto substrates containing labile hydrogen
atoms.
17,24
To obtain high surface coverage of our coatings, we had to ensure that enough BP
saturated the surface. Martin et al. previously showed that a longer BP deposition time leads to a
nonlinear increase in the thickness of the grafted polymer layer at a given substrate
temperature.
12,13
We used a quartz crystal microbalance (QCM) to estimate that 30 min is required
for benzophenone to saturate the surface at our TEC temperature (40 ºC). After 30 minutes of
27
adsorption, the BP flow was ceased and UV light was used to create benzophenone diradicals that
abstract hydrogen from the Parylene C surface. The benzophenone and UV light were introduced
sequentially to prevent the formation of excess BP radicals which can lead to the termination of
the radicals formed on the Parylene C surface.
75

Figure 2-1. Schematic of the process to deposit grafted coatings. Benzophenone (BP) is used as
the photoinitiator to create radicals on the surface of Parylene C and EGDA is grafted from these
surface radicals. TBPO initiates polymerization of monomer (M) from the unreacted vinyl bonds
of the PEGDA surface.

After the UV pretreatment time, the cross-linker ethylene glycol diacrylate (EGDA) was
flown into the reactor in the vapor phase in the presence of the UV light to initiate polymerization
off the radicals on the surface of the Parylene C (Figure 2-1). Using a cross-linker as the anchoring
layer provides the ability to obtain high grafted coverage of functional polymers onto the Parylene
C surface because the EGDA molecule has two vinyl groups, thereby increasing the ability to bind
to the radicals on the surface of the Parylene C and also bind to other EGDA molecules that are
28
already grafted to the surface. The monomer partial pressure was kept relatively low (60 mTorr)
because previous work has shown that a high monomer concentration at the surface promotes
homopolymerization of ungrafted polymer, diminishing the attachment to the surface.
17,24
In order
to verify that the gPEGDA was grafted onto Parylene C, we conducted XPS and contact angle
goniometry analysis on samples E1-E12.

Our XPS analysis of the top 5 nm of the surface of the
sample after a methanol and DI water wash matches that of a PEGDA reference film indicating
successful attachment of gPEGDA onto Parylene C (Table 2-1). Contact angles were also
measured to verify the successful attachment of gPEGDA by the increase in the hydrophilicity of
the sample. Uncoated Parylene C has a contact angle of approximately 90º,
22
whereas the contact
angle on the sample was 55º ± 2º which is similar to the contact angle on the PEGDA reference
film (53º ± 2º). This evidence, coupled with the XPS survey scan data, shows that our gPEGDA
coating completely covers the Parylene C substrate.
Table 2-1. XPS survey spectra of the gPEGDA samples (E1-E6) and gPVP samples (V1-V6)
versus the iCVD reference films.
a
Samples were washed with methanol and DI water.

Atomic Compositions

Sample %C %O %N %Cl
Parylene C
reference
90 ± 1 0 ± 0 0 ± 0 10 ± 1
PEGDA reference 70 ± 1 30 ± 1 0 ± 0 0 ± 0
gPEGDA
a
69 ± 3 29 ± 4 0 ± 0 0 ± 0
PVP reference 75 ± 1 15 ± 1 10 ± 1 0 ± 0
gPVP-noXL
a
86 ± 2 6 ± 1 2 ± 1 6 ± 2
gPVP
a
75 ± 1 16 ± 1 9 ± 1 0 ± 0

29
To fabricate samples with the functional top layer (V1-V12), the gPEGDA layer was
deposited as described above. After the deposition of the gPEGDA layer, the initiator and
monomer were introduced into the chamber immediately, the reactor pressure was set, and the
filament was turned on to deposit the grafted-poly(vinyl pyrrolidone) (gPVP) top layer using
conventional iCVD (Figure 2-1). The EGDA flow was gradually turned off while maintaining the
reactor pressure which allows for continuity from the piCVD process to the conventional iCVD
process. TBPO radicals were used to attach the gPVP layer to the gPEGDA layer instead of BP
radicals because TBPO radicals are not reactive enough to abstract hydrogen and therefore there
are no side reactions during the iCVD process and the functionality of the monomer is kept intact.
TBPO radicals initiate polymerization from the unreacted vinyl groups of the gPEGDA layer,
leading to the formation of the gPVP layer. After the EGDA flow was ceased, the deposition
continued with only VP flow to ensure maximum coverage. Since this top layer was deposited
using conventional iCVD, coatings grew significantly thicker than the gPEGDA layer but only a
small portion of the thickness was grafted, while the rest was washed away. XPS analysis shows
that the elemental composition of the gPVP layer after a methanol and DI water wash matches that
of our iCVD PVP reference coatings (Table 2-1), demonstrating the successful attachment of the
gPVP layer. In order to demonstrate the necessity of the cross-linked anchoring layer, we deposited
gPVP via piCVD with no cross-linker layer (gPVP-no XL). In this case, Parylene C is still exposed
because of the finite grafting sites available.
17,22,24,75
The XPS survey scan of gPVP-noXL shows
both chlorine (6%) and nitrogen (2%) are present, indicating that PVP is present with low graft
density emphasizing the importance of the cross-linked anchoring layer.
Contact angle hysteresis was used to obtain a true measure of hydrophilicity because it has
been previously shown that grafted polymer systems tend to rearrange, depending on their dry or
30
wet conditions
81
and the functionality of the polymer.
82,83
The static contact angles of 41º ± 5º of
our gPVP coatings do not match PVP reference static contact angles of 11º ± 1º (Table 2-2).
However, at 90º tilt, the receding angle of our gPVP coatings approached that of the PVP reference
static contact angle, demonstrating that some rearrangement is occurring.
83
PVP is extremely
hydrophilic, therefore it tends to orient itself away from the dry environment (air) and when a
water droplet is placed onto the surface, some molecules orient into the water droplet. The captive
bubble method was used to elucidate the wet environment hydrophilicity of the coatings by
dispensing an air droplet onto the coating in a DI water bath. This data is also useful because
implantable Parylene C-based biomedical devices are constantly exposed to an aqueous
environment.
64–66,70,72,84,85
The contact angle is immediately lowered after immersion from that of
the static sessile drop contact angle, from 41º ± 5º to 27º ± 2º. The coatings were immersed for 60
minutes to allow time for rearrangement of the gPVP molecules, ultimately reaching an angle of
19º ± 3º (Table 2-2). In comparison, uncoated Parylene C show no decrease in contact angle after
immersion because there is no other favorable orientation. The gPEGDA coatings decrease by only
10º ± 1º after 60 minutes of immersion showing that some rearrangement is occurring. Figure 2-2
depicts the increase in hydrophilicity in the coatings after the various stages in the process.

31
Table 2-2. Contact angle measurements for the gPVP samples (V7-V12) before and after
durability testing.
a
Samples were sonicated in methanol for five 1 hour rounds.
b
Samples were
soaked in 1X PBS solutions at 37 ºC for 30 days.
Parylene C gPVP
Sonicated
gPVP
a
Soaked
gPVP
b
Captive Bubble 90º ± 5º 19º ± 3º 19º ± 3º 18º ± 2º
Static Sessile
Drop
89º ± 1º 41º ± 5º 45º ± 5º 47º ± 4º
Advancing

97º ± 2º 49º ± 6º 52º ± 7º 55º ± 5º
Receding 79º ± 1º 28º ± 7º 33º ± 5º 34º ± 5º

Figure 2-2. Representative static sessile droplet and captive bubble goniometer images for
Parylene C, gPEGDA samples, and gPVP samples.

In order to show the durability of our grafting process, the coatings were tested by
sonicating them for 5 hours in methanol to show that they can withstand physical forces. The
durability of the coatings is evident since after 5 hours of sonication in methanol, little to no
functionality is lost in the gPVP coatings (Table 2-2) as evidenced by the negligible change in
contact angle. The elemental composition from the XPS spectra also confirms that our coatings
are still on the surface of the Parylene C substrate evidenced by the lack of a chlorine signal
showing that our coatings still cover the Parylene C (Table 2-3). Since Parylene C is used for
32
biomedical implants,
64,65,67,68,84
the coatings were also soaked in 1X PBS solution at 37 ºC for 30
days to show that the functionality can be retained, long-term in in vitro environments. After 30
days in the 1X PBS soak at 37 ºC, the coatings showed little to no degradation in functionality, as
evidenced by the lack of change in contact angle (Table 2-2) and elemental composition (Table 2-
3).

