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Understanding organic and inorganic semiconductor surfaces by coherent nonlinear spectroscopy

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Understanding organic and inorganic semiconductor surfaces by coherent nonlinear spectroscopy

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Content UNDERSTANDING ORGANIC AND INORGANIC SEMICONDUCTOR
SURFACES BY COHERENT NONLINEAR SPECTROSCOPY
by
Purnim Dhar
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)
May 2016
Copyright 2016 Purnim Dhar
To all who helped me see this through...
ii
Abstract
Although molecularly thin in nature, the surfaces and interfaces play an important role in
many physical and chemical processes occurring around us. From industrially important
processes such as heterogeneous catalysis and charge generation in organic photovoltaic
materials to the basic metabolic mechanisms such as respiration - all of them take place
at some interfaces, either living or dead. But it is often difficult to probe those interfaces
because of their buried nature, complex environment, and problem in isolating signals
from the bulk. A molecular level understanding of the structure and organization of
the surfaces and interfaces is necessary to gain insights into such processes. Even-
order nonlinear spectroscopic techniques such as second harmonic generation (SHG)
and sum frequency generation (SFG) possess surface selectivity and thus can specifically
provide information about the surface and surface-bound processes. In this dissertation,
vibrational sum frequency generation spectroscopy is used as a surface selective probe to
study a variety of inorganic and organic semiconductor surfaces that have applications in
energy conversion, communication, computing, catalysis, sensing, biotechnology, and
life sciences. A systematic understanding of the interrelationship between the structure
iii
and property of these semiconductor surfaces is sought throughout the course of this
work.
iv
Acknowledgments
Graduate school has been a tremendous learning experience and a place of self discovery
for me. I will always remember it as one of the most defining chapters of my life.
My graduate life would not be the same without the continuous support and encour-
agement from my research advisor Alex Benderskii. I am truly indebted to him for
giving me the right platform and guidance to prosper - both intellectually and person-
ally. Alex is an absolutely fantastic mentor who not only taught me how science is done,
but was there during the highs and lows of my graduate life. He gave me utmost freedom
to try different projects in his lab and guided me throughout with his unique scientific
insights. I have thoroughly enjoyed working with him.
The lively scientific environment of the physical chemistry department of USC had
been a delightful experience for me, mainly because of the incredibly talented and help-
ful professors it has. I especially feel lucky to be a part of the wonderful nonlinear spec-
troscopy community of SSC captained by Professors Steve Bradforth, Alex Benderskii
and Jahan Dawlaty. Steve is an excellent teacher and has always set a high standard
v
of clarity of thought and expression. I have always admired his knowledge and pub-
lic speaking prowess. Jahan always had a listening ear for my ever-so-dumb scientific
questions and was enthusiastic about everything on this face of earth. I enjoyed having
scientific discussions with him. This list would not be complete without thanking Pro-
fessors Hanna Reisler and Anna Krylov, and teaching faculty Dr. Jessica Parr for letting
me teach their Advanced General Chemistry courses. Especially, Hanna, who always
had her door open for me for help and advice.
During my early days at USC, I was fortunate to work with some really talented
graduate students and post-docs in the Benderskii lab. Sergey Malyk and Fadel Shalhout
were patient enough to teach a theoretical chemist the basics of nonlinear spectroscopy.
Sergey was a master of the lasers and optics in lab 616 and taught me a great deal of
instrumentation. He also kept me entertained with a constant supply of funny Russian
videos. Ian Craig educated me about organic electronics in my early years. Here, I
would like to specially thank Sean Roberts, who was a joint post-doc of Steve and Alex,
for introducing me to the world of ultrafast spectroscopy of organic photovoltaics. I
performed my first few experiments under his guidance. Sean was really helpful during
my preparation for the qualifying exam. He was a person of great scientific intellect and
working with him was a real pleasure for me. David Valley helped me in some of my
organic photovoltaic experiments and shared his Mathematica tricks. And of course, I
would like to thank my friend and colleague, Mikhyl Vinakyn. Misha and I started our
graduate school together and I am glad that I shared lab and office space with him. His
vi
enthusiasm for science, curiosity and sense of humor were infectious. I have a great deal
of respect for him.
As I moved to the senior graduate student position, I took the role of helping and
mentoring students in the Benderskii lab. Here, I met a talented bunch of students over
the course of next few years. Chayan Dutta, Dhritiman Bhattacharya, Angelo Mon-
tenegro, and Muhammet Mammetkuliyev always kept the lab environment happy and
fun-filled. Amidst these insane bunch of people, only Chayan helped in keeping some
sanity in the lab. Dhritiman took over some of my projects and I am sure that he is going
to do very well in them. Angelo and Muhammet were the newest addition to the water
family and I wish them success. I will fondly remember Angelo’s wedding ceremony
where we infused Indian wedding flavor with some mad Bollywood dancing. It was way
too much fun. I will cherish Angelo and his wife, Kim’s friendship. I would also like to
thank my talented REU summer student Shuhui Yin with whom I did some of the works
of my organic light emitting diode project.
I am also thankful to all the SSC 6th and 7th floor folks who had made my graduate
life truly enjoyable. I am glad that I got to know Shayne Sorenson, Eric Driscoll, Shima
Haghighat, Konstantin Kudinov, Jimmy Joy, Robert Siedel, Lee Ch’ng, and the mem-
bers of our small SSC Indian community, Anirban Roy, Saptaparna Das, Amit Samanta,
Parichita Majumdar, Anishka Samanta, Gaurav Kumar, Subhasish Sutradhar, Chinran-
tha Rodrigo, Saugata Pal, and Bibek Samanta during my stay at USC. I am happy to have
met and been friends with Anirban and Saptaparna for the past eleven years. Amit was
vii
always there for me for all the scientific and non-scientific discussions and had given me
valuable inputs in my research time to time. He also helped me a lot in proof-reading my
thesis. I am going to fondly remember my 10-minute coffee breaks with Gaurav which
would eventually last for an hour with discussions ranging from the plights of graduate
students to Indian politics and economy.
None of the works described below would have been possible without the help of my
wonderful collaborators who were gracious enough to provide me with all the materials
and keep me out of the synthesis lab. I would like to thank Professor Barry Thompson
and his students, Petr Khlyabich, Beate Burkhart, Alia Latif for the Chapter 3 work;
Professor Mark Thompson, his students, Yifei Liu, Andrew Bartynskii, Matthew Jurow,
and his research scientist, Dr. Peter Djurovich for the Chapter 4 and 5 work; Professor
Nathan Lewis and his student, Noah Plymale for the Chapter 6 work; Professor Jongse-
ung Yoon and his student, Sung-Min Lee for the work on silicon solar cell (not described
in the thesis). I have enjoyed an interdisciplinary research experience at USC because
of my excellent collaborators.
I just can not fail to thank our department’s administrative assistants, Michele Dea,
Magnolia Benitez, Valerie Childress, and Katie McKissick, who, over the years, made
my graduate life totally stress-free. Michele needs a special shout out for being the go-to
person for any administrative work that ever exists for a graduate student.
I can not say enough about my awesome roomamtes, Atanu Acharya, Subodh Tiwari
and Piyush Deokar, who had always made sure that I was having a great day no matter
viii
how bad the situation was in the school. There was hardly any dull moment with them
being around. From our midnight errands in the streets of LA to satiate our after-dinner
food craving to the impromptu trips to the distant places in California and endless hours
of Borderland, the list of our activities were endless. While I am excited to move on to
the next chapter of my life, my heart sinks with the thought of leaving them all.
I want to take this opportunity to thank my parents, Aruna Dhar and Kamalendu
Dhar, who have always stressed the importance of being a good person above everything
else. They have instilled in me a strong sense of right and wrong from a very early age
and those traits are my absolute strengths in whatever I do in my life. Without their
sacrifices, I would not be here today. I would also like thank my sister, Ankita Dhar, and
sister-in-law, Dyuti Coomar, for their constant love and support in all my endeavors.
Finally, I would like to acknowledge Arunima Coomar, my wonderful wife and
partner-in-all-crime for her continuous support, understanding and love for all these
years. She has taught me that any problem on earth can be solved with a generous dose
of chocolate, genuineness and a smile on the face. Because of her, I have more reasons
to smile now. Life has been rock-n-roll so far with her being around and I can not wait
to explore more.
Purnim Dhar
May 2016
Los Angeles, CA
ix
Table of Contents
Dedication ii
Abstract iii
Acknowledgments v
List of Tables xiii
List of Figures xv
Abbreviations xxii
Chapter 1: Introduction 1
1.1 Surfaces and interfaces: A general perspective . . . . . . . . . . . . . . 1
1.2 Interfaces in organic and inorganic semiconductor materials . . . . . . . 4
1.3 Characterization of semiconductor surfaces . . . . . . . . . . . . . . . 7
1.4 Second order nonlinear spectroscopy - The surface probe . . . . . . . . 10
1.4.1 Surface selectivity of the second order processes . . . . . . . . 12
1.4.2 Sum frequency generation spectroscopy . . . . . . . . . . . . . 13
1.5 Outline of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.6 Chapter 1 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Chapter 2: Vibrational Sum Frequency Generation Spectroscopy: Formal-
ism and Experimental Details 23
2.1 Vibrational sum frequency generation spectroscopy . . . . . . . . . . . 23
2.1.1 Second order susceptibility and selection rule . . . . . . . . . . 23
2.1.2 Polarization dependencies in VSFG . . . . . . . . . . . . . . . 26
2.2 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.3 Chapter 2 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Chapter 3: Annealing-Induced Changes in the Molecular Orientation of Poly-
3-hexylthiophene at Buried Interfaces 38
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
x
3.2 Morphology and structure of P3HT at buried interfaces . . . . . . . . . 39
3.3 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.3.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.3.2 Sample preparation and handling . . . . . . . . . . . . . . . . . 42
3.3.3 FTIR and Raman measurements . . . . . . . . . . . . . . . . . 42
3.3.4 VSFG spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 43
3.4 Results and discussions . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.4.1 FTIR and Raman studies of rrP3HT films . . . . . . . . . . . . 44
3.4.2 VSFG studies of rrP3HT films at buried interfaces . . . . . . . . 44
3.4.2.1 rrP3HT films at rrP3HT/SiO
2
interface . . . . . . . . 46
3.4.2.2 rrP3HT films at rrP3HT/AlO
X
interface . . . . . . . . 48
3.4.3 Orientational analysis . . . . . . . . . . . . . . . . . . . . . . . 50
3.4.3.1 Hexyl side chain of rrP3HT . . . . . . . . . . . . . . 50
3.4.3.2 Backbone of rrP3HT at P3HT/SiO
2
interface . . . . . 52
3.4.3.3 Backbone of rrP3HT at P3HT/AlO
X
interface . . . . . 57
3.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.6 Appendix A: Control experiments showing VSFG is generated from the
buried interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.7 Appendix B: Annealing induced changes in the alkyl stretches of rrP3HT 64
3.8 Appendix C: Contribution of C-H bending modes to the C=C symmetric
stretching lineshapes . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.9 Appendix D: Orientational analysis: Hyperpolarizability ratios for the
thiophene backbone at rrP3HT/SiO
2
interface . . . . . . . . . . . . . . 67
3.10 Appendix E: Orientational analysis: Hyperpolarizability ratios for the
thiophene backbone at rrP3HT/AlO
X
interface . . . . . . . . . . . . . . 69
3.11 Appendix F: Tables for data fitting . . . . . . . . . . . . . . . . . . . . 71
3.12 Chapter 3 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Chapter 4: Molecular Orientation of Typical Light Emitting Diode Materials 81
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.2 Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.3 Results and discussions . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.3.1 Spectroscopic characterizations of CBP film . . . . . . . . . . . 88
4.3.2 Thickness dependence VSFG study of CBP films . . . . . . . . 90
4.3.3 Modeling optical interference for CBP films . . . . . . . . . . . 91
4.3.4 VSFG studies of NPD and DIP films . . . . . . . . . . . . . . . 96
4.3.5 Rotational anisotropy studies of CBP and NPD surfaces . . . . . 98
4.3.6 On the origin of structural anisotropicity at the surfaces . . . . . 99
4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.5 Chapter 4 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
xi
Chapter 5: Beyond Electric-Dipole Approximation: Sum Frequency Gener-
ation from Centrosymmetric Molecules 110
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
5.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5.2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5.2.2 FTIR and Raman measurements . . . . . . . . . . . . . . . . . 113
5.2.3 VSFG spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 113
5.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 114
5.3.1 FTIR, Raman and VSFG studies of CBP films . . . . . . . . . . 114
5.3.2 On the origin of sum frequency response . . . . . . . . . . . . . 115
5.3.2.1 Symmetry breaking of CBP molecules at the Interface 116
5.3.2.2 Surface vs. bulk contribution: Higher-order response . 117
5.3.2.3 Polarization dependence of bulk response . . . . . . . 119
5.3.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
5.4 Appendix A: Crystal structure of CBP . . . . . . . . . . . . . . . . . . 123
5.5 Chapter 5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
Chapter 6: Vibrational Sum Frequency Spectroscopic Investigation of Func-
tionalized Si(111) Surface 129
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
6.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
6.2.1 Materials and methods . . . . . . . . . . . . . . . . . . . . . . 132
6.2.1.1 Preparation of H-Si(111) surfaces . . . . . . . . . . . 133
6.2.1.2 Preparation of Cl-Si(111) surfaces . . . . . . . . . . . 133
6.2.1.3 Preparation of CH
3
-CC-Si(111) surfaces . . . . . . 133
6.2.2 VSFG spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 134
6.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 134
6.3.1 VSFG studies of CH
3
-CC-Si(111) surfaces . . . . . . . . . . 134
6.3.2 Anisotropy studies of CH
3
-CC-Si(111) surfaces . . . . . . . . 137
6.3.3 Effect of annealing on adsorbate-substrate coupling interaction . 141
6.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
6.5 Appendix A: VSFG spectra of baked CH
3
-CC-Si(111) samples . . . . 145
6.6 Appendix B: VSFG spectra of baked CH
3
-CC-Si(111) samples . . . . 146
6.7 Appendix C: Tables for data fitting . . . . . . . . . . . . . . . . . . . . 147
6.8 Chapter 6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
xii
List of Tables
3.1 Fitting parameters for the SPS and SSP spectra from the SI of Ref 8. . . 65
3.2 VSFG fitting parameters for the alkyl stretches of unannealed rrP3HT
samples on SiO
2
for PPP and SSP polarization combinations. . . . . . 71
3.3 VSFG fitting parameters for the ring modes of unannealed and annealed
rrP3HT samples on SiO
2
for PPP, SSP and SPS polarization combina-
tions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.4 VSFG fitting parameters for the ring modes of unannealed and annealed
rrP3HT samples on AlO
X
for PPP, SSP and SPS polarization combina-
tions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.5 Optical parameters used in VSFG orientational analysis and Fresnel
coefficient calculations. . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.1 Calculated quadrupole moments of CBP and NPD. Calculations were
performed using the Jaguar 8.5 (release 13) software package on the
Schrodinger Material Science Suite (version 2014). Gas-phase geom-
etry optimization was calculated using the B3LYP functional with the
6-31g

basis set as implemented in Jaguar. Quadrupole moments of
C
6
H
6
and C
6
F
6
are also tabulated for comparison. . . . . . . . . . . . 120
6.1 Fitting parameters for the rotational anisotropy in the resonant ampli-
tudes of the PPP spectra. . . . . . . . . . . . . . . . . . . . . . . . . . 147
6.2 Fitting parameters for PPP spectra for as prepared CH
3
-CC-Si(111)
samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
6.3 Fitting parameters for PPP spectra for baked CH
3
-CC-Si(111) sam-
ples at 320

C under vacuum Values for only the CH
3
symmetric and
asymmetric stretches are given. . . . . . . . . . . . . . . . . . . . . . 149
6.3 Fitting parameters for PPP spectra (Continued).. . . . . . . . . . . . . 150
xiii
6.3 Fitting parameters for PPP spectra (Continued).. . . . . . . . . . . . . 151
xiv
List of Figures
1.1 (A) Important interfaces in an organic bulk heterojunction (BHJ) solar
cell that is composed of an electron donor and an electron acceptor mate-
rial (Figure is adapted from Phys. Chem. Chem. Phys., 2009, 11,
2575-2591). (B) and (C) Functionalized semiconductor surfaces have
applications in semiconductor technology, energy conversion, molecu-
lar electronics and catalysis (Figure by Noah T. Plymale, Caltech). . . . 5
1.2 The optical process describing sum frequency generation from a flat sur-
face is shown above. SFG is a second order nonlinear optical process
where two laser beams having frequenciesw
1
andw
2
interact to gener-
ate a second order polarization P
(2)
at the surface that radiates a signal
field at the sum of two input frequencies (w
SF
=w
1
+w
2
) in a phase-
matched direction. Because of the cetrosymmetric nature of the bulk,
P
(2)
is bulk-forbidden under the electric-dipole approximation. Thus,
sum frequency generation spectroscopy can be used to selectively probe
surfaces and interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.3 The figure shows the process of sum frequency generation at a molecular
level. The left panel shows the interactions involved in vibrational sum
frequency generation (VSFG) process. The first interaction in VSFG is
a resonant infrared interaction that excites a molecular vibration. This
is then followed by a nonresonant interaction with a visible photon that
induces a second order polarization in the medium. Subsequently, the
molecule returns to its ground state and emits a sum frequency photon
which carries information about the surface. The left panel describes
the interactions involved in electronic sum frequency generation (ESFG)
process. In case of ESFG, the two interactions lead to an electronic
transition in a system. ESFG provides information about the surface
electronic states which can be very different from that of the bulk. . . . 14
2.1 Illustrations of vibrational sum frequency generation spectroscopy from
a surface and the associated molecular process. . . . . . . . . . . . . . 24
xv
2.2 Schematic showing the surface-fixed and the molecule-fixed coordinate
system used in the vibrational sum frequency generation experiments.
The polarizations (P or S) of the incoming and outgoing beams with
respect to the coordinate system are also shown. The surface is assumed
to be rotationally invariant (C
¥
v symmetry). . . . . . . . . . . . . . . . 27
2.3 A three-layer model for the interface VSFG showing the experimental
parameters and refractive indices. The interfacial layer is assumed to be
molecularly thin in nature and has an effective refractive index n
0
which
is different from that of the bulk regions. n
0
is unknown parameter in the
calculation of the Fresnel factors and has to be estimated for the material
system under study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.4 The above diagram describes how one can get quantitative molecular
information from experimental VSFG spectroscopy. The macroscopic
second order susceptibilities (c
(2)
i jk
) and the microscopic molecular hyper-
polarizabilities (b
(2)
abc
) can be assessed via VSFG spectroscopic measure-
ments and ab initio calculations, respectively. The two can then be
related via coordinate transformation which gives orientation informa-
tion of the molecules present at the surface. . . . . . . . . . . . . . . . 31
2.5 Schematic of the vibrational SFG setup. . . . . . . . . . . . . . . . . . 32
2.6 (A) Cross-correlation image of the femtosecond IR pulse and picosec-
ond visible pulse. (B) Temporal profile of the narrowband visible pulse
after the etalon. (C) Frequency-domain spectrum of the visible pulse
having a FWHM17 cm
1
. . . . . . . . . . . . . . . . . . . . . . . . 33
2.7 IR spectrum in CH stretch region taken on MCT detector. Two Gaus-
sians are used to fit the experimental spectrum (black line). The IR pulse
has a FWHM300 cm
1
. . . . . . . . . . . . . . . . . . . . . . . . . 34
xvi
3.1 FTIR (red) and Raman (blue) spectra showing vibrational resonances
of the C-C (A) and C-H (B) stretching regions of as-cast rrP3HT films
on CaF
2
, respectively. The C-C and C-H stretching modes are high-
lighted in different colors so that they can be identified easily in VSFG
spectra later. Resonant vibrational modes in the ring stretching region
are identified as C-C inter-ring stretch (blue band,1380 cm
1
), C=C
symmetric stretch (orange band,1450 cm
1
), and C=C antisymmet-
ric stretch (green band,1510 cm
1
). The main C-H stretching modes
are assigned as CH
2
symmetric stretch, d+ (pink band,2854 cm
1
),
CH
3
symmetric stretch, r
+
(yellow band,2885 cm
1
), CH
2
Fermi
resonance (FR) (dark green band,2925 cm
1
), and CH
3
asymmet-
ric stretch, r

(violet band,2955 cm
1
). The structure of rrP3HT is
shown in the inset in panel A. . . . . . . . . . . . . . . . . . . . . . . . 45
3.2 VSFG spectra of the hexyl side chains of P3HT spuncast on SiO
2
for
PPP and SSP polarization combinations. The spectra show CH
2
d
+
(blue
band,2850 cm
1
), CH
3
r
+
(orange band,2876
1
), and CH
3
r

(violet band,2963 cm
1
). . . . . . . . . . . . . . . . . . . . . . . . . 47
3.3 Annealing induced changes in the VSFG spectra showing ring modes
of rrP3HT at the P3HT/SiO
2
buried interface for SSP, SPS, and PPP
polarization combinations. The vibrational modes are identified as C-
C inter-ring stretch (1380 cm
1
) and C=C symmetric stretch (1440
cm
1
). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.4 VSFG spectra of unannealed and annealed rrP3HT thin films on AlO
X
collected with SSP, SPS, and PPP polarization combinations. The spec-
tra consist of the C=C symmetric (1440 cm
1
) and C=C asymmetric
(1510 cm
1
) stretches. Significant changes in the intensity as well as
narrowing of the resonant modes are observed following thermal anneal-
ing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.5 The coordinate system for the CH
3
moiety assuming C
3v
symmetry
where q is the tilt angle of the CH
3
group with respect to the surface
normal. The tilt angle of the main chain axis is different from that of the
CH
3
group. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.6 Calculated values of the susceptibility tensor ratio for the CH
3
group of
the hexyl side chain of rrP3HT as a function of both the tilt angle (q)
and refractive index of the interfacial layer (n
0
) for as-spun P3HT on
SiO
2
. The blue rectangle shows physically acceptable solutions for the
experimental values (black dotted line). The average tilt angle of the
CH
3
moiety found from the analysis lies in the range of 52

q 28

. 52
xvii
3.7 The direction of the net transition dipole moment of the P3HT dimer
and its relation to the molecular frame of reference are shown . The
dipole moment vector is orthogonal to the average plane of the polymer
backbone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.8 Cartoons showing annealing induced changes in the orientation of the
P3HT backbone at the SiO
2
interface. The direction of the net transition
dipole moment of the P3HT dimer and its relation to the molecular frame
of reference are shown in the top diagram. The dipole moment vector is
orthogonal to the average plane of the polymer backbone. P3HT adopts
an edge-on orientation after thermal annealing at the P3HT/SiO
2
inter-
face with a change in tilt angle of 3-10