Table 2-3. XPS survey spectra for the gPVP samples (V1-V6) before and after durability testing.
a
Washed with methanol and DI water.
b
Sonicated in methanol for five 1 hour rounds.
c
Soaked in
1X PBS solution at 37 ºC for 30 days.

Atomic Compositions

Sample %C %O %N %Cl
PVP reference 75 ± 1 15 ± 1 10 ± 1 0 ± 0
gPVP
a
75 ± 1 16 ± 1 9 ± 1 0 ± 0
Sonicated gPVP
b
76 ± 1 14 ± 1 10 ± 1 0 ± 0
Soaked gPVP
c
74 ± 2 17 ± 2 8 ± 1 0 ± 0

To show the generality of our procedure, the photo-responsive polymer poly(ortho-
nitrobenzyl methacrylate) (PoNBMA) was also deposited as the functional top layer. This photo-
responsive polymer can be converted from hydrophobic to hydrophilic by exposure to UV light
which cleaves some of the nitrobenzyl groups and thereby converts them to methacrylic acid
groups.
80
To create the grafted-PoNBMA (gPoNMBA) layer, we first deposited the cross-linked
anchoring layer onto Parylene C using the photoinitiator and then grafted PoNBMA using the
thermal initiator as described above. The grafted samples were washed with methanol and DI water
to remove ungrafted homopolymer. The static sessile drop angle was measured to be 77º ± 3º,
33
which changed to 56º ± 3º after 1 hour of UV light exposure (Figure 2-3), confirming that the
photoresponsiveness is retained. In comparison, the reference of ungrafted PoNMBA coatings on
Parylene C had a static sessile drop angle of 102º ± 4º, which decreased to 72º ± 3º after 1 hour of
UV light irradiation. Since the gPoNBMA coatings can rearrange, captive bubble measurements
were taken to see the extent of rearrangement of the functional groups in an aqueous environment
before and after UV exposure. We found that the captive bubble contact angles before UV light
exposure decreased from 70º ± 3º to 20º ± 3º after exposure (Figure 2-3), showing the gPoNBMA
coatings after exposure showed significant rearrangement when placed into an aqueous
environment.

Figure 2-3. Static sessile drop and captive bubble measurements of gPoNBMA samples before
and after 1 hour UV light exposure.
2.5 Conclusion
We have demonstrated a novel vapor deposition process where we use photoinitiated CVD,
followed by conventional iCVD to graft functional polymers onto Parylene C. We used XPS and
contact angle goniometry to show that our grafted coatings retained the desired functionality and
fully covered the Parylene C substrate. Durability testing showed that our grafted coatings retained
their functionality and elemental composition, showing promising long-term efficacy as an
implantable material. Since our process uses standard iCVD parameters and precursors, this work
could be used to deposit any cross-linker and functional polymer combination that has been
34
deposited using iCVD. Therefore, the generality of this process gives access to a wide library of
iCVD precursors that could be used. Additionally, this process can be extended to any substrate
with a labile hydrogen atom. Also, given the conformal nature of the iCVD process, our process
can be used on substrates with a wide array of geometries.
2.6 Acknowledgements
This work was supported by the National Science Foundation Award Number 1332394. M. D. L.
is supported by a National Science Foundation Graduate Research Fellowship under grant DGE-
1418060.
35

Chapter 3: Effects of Surface Tension and Viscosity on Gold
and Silver Sputtered onto Liquid Substrates

M. M. De Luna, M. Gupta. “Effects of surface tension and viscosity on gold and silver sputtered
onto liquid substrates.” Applied Physics Letters, 2018, 112, 201605.

36
3.1 Abstract
In this chapter, we study DC magnetron sputtering of gold and silver onto liquid substrates
of varying viscosities and surface tensions. We were able to separate the effects of viscosity from
surface tension by depositing the metals onto silicone oils with a range of viscosities. The effects
of surface tension were studied by depositing the metals onto squalene, poly(ethylene glycol), and
glycerol. It was found that dispersed nanoparticles formed on liquids with low surface tension and
low viscosity whereas dense films formed on liquids with low surface tension and high viscosity.
Nanoparticles formed on both the liquid surface and within the bulk liquid for high surface tension
liquids. Our results can be used to tailor the metal and liquid interaction to fabricate particles and
films for various applications in optics, electronics, and catalysis.
3.2 Introduction
Metal nanoparticles and films can be produced via sputtering onto low vapor pressure
liquids such as oils and ionic liquids for a range of applications in optics,
44
electronics,
47,86
and
catalysis.
87
The sputtering process parameters
88
and the physical and chemical properties of the
liquid can affect the morphology of the deposited metal.
48,89
For example, Hatakeyama et al.
studied the effects of sputtering conditions on the deposition of gold nanoparticles onto 1-butyl-3-
methylimidazolium tetrafluoroborate and concluded that sputtering time, distance from the target,
discharge current, and argon pressure had an insignificant effect on the size of the gold
nanoparticles whereas the temperature of the target and the temperature of the ionic liquid had a
strong influence on the size.
88
In contrast to Hatakeyama, Sugioka et al. found that the discharge
current and sputtering time had a significant effect on particle size.
90
Sugioka et al. also found that
sputtering gold, silver, palladium, or platinum onto room temperature ionic liquids that contained
hydroxyl groups led to the formation of a film composed of a monolayer of nanoparticles.
90
Xie et
37
al. showed that gold and silver sputtered onto 175 cst silicone oil formed microstructured films
composed of ramified aggregates which had similar grain sizes when comparing the liquid and
vapor surfaces.
48
Additionally, Fang et al. showed that sputtering aluminum onto 175 cst silicone
oil formed ramified aggregates with grain sizes that are independent of film thickness.
91
Richter et
al. explained that while some studies acknowledged the effects of viscosity or surface tension on
nanoparticle size and film structure, other studies showed no effect or neglected the effects of the
liquid properties.
89
While some studies acknowledged the effects of viscosity or surface tension
on nanoparticle size and film structure, other studies showed no effect or neglected the effects of
the liquid properties.
89
In order to resolve this conflicting information, we have systematically
studied DC magnetron sputtering of gold and silver onto a wide range of liquids in order to
deconvolute the effects of surface tension and viscosity.
DC magnetron sputtering is advantageous because of its ability to confine the target atoms
to the substrate and allow for efficient deposition without degrading the substrate.
42
We kept the
sputtering conditions constant and chose a series of low vapor pressure liquids to isolate the
physical properties from chemical interactions such as electrostatic interactions.
45,90
We chose a
series of silicone oils with viscosities of 48, 96, 339, and 485 cP because their similar chemical
makeup and surface tension allows us to isolate the effects of viscosity. In order to study the effects
of surface tension, squalene, poly(ethylene glycol) (400 MW PEG), and glycerol were chosen
because of their lack of charged chemical moieties. Our previous studies have shown that the
deposition of polymers onto liquid surfaces is governed by the spreading coefficient 𝑆 =
𝛾
56
(1+𝑐𝑜𝑠𝜃
=
)− 2𝛾
?6
, where 𝛾
56
is the liquid-vapor surface tension, 𝜃
=
is the advancing
contact angle of the liquid on the polymer, and 𝛾
?6
is the polymer-vapor surface tension.
26,32

Particles form when the spreading coefficient is negative whereas films form when the spreading
38
coefficient is positive. The key difference between the deposition of polymers and metals is that
the surface energy of metals is much higher than polymers and therefore the spreading coefficient
will always favor particle formation.
92