. . . . . . . . . . . . . . . . . . 54
3.9 Thermal annealing induces face-on orientation of the P3HT backbone
at the P3HT/AlO
X
interface. The change is more pronounced and in a
direction opposite to those observed for films spin-cast on SiO
2
. . . . . 58
3.10 For thin film VSFG, spectroscopic contribution can arise from both (A)
polymer/air interface as well as from (B) polymer/substrate interface. . . 62
3.11 (A) FTIR spectra of 60 nm and 120 nm thick as-spun rrP3HT samples
on CaF
2
taken in transmission geometry. (B) VSFG spectra of 60 nm
and 300 nm thick unannealed rrP3HT samples on CaF
2
collected for
SSP polarization. The decrease in VSFG intensity for the 300 nm thick
sample indicates that the signal contains contributions from the buried
interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.12 (A) VSFG spectra of as-spun rrP3HT films of different thicknesses on
AlO
X
for PPP polarization. (B) A comparison of PPP spectra between
rrP3HT on different substrates shows a substantial increase in intensity
for rrP3HT on AlO
X
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.13 PPP and SSP spectra of the hexyl side chain of rrP3HT annealed for
different amount of time. . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.14 Comparison between (A) SPS and (B) SSP VSFG data of as-spun DP3HT
on SiO2 reconstructed from the SI of Ref. 8 and our experimental results
of P3HT on SiO
2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
xviii
3.15 Calculated values of the susceptibility tensor ratios for the C=C symmet-
ric stretch of rrP3HT as a function of tilt angle (q), refractive index of
the interfacial layer (n
0
) and tilt angle distribution width (s) for as-spun
P3HT on SiO
2
. The blue bars show physically acceptable solutions for
the experimental values (black dotted line). The average tilt angle is 54

q 62

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.16 Calculated ratios of the susceptibility tensor elements along with the
experimental values give average tilt angle 57

q 73

for annealed
P3HT spin coated on SiO
2
. . . . . . . . . . . . . . . . . . . . . . . . . 68
3.17 Orientational analysis suggests average tilt angle to be in the range 47

q 75

for as-cast P3HT on AlO
X
. The tilt angle distribution width
is also narrower compared to that of P3HT films on SiO
2
. This indicates
better structural reorganization of P3HT backbone in presence of AlO
X
. 69
3.18 The susceptibility tensor ratios give average tilt angle to be in the range
44

q 49

for annealed P3HT on AlO
X
. . . . . . . . . . . . . . . . 70
4.1 (A) FTIR, (B) Raman and (C) Polarization-selective VSFG spectra show-
ing vibrational resonances of a 100 nm thin film of 4,4-bis(N-carbazolyl)-
1,1-biphenyl (CBP) in the ring stretching region. The main vibrational
mode of CBP, which we are interested in is highlighted in the figure
(blue band) for easy comparison between linear (FTIR and Raman) and
nonlinear (VSFG) measurements. The mode is identified as the C=C
symmetric stretching mode localized mainly on the biphenyl backbone
of CBP. The structure of CBP is shown in the inset of Fig.(A). . . . . . 88
4.2 SSP spectra of the vapor deposited CBP films of varied thickness. The
dotted lines represent the VSFG measurements of the same film recorded
on different days. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.3 Optical interference between the signals generated at the buried surface
and the free surface in thin film SFG. The diagram is adapted from Ref-
erence 69. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
xix
4.4 Simulated results of the modulus of the Fresnel factors for the (A) CBP/air
and (B) CaF
2
/CBP interface for SSP polarization combination as a func-
tion of CBP film thickness and refractive index of the interfacial layer.
(C) and (D) Simulated Fresnel factors for these two interfaces for the
n
0
values of 1.35 and 1.60, respectively. The horizontal cuts along n
0
are shown in (A) and (B). (E) Plot showing the total Fresnel factor aris-
ing from the top and bottom surface of the films versus film thickness.
(F) Comparison between the simulated result (red dotted line) and the
observed SSP spectra (blue dots). . . . . . . . . . . . . . . . . . . . . 93
4.5 SSP spectrum of vapor deposited 100 nm NPD film on CaF
2
. The struc-
ture of NPD is also shown in the inset. . . . . . . . . . . . . . . . . . . 96
4.6 SSP spectra of vapor deposited 25 nm DIP film on CaF
2
and SiO
2
. The
structure of DIP is also shown in the inset. . . . . . . . . . . . . . . . . 97
4.7 In plane rotational anisotropy of vapor deposited 100 nm thin films of
CBP (left) and NPD (right) for the SSP spectra of the C=C symmetric
stretch mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
5.1 (a) FTIR, (b) Raman, and (c-f) polarization-selective VSFG spectra show-
ing vibrational resonances of a 100 nm thin film of 4,4-bis(N-carbazolyl)-
1,1-biphenyl (CBP) in the ring stretching region. The film is fabricated
by vacuum vapor deposition on CaF
2
. The structure of CBP is shown in
the inset of Fig. (a). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
5.2 Polarization dependent VSFG spectra of (A) 100nm thin film of NPD
and (B) 25nm thin film of DIP vapor deposited on CaF2. The molecular
structures of NPD and DIP are shown in the inset. The sum frequency
responses for SPS and PSS input/output polarization combinations are
similar for both NPD and DIP. . . . . . . . . . . . . . . . . . . . . . . 121
5.3 Crystal structure of CBP showing non-planarity of the molecule. . . . . 123
6.1 PPP (red) and SSP (blue) polarized VSFG spectra of the propynyl-terminated
Si(111) surface for the methyl stretch region. The time delay between
the IR and the visible pulses was chosen to be 270 fs for the measure-
ments. The corresponding molecular motions are shown alongside the
VSFG spectra. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.2 Polar plot showing the azimuthal dependence of the CH
3
(A) symmetric
stretch (r
+
) amplitude and (B) asymmetric stretch (r

) amplitude of the
PPP spectra of as prepared CH
3
-CC-Si samples for a complete 360

in-plane rotation. The blue solid lines are fits. . . . . . . . . . . . . . . 137
xx
6.3 Polar plot showing the azimuthal dependence of the CH
3
(A) symmetric
stretch (r
+
) amplitude and (B) asymmetric stretch (r

) amplitude of the
PPP spectra of CH
3
-Si samples for a complete 360

in-plane rotation.
The blue solid lines are fits. The figure is taken from Reference 42. . . . 139
6.4 Polar plot showing the azimuthal dependence of the CH
3
(A) symmet-
ric stretch (r
+
) amplitude and (B) asymmetric stretch (r

) amplitude of
CH
3
-CC-Si samples annealed at 200

for 16-20 hours under vacuum.
The blue solid lines are fits. . . . . . . . . . . . . . . . . . . . . . . . . 141
6.5 Annealing causes removal of the few CH
3
-CC- units from the Si(111)
surface creating surface defects. To reduce the steric strain, a CH
3
-CC-
unit adjacent to the void space tilts towards the Si(111) surface. This
results in an increase in coupling between the CH
3
-CC- units and the
Si(111) surface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
6.6 PPP and SSP spectra of the methyl stretch region of CH
3
-CC-Si(111)
samples baked at 320

C for 16-20 hours under vacuum. The decrease
in intensity is ascribed to the removal of the CH
3
-CC- units from the
Si(111) surface. The slight blue shift of the peaks is denoted to the
increased coupling between the propynyl units and the Si(111) substrate
as a result of closer proximity of the adsorbate to the Si surface. . . . . . 145
6.7 Transmission infrared spectroscopy (TRIS) data for CH
3
-CC-Si(111)
surfaces collected at 74

(bottom) and 30

(top) from the surface nor-
mal. (a) High energy region and (b) Low energy region. The high energy
region exhibited three distinct C-H stretching peaks at 2958, 2934 and
2872 cm
1
. The absorbance features at 2934 and 2872 cm
1
were
observed only at the 74

incidence angle, which indicated that those
features arose from modes perpendicular to the surface, whereas the
absorbance at 2957 cm
1
was observed at both angles and was, there-
fore, not perpendicular to the surface. The Figure is taken from Refer-
ence 24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
xxi
Abbreviations
AFM Atomic Force Microscopy
BHJ Bulk Heterojunction
CBP 4,4-bis(N-carbazolyl)-1,1-biphenyl
DIP Diindenoperylene
DFT Density Functional Theory
ESFG Electronic Sum Frequency Generation
FTIR Fourier Transform Infrared Spectroscopy
HREELS High Resolution Electron Energy Loss Spectroscopy
NPD N,N-di(1-naphthyl)-N,N-diphenyl-(1,1-biphenyl)-4,4-diamine
OFET Organic Field Effect Transistor
OLED Organic Light Emitting Didoe
OPV Organic Photovoltaic
PCBM Phenyl-C
61
-butyric acid methyl ester
PES Photoelectron Spectroscopy
PLS Photoluminiscence Spectroscopy
xxii
rrP3HT Regioregular Poly-3-hexyl-thiophene
SHG Second Harmonic Generation
SE Surface Ellipsometry
SEM Scanning Electron Microscopy
SERS Surface Enhanced Raman Spectroscopy
SFG Sum Frequency Generation
SPA Surface Photoabsorption
STM Scanning Tunneling Microscopy
TEM Transmission Electron Microscopy
V ASE Variable Angle Spectroscopic Ellipsometry
VSFG Vibrational Sum Frequency Generation
UHV Ultra High Vacuum
xxiii
Chapter 1: Introduction
1.1 Surfaces and interfaces: A general perspective
Surfaces and interfaces are omnipresent in nature and in most cases the first place of
interaction of a medium or a system with its surrounding environment. They are the
doorway from one medium to another and thus nature has evolved materials and molec-
ular systems to carry out important and complex tasks at various surfaces. The transport
of ions, enzymes and proteins through the cell membrane, the intake and the release of
oxygen and carbon dioxide in the circulating medium of living organisms, the adsorp-
tion and desorption of gases in amorphous solid water present in the interstellar clouds
are the examples of few such processes that are being actively controlled by the surface.
Surface is also the place of broken symmetry and reduced dimensionality. These
have profound implications in the electronic, vibrational and optical properties of the
surface. In the age of ”smart materials”, where artificially tailored materials possess
fascinating and complex properties, fine tuning their performances often require control
over specific surface properties. Processes like molecular recognition, signal transduc-
tion, sensing, catalysis, phase transition, and information processing can be extensively
controlled via the modification of material surfaces. It is thus no surprise that surfaces
and surface-bound processes are the areas of active research in variety of scientific and
technological fields.
1
The uniqueness of the surfaces stems from the fact that the endless repetitions of
atoms and molecules in the bulk come to a halt at the surface. This leads to energy mis-
match, excess force and unsatisfied bonds for the interfacial molecules and also makes
the surfaces, whose spatial extent are of only few atomic layers, a strongly inhomoge-
neous region. The physical and thermodynamic properties properties such as pressure,
viscosity, wettability, conductivity, and refractive index that characterize any surface
layer thus differ considerably than that of the bulk. One such example is the inter-
facial region of neat water, the molecular liquid that we encounter on a daily basis.
The surface structure of water is highly complex with water having unusually high sur-
face tension
1
, an intricate network of hydrogen bonds that fluctuates on femtosecond to
picosecond timescale,
2
and specific surface-bound water structures that are not present
in the bulk.
3, 4
Such diversity often makes it hard to characterize surfaces that play an
essential role in many physical, chemical and biological processes.
A majority of the aforementioned basic and technologically important surface active
processes take place at buried or hidden interfaces; the interfaces that are not exposed
to the air, but rather buried between two bulk media. For example, (1) charge trans-
port in organic field effect transistors (OFETs) is influenced by the polymer structure
and morphology of the first few monolayers at the buried dielectric interface of the gate
electrode;
5–8
(2) the efficiency of charge carrier separation and rate of career recombi-
nation in organic photovoltaics (OPVs) are determined by the conformation and relative
molecular orientations at electron donor/acceptor interfaces;
9–11
(3) the biocompatibility
and proper functioning of the polymeric materials that are used as biomedical implants
rely upon the structure and morphology of the polymeric surfaces that are in contact
with the body.
12–14
. When considering such buried interfaces, the traditional surface
2
characterization techniques that require direct contact of the probing agent with the sur-
faces have limited applicability. The method of choice in such scenario is to somehow
remove one of the intervening bulk media to expose the surface of interest to the probing
agent.
15
The problem with such an approach is that the prepared surface may not be the
true representation of the actual working surface we were interested in, as the chemical
and/or mechanical treatments have altered the structure and morphology of the surface.
Another limitation of such techniques is that they can not be performed in situ because
of their invasive nature.
Among the vast majority of surface analytical techniques that have been developed
over the years, optical techniques have definite advantages owing to their non-perturbing
nature, reliability of use and the ability to characterize in situ. Even in the case of buried
or hidden interfaces, if one of the two bulk media is transparent in the considered spectral
region, optical techniques can provide valuable information regarding those surfaces.
With the advent and commercialization of the lasers, we can routinely produce light
sources with high spectral intensity, monochromaticity and directivity. The application
of laser based surface characterization techniques have revolutionized the field of surface
science.
In this thesis, I present coherent laser-based spectroscopic studies of organic and
inorganic semiconductor surfaces that have a wide range of scientific and engineering
applications. Surface-selective vibrational sum frequency generation technique has been
used to unravel the molecular level picture of the semiconductor surfaces.
3
1.2 Interfaces in organic and inorganic semiconductor
materials
Semiconductor materials are of interest in a variety of fields such as communication,
computing, energy conversion, drug delivery, biotechnology and life sciences.
16–22
The
applications and advances in these fields are continuously being pushed forward by a
systematic understanding of the interrelationship between the structure and property of
the semiconductor materials. The surfaces of these materials often play important roles
in determining the optical, electrical and overall performances of the devices and pro-
cesses. Such interfaces can be varied in nature, ranging from the well-defined single
crystal surfaces to the chemically modified semiconductor surfaces (See Fig. 1.1(B) and
(C)) and extremely heterogeneous donor/acceptor interfaces found in bulk heterojunc-
tion (BHJ) OPVs (See Fig. 1.1(A)).
The use of inorganic semiconductor materials in electronic and optoelectronic
devices depends on their ability to form efficient electrical junctions and contact surfaces
such as: (1) p-n homojunction (junction between two semiconductor regions of opposite
doping), (2) metal-semiconductor junction (or Schottky barrier) and (3) heterojunction
(junction between two dissimilar semiconductor materials).
23
The photocatalytic activ-
ity of oxide semiconductors is also known to be driven by their surface chemistry.
24, 25
While exceptional progresses have been made in understanding the structure and reac-
tivity of these semiconductor surfaces (e.g. TiO
2
), much of this knowledge is only
limited to the pristine surfaces or surfaces under vacuum, but not the surfaces that are in
action. For example, little is known about the semiconductor surfaces in the presence
4
Figure 1.1: (A) Important interfaces in an organic bulk heterojunction (BHJ) solar cell
that is composed of an electron donor and an electron acceptor material (Figure is
adapted from Phys. Chem. Chem. Phys., 2009, 11, 2575-2591). (B) and (C) Function-
alized semiconductor surfaces have applications in semiconductor technology, energy
conversion, molecular electronics and catalysis (Figure by Noah T. Plymale, Caltech).
5
of air/water or under ambient condition which is generally the case for the photocat-
alytic reactions.
25
Another important aspect of the semiconductor surfaces is the pres-
ence of defect sites.
26
Surface defects can play a crucial role in the functioning of the
semiconductor materials by modulating their optoelectronic properties and influencing
the carrier mobility and lifetime. The overall semiconductor property is thus greatly
influenced by its surface property. Recently, tailoring the chemical, physical and elec-
tronic properties of semiconductor materials by surface functionalization has seen an
explosion of activity due to its promise in fundamental as well as technological appli-
cations.
16–18, 27–29
A detailed understanding of the inorganic semiconductor surfaces is
thus necessary to rationalize the surface reactivity and optimize device performances.
On the other hand, surface and interfaces are prevalent in devices and processes
which are based on organic semiconductor materials. The molecular structure of poly-
mers at the solid/liquid (e.g. polymer/water) and solid/solid (e.g. polymer/polymer)
interface is important in fields like adhesives, paints, coatings, composites, biomedi-
cal implants and anticorrosives. The performance and efficiency of organic electronics
are affected by the properties of the interfacial layers and selective contacts.
6, 30
This is
because of the fact that a vast majority of the rate determining processes, such as, charge
transport at the organic/dielectric interface in case of OFETs, exciton dissociation at the
donor/acceptor interface and charge extraction at the organic/electrode interface that
control the device performance take place at interfaces.
31–35
Rate limiting processes
aside, selective interfaces in organic electronics have a multitude of fundamental and
technological applications. They adjust the energetic barrier height between the pho-
toactive layer and the electrode, form selective contacts, determine polarity of the device
and act as optical spacers. Therefore, a through understanding (macroscopic as well as
6
microscopic) of such interfaces is essential for improving the performances of the next
generation electronic and optoelectronic devices.
1.3 Characterization of semiconductor surfaces
The development of surface science over the years has largely been relied upon the
development of novel surface analytical tools capable of providing new insights into
the surfaces and surface-bound processes. Characterization of semiconductor surface,
in fact, any surface, has always been a great challenge due to the fact that the surface
constitutes of only a small fraction of molecules compared to that of the bulk. Thus,
any analytical technique employed to characterize surface will inevitably sample a large
number of bulk molecules that generally obscure any signal originated from the interfa-
cial region. One way to overcome such problem is to prepare and maintain a surface in a
clean enough environment so that even small amount of surface signal can be detected.
Historically, the exploration of the semiconductor surfaces took the first mature step with
the progress in vacuum technology
36
and the advent of electron microscopy techniques
(in the 1930s) such as transmission electron microscopy (TEM) and scanning electron
microscopy (SEM).
37, 38
These probing techniques made it possible to visualize semi-
conductor surfaces at the atomic level, giving both compositional and topographical
information. The field witnessed the next major breakthrough in the late 1950s with the
introduction of the ultra high vacuum (UHV) technology that resulted in the study of
clean single-crystal surfaces. Most of these processes include bombardment of surfaces
with high energy particles (e.g. electrons, neutrons, photons or ions) that give informa-
tion about the surface structure and energetics. Auger electron spectroscopy, photoelec-
tron spectroscopy (PES), high resolution electron energy loss spectroscopy (HREELS),
neutron scattering, X-ray scattering etc. are examples of such techniques. One of the
7
major limitations of these processes was the use of extreme environmental conditions
(e.g. low pressure or temperature) to characterize the surface. Later, in the 1980s, the
inventions of various scanning probe microscope techniques resolved this shortcoming
and contributed greatly towards the continuing advances in semiconductor technology.
Techniques like scanning tunneling microscopy (STM) and atomic force microscopy
(AFM) not only opened new avenues in surface science by providing a zoomed in view
of the semiconductor surfaces held at ambient conditions, but expanded the territory of
this field with subsequent inventions of dip-pen nanolithography and nanofabrication.
Although the importances and the contributions of the aforementioned surface char-
acterization techniques in the growth of surface science can not be overstated, the
requirement that the surface needs to be adjacent to or in contact with the instrument and
the perturbing nature of most of the techniques pose some serious limitations in employ-
ing these methods in quantitative surface analysis. All-optical techniques (where pho-
tons act as both probe and signal), on the other hand, have the advantages of being non-
destructive and label-free, and can be applied to any surface (exposed or buried) under
any condition. The use of optical techniques in the study of semiconductor surfaces
dates back to 1890 when Drude first studied the reflection of light from crystal surfaces.
Over the years, the field has undergone steady and permanent progress and different
parts of the electromagnetic spectrum have been exploited to get different type of infor-
mation about the surface and surface adsorbates. Earlier accounts of optical techniques
were primarily based on linear optical phenomena, where the amplitude of the inci-
dent light wave, E
i
was negligible in comparison with the intra-atomic field strengths.
A wide variety of linear optical techniques that have been successfully employed to
study semiconductor surfaces include spectroscopic ellipsometry (SE), surface differen-
tial reflectivity (SDR), surface photoabsorption (SPA), photoluminiscence spectroscopy
8
(PL), fluorescence spectroscopy, IR and Raman spectroscopy, surface-enhanced Raman
scattering (SERS) and surface plasmon polariton spectroscopy. Each of these techniques
has its unique strengths and limitations. One of the major limitations of these techniques
is that they are not truly surface specific.
The limitation of surface-selectivity can be overcome if we consider higher order
optical spectroscopic techniques, specifically even order nonlinear spectroscopic tech-
niques. The second order nonlinear spectroscopic techniques such as second harmonic
generation (SHG) and sum frequency generation (SFG) spectroscopies are the lowest
even order spectroscopic techniques that are truly surface-selective.
39–42
The inherent
surface selectivity of such second order nonlinear optical processes arise from different
structural symmetries of the material sections under consideration. Under the electric-
dipole approximation and proper experimental geometry, the second order response of
any section of the material that possesses a center of inversion (such as bulk) can be
suppressed compared to the material section that does not have a center of inversion
(such as surface or interface). Thus, the SFG and SHG spectroscopic techniques can
selectively and specifically provide information about the surface and surface-bound
processes. The inherent surface selectivity of SFG and SHG techniques have enabled
them to be versatile surface probes that have found numerous applications in wide range
of scientific, engineering and industrial problems. SFG spectroscopy, as we will see
later, will be our choice of technique for looking into the surfaces and interfaces of a
number of semiconductor materials.
43–47
9
1.4 Second order nonlinear spectroscopy - The surface
probe
In order to appreciate the inherent surface selectivity of even order nonlinear spectro-
scopic techniques, we first need to consider how polarization P
i
, of a material system
depends on the strength E
j
of an applied optical field. When optical field of sufficiently
low intensity (compared to the intra-atomic field strength) is used to excite a material
system, the induced polarization generated inside the medium depends linearly on the
applied electric field strength. Such a process can be described by the relationship
P
i
=c
(1)
jk
E
k
(1.1)
where the constant of proportionalityc
(1)
jk
is known as linear susceptibility.c
(1)
jk
is mate-
rial specific and it accounts for the molecular response induced by the optical field. The
polarization acts as a source to radiate a new electromagnetic field, which we call the sig-
nal E
Sig
. Conventional optical techniques such as reflection or refraction belong to such
linear optical regime. However, when an optical source of sufficient intensity (where
the intensity is comparable to intra-atomic field strength) is used to excite a material
system, the induced polarization no longer depends linearly on the applied electric field
strength. In such a case, the induced polarization can be expressed as a power series in
the field strength E
j
as
48, 49
P
i
=c
(1)
jk
E
j
+c
(2)
i jk
: E
j
E
k
+c
(3)
i jkl
: E
j
E
k
E
l
+::: (1.2)
= P
(1)
+ P
(2)
+ P
(3)
+::: (1.3)
10
Figure 1.2: The optical process describing sum frequency generation from a flat surface
is shown above. SFG is a second order nonlinear optical process where two laser beams
having frequencies w
1
and w
2
interact to generate a second order polarization P
(2)
at
the surface that radiates a signal field at the sum of two input frequencies (w
SF
=w
1
+
w
2
) in a phase-matched direction. Because of the cetrosymmetric nature of the bulk,
P
(2)
is bulk-forbidden under the electric-dipole approximation. Thus, sum frequency
generation spectroscopy can be used to selectively probe surfaces and interfaces.
where each P
(n)
(n2) describes one or more nonlinear optical processes that are pro-
portional to the nth power of the incident field amplitude. Here, c
(n)
is nth order sus-
ceptibility and i, j, k, .. are placeholders for any laboratory-frame Cartesian coordinates
(x, y, z). In second order nonlinear spectroscopy, two optical fields are used to drive
polarization in a molecular system which is followed by the emission of a signal field
in a pre-calculated direction. c
(2)
i jk
describes the material response under the influence
of two optical fields and can be measured with proper experimental geometry. c
(2)
i jk
is
related to the so called molecular hyperpolarizability, as will be discussed in more detail
in Chapter 2. In third order nonlinear spectroscopy, three optical fields are used and so
on. Nonlinear optical techniques are particularly useful for complex molecular systems
because multiple light fields with independent control over frequency or time-ordering
can be employed to dissect molecular interactions and dynamics, which is otherwise not
possible with linear optical techniques that typically use a single frequency or time axis.
In this thesis, we will mainly be dealing with different variations of second order non-
linear spectroscopy and show their usefulness in providing rich molecular information
about surfaces and interfaces.
11
1.4.1 Surface selectivity of the second order processes
Lets take a closer look at the second order polarization described in 1.3 and what hap-
pens to it when we apply the inversion operator ˆ ı. The inversion operation is going to flip
the signs of the two electric fields as well as the second order polarization: ˆ ıE
j
=E
j
,
ˆ ıE
k
=E
k
and ˆ ıP
(2)
i
=P
(2)
i
. However, as material properties are invariant under inver-
sion,c
(2)
i jk
will remain unchanged under such operation. This now gives us
P
(2)
i
=c
(2)
i jk
: E
j
E
k
(1.4)
Comparing 1.4 with 1.2 gives
c
(2)
i jk
: E
j
E
k
=c
(2)
i jk
: E
j
E
k
(1.5)
which can only be true simultaneously ifc
(2)
i jk
= 0. Thus for centrosymmetric medium
such as bulk material, the second order response is zero. At the surface or interface,
the inversion symmetry is usually broken and this make the second order response to
be non-zero. This is the reason for the second order nonlinear spectroscopy techniques
to be intrinsically surface-selective. However, one should keep in mind that the above
discussion is strictly valid only under the electric-dipole approximation. Beyond the
dipole approximation, higher-order nonlocal contributions (such as electric-quadrupole
and magnetic-dipole contributions) do not vanish even in a medium with inversion sym-
metry and their contributions towards the overall signal cannot always be ignored. This
is described in more detail in Chapter 5. Nevertheless, it has been shown that under
proper experimental conditions (co-propagating experimental arrangement where the
signal is collected in a reflection geometry), second order spectroscopic techniques serve
12
as sensitive surface-specific probes having excellent molecular-specificity and chemical-
selectivity.
1.4.2 Sum frequency generation spectroscopy
Sum frequency generation spectroscopy is a second order nonlinear optical spectro-
scopic technique where two input optical fields (say of frequenciesw
1
andw
2
) induce
a second order polarization P
(2)
in a sample or nonlinear medium which subsequently
emits a signal fieldw
SF
in a phase-matched direction with an energy equal to the sum of
the two incident optical field energies.
41, 42
The sum frequency signal can be expressed
as
w
SF
=w
1
+w
2
(1.6)
A special case of sum frequency generation is second harmonic generation (SHG) spec-
troscopy where the two input optical fields have the same frequencies, i.e. w
SHG
=
w+w= 2w. In SFG or SHG spectroscopy, the first interaction with a laser field gen-
erates a first order polarization throughout the sample. The second interaction, which
can be either instantaneous or time delayed upconverts P
(1)
to P
(2)
, imparting surface
selectivity in the detection step (P
(2)
is only present at the surface). When the interact-
ing optical fields have the same energies as the differences between energy levels of the
surface molecules, one gets molecular level information via SFG or SHG spectroscopy.
In case of vibrational sum frequency generation (VSFG) spectroscopy, the vibrational
modes of the surface bound molecules are excited via a tuneable infrared (IR) laser field
(w
1
=w
IR
). This is then followed by a second interaction with a fixed frequency visible
field (w
2
=w
Vis
). By detecting the resultant sum frequencyw
V SFG
=w
IR
+w
Vis
light
as a function of infrared frequency, a surface-specific vibrational spectrum is obtained
13
Figure 1.3: The figure shows the process of sum frequency generation at a molecular
level. The left panel shows the interactions involved in vibrational sum frequency gen-
eration (VSFG) process. The first interaction in VSFG is a resonant infrared interaction
that excites a molecular vibration. This is then followed by a nonresonant interaction
with a visible photon that induces a second order polarization in the medium. Sub-
sequently, the molecule returns to its ground state and emits a sum frequency photon
which carries information about the surface. The left panel describes the interactions
involved in electronic sum frequency generation (ESFG) process. In case of ESFG, the
two interactions lead to an electronic transition in a system. ESFG provides information
about the surface electronic states which can be very different from that of the bulk.
in the fingerprint mid-IR region. In case of second harmonic generation (SHG) or elec-
tronic sum frequency generation (ESFG) spectroscopy, resonant laser interactions drive
electronic transitions of the surface-bound species and thus one can obtain information
about the surface electronic states of the molecules. VSFG and ESFG spectroscopic
techniques can often act as complementary techniques to provide a complete molecular
picture of the surface.
14
1.5 Outline of the thesis
The aim of this thesis is to exploit the second order nonlinear spectroscopic techniques to
explore the surfaces and interfaces of semiconductor materials that have applications in
photovoltaics, field effect transistors, light emitting diodes, and silicon-based electron-
ics. Such nonlinear spectroscopic techniques are quite powerful in unraveling molecular
level information of the semiconductor surfaces and thus, lead to a better understanding
of those surfaces.
In Chapter 2, the background needed to describe and subsequently interpret the
vibrational sum frequency generation spectroscopic data is presented. Here, I discuss
how polarization selected VSFG data along with a knowledge of molecular hyperpo-
larizability can help in deducing orientation information of the molecules present at the
surface. In case of VSFG from a thin-film system, multiple interfaces can contribute
towards the overall VSFG signal and this makes data interpretation non-trivial. In this
scenario, careful consideration of thin-film interference and the evaluation of contribu-
tion from each surface are required.
In Chapter 3, polarization-selected VSFG spectroscopy is used to characterize the
molecular orientation of an organic semiconductor material, poly-3-hexylthiophene
(P3HT) at buried polymer/substrate interfaces. The measured VSFG spectra of the C=C
stretch of the thiophene ring gave direct information about the orientation of the con-
jugated backbone of P3HT, which is directly relevant to the electronic properties at the
interface. The molecular orientation at buried polymer/substrate interfaces was then
compared for the films spin-cast on SiO
2
and AlO
X
substrates, before and after thermal
annealing. Thermal annealing is known to alter the microstructure of the polymers in the
active layers of the OPVs and thus affect performance and carrier mobility. The study
15
revealed that annealing resulted in a more edge-on orientation of the thiophene rings
on SiO
2
substrate. An opposite change was observed for the films deposited on AlO
X
.
Here, annealing led to a more face-on orientation of the thiophene rings. Although sub-
tle, such orientational changes may significantly affect the electron transfer rates across
interfaces and hence the overall photovoltaic efficiency.
In Chapter 4, VSFG studies on typical organic light emitting diode (OLED) materi-
als are discussed in the context of ordering and packing of these molecules in the active
layers of OLEDs. The molecular orientation of organic semiconductor materials plays a
crucial role in the charge transport and light outcoupling efficiencies of the OLEDs. In
our study, we have found that contrary to the general notion of isotropic molecular dis-
tribution in vapor deposited thin-film OLEDs, small organic semiconductor molecules
have anisotropic molecular arrangement in such films. We studied thin films three
OLED materials, 4,4-bis(N-carbazolyl)-1,1-bipheny (CBP), N,N-di(1-naphthyl)-N,N-
diphenyl-(1,1-biphenyl)-4,4-diamine (NPD) and diindenoperylene (DIP) prepared via
vacuum vapor deposition. These semiconductor materials are frequently used as charge
transport materials in the OLED active layer. A general mechanism is proposed to
explain the order and packing of these materials.
In Chapter 5, the role bulk contribution in surface sum frequency generation spec-
troscopy is discussed. The use of SFG spectroscopy as a surface-selective technique
relies on certain assumptions (electric-dipole approximation) and experimental condi-
tions (reflection vs transmission geometry). Beyond such restrictions, SFG provides
information about both the surface and the bulk. In this chapter, VSFG studies on three
centrosymmetric molecules, CBP, NPD and DIP were used to highlight the importance
of bulk contribution in nonlinear SFG measurements. We also showed here that the
16
surface and the bulk contributions depend on the polarizations of the input and output
optical fields and thus, they can separated in a VSFG measurement.
Finally, in Chapter 6, VSFG measurements on functionalized Si(111) surfaces are
discussed. Covalent attachment of organics to oxide-free crystalline Si surfaces, while
still preserving ideal electrical and electronic properties of the H-terminated Si surfaces
is an attractive route for the fabrication of highly passivated Si surfaces for applica-
tions in micro- and nanoelectronics, photonics, solar energy conversion applications,
and chemical and biological sensors. In this chapter, VSFG spectroscopy is used to
investigate the molecular structure and the adsorbate-substrate interaction picture of the
propynyl-terminated Si(111) surface. The well-resolved vibrational C-H stretch reso-
nances of the terminal methyl groups, recorded with the PPP and SSP polarization com-
binations of the laser beams, gave us information about the orientation of the CH
3
CC-
units on the crystalline Si(111) surface. Furthermore, the rotational anisotropy of the
methyl vibrational modes and the propynyl-Si(111) coupling were also investigated by
the VSFG technique.
17
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22
Chapter 2: Vibrational Sum Frequency
Generation Spectroscopy: Formalism
and Experimental Details
2.1 Vibrational sum frequency generation spectroscopy
Vibrational sum frequency generation spectroscopy (VSFG) is a surface-specific tech-
nique that provides vibrational spectra of molecules at interfaces. Since the first exper-
imental demonstration of VSFG spectroscopy via infrared-visible sum frequency gen-
eration by the Shen group in 1987
1
, the field has grown substantially over the years to
become a powerful surface characterization technique.
2–7
In this section, I briefly intro-
duce some of the basic concepts behind VSFG that will be essential for the discussions
in the later chapters.
2.1.1 Second order susceptibility and selection rule
VSFG is a second order process involving two input laser fields, one of fixed visible fre-
quency,w
Vis
, and the other of tuneable infrared frequency,w
IR
, that induce a nonlinear
polarization in a medium given by
8, 9
23
Figure 2.1: Illustrations of vibrational sum frequency generation spectroscopy from a
surface and the associated molecular process.
P
(2)
i
(w
SF
)=c
(2)
e f f;i jk
(w
Vis
;w
IR
) : E
j
(w
Vis
)E
k
(w
IR
) (2.1)
where c
(2)
i jk
(w
Vis
;w
IR
) is the macroscopic average of the molecular polarizability and
is known as second order susceptibility. This 27-element rank-3 tensor couples the
incident fields E
j
and E
k
to the i-th component of the induced polarization P
(2)
i
. The P
(2)
i
then radiates the sum frequency signal whose intensity is proportional to the two incident
laser intensities and the magnitude square of the effective second order susceptibility
c
(2)
e f f;i jk
. The intensity of the generated SF signal is given by
10–14
I
i
(w
SF
)µjc
(2)
e f f;i jk
j
2
I
j
(w
Vis
)I
k
(w
IR
) (2.2)
Elements of the effective susceptibility are related to the actualc
(2)
i jk
via the local field
correction factors that relate the freely propagating incoming/outgoing electric fields
and the surface-bound fields.
c
(2)
e f f;i jk
=[ ˆ e
i
L
ii
(w
SF
)]c
(2)
i jk
:[L
j j
(w
Vis
) ˆ e
j
][L
kk
(w
IR
) ˆ e
k
] (2.3)
24
where L
ii
s are the frequency dependent Fresnel factors and ˆ e
i
is the unit polarization vec-
tor. The macroscopic susceptibilityc
(2)
e f f;i jk
can be decomposed into two parts depending
on their origins with respect to the molecular system under consideration.
c
(2)
e f f
=c
(2)
NR
+
å
v
c
(2)
R
(2.4)
wherec
(2)
R
is the molecular part of the nonlinear susceptibility which describes the res-
onant response of the interfacial chromophores and c
(2)
NR
is the non-resonant response
generated from the substrate (electronic response from the substrate). The summation
sign takes into account more than one vibrational resonances for a particular molecular
system. In general, c
(2)
NR
is small and real as long as the substrate is not in resonance
with either of the three frequencies, w
IR
, w
Vis
and w
SF
. This is the case for dielec-
tric interfaces that are generally far from any electronic resonance. However, for metal
or semiconductor substrates,c
(2)
NR
is a large-valued complex function.
15, 16
The macro-
scopic c
(2)
R
, on the other hand, is the ensemble average of the microscopic molecular
hyperpolarizabilities and is given by
c
(2)
R;i jk
µb
(2)
abc
(2.5)
c
(2)
R;i jk
and b
(2)
abc
are related via a rotational transformation matrix that connects the
molecule-fixed coordinate (a, b, c) and the laboratory coordinate (x, y, z) (See Fig.
2.4).
10, 11, 17, 18
c
(2)
R;i jk
(q;f;y)= N
S
*
å
abc
U
i jk;abc
(q;f;y)b
(2)
abc
+
(2.6)
where N
S
is the number density of molecules at the interface and (q;f;y) are the Euler
angles for molecular transformation. hi indicates orientational averaging arising from
25
the transformation from the molecular property to the macroscopic property of the sam-
ple.
A quantum mechanical expression for b
(2)
abc
can be derived via perturbation the-
ory.
8, 19
It can be expressed as
b
(2)
abc
=
å
q
hgja
ab
jvihvjμ
c
jgi
w
q
w
IR
iG
q
(2.7)
where w
IR
is the frequency of the tunable infrared beam, w
v
is the frequency of the
vibrational resonance and G
1
v
is the relaxation time of the qth vibrationally excited
state.hgja
ab
jvi andhvjμ
c
jgi are the Raman and infrared transition moments involving
ground state g and the excited state v. Equation 2.7 tells that a vibrational mode must
be both Raman and infrared active in order for it to have a nonzero SFG response. This
is the sum frequency selection rule. b
(2)
abc
can be substituted back into the Equation 2.5
to get the final expression forc
(2)
e f f
. The final expression for the sum frequency intensity
that can be used to analyze measured data is given below.
I(w
SF
)µ