Our hypothesis is that viscosity and surface tension impact the rate at which the liquid wets
the deposited metal which determines whether the sputtered nanoparticles deposited onto the liquid
surface will diffuse and aggregate, submerge into the bulk liquid, and diffuse and aggregate within
the bulk. Nakamura et al. showed that the rate of wetting for liquid argon on platinum at the
nanoscale was proportional to the liquid surface tension and inversely proportional to the liquid
viscosity.
93
Therefore, we expect the high surface tension liquids to have a faster rate of wetting
than the low surface tension liquids and high viscosity liquids to have a slower rate of wetting than
the low viscosity liquids. Additionally, the liquid viscosity is an important factor because it directly
impacts the nanoparticle diffusion velocity and aggregation behavior. The nanoparticles in our
study are approximately 4 nm in radius which is similar to the size measured by others in
comparable sputtering systems
94
and therefore the energy required for these particle to penetrate
the liquid surface into the bulk varies from ~20-60 eV. Previous studies have found that sputtered
metals have energies in the range of 5-15 eV irrespective of the applied voltage
95
, and therefore
the sputtered metal nanoparticles likely do not penetrate the surface. These nanoparticles impinge
on the surface with a stagnation pressure (~10 MPa) less than their hardness values (~3 GPa)
96
,
and therefore the nanoparticles should not deform. The nanoparticles at the vapor-liquid interface
may submerge into the bulk liquid leading to aggregation behavior within the bulk.
49
The minimum
energy required for a particle to submerge into the liquid is given by the equation ∆G = πr
2
γ (1 -
cosθ)
2
for 0 ≤ θ ≤ 90º where r is the particle radius, γ is the liquid-vapor surface tension, and θ is
the equilibrium three phase contact angle.
97
Our results indicate that varying the viscosity and
39
surface tension can lead to nanoparticles within the bulk liquid, aggregated nanoparticles within
the bulk, or films comprised of branch-like (ramified) aggregates or dense (clustered) aggregates.
3.3 Experimental
Silicone oils (48, 96, 339, and 485 cP, Sigma-Aldrich), glycerol (Omnipur), poly(ethylene
glycol) average Mn 400 (400 MW PEG) (Sigma-Aldrich), and squalene (Aldrich) were used as
received without further purification. A commercial sputter coater (Ted Pella, Inc., 108 Auto
Sputter Coater) was used for all depositions. A gold target (99.999% Au, Ted Pella Inc.) and silver
target (99.99% Ag, Ted Pella Inc.) were both used without further processing. Approximately 0.5
mL of liquid was evenly spread on a 2.5 x 2.5 cm glass slide and placed inside the sputter
deposition chamber at a working distance of 50 mm. The glass slides were roughened by scratching
the edges for approximately 10 seconds using an X-Acto blade (Elmer’s Products, Inc.) to prevent
the liquid from dewetting onto the stage. The systematic depositions were sputtered for 30 seconds
at a power of 2.2 W and at a chamber pressure of 0.8 mbar using argon gas. For the long time (t)
depositions, the sputtering time was increased to 5 minutes while keeping the other parameters
constant.
Camera images were taken immediately after deposition by a DSLR camera (Nikon,
D2100) at a distance of 50 cm from the sputter coater stage. The microscope images were taken 5
minutes after deposition was complete using an optical microscope (OMAX) at a magnification of
40X. Transmission electron microscope (TEM) micrographs were taken using a JOEL JEM-2100
electron microscope at an accelerating voltage of 200 kV and a magnification of 200,000X. TEM
grids of the bulk of the liquid were prepared by removing any film from the surface of the liquid
and dipping the grid perpendicular into the liquid surface until the grid was completely submerged
within the bulk. TEM grids of the surface of the liquid were prepared by lightly touching the grid
40
to the surface of the sample. All grids were prepared immediately following camera imaging and
were rinsed in a hexane bath for 30 minutes and dried overnight before analysis. Three different
samples were imaged for each data point and a representative image from each set was chosen.
Ultraviolet-visible light (UV-VIS) spectroscopy was performed using a spectrometer
(Agilent, Model 8453) in a disposable cuvette with a 1 cm path length. Concentrated samples were
prepared by increasing the sputtering time to 15 minutes while keeping all other sputtering
parameters constant. Image J analysis was used to deduce the particle size distributions by
measuring the diameters of 100 particles from representative TEM micrographs.
3.4 Results & Discussion
In order to systematically study the effect of the viscosity of the liquid substrate on the
sputtered gold, we performed depositions onto 48, 96, 339, and 485 cP silicone oils at constant
sputtering conditions. In order to visualize the deposited gold at various length scales, the samples
were imaged using a camera, an optical microscope, and a transmission electron microscope
immediately after deposition (Figure 3-1). The camera images of the 48 and 96 cP silicone oil
systems show gold nanoparticles suspended in solution (Figure 3-1a). We confirmed the presence
of nanoparticles dispersed throughout the z-direction using optical microscopy (Figure 3-1b).
Since silicone oil has a very low contact angle of approximately 4º ± 2º on gold, it is energetically
favorable for the particles to submerge into the bulk liquid rather than aggregate on the surface of
the liquid to form a film. No film was formed on the surface of the liquid even when the deposition
time was increased to 5 minutes because the gold nanoparticles deposited at the surface
continuously submerge into the bulk silicone oil. The optical microscope image of the 48 cP
silicone oil system shows aggregates on the order of tens of microns within the bulk liquid. These
aggregates are bigger than in the 96 cP silicone oil because the diffusion of the nanoparticles is
41
inversely proportional to viscosity (Figure 3-1b). The aggregation behavior in the 48 cP silicone
oil system is consistent with work by Cai et al.
28
that showed that the diffusion velocity of non-
sticky nanoparticles in polymeric liquids is mainly dependent on the solvent viscosity. While the
camera and microscope images were taken of the nanoparticles as deposited, the TEM images are
from grids that were gently dipped into the bulk liquid immediately after sputtering, rinsed with
hexane, and allowed to dry for 1 day. The TEM images show that nanoparticles aggregated in the
48 cP silicone oil system, whereas the nanoparticles are more dispersed in the 96 cP silicone oil
system, which is consistent with the microscope images (Figure 3-1c). Image J analysis of 100
nanoparticles and nanoparticle aggregates from representative TEM images show that both
systems consistently have sub-20 nm diameter individual nanoparticles and aggregates, with the
48 cP silicone oil resulting in more nanoparticle and aggregate sizes greater than 9 nm indicating
a greater degree of aggregation (Figure 3-1c). By increasing the sputtering time to 15 minutes to
increase the gold concentration within the bulk, we were able to use UV-Vis spectroscopy to
determine the relative size and relative concentration of gold nanoparticles.
98
Since the TEM
images show that the nanoparticle diameters are less than 20 nm, the larger ratio of the absorbance
at the surface plasmon resonance peak (Aspr) to the absorbance at 450 nm wavelength (A450) in the
48 cP silicone oil system (1.14) compared to the 96 cP silicone oil system (1.06) confirms a higher
degree of aggregation in the 48 cP silicone oil system. After 48 hours, the gold nanoparticles form
larger aggregates so the surface plasmon resonance peaks can be directly compared to give relative
sizes. A shift to 638 nm for the 48 cP silicone oil system indicated a greater degree of aggregation
when compared to the shift to 551 nm for the 96 cP silicone oil system.
42

Figure 3-1. (a) Camera images, (b) microscope images, (c) TEM images of the bulk, and particle
size count (inset) of gold sputtered onto 48 cP (left column) and 96 cP (right column) silicone oils.

Thermodynamically, sputtered gold always favors nanoparticle submersion into the bulk
silicone oil because the ∆G for submerging is close to zero. However, the camera and optical
images of the 339 cP and 485 cP oils show that a film forms on the surface of the liquid. (Figures
3-2a and 3-2b) Our hypothesis is that the film forms because the increase in viscosity results in a
lower rate of wetting, and therefore nanoparticles at the vapor-liquid interface accumulate due to
adparticle deposition. In the microscope images, we observe wrinkling of the films along the edges
in both the 339 cP silicone oil system and the 485 cP silicone oil system likely because the sputter
deposition process slightly heats the surface causing the liquid surface to expand during deposition.
(Figure 3-2b) The wrinkles form as the surface cools along the stress-relieving edges similar to the
wrinkles Yang et al. observed when using thermal evaporation to deposit gold onto 175 cSt silicone
oil.
99
TEM images show that the surface structures of the 339 cP and 485 cP silicone oil systems
have significantly different morphologies compared to their bulk counterparts. We observe films
composed of dense nanoparticle aggregates with crack-like features. (Figure 3-2c) There is more
empty space between the structures in the 339 cP silicone oil system than in the 485 cP silicone
43
oil system as evidenced by the light grey areas in the TEM images. (Figure 3-2c) These crack-like
features are likely due to a combination of wetting of the gold by the silicone oil and surface
diffusion of gold aggregates. The TEM images of the 339 cP silicone oil bulk and the 485 cP
silicone oil bulk shows a lower concentration of gold and less aggregation than the lower viscosity
systems. (Figure 3-2d) Image J analysis shows that the bulk contains mostly sub-8 nm gold
nanoparticles. (Figure 3-2d) The absence of a peak in the UV-Vis spectra of 339 cP and 485 cP
silicone oil systems after 15 minutes of deposition further confirms that the concentration of gold
in the bulk is insignificant because most of the deposited gold is likely incorporated into the films.

44

Figure 3-2. (a) Camera images, (b) microscope images, (c) TEM images of the surface, (d) TEM
images of the bulk, and particle size count (inset) of gold sputtered onto 339 (left column) and 485
cP (right column) silicone oils.

In order to study the effects of increasing the surface tension of the liquid, we used constant
sputtering parameters and chose squalene (12 cP, 31.2 mN/m), 400 MW PEG (112 cP, 43.2
mN/m), and glycerol (946 cP, 64.3mN/m). These liquids were selected instead of ionic liquids to
minimize electrostatic interactions with the deposited metals. While increasing viscosity reduces
diffusion and aggregation of the metal nanoparticles on the surface and within the bulk, increasing
the liquid surface tension has the opposite effect since the liquid molecules prefer to minimize the
surface contact area with the gold in order to minimize the free energy of the system thereby
causing aggregation. Nakamura et al. showed that the rate of wetting is proportional to the liquid
surface tension.
93
However, since the energy required for submerging (∆G) is proportional to
surface tension and the liquids have a nonzero contact angle on gold (14º ± 3º, 12º ± 2º, and 11º ±
45
3º for squalene, 400 MW PEG, and glycerol, respectively), nanoparticle aggregates are generated
on both the surface and within the bulk. The camera image of gold deposited onto squalene shows
ramified aggregates along the edges of the liquid and a continuous film-like structure in the center
on the liquid surface (Figure 3-3a). The center structure likely forms due to fast surface diffusion
due to a combination of low viscosity and high surface tension (Figure 3-3b). The TEM of this
surface structure shows that the film is comprised of a network of ramified aggregates (Figure 3-
3c). Dispersed gold nanoparticles are observed within the bulk of squalene likely because energy
may be supplied to the system through heating of the liquid surface to overcome the ∆G
requirement to submerge. Sputtering gold onto the 400 MW PEG results in large ramified
aggregates on the surface of the liquid and smaller ramified aggregates within the bulk of the liquid
(Figures 3-3a and 3-3b). In comparison to squalene, 400 MW PEG has a higher surface tension,
resulting in denser structures both on the surface and within the bulk of the liquid (Figures 3-3c
and 3-3d).

46

Figure 3-3. (a) Camera images, (b) microscope images, (c) TEM images of the surface, and (d)
TEM images of the bulk of sputtered gold onto squalene (first column), 400 MW PEG (second
column), and glycerol (third column).

Glycerol has the highest surface tension (~63.4 mN/m) and viscosity (~946 cP) of all the
liquids tested. A film formed on the surface since the high viscosity hinders diffusion and
aggregation (Figure 3-3a). The film is unstable because the system wants to reduce the interfacial
area between the gold and the glycerol (Figures 3-3a and 3-3b). The microscope images show
fragments of the gold film which have aggregated (Figure 3-3b). Since the energy to break gold
bonds
100
is much higher than the energy gained from aggregating the gold,
101,102
we may conclude
that the aggregation occurs during deposition. The TEM image of the surface shows dense
aggregate structures (Figure 3-3c). The TEM image of the bulk liquid shows ramified aggregates
on the order of 100 nm despite the high viscosity (Figure 3-3d).
Our data demonstrates that surface tension has an effect on the growth and aggregation
behavior because dispersed nanoparticles readily form in the 96 cP silicone oil (Figure 3-1c)
47
whereas ramified aggregates form on the surface and within the bulk of the 400 MW PEG at a
similar viscosity but nearly doubled surface tension (Figures 3-3c and 3-3d). While PEG may have
a small electrostatic interaction with the gold, the effect is minimal and would promote
nanoparticle stability rather than aggregation allowing us to conclude that the ramified aggregates
are a result of the high surface tension. The surface structures on each of the liquids (339 cP, 485
cP, squalene, 400 MW PEG, and glycerol) can be described by the diffusion-limited regime or
reaction-limited regime. In the diffusion-limited regime, nanoparticles are strongly attracted to one
another and tend to form open ramified aggregate systems.
103
In the reaction-limited regime,
clustered or dense aggregates form more favorably since nanoparticle attraction is weak and
surface diffusion is slow.
104
The surface tension data shows the diffusion-limited regime at low
viscosity (squalene, 400 MW PEG) form open ramified aggregates (Figure 3-3c). The system
switches to a reaction-limited regime as the viscosity increases (glycerol), evidenced by the dense
aggregates seen as black spots in the TEM images. (Figures 3-3c and 3-3d) High viscosity silicone
oils (339 cP and 485 cP) also form compact clusters because of the high viscosity and low surface
tension driving force of the systems (Figure 3-2c).
In order to understand whether our observed trends could be generalized to high surface
energy materials, we repeated our set of experiments using silver. The camera images for both the
48 and 96 cP silicone oil systems show dispersed nanoparticles (Figure 3-4a). The TEM images
show greater aggregation in the 48 cP silicone oil, which is in agreement with the gold results.
(Figure 4c) Similar to the gold system, film formation occurs on the higher viscosity systems (350
and 485 cP) (Figures 3-4a and 3-4b). TEM images of the 339 cP and 485 cP bulk shows only sub-
8 nm nanoparticles (Figure 3-4c). Since the 339 cP silicone oil system allows for a faster rate of
wetting than in the 485 cP silicone oil system, more empty space is observed. Figure 3-5 shows a
48
summary of the results for sputtering silver onto liquids of varying surface tension. Similar to gold,
sputtered silver diffuses and aggregates according to the properties of the liquid. Squalene causes
the silver to aggregate in the diffusion limited regime, creating ramified aggregates whereas the
glycerol causes the silver to aggregate in the reaction-limited regime, creating densely packed
aggregates. 400 MW PEG has an intermediate effect on the aggregation behavior of the silver. We
can conclude that our results are generalizable to high surface energy materials sputtered onto low
vapor pressure liquids. A morphological phase diagram to summarize our results is given in Figure
3-6.

Figure 3-4. (a) Camera images, (b) microscope images, and (c) TEM images of the bulk of
sputtered silver onto 48 cP (first column), 96 cP (second column), 339 cP (third column) and 485
cP (fourth column) silicone oil systems.

49

Figure 3-5. (a) Camera images, (b) microscope images, (c) TEM images of the surface, and (d)
TEM images of the bulk of sputtered silver onto squalene (first column), 400 MW PEG (second
column), and glycerol (third column).

50

Figure 3-6. Morphological phase diagram of the resulting structures. X’s represent the data points
in this study.

3.5 Conclusion
In conclusion, we elucidated the influence of viscosity and surface tension on the resulting
morphologies of sputtered gold and silver onto low vapor pressure liquids. The resulting
morphologies were nanoparticles within the liquid bulk, aggregated nanoparticles within the bulk,
and films comprised of sparsely or densely aggregated nanoparticles on the liquid surface.
Sputtered gold and silver were shown in this study, but our results are generalizable to other high
surface energy materials, such as platinum. These results allow for tuning the liquid properties to
fabrication structures for applications in optics, catalysis, and electronics.
3.6 Acknowledgements
This work was supported by the U.S. Department of Energy, Office of Science, Basic
Energy Sciences under Award #DE-SC0012407. M. D. L. was supported by a National Science
Foundation Graduate Research Fellowship under grant DGE-1418060.
51

Chapter 4: Synthesis of Inorganic/Organic Hybrid Materials
via Vapor Deposition onto Liquid Surfaces

M. M. De Luna, P. Karandikar, M. Gupta. “Synthesis of Inorganic/Organic Hybrid Materials via
Vapor Deposition onto Liquid Surfaces.” ACS Applied Nano Materials, 2018, 1, 6575-6579.