c
(2)
NR
+
å
q
B
q
w
q
w
IR
iG
q

2
(2.8)
2.1.2 Polarization dependencies in VSFG
When describing VSFG from a planar surface, the electric fields associated with the
incoming and outgoing beams can be resolved into parallel (P) and perpendicular (S)
components. A right handed coordinate system is used throughout this thesis where z
direction is normal to the surface plane and all the light beams propagate in the xz plane.
According to this convention, P denotes the polarization of the optical fields in the xz
plane, whereas S denotes the polarization perpendicular to the xz plane (see Fig. 2.2). If
26
Figure 2.2: Schematic showing the surface-fixed and the molecule-fixed coordinate sys-
tem used in the vibrational sum frequency generation experiments. The polarizations (P
or S) of the incoming and outgoing beams with respect to the coordinate system are also
shown. The surface is assumed to be rotationally invariant (C
¥
v symmetry).
we now reconsider Equation 2.1 with respect to the polarizations of the electric fields,
we see that P
(2)
i
(w
SF
)=c
(2)
e f f;i jk
: E
j
(w
Vis
)E
k
(w
IR
) denotes the polarization induced in
the i direction by the E fields along the j and k axes, e.g. P
(2)
x
(w
SF
) induced by E
y
(w
Vis
)
and E
z
(w
IR
). Under such circumstance,c
(2)
xyz
(w
Vis
;w
IR
) describes the material response
and has components in all three directions for a given pair of electric field components.
Since c
(2)
is a rank-3 tensor, there are 27 such equations that fully describe a surface
under consideration. But the assumption of a rotationally isotropic interface having
C
¥
v symmetry reduces the number of non-zero susceptibility elements to seven among
which only four are independent:
11, 17, 18, 20, 21
c
(2)
xxz
c
(2)
yyz
c
(2)
xzx
c
(2)
yzy
c
(2)
zxx
c
(2)
zyy
c
(2)
zzz
(2.9)
In a VSFG experiment, these four independent susceptibility tensors can be probed via
four polarization combinations of the input and output laser fields, namely, SSP, SPS,
27
PSS and PPP. Here, the first, second and third letters correspond to the polarizations of
the SFG, visible and IR beams, respectively.
c
(2)
e f f;SSP
= L
yy
(w
SF
)L
yy
(w
Vis
)L
zz
(w
IR
)sina
IR
c
(2)
yyz
(2.10)
c
(2)
e f f;SPS
= L
yy
(w
SF
)L
zz
(w
Vis
)L
yy
(w
IR
)sina
Vis
c
(2)
yzy
(2.11)
c
(2)
e f f;PSS
= L
zz
(w
SF
)L
yy
(w
Vis
)L
yy
(w
IR
)sina
SF
c
(2)
zyy
(2.12)
c
(2)
e f f;PPP
=L
xx
(w
SF
)L
xx
(w
Vis
)L
zz
(w
IR
)cosa
SF
cosa
Vis
cosa
IR
c
(2)
xxz
L
xx
(w
SF
)L
zz
(w
Vis
)L
xx
(w
IR
)cosa
SF
sina
Vis
cosa
IR
c
(2)
xzx
+ L
zz
(w
SF
)L
xx
(w
Vis
)L
xx
(w
IR
)sina
SF
cosa
Vis
cosa
IR
c
(2)
zxx
+ L
zz
(w
SF
)L
zz
(w
Vis
)L
zz
(w
IR
)sina
SF
sina
Vis
sina
IR
c
(2)
zzz
(2.13)
Here,a
Vis
,a
IR
anda
SF
are the incidence angles for the visible and the IR beams and the
outgoing angle for the sum frequency beam, respectively (See Fig. 2.3). From momen-
tum conservation criterion, they can be calculated from the following relationship:
w
SF
sina
SF
=w
Vis
sina
Vis
+w
IR
sina
IR
(2.14)
The tensorial Fresnel factors can be represented as (considering a three-layer model for
the interface):
20, 22
28
Figure 2.3: A three-layer model for the interface VSFG showing the experimental
parameters and refractive indices. The interfacial layer is assumed to be molecularly
thin in nature and has an effective refractive index n
0
which is different from that of the
bulk regions. n
0
is unknown parameter in the calculation of the Fresnel factors and has
to be estimated for the material system under study.
L
xx
(w
i
)=
2n
1
(w
i
)cosg
i
n
1
(w
i
cosg
i
)+ n
2
(w
i
cosa
i
)
L
yy
(w
i
)=
2n
1
(w
i
)cosa
i
n
1
(w
i
cosa
i
)+ n
2
(w
i
cosg
i
)
(2.15)
L
zz
(w
i
)=
2n
2
(w
i
)cosa
i
n
1
(w
i
cosg
i
)+ n
2
(w
i
cosa
i
)

n
1
(w
i
)
n
0

2
whereg
i
is the refractive angle into medium 2 (See Fig. 2.3) and n
i
(w
i
) is the refractive
index of medium i at wavelengthw
i
. n
0
(w
i
) is the effective refractive index of the inter-
facial layer. n
0
is the only unknown parameter in the VSFG quantitative analysis.
20, 23, 24
g
i
can be obtained via the following relationship:
29
n
1
(w
i
)sina
i
= n
2
(w
i
)sing
i
(2.16)
In a VSFG experiment, the polarizations of the input and output laser fields can
be judiciously chosen to preferentially excite molecular dipoles present at the inter-
face. The polarization dependent data can then be used to deduce the orientations of the
surface-bound molecules.
2.2 Experimental setup
Our VSFG setup is based on a Ti:sapphire laser system (Spectra Physics Spitfire) that is
retrofitted with a Coherent Legend regenerative amplifier cavity, pumped with a Nd:YLF
laser (18 W, 1 kHz, Evolution 30, Spectra Physics) and seeded with a Ti:sapphire oscil-
lator (350 mW, 82 MHz, Kapteyn-Murnane Laboratotires) centered at800 nm (full
width half maximum, fwhm45 nm). The oscillator is pumped with a Nd:YVO
4
laser
(2.6 - 3 W, Millennia Vs J, Spectra Physics). The laser outputs<100 fs pulses at 800 nm
with an average power of 4 W at a 1 kHz repetition rate. Sixty percent of the fundamen-
tal is passed through a compressor producing60 fs pulses (1.8 mJ,796 nm, fwhm
27 nm) that are subsequently used to pump an optical parametric amplifier (TOPAS-C,
Light Conversion). The signal and idler pulses generated from the TOPAS are mixed in
a difference frequency generator (NDFG, Light Conversion) to yield broad band (fwhm
350 cm
1
) IR pulses tunable from 3 to 20 μm (3000-500 cm
1
). The remaining 40%
of the fundamental pulse is compressed again and then passed through an etalon (TecOp-
tics; fwhm17 cm
1
, free spectral range480 cm
1
and finesse65) to produce nar-
rowband picosecond visible pulses with an exponential time-domain profile centered at
30
Figure 2.4: The above diagram describes how one can get quantitative molecular information from experimental VSFG spec-
troscopy. The macroscopic second order susceptibilities (c
(2)
i jk
) and the microscopic molecular hyperpolarizabilities (b
(2)
abc
) can
be assessed via VSFG spectroscopic measurements and ab initio calculations, respectively. The two can then be related via
coordinate transformation which gives orientation information of the molecules present at the surface.
31
comp
IR OPA
DFG
800 nm,
1.8W compressed
1.6W uncompressed
0
50cm
Vis Delay Stage
Evolution
KML Oscillator
Monochromator
CCD
Stage
λ/2 Plate
Polarization Selector
λ/2 Plate
800nm Filter
Ge
λ/2
Etalon
Figure 2.5: Schematic of the vibrational SFG setup.
796 nm. The IR and visible pulses are overlapped temporally and spatially on the sam-
ple to generate the SFG signal. The laser power at the sample is typically 8-9 μJ/pulse
for the IR (IR centered at3 μm) and20 μJ/pulse for the visible. The intensity of the
visible is adjusted by a variable neutral density filter. The time delay between the visible
and IR pulses is adjusted by a joystick-controlled translation stage (Newport VX-25, 0.1
μm (0.67 fs) accuracy). The polarizations of the IR, visible, and SFG pulses are con-
trolled by waveplates and polarizers (zero-order quartz half-wave plate, 800 nm, CVI
Melles Griot for visible beam; zero-order CdSe half-wave plate, 1000-19000 nm, 5 mm
32
Figure 2.6: (A) Cross-correlation image of the femtosecond IR pulse and picosecond
visible pulse. (B) Temporal profile of the narrowband visible pulse after the etalon. (C)
Frequency-domain spectrum of the visible pulse having a FWHM17 cm
1
.
thick, Alphalas for IR beam; zero-order quartz half-wave plate, 670 nm, CVI Melles
Griot for SFG beam).
The VSFG signal is spatially and spectrally filtered (a Raman notch filter and a 800
high reflector is used) before entering monochromator (Princeton Instruments Acton
SP2500 and SpectraPro 300i triple grating monochromator) and liquid-nitrogen-cooled
CCDs (Princeton Instruments, Spec-10:100B, 1340 X 100 pixels). For reference spec-
trum, the narrow-band visible and broad-band IR pulses are overlapped on the surface of
a gold sample Collected VSFG spectra for all samples are normalized with respect to the
gold nonresonant signal measured following each sample which partially deconvolutes
the IR pulse spectrum. For each acquisition, a background spectrum is collected sepa-
rately by blocking the IR beam and is subtracted from each VSFG spectrum to remove
33
Figure 2.7: IR spectrum in CH stretch region taken on MCT detector. Two Gaussians
are used to fit the experimental spectrum (black line). The IR pulse has a FWHM300
cm
1
.
contributions from scatter and dark counts to the measured signals. In order to get rid
of the contributions from the atmospheric CO
2
and H
2
O in the measured IR spectrum,
plexi-glass boxes have built around the NDFG, IR delay, and the SFG setup with an
option to continuously purge them with dry air.
34
2.3 Chapter 2 References
[1] Zhu, X. D.; Suhr, H.; Shen, Y . R. Surface vibrational spectroscopy by infrared-
visible sum frequency generation Phys. Rev. B 1987, 35, 3047–3050.
[2] Chen, Z.; Shen, Y . R.; Somorjai, G. A. Studies of polymer surfaces by sum fre-
quency generation vibrational spectroscopy. Annu. Rev. Phys. Chem. 2002, 53,
437–465.
[3] Tian, C. S.; Shen, Y . R. Recent progress on sum-frequency spectroscopy Surf. Sci.
Rep. 2014, 69, 105–131.
[4] Yan, E. C. Y .; Fu, L.; Wang, Z.; Liu, W. Biological macromolecules at interfaces
probed by chiral vibrational sum frequency generation spectroscopy. Chem. Rev.
2014, 114, 8471–8498.
[5] Roy, S.; Covert, P.; FitzGerald, W.; Hore, D. Biomolecular structure at solid-liquid
interfaces as revealed by nonlinear optical spectroscopy Chem. Rev. 2014, 114,
8388–8415.
[6] Nihonyanagi, S.; Mondal, J. A.; Yamaguchi, S.; Tahara, T. Structure and dynam-
ics of interfacial water studied by heterodyne-detected vibrational sum-frequency
generation. Annu. Rev. Phys. Chem. 2013, 64, 579–603.
[7] Richmond, G. L. Molecular bonding and interactions at aqueous surfaces as probed
by vibrational sum frequency spectroscopy molecular bonding and interactions at
aqueous surfaces as probed by vibrational sum frequency spectroscopy Chem. Rev.
2002, 102, 2693–2724.
[8] Y .R.Shen; Principles of nonlinear optics Wiley-Interscience, New York, NY, USA
1984.
[9] Boyd, R. W. Nonlinear Optics Academic Press, Cambridge, USA 2005.
[10] Lambert, A. G.; Davies, P. B.; Neivandt, D. J. Implementing the theory of sum
frequency generation vibrational spectroscopy: A tutorial review Appl. Spectrosc.
Rev. 2005, 40, 103–145.
[11] Wang, H.-F.; Gan, W.; Lu, R.; Rao, Y .; Wu, B.-H. Quantitative spectral and ori-
entational analysis in surface sum frequency generation vibrational spectroscopy
(SFG-VS) Int. Rev. Phys. Chem. 2005, 24, 191–256.
35
[12] Vidal, F.; Tadjeddine, A. Sum-frequency generation spectroscopy of interfaces
Reports Prog. Phys. 2005, 68, 1095–1127.
[13] Wei, X.; Hong, S.; Zhuang, X.; Goto, T.; Shen, Y . Nonlinear optical studies of
liquid crystal alignment on a rubbed polyvinyl alcohol surface Phys. Rev. E. Stat.
Phys. Plasmas. Fluids. Relat. Interdiscip. Topics 2000, 62, 5160–72.
[14] Shen, Y . R. Basic theory of surface sum-frequency generation J. Phys. Chem. C
2012, 116, 15505–15509.
[15] Braun, R.; Casson, B. D.; Bain, C. D.; van derHam, E. W. M.; Vrehen, Q. H. F.;
Eliel, E. R.; Briggs, A. M.; Davies, P. B. Sum-frequency generation from thiophe-
nol on silver in the mid and far-IR J. Chem. Phys. 1999, 110, 4634.
[16] Covert, P. A.; Hore, D. K. Assessing the Gold Standard: The Complex Vibrational
Nonlinear Susceptibility of Metals J. Phys. Chem. C 2015, 119, 271–276.
[17] Hirose, C.; Akamatsu, N.; Domen, K. Formulas for the analysis of the surface
SFG spectrum and transformation coefficients of cartesian SFG tensor components
Appl. Spectrosc. 1992, 46, 1051–1072.
[18] Hirose, C.; Akamatsu, N.; Domen, K. Formulas for the analysis of surface sum-
frequency generation spectrum by CH stretching modes of methyl and methylene
groups J. Chem. Phys. 1992, 96, 997–1004.
[19] Hunt, J.; Guyot-Sionnest, P.; Shen, Y . Observation of C-H stretch vibrations
of monolayers of molecules optical sum-frequency generation Chem. Phys. Lett.
1987, 133, 189–192.
[20] Zhuang, X.; Miranda, P.; Kim, D.; Shen, Y . Mapping molecular orientation and
conformation at interfaces by surface nonlinear optics Phys. Rev. B 1999, 59,
12632–12640.
[21] Lu, R.; Gan, W.; Wu, B. H.; Chen, H.; Wang, H. F. Vibrational polarization spec-
troscopy of CH stretching modes of the methylene group at the vapor/liquid inter-
faces with sum frequency generation J. Phys. Chem. B 2004, 108, 7297–7306.
[22] Guyot-Sionnest, P.; Chen, W.; Chen, W.; Shen, Y . R. General considerations on
optical second-harmonic generation from surfaces and interfaces Phys. Rev. B
1986, 33, 8254–8263.
[23] Feller, M. B.; Chen, W.; Shen, Y . R. Investigation of surface-induced alignment of
liquid-crystal molecules by optical second-harmonic generation Phys. Rev. A 1991,
43, 6778–6792.
36
[24] Ye, P.; Shen, Y . Local field effect on linear and nonlinear optical properties of
adsorbed molecules Phys. Rev. B 1983, 28, 4288.
37
Chapter 3: Annealing-Induced
Changes in the Molecular Orientation
of Poly-3-hexylthiophene at Buried
Interfaces
3.1 Introduction
The interfacial orientation and organization of organic semiconducting materials play
crucial roles in the performance of organic eld-eect transistors (OFETs) and organic
photovoltaics (OPVs).
1–5
In OFETs, charge transport is influenced by the polymer
structure and morphology of the first few monolayers at the buried dielectric interface
of the gate electrode.
5–8
In OPVs, the efficiency of charge carrier separation and the
rate of carrier recombination are determined by the conformation and relative molecu-
lar orientations at electron donor/acceptor interfaces; further charge extraction depends
critically on the contact resistance at the donor/electrode and acceptor/electrode inter-
faces.
9–11
For example, recent theoretical calculations showed that subtle changes (a few
degrees) in the molecular orientation at the donor/acceptor interface can result in order-
of-magnitude changes in the (forward or reverse) electron transfer rate.
12
A molecular
38
level understanding of these interfaces is necessary to gain insight into the mechanistic
details that affect device operation. In this work, we specifically focus on measurements
of the molecular orientation of the conjugated backbone of the donor polymer (poly-3-
hexylthiophene, P3HT) at oxide interfaces.
3.2 Morphology and structure of P3HT at buried inter-
faces
Among the vast library of organic semiconducting materials, poly-3-hexylthiophene
(P3HT) has been extensively used in the active layers of both thin film OPVs and OFETs
because of its relatively high carrier mobility, ease of solution processability, and high
absorptivity in the visible region.
13–15
The self-assembly of P3HT with a high degree
of regioregularity mainly depends on the p-p interactions between the backbones of
neighboring P3HT chains and steric effects associated with the alkyl side chains.
16–18
Although regioregular P3HT (rrP3HT) spin-cast from a suitable solvent is known to
self-assemble into microcrystalline domains with stacked polymer backbones, a large
degree of morphological disorder coexists in these domains.
16, 19–23
The molecular
level order and relative orientation of rrP3HT also depend on molecular weight, the
choice of substrate, the nature of surface functionalization, and a number of other fac-
tors associated with device fabrication, such as deposition technique, solvent choice, and
solvent or thermal annealing. In particular, solvent and/or thermal annealing has a pro-
nounced effect on the morphology of rrP3HT films whose degree of phase separation,
crystallinity, and crystal orientation depend on the rate, solvent choice, heating time,
and temperature of the annealing process.
23–26
While judicious annealing of bulk het-
erojunction (BHJ) OPVs with rrP3HT:PCBM has been shown to form microcrystalline
39
domains and a controlled interpenetrating network between the constituents leading to
increased device efficiency,
21, 22, 27
the performance and carrier mobility of OFETs are
altered significantly by thermal treatment.
23, 28
Clearly, the optimization of the effi-
ciencies of such devices requires a greater understanding of how annealing alters the
microstructure of rrP3HT films, in particular molecular orientation at the interfaces.
Apart from electron donoracceptor interfaces that mediate exciton dissociation in
BHJ OPVs, contact interfaces have been shown to be important for efficient and sta-
ble OPVs and OFETs.
11, 29–31
Such selective contacts optimize charge injection, influ-
ence band alignment between the active layer and electrodes and hence carrier extrac-
tion, alter open-circuit voltage, determine polarity of the device, and potentially act as
a blocking layer for carriers of one type preventing carrier recombination. In particular,
metal and metal oxides are ubiquitous in organic electronics as selective interfacial lay-
ers, electrodes, and dielectric materials and greatly affect charge injection/extraction and
performance of these devices. Modification and optimization of the interfaces between
organic semiconductors and metal or metal oxides are ongoing challenges in the quest of
improving efficiency and large scale production of such devices. Also, growing interest
in organic inorganic hybrid structures for organic electronics
32, 33
necessitates a better
understanding of the molecular structure at heterointerfaces.
While spectroscopic techniques offer a wealth of information on molecular struc-
ture, the difficulty in accessing these buried interfaces makes the application of tra-
ditional characterization techniques a challenge. Additionally, interfacial regions are
molecularly thin, and thus spectroscopic signals are typically dominated by the bulk
response. Surface selective nonlinear optical spectroscopic techniques, such as vibra-
tional sumfrequency generation (VSFG) spectroscopy, are capable of noninvasive in
40
situ probing of molecular organization at buried interfaces.
34, 35
VSFG is a second-
order nonlinear optical process that is forbidden for materials with inversion symmetry
but allowed for molecular surfaces or interfaces wherein an inherent loss of symme-
try occurs. VSFG has thus found numerous applications in characterizing the molec-
ular organization of polymers at buried interfaces, such as polymer/polymer,
36
poly-
mer/dielectric or metal,
37, 38
and polymer interfaces in organic electronics.
39–41
In this work, we investigate the buried interfaces between rrP3HT and a dielectric
material (SiO
2
) as well as rrP3HT and a metal oxide (AlO
X
), utilizing surface-selective
VSFG spectroscopy, to understand the organization and orientation of rrP3HT at these
interfaces. SiO
2
and AlO
X
are often used as insulator/dielectric layers in OFETs, as
highly stable dielectric encapsulation layers, and as a host or template in semiconductor
insulator nanocomposites.
42–46
We also examine the effect of thermal annealing on the
orientation of rrP3HT at these interfaces.
3.3 Experimental details
3.3.1 Materials
All reagents from commercial sources were used without further purification, unless oth-
erwise noted. Solvents were purchased from VWR and used without additional purifica-
tion except for THF which was dried over sodium/ benzophenone before being distilled.
Poly-3-hexylthiophene (P3HT) was synthesized using previously reported procedures
without modification.
47
41
3.3.2 Sample preparation and handling
All steps of device fabrication were performed in air. Glass substrates were sequen-
tially cleaned by sonication (10 min in detergent and deionized water each and 5 min
in tetrachloethylene, acetone, and isopropyl alcohol each) and subsequently dried in a
nitrogen stream. Solutions of 10 mg/mL P3HT were prepared in chlorobenzene sol-
vent and were stirred for 24 h before spin-coating. P3HT/SiO
2
interfaces were prepared
by spin-coating P3HT on top of glass (with a 0.45 μm PTFE syringe filter, Pall Life
Sciences). Samples selected for thermal annealing were placed in a nitrogen oven for
30 min at 145

C following spin-coating. For measurements on AlO
X
substrates, pre-
cleaned glass substrates were placed in a vacuum chamber for aluminum deposition and
pumped down to high vacuum (< 9 X 10
7
Torr). Aluminum (100 nm) was thermally
evaporated at 3-4
˚
A/s using a Denton Benchtop Turbo IV Coating System. The P3HT
active layer was spin-coated (with a 0.45 μm PTFE syringe filter, Pall Life Sciences)
on top of the AlO
X
. As was done for films spin-coated on SiO
2
, samples selected for
thermal annealing were placed in a nitrogen oven for 30 min at 145

C.
Samples used for VSFG experiments were housed within a home-built airtight cell
under a continuous flow of nitrogen to ensure that samples were not directly exposed
to air during the experiment. The intensities of the VSFG signals obtained from P3HT
samples were checked and compared to a nonresonant signal measured on a thin gold
film before and after each scan to confirm that no photo-degradation had occurred.
3.3.3 FTIR and Raman measurements
In order to identify the SFG active vibrational modes, FTIR and Raman measurements
were performed on the thin films of rrP3HT spin coated on IR grade CaF
2
windows.
42
FTIR spectra were recorded using a Bruker Vertex 80 FTIR spectrometer under vac-
uum in a transmission geometry. Baseline correction was done by measuring the FTIR
spectrum of a clean CaF
2
window and subsequently subtracting it from the sample spec-
tra. Raman spectra were collected using a Renishaw spectrometer with a 633 nm laser
focused to a 0.5 μm spot through a Leica microscope with a 100X objective lens. The
baseline of each spectrum was determined by fitting the background to a polynomial
function.
3.3.4 VSFG spectroscopy
VSFG is performed with two laser pulses: a tunable broadband infrared (IR) pulse
whose spectrum covers the vibrational resonances of functional groups at the interface
and a narrow-band visible pulse centered at 800 nm that is far off of resonance. The
orientation of surface molecular groups can now be determined by performing VSFG
experiments with different polarization combinations of the infrared, visible and SFG
fields. Polarization combinations in a VSFG experiment are described by three polar-
ization vectors (i.e., PPP, SSP, and SPS); the first describes the polarization of the gen-
erated VSFG signal relative to the sample surface, the second denotes the polarization
of the nonresonant visible pulse, and the last specifies the polarization of the infrared
pulse resonant with the vibrational transition of interest. The intensities of the polariza-
tion dependent VSFG signals can be exploited to obtain the relative orientation of sur-
face molecular groups. VSFG has thus found numerous applications in characterizing
the molecular organization of polymers at buried interfaces, such as polymer/polymer,
polymer/dielectric or metal and polymer interfaces in organic electronics. A detailed
description of our VSFG setup is given in the Chapter 2 of this thesis. For our studies
of the C-H and ring stretching regions of rrP3HT, we tuned our IR lights so that they
43
were centered at2900
1
and1450 cm
1
, respectively. The laser powers for the
visible and IR light sources were5-7 mW and9-10 mW, respectively In order to
prevent photo-degradation of the rrP3HT thin films while performing the experiment, a
home-built air-sealed sample cell with continuous N
2
purging was used.
3.4 Results and discussions
3.4.1 FTIR and Raman studies of rrP3HT films
Selection rules for VSFG require that the vibrational modes of molecules present at
the interface be both IR and Raman active. FTIR and Raman spectra of a 60 nm thick
rrP3HT film, spincast from cholorobenzene solvent on a CaF
2
window are first collected
to identify the active vibrational modes in the C-C and C-H stretching regions. (See
Fig.3.1) The resonant peaks in the ring stretching region are assigned as the C-C inter-
ring stretch at1380 cm
1
, the C=C symmetric stretch at1450 cm
1
, and the C=C
antisymmetric stretch at1510 cm
1
. Among these in-plane ring skeleton modes, the
C-C and the C=C modes are known to be sensitive to the degree of molecular order in the
P3HT phase.
48, 49
The main C-H stretching modes are identified as the CH
2
symmetric
stretch (d
+
) at2854 cm
1
, the CH
3
symmetric stretch (r
+
) at2885 cm
1
, the CH
2
Fermi resonance (FR) at2925 cm
1
, and CH
3
asymmetric stretch (r

) at2955 cm
1
.
These assignments follow those reported in previous studies of rrP3HT.
7, 49
3.4.2 VSFG studies of rrP3HT films at buried interfaces
After identifying the spectral regions where the characteristic vibrational modes of
rrP3HT appear, polarization selective VSFG is performed to record the vibrational spec-
tra of rrP3HT at buried P3HT/SiO
2
and P3HT/AlO
X
interfaces. In the case of VSFG
44
Figure 3.1: FTIR (red) and Raman (blue) spectra showing vibrational resonances of the
C-C (A) and C-H (B) stretching regions of as-cast rrP3HT films on CaF
2
, respectively.
The C-C and C-H stretching modes are highlighted in different colors so that they can be
identified easily in VSFG spectra later. Resonant vibrational modes in the ring stretching
region are identified as C-C inter-ring stretch (blue band,1380 cm
1
), C=C symmetric
stretch (orange band,1450 cm
1
), and C=C antisymmetric stretch (green band,1510
cm
1
). The main C-H stretching modes are assigned as CH
2
symmetric stretch, d+
(pink band,2854 cm
1
), CH
3
symmetric stretch, r
+
(yellow band,2885 cm
1
), CH
2
Fermi resonance (FR) (dark green band,2925 cm
1
), and CH
3
asymmetric stretch, r

(violet band,2955 cm
1
). The structure of rrP3HT is shown in the inset in panel A.
from thin films, spectroscopic signals can arise from multiple interfaces; thus, contri-
butions from all surfaces present in the film need to be taken into account.
34
In the
45
present study, we have P3HT/air as well as P3HT/SiO
2
or P3HT/AlO
X
interfaces that
can contribute to the overall VSFG spectra. Control experiments and Fresnel factor
calculations (Presented in Appendix A; See Fig.3.10, Fig.3.11, Fig.3.12) show that the
majority of the signals measured in our study arise from the buried polymer/substrate
interface. Suppression of the nonresonant background in the collected VSFG spectra
is done by introducing a 270 fs time delay between the resonant IR and nonresonant
visible pulse.
50–52
3.4.2.1 rrP3HT films at rrP3HT/SiO
2
interface
Figure 3.2 shows VSFG spectra of the hexyl-side chains of as-spun rrP3HT on SiO
2
collected with PPP and SSP polarization combinations. The VSFG spectra consist of a
number of vibrational modes of the rrP3HT hexyl side chains and are identified as the
CH
2
symmetric stretch d
+
(2850 cm
1
), CH
3
symmetric stretch r
+
(2876 cm
1
),
and CH
3
asymmetric stretch r

(2963 cm
1
) modes. The appearance of the d
+
mode in
the VSFG spectra is an indication of gauche defects in the hexyl side chain.
53–55
It can
be seen from Figure 3.2 that the relative intensities of the modes vary greatly depend-
ing on the polarizations of the IR and visible pulses used in the VSFG experiment.
The intensity ratios of the r
+
mode for different polarization combinations, along with
knowledge of its molecular hyperpolarizability can be used to calculate the orientation
of the CH
3
group, under the assumption that it possesses C
3v
symmetry.
The C-H vibrational modes are similar for rrP3HT films thermally annealed at 145

C for 30 min under a N
2
environment (See Appendix B: Fig.3.14). The change in
VSFG spectra is mainly due to a change in the nonresonant contribution, whereas vibra-
tional resonances remain almost unaltered in their relative amplitudes. This suggests
46
Figure 3.2: VSFG spectra of the hexyl side chains of P3HT spuncast on SiO
2
for PPP
and SSP polarization combinations. The spectra show CH
2
d
+
(blue band,2850
cm
1
), CH
3
r
+
(orange band,2876
1
), and CH
3
r

(violet band,2963 cm
1
).
that thermal annealing does not induce significant orientational reorganization of the
hexyl side chains of pristine rrP3HT films.
Next we look into annealing induced changes in the VSFG response of the ring
modes of thin rrP3HT films deposited on SiO
2
. VSFG spectra of the P3HT/SiO
2
buried
interface are dominated by C-C inter-ring stretch (1380 cm
1
) and C=C symmetric
stretch (1440 cm
1
) (See Fig. 3.3). For each polarization combination, the spectra are
shown for unannealed (red markers) and thermally annealed (blue markers) films. The
intensity of the VSFG signal as well as the full-width at half-maximum (fwhm) of the
C-C and C=C modes remain almost identical for both annealed and unannealed rrP3HT
films deposited on SiO
2
for SSP and PPP polarization combinations. However, the SPS
signal shows a60% increase in intensity with thermal annealing.
47
Figure 3.3: Annealing induced changes in the VSFG spectra showing ring modes of
rrP3HT at the P3HT/SiO
2
buried interface for SSP, SPS, and PPP polarization combina-
tions. The vibrational modes are identified as C-C inter-ring stretch (1380 cm
1
) and
C=C symmetric stretch (1440 cm
1
).
3.4.2.2 rrP3HT films at rrP3HT/AlO
X
interface
In contrast, annealing induces a much greater structural reorganization of the polymer
backbone on the AlO
X
substrate, as clearly evidenced by pronounced changes in the
48
Figure 3.4: VSFG spectra of unannealed and annealed rrP3HT thin films on AlO
X
col-
lected with SSP, SPS, and PPP polarization combinations. The spectra consist of the
C=C symmetric (1440 cm
1
) and C=C asymmetric (1510 cm
1
) stretches. Signif-
icant changes in the intensity as well as narrowing of the resonant modes are observed
following thermal annealing.
VSGF spectra (See Fig.3.4). The intensity and fwhm of the C=C mode decreases signif-
icantly for all three polarization combinations, PPP, SPS, and SSP for rrP3HT prepared
on AlO
X
. The smaller fwhm of the C=C mode indicates an increase in the molecular
order of rrP3HT at the rrP3HT/AlO
X
interface. This could indicate the higher degree
49
of crystallinity of P3HT, a well-known effect of annealing, propagating all the way to
the interface, although we note that VSFG averages over the crystalline and amorphous
domains to yield an ensemble averaged molecular orientation.
3.4.3 Orientational analysis
3.4.3.1 Hexyl side chain of rrP3HT
Figure 3.5 shows the surface fixed coordinate system used in the quantitative estimation
of the orientation of the CH
3
group, whereq is the tilt angle from surface normal. The
choice of a molecule-fixed frame also determines the number of nonzero microscopic
hyperpolarizability elements for the r
+
mode of CH
3
. To calculate the orientation of the
hexyl side chain of rrP3HT, we focus on the terminal methyl group. We approximate the
CH
3
methyl group as having C
3v
symmetry, with c axis being oriented along the C
3
axis
and a and b axes mutually perpendicular to each other (CH
3
fixed frame). Considering a
rotationally isotropic interface, with both the sum and visible frequencies off resonance,
there are 11 non-zero microscopic hyperpolarizability tensor elements for the sym-
metric and asymmetric stretching modes of a group having C
3v
symmetry.
56–58
The
macroscopic susceptibity elements are obtained through integration over Euler angles
as detailed in the previous works of Wang and co-workers.
57
In our analysis, we con-
centrate on the CH
3
r
+
mode centered at 2876 cm
1
. This is due to the fact that the
hyperpolarizability ratio, R =b
aac
/b
ccc
=b
bbc
/b
ccc
for the CH
3
group is well known.
Although, the value of R for the methyl group has been reported to be in the range of
1.66 - 4.0, we have used the value of R=3.4 calculated for ethanol and longer chain
1-alcohols.
56, 57, 59
We assume that the orientational distribution to be Lorentzian along
q, the tilt angle, and simulated c
r
+
PPP
/ c
r
+
SSP
as a function of n
0
and q (See Fig 3.6).
The value of n
0
is a matter of uncertainty in VSFG orientational analysis.
56, 57
It is
50
Figure 3.5: The coordinate system for the CH
3
moiety assuming C
3v
symmetry where
q is the tilt angle of the CH
3
group with respect to the surface normal. The tilt angle of
the main chain axis is different from that of the CH
3
group.
generally accepted that n
0
lies in between the refractive indices of the two bulk media
associated with the interface. Here and subsequent analysis, the refractive index of the
interfacial layer is taken in the range 1.7 n
0
2.0 as detailed in the Figure 3.6 (for
reference, the refractive indices of rrP3HT and SiO
2
are taken to be2.2 and1.45,
respectively).Analysis of the amplitude ratios of the CH
3
r
+
stretch probed by different
polarization combinations gives us an average tilt angle of the CH
3
group in the range
52

q 28

. We should emphasize that the tilt angleq describes the C
3
symmetry
axis, i.e., the C-C bond of the terminal CH
3
group, not the orientation of the hexyl side
chain as a whole. For an all-trans alkyl chain, the angle of the terminal C-C bond with
respect to the chain axis is35

.
58
Our result suggests that the hexyl chain of poly-
thiophene is significantly tilted from the surface normal. The applied model assumes a
delta-function distribution for the tilt angle, whereas structural disorder is expected to
give rise to a broad distribution of side chain orientations in solution cast rrP3HT films.
51
Figure 3.6: Calculated values of the susceptibility tensor ratio for the CH
3
group of the
hexyl side chain of rrP3HT as a function of both the tilt angle (q) and refractive index of
the interfacial layer (n
0
) for as-spun P3HT on SiO
2
. The blue rectangle shows physically
acceptable solutions for the experimental values (black dotted line). The average tilt
angle of the CH
3
moiety found from the analysis lies in the range of 52

q 28

.
The effects of introducing the distribution width in the orientational analysis of SFG and
SHG (second harmonic generation) measurements has been considered in detail previ-
ously.
60, 61
Nevertheless, the calculated tilt angle agrees well with the value of 50.1

reported recently by Maillard et al.
62
3.4.3.2 Backbone of rrP3HT at P3HT/SiO
2
interface
To calculate the absolute orientation of the P3HT backbone at the buried interface, it is
necessary to determine the direction of the net transition dipole vector with respect to
the polymer backbone in the molecular frame of reference. Considering that the average
conjugation length of rrP3HT is extended to10 thiophene units,
63
we adopt the model
proposed by Anglin et al.
8
In this model, the lateral components of the transition dipole
52
Figure 3.7: The direction of the net transition dipole moment of the P3HT dimer and its
relation to the molecular frame of reference are shown . The dipole moment vector is
orthogonal to the average plane of the polymer backbone.
of two thiophene units connected at an angle of 165

(dihedral angle,\S-C-C-S) can-
cel, while out-of-plane components add to give a net transition dipole vector pointing
upward, orthogonal to the average plane of the ring (See Fig.3.7). We can now define
the angle q made by the transition dipole moment vector from the surface normal to
extract orientational information related to the rrP3HT backbone.
To quantify the influence of thermal annealing on the thiophene ring orientation,
emphasis is placed on the most prominent VSFG feature, the symmetric C=C mode,
which is present in all three polarization combinations for P3HT/SiO
2
interfaces (See
Fig. 3.3). The amplitude of this mode can be related to the macroscopic susceptibility
terms probed by VSFG experiments and the microscopic hyperpolarizability elements
of the mode to deduce the orientation of the polymer backbone. However, one potential
complication to this analysis is that several C-H bending modes are also known to be
present in the same spectral window as the ring C-C stretches and contributions from
53
Figure 3.8: Cartoons showing annealing induced changes in the orientation of the P3HT
backbone at the SiO
2
interface. The direction of the net transition dipole moment of the
P3HT dimer and its relation to the molecular frame of reference are shown in the top dia-
gram. The dipole moment vector is orthogonal to the average plane of the polymer back-
bone. P3HT adopts an edge-on orientation after thermal annealing at the P3HT/SiO
2
interface with a change in tilt angle of 3-10

.
them may appear in VSFG spectra.
64
However, a careful comparison between the recon-
structed VSFG spectra from the fitting parameters given in the Supporting Information
54
of ref 8 and the SSP and SPS spectra of the unannealed rrP3HT samples obtained in
our experiment shows comparable fwhm for the C=C mode (See Fig. 3.7), suggest-
ing that C-H bending modes only make negligible contributions to our VSFG spectra.
This allows an orientational analysis of the rrP3HT backbone at the SiO
2
interface to be
performed by fitting the polarization dependent spectra shown in Figure 3.3.
For the C=C mode, the assumption of C
2v
symmetry of the polythiophene ring leaves
three nonvanishing hyperpolarizability tensor elements, b
aac
, b
bbc
and b
ccc
, that can
each contribute to the measured VSFG signals.
57
The values of the relative hyperpolar-
izabilites for the C=C mode were calculated by Anglin et al. where they have assumed
b
aac
b
ccc
-b
bbc
.
8
The calculated ratios of the hyperpolarizability tensor elements
were on the order ofb
aac
/b
ccc
70 andb
bbc
/b
ccc
= -1 assuming that the c axis lies
along the direction of the net transition dipole, the a axis along the conjugation length
of polythiophene, and the b axis to be mutually perpendicular to both of them. The
same coordinate system and the ratios of tensor elements were used in our orientational
analysis of the backbone of rrP3HT (see Fig.3.7). The exact forms of the non-zero
macroscopic susceptibility elements for the C
2v
point group were given by Wei et. al.
65
and Wang et.al.
57
and are not derived here again. To calculate the surface susceptibility
elements, we averaged over the orientational distribution f (q,y,f ),
c
(2)
XY Z
(q;y;f)= N
ZZZ
dqdydf f(q;y;f)
å
abc