52
4.1 Abstract
Hybrid materials have a multitude of uses in electronics, catalysis, and drug delivery.
Here, we combine sputter deposition and initiated chemical vapor deposition onto low vapor
pressure liquids to fabricate inorganic/organic hybrid materials. We demonstrate that polymer
nanoparticles can be decorated with metal nanoparticles and nanoparticle dispersions can be
encapsulated within polymer shells or within gel beads. The generality of our synthetic route
allows for a variety of hybrid materials to be fabricated by tuning the viscosity of the liquid, the
solubility of the monomer, and the surface tension of the polymer and the liquid.
4.2 Introduction
The development of new synthetic routes for the production of hybrid materials
composed of inorganic and organic components is important for the fabrication of materials with
enhanced properties. Hybrid materials have been shown to have a variety of applications in
optics,
51
sensors,
52
electronics,
53
and catalysis.
55
For instance, Winter and coworkers incorporated
iron oxide nanoparticles and quantum dots into amphiphilic polymeric micelles via solution phase
self-assembly which enabled magnetic control and fluorescent tracking of these hybrid
nanostructures for applications in theraputics.
105
Additionally, Álvarez-Paino et al. decorated
poly(dopamine methacrylamide-co-ethylene glycol dimethacrylate) particles with gold and iron
oxide nanoparticles via solution phase chemical reactions for the catalytic reduction of 4-
nitrophenol and 4-aminophenol.
106

In this paper, we demonstrate a versatile method to create hybrid materials with control
over the size, functionality, and morphologies of both the organic and inorganic phases. We
combine two vapor phase deposition processes, which eliminates the need for surfactants and
organic solvents. The polymers and metals are deposited onto re-useable low vapor pressure
53
liquids (silicone oils and ionic liquids (ILs)) which allows for the formation of a variety of hybrid
materials such as polymer nanoparticles decorated with metal nanoparticles, encapsulated
nanoparticle dispersions, and polymer gel beads with embedded metal nanoparticles. The polymer
is deposited onto liquid surfaces via initiated chemical vapor deposition (iCVD). Initiator and
monomer precursors are flown into a vacuum chamber where the initiator is thermally cleaved by
a heated filament array to initiate free-radical polymerization.
16
Polymer deposition occurs at
moderate substrate temperatures and filament temperatures which allows for the retention of the
chemical functionality as confirmed by Fourier transform infrared spectroscopy and X-ray
photoelectron spectroscopy.
1
Our previous studies have shown that deposition of polymers onto
low vapor pressure liquids via the iCVD process yielded different morphologies such as
nanoparticles, films, and gels based on the surface tension and the viscosity of the liquid and the
monomer solubility in the liquid.
26,37,38
The inorganic materials are deposited onto the low vapor
pressure liquids via DC magnetron sputtering, which has been shown to create inorganic thin
films,
107
nanoparticles at the vapor-liquid interface,
90
or nanoparticles within the bulk of the
liquid.
108
Recently, our group elucidated the effects of liquid viscosity and surface tension on the
resulting metal morphology in DC magnetron sputtering.
50
Despite the advantages of vapor-phase
deposition methods, little work has been done to combine inorganic and organic deposition
methods to create hybrid structures. Here, we demonstrate for the first time that iCVD and
sputtering can be combined to fabricate a variety of unique hybrid materials composed of inorganic
nanoparticles and polymers by tuning the viscosity of the liquid, the solubility of the monomer,
and the surface tension of the polymer and the liquid.
4.2 Experimental
Materials: Silicone oil (100 cSt, Sigma-Aldrich), 4-vinyl pyridine (4VP) (Sigma-Aldrich,
54
99%), 2-hydroxyethyl methacrylate (HEMA) (Sigma-Aldrich, 99%), ethylene glycol diacrylate
(EGDA) (Polysciences, Inc.), 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) (SynQuest
Laboratories, 97%), tert-butyl peroxide (TBPO) (Sigma-Aldrich, 98%), and
poly(tetrafluoroethylene) particles (PTFE) (35 µm, Sigma-Aldrich) were all used as received. 1-
Ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) (Sigma-Aldrich, 97%) was dried at
50 ºC for 24 hours in a vacuum chamber before use. All sputter depositions were carried out in a
commercially available sputter coater (108 Auto Sputter Coater, Ted Pella Inc.) with a current of
20 mA for 30 s at an argon (99.999%) pressure of 0.8 mbar using a gold target (Ted Pella Inc.,
99.999% Au).
Decorated Polymer Nanoparticles: 100 µL of 100 cSt silicone was dispensed on a 3 x 3
cm piece of silicon (Wafer World) and placed in the sputter deposition chamber. Gold was
sputtered at the conditions described above. The resulting dispersion was placed in a custom-built
iCVD reactor (GVD Corporation, 250 mm diameter, 48 mm height). For the poly(2-hydroxyethyl
methacrylate) (PHEMA) depositions, the HEMA monomer was heated to 50 ºC and the TBPO
initiator was kept at room temperature to produce a flow rate of 1 sccm and 0.8 sccm, respectively.
The TBPO flow rate was set using a mass flow controller (MKS Type 1479A). The substrate
temperature was set to 30 ºC using a recirculating chiller, the pressure was set to 150 mTorr, and
the filament temperature (80% Ni, 20% Cr, Omega Engineering) was set to 250 ºC. The pressure
was measured using a pressure transducer (MKS Baratron 622A01TDE) and regulated via an
automated butterfly valve connected to a vacuum pump (25.9 CFM, Edwards Model E2M40). 200
nm of polymer was deposited as measured by an He-Ne laser interferometer (Industrial Fiber
Optics, 633 nm) on a reference silicon wafer. For the poly(4-vinyl pyridine) (P4VP) depositions,
the 4VP monomer was heated to 23 ºC and TBPO was kept at room temperature to produce a flow
55
rate of 5 sccm and 0.8 sccm, respectively. The substrate temperature was set to 25 ºC, the pressure
was set to 300 mTorr, the filament temperature was set to 250 ºC, and 400 nm of polymer was
deposited as measured on a reference silicon wafer. For both polymers, the resulting dispersion
was placed in a microcentrifuge tube and centrifuged for 30 min at 6400 RPM (DW-41-115,
Qualitron, Inc.). To fabricate polymer particles via condensation, first gold was sputtered onto 100
µL of 100 cSt silicone oil on a 3 x 3 cm piece of silicon and then the dispersion was placed in the
iCVD reactor. The dispersion was pre-cooled inside the reaction chamber using a thermoelectric
cooler (TEC) (Custom Thermoelectric) for 30 minutes at -10 °C prior to introducing the monomer
and was maintained at -10 °C for the entire process. 4VP was heated to 25 °C and TBPO was kept
at room temperature to produce a flow rate of 13 sccm and 0.7 sccm, respectively. The monomer
was condensed at Pm/ Psat = 1.5 by setting the reactor pressure to 225 mTorr. The monomer flow
was turned off after 15 s and the reactor pressure was increased to 1 Torr to prevent evaporation
of the condensed monomer. The condensed droplets were polymerized for 15 min at a TBPO flow
rate of 5 sccm and a filament temperature of 250 °C.
Encapsulation of Nanoparticle Dispersions: 200 µL of [emim][BF4] was dispensed onto a
3 x 3 cm piece of silicon and gold was sputtered using the conditions described above. Marbles
were then fabricated by pipetting 10 µL of the resulting dispersion onto a Petri dish (5 cm diameter)
containing 0.5 g of PTFE particles. The Petri dish was carefully tilted back and forth to coat the
entirety of the droplet with PTFE particles. The marble was then transferred using tweezers to
another Petri dish (5 cm diameter) containing a bed of PTFE particles (1.5 g) and placed in the
iCVD chamber. PFDA and EGDA were heated to 50 ºC and 40 ºC to produce flow rates of 0.4 and
0.1 sccm, respectively, and the TBPO was kept at room temperature to produce a flow rate of 1
sccm. The polymerization was carried out at a substrate temperature of 30 ºC, reactor pressure of
56
100 mTorr, and filament temperature of 250 ºC until 3 µm of polymer was deposited as measured
on a reference silicon wafer.
Gels with Embedded Nanoparticles: First, Whatman chromatography paper was placed in
the iCVD chamber to coat the surface with PPFDA polymer to render the surface hydrophobic.
PFDA was heated to 50 ºC to produce a flow rate of 0.4 and TBPO was kept at room temperature
to produce a flow rate of 1 sccm. The substrate temperature was set to 30 ºC, the pressure was set
to 80 mTorr, and the filament temperature was set to 250 ºC. The polymerization was terminated
once 400 nm of polymer was deposited as measured on a reference silicon wafer. To fabricate the
gel beads with embedded nanoparticles, 200 µL of [emim][BF4] was dispensed onto a 3 x 3 cm
piece of silicon and gold was sputtered using the conditions described above. 1.5 µL of the
resulting dispersion was pipetted onto the PPFDA coated paper and placed in the iCVD chamber.
In order to absorb HEMA monomer into the IL, the HEMA was heated to 50 ºC to achieve a flow
rate of 1 sccm and flown into the reactor for 2 hours at a reactor pressure of 100 mTorr and a
substrate temperature of 25 ºC. The monomer line was then closed, and polymerization was carried
out for 1 hour at a TBPO flow rate of 3 sccm, chamber pressure of 1 Torr, and a filament array
temperature of 250 ºC.
Characterization: Dynamic light scattering (DLS) (Wyatt Technology, Möbuiz) was
performed to obtain the particle size distribution of the decorated nanoparticles. 60 µL of the
dispersion was pipetted into a quartz cuvette. Measurements were taken in triplicate and performed
at 25 ºC with an acquisition time of 10 s for a total of 20 acquisitions. Transmission electron
microscopy (JEOL JEM-2100F) was performed using an operating voltage of 200 kV equipped
with a Gatan Orious CCD camera. After each dispersion was placed on the grid, the dispersion
was left to settle on the grid for 1 hour. To visualize the decorated nanoparticles, 1 µL of the
57
dispersion was pipetted onto a copper grid (Ted Pella, Inc.) and rinsed with hexane. To image the
dispersed nanoparticles within the encapsulated marble, the polymer shell was cut open using
tweezers, the exposed liquid was allowed to contact the grid, and the grid was rinsed with acetone.
X-ray photoelectron spectroscopy (Kratos Axis Ultra DLD) was performed on the polymer shell
using a monochromatic Al Ka x-ray source. The encapsulated nanoparticle dispersion was
carefully cut open, the polymer shell was placed on carbon tape (Ted Pella, Inc.) with either the
top or bottom side exposed, and the polymer shell was carefully rinsed with acetone to remove any
excess liquid. Survey spectra were collected from 0 to 1000 eV with a step size of 1 eV, sweep
time of 60 s, and a total of 5 scans. For the gel beads, static contact angles were measured using a
contact angle goniometer (ramé-hart Model 290-FI). Energy dispersive spectroscopy (EDS)
(JEOL JSM-7001F-LV) was performed by flattening the gel bead onto a 1 x 1 cm piece of silicon,
which was then placed in an acetone bath to remove the ionic liquid, and after drying with
compressed air, a razor blade was used to cut open the flattened bead to expose the middle.
4.3 Decorated Polymer Nanoparticles
In order to synthesize polymer nanoparticles decorated with metal nanoparticles, we
deposited the metal and polymer onto low surface energy liquids which allowed for the formation
and submergence of the inorganic and organic components into the bulk liquid. As shown in Figure
4-1a, metal was first sputtered onto the liquid to create dispersed nanoparticles. Polymer
nanoparticles were deposited onto the resulting dispersion which was then subsequently
centrifuged to fabricate decorated nanoparticles. We have previously shown that a negative
spreading coefficient (S) is required for particle formation via iCVD of polymers onto liquid
substrates in which S is defined as 𝑆 = 𝛾
56
(1+𝑐𝑜𝑠𝜃
=
)− 2𝛾
?6
, where 𝛾
56
is the liquid-vapor
surface tension, 𝜃
=
is the advancing contact angle of the liquid on the polymer, and 𝛾
?6
is the
58
polymer-vapor surface tension.
26
The polymer particles will either remain at the vapor-liquid
interface
31,32
or submerge into the bulk
14,31
depending on the energy required for submerging (∆𝐺)
which is defined as ∆𝐺 = 𝜋𝑟
C
𝛾
56
(1−|𝑐𝑜𝑠𝜃
E
|)
C
where r is the particle radius and 𝜃
E
is the
equilibrium contact angle. We chose to deposit poly(4-vinylpyridine) (P4VP) and poly(2-
hydroxyethyl methacrylate) (PHEMA) onto 100 cSt silicone oil because the spreading coefficient
of these systems is negative ensuring the formation of polymer particles, and the energy required
for submerging is zero due to the 0º contact angle of silicone oil on these polymers which causes
the particles to submerge into the liquid. Similarly, 100 cSt silicone oil readily wets gold surfaces
(4º contact angle) due to the high surface energy of the gold (1500 mN/m)
109
which results in the
submersion of gold nanoparticles sputtered onto the silicone oil surface.
50
A low viscosity silicone
oil (100 cSt) was chosen because gold sputtered onto higher viscosities ( ≥ 350 cSt) leads to the
formation of dense films rather than submerging nanoparticles.
50