U
XY Z;abc
(q;y;f)b
(2)
bbc

(3.1)
where N is the number density at the interface and U
XY Z;abc
(q;y;f) is a product of three
Euler matrices. To obtain a more realistic picture of the orientational distribution of
molecular geometries present at the interface, we consider the tilt angle for the polymer
55
backboneq to have a Gaussian distribution.
58, 61
Assuming a uniform distribution ofy
andf, the (See Eq. 3.1) integral can be represented as
c
(2)
XY Z
(q;y;f)= N
Z
p
0
f(q;y;f)sinq;dq
å
abc

U
XY Z;abc
(q;y;f)b
(2)
bbc

(3.2)
where f(q;y;f)=
1
4p
2
p
2ps
exp

qq
0
2s
2

. Here, q
0
is the average tilt angle and s is
the tilt angle distribution width. We then plot the calculated ratios of the macroscopic
susceptibility elements for the C=C symmetric stretch as a function of average tilt angle
(q
0
), the refractive index of the interfacial layer (n
0
) and tilt angle distribution width
(s) and compared them with the ratios extracted from fitting the polarization selective
VSFG data. The analysis gave us a range of solutions forq
0
ands for different values
of n
0
. Only a range of possible solutions for tilt angle and distribution width can be
obtained from such plots, providing a quantitative estimation of molecular order at the
interfaces.
Orientational analysis, considering the assumptions mentioned above suggests that
the mean tilt angle of the backbone of as-spun rrP3HT on SiO
2
lies between 54

and
62

with a distribution width,s
q
20

(See Fig. 3.8 and Appendix D: Fig.3.15). For a
P3HT chain, consisting of both long and short conjugated segments (considering defects
in the conjugation plane), the angle between the net transition dipole and the plane of the
backbone has been predicted to be between 42

and 90

with an average angle of 68

.
8
This value is very close to the one we obtain from our calculations. After annealing at
145

C for 30 min, rrP3HT adopts a more edge-on orientation with the backbone tilt in
the range between 57

and 73

(See Fig. 3.8 and Appendix D: Fig.3.16), yielding a net
change ofDq = +(3-10)

of the tilt angle as a result of thermal annealing. As previously
mentioned, annealing has little effect on the intensities and fwhm of the C=Cn
s
mode in
56
the PPP and SSP polarized VSFG spectra of rrP3HT, whereas the SPS spectrum shows a
change in intensity. In prior studies, the behavior of the C=C mode of rrP3HT has been
used to quantify the degree of molecular order in rrP3HT.
22, 66
The very similar VSFG
spectra of as-prepared and annealed rrP3HT films on SiO
2
indicate minimal structural
reorganization of rrP3HT near the P3HT/SiO
2
interface as a result of thermal annealing.
3.4.3.3 Backbone of rrP3HT at P3HT/AlO
X
interface
In contrast, annealing induces a much greater structural reorganization of the polymer
backbone on the AlO
X
substrate, as clearly evidenced by pronounced changes in the
VSGF spectra. The intensity and fwhm of the C=C mode decreases significantly for all
three polarization combinations, PPP, SPS, and SSP for rrP3HT prepared on AlO
X
(See
Fig.3.4). The smaller fwhm of the C=C mode indicates an increase in the molecular
order of rrP3HT at the rrP3HT/AlO
X
interface. This could indicate the higher degree of
crystallinity of P3HT, a well-known effect of annealing,
23–25
propagating all the way to
the interface, although we note that VSFG averages over the crystalline and amorophous
domains to yield an ensemble averaged molecular orientation. The orientational analy-
sis estimates the average tiltq of the polymer backbone to be in the range between 47

and 75

for unannealed rrP3HT films on AlO
X
(See Fig.3.9 and Appendix E: Fig.3.17).
After annealing, the thiophene rings tilt away from the surface normal in a direction
opposite to what was observed for the rrP3HT/SiO
2
interface to adopt a more face-on
orientation(See Fig.3.9 and Appendix E: Fig.3.18). The net change in the tilt angle for
the annealed samples is Dq = -(3-26)

, which is also higher compared to the rrP3HT/
SiO
2
interface. We note that while there is evidence of roughening of the film surface
67
57
Figure 3.9: Thermal annealing induces face-on orientation of the P3HT backbone at the
P3HT/AlO
X
interface. The change is more pronounced and in a direction opposite to
those observed for films spin-cast on SiO
2
.
and a slight decrease in film thickness upon annealing, these are not likely to signif-
icantly affect the morphology at the bottom polymer/substrate interface probed in the
present study.
58
The large change in backbone orientation in the presence of AlO
X
could be due to a
more specific interaction between rrP3HT and AlO
X
, compared to the SiO
2
substrate, as
strong Al-C and Al-O-C bonds are formed in thea positions of the thiophene ring.
68, 69
Al is also known to participate in charge transfer to the conjugated backbone of poly-
thiophene, in which thep-electronic system of the chain is significantly perturbed.
68
On
the other hand, studies on the nature of metal/metalloid oxides suggest that delocaliza-
tion of one of the electrons of O
2
at neighboring sites can also affect thep levels of the
polymer.
70
Finally, thermal annealing can lead to diffusion of Al/AlO
X
into the rrP3HT
network, which forms a better contact and eventually changes the microstructure of the
polymer at the contact surface.
21, 70
3.5 Conclusions
In conclusion, we have demonstrated that the underlying substrate influences the orien-
tation of the conjugated backbone of rrP3HT at buried interfaces and how this molec-
ular organization changes upon thermal annealing. Our VSFG experiments reveal that
annealing induces a more edge-on orientation of the thiophene rings on the SiO
2
sub-
strate. In contrast, at the AlO
X
interface annealing results in more face-on orientation of
the thiophene rings of rrP3HT. The decrease in intensity and narrowing of the C=C sym-
metric stretch mode of samples annealed on AlO
X
indicate structural reorganization and
increased molecular order at the metal-oxide interface upon annealing. Since charge car-
rier generation, recombination, and transport efficiency are all intimately related to the
molecular scale morphology, in particular orientation of the conjugated cores of organic
semiconductors at buried interfaces, the results presented above can be used as a struc-
tural input for theoretical calculations of the electron transfer processes,
9, 10, 12
and thus
help elucidate the structurefunction relationship governing the photovoltaic efficiency.
59
Future experiments will be directed toward understanding other OPV interfaces, such as
P3HT/ C
60
, P3HT/PEDOT, etc. that are directly relevant to device architecture.
60
3.6 Appendix A: Control experiments showing VSFG is
generated from the buried interface
In the case of SFG from thin polymer films, spectroscopic responses can arise from
both the top polymer/air interface as well as from the buried polymer/substrate inter-
face. However, control experiments that examine the thickness dependence of the VSFG
signal demonstrate that the VSFG response is dominated by the contribution from the
polymer/substrate interface, with the polymer/air interface and bulk polymer contribut-
ing little to the measured signal. Figure 3.10 plots FTIR spectra of rrP3HT samples of
different thicknesses in a transmission geometry. The features that appear in the spec-
trum scale linearly with sample thickness, as expected for a weakly absorbing sample.
Figure 3.11 displays VSFG spectra of 60 nm and 300 nm thick unannealed rrP3HT
films spun-cast on SiO
2
. If the VSFG signal originated from the bulk of the mate-
rial, we would expect to see an increase in the signal strength with thickness, similar
to the FTIR spectra in Figure 3.10. Instead, the VSFG signal is seen to decrease with
film thickness. This observation also argues against signal generation coming from the
air/polymer interface since such signals should not depend in any way on film thick-
ness. The only logical conclusion is that the VSFG signal is primarily created at the
buried polymer/SiO
2
interface, with the attenuation of the signal seen with increasing
film thickness resulting from the absorption of the VSFG signal by the P3HT layer as it
exits the film.
Next, we collected the VSFG spectra of as-spun rrP3HT films deposited on AlO
X
.
Here also, the thicker film showed attenuation of the VSFG signal intensity measured
at for PPP polarization (See Fig. 3.12). Finally, we compared the PPP spectra of 60 nm
rrP3HT films spun cast on SiO
2
and AlO
X
. Almost a fifty fold increase in PPP signal
61
was observed for the rrP3HT on AlO
X
compared to that on SiO
2
(See Fig. 3.12). This
can be explained if we consider that the AlO
X
surface can act as a reflecting plane and
the majority of the signal is originating from the rrP3HT/AlO
X
interface.
Figure 3.10: For thin film VSFG, spectroscopic contribution can arise from both (A)
polymer/air interface as well as from (B) polymer/substrate interface.
Figure 3.11: (A) FTIR spectra of 60 nm and 120 nm thick as-spun rrP3HT samples on
CaF
2
taken in transmission geometry. (B) VSFG spectra of 60 nm and 300 nm thick
unannealed rrP3HT samples on CaF
2
collected for SSP polarization. The decrease in
VSFG intensity for the 300 nm thick sample indicates that the signal contains contribu-
tions from the buried interface.
62
Figure 3.12: (A) VSFG spectra of as-spun rrP3HT films of different thicknesses on
AlO
X
for PPP polarization. (B) A comparison of PPP spectra between rrP3HT on dif-
ferent substrates shows a substantial increase in intensity for rrP3HT on AlO
X
.
63
3.7 Appendix B: Annealing induced changes in the alkyl
stretches of rrP3HT
VSFG spectra of the hexyl side chain of rrP3HT annealed at 145

C for different times
are shown below for PPP and SSP polarization combinations. Spectra for the unannealed
sample are also included for comparison.
Figure 3.13: PPP and SSP spectra of the hexyl side chain of rrP3HT annealed for differ-
ent amount of time.
64
3.8 Appendix C: Contribution of C-H bending modes to
the C=C symmetric stretching lineshapes
One potential complication of analyzing the C=C symmetric stretching mode of rrP3HT
is the presence of several C-H bending modes in the same region. Anglin et al. stud-
ied deuteriated P3HT (DP3HT) to avoid interference of the abovementioned modes.
8
Since we have used P3HT in our experiment, we thought that it would be worthwhile to
compare the spectral characteristics of our VSFG data with that previously reported by
Anglin et al. We have used the fitting parameters from the SI of Ref. 8 to reconstruct
the SPS and SSP VSFG spectra of the C=C symmetric stretch mode of as-cast DP3HT
on SiO
2
and compared them with our SPS and SSP data of P3HT on SiO
2
. We found
that the FWHM of the C=C mode were comparable, suggesting that C-H bending modes
only make negligible contributions to our VSFG spectra. The fitting parameters for SPS
and SSP data taken from the SI of Ref. 8 are given in the Table 3.1.
Table 3.1: Fitting parameters for the SPS and SSP spectra from the SI of Ref 8.
SPS
c
NR
C-Cn
S
C=Cn
S
C=Cn
AS
A f B G w B G w B G w
SiO
2
(a) 0.7 2.4 - - - -25.9 11.0 1448.9 - - -
SiO
2
(b) 0.6 3.8 - - - -55.0 26.9 1430.4 - - -
SiO
2
(c) 1.8 6.0 - - - 64.8 27.0 1452.6 - - -
SSP
SiO
2
(a) 2.3 5.4 -25.9 -4.3 -1359.4 -78.0 14.5 1444.8 15.3 16.1 1502.0
SiO
2
(b) 2.8 4.9 -54.6 -15.0 -1349.1 -116.4 20.9 1451.4 43.4 12.7 1508.4
SiO
2
(c) 3.1 4.8 -48.1 -12.3 -1354.2 108.9 17.8 1448.4 32.5 14.5 1503.4
65
Figure 3.14: Comparison between (A) SPS and (B) SSP VSFG data of as-spun DP3HT
on SiO2 reconstructed from the SI of Ref. 8 and our experimental results of P3HT on
SiO
2
.
66
3.9 Appendix D: Orientational analysis: Hyperpo-
larizability ratios for the thiophene backbone at
rrP3HT/SiO
2
interface
Figure 3.15: Calculated values of the susceptibility tensor ratios for the C=C symmetric
stretch of rrP3HT as a function of tilt angle (q), refractive index of the interfacial layer
(n
0
) and tilt angle distribution width (s) for as-spun P3HT on SiO
2
. The blue bars show
physically acceptable solutions for the experimental values (black dotted line). The
average tilt angle is 54

q 62

.
67
Figure 3.16: Calculated ratios of the susceptibility tensor elements along with the exper-
imental values give average tilt angle 57

q 73

for annealed P3HT spin coated on
SiO
2
.
68
3.10 Appendix E: Orientational analysis: Hyperpo-
larizability ratios for the thiophene backbone at
rrP3HT/AlO
X
interface
Figure 3.17: Orientational analysis suggests average tilt angle to be in the range 47

q 75

for as-cast P3HT on AlO
X
. The tilt angle distribution width is also narrower
compared to that of P3HT films on SiO
2
. This indicates better structural reorganization
of P3HT backbone in presence of AlO
X
.
69
Figure 3.18: The susceptibility tensor ratios give average tilt angle to be in the range 44

q 49

for annealed P3HT on AlO
X
.
70
3.11 Appendix F: Tables for data fitting
Table 3.2: VSFG fitting parameters for the alkyl stretches of unannealed rrP3HT sam-
ples on SiO
2
for PPP and SSP polarization combinations.
Unannealed rrP3HT on SiO
2
PPP SSP
c
NR
A 22.20 14.07
G
0
285.32 413.91
w
0
2772.00 2763.70
f -1.48 -1.00
CH
2
(d
+
) B
1
12.48 24.60
G
1
9.00 9.01
w
1
2853.10 2830.00
CH
3
(r
+
) B
2
107.66 150.82
G
2
14.08 11.94
w
2
2875.60 2873.20
CH
3
(r

) B
3
368.65 -
G
3
10.75 -
w
3
2961.90 -
CH
3
(r

) B
4
- 7.57
G
4
- 4.89
w
4
- 2937.00
- B
5
- 47.13
G
5
- 16.52
w
5
- 3043.60
71
Table 3.3: VSFG fitting parameters for the ring modes of unannealed and annealed
rrP3HT samples on SiO
2
for PPP, SSP and SPS polarization combinations.
Unannealed rrP3HT on SiO
2
Annealed rrP3HT on SiO
2
PPP SSP SPS PPP SSP SPS
c
NR
A 0.03 0.01 0.02 0.03 -0.01 0.02
f 1.28 2.28 1.42 1.28 2.20 1.42
C-Cn
S
B
1
0.89 0.21 - 0.94 0.25 -
G
1
17.99 6.05 - 13.91 7.47 -
w
1
1382.60 1380.50 - 1382.00 1381.90 -
C=Cn
S
B
2
3.32 4.13 2.46 3.38 3.99 2.97
G
2
17.07 14.13 15.80 17.29 13.82 15.72
w
2
1443.30 1438.70 1439.20 1444.90 1439.90 1439.40
Table 3.4: VSFG fitting parameters for the ring modes of unannealed and annealed
rrP3HT samples on AlO
X
for PPP, SSP and SPS polarization combinations.
Unannealed rrP3HT on AlO
X
Annealed rrP3HT on AlO
X
PPP SSP SPS PPP SSP SPS
c
NR
A 0.12 -0.10 0.05 0.32 -0.12 -0.01
f 0.50 0.95 1.52 0.60 1.46 1.50
C-Cn
S
B
1
2.47 -3.94 -0.29 - -0.68 -1.23
G
1
30.00 30.00 23.43 - 35.00 38.03
w
1
1376.00 1381.1 1385.00 - 1387.00 1382.90
C=Cn
S
B
2
-31.80 8.81 2.70 -16.88 5.63 1.18
G
2
24.19 16.40 16.88 23.51 16.61 17.53
w
2
1442.90 1438.00 1439.20 1435.00 1438.10 1442.00
C=Cn
AS
B
3
-0.73 2.15 - 0.25 1.16 -
G
3
9.50 11.50 - 6.24 9.91 -
w
3
1510.00 1509.00 - 1505.90 1506.80 -
- B
4
- - - 12.85 - -
G
4
- - - 12.75 - -
w
4
- - - 1473.00 - -
72
Table 3.5: Optical parameters used in VSFG orientational analysis and Fresnel coeffi-
cient calculations.
Refractive Indices
Beam Angle of Incidence P3HT
71, 72
Air SiO
2
8, 73, 74
AlO
X
75
IR (3300 nm) 66