59

Figure 4-1. (a) Schematic representation of the fabrication process to decorate polymer
nanoparticles with metal nanoparticles, (b) a TEM micrograph of a PHEMA nanoparticle
decorated with gold nanoparticles, and (c) a TEM micrograph of a P4VP nanoparticle decorated
with gold nanoparticles.

After the deposition of gold and polymer onto the silicone oil, the silicone oil was placed
in a centrifuge tube and centrifuged for 30 min to complex the gold nanoparticles with the polymer
nanoparticles. Computational models have shown that neutral sputtered metals, such as gold,
complex through weak van der Waals interactions with the lone pairs in nitrogen and oxygen
atoms,
110,111
and therefore we expect that gold interacts with the pyridine moieties of P4VP and
the hydroxyl groups of PHEMA. Sputtering gold onto the silicone oil yielded nanoparticles with
an average radius of 5 nm. Dynamic light scattering (DLS) data showed that PHEMA nanoparticles
decorated with gold nanoparticles have an average radius of 110 nm ± 38 nm. Figure 4-1b shows
a representative decorated PHEMA nanoparticle with a radius of 150 nm. DLS data showed that
P4VP nanoparticles decorated with gold nanoparticles have an average radius of 82 nm ± 20 nm.
Figure 4-1c shows a representative decorated P4VP nanoparticle with a radius of 95 nm. As
60
exhibited by the lower number of gold nanoparticles in Figure 4-1b, PHEMA complexes less
readily with the gold because of the weaker interaction of the lone pairs of the oxygen groups with
gold compared to the lone pair of the pyridine groups with gold.
110
The iCVD process parameters
can be optimized to achieve a narrower size distribution. For example, previous studies have
shown that lowering the deposition rate of the polymer leads to a lower standard deviation in
particles sizes.
32
The silicone oil serves as a model liquid system due to the neutral nature of the
oil which allows the polymer and metal to interact freely compared to ILs where electrostatic
repulsion can inhibit interactions between the polymer and the metal.
88
Since the silicone oil does
not react with the polymer or the metal, the decorated nanoparticles can be separated out and the
oil can be reused, allowing for large scale synthesis of these particles. Polymer nanoparticles
decorated with gold nanoparticles are useful for catalytic reactions such as the reduction of
nitrophenols,
106
which is important for applications in pharmaceutical and organic synthesis
industries. We estimate that we can fabricate ~10
11
decorated P4VP nanoparticles on a single 3 x
3 cm wafer containing 100 µL of silicone oil over 10 minutes of deposition time.
We can fabricate larger hybrid particles by modifying the iCVD parameters. Typically, the
iCVD process uses reactor conditions in which the ratio of the monomer partial pressure (Pm) to
the monomer saturation pressure (Psat) is less than one which leads to undersaturation of the
precursors.
16
We previously demonstrated that we can form larger polymer particles by introducing
the monomer into the deposition chamber at Pm/Psat > 1 leading to condensation of monomer
droplets onto the silicone oil.
14
During condensation, the monomer droplets nucleate at the vapor-
liquid interface and are readily wet by the silicone oil which leads to the engulfment of these
droplets into the liquid. The monomer droplets grow via coalescence in the bulk liquid. The
monomer molecules within the droplets are subsequently polymerized via a free-radical
61
mechanism to form polymer chains. The molecular weight of a polymer chain is typically ~100
kDa in the iCVD process,
32
and therefore there are approximately ~10,000 polymer chains within
each resulting polymer particle. In order to fabricate larger hybrid particles, first we deposited gold
nanoparticles onto 100 cSt silicone oil via sputtering. Then we condensed 4-vinylpyridine (4VP)
droplets onto this dispersion and subsequently polymerized the monomer to fabricate decorated
nanoparticles (Figure 4-2a). Figure 4-2b shows that we form particles with a heterogeneous size
distribution due to the coalescence of the monomer droplets prior to polymerization. Figure 4-2c
shows a representative decorated nanoparticle with a diameter of 500 nm which demonstrates that
we can achieve a larger particle size by changing process parameters independent of substrate
viscosity. A narrower particle size distribution can be achieved by increasing the substrate
viscosity in order to inhibit the coalescence of monomer droplets prior to polymerization.
14

62

Figure 4-2. (a) Schematic representation of the fabrication process to decorate polymer
nanoparticles with metal nanoparticles via condensing, (b) a TEM micrograph of P4VP
nanoparticles decorated with gold nanoparticles, and (c) a magnified view of a representative
nanoparticle.

4.4 Encapsulated Nanoparticle Dispersions
We can combine the iCVD and sputtering processes to encapsulate nanoparticle
dispersions within polymer shells by coating liquid marbles. The ability to encapsulate
nanoparticles can be useful for applications such as drug delivery and imaging, which require
controlled release of nanoparticles at specific locations.
112
In our process, the chemical
composition and thickness of the polymer shell can be tuned to include functionality such as
thermo-responsiveness or pH-responsiveness to control the release. One approach to fabricate
spherical dispersions which can be encapsulated is to use the concept of liquid marbles.
113,114

Liquid marbles are fabricated by rolling hydrophilic droplets on hydrophobic grains
113
or
particles
114
to stabilize the liquids in a spherical shape. In order to encapsulate gold nanoparticles,
we first sputtered gold onto an IL. We chose 1-ethyl-3-methylimidazolium tetrafluoroborate
63
[emim][BF4] because of its ability to form nanoparticle dispersions.
45
A 10 µL droplet of this
solution was rolled in 35 µm diameter PTFE particles to fabricate spherical marbles. (Figure 4-
3a). Poly(1H,1H,2H,2H-perfluorodecyl acrylate-co-ethylene glycol diacrylate) P(PFDA-co-
EGDA) was deposited onto the marbles via the iCVD process in order to create a polymer shell.
36

The marbles were coated on a bed of PTFE particles to prevent the marbles from attaching to the
Petri dish during deposition (Figure 4-3b). 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) was
chosen as the monomer because poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) has a
positive spreading coefficient and therefore a film is formed at the vapor-liquid interface.
26,36
The
cross-linking molecule ethylene glycol diacrylate (EGDA) was copolymerized with PFDA to
increase the robustness of the resulting polymer shell. The PTFE particles on the surface of the
marble get incorporated into the polymer shell which enhances the mechanical strength of the
shell. XPS analysis showed that the atomic composition of the top (polymer-vapor) and bottom
(polymer-liquid) side of the shell consisted of only carbon, oxygen, and fluorine, and no gold was
present in the shell. Figure 4-3c shows the order of the steps reversed by first fabricating the liquid
marbles from pure IL and then subsequently depositing gold before performing polymer
deposition. The resulting encapsulated marble is darker, indicating that the gold remains mostly
on the surface of the marble likely because the PTFE particles act as a solid barrier, preventing
most of the sputtered gold from getting incorporated into the IL (Figure 4-3d). We performed TEM
analysis on the liquid extracted from the marbles which confirmed a much higher concentration of
gold nanoparticles when the marble is fabricated by first sputtering gold onto the IL versus when
gold is sputtered after marble formation (Figure 4-3e and 4-3f). Additionally, the extracted liquid
was also spotted onto chromatography paper. There are more nanoparticles when sputtering occurs
before marble formation, as evidenced by the darker area on the paper (insets of Figures 4-3e and
64
4-3f). We can easily encapsulate ~500 nanoparticle dispersions of 10 µL each in a single deposition
since our reactor chamber has a deposition area of ~400 cm
2
.

Figure 4-3. (a) Schematic diagram of the process to encapsulate dispersed gold nanoparticles
within a polymer shell and (b) the resulting encapsulated marble. (c) Schematic diagram of the
process with the steps reversed and (d) the resulting encapsulated marble. (e) TEM micrograph of
the extracted liquid from the coated marble in 3b and the liquid blotted on chromatography paper
(inset). (f) TEM micrograph of the extracted liquid from the coated marble in 3d and the liquid
blotted on chromatography paper (inset).

4.5 Gels with Embedded Nanoparticles
An advantage of using liquid substrates instead of solid substrates during vapor deposition
is the ability of precursors to absorb into the liquid. Our previous work showed that [emim][BF4]
readily absorbs the monomer 2-hydroxyethyl methacrylate (HEMA), which can be subsequently
polymerized to form a gel bead.
39
We can use this absorbing system to embed dispersed
65
nanoparticles within gels as shown in Figure 4-4a. Gold was first sputtered onto [emim][BF4] to
create a nanoparticle dispersion. In order to keep this dispersion spherical during iCVD coating,
we fabricated a hydrophobic surface by coating chromatography paper with PPFDA. The
hydrophobicity of the coated paper was achieved through the combination of the roughness of the
paper and the low surface energy of the PPFDA polymer.
115
The contact angle of [emim][BF4]
with and without dispersed gold on the PPFDA coated paper was 130º. After the gold dispersion
was placed on the coated paper, HEMA monomer was absorbed into the spherical droplet and was
subsequently polymerized by introducing free radicals (Figure 4-4a). We chose to first absorb the
monomer before polymerization because simultaneous introduction of the monomer and free
radicals results in PHEMA deposition onto the PPFDA coated paper which changes the paper back
to hydrophilic and thereby causes the IL droplet to spread. By sequentially absorbing monomer
and then subsequently polymerizing, the gel beads maintained the same contact angle (130º) after
fabrication (Figure 4-4b). The presence of gold within the gel was confirmed via EDS analysis.
Reversing the steps by sputtering onto a gel bead after polymerization instead of adding the gold
before absorption causes the gel bead to wet through the chromatography paper since the surface
properties of the PPFDA coated paper changes back to hydrophilic during sputter deposition
because of the high surface energy of the metal (Figure 4-4c). However, the spreading of the gel
bead is inhibited due to the viscoelastic properties of the gel,
38
evidenced by the hemispherical
shape. We expect that the gold remains at the surface of the gel bead in this case because the
gelation process increases the viscosity of the IL droplet, preventing the submersion of the
sputtered gold.
50
Therefore, in order to embed gold nanoparticles within a gel bead, the gold must
first be dispersed within the IL droplet before monomer absorption and subsequent polymerization.
The ability to embed metal nanoparticles within gel networks is useful for applications in catalysis,
66
electronics, optics, and sensing.
116–118
However, common synthesis methods lack versatility in
changing shape of the gel, limiting their applications.
118
In contrast, we can easily extend our
process to other geometries, such as thin films or microspheres.

Figure 4-4. (a) Schematic of the process to embed gold nanoparticles within gel beads, (b)
stereoscope (left) and contact angle (right) images of the resulting gel bead, and (c) stereoscope
(left) and contact angle (right) images of the gel bead that is formed when the gold is sputtered
after the formation of the gel bead.

4.6 Conclusion
In conclusion, we showed that we can combine two vapor phase deposition processes,
iCVD and DC magnetron sputtering, to create novel hybrid materials with embedded inorganic
nanoparticles. Polymer nanoparticles decorated with metal nanoparticles were fabricated by using
a low viscosity and low surface energy silicone oil which caused both the metal nanoparticles and
polymer nanoparticles to submerge. We showed that the size of the decorated particles could be
increased by condensing the monomer during the iCVD process. Dispersions of gold nanoparticles
were encapsulated by fabricating liquid marbles and coating the marble with a cross-linked
fluoropolymer. We also embedded gold nanoparticles within gel beds by sputtering metal onto an
ionic liquid, absorbing monomer into the resulting dispersion, and polymerizing the monomer. The
67
versatility of the iCVD and sputter deposition methods allows for the fabrication of other
functional hybrid materials. For example, the functionality of the monomer can be varied to make
polymers with properties such as thermo- or photo-responsiveness and the inorganic phase can be
independently tuned by switching the sputtering target to another metal such as silver or platinum.
4.7 Acknowledgments
M. D. L. was supported by a National Science Foundation Graduate Research Fellowship
under grant DGE-1418060.