2.10 1.0 1.4116 -
Vis (796 nm) 63

2.27 1.0 1.4533 -
SFG (650 nm) - 2.41 1.0 1.4567 -
IR (6900 nm) 66

2.12 1.0 1.1400+0.0007i 1.4850+0.005i
Vis (796 nm) 63

2.27 1.0 1.4533 1.7598
SFG (714 nm) - 2.37 1.0 1.4550 1.7628
73
3.12 Chapter 3 References
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80
Chapter 4: Molecular Orientation of
Typical Light Emitting Diode Materials
4.1 Introduction
Small molecule organic semiconductor materials are promising candidates for organic
electronics such as organic light emitting diodes (OLEDs) because they offer functional
tunability, morphological homogeneity, structural flexibility and ease of processability
for large area modules.
1–3
The active layer of the small-molecule OLED devices is
generally prepared via vacuum vapor deposition. This simple high-purity fabrication
process offers some great advantages such as preparation of high-quality pinhole-free
active layer, nanometer scale surface smoothness, easy control over layer thickness, the
ability to deposit multiple stacked layers having different properties and flexibility in
the choice of substrate materials. The design and synthesis of thermally robust emissive
and charge transport materials for the OLED active layer and optimization of the deice
fabrication parameters have witnessed a lot of activity in recent years because of their
importances in high performance, stable and cost-effective OLED devices.
4, 5
However,
one area that has not been appreciated until very recently is the orientation and orga-
nization of these vapor deposited organic molecules in the active layer of OLEDs and
their effect on the optoelectronic properties and overall performances of the devices.
6–11
81
Although molecular orientation and ordering of the organic semiconductor materi-
als are of great interests in the polymer-based organic electronics community,
12–14
these
factors have commonly been disregarded in small molecule-based OLEDs for a long
time. Such general underappreciation for the role of molecular orientation in the fun-
damental operating principle of OLEDs stems from couple of factors. In contrast to
polymer-based OLEDs, where strong intermolecular interactions govern the organiza-
tion and packing of the organic polymeric materials,
15–17
the morphology and structure
of the active layer of the small molecule OLEDs are clearly dictated by the weak inter-
molecular interactions (van der Waals type inteaction) and some degree of structural
independency (due to the localization of electrons within the molecule). In addition
to that, the possibility of the small aromatic organic molecules to have a large num-
ber of conformational structures ensures that the vapor deposited thin films do not have
any periodic structure with long-range order. So, the general assumption regarding the
molecular orientation in OLEDs was that it is random and isotropic. This, so called
“zeroth order approximation” of molecular orientation was adequate enough during the
early days of OLED research as it was able to explain the basic morphology (crystallite
free homogeneous layer with nanometer scale surface smoothness) and optoelectronic
properties (luminescence quantum efficiency, charge conduction etc.) of the thin-film
OLED devices and captured the underlying device physics without going into too much
details about the microscopic structure of the active layer. Such a simplistic macroscopic
model supported the development of the first generation of OLED devices. However,
with the maturation of the field and the device power conversion efficiency reaching the
theoretical limit,
3, 18
there have been a sudden explosion of active scientific research in
82
this community aiming at understanding the microscopic picture of the energy conver-
sion process, specifically, the orientation and packing of the guest and the host molecules
in the active layer and their roles in the operation and performance of the devices.
When considering the performance and efficiency of the small-molecule OLEDs,
the importance of molecular orientation can hardly be overstated. In terms of device
physics, the external quantum efficiency (EQE) of the OLEDs depend critically upon
the distribution of the emitting dipoles in the active layer and light outcoupling effi-
ciency. Both these factors, in turn, are influenced by the device architecture, processing
conditions and more importantly, by the optical properties of the layered system, such
as material refractive indices, profile of the emission zone and orientation of the tran-
sition dipole moments of the emitting materials. It has been shown that depending on
the OLED structure, the light outcoupling efficiency can be increased by a factor of 1.5
just by controlling the direction of the emissive dipoles.
9, 10, 19, 20
Furhthermore, charge
injection and charge transport characteristics are also known to be effected (sometimes
of the order of 30 or more) by the structure and orientation of the small molecules in
the active layer of the OLEDs.
21, 22
Therefore, our efforts in increasing the efficiency
of the OLEDs, in order to make them cost-effective commercial alternative to incan-
descent lighting, should find their basis in understanding the fundamentals of molecular
interaction and ordering in these thin-film devices.
Contrary to the general assumption of isotropic molecular distribution in vacuum
deposited thin-film OLEDs, Lin and coworkers first pointed out that a large uniaxial
anisotropy exists in vapor-deposited fluorene films.
6
Later, findings from several other
groups further reinforced the idea of anisotropic molecular orientation in the amor-
phous films of various kinds of OLED materials, such as emitters, hole transport and
electron transport materials.
7, 9, 10, 23–27
Although it has been more or less established
83
that organic molecules with large optical anisotropies (large planar-shaped molecules)
prefer to orient horizontally,
7, 23
such generalizations are not valid for the orientations
of relatively low molecualr weight OLED materials, such as 4,4-bis(N-carbazolyl)-
1,1-biphenyl (CBP) and N,N-di(1-naphthyl)-N,N-diphenyl-(1,1-biphenyl)-4,4-diamine
(NPD). CBP and NPD are excellent hole transport materials that also act as hosts for
various phosphorescent dopants in OLEDs.
28–30
The vapor deposited neat films of CBP
and NPD are considered to have a completely random molecular orientation.
7, 31
This
was mainly attributed to the small intermolecular interactions in these molecules aris-
ing due to the presence of bulky substituents at both ends of their long molecular axes.
Recently, however, it has been found that under controlled experimental conditions, it
is possible to achieve a kinetically-controlled anisotropic molecular distribution in the
low molar-mass semiconductor molecules including CBP and NPD.
11, 32–35
This trend
is particularly noticeable at or near the substrate surface where enhanced mobility of the
free surface promotes the formation of stable molecular arrangements with unusually
different thermodynamic and kinetic properties unattainable by the bulk material.
36–40
Substrate temperature has been identified as one of the key determining factors behind
such stable molecular arrangements as it lets the vapor deposited molecules access a
wide variety of conformations before they settle to a stable configuration.
11, 32, 33, 37
The
first few layers near the surface thus can achieve a unique packing arrangement which
then serves as a template for the incoming molecules to control the overall packing of
the molecules in the bulk. This is particularly important for the guest-host OLED sys-
tems where orientations of the host molecules significantly impact the interactions and
subsequent alignment of the guest molecules.
33, 41, 42
Not only the buried surface, but
the inherent asymmetry of the vapor-deposited free surface of organic host materials
can also drive the orientation of the incoming dopants as shown recently by Jurow et
84
al.
27
Surprisingly, not much studies have been aimed at understanding the orientation
and conformation of the vapor deposited OLED materials at or near the surface, be it
the substrate/material surface or the material/air free surface.
The tool of the trade to measure the molecular orientation in OLED materials is vari-
able angle spectroscopic ellipsometry (V ASE) which provides information on the bire-
fringence of a molecular system as a measure of optical anisotropy.
7
Although V ASE
is used quite frequently in OLED research, we should keep in mind that this technique
gives bulk-averaged optical properties and is not particularly surface sensitive. The same
can be said about angle-dependent photoluminescent emission spectroscopy measure-
ment which is sensitive enough to measure small fractions of guest molecules doped into
a host, but does not provide structural information of the surface-bound molecules.
8, 43
Grazing incidence X-ray scattering (GIXS), on the other hand, is performed with the
light source incident on the top surface of a material system and thus structural param-
eters of the buried surface can no be obtained. Moreover, GIXS is more sensitive to
the crystalline domains present in a semiconductor film than the amorphous regions in
it.
34, 44, 45
So, there is a need for an interface-specific structural probe that can directly
measure molecular structure and orientation of the OLED materials at varoius interfaces.
Recently, vibrational sum frequency generation (VSFG) spectroscopy has been used as
an interface-specific technique to look into the structural changes of the semiconductor
materials at buried interfaces in operational multilayer OLED devices.
46, 47
VSFG is
a second order nonlinear spectroscopic technique that is forbidden in centrosymmetric
media (bulk of a material), but is allowed at interfaces where local symmetry is bro-
ken.
48–50
VSFG, thus, serves as a non-invasive interface-specific technique that can
provide molecular level information of the surfaces and interfaces. Although VSFG
has been extensively used for the in situ characterization of molecular structure at the
85
buried interfaces of polymeric and small-molecule organic photovoltaics (OPVs) and
organic field effect transistors (OFETs),
51–57
the application of this technique in case of
OLEDs is only limited to the measurements of electric field distributions at the buried
interfaces
46, 58–60
and the operational degradation of the devices.
47
In this study, polarization-selective VSFG spectroscopy was used to investigate the
structure and orientation of two commonly used low molecular weight OLED materials,
CBP and NPD in their vapor deposited thin films. Changes in VSFG spectra with respect
to film thickness were monitored to evaluate the origin of spectroscopic response from
these films. Spectral analysis and theoretical modeling suggest that the overall VSFG
activity can be attributed to the contributions from both the substrate/material (buried
surface) and material/air (free surface) interfaces. Furthermore, the collected spectra
indicate a non-isotropic orientation of interfacial dipoles at the film interface. To support
our experimental findings, VSFG measurements of thin films of diindenoperylene (DIP),
an organic semiconductor material known to possess extraordinary structural order in
its vapor deposited films, were performed. A comparison among the measured VSFG
spectra of CBP, NPD and DIP confirms that the amorphous neat films of low molecular
weight OLED materials can have structural order at or near the surface even when they
are deposited on dielectric substrates at room temperature. We ascribe the observed
structural anisotropy of the molecules to two factors: (1) reduced surface mobility of the
molecules due to room temperature deposition condition and (2) a competition between
molecule/substrate and molecule/molecule interactions.
4.2 Materials and methods
CBP and NPD were purchased from Sigma-Aldrich. DIP was obtained from Lumtec.
All organic materials were purified by gradient sublimation before use. Films were
86
deposited on IR grade CaF
2
windows (CeNing Optics, Diameter: 25.40.1 mm, Thick-
ness: 1.00.1 mm, Surface Quality: 40-20S/D). All layers were deposited by vacuum
thermal evaporation (system base pressure of 1-3 X 10
6
Torr) in an Angstrom Engi-
neering EvoVac 800 VTE deposition system attached to a glove box. Inficon SQS-242
deposition software was used to control the deposited material thickness using a 6 MHz
Inficon quartz gold-coated crystal sensor. All film deposition in the VTE were per-
formed at rates between 0.02 and 0.2 nm s
1
while the substrates were held at room
temperature. Films were stored under a nitrogen atmosphere after the preparation.
FTIR spectra of CBP, NPD and DIP were recorded using a Bruker Vertex 80 FTIR
spectrometer under vacuum in transmission geometry. FTIR spectrum of a clean CaF
2
window was used for baseline correction. Raman spectra of the samples were collected
using a Horiba XploRA Raman Microscope system (Model) with a 633 nm laser focused
to a 0.5 μm spot through a Leica microscope with a 100X objective lens.
VSFG was performed with two laser pulses, one picosecond visible laser pulse (nar-
rowband visible pulse) fixed at795 nm and another femtosecond IR pulse (broadband
IR pulse) that can be tuned via a OPA-NDFG system to the desired vibrational reso-
nances of the molecules under study. For our studies of mainly the C=C stretch of CBP,
NPD and DIP, we chose the IR pulse to be centered at 1550 cm
1
. The two laser beams
were spatially and temporally overlapped into the sample surface. The intensities of
the visible and the IR beams used in the experiments were chosen to be6-7 mW and
6-9 mW, respectively. The experimental details of our VSFG spectrometer is detailed
in Chapter 2.
All VSFG spectra were collected and recorded via WinSpec Software (Version 2.5)
and processed in IgorPro (Version 6.2) and Matlab (Matlab 2012b). Optical modeling
was done in Mathematica 10.
87
4.3 Results and discussions
4.3.1 Spectroscopic characterizations of CBP film
Figure 4.1: (A) FTIR, (B) Raman and (C) Polarization-selective VSFG spectra show-
ing vibrational resonances of a 100 nm thin film of 4,4-bis(N-carbazolyl)-1,1-biphenyl
(CBP) in the ring stretching region. The main vibrational mode of CBP, which we are
interested in is highlighted in the figure (blue band) for easy comparison between linear
(FTIR and Raman) and nonlinear (VSFG) measurements. The mode is identified as the
C=C symmetric stretching mode localized mainly on the biphenyl backbone of CBP.
The structure of CBP is shown in the inset of Fig.(A).
The selection rule for the VSFG spectroscopy tells that a vibrational mode needs to
have both nonzero IR and Raman transition moments in order for it to be VSFG active.
To identify the vibratonal modes of CBP, FTIR and steady-state Raman spectra were
first recorded for a 100 nm CBP film deposited on CaF
2
. The result is shown in Figure
4.1(A) and 4.1(B). Although several vibrational resonances of CBP can be seen in the
figure, for the purpose of this study, we will only be considering the vibrational mode
88
at1602 cm
1
(highlighted by the blue line) which is present in both the FTIR and
Raman measurements. The mode at1602 cm
1
is identified as the C=C symmetric
stretch mode which is localized mainly on the biphenyl backbone of CBP and coupled to
the in-plane C-H deformation modes of the molecule.
61
The reason behind choosing this
particular C=C stretch mode in our subsequent analysis is twofold. Firstly, the transition
dipole associated with this mode lies along the long molecular axis of CBP and thus,
the orientation of CBP at the interface can aptly be described by the orientation of this
mode. Secondly, according to our recent findings, only this vibrational mode of CBP has
interfacial origin, whereas the other modes are originating from the bulk of the sample.
62
This is mainly because to the quadrupole transitions of the interfacial molecules that
contribute towards the surface nonlinear susceptibility. The surface orientation of CBP
can not be described with these quadrupole induced transitions.
The appearance of a particular vibrational mode in both FTIR and Raman measure-
ments is a necessary condition for its VSFG activity, but not a sufficient one. In order
to have non-zero VSFG response, a material section must possess an anisotropic distri-
bution of vibrational dipoles. In case of an isotropic material section, such as the bulk
of a material, the randomly oriented dipoles cancel each other and this results in no
VSFG activity from that part of the system. However, we would like to point out here
that such generalization is strictly valid under the framework of electric-dipole approx-
imation where the spatial influence of the applied electric field is not considered.
63–66
For the present study, we will only be concentrating on the vibrational mode of CBP
that follows the aforementioned dipole approximation (the1602 cm
1
band) and thus
can give microscopic information about the surface. Figure 4.1(C) shows the VSFG
spectra of a 100 nm CBP film recorded with all four polarization combinations, namely,
PPP, SSP, SPS and PSS. Here, the VSFG spectrum taken with, say PSS polarization
89
combination, utilizes S-polarized IR beam, S-polarized visible beam and P-polarized
sum frequency beam (from right to left). The observation of VSFG signals in Fig-
ure 4.1(C) strongly indicates anisotropic molecular arrangement in the vapor deposited
CBP film. The presence of the C=C symmetric stretch mode in both the linear (FTIR
and Raman) and nonlinear (VSFG) measurements indicates its dipolar origin (surface
origin). Whereas the one at1507 cm
1
, which is present in either FTIR or Raman
and VSFG indicates its bulk origin. This has been described in detail in our recent find-
ings of VSFG from centrosymmetric molecular systems, such as CBP.
62
Figure 4.1(C)
shows that only PPP and SSP contain the1602 cm
1
band. Between the PPP and
SSP spectra, the SSP spectrum has only the C=C stretch mode and was taken for further
analysis. Nevertheless, the strong VSFG response observed for both the PPP and SSP
spectra tells us that the vapor deposited films of CBP has substantial anisotropy at or
near the surface.
4.3.2 Thickness dependence VSFG study of CBP films
To further investigate the origin of VSFG response from the vapor deposited CBP films,
SSP spectra of the samples were recorded as a function film thickness. Figure 4.2 shows
the changes in intensities of the SSP spectra for five different CBP films of increas-
ing thicknesses. The very similar SSP spectra for all the samples is an indication of
minimal bulk contribution in the VSFG response. Otherwise, the spectrum of the thin
CBP sample would have been very different from the thick one. Additionally, the bulk
response is, in general, proportional to the volume of the material under study and is thus
expected to increase monotonically with the film thickness. The observed spectra do not
follow this trend either. The Figure 4.2 shows that the intensity first increases with the
film thickness upto 100 nm, stays the same for the 150 nm film and then decreases as
90
Figure 4.2: SSP spectra of the vapor deposited CBP films of varied thickness. The
dotted lines represent the VSFG measurements of the same film recorded on different
days.
we increase the film thickness to 200 nm. Such a change in intensity as a function of
sample thickness also suggests that the SSP spectra are not dominated by the bulk sig-
nal, rather demonstrates the incident of optical interference in thin film SFG arising due
to the signals generated from both the free CBP/air and the buried CaF
2
/CBP interface
(See Fig.4.3). Optical interference is a well studied phenomenon for VSFG from thin
films where spectroscopic signals arising from multiple interfaces either constructively
or destructively interfere in the far field.
67–73
4.3.3 Modeling optical interference for CBP films
In order to simulate the thickness dependent VSFG spectra, a theoretical model needs
to be constructed that resembles the thin film system under consideration. Several
approaches have been established to model thickness interference effects in VSFG spec-
troscopy.
67–75
Here, we adopt the interference model proposed by Tong et al.where the
91
Figure 4.3: Optical interference between the signals generated at the buried surface and
the free surface in thin film SFG. The diagram is adapted from Reference 69.
total VSFG intensity is calculated by simulating the thickness dependent Fresnel coeffi-
cients of each of the electric fields at the interface and the frequency dependent second
order nonlinear susceptibilities of the sum frequency active molecules.
70, 71
A four layer
model consisting of two interfaces, CBP/air and CaF
2
/CBP, was adopted for this purpose
and the linear and nonlinear Fresnel factors for the incoming and outgoing beams were
calculated at each of these interfaces.
76
While calculating the Fresnel factors, multiple
reflections within the CBP film and the phase factor accounting for the geometrical path
difference between the successive reflections or transmissions were considered. The
VSFG spectral intensity for the SSP input/output polarization combination can then be
expressed as:
I
SSP
µj L
I
yy
(w
SFG
)L
I
yy
(w
Vis
)L
I
zz
(w
IR
)sinq
IR
c
(2);I
yyz
+ L
II
yy
(w
SFG
)L
II
yy
(w
Vis
)L
II
zz
(w
IR
)sinq
IR
c
(2);II
yyz
j
2
(4.1)
µj L
I
yyz
sinq
IR
c
(2);I
yyz
+ L
II
yyz
sinq
IR
c
(2);II
yyz
j
2
(4.2)
92
Figure 4.4: Simulated results of the modulus of the Fresnel factors for the (A) CBP/air
and (B) CaF
2
/CBP interface for SSP polarization combination as a function of CBP film
thickness and refractive index of the interfacial layer. (C) and (D) Simulated Fresnel
factors for these two interfaces for the n
0
values of 1.35 and 1.60, respectively. The
horizontal cuts along n
0
are shown in (A) and (B). (E) Plot showing the total Fresnel
factor arising from the top and bottom surface of the films versus film thickness. (F)
Comparison between the simulated result (red dotted line) and the observed SSP spectra
(blue dots).
where L
ii
, L
j j
and L
kk
(i, j, j = x, y, z) in Equation 4.1 are the Fresnel coeeficients or local
field factors (L factors) relating the input macroscopic electric fields to the macroscopic
fields at the interface and the superscripts I and II denote the CBP/air and the CaF
2
/CBP
93
interfaces, respectively. c
(2);I
yyz
and c
(2);II
yyz
are the second order nonlinear susceptibility
tensor components of the C=C symmetric stretch mode of CBP at the above two inter-
faces, respectively. The L factors depend on the film thickness, refractive indices and
incident beam angles, whereas the susceptibility parameters only depend on material
structure at the interface. q
IR
is the angle of incidence of the IR input beam. Here, the
positive or negative sign before the L factors in Equations 4.1 and 4.2 denote the abso-
lute orientation of the molecules (up vs. down) at the interfaces. Figure 4.4 shows the
simulated Fresnel factors for the (A) CBP/air and (B) CaF
2
/CBP interfaces as a function
of film thickness and refractive index of the interfacial layer (n
0
). For the simulation,
the value n
0
was varied keeping it consistent with the other optical constants of the CBP
film. The variation of Fresnel coefficients with the change in film thickness shown in
Figure 4.4 depicts optical interference pattern for the multilayer system under study. The
thickness dependence intensity modulation in the collected SSP spectra (See Fig. 4.2)
can now be reproduced if we consider that both the CBP/air and CaF
2
/CBP interfaces
are contributing towards the overall VSFG signal. To do this, two particular values of
n
0
were chosen, one each for the CBP/air interface (n
0
=1.35) and the CaF
2
/CBP inter-
face (n
0
=1.60). The refractive index of the interfacial layer, n
0
is the subject of the usual
uncertainty in VSFG analysis.
77
Here, the values of n0 were taken approximately in
between the bulk refractive indices of the materials (in this case CaF
2
, CBP and air).
For the two particular values of n
0
, Figure 4.4(C) and 4.4(D) show the plots of the sim-
ulated Fresnel coefficients for the CBP/air and CaF
2
/CBP interfaces, respectively. A
comparison between Figure 4.4(C) and 4.4(D) reveals that the SSP spectrum is domi-
nated by the L factor at the CBP/air interface, L
I
yyz
which is almost 5 times greater than
the L factor at the CaF
2
/CBP interface, L
II
yyz
(L
I
yyz
0.06 and L
I
yyz
0.015). Never-
theless, contributions from both these interfaces were taken into account and the total
94
Fresnel factor was then calculated according to the Equation 4.2. The Figure 4.4(E)
shows the sum of Fresnel factor amplitudes, L
I
yyz
+ L
II
yyz
which varies periodically with
the CBP film thickness. The calculated result resembles the thickness dependent inten-
sity variation of the CBP SSP spectra shown in Figure 4.2. Now, in order to compare
the calculated and observed spectra, The spectra were fitted according to the following
equation:
I
(2)
SSP
µ

A
NR
e
if
+
å
q
B
q
w
q
w
IR
iG
q

2
(4.3)
The first term in the Equation 4.3 accounts for the nonresonant part of the response
(instantaneous response from the substrate CaF
2
) with amplitude A
NR
and a phase dif-
ference f with respect to the vibrationally resonant part of the response. The second
part describes the vibrational resonances as a summation of Lorentzian line shapes with
amplitude B
q
, linewidths G
q
and transition frequency w
q
. The amplitude square of
the C=C symmetric stretch mode is then plotted as a function of film thickness (See
Fig.4.4(F)). The plot qualitatively corroborates the thickness dependence Fresnel factor
change shown in Figure 4.4(E). The quantitative difference between the two plots mainly
arise due to the fact that we have not included the intensities of the input beams (visible
and IR) and a constant of proportionality while calculating the overall signal intensity
according to the Equation 4.2. Nevertheless, the agreement between the observed and
calculated results are a clear indication of VSFG activity from the free and buried sur-
faces of the CBP films. Since anisotropicity of the interfacial molecules is a requirement
for VSFG activity, our results suggest that the vacuum deposited CBP molecules possess
a non-isotropic arrangement of molecular dipoles at the surfaces.
95
4.3.4 VSFG studies of NPD and DIP films
Figure 4.5: SSP spectrum of vapor deposited 100 nm NPD film on CaF
2
. The structure
of NPD is also shown in the inset.
To inquire more about the orientations of the small molecule OLED material,
polarization-depent VSFG studies were performed on the vapor deposited thin films of
NPD. Figure 4.5 shows the VSFG spectra of NPD recorded with SSP input/output polar-
ization combinations. The spectrum is dominated by the C=C stretching mode at1610
cm
1
which involve the core biphenyl group of NPD.
78
Here also, the strong SSP signal
from NPD indicates that the molecules are not randomly oriented at the interfaces, but
aligned with preferential dipole orientations.
Having established that we are observing the interfaces of the thin films of CBP and
NPD, and that the molecular arrangements at these interfaces can be quite anisotropic,
we now consider another OLED material, DIP which is know to possess exceptional
structural order in its vapor deposited films. Several studies have shown that thin films of
96
Figure 4.6: SSP spectra of vapor deposited 25 nm DIP film on CaF
2
and SiO
2
. The
structure of DIP is also shown in the inset.
DIP grown on dielectric substrates exhibit high orientational order with DIP molecules
standing upright on the substrate surface.
79–82
This makes DIP an excellent candidate
for elucidating growth and structural order in organic thin films. VSFG spectra of 25
nm films of DIP deposited on CaF
2
and SiO
2
were recorded next and the results are
presented in the Figure 4.6. As expected, both the films show strong SSP response
indicating large structural anisotropicity of the DIP films. The vibrational resonances
observed at1447 cm
1
and1487 cm
1
are assigned to the ring C=C stretching
modes of DIP centered at the indeno and the perylene cores, respectively.
It has been reported previously that in VSFG, the relative phase between the non-
resonant background and the resonant peak can help deduce the absolute orientation
direction (up vs. down) of any vibrational resonance since they interfere with each other
in the detected signal.
83–85
In other words, the vibrational chromophores which are
97
‘pointing up’ and ‘pointing down’ generate two out-of-phase vibrations with respect to
the nonresonant background. For dielectric substrates like CaF
2
and SiO
2
, the nonres-
onant response, A
NR
(See Eq. 4.3) can be assumed have a small positive value. We
may thus deduce the orientation of the studied molecules considering the sign of the
resonance amplitude, B
q
described in Equation 4.3. For the DIP SSP spectra shown
in Figure 4.6, the positive peak of the C=C symmetric stretching mode indicates the
upright alignment of the DIP molecules in its vapor deposited films. We can now com-
pare the relative signs of the resonance C=C mode of CBP and NPD (See Fig.4.1 and
4.5) with that of the DIP spectra. Both CBP and DIP show positive C=C peak similar
to DIP and thus provide strong evidence for the upright molecular orientations at the
interfaces. One point should be noted here. CBP, NPD and DIP being centrosymmetric,
the consideration of up vs. down alignment is not necessary and it is suffice to say that
the molecules have a net preferential orientation at the surfaces.
4.3.5 Rotational anisotropy studies of CBP and NPD surfaces
Figure 4.7: In plane rotational anisotropy of vapor deposited 100 nm thin films of CBP
(left) and NPD (right) for the SSP spectra of the C=C symmetric stretch mode.
98
Next, we studied rotational anisotropy of the CBP and NPD film surfaces for a com-
plete 360

in plane rotation for the SSP input/output polarization of the laser beams. The
result is shown in the Figure 4.7. Each data point in the figure represents the resonant
amplitude of the SSP spectrum of the C=C symmetric stretch mod recorded at any given
azimuthal angle. The vapor deposited small molecules standing upright on the substrate
are not expected to have any in-plane anisotropicity. The isotropic in-plane order is con-
firmed by the observation of the completely symmetric plots for both CBP and NPD as
in Figure 4.7.
4.3.6 On the origin of structural anisotropicity at the surfaces
As possible explanation for the observed structural anisotropicity in the vapor deposited
CBP and NPD films near the surface, we propose an interplay of two competing factors
involved in the film deposition process: the mobility of the molecules near the surface
and the intermolecular interactions between the molecules. In general, surface mobil-
ity is governed by the deposition temperature where an elevated temperature (tempera-
ture close to glass transition temperature) promotes an isotropic molecular arrangement
due to the enhanced mobility of the molecules. This allows the molecules to sample a
large configuration space before being settled to a thermodynamically stable equilibrium
structure. On the other hand, when the molecular deposition takes place at a temperature
much lower than the glass transition temperature, as in the case of CBP and NPD in this
study, the surface structure is dominated by a nonequlibrium configuration which is gov-
erned by the reduced surface mobility of the molecules. This is a kinetically controlled
structure where the conformations of the molecules are locked as they only have a subset
of the total configuration space available to them. In this kinetically controlled regime,
99
the key factor that governs the molecular orientation is the stabilization energy aris-
ing from a competition between the molecule-molecule and molecule-substrate inter-
actions. For dielectric substrate like CaF
2
, the molecule-substrate interaction is much
less than the intermolecular interaction which is dictated by the molecular p-p stabi-
lization energy. Thus, the aligned molecules at or near the surface mainly represent the
kinetically trapped molecules. In case of nonplanar CBP and NPD molecules, thep-p
stabilization energy is considerably lower than the planar DIP molecule. This becomes
apparent as the films of neat CBP and NPD prepared via vacuum deposition quickly
loose orientational order as the film thickness increases and the incoming molecules
start randomizing. This is contrast to the thin films of DIP which is known to retain its
molecular orientation for several hundred angstroms from the substrate surface.
4.4 Conclusions
Surface-selective vibrational sum frequency generation spectroscopy was used to probe
the structural order of vapor deposited glasses of CBP and NPD, two important hole
transport materials frequently used in organic light emitting diodes, at various interfaces.
Thin films of CBP and NPD were prepared on dielectric substrates such as CaF
2
and
SiO
2
at room temperature for this study. The nonzero VSFG response observed for the
samples was an indication of anisotropic molecular arrangements in the vapor deposited
films which were previously been considered to have a random or isotropic distribu-
tion of molecules. In order to inquire about the origin of VSFG response, polarization
dependent VSFG spectra were collected from films of different thicknesses. Thick-
ness dependent changes in the VSFG spectra indicate contributions from both the sub-
strate/material and material/air interfaces. Optical modeling was performed to repro-
duce the experimental trends and explain the molecular orientations of the deposited
100
molecules. A comparison between the VSFG spectra of a highly anisotropic material,
DIP and the spectra obtained from CBP and NDP confirms our inference on structural
order of the vapor deposited low molecular weight OLED materials at or near the sur-
face. The observed molecular arrangement at the surface is attributed to two factors: (1)
Reduced surface mobility of the vapor deposited glasses, and (2) A competition between
the molecule/substrate and molecule/molecule interactions.
101
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109
Chapter 5: Beyond Electric-Dipole
Approximation: Sum Frequency
Generation from Centrosymmetric
Molecules
5.1 Introduction
Sum-frequency generation (SFG) and second-harmonic generation (SHG) spectroscopy
have been established as powerful techniques for studying surface vibrational and elec-
tronic states with intrinsic surface selectivity, sub-monolayer sensitivity and chemical
specificity at the molecular level.
1–3
The interface selectivity of SFG/SHG relies on the
facts that these techniques are even-order nonlinear optical processes (SFG/SHG being
second order with respect to the applied electric fields) and under electric dipole approx-
imation, they occur only in the sections of a material wherein an inherent loss of inver-
sion symmetry occurs. Considering the bulk of a material to be random and isotropic,
such a loss of inversion symmetry occurs only at the surfaces and interfaces and this
ensures SFG/SHG to be surface-selective. Because of the intrinsic surface specificity
and sensitivity, SFG/SHG has found applications in many different areas of science and
110
technology as a non-invasive, label-free and in-situ surface analytical technique to quan-
titatively determine surface molecular structures and dynamics.
4–8
The above-mentioned selection rule implies that these even-order spectroscopic
techniques cannot be applied to study systems consisting of centrosymmetric molecules,
as the nonpolar interfaces are expected to preserve the center of inversion. The absence
of SFG/SHG from such systems can also be interpreted if we consider the mutual exclu-
sion principle: any particular vibrational or electronic transition is either one-photon or
two-photon allowed in a molecule that possesses a center of inversion. Thus, the molec-
ular hyperpolarizability tensor elements, which consist of one-photon and two-photon
transition products and are probed by different input/output polarization combination of
electric fields in SFG/SHG, will be zero for such nonpolar interfaces. Contrary to the
above arguments, several experimental and theoretical results have been published in the
last two decades on the SFG/SHG activity from systems comprised of centrosymmetric
molecules.
9–21
This clearly indicates that the selection rule within the framework of
electric-dipole approximation is not adequate enough to explain SFG/SHG generation
from such interfaces.
However, we would like to point out that, SFG/SHG is not strictly forbidden for
materials with inversion symmetry. Beyond the dipole approximation, higher-order non-
local contributions (such as electric-quadrupole and magnetic-dipole contributions) do
not vanish even in a medium with inversion symmetry and their contributions towards
the overall signal cannot always be ignored.
22–29
Although such nonlocal contribu-
tions arise from higher-order mechanisms, they can become significant for the bulk of a
material, as the number of molecules representing the bulk is many orders of magnitude
higher than the few monolayers of interfacial molecules. Not only the bulk nonlinearity,
but the surface nonlinearly can also be dominated by electric-quadrupole contributions
111
due to the rapid fluctuations of electric fields and variation of refractive indices across
the interface.
19, 20, 30
Thus, successful implementation of SFG/SHG as a surface-specific
technique relies on isolating the true surface signal from that of the bulk. Unfortunately,
such a separation between surface and bulk nonlinearities is not always trivial as certain
parts of the bulk response are indistinguishable from the surface response.
29, 31
Never-
theless, it has been established that for sufficiently polar molecules (more specifically,
polar moieties within molecules), where the geometric dimensions of the SFG/SHG
active groups are much smaller than the surface layer thickness, the bulk contribution to
the reflected SFG/SHG spectra is usually negligible.
25, 26
In the present work, we report measurements of vibrational SFG (VSFG) from a
series of centrosymmetric molecules having different complexity and try to rationalize
the results on a common footing so that the applicability of VSFG can be extended to
study such systems. In this regard, polarization-selective VSFG measurements have
been performed on the thin films of 4,4-bis(N-carbazolyl)-1,1-biphenyl (CBP), N,N-
di(1-naphthyl)-N,N-diphenyl-(1,1-biphenyl)-4,4-diamine (NPD) and diindenoperylene
(DIP). Small molecule based organic semiconducting materials, such as CBP, NPD
and DIP have been in the forefront of research these days due to their applications in
organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic
photovoltaics (OPVs), and organic spintronics.
32–41
The performances and efficien-
cies of such organic-electronic and optoelectronic devices have been shown to be crit-
ically dependent on molecular structure and organization of the constituent molecules
at surfaces and buried dielectric interfaces.
42–46
Although VSFG has been successfully
applied in the past to probe interfacial orientation and organization of organic poly-
meric and small molecules,
47–53
its application in the present scenario poses a challenge,
as centrosymmetric environments, by the virtue of their symmetry, would be silent in
112
VSFG. Also, amorphous organic semiconductor films of CBP and NPD, which are fab-
ricated by vacuum vapor deposition, are known to be inherently random and isotopic.
54
But surprisingly, we observe strong VSFG response from optical quality thin films of
CBP, NPD and DIP.
5.2 Experimental details
5.2.1 Materials
CBP and NPD were obtained from Sigma-Aldrich. DIP was obtained from Lumtec.
All organic materials were purified by gradient sublimation before use. Films were
deposited on CaF
2
substrates. All layers were deposited by vacuum thermal evaporation
(system base pressure of 1-3 10
6
Torr) at rates between 0.02 and 0.2 nm s
1
.
5.2.2 FTIR and Raman measurements
FTIR spectra of CBP, NPD and DIP were recorded using a Bruker Vertex 80 FTIR
spectrometer under vacuum in transmission geometry. FTIR spectrum of a clean CaF
2
window was used for baseline correction. Raman spectra of the samples were collected
using a Horiba XploRA Raman Microscope system (Model) with a 633 nm laser focused
to a 0.5 μm spot through a Leica microscope with a 100X objective lens.
5.2.3 VSFG spectroscopy
VSFG is performed with two laser pulses, one picosecond visible laser pulse (narrow-
band visible pulse) fixed at795 nm and another femtosecond IR pulse (broadband IR
pulse) that can be tuned via a OPA-NDFG system to the desired vibrational resonances
113
of the molecules under study. For our studies of mainly the C=C stretch of CBP, NPD
and DIP, we chose the IR pulse to be centered at 1550 cm
1
. The two laser beams are
spatially and temporally overlapped into the sample surface. The experimental details
of our VSFG spectrometer is detailed in Chapter 2.
5.3 Results and discussion
5.3.1 FTIR, Raman and VSFG studies of CBP films
Figure 5.1 shows the FTIR (a) and steady-state Raman (b) spectra of a 100 nm thin film
of CBP vacuum vapor deposited on a 1 mm thick IR-grade CaF
2
window. Although,
several vibrational resonances are observed in the FTIR and Raman spectra, here we
will only be considering the ones guided by the blue lines (See Fig. 5.1), as these peaks
are present in both linear (FTIR and Raman) and nonlinear (VSFG) measurements. The
active vibrational mode centered at1507 cm
1
is identified as the C-N stretching
mode coupled to the in-plane C-H deformation mode on the biphenyl ring. The mode at
1602 cm
1
is assigned as the C-C stretching mode localized mainly on the biphenyl
backbone and coupled to the in-plane C-H deformations. The peak assignments are
based on the previous infrared study on CBP films
55
and verified by density functional
theory (DFT) calculations. Figure 5.1 (c-f) shows the vibrational bands observed in
VSFG with different polarization combinations, namely PPP, SSP, SPS and PSS, where
the first, second, and third letters correspond to the polarizations of the sum frequency,
visible, and IR input beams, respectively. In VSFG, the appearance of any particular
vibrational mode, measured with different input and output polarization combinations,
is governed primarily by the symmetry of that mode and the orientation of the molecule
with respect to a frame of reference (generally surface or interface).
56–58
The intensity
114
ratios of the active vibrational modes can then be taken into account to quantitatively
determine the average molecular orientation, provided the origin of the VSFG response
can be uniquely identified (surface vs. bulk). Here, in case of CBP, we found that the
measured VSFG signal comprises of contributions from both the surface and bulk.
Figure 5.1: (a) FTIR, (b) Raman, and (c-f) polarization-selective VSFG spectra show-
ing vibrational resonances of a 100 nm thin film of 4,4-bis(N-carbazolyl)-1,1-biphenyl
(CBP) in the ring stretching region. The film is fabricated by vacuum vapor deposition
on CaF
2
. The structure of CBP is shown in the inset of Fig. (a).
5.3.2 On the origin of sum frequency response
In the present study, there seem to be few possibilities underlying the observed VSFG
from the vapor deposited CBP films. The first possibility is the symmetry breaking
115
of the CBP molecules at the interface. Symmetry breaking leads to mode mixing and
subsequently induces a nonlinear polarization at the interface.
16, 19, 28
The second pos-
sibility is the generation of VSFG from the bulk of the system by some higher order
mechanisms, mainly electric-quadrupole transitions. Bulk contribution, generated via
quadrupole transitions has been shown to be particularly important for VSFG performed
in transmission geometry.
25, 26
Not only the bulk response, but the surface response can
also be dominated by such quadrupole transitions.
19
The experimental results also allow
us to infer that in a polarization-selective VSFG study, the surface and the bulk contri-
butions from such a centrosymmetric system have their unique polarization dependence.
We think that these findings will be of general significance toward understanding VSFG
measurements performed on the assemblies of centrosymmetric molecules.
5.3.2.1 Symmetry breaking of CBP molecules at the Interface
Considering the CBP molecules to be centrosymmetric, the same vibrational mode is
not expected to occur simultaneously in both FTIR and Raman measurements. The
observation of the1602 cm
1
mode in both FTIR and Raman spectra (See Fig. 5.1(a)
and 5.1(b)) indicates the possibility of symmetry breaking of the CBP molecules in the
film. In fact, small molecules like CBP are known to exist in multiple conformational
structures in the active layer of OLEDs.
54
This is due to the fact that two aromatic rings
connected by a single bond can have more than one dihedral angle between them. The
crystal structure of CBP also confirms non-planarity of the molecule.(See Appendix
A: Fig. 5.3) Such symmetry breaking leads to the mixing of the degenerate/quasi-
degenerate normal modes via anisotropic perturbation and forms local modes at the
116
interface.
16, 19, 28
Under the dipole approximation, this can result in induced hyperpolar-
izability at the interface. In a previous study, Hommel and Allen performed VSFG mea-
surements at the benzene/air interface and attributed the observed signal to the induced
dipole arising from the distortion of the benzene molecules at the interface.
16
Also
recently, Tahara and co-workers have reported that the simulated SSP spectrum of the
benzene/air interface has substantial contributions from the local modes which originate
from the mixing of the quasi-degenerate IR activen
20
and the Raman activen
2
modes
of benzene.
19
In the present case, the appearance of the1602 cm
1
band in the PPP
and SSP spectra of CBP can be argued to be originating from the mixing of the closely
lying FTIR and Raman bands centered around1600 cm
1
. Such mode mixing is par-
tially facilitated by the symmetry breaking near the surface or interface. In fact, we can
think of an artificial dividing surface, which serves as a boundary between the surface
and the bulk of the system, as an imaginary symmetry-breaking line. This imaginary
dividing plane breaks the symmetry of the molecules and creates anisotropic environ-
ments (above and below the dividing surface). This in turn promotes coupling of the
near degenerate modes via anisotropic perturbation. Although, such symmetry breaking
may induce a nonlinear polarization at the surface, this is not the case for the bulk of the
system, as it will still remain random and isotropic.
5.3.2.2 Surface vs. bulk contribution: Higher-order response
According to the classical macroscopic theory, the effective nonlinear susceptibility, that
generates the sum frequency output, can be expressed as:
24–26, 29, 31
~ c
(2)
S;e f f
=~ c
(2)
S
+
~ c
(2)
B
iDk
(5.1)
117
where~ c
(2)
S
and~ c
(2)
B
are the second order surface and bulk nonlinear susceptibilities,
respectively and Dk k
SF
k
1
k
2
is the wave vector mismatch between input and
output beams. In this expression, the bulk term,
~ c
(2)
B
iDk
consists of three separate contribu-
tions: (1) An electric-quadrupole (and magnetic dipole) contribution of the interfacial
molecules due to the rapid variation of electric field gradient and change of refractive
indices across the interface. This is considered to be a part of the surface dipole; (2)
A surface-like bulk contribution,~ c
(2)
BS
which is redistributed over~ c
(2)
S
and can not be
separated from~ c
(2)
S
. This is mainly because of the non-uniqueness in dividing a given
molecular structure into dipole and quadrupole contributions; (3) A true bulk contri-
bution,~ c
(2)
BB
which can be unambiguously determined from measurements from for-
ward and backward SFG. All these contributions comprising~ c
(2)
B
originate from the
induced quadrupole. Among these three terms, quadrupole transitions of the interfacial
molecules can play a dominant role in the surface nonlinear susceptibility due to the
field discontinuity at the interface. This is particularly important when the surface layer
is structurally not very dissimilar and does not have a stronger polarization compared to
the bulk; e.g. nonpolar interface. It is the case for the nonpolar benzene/air interface as
reported by Matsuzaki and coworkers.
20
In the present study, we also think that part of
the surface nonlinearity is coming from the quadrupole transitions associated with the
change of field gradient across the interface, whereas induced polarization due to sym-
metry breaking do contribute as well. But, we will see below that surface susceptibility
alone is not sufficient to fully account for the observed VSFG from the CBP samples;
the bulk contribution,~ c
(2)
BB
also needs to be taken into account.
118
5.3.2.3 Polarization dependence of bulk response
The observation of the VSFG transitions exactly at the bulk frequency indicates bulk
quadrupole contribution in the signal.
20
In vibrationally resonant SFG from an isotropic
media, the true bulk response,~ c
(2)
BB
can be shown to be nearly the same in the SPS and
PSS and negligible in the SSP polarization combinations, provided the visible pulse is
far away from any electronic resonances.
26
A detailed theoretical derivation of the above
is presented in the Appendix B. In order to compare the intensities of the observed VSFG
signal, all collected spectra are normalized by their acquisition time and with respect to
a gold non-resonant signal. For the SPS and PSS polarization combinations, Figure
5.1(e) and 5.1(f) show the spectra to be alike with a prominent band at1507 cm
1
and a shoulder at1500 cm
1
. The same peaks are also observed in the FTIR spec-
trum of the CBP film (See Fig. 5.1(a)). The similar SPS and PSS spectra indicate that
in a VSFG measurement of a centrosymmetric system, SPS and PSS can selectively
probe the quantifiable bulk response and they are equal in magnitude. The observation
of such a strong bulk quadrupole contribution in the VSFG response is not surprising
considering that the CBP molecule has a considerable quadrupole moment in its equilib-
rium geometry (See Table 5.1). The SSP response (See Fig. 5.1(d)), on the other hand,
mainly consists of: (1) induced dipole due to the symmetry breaking at the interface; (2)
induced quadrupole due to the field discontinuity at the interface; (3) the surface-like
bulk contribution,~ c
(2)
BS
. But, there is negligible~ c
(2)
BB
contribution in the SSP response.
The PPP response (See Fig. 5.1(c)) has contributions from both the surface and the bulk
of the system.
In order to validate the above-mentioned polarization dependence of the VSFG
response, we have performed sum frequency measurements on two other centrosymmet-
ric molecules, namely NPD and DIP. These two molecules are structurally very different
119
Table 5.1: Calculated quadrupole moments of CBP and NPD. Calculations were per-
formed using the Jaguar 8.5 (release 13) software package on the Schrodinger Material
Science Suite (version 2014). Gas-phase geometry optimization was calculated using
the B3LYP functional with the 6-31g

basis set as implemented in Jaguar. Quadrupole
moments of C
6
H
6
and C
6
F
6
are also tabulated for comparison.
Molecules Quadrupole moment (C.m
2
)
C
6
H
6
59
-(29.01.7) X 10
40
C
6
F
6
59
(31.71.7) X 10
40
XX = -719.1 X 10
40
CBP YY = -686.8 X 10
40
ZZ = -677.1 X 10
40
XX = -801.6 X 10
40
NPD YY = -769.5 X 10
40
ZZ = -824.3 X 10
40
when compared to CBP. NPD is structurally more flexible and can have many more dif-
ferent conformational structures compared to CBP.
54
On the other hand, DIP is primarily
a planar molecule with no such structural flexibility. Figure 5.2 shows the VSFG spectra
of the vapor deposited thin films of (A) NPD and (B) DIP for all four polarization combi-
nations. The VSFG spectra of NPD are dominated by the C=C stretching modes which
involve the naphthyl groups (1576 cm
1
), the terminal phenyl groups (1594 cm
1
)
and the core biphenyl group (1610 cm
1
) of NPD (See Fig. 5.2(A)).
60
For Figure
5.2(B), the observed vibrational resonances at1447 cm
1
and1487 cm
1
are iden-
tified as the ring C=C stretching modes of DIP centered at the indeno and the perylene
cores, respectively. We see from Figure 5.2(A) and 5.2(B) that the VSFG responses are
similar in SPS and PSS input/output polarization combinations, whereas the SSP spec-
tra look completely different. The minor variations between the SPS and PSS spectra of
NPD (See Fig. 5.2(A)) can be accounted for if we consider that these two polarization
combinations probe two different susceptibility tensor elements with slightly different
120
Figure 5.2: Polarization dependent VSFG spectra of (A) 100nm thin film of NPD and
(B) 25nm thin film of DIP vapor deposited on CaF2. The molecular structures of
NPD and DIP are shown in the inset. The sum frequency responses for SPS and PSS
input/output polarization combinations are similar for both NPD and DIP.
Fresnel factor coefficients. Nevertheless, the almost identical SPS and PSS spectra of
NPD and DIP corroborate the fact that the induced bulk polarization~ c
(2)
BB
is polarization
selective and it has similar response in SPS and PSS configurations. The situation is
different for SSP polarization combination and it has negligible bulk contribution.
121
5.3.3 Conclusions
In conclusion, a general prescription for the analysis of VSFG response from assemblies
of centrosymmetric molecules is provided. VSFG studies of three different centrosym-
metric molecules, namely CBP, NPD and DIP indicate that the sum frequency activ-
ities from these systems cannot be explained within the framework of electric-dipole
approximation. Comparisons between linear (FTIR and Raman) and nonlinear (VSFG)
spectroscopic measurements indicate that the overall VSFG signal has meaningful con-
tributions from both the interface and the bulk of the materials. This observation is of
general significance and is applicable to any vibrationally resonant SFG measurement
where the geometric dimension of the SFG active group is comparable to the molec-
ular dimension. In the present study, the observation of VSFG signal is explained on
the basis of symmetry breaking at the interface and nonlinear polarization induced by
the quadrupole transitions. Polarization dependent VSFG measurements have allowed
us to demonstrate that the surface and the bulk response from an isotropic media have
unique polarization dependence: the quantifiable bulk contribution is almost equal in
magnitude in SPS and PSS and is negligible SSP polarization combinations. This is
only true when the molecule is far away from any electronic resonance. Although these
results are general and can be extended to account for the VSFG generation from any
centrosymmetric medium, rigorous analysis based on molecular dynamics simulations
and quantum chemical calculations are required in order to comment on the relative
significance of different proposed mechanisms.
122
5.4 Appendix A: Crystal structure of CBP
Figure 5.3: Crystal structure of CBP showing non-planarity of the molecule.
123
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128
Chapter 6: Vibrational Sum Frequency
Spectroscopic Investigation of
Functionalized Si(111) Surface
6.1 Introduction
Functionalization and selective chemical modification of inorganic semiconductor sur-
faces are of interest because of their ability to tune band gaps, manipulate density of
surface states and defect sites, increase atmospheric stability and alter overall optoelec-
tronic properties of the resulting materials.
1–8
The conformation and organization of
chemi- or physisorbed molecules at such heterointerfaces often play important roles in
dictating the coupling between the attached chromophore and the semiconductor mate-
rial, therefore regulating interfacial electron transfer, charge injection and extraction,
charge transport, and electron tunneling at the molecular electronic junctions.
9, 10
In
addition, the nature and the extent of the coupling can be instrumental in modulating
energy flow from the chromophore to the substrate and determining the reaction rates
as well as the reaction mechanisms at the surfaces. A molecular level understanding of
the functionalized semiconductor surfaces is thus necessary to elucidate the interfacial
129
chemistry and chromophore-semiconductor interactions, which in turn can be used as
an input in device engineering.
Covalent attachment of organics to oxide-free crystalline Si surfaces, while still pre-
serving ideal electrical and electronic properties of the H-terminated Si surfaces is an
attractive route for the fabrication of highly passivated Si surfaces for applications in
micro- and nanoelectronics,
6, 11
photonics, solar energy conversion applications,
12
and
chemical and biological sensors.
13, 14
Due to the steric constraints, only the organic
moieties that have the van der Waals diameters comparable to the distances between
the Si atop sites (3.8
˚
A for an unreconstructed 1X1 Si(111) surface
15
) can ensure an
atomically flat and completely terminated Si(111) surface. Methyl group (-CH
3
), hav-
ing a van der Waals diameter of 2.3
˚
A have been shown to achieve a complete coverage
on Si(111) surface through a two-step chlorination/alkylation process.
16–18
Although
CH
3
-terminated Si(111) surfaces have exhibited enhanced resistance to air oxidation
compared to the Si-H surfaces,
19, 20
one of the major limitations of the CH
3
termina-
tion is its kinetic inertness towards further functionalization via subsequent chemical
reactions. To this end, Lewis et al. and several other groups have demonstrated that
functionalization of the Si(111) surfaces with acetylenic moieties has the unique advan-
tage of elaboration of interfacial chemistry through the unsaturated -CC- units while
still maintaining a nearly full surface coverage.
4, 21–24
Alkyne-terminated Si(111) sur-
faces have shown excellent chemical stability towards oxidizing agents compared to the
H- and CH
3
-terminated Si(111) surfaces,
25–27
low surface recombination velocities of
(61) X 10 cm s
1
,
22
low surface defect sites, and offer the possibility of introducing
molecular connectors between Si surfaces and other functional groups or nanostruc-
tures.
28–30
Design rationale specific to the alkyne terminated Si(111) surfaces are being
explored to extend the possibilities of Si-based electronics, but a detailed understanding
130
of such hybrid surfaces is still lacking in particular the conformations and organizations
of the grafted alkyne at the heterointerfaces and the perturbations or electronic effects
of the underlying bulk on the surface. This is mainly due to the lack of available tech-
niques that can specifically probe molecular structures and interactions at the surfaces
and buried dielectric interfaces, which are molecularly thin and often not accessible by
traditional spectroscopic techniques.
Vibrational sum frequency generation (VSFG) spectroscopy offers the required sur-
face selectivity and submonolayer sensitivity for non-invasive in-situ investigation of the
molecular structure and dynamics at the surface and has been proven to be quite useful
in many different areas of chemistry, physics, and biology.
31–34
VSFG spectroscopy
is a second order nonlinear spectroscopic technique and under the dipole approxima-
tion, it occurs only in the region of the material where the inversion symmetry is bro-
ken, particularly at the surface or interface. Thus, vibrational spectra of the interfacial
species can selectively be obtained by this technique without any interference form the
bulk of the system. Since vibrational signatures of a chromophore in the fingerprint
mid-IR region are very sensitive to its local environment, VSFG spectroscopy can be
quite powerful in unraveling the interactions of the chromophore with its neighboring
molecules and/or substrate (adsorbate-substrate interaction) as such perturbations get
imprinted on the recorded surface IR spectra.
35–40
In addition to that, VSFG technique
offers the advantage of polarization selectivity, as in the experiment, the input and output
beam polarizations can independently be controlled to extract information about molec-
ular orientations of the surface species.
41
Recently, we have used polarization-selected
VSFG spectroscopy to investigate the molecular structure and rotational dynamics of
the methyl groups on CH
3
-terminated Si(111) surfaces.
42
An important conclusion
of our previous work is that the methyl group vibrations are strongly coupled to the
131
above-band-gap Raman polarizability of the Si substrate. When the methyl groups are
vertically displaced from the Si(111) surface by linear -CC- type units, as in the case
of propynyl-terminated Si(111) surfaces, the surface structure changes (due to the strain
relief compared to the CH
3
-terminated surface) and so do the nature of interaction of the
methyl groups with the Si substrate. Although, quantum chemical calculations based on
cluster models and periodic boundary conditions have been used in the past to compute
the vibrational frequencies of the propynyl-terminated Si(111) surfaces,
43
to the best
of our knowledge, any experimental observation of the surface vibrational spectrum of
this functionalized surface has not yet been reported. The present chapter describes a
polarization-dependent VSFG study of the propynyl-terminated Si(111) surface to shed
light on the surface structure and the adsorbate-substrate interaction of this important
heterointerface.
6.2 Experimental details
6.2.1 Materials and methods
Water with a resistivity of 18.2 MW cm was obtained from a Barnstead Nanopure
system. Ammonium fluoride (NH
4
F(aq), 40%, semiconductor grade, Transene Co.,
Inc., Danvers, MA) was purged with Ar(g) (99.999%, Air Liquide) for 1 h prior to use.
All other chemicals were used as received. Czochralski-grown n-Si wafers (Virginia
Semiconductor, Fredericksburg, V A) were double-side polished, doped with phosphorus
to a resistivity of 1W cm, 381 25 μm thick, and oriented to within 0.1

of the (111)
crystal plane.
132
6.2.1.1 Preparation of H-Si(111) surfaces
The wafers were cut into 1 cm X 4 cm pieces and rinsed sequentially with water,
methanol ( 99.8%, BDH), acetone ( 99.5%, BDH), methanol, and water. The wafers
were oxidized and organic contaminants were removed by immersing the wafers in a
piranha solution (1:3 v/v of 30% H
2
O
2
(aq) (EMD): 18 M H
2
SO
4
(EMD)) at 95

C
for 10 min. The wafers were removed from the piranha solution and rinsed with copi-
ous amounts of water. The oxide was removed by immersing the wafers in buffered
hydrofluoric acid (HF(aq), Transene Co. Inc., Danvers, MA) for 18 s, rinsing with
water, and immediately placing the wafers in an Ar(g)-purged solution of NH
4
F(aq) for
9.0 min.
24, 44, 45
The wafers were agitated at the start of each minute of etching to remove
bubbles that formed on the surface. The samples were then removed, rinsed with water,
and dried under Ar(g).
6.2.1.2 Preparation of Cl-Si(111) surfaces
The H-Si(111) samples were transferred to a N
2
(g)-purged glove box with < 10 ppm
O
2
(g). An initiating amount (< 1 mg/mL) of benzoyl peroxide 98%, Sigma-
Aldrich) was added to a saturated solution of PCl
5
(99.998% metal basis, Alfa Aesar)
in chlorobenzene (anhydrous, 99.8%, Sigma-Aldrich). The wafers were rinsed with
chlorobenzene and immersed in the PCl
5
solution at 90 2

C for 45 min.
24, 45, 46
The
wafers were removed from the reaction and rinsed with chlorobenzene, then hexanes
(mixture of isomers, anhydrous 99%, Sigma-Aldrich).
6.2.1.3 Preparation of CH
3
-CC-Si(111) surfaces
The Cl-Si(111) surfaces were immersed in a 1.0 M solution of 1-propynyllithium
(CH
3
CCLi, BOC Sciences, Shirley, NY) at 45 2

C for 15 h inside foil-wrapped
133
test tubes. The wafers were removed from the solution and rinsed with hexane, fol-
lowed by methanol (anhydrous, 99.8%, Sigma-Aldrich), submerged in methanol, and
removed from the glove box. The samples were sonicated sequentially for 10 min in
methanol, rinsed with water, and dried under Ar(g). The samples were broken into 1
cmX1 cm squares, rinsed again with water, dried with Ar(g), and sealed under Ar(g)
inside polypropylene centrifuge tubes for transport from Caltech to the University of
Southern California.
6.2.2 VSFG spectroscopy
A detailed description of the VSFG setup is given in Chapter 2. For our studies of
mainly the CH
3
stretch region of CH
3
-CC-Si(111) samples, we chose the IR pulse to
be centered at 3000 cm
1
. The intensities of the visible and the IR beams used in the
experiments were chosen to be6-7 mW and6-9 mW, respectively.
6.3 Results and discussion
6.3.1 VSFG studies of CH
3
-CC-Si(111) surfaces
To study the propynyl-terminated Si(111) surfaces, we recorded the surface vibrational
spectra of the samples in the C-H stretch region with PPP and SSP polarization combina-
tion of the laser beams, where the first, second, and third letters correspond to the polar-
izations of the sum frequency, visible, and IR input beams, respectively. The spectra
are presented in Figure 6.1. A suitable time delay between the IR and the visible pulses
(for the present study, its 270 fs) was employed for collecting both the PPP and the SSP
spectra in order to suppress the nonresonant response of the silicon substrate.
42, 47, 48
134
Figure 6.1: PPP (red) and SSP (blue) polarized VSFG spectra of the propynyl-
terminated Si(111) surface for the methyl stretch region. The time delay between the IR
and the visible pulses was chosen to be 270 fs for the measurements. The corresponding
molecular motions are shown alongside the VSFG spectra.
The PPP spectrum in Figure 6.1 shows three resonant vibrational modes of the termi-
nal CH
3
group and they are identified as: CH
3
symmetric stretch (r
+
) at2865 cm
1
,
CH
3
asymmetric stretch (r

) at2962 cm
1
, and Fermi resonance of CH
3
symmetric
stretch with CH
3
symmetric bending overtone (r
+
FR
) at2933 cm
1
.
24, 42, 43, 49, 50
SSP
spectrum, on the other hand, shows only the CH
3
symmetric stretch (r
+
), at2863
cm
1
and the Fermi resonance (r
+
FR
), at2935 cm
1
, but no CH
3
asymmetric stretch
(r

). To analyze the sum frequency spectra of the CH
3
-CC-Si(111) samples, macro-
scopic averaging over the resonant hyperpolarizability tensor elements (b
(2)
lmn
, defined
in the molecular frame (a, b, c)) representing the methyl group vibrations need to be
135
performed.
41
The molecular hyperpolarizability tensor element,b
(2)
lmn
for the i-th vibra-
tional mode is expressed as a product of the vibrational transition dipole moment and
the Raman polarizability tensor
b
(2)
lmn;i
µ
da
lm
dq
i
dμ
n
dq
i
(6.1)
where q is the normal coordinate and the indices l, m, and n represent axes in the molec-
ular frame of reference. Now, if we consider the fact that different vibrational modes
have different symmetries and the PPP and SSP polarization combinations sample dif-
ferent b tensor elements,
51–53
which are also governed by the molecular symmetries,
the absence of the r

mode in the SSP spectrum gives us a sense of orientation (albeit
qualitative) of the CH
3
-CC- moieties on the Si(111) surface. We approximate the CH
3
terminal methyl group as having C
3v
symmetry, with c being the C
3
symmetry axis and
ac plane lying on one of the s
v
symmetry planes.
51, 53
With respect to this molecule
fixed coordinate system, the three C-H vibrational modes observed in the SFG spec-
tra can now be categorized into two different types of vibrations with very different
hyperpolarizability tensor elements.
54, 55
The r
+
and the r
+
FR
modes account for changes
in the transition dipole moments along the c axis (parallel vibrations), whereas the r

mode accounts for changes along a direction, which is perpendicular to the c axis (per-
pendicular vibration). The correspondingb tensor elements for these modes can also be
categorized into two groups: the totally symmetric r
+
mode corresponds to two isotropic
independentb tensor elements, b
(2)
aac
=b
(2)
bbc
, andb
(2)
ccc
, and the r

mode corresponds to
only one independentb tensor element,b
(2)
aca
=b
(2)
caa
. With the symmetries of theb tensor
elements for the r
+
and r

modes taken into account, along with a knowledge of the
input/output polarization combinations, we can say that if the CH
3
-CC- units were
136
all lying flat on the Si surface, the SSP spectrum (which samples c
(2)
yyz
tensor element
defined in the laboratory frame of reference (x, y, z)) would contain no peaks. On the
other hand, if the CH
3
-CC- units were all standing perpendicular to the Si surface, the
SSP spectrum would contain only peaks corresponding to the parallel vibrations. This
is exactly what we observed in the SSP spectrum displayed in Figure 6.1 indicating that
the CH
3
-CC- groups are oriented perpendicular to the Si(111) surface. The PPP spec-
trum contains peaks corresponding to both the parallel and perpendicular vibrations, as
it is comprised of four different susceptibility tensor elements (c
(2)
xxz
,c
(2)
xzx
,c
(2)
zxx
andc
(2)
zzz
)
sampling molecular hyperpolarizabilities in all possible directions.
Figure 6.2: Polar plot showing the azimuthal dependence of the CH
3
(A) symmetric
stretch (r
+
) amplitude and (B) asymmetric stretch (r

) amplitude of the PPP spectra of
as prepared CH
3
-CC-Si samples for a complete 360

in-plane rotation. The blue solid
lines are fits.
6.3.2 Anisotropy studies of CH
3
-CC-Si(111) surfaces
Next, the rotational anisotropy of the CH
3
-CC-Si(111) sample was measured,
42, 56–58
for azimuthal angle, f varying from 0

to 360

for the PPP polarization combination
137
and thef dependencies of the r
+
and r

modes are presented in Figure 6.2. The data
points (solid red circles) in the Figure 6.2(A) and 6.2(B) represent the resonant ampli-
tudes of the r
+
and r

modes, respectively and are obtained from the fits of the PPP
spectra collected by in-plane rotation of the sample at 30

intervals. The polar plot cor-
responding to the r
+
mode shown in Figure 6.2(A) is completely symmetric, whereas
the one corresponding to the r

mode in Figure 6.2(B) shows a 3-fold dependence on
the azimuthal angle. The azimuthal dependence of the vibrationally resonant signals
in the observed VSFG spectra can be accounted for if we consider that the effective
second order response from the functionalized Si(111) surface has contributions from
both the surface and the bulk of the system. Each of these contributions comprises of
an isotropic component and an anisotropic component, and they depend on the crystal
symmetry. For the Si(111) surface that has 3-fold rotation symmetry, the generated sum
frequency field has a form
E
SFG
=(a+ ccos(3f+f
0
))E
Vis
E
IR
(6.2)
where a and c are the isotropic and anisotropic contributions to the response, respec-
tively; f is the azimuthal angle within the (111) plane, with a phase correction of f
0
;
E
Vis
and E
IR
are the input visible and the mid-IR fields, respectively. The observed
azimuthal dependence of the VSFG signals can now be connected to the resonance
vibrational modes via the uniqueb tensor elements describing the methyl group vibra-
tions. As mentioned in the previous section, the non-zerob tensor elements for the r
+
mode are symmetric with respect to the C
3
axis (b
(2)
aac
=b
(2)
bbc
andb
(2)
ccc
). This is because
of the facts that the IR transition moment for the r
+
mode is along the C
3
axis (which
is along c) and the Raman polarizability tensor is totally isotropic about the symmetry
138
axis. Hence, if the C
3
axis of the terminal methyl group is normal to the surface (which
we have shown is the case here), no azimuthal anisotropy for the r
+
mode is expected
regardless of the in-plane orientation of the methyl group. A totally symmetric plot
shown in Figure 6.2(A) corroborates the above hypothesis. On the other hand, the b
tensor elements for the r

mode comprise of off-diagonal Raman polarizability tensor
elements (a
ac
=a
ca
), which are not symmetric about the C
3
axis. Thus, the r

mode
shows a 3-fold rotational anisotropy (See Fig. 6.2(B)) reflecting the 3-fold symmetry
of the crystalline Si(111) surface. The 3-fold anisotropy of the r-mode also indicates
that the methyl groups do not rotate freely at room temperature, but locked in one of the
three minima in registry with the Si(111) surface.
A B
Figure 6.3: Polar plot showing the azimuthal dependence of the CH
3
(A) symmetric
stretch (r
+
) amplitude and (B) asymmetric stretch (r

) amplitude of the PPP spectra of
CH
3
-Si samples for a complete 360

in-plane rotation. The blue solid lines are fits. The
figure is taken from Reference 42.
Here, we would like to point out one key observational difference between the
azimuthal anisotropy of the r
+
mode observed in the PPP spectra of the CH
3
-terminated
and CH
3
-CC-terminated Si(111) surfaces. In our previous study, we showed that in
case of the CH
3
-terminated surfaces, a clear 3-fold azimuthal anisotropy was observed
for the r
+
mode (considering the mode is parallel to the C
3
symmetry axis; see Fig.
139
6.3).
42
But no such anisotropy is observed for the CH
3
-CC-terminated Si(111) sur-
faces (See Fig. 6.2(A)). This can be explained on the basis of coupling between the
methyl group vibrations and the electronic degrees of freedom of the underlying Si(111)
substrate. In the CH
3
-Si(111) system, CH
3
molecular vibrations are strongly coupled to
the Si substrate due to the close proximity of the methyl groups to the Si surface, and
thus, the resultant Raman polarizability derivative for the methyl group vibrations can
be expressed as
da
lm
dq
i
=
da
Si
dq
i
+
da
Me
dq
i
(6.3)
wherea
Si
anda
Me
are the electronic polarizability of the Si substrate and the Raman
polarizability of the -CH
3
vibration, respectively. Since polarizability scales with vol-
ume, the magnitude of a
Si
is much greater than that of the methyl group polarizabil-
ity, a
Me
and the measured VSFG signal is mainly dominated by the above-band-gap
electronic response of the Si substrate. As a result, the resultant molecular hyperpolar-
izabilities corresponding to the r
+
and r

modes show the same 3-fold anisotropy of
the Si(111) bulk irrespective of the orientations of their transition moments (parallel vs.
perpendicular). However, the extent of the adsorbate-substrate coupling decreases as the
methyl groups are further displaced from the Si(111) surface by -CC- type units, and
thus, the hyperpolarizability tensor elements for the r
+
mode corresponding to the CH
3
-
CC-Si(111) samples only reflect the totally symmetric Raman polarizability tensor of
the methyl vibrations.
140
Figure 6.4: Polar plot showing the azimuthal dependence of the CH
3
(A) symmetric
stretch (r
+
) amplitude and (B) asymmetric stretch (r

) amplitude of CH
3
-CC-Si sam-
ples annealed at 200

for 16-20 hours under vacuum. The blue solid lines are fits.
6.3.3 Effect of annealing on adsorbate-substrate coupling interac-
tion
Lastly, we investigated the effect of annealing on the stability of the propynyl-terminated
Si(111) samples. To examine this, the samples were heated at 160-200

C for 16-20
hours under vacuum, and then cooled it down, and immediately recorded the VSFG
spectra at room temperature. The baked samples show the same set of SFG transitions
that we have seen previously for the untreated samples (See Fig. 6.1), namely, the
CH
3
symmetric stretches and the Fermi resonances for both the SSP and PPP spectra,
and an additional CH
3
asymmetric stretch for the PPP spectrum (See Appendix A: Fig.
6.7). The spectra show decreases in peak intensities along with shifts in the peak posi-
tions (5-9 cm
1
blue shift). But, what is rather interesting is the azimuthal rotational
anisotropy of the r
+
and r

modes of the thermally treated samples. The anisotropy
data presented in Figure 6.4 now shows a 3-fold rotational anisotropy for the r
+
mode
141
(See Fig. 6.4(A)) and this clearly contradicts our previous explanation based on Figure
6.2(A). On the other hand, the r

mode shows the same 3-fold rotational anisotropy
(See Fig. 6.4(B)), as predicted before, but it is now even more pronounced compared to
Figure 6.2(B). The observations of such azimuthal dependency of the vibrational modes
(especially in case of the r
+
mode) can be logically explained if we consider that the
annealing process removes some of the CH
3
-CC- units from the Si(111) surface creat-
ing surface defect sites. A decrease in SFG signal intensity of the C-H stretching modes
supports our statement. Removal of the CH
3
-CC- units creates void spaces on top
of Si(111) surface, in an otherwise closely packed structure (propynyl achieves a 100%
coverage on the Si(111) surface) and thus, some of the CH
3
-CC- units adjacent to the
defect sites tilt away from the surface normal. The whole situation is depicted in Fig-
ure 6.5. As the direction of the C
3
symmetry axis of the terminal methyl group is no
longer perpendicular to the Si surface, the representative b tensor elements for the r
+
mode may now have resolvable components parallel to the surface and thus, can couple
to the electronic response of the Si(111) surface. A 3-fold azimuthal anisotropy is thus
observed (See Fig. 6.4(A)) for the symmetric r
+
mode. The tilt of the C
3
symmetry axis
also brings the CH
3
-CC- units closer to the Si surface and this results in an increase
in coupling between the CH
3
-CC- units and the Si surface. We believe that the higher
modulation depth of the r

azimuthal response (See Fig. 6.4(B)) is due to this increased
propynyl-Si coupling.
6.4 Conclusions
In summary, a detailed understanding of the surface structure and the adsorbate-
substrate interaction picture are provided for the propynyl-terminated Si(111) surfaces
142
Δ
Figure 6.5: Annealing causes removal of the few CH
3
-CC- units from the Si(111)
surface creating surface defects. To reduce the steric strain, a CH
3
-CC- unit adjacent
to the void space tilts towards the Si(111) surface. This results in an increase in coupling
between the CH
3
-CC- units and the Si(111) surface.
with the aid of surface-selective VSFG spectroscopy. The well-resolved vibrational C-
H stretch resonances of the terminal methyl groups, recorded with the PPP and SSP
polarization combinations of the laser beams, gave us information about the orienta-
tion of the CH
3
-CC- units on the crystalline Si(111) surface. Furthermore, the rota-
tional anisotropy of the methyl vibrational modes were also investigated. The observed
azimuthal dependencies of the C-H modes were found to be quite insightful regarding
the coupling between the adsorbate vibrational degrees of freedom and the electronic
response from the Si substrate. Our findings suggest that the terminal CH
3
hydro-
gens are locked in registry with the 3-fold symmetry of the Si(111) substrate. These
results, when compared to our previous studies of the methyl terminated Si(111) sur-
faces, gave us an opportunity to systematically explore the adsorbate-substrate interac-
tion picture for the functionalized Si surfaces. To further study the coupling between the
CH
3
-CC- units and the Si substrate, we performed VSFG measurements on thermally
treated propynyl-terminated Si(111) samples. In case of the annealed samples, different
143
anisotropic behaviors were observed, which are explained on the basis of the formation
of surface defect sites that are created following the heat treatment and the subsequent
structural relaxation of the propynyl units adjacent to the defect sites. As new synthetic
routes are being explored to directly attach controlled functionality on the Si surfaces via
the reactive alkyne units, we think that our findings for the propynyl-terminated Si(111)
surfaces will be of general significance towards understanding such functionalized Si
heterointerfaces, and thus, provide valuable input in the Si-based device engineering
process.
144
6.5 Appendix A: VSFG spectra of baked CH
3
-CC-
Si(111) samples
Figure 6.6: PPP and SSP spectra of the methyl stretch region of CH
3
-CC-Si(111)
samples baked at 320

C for 16-20 hours under vacuum. The decrease in intensity is
ascribed to the removal of the CH
3
-CC- units from the Si(111) surface. The slight blue
shift of the peaks is denoted to the increased coupling between the propynyl units and
the Si(111) substrate as a result of closer proximity of the adsorbate to the Si surface.
145
6.6 Appendix B: VSFG spectra of baked CH
3
-CC-
Si(111) samples
Figure 6.7: Transmission infrared spectroscopy (TRIS) data for CH
3
-CC-Si(111) sur-
faces collected at 74

(bottom) and 30

(top) from the surface normal. (a) High energy
region and (b) Low energy region. The high energy region exhibited three distinct C-H
stretching peaks at 2958, 2934 and 2872 cm
1
. The absorbance features at 2934 and
2872 cm
1
were observed only at the 74

incidence angle, which indicated that those
features arose from modes perpendicular to the surface, whereas the absorbance at 2957
cm
1
was observed at both angles and was, therefore, not perpendicular to the surface.
The Figure is taken from Reference 24.
146
6.7 Appendix C: Tables for data fitting
Table 6.1: Fitting parameters for the rotational anisotropy in the resonant amplitudes of
the PPP spectra.
As prepared CH
3
-CC-Si(111) samples
a (a.u.) c (a.u.) f
0
(rad)
CH
3
r
+
0.014 0.286 104.94
CH
3
r

0.103 0.300 84.520
Baked CH
3
-CC-Si(111) samples
a (a.u.) c (a.u.) f
0
CH
3
r
+
0.009 0.033 75.463
CH
3
r

0.024 0.500 80.905
147
Table 6.2: Fitting parameters for PPP spectra for as prepared CH
3
-CC-Si(111) samples.
Rotation A
NR
f B
1
G
1
w
1
B
2
G
2
w
2
B
3
G
3
w
3
Angle (deg) (a.u.) f (a.u.) (cm
1
) (cm
1
) (a.u.) (cm
1
) (cm
1
) (a.u.) (cm
1
) (cm
1
)
0 -0.006 0.6 0.3 11.1 2869.7 0.5 15.5 2930.8 0.3 13.5 2958.5
30 -0.003 0.1 0.3 10.7 2867.7 0.4 13.7 2928.2 0.2 11.8 2954.9
60 -0.002 0.5 0.3 10.6 2868.3 0.4 14.1 2928.7 0.3 13.6 2956.9
90 -0.005 0.9 0.2 10.9 2870.0 0.3 13.8 2931.6 0.3 14.7 2961.5
120 -0.005 0.9 0.2 10.9 2870.0 0.3 13.7 2931.6 0.3 14.7 2961.5
150 -0.008 1.0 0.3 11.5 2870.0 0.4 14.7 2931.6 0.2 12.4 2958.7
180 -0.004 0.8 0.3 11.2 2870.0 0.4 14.7 2931.9 0.3 14.4 2960.8
210 -0.005 1.5 0.3 10.2 2869.7 0.4 15.0 2929.9 0.4 16.8 2960.0
240 -0.004 0.5 0.3 11.4 2868.1 0.4 15.3 2928.7 0.3 13.6 2957.2
270 -0.014 1.1 0.3 11.6 2867.0 0.4 15.0 2928.5 0.2 11.7 2955.5
300 -0.005 0.5 0.3 11.5 2869.4 0.5 15.8 2930.2 0.3 14.2 2959.1
330 -0.002 1.5 0.3 11.3 2870.0 0.4 14.5 2932.0 0.4 16.0 2961.6
360 -0.007 1.0 0.3 11.7 2870.0 0.4 15.2 2932.0 0.3 14.3 2960.9
148
Table 6.3: Fitting parameters for PPP spectra for baked CH
3
-CC-Si(111) samples at 320

C under vacuum Values for only
the CH
3
symmetric and asymmetric stretches are given.
Rotation A
NR
f B
1
G
1
w
1
B
4
G
4
w
4
Angle (deg) (a.u.) X10
4
f (a.u.) X10
2
(cm
1
) (cm
1
) (a.u.) X10
2
(cm
1
) (cm
1
)
0 4.4 2.1 4.1 11.1 2878.0 6.0 12.3 2972.6
10 8.3 3.5 4.0 11.5 2878.0 4.3 10.8 2971.7
20 14 0.8 3.9 11.4 2877.1 3.2 9.7 2970.3
30 25 1.1 3.1 11.2 2875.9 2.4 9.2 2970.4
40 27 1.1 2.8 11.2 2875.9 2.3 9.2 2971.1
50 12 0.8 3.2 11.2 2877.2 3.5 10.4 2971.5
60 10 -1.0 5.8 12.8 2877.1 6.3 12.2 2973.1
70 9.4 1.6 3.5 11.8 2878.0 6.1 12.2 2971.6
80 14 1.1 3.7 12.6 2878.0 6.2 11.7 2971.3
90 18 1.0 3.4 11.7 2877.9 6.3 11.6 2971.0
100 17 1.1 3.3 11.5 2878.0 6.5 11.7 2971.0
110 13 1.3 3.4 12.5 2877.9 6.4 11.5 2971.7
120 8.3 1.9 2.9 11.1 2877.3 6.1 11.5 2972.1
149
Table 6.3: Fitting parameters for PPP spectra (Continued)..
Rotation A
NR
f B
1
G
1
w
1
B
4
G
4
w
4
Angle (deg) (a.u.) X10
4
f (a.u.) X10
2
(cm
1
) (cm
1
) (a.u.) X10
2
(cm
1
) (cm
1
)
130 8.6 3.6 2.8 11.4 2874.9 4.4 10.3 2972.2
140 23 -2.1 2.2 10.7 2873.3 3.1 9.6 2972.1
150 42 1.1 1.6 10.3 2871.9 2.0 8.7 2971.3
160 38 1.2 1.8 10.7 2871.7 2.2 8.8 2971.3
170 24 1.1 2.2 11.8 2872.1 2.7 9.1 2972.5
180 6.2 0.8 2.4 10.6 2875.9 5.3 10.8 2971.9
190 12 1.5 3.3 11.6 2878.0 6.2 11.8 2971.6
200 15 1.1 3.1 9.5 2876.0 6.0 11.4 2971.7
210 20 1.0 3.8 10.9 2878.0 6.5 12.2 2970.6
220 18 1.0 4.2 11.9 2878.0 6.5 12.2 2970.9
230 17 1.1 3.3 11.5 2878.0 6.5 11.7 2971.0
240 4.9 2.2 3.9 12.1 2878.0 5.6 12.1 2972.3
250 8.6 3.6 3.8 12.1 2877.3 4.4 11.0 2971.3
150
Table 6.3: Fitting parameters for PPP spectra (Continued)..
Rotation A
NR
f B
1
G
1
w
1
B
4
G
4
w
4
Angle (deg) (a.u.) X10
4
f (a.u.) X10
2
(cm
1
) (cm
1
) (a.u.) X10
2
(cm
1
) (cm
1
)
260 28 1.1 2.7 11.3 2875 2.4 9.5 2971.7
270 3.9 1.0 2.8 12.4 2875.1 2.7 11.5 2971.3
280 32 1.1 2.7 11.7 2874.8 2.1 9.4 2970.8
290 15 0.8 3.6 11.7 2876.5 3.5 10.3 2971.2
300 5.9 6.0 4.3 12.7 2877.4 4.9 11.6 2971.8
315 15 1.4 5.1 11.4 2878.0 7.8 13.3 2971.4
330 19 1.1 4.5 10.8 2878.0 6.6 12.3 2970.7
340 19 1.2 5.0 11.0 2878.0 7.7 13.4 2970.8
350 13 1.4 5.0 11.5 2878.0 7.4 12.9 2971.5
360 5.9 6.0 4.3 12.7 2877.4 4.9 11.6 2971.8
151
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Abstract (if available)

Abstract Although molecularly thin in nature, the surfaces and interfaces play an important role in many physical and chemical processes occurring around us. From industrially important processes such as heterogeneous catalysis and charge generation in organic photovoltaic materials to the basic metabolic mechanisms such as respiration—all of them take place at some interfaces, either living or dead. But it is often difficult to probe those interfaces because of their buried nature, complex environment, and problem in isolating signals from the bulk. A molecular level understanding of the structure and organization of the surfaces and interfaces is necessary to gain insights into such processes. Even-order nonlinear spectroscopic techniques such as second harmonic generation (SHG) and sum frequency generation (SFG) possess surface selectivity and thus can specifically provide information about the surface and surface-bound processes. In this dissertation, vibrational sum frequency generation spectroscopy is used as a surface selective probe to study a variety of inorganic and organic semiconductor surfaces that have applications in energy conversion, communication, computing, catalysis, sensing, biotechnology, and life sciences. A systematic understanding of the interrelationship between the structure and property of these semiconductor surfaces is sought throughout the course of this work.

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Creator Dhar, Purnim (author)

Core Title Understanding organic and inorganic semiconductor surfaces by coherent nonlinear spectroscopy

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 05/03/2017

Defense Date 12/03/2015

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

Tag chemistry (physical chemistry),nonlinear spectroscopy,OAI-PMH Harvest,renewable energy,semiconductor materials,solar cell,sum-frequency generation,surface characterization,surface-selective spectroscopy

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Language English

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Advisor Benderskii, Alexander V. (committee chair), Bradforth, Stephen E. (committee member), Nakano, Aiichiro (committee member)

Creator Email pdhar@usc.edu,purnimd@gmail.com

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