68

Chapter 5: Conclusions & Future Work

69
5.1 Conclusions
The first part of this thesis studied the fabrication of grafted functional polymer coatings
onto Parylene C via photoinitiated chemical vapor deposition for biomedical applications. This
work showed that by using a partially unreacted, cross-linked intermediate layer, it is possible to
achieve a top functional layer that is covalently attached to the underlying Parylene C substrate.
By using a type-II photoinitiator, free-radicals are generated on the Parylene C surface after the
photoinitiator abstracts a labile hydrogen atom from the surface. While Parylene C was the sole
substrate in this study, the process can be extended to any substrate with a labile hydrogen atom,
such as PDMS or PTFE. Additionally, the generality of the procedure was demonstrated by
grafting two different functional polymer coatings. The hydrophilic polymer, poly(vinyl
pyrrolidone) was grafted and the hydrophilicity was demonstrated in both a dry and wet
environment. The photo-responsive polymer, poly(o-nitrobenzyl methacrylate), was grafted and
the photo-responsiveness was demonstrated by showed a change in functionality from
hydrophobic (before UV irradiation) to hydrophilic (after UV irradiation). It must be emphasized
that this process may be used to graft other functionalities to the underlying substrate, such as the
hydrogel poly(2-hydroxyethyl methacrylate) or the thermo-responsive poly(N-isopropyl
acrylamide).
The second part of this work focused on sputter deposition of gold and silver onto low
vapor pressure liquids and the effects of the liquid viscosity and surface tension. The liquid
viscosity is important because the rate at which the liquid wets the metal is inversely proportional
to liquid viscosity. In low viscosity silicone oils, the sputtered metal forms dispersed nanoparticles
because the sputtered nanoparticles submerge into the bulk liquid quickly. In high viscosity
silicone oils, the rate at which the liquid wets the metal slows significantly, allowing for adparticle
deposition of the sputtered metal. The slowed rate of wetting allows for dense films to form on the
70
liquid surface. At high surface tension, the rate at which the liquid wets the metal increases.
Ramified aggregates formed rather than dense surface structures at high surface tensions because
it was more energetically favorable for the metal to aggregate in high surface tension liquids. In
the high surface tension liquids tested, the viscosity of the liquid increased with the surface tension
so that the structures became denser as the surface tension increased. However, the structures were
comprised of ramified aggregates when compared to liquids with similar viscosities but much
lower surface tensions.
The last part of this work describes the fabrication of hybrid materials in a variety of
geometries. This work used dispersed metal nanoparticles as the inorganic phase. Polymer
nanoparticles decorated with gold nanoparticles were created by sputtering gold onto a low
viscosity silicone oil and depositing a submerging particle forming system via iCVD. The average
polymer particle diameter was increased by adjusting the iCVD parameters. Additionally, metal
nanoparticles were sputtered onto an ionic liquid to form stable nanoparticle dispersions. By using
a film-forming polymer system, encapsulated nanoparticle dispersions were fabricated by using
the concept of liquid marbles. By using a monomer soluble in the ionic liquid, gel bead with
embedded nanoparticles were formed. The sequential sputter deposition and iCVD process
developed in this work can be modified to form hybrid films by simply sputtering onto a higher
viscosity liquid. Our understanding of the liquid interactions with both the sputtered metal and
polymer deposited via iCVD is key to developing new reactor designs that have the capability to
deposit inorganic and organic phases sequentially without breaking vacuum or simultaneous. This
will improve upon our understanding of the interaction of the high surface energy metal with the
low surface energy polymer when deposited onto liquid substrates.
71
5.2 Future Work
This work studied the effects of combining the iCVD process with high energy radicals
and high surface energy materials. This work will encourage and guide the development hybrid
reactor designs that can deposit both inorganic and organic phases, either simultaneously or
sequentially. One proposed design is to use an RF power source to generate a plasma within an
iCVD reactor. Since a typical iCVD reactor uses a copper lead to connect an external DC power
source to resistively heat the filament array, we can use one of these leads to send an RF signal
(13.56 MHz) through the filament array while the other lead is grounded. Then, an inert gas, such
as Ar or N2, can be flown into the iCVD reactor to be ionized and to generate the plasma. The
plasma can initiate polymerization through a process known as plasma-enhanced chemical vapor
deposition (PECVD). As an example, Figure 5-1 shows an RF-generated Ar plasma within the
iCVD reactor used in this work. In addition to depositing novel chemistries, such as
organosilicons
119
and hybrid layered coatings,
120
the plasma can be used to generate free-radicals
on the surface of the substrate. These free radicals can enable the grafting of polymers deposited
via iCVD without breaking vacuum or without using a high energy radical, such as the type-II
photoinitiator used in this work.

72

Figure 5-1. Argon plasma generated within an iCVD reactor. The plasma is generated by sending
a 50 W, 13.56 MHz RF signal through the filament array using an external RF power source and
matching network.

We can also modify our iCVD reactor by incorporating a metal target and magnetron to
enable the sputtering of metal in situ. To study the effects of sequential or simultaneous deposition,
our reactor can be fitted with a second filament array, which can be placed into the reactor to be
resistively heated to thermally cleave the TBPO radical to initiate the standard iCVD process
(Figure 5-2). By sequentially depositing metals and polymers, we can form many of the hybrid
structures demonstrated in this work in an efficient manner because the pump-down and prep time
are greatly reduced. Additionally, more complex structures such as hybrid layered thin films can
be formed on substrates that require much milder deposition conditions, such as electronics and
biomedical implants. Simultaneous deposition of sputtered metal and iCVD deposited polymers
could enable us to understand the fundamentals of how high surface energy metals interact with
low surface energy materials at a molecular scale. I hypothesize that these hybrid thin films could
have properties of typical hybrid structures without the drawbacks or current hybrid structures,
such as phase separation or delamination, because of the possible molecular intercalation. Lastly,
by depositing both metal and polymers onto low vapor pressure liquids simultaneously, other
73
unique hybrid structures could arise. Future work will study and characterize these interactions
and structures, furthering our understanding of hybrid materials and the processing conditions
required to achieve certain properties and morphologies.

Figure 5-2. Proposed hybrid reactor set up. Filament 1 is connected to an external RF power supply
and matching network to generate and control the plasma. The plasma is localized near the metal
target using a magnetron to perform RF magnetron sputtering. Filament 2 is connected to an
external DC power supply and resistively heated to enable iCVD.

74
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Abstract (if available)

Abstract This dissertation details the fabrication of grafted coatings and inorganic/organic hybrid materials deposited via initiated chemical vapor deposition (iCVD) and DC magnetron sputtering. Chapter 1 introduces vapor phase deposition techniques, in particular iCVD and DC magnetron sputtering. Background on the use of low vapor pressure liquids in both the iCVD and DC magnetron sputtering processes is discussed. Lastly, the utility and current fabrication methods of hybrid materials are introduced. Chapter 2 focuses on modification of the iCVD process by using a photoinitiator to covalently attach functional polymer coatings onto Parylene C. The grafted coatings are characterized using X-ray photoelectron spectroscopy and contact angle goniometry. Chapter 3 studies the effects of viscosity and surface tension of gold and silver sputtered onto low vapor pressure liquids. A morphological phase diagram is given to guide the selection of the required liquid properties for a desired morphology. Chapter 4 demonstrates the facile fabrication of inorganic/organic hybrid materials by combining DC magnetron sputtering and iCVD onto low vapor pressure liquids. Using the various metal-liquid and polymer-liquid interactions, the fabrication of several hybrid materials is demonstrated. Chapter 5 discusses the conclusions derived from this work and how this work can impact and guide future hybrid reactor designs.

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Asset Metadata

Creator De Luna, Mark Miguel (author)

Core Title Inorganic/organic hybrid materials and grafted coatings via vapor phase deposition processes

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Chemical Engineering

Publication Date 09/23/2019

Defense Date 06/24/2019

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

Tag grafting,hybrid,OAI-PMH Harvest,polymers,sputtering,vapor deposition

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Gupta, Malancha (committee chair), Nakano, Aiichiro (committee member), Ravichandran, Jayakanth (committee member)

Creator Email markdelunacu2@gmail.com,mmdeluna@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-223020

Unique identifier UC11675073

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Legacy Identifier etd-DeLunaMark-7812.pdf

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Document Type Dissertation

Rights De Luna, Mark Miguel

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Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA