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NANOPARTICLE ASSEMBLY IN POLYMER THIN FILMS
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TZU-CHIA (ERICA) TSENG

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5/08 KIPrq/AccaiPrelelRC/Dateoueindd

NANOPARTICLE ASSEMBLY
IN POLYMER THIN FILMS AND POLYMER MELTS

By

Tzu-Chia (Erica) Tseng

A DISSERTATION

Submitted to
Michigan State University
in partial ﬁtlﬁllment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
Materials Science and Engineering

2010

ABSTRACT

NANOPARTICLE ASSEMBLY
IN POLYMER THIN FILMS AND POLYMER MELTS

By

Tzu-Chia (Erica) Tseng

The assembly of nanoparticles in polymeric materials has drawn great attention,
due to its extraordinary potential for a variety of nano-scale applications. In this work, we
ﬁrst investigated the assembly of nanoparticles to interfaces in polymer thin ﬁlms, as well
as with the presence of large subjects on the substrate. According to cross-sectional
transmission electron microscopy (TEM) images, it was found that pyridine stabilized
cadmium selenide nanoparticles assemble on the substrate and also along large subjects
such as silica particles or nanotubes, after being thermal annealed above the glass
transition temperature of the polymer. The assembly of nanoparticles is driven by the
interplay between entropic and enthalpic forces, which dictate their assembly direction in
polymer thin ﬁlms. Remarkably, even when ﬁlm thicknesses were reduced to be thinner
than the surface subject sizes, the ﬁlms followed the curved features instead of dewetting
ﬁ'om the substrate, indicating very strong assembly energy. We demonstrated these three-
dimensional surfaces through atomic force microscopy (AFM). Finally, we explored the
assembly of nanoparticles in polymer melts with the presence of large silica particles and
found that nanoparticle assembly around the silica particles increased with longer
annealing time. This is because once the nanoparticles diffuse in the polymers and reach

the surfaces of silica particles, they adhere on the curved surfaces due to entropic forces.

This dissertation is dedicated to my family and friends,

for their love and support over the years.

iii

ACKNOWLEDGEMENTS

This Ph.D. work would not be accomplished without many people’s support. First
of all, I would like to acknowledge my advisor Professor Michael Mackay for his
guidance on my research at Michigan State and University of Delaware. Professor Phil
Duxbury is always there supporting me for a wide range of physics knowledge to help
my understanding. Moreover, my group members, Jon Kiel and Jon Seppala, are my best
friends in the lab from MSU to UD, and thank for their patience teaching me a lot of lab
skills as well as other knowledge; it was always fun in the lab with their accompanying.

I am also grateful for Dr. Erin McGarrity’s help in the theory and very helpful
discussions. Dr. Alicia Pastor is my mentor for transmission electron microscopy; she and
Ewa Danielewicz always motivated me a lot when I was in the microscope lab at MSU. I
want to especially thank for Dr. Tiffany Bohnsack, previous group member, for
instructing me the operation of atomic force microscope. Also, other Mackay group
members, Dr. Dave Bohnsack, Dr. Melissa Yaklin, Dr. R.S. Krishnan, Megan
Romanowich, inspired me in the early stage of my Ph.D. study.

After moving to University of Delaware, Frank Kriss and Dr. Chao Ni became my
strongest support when using TEM there. At the Materials Science & Engineering
Department, I got a lot of help ﬁ'om the staff and graduate students building precious
friendships. Particularly, I want to thank for Dr. Conan Weiland’s help in the
proofreading of this manuscript. Also, new group members at UD - Brett Guralnick,
JeongJae Wie, Hao Shen, and Wenluan Zhang — shared their enthusiasm and it was great

to brainstorm ideas with them.

iv

Finally, my parents and my brother give me endless love and always cheer for me.
And, last but not least, I want to thank for all friends of mine from MSU to UD, as well
as those in Taiwan or anywhere around the world, providing me the courage to face those

difﬁculties through my PhD life and enjoy fun things too.

TABLE OF CONTENTS

LIST OF TABLES ................................................................................ viii
LIST OF FIGURES ................................................................................. ix
CHAPTER 1: Introdcution ........................................................................... 1
1 . 1 Motivation .................................................................................... 1
1.28ackground ............................................................................................................. 3
CHAPTER 2: Nanoparticle Assembly in Thin Polymer Films ................................ 6
2.1 Introduction ................................................................................. 6
2.2 Experimental Methods .................................................................... 10
2.2.1 Materials ............................................................................. 10
2.2.2 Transmission Electron Microscopy .............................................. 11
2.3 Results and Discussion ................................................................... 12
2.3.1 Nanoparticle Assembly in Polymer Films in T wo- and T hree- Dimensions 12
2.3.2 Assembly of Multi—Nanoparticles in Polymer Films ............................ 16
2.4 Conclusion ................................................................................. 20
CHAPTER 3: Three-Dimensional Liquid Surfaces through Nanoparticle Assembly 21
3.1 Introduction ................................................................................ 21
3.2 Experimental Methods ................................................................... 23
3.3 Results and Discussion ................................................................... 26
3.4 Conclusion ................................................................................. 42
CHAPTER 4: Nanoparticle Assembly along Single-Walled Carbon Nanotubes ......... 43
4.1 Introduction ................................................................................ 43
4.2 Experimental Methods .................................................................... 45
4.3 Results and Discussion ................................................................... 47
4.3.1 One-Dimensional Assembly of Nanoparticles on Nanotubes ................. 47
4.3.2 Two-Dimensional Assembly of Nanoparticles along Nanotubes 49
4.4 Conclusion ................................................................................. 57
CHAPTER 5: Nanoparticle Assembly on Silica Particles in Polymer Melts .............. 58
5.1 Introduction ................................................................................ 58
5.2 Experimental Methods ................................................................... 60
5.3 Results and Discussion ................................................................... 62
5.4 Conclusion ................................................................................. 74
CHAPTER 6: Summary and Conclusions ....................................................... 75
Appendix A: Derivation of Einstein Diffusion Equation for Free Difﬁrsion around a
Spherical Object ...................................................................................... 77
Appendix B: Adhesion of Nanoparticle-ﬁlled Polymer Films to Substrates ............... 81

vi

Appendix C: X-ray Photoelectron Spectroscopy Spectrum .................................. 83
Appendix D: Additional Data .................................................................... 85

References ............................................................................................ 86

vii

LIST OF TABLES

Table 2.1. The refractive index ofthe materials used in chapter 2 17

Table 3.1. Different concentration of the stock solution is required to manufacture a
given ﬁlm thickness when spin coated at 5000 RPM for 40 seconds. The weight
percentage of p-QD in the stock solution is calculated based on the ﬁlm thickness and
that required to coat the silica particles (121 d: 14 nm) and the substrate with a

monolayer (6 = 1.007, 2a = 5.3 nm) ...................................................................... 28

Table 3.2. Different concentration of the stock solution is required to manufacture a
given ﬁlm thickness to coat the silica particles (39 i 7 nm) and the substrate with a
monolayer (6 = 1.0007, 2a = 5.3 nm) .. ........................................................................ 39

Table 4.1. The reﬁ'active index ofthe materials used in chapter 4 52

viii

LIST OF FIGURES

Figure 1.1. Entropy pushes nanoparticles assemble on the substrate — polymer
interface .. ....................................................................................................................... 4

Figure 2.1. A layer of PS and CdSe quantum dots stabilized with a pyridine steric layer
(p-QD) was spin coated onto a silicon wafer and thermally annealed at 160°C for 16 hrs.
A cross-sectional transmission electron micrograph revealed a p-QD layer at the substrate
(a), due to a combination of their larger surface energy than polystyrene (enthalpy) as
well as the entropy gain through releasing the polymer ﬁom the substrate constraint. A
plan view of a similar ﬁlm demonstrates a densely packed layer of p-QD’s (b) although a
Fourier transform of the image (inset) shows no long range

order........... ................................................................................................................. 13

Figure 2.2. Cross-sectional TEM images: (a) If the silicon substrate was decorated
sparsely with Si02 particles and a layer of 12.3 wt% p—QD’s and crosslinkable PS was
spin coated on top and annealed, p-QD’s assembled along the substrate and around the
silica particle. (b) A similar sample to a) but with a higher concentration of p-QD’s (21.6
wt%) .................................................................................................. 14

Figure 2.3. Cross-sectional TEM image of a polymer ﬁlm resulting from spin coating a
solution blend of 75kD polystyrene, p-QD, and SiOz particle. The ﬁlm was thermally
annealed at 160°C, for 17 hrs ..................................................................... 17

Figure 2.4. (a) Cross-sectional TEM image of a polymer ﬁlm resulting from spin coating
a solution blend of crosslinkable polystyrene, p-QD, and polystyrene particle. The ﬁlm
was crosslinked at 220°C for 30 mins. (b) The same sample as (a) but at a different

area ................................................................................................... 19

Figure 3.]. AFM images: (a) A silicon wafer was sparsely decorated with 121 i 14 nm
diameter silicon dioxide (SiOz) particles by spin coating a dilute solution of them in water.
(b) The nominal 63 nm thick polystyrene ﬁlms containing pyridine-coated cadmium
selenide quantum dots (p-QD’s) spin coated onto (a) and thermally annealed for 24 hrs
(0). Scan size is 10 um * 10 pm in all images ................................................... 27

Figure 3.2. A sparse layer of SiOz particles 121 i 14 nm in diameter were deposited onto
a silicon wafer, then various thickness ﬁlms of p-QD and 75kD PS were spin coated onto
this rough substrate. The system was thermally aged for 24 hrs at 160°C. The resulting
AF M images of the ﬁlms (a) show a relatively ﬂat surface for a ﬁlm thickness of 180 nm
which becomes rougher as the nominal ﬁlm thickness is decreased to 120 nm and then 40
nm. A similar ﬁlm was prepared (b) with polystyrene that was crosslinked”5 then another
layer of p—QD and polystyrene was deposited on top and thermally annealed causing the
p-QD to deposit on top of the ﬁrst layer outlining the original air interface. It is clear that
the p-QD assemble to the silicon substrate and additionally around the silica particle 30

ix

Figure 3.3. Scanning electron microscope (SEM) images: (a) A silicon dioxide (SiOz)
particle spin coated on a silicon wafer substrate. (b) A 67 nm thick polystyrene ﬁlms
containing pyridine-coated cadmium selenide quantum dots (p-QD’s) spin coated on to
(a) and thermally annealed at 160°C for 24 hrs .................................................. 32

Figure 3.4. (a) Optical micrograph of a nominal 85 nm thick PS ﬁlm containing 13.5 wt
% p-QD’s spin coated onto a silicon substrate sparsely decorated with silica particles and
thermally annealed and (b) corresponding AFM image. (0) Optical micrograph of a
similar ﬁlm to (a) but without p-QD’s and (d) an AFM scan near the rim of a dewetting
hole ..................................................................................................... 33

Figure 3.5. (a) Optical micrograph of a 63 nm thick polystyrene ﬁlm containing 42 wt%
of o-QD spin coated onto a silicon substrate sparsely decorated with silica particles and
thermally annealed and (b) corresponding AFM image. (c) A cartoon showing the o-
QD’s assemble to the air interface causing thermal ﬂuctuations so the ﬁlms

dewet .................................................................................................. 34

Figure 3.6. Optical micrographs of (a) a 63 nm thick polystyrene ﬁlm containing p-QD’s
spin coated onto the rough silanized substrate and thermally annealed, (b) a similar ﬁlm
to (a) but with o-QD’s .............................................................................. 36

Figure 3.7. Calculated ﬁlm proﬁles, using independently determined parameters, agree
well with the experimental observations. A cartoon of the system describes the
parameters (a) that used to calculate the results presented in b and c. (b) The height
proﬁle as a ﬁrnction of distance from the center of the sphere agrees well with the
experimentally determined values for a nominal 92 nm thick ﬁlm. (c) The ﬁlm height
above the sphere apex (ho) is similar to the ﬁlm thickness far ﬁom the sphere (h...) for
large ﬁlm thicknesses while for thinner ﬁlms it approaches a value of ca. 25 nm. The
error bars are larger for thinner ﬁlms as the size dispersity of the silica particles

(121 d: 14 nm) leads to variations in the measured height at the apex ..................... 38

Figure 3.8. (a) AP M measured values of the 32 nm ﬁlm height proﬁle as a ﬁrnction of
distance from the center of the 39 nm silica particle. (b) The ﬁlm height above the 32 nm
particle apex (ho) plotted versus the inﬁnite ﬁlm thickness (hog). (c) The thickness of the
ﬁlms located above the silica particle apex (ho- ds) plotted versus the inﬁnite ﬁhn
thickness (h...) for ﬁlms coating the 39 nm or 121 nm silica particles ....................... 41

Figure 4.1. AF M height images of (a) single-walled carbon nanotubes spin coated onto a
mica substrate. (b) I gG nanoparticles deposited onto (a) showing the one-dimensional
assembly of nanoparticles on the nanotubes .................................................... 48

Figure 4.2. Top-view TEM images: (a) A 70 nm thick polystyrene ﬁlm containing 0.74
wt% pyridine-coated CdSe quantum dots was spin coated onto nanotubes adhered to a
mica substrate showing randomly oriented nanotube bundles and distributed
nanoparticles. (b) After annealing a similar ﬁlm to (a) at 160°C for 24 hrs, it is found that
the nanoparticles assemble along the nanotube bundles; (inset) with higher magniﬁcation.

(c) The same sample as (b) but imaged at a different area; some nanoparticles were left in
the ﬁlm and not assembling on the nanotubes ................................................ 51

Figure 4.3. Top-view TEM images: (a) A 20 nm thick polystyrene ﬁlm containing 0.74
wt% pyridine-coated CdSe quantum dots was spin coated onto nanotubes adhered to a
mica substrate. (b) After annealing a similar ﬁlm to (a) at 160°C for 24 hrs, it is found
that some of the nanoparticles assemble along the nanotubes ................................ 54

Figure 4.4. Top-view TEM images: (a) A 70 nm thick polystyrene ﬁlm containing 0.74
wt% oleic acid coated CdSe quantum dots was spin coated onto nanotubes adhered to a
mica substrate. (b) A similar ﬁlm to (a) and was annealed at 160°C for 24 hrs ........... 55

Figure 4.5. Top-view TEM images: (a) A 20 nm thick polystyrene ﬁlm containing 0.74
wt% oleic acid coated CdSe quantum dots was spin coated onto nanotubes adhered to a
mica substrate. (b) A similar ﬁlm to (a) and was annealed at 160°C for 24 hrs ............. 56

Figure 5.1. TEM images of 0.3vol% pyridine-coated CdSe QDs in bulk polystyrene
matrix. The samples were thermally annealed at 170°C for different durations: (a) 0 hr
(b) 1.75 hr and (c) 24 hr .......................................................................... 63

Figure 5.2. A cartoon showing the materials used for this work: QD’s with the presence
of Si02 particles in polystyrene ................................................................. 65

Figure 5.3. TEM images of O.3vol% p-QD’s blended with 0.23vol% Si02 particles in
bulk PS matrix. The samples were thermally annealed at 170°C for different durations:
(a) 0hr(b) 1.75hr(c)4hrand (d) 24hr ....................................................... 66

Figure 5.4. (a) Annealing time (t) plotted versus the “fraction of un-assembled
nanoparticles (Nun); solid data points are from TEM measurements and the dashed line
represents the theory calculation. (b) The same plot as (a) in log scale .................... 68

Figure 5.5. TEM images of 0.03vol% p-QD’s blended with 0.23vol% SiOz particles in
bulk PS matrix. The samples were thermally annealed at 170°C for different durations:
(a) 1.75 hr (b) 4 hr .................................................................................. 70

Figure 5.6. TEM images of 0.3vol% oleic acid coated CdSe QDs blended with 0.23vol%
SiOz particles in bulk PS matrix. The samples were thermally annealed at 170°C for
different durations: (a) 1.75 hr (b) 24 hr ........................................................ 71

Figure 5.7. TEM image of a 1% p-QD/ 1% SiOz/PS composite thermally annealed for
1.75 hrs at 170°C .................................................................................. 72

Figure 5.8. TEM images of samples thermally annealed for 1.75 hrs at 170°C:
(a) 0.23% SiOz/PS composite. (b) 0.3% p-QD/0.23% SiOz/PS composite 73

Figure B.1. Peel energy plotted versus the surface coverage of nanoparticles on the

xi

substrate, for nanoparticle-ﬁlled polymer ﬁlms ................................................ 82

Figure C.l. XPS spectra of a nominal 63 nm thick polystyrene ﬁlms containing pyridine-
coated cadmium selenide quantum dots (p-QD’s) spin coated onto a silicon substrate
sparsely decorated with 120 nm-sized SiOz particles ........................................ 83

Figure C.2. XPS spectra of a nominal 63 nm thick polystyrene ﬁlms containing pyridine-
coated cadmium selenide quantum dots (p—QD’s) spin coated onto a silicon substrate
sparsely decorated with 120 nm-sized SiOz particles, after being thermally annealed at
160°C for 24 hrs ..................................................................................... 84

Figure D.1. Cross-sectional transmission electron microscopy (TEM) images: (a) 26 wt%
of polystyrene-coated ferromagnetic cobalt nanoparticles (CoNP) were spin coated with
393 kD polystyrene and (b) thermally annealed under vacuum at 160°C for 24 hrs, under
a magnetic ﬁeld of 26 MGOe applied in the vertical direction of the ﬁlms (the disk
magnets were placed on the top and bottom of the sample) .................................. 85

xii

Chapter 1

Introduction

1.1 Motivation

The motivation of this work is from polymer-based solar cells, as solar energy is
one of the alternative energy resources poised to replace fossil ﬁrels. Polymer solar cells
have been extensively researched over the past decade as an alternative to conventional
inorganic (mostly silicon) solar cells, since polymers are low-cost, lightweight, and can
be easily-manufactured onto ﬂexible substrates. However, the highest reported power-
conversion efficiency, deﬁned as the ratio of power output to input,1 is much lower2 than
that of inorganic solar cells.3 There are many technological difﬁculties to overcome to
achieve optimum performances of polymer solar cells.

One of the reasons for the poor efﬁciency of organic photovoltaics is the
undesirable interior morphology of the polymer thin ﬁlm photoactive layer. After spin
coating the photoactive layer, which is generally composed of an electron donor (polymer)
and an electron acceptor (nanoparticle), the two components phase segregate to form an
unfavorable morphology4 for electron transport properties of the device. This is because
in polymer solar cells, excitons (i.e. Coulombically bound electron-hole pairs) only
dissociate at the interface of electron donor and acceptor materials}.6 and their diffusion
length in conducting polymers is extremely short, on the order of 10 nm,7'9 so excitons
cannot reach the interface and electron-hole recombines. Also, the polymer active layer

11-12 .
1

should be thick enough,10 thicker than the optical absorption length ~ 200 nrrr, n

order to assure high absorption of light. The exciton diffusion lengths and the respective

optical absorption length of organic semiconductors cannot be matched.10 Thus,
designing an appropriate structure by optimizing the morphology of the photoactive layer
for the polymer solar cells is needed.

In the literature, McGehee et al.12 proposed an ordered structure to overcome the
mismatch of both important parameters discussed above. In this ordered structure, the
two phases of donor and acceptor within the photoactive layer have to be interspaced
with an average length scale of around 10 nm. Furthermore, the two phases need to be
interdigitated in percolated highways to ensure charge carrier transport with reduced
recombination. However, achieving this well—organized nanostructure is Changeling.
Today’s lithography techniques cannot be used to create structures on the scale of a few

nanometers.

1.2 Background

Previous studies in our group have shown the capability to control the assembly
and dispersion of nanoparticles in polymers”)-14 This nanoparticle assembly technique
is suitable for creating a nanostructure of the active layer in polymer solar cells.

Nanoparticles would assemble to the substrate interface in thin polymer ﬁlms
after thermal or solvent annealing”.16 as they are stabilized by entropic and enthalpic
effects; this nanoparticle assembly suppresses the dewetting of polymer ﬁlms. Numerous
experiments have been performed on thin ﬁlms made from different combination of
polymers and nanoparticles.13’15’17

The ﬁrst example by Krishnan15 used linear polystyrene (PS) mixed with
polystyrene nanoparticles.18 According to neutron reﬂectivity data, it was shown that
nanoparticles were uniformly distributed in PS ﬁlms after spin coating; yet after
annealing above the glass transition temperature, nanoparticles assembled to the solid
substrate. Since the nanoparticles and the linear polymer have the same monomer
(styrene), the enthalpic effect is minimal and the entropic terms dominate in this case.
The reason for the entropic assembly is that the nanoparticles lose at most three degrees
of freedom when pinned to a solid substrate, while a polymer molecule loses many more
degrees of conﬁgurational entropy19 (Figure 1.1). By removing the polymers away from
the substrate and exchanging nanoparticles for polymer, the free energy of the system

increases.

 

 

Figure 1.1. Entropy pushes nanoparticles assemble on the substrate — polymer interface.

In addition, Krishnan had shown another system with a blend mixture of cadimum
selenide (CdSe) quantum dots (QD’s) and PS.13 The QD’s used in this work were
sterically stabilized with oleic acid chains to make them soluble in toluene,20 denoted o-
QD’s. After spin coating the blend and thermal annealing, o-QD’s were found to
assemble to the air interface instead of to the substrate, proved by cross-sectional
transmission electron microscopy (TEM) images. Here besides the entropic effect,
another driving force for the assembly in nanoparticle/polymer ﬁlms arises from enthalpy.
The enthalpic contribution to the assembly in the blends are due to dispersion forces,21
which cause the components to order in layers according to their dielectric properties in a
typical composite ﬁlm. An approximation to predict the ordering of the components can
be performed through a minimization of the trilayer Hamaker constant, which dictates the

ordering to be air — o-QD — PS, corresponding to the TEM experimental result. In this

4

system the enthalpic term favoring segregation of nanoparticles to the air surface
dominates the assembly, while the entropic term favors segregation to the substrate.

In the literature on the incorporation of nanoparticles into polymer ﬁlms, Gupta et
al.22 demonstrated the segregation of the nanoparticles to cracks, driven by both
enthalpic (tailoring the ligands on the nanoparticles) and entropic interactions between
the polymer and nanoparticles. Similar entropic inﬂuence has also been shown on the
control of nanoparticle location in block copolymers,23-24 which behave as templates for
the assembly of nanoparticles.

In this work, the assembly of nanoparticles in polymer ﬁlms was investigated.
Particularly, we looked into their assembly in three-dimensions, i.e. with the presence of
large objects such as SiOz particles and single-walled nanotubes. Finally, we extended
our research interest by studying nanoparticle assembly in polymer melts with the
presence of Si02 particles forming nanocomposite. By controlling the assembly of
nanoparticles in polymer thin ﬁlms or polymer melts, we believe this will provide a

gateway to potential nano-scale applications.

Chapter 2

N anoparticle Assembly in Thin Polymer Films

2.] Introduction

It has been shown the capability to control the assembly and dispersion of
nanoparticles in polymers.”-15 In particular, the unique ability of nanoparticles to
suppress dewetting25 of polymer thin ﬁlms was found due to their assembly to the
interfaces, while without nanoparticles the ﬁlms will dewet on a substrate after being
thermally annealed. A large amount of experiments were preformed on thin ﬁlms made
from blends of polymers with nanoparticles showing this effect. Particularly
nanoparticles also enable formation of continuous, stacked polymer ﬁlms13 and provide
an avenue to three-dimensional assembly of complex nanostructures and complementing
other assembly methods26 to great effect.”-29

In the literature, several research groups have studied that when nanoparticles are
dispersed in a polymer film, the polymers induce an entropic depletion attraction between
the particles and the substrate and it make the segregation of the nanoparticles to the

9

33 ﬂuid density functional theory (DFT) calculations of
24,32

substrate”.33 Simulations,3
nanoparticles in polymer melts,34 and also nanoparticle/diblock copolymer mixtures
have shown this entropic effect. Additionally, experimental work has shown that

9

nanoparticles segregate to cracks in a substrate2 5 also due to entropy.
Indeed, the forces driving the nanoparticle assembly in polymer ﬁlms is important

as it will change the morphology of the nanoparticle-ﬁlled polymer ﬁhns or so-called

nanocomposites. We stated that the assembly of nanoparticles to interfaces is driven by
both entropic and enthalpic forces.1 ’
For the entropic part of these self-assembly forces,22 we used polystyrene

13,15,17 . .
Since the constituents are

nanoparticles18 mixed with linear polystyrene.
chemically similar, the blends are dominated by entropy. The nanoparticles lose at most
three degrees of freedom when pinned to a solid substrate, while a polymer molecule
loses many more degrees of conﬁgurational entropy.19 By removing the polymers away

from the substrate and exchanging nanoparticles for polymers, the system can gain a free

energy (vr) per nanoparticle,

vr = akBT x VNP/Vseg = arkBT, (2.1)
where 0: represents the degrees of freedom gained by a statistical polymer segment,“-37
k3 is Boltzmann’s constant, T is temperature, V Np and V56.g are the nanoparticle and
segment volumes, respectively, and a, is an overall measure of the strength of the
entropic terms. Each polymer segment was estimated to gain of order 0.1 to 1 degrees of
ﬁ'eedom using the experimental data17 and ﬂuids DFT calculations,19 so a, is in the
range 10-1000 for the nanoparticles used in our following experiments.

The enthalpic driving force for nanoparticle assembly is due to dispersion
terms,38’21 which in a typical composite ﬁlm causes the components to order in layers
according to their dielectric properties. A ﬁrst approximation to this effect is based on the
refractive indices (n) of the blend components.17 For a sphere (e.g. nanoparticle) of
refractive index it; near a substrate of refractive index n 1, in a medium with reﬁ'active

index n 3, the enthalpic contribution ﬁ'om dispersion forces is approximately —A132 a/6l,

where a is the radius of the sphere, l is the distance ﬁom the substrate, A132 is the

Hamaker constant for media 1 and 2 interacting across medium 3 and it can be simpliﬁed
to

A132 ~ (m2 — n32) (nz2 - n32) (2.2)
The formula is valid provided 1 « a. Note that the sign of the Hamaker constant can give
us an indication of the stability of the system, i.e. if A132 is negative, the system is stable
for the trilayers with the effective interface potential positive.39 Thus, the sign of the
dispersion force depends on the ordering of the values of the refractive indices, so the
force on a nanoparticle next to a surface or interface can be either attractive or repulsive.
Therefore, it is possible to assemble nanoparticles to a solid substrate interface or the air
interface dependent solely on the dispersion forces.

One example demonstrating this refractive index (dispersion forces) ordering was
performed by using the blend mixture of cadimum selenide (CdSe) quantum dots (QD’s)
and polystyrene.17 The QD’s used in this work were sterically stabilized with oleic acid
chains to make them soluble in toluene,20 denoted o-QD’s. After spin coating the blend
and thermal annealing, o-QD’s were found to assemble to the air interface, proof was
obtained by cross-sectional transmission electron microscopy (TEM) images.13 If we
used Equation 2.2 and the refractive index of each component with the following
approximate values: 1.0 (air), 1.54 (o-QD), 1.59 (PS), we found that A132 is negative
with a trilayer of air (component 1) — o-QD (3) — PS(2), indicating the system is stable.
This ordering corresponds with our TEM result and thus one can sinfply use the refractive
index of each component in the system to predict the nanoparticle assembly direction. In

this case the dispersion term favors segregation to the air interface, while the entropic

term favors segregation to the substrate, leading to competing interactions on the
nanoparticles.

In this chapter we followed previous studies but used another kind of CdSe
quantum dots that were stabilized with pyridine ligands instead of oleic acids; in this way
the refractive index of the nanoparticles is changed, alternating the assembly direction in
polymer thin ﬁlms. Additionally, we investigated multi-nanoparticle assembly in polymer
ﬁlms by simultaneously spin coating small nanoparticles and large particles along with
polymers onto the substrates. Visualizing the nanoparticles assembly in polymer ﬁlms

was completed through cross-sectional TEM images.

2.2 Experimental Methods
2.2.1 Materials

Cadmium selenide (CdSe) quantum dots (QD’s) with a core radius of 2.2 nm were
prepared according to a well known procedure20 in Prof. Michael Wong’s laboratory at
Rice University and were stabilized with a pyridine layer (0.5 nm in radius), denoted p-
QD’s. The pyridine ligand thickness was determined by dynamic light scattering of a
dilute pyridine solution and the CdSe core radius was measured by transmission electron
microscopy averaged over 20 particles.

Silicon dioxide (SiOz) (Bangs Laboratories, Inc.) particles were diluted with
deionized water to 0.5 mg/ml, sonicated and ﬁltered through a 200 nm PTFE ﬁlter. The
SiOz particle solution were then spin coated onto silicon substrates and dried in vacuum
at 40°C for at least 12 hrs prior to polymer ﬁlm deposition, in order for the water to
evaporate completely. AFM measurements of more than 50 particles found the particle
height (diameter) above the substrates to be 121 i 14 nm

Polystyrene latex particles were 2% covalently crosslinked and purchased from
Molecular Probes, Inc. Prior to use the as received solutions were diluted with pyridine to
achieve the needed weight fraction relative to polystyrene polymer for use in the cross-
sectional TEM experiment (Section 2.3.2).

Polystyrene/nanoparticle solutions were prepared by dissolving 75kD polystyrene
(Scientiﬁc Polymer Products Inc.) and nanoparticles in solvent (pyridine or toluene) in
concentrations ranging from 45 to 50 mg/ml to reach certain ﬁlm thicknesses. Solutions
were mixed on a rocking platform for at least 12 hrs before use, sonicated and ﬁltered

through a 200 nm PTFE ﬁlter. Films were deposited by spin coating at 5000 RPM for 40

10

seconds and ﬁlm thicknesses were determined through ellipsometry (1A Woolam M-44).
For cross-sectional transmission electron microscopy (TEM) images (see below) a
thermally cross-linkable polystyrene of molecular weight comparable to the 75kD PS was

used.18 This was the linear precursor used to make the polystyrene nanoparticles.

2.2.2 Transmission Electron Microscopy (TEM)

To make cross-sectional images thin ﬁlms of polystyrene, SiOz particles and
quantum dots were prepared on silicon wafers as described above. A gold layer of ~ 40
nm was sputtered on top of the ﬁlm as a marker for viewing. A drop of epoxy resin
(Poly-Bed® 812, Ted Pella, Inc.) was deposited onto the sample and then cured at 60°C
for 24 hrs. The sample was placed in liquid nitrogen and the epoxy along with the ﬁlm
was peeled from the silicon substrate and re-embedded into resin for ultramicrotomy. A
diamond knife was used and the samples were sectioned to ~ 70 nm in thickness,
estimated by the interference colors of transmitted light.40 All ultramicrotomy was
performed on a Power Tome XL (RMC) and images were taken with a JEOL lOOCX
transmission electron microscope operated at 100kV. For Figure 2.4 the sample were
made by spin coating the polymer/PS particle/p-QD solution onto a silicon substrate with
100 nm thick grown oxide layer, which is then dissolved after soaking in 8% hydroﬂuoric
acid (HF) for one hour, followed by rinsing with deionized water. In this way the ﬁlms

would be detached ﬁ'om the substrate more easily when placing in the liquid nitrogen.

11

2.3 Results and Discussion

2.3.1 Nanoparticle Assembly in Polymer Films in Two- and Three- Dimensions

Following previous studies, in this work we used another kind of CdSe quantum
dots (QD’s), which is stabilized with a pyridine layer,20 denoted p-QD, to demonstrate its
assembly in two-dimensions in thin polystyrene ﬁlms (~ 200 — 300 nm). By replacing the
steric layer from oleic acid to a thinner ligand of pyridine, the refractive index of the
CdSe QD increases from 1.54 to approximately 2.2. The value for the quantum dots’
refractive index was obtained by computing a volume average of CdSe inner core with a
2.2 nm radius (reﬁactive index of 2.8) surrounded by a pyridine layer which is 0.5 nm
thick (reﬁactive index of 1.5). If we use the refractive index ordering as discussed before
(Eq. 2.2), the ordering will be air (1) — PS (1.59) — p-QD (2.2). Thus the p-QD’s are
predicted to assemble to the substrate interface in PS ﬁlms and we found the prediction
was in correspondence with our following experimental results. In this system both the
entropic free energy and dispersion forces favor the nanoparticles segregation to the
substrate.

The cross-sectional transmission electron microscopy (TEM) image in Fig. 2.1a
shows that the p-QD’s assemble onto the substrate after being spin coated along with
polystyrene onto a silicon substrate. The resulting ﬁlms were thermally annealed at
160°C for 16 hrs in air and crosslinked at 250°C for 30 mins in air. 140kD 20%-
crosslinkable polystyrene18 was used since it is easier to microtome than normal PS. The
crosslinkable PS was dissolved in pyridine achieving the concentration of 45 mg/ml and
after spin coating made a ﬁlm thickness of approximately 290 nm according to the cross-

sectional TEM image. The mass concentration of p-QD relative to PS was made to be

12

12.25% in order to achieve at least one monolayer surface coverage if all nanoparticles
assembled at the substrate.

Fig. 2.1b reveals a plan view of the densely packed QD layer at the substrate
though a Fourier transform of the image (inset) shows no long range order. The sample in
Fig. 2.1b was a similar ﬁlm to Fig. 2.1a but was spin coated onto a mica substrate, ﬂoated
onto deionized water then picked up with TEM grids and thermally annealed. Thus we

observed the plane view of the sample under TEM.

 

  

Air interface

 

 

 

 

Figure 2.1. A layer of PS and CdSe quantum dots stabilized with a pyridine steric layer
(p-QD) was spin coated onto a silicon wafer and thermally annealed at 160°C for 16 hrs.
A cross-sectional transmission electron micrograph revealed a p-QD layer at the substrate
(a), due to a combination of their larger surface energy than polystyrene (enthalpy) as
well as the entropy gain through releasing the polymer from the substrate constraint. A
plan view of a similar ﬁlm demonstrates a densely packed layer of p-QD’s (b) although a
Fourier transform of the image (inset) shows no long range order.

In the published work“ we found that when a blend consisting of both p-QD and
o-QD was spin coated along with polystyrene and then thermally annealed, the o-QD
assembled to the air interface while the p-QD segregated to the solid substrate giving a

ﬁnal order of air (1) — o-QD (1.54) - polystyrene (1.59) — p-QD (2.2). The result also

13

corresponds with the refractive index ordering. In this case the dispersion term favors
segregation to the air surface, while the entropic term favors segregation to the substrate,
leading to competing interactions on the nanoparticle.

To further test the capability of the nanoparticle assembly in three-dimensions, we
sparsely decorated a silicon substrate with SiOz particles by spin coating a dilute solution
of them in deionized water (0.5 mg/ml). The sample was dried for at least 12 hrs in
vacuum at 400C. Then a similar mixture of 140kD 20%-crosslinkable PS and 12.25 wt%
p-QD dissolved in pyridine was then spin coated on to the SiOz particle-silicon substrate.
The sample was again thermally annealed at 160°C for 16 hrs and crosslinked at 250°C
for 30 mins both in air. P-QD’s were found to assemble at the substrate as well as around
the silica particle, as shown in Fig. 2.2a. It shows that after thermal annealing the
nanoparticles will assemble at the substrate interface as well as on a 3D object located at

the substrate.

‘ .

Polystyren

;\ f Gold layer

\‘é

p-QD Sic, particle

Substrate

 

sparsely with Si02 particles and a layer of 12.3 wt% p-QD’s and crosslinkable PS was
spin coated on top and annealed, p-QD’s assembled along the substrate and around the
silica particle. (b) A similar sample to a) but with a higher concentration of p-QD’s (21.6
wt%).

14

Fig. 2.2b shows a similar sample to Fig. 2.2a but with a higher concentration of p-
QD (21.6 wt%). However, this sample was treated in a different way than the previous
one and only crosslinked at 220°C for 30 min in vacuum, with no preceding annealing.
One can see that more p—QD’s were trapped in the polystyrene ﬁlm, probably due to the
short crosslinking time making them kinetically trapped and unable to segregate to the
substrate interface. Moreover, a different molecular weight (60kD) of 20%-crosslinkable
PS was used and its concentration in pyridine was 52 mg/ml, making the ﬁlm about 245
nm thick according to the TEM image. Nevertheless, p-QD still assembled around the
SiOz particle as well as on the substrate. More p-QD’s assembled around the SiOz as
compared to Figure 2.2a most likely due to the higher concentration of p-QD likely.

From the results we have shown the assembly of p-QD to the solid substrate is
robust as entropy and enthalpy operate in concert and the free energy gain per
nanoparticle is much larger than kBT. In contrast a simple surfactant has an assembly
energy to an air — liquid interface of 4rtr2 X S, where S is the surface tension and r is the
molecular size,38 to yield a value of order kBT. Moreover, self-assembly through entropic
surfactant behavior will only occur when the liquid is polymeric as there is no such
entropic driving force with simple liquids. Nanoparticles will assemble to small molecule
liquid — liquid interfaces,42 yet, they are not pinned as strongly to the interface since they
rapidly exchange to the surrounding environment. We believe it is not the case for our
system. From Figure 2.2a one can clearly tell that the nanoparticles stay at the polymer —
substrate interface since very few are found within the ﬁlm itself and they even assemble

around a solid protrusion (Si02 particle).

15

2.3.2 Assembly of Multi-Nanoparticles in Polymer Films

In the previous section of nanoparticle assembly in three-dimensions, Si02
particles were placed on the silicon substrate prior to the deposition of nanoparticle-
polymer ﬁlms. In this section all components —- SiOz particles or PS particles, pyridine-
coated CdSe QD (p-QD), and polystyrene — were suspended in the same solvent and then
spin coated onto the substrates simultaneously. We would like to observe the ordering of
each component after thermal annealing well above the glass transition temperature of the
polymer.

The ﬁrst experiment was spin coating a blend of SiOz particles, p-QD and
polystyrene onto a silicon substrate and the resulting polymer ﬁlm was thermally
annealed at 160°C for 17 hrs. Instead of using a crosslinkable PS, normal PS (75kD) was
used so any crosslinking factors were eliminated; its concentration in pyridine was 52
mg/ml. The SiOz particles were diluted with pyridine to become 0.7 wt% in PS (0.3
myml in the ﬁnal solution). The concentration of p-QD in PS was made to be 12.9 wt%
(7.4 mg/ml in the solution), which was calculated to give a monolayer of surface
coverage if all nanoparticle assemble to the substrate by using a simple mass balance.15
The TEM image in Figure 2.3 provides a cross-sectional view of this sample. The silicon
substrate was detached ﬁom the ﬁlm for microtomy purposes only. One can clearly see
that the p-QD’s assemble around the Si02 particle in the polystyrene ﬁlm, while the Si02
particle is located near the air interface. The reason for the agglomerations of p-QD in the

ﬁlm is unknown but it is probably due to the large attraction forces between the quantum

dots.

l6

Air interface

Polystyrene

 

 

Figure 2.3. Cross-sectional TEM image of a polymer ﬁlm resulting from spin coating a
solution blend of 75kD polystyrene, p-QD, and Si02 particle. The ﬁlm was thermally
annealed at 160°C, for 17 hrs.

Table 2.1. The refractive index of the materials used in chapter 2

 

 

 

 

 

 

Material Reﬁactive Index (n)
Air 1 .0
Si02 l .37
PS 1 .59
p-QD 2.0
Silicon 3.8

 

 

 

 

Here we use the same refractive index ordering as discussed before to explain the
assembly of nanoparticles in the system. Table 2.1 lists the values of each material that is
used in this experiment. For SiOz particles they tend to be close to the air to make the
trilayer system to be air (1) — SiOz (1.37) — PS (1.59) so it is stable according to

Equation 2.2. This explains why the Si02 particle is located close to the air interface in

17

Figure 2.3. For p-QD’s the best trilayer makeup is Si02 (1.37) — PS (1.59) — p-QD
(2.0), however we did see p-QD assembled on the SiOz particle in the TEM image, which
is contradictory to the refractive index ordering. Remember that there are also entropic
forces inﬂuencing the assembly of nanoparticles in polystyrene ﬁlms. In this case we
treat rather large SiOz particles as buried curved surfaces and the polymer chains gain
entropy when being pushed away ﬁ'om these SiOz curved surfaces. Clearly entropy
dominates the segregation of p-QD onto the Si02 particles’ surfaces, while the dispersion
forces (refractive index ordering) do not. From the results in the previous section we saw
that p—QD assembled to the substrate in polystyrene ﬁlms (Figure 2.2), however we did
not see p-QD on the substrate in this experiment when all components were spin coated
at once. One way to interpret this is that the attraction forces from the Si02 particle
curved surfaces to the quantum dots may be larger than those from the silicon ﬂat
substrate, or the system entropy is most minimized when p-QD’s assemble around the
SiOz particle instead of on the substrate.

The other experiment was spin coating a blend of polystyrene particles, p-QD and
polystyrene onto a silicon substrate and the resulting polymer ﬁlm was crosslinked at
220°C for 30 mins. A 60kD 20%-crosslinkable polystyrene (50 mg/ml) was used and the
concentrations of p-QD and polystyrene particles are 12.5% (7.4 mg/ml) and 3.4% (2.1
mg/ml), respectively; all three components were suspended in pyridine. One can clearly
see from the cross-sectional TEM image in Figure 2.4 that the p-QD’s assembled around
the polystyrene particles, which are around 100 nm in diameter in both images. The
reason we did not see a clear sphere shape of the polystyrene particle under TEM is due

to its presence in the polystyrene ﬁlm matrix and they have the same electron density.

18

However, we do believe that those are PS particles since there are circle-shaped patterns

made of p-QD’s appearing in the polystyrene ﬁlms.

 

 

 

Figure 2. 4. (a) Cross. sectional TEM Image of polymer ﬁlmresulting ﬁ'om spin coating
a solution blend of crosslinkable polystyrene, p-QD, and polystyrene particle. The ﬁlm
was crosslinked at 220°C for 30 mins. (b) The same sample as (a) but at a different area.

The ﬁrst interesting thing we notice ﬁom both images (Figure 2.3 and 2.4) is that
although the annealing conditions were different, i.e. shorter time (30 mins) than the
previous experiment (17 hrs) and higher temperature (220°C V.S. 160°C), the p-QD still
assembled around the large PS particle. We also notice that the PS particles are located
near the air interfaces, which corresponds with the ordering of the refractive index, that is
air (1) — PS (1.59) — Silicon (3.8). For p-QD’s it is believed that the polymer chain
gains entropy when being pushed away ﬁom the polystyrene particle curved surfaces (the
PS particle are ~ 2% covalently crosslinked) so the p-QD’s assembled arormd the PS
particle. The enthalpic terms did not play a role here for the p-QD’s assembly since the

PS particles and polystyrene are chemically similar.

19

2.4 Conclusion

The assembly of nanoparticles to interfaces in liquid polymer thin ﬁlms is driven
by both entropic and enthalpic forces, either of which will dominate the assembly
direction. Pyridine-coated cadmium selenide quantum dots (p-QD) were found to
assemble on the substrate in polystyrene ﬁlms after thermal annealing well above the
glass transition temperature of the polymer; in this case both entropic and enthalpic
forces favored the assembly. Furthermore, the same blend was spin coated onto a
substrate containing large silica particles and the p-QD’s not only assembled on the
substrate but also around the large silica particles thus showing the capability of
nanoparticles assembly in three-dimensions.

We further investigated the assembly of nanoparticles in the polymer ﬁlms
containing large particles. To do this we simultaneously spin coated p-QD, polystyrene,
and large silica or polystyrene particles onto a substrate and thermally annealed the
samples. In both cases p-QD’s were found to assemble around the large particles in the
polystyrene ﬁlms and we believe that the entropic terms dominated the assembly as the

polymer chains are pushed away from the buried particle surfaces.

20

Chapter 3

Three-Dimensional Liquid Surfaces through N anoparticle Assembly

3.1 Introduction

In the previous chapter, we found the capability of nanoparticles to assemble
around a three-dimensional object located on the substrate in 200 — 300 nm thick polymer
ﬁlms, and to stabilize the ﬁlms. In this work, we would like to further challenge the
strength of the nanoparticle assembly as the ﬁlms are made to be thinner, even thinner
than the sizes of the three-dimensional objects.

In the literature, it has be found that even fairly large protrusions on a rough
surface can be wetted by a liquid drop, resulting in a ﬂat, uniform ﬁlm.43 The reason is
that the liquid drop will spread out and ﬂatten to a height h related to its surface tension S
and density p,44 and this spreading behavior occurs on ca. 1 mm length scale. In this so-
called thick-thin ﬁlm regime,43 the ﬁlm height has a simple form; h = \l(S/pg), where g is
the gravitational constant.

If thinner ﬁlms are made, resulting ﬁom the competition between van der Waals
forces and surface tension, the substrate —- liquid — air interactions become important and
a new length scale arises, that is called the healing length 5 (= hz/b E h2><\/(21tS/A132)).
A132 is the trilayer Hamaker constant for the liquid (component 3) sandwiched between
the substrate (1) and air (2) and b is a microscopic length scale. For polystyrene at 160°C
on a silicon substrate in air, as used in the following experiments, b is on the order of 1

45-47
m.

n It has been shown48 that the ﬁlm would follow surface features with lateral

21

length scales larger than c“ and ﬂatten over features smaller than 6, based on a linear
analysis of thin liquid wetting ﬁlms on a regular, corrugated surface.

However, in this work the surface protrusions are irregular, on the order of 100
nrrr, and typically thought to be un-wettablefw'50 By using the nanoparticle assembly
technique developed previously, we enable liquid polymer ﬁlms to cover these non-
wetting protrusions even when the nominal ﬁlm thickness is less than half the size of the
protrusion, which is a regime far from the linear limit. This assembly technique was
demonstrated by using blends of polystyrene and cadmium selenide nanoparticles spin
coated onto the substrates containing sparsely distributed SiOz particles. Film proﬁles of
different thicknesses were characterized using atomic force microscopy (AF M) while
cross-sectional transmission electron microscopy (TEM) was performed to provide the
three dimensional ﬁlm proﬁle contour and the layered assembly of nanoparticles. These
three-dimensional surfaces are optimal for constructing the active layer of nanoparticle-

polymer hybrid solar cells among other uses.“-52

22

3.2 Experimental Methods
3.2.1 Materials

Cadmium selenide (CdSe) quantum dots (QD’s) with a core radius of 2.2 nm were
prepared according to a well known procedure20 in Prof. Michael Wong’s laboratory at
Rice University, and were stabilized with a pyridine layer, denoted p-QD’s. The pyridine
ligand thickness was determined by dynamic light scattering of a dilute pyridine solution,
and the CdSe'core radius was measured by transmission electron microscopy averaged
over 20 particles.

One source of silicon dioxide (SiOg) (Bangs Laboratories, Inc.) particles we used
were diluted with deionized water to 0.5 mg/ml, sonicated and ﬁltered through a 200 nm
PTFE ﬁlter. The SiOz particle solution was then spin coated onto silicon substrates and
dried in vacuum at 40°C for at least 12 hrs prior to polymer ﬁlm deposition, to evaporate
water completely. AF M measurements of more than 50 particles found the particle height
(diameter) above the substrates to be 121 i 14 nm. The second set of 8102 particles were
purchased from Kisker-Biotech. Prior to use, the as received solution was diluted with
deionized water to 16 pg/ml, and otherwise prepared as above. AFM measurements
provided the particle height (diameter) above the substrates to be 39 :1: 7 nm.

Polystyrene/nanoparticle solutions were prepared by dissolving 75kD PS and
nanoparticles in solvent, pyridine or toluene, in concentrations ranging from 10 to 26
mg/ml. Solutions were mixed on a rocking platform for at least 12 hrs before use,
sonicated and ﬁltered through a 200 nm PTFE ﬁlter. Films were deposited by spin
coating at 5000 RPM for 40 seconds and ﬁlm thicknesses were determined through

ellipsometry (JA Woolam M-44). For cross-sectional transmission electron microscopy

23

 

(TEM) images (see below) a thermally cross-linkable polystyrene of molecular weight
comparable to the 75kD PS was used.18 This was the linear precursor used to make the
polystyrene nanoparticles.

For dewetting experiments that used silanized wafers, SiOz particles were
deposited onto silicon wafers as described above. The wafers and SiOz particles were
then silanized with Sigmacote ((SiC12C4H9)20, Sigma-Aldrich) by spin coating the as
received Sigmacote solution onto the SiOz particles/silicon wafers at 5000 rpm for 40
seconds. The wafer was then rinsed with copious amounts of deionized water to eliminate
the excess solution, followed by drying with N2. The silanized wafer was checked by
AFM to ensure the coating provided a smooth surface with an RMS rouglmess below 0.5
mm Sigmacote is a solution consisting of 2.5% chlorosiloxane ((SiC12C4H9)zO) and

97.5% heptanes that ﬁrnctionalizes the surface with short alkane chains.

3.2.2 Atomic force microscopy (AF M)

The distribution of Si02 particles on the substrates and the ﬁlm proﬁle
measurements were performed by a Paciﬁc Nanotechnology Nano—R atomic force
microscope in close contact (oscillating) mode. Silicon probes were used for all

measurements.

3.2.3 Transmission electron microscopy (TEM)
To make cross-sectional images thin ﬁlms of polystyrene, SiOz particles and
quantum dots were prepared on silicon wafers as described above. A gold layer of ~ 40

nm was sputtered on top of the ﬁlm as a marker for viewing. A drop of epoxy resin

24

 

(Poly-Bed® 812, Ted Pella, Inc.) was deposited onto the sample and then cured at 60°C
for 24 hrs. The sample was placed in liquid nitrogen and the epoxy along with the ﬁlm
was peeled from the silicon substrate and re-embedded into resin for ultrarrricrotomy. A
diamond knife was used and the samples were sectioned to ~70 nm in thickness. All
ultramicrotomy was performed on a Power Tome XL (RMC) and images were taken with
a JEOL lOOCX transmission electron microscope operated at 100kV. Top view images in
TEM were taken by spin coating the polymer ﬁlms onto freshly cleaved mica substrates
(Electron Microscopy Sciences, EMS), ﬂoated off onto the surface of deionized water

and picked up by 300-meshed copper grids (EMS).

25

3.3 Results and Discussion

Following the previous chapter, here we investigate the behavior of the
polymer/nanoparticle ﬁlm in the presence of a surface protrusion, particularly for ﬁlms
that are thinner than the protrusion sizes. To create surface protrusions, a silicon wafer
was sparsely decorated with 121 i 14 nm diameter silicon dioxide (SiOz) particles by
spin coating a dilute solution of them in water. The typical distance between the Si02
particles is on the order of several microns; an atomic force microscope (AF M) image is
shown in Figure 3.1a as an example. After drying 12 hrs in a vacuum oven at 400C to
evaporate water completely, a mixture of 75kD polystyrene with pyridine-coated
cadmium selenide quantum dots (p-QD’s) in toluene was spin coated on top (Figure 3.1b)
to make a 63 nm thick ﬁlm, and subsequently annealed at 160°C (Figure 3.10), well
above the polymer’s glass transition temperature of 105°C, for 24 hrs under air. One can
clearly see that the ﬁlms appear rough after spin coating, however they are much
smoother after annealing. This result is interesting since the annealing process made a
signiﬁcant change in the ﬁlm morphology according to the AFM images. It seemed that
the ﬁlms “wrapped” up around the silica particles. More remarkably, the ﬁlms did not
dewet but remained three-dimensional stable although without the quantum dots the Si02
particles would be nucleation sites for dewetting in such thin ﬁlms (data shown later in
this chapter).

X-ray photoelectron spectroscopy (XPS) experiments were performed both before
and after annealing samples (Figure 3.1 b and c) in order to obtain more information
about the surface composition. We found that the silica particles were covered with

polymer above the particle apex since there was no Si detected within the probe depth of

26

about 10 run; that means the polymers covered the silica particles for about 10 nm after
being spin coated and also thermally annealed. The XPS spectra are presented in the

Appendix section.

154nm

Onm

 

83nm 118nm

   

Figure. 3.1. AFM images: (a) A silicon wafer was sparsely decorated with 121 d: 14 nm
diameter silicon dioxide (SiOz) particles by spin coating a dilute solution of them in
water. (b) The nominal 63 nm thick polystyrene ﬁlms containing pyridine-coated
cadmium selenide quantum dots (p-QD’s) spin coated onto (a) and thermally annealed for
24 hrs (c). Scan size is 10 pm * 10 pm in all images.

The previous sample we made had the concentration of p—QD’s to provide one
monolayer if all nanoparticles assemble on the substrate. The reason we used at least one
monolayer of nanoparticles is that it is the surface coverage needed to inhibit the
dewetting of polymer ﬁlms.15 In this system, provided all CdSe p-QD’s assemble at the
substrate interface, we estimate the surface aerial coverage (6) of the nanoparticles at the
substrate, based on a simple mass balance, by

6 = (h/Za) x v. (3.1)

27

where h is the ﬁlm thickness, a is the nanoparticle radius and (p is the bulk nanoparticle
volume ﬁaction. 6 z 1 corresponds to a dense packed monolayer of nanoparticles at the
substrate. In our samples, the surface coverage of $102 particles at the substrate also
needs to be taken into consideration. Based on the number of SiOz particles placed on the
silicon wafer substrate, we ﬁnd 6 2 1.007 is required for a monolayer surface coverage of
p-QD’s on top of Si02 particles as well as the substrate. By knowing the value of 6, the
ﬁlm thickness (h) and the radius (a) of p-QD, one can calculate the bulk volume fraction
((p). After converting the volume ﬁ'action to weight fraction (wt%), different values of

wt% of QD can be obtained for the samples of different ﬁlm thicknesses (Table 3.1).

Table 3.1. Different concentration of the stock solution is required to manufacture a
given ﬁlm thickness when spin coated at 5000 RPM for 40 seconds. The weight
percentage of p-QD in the stock solution is calculated based on the ﬁlm thickness and
that required to coat the silica particles (121 i 14 nm) and the substrate with a monolayer
(6 = 1.007, 2a = 5.3 nm).

 

 

 

 

 

 

 

 

 

Film p-QD volume p-QD Concentration of
thickness fraction ( ) wt‘y the stock solution
In (um) l’ ° (mg/ml)
22 0.241 65 5
32 0.166 54 7
40 0.132 47 10
63 0.084 35 15
92 0.058 26 20
120 0.044 21 26
148 0.036 18 32
180 0.029 15 35

 

 

 

 

 

 

Based on the calculated concentrations, polystyrene ﬁlms with various nominal

thicknesses (listed in Table 3.1) were made by spin coating the nanoparticle/polymer

28

stock solutions onto the silicon substrates that were sparsely decorated with Si02
particles. Again, all of the samples did not dewet after thermal annealing and some of
them are shown in Figure 3.2a. The AFM images provide the surface topography
showing a relatively ﬂat surface for a ﬁlm thickness of 180 nm which becomes rougher
as the nominal thickness is decreased to 120 nm and then 40 nm.

To show the ﬁlm contour around the Si02 particle, TEM cross-section
experiments were preformed (Figure 3.2b). Here we used crosslinkable PS so stacked
polymer ﬁlms could be manufactured.13 First, the silicon substrate was sparsely
decorated with $102 particles. Next, a nominal 45 nm thick layer of the mixture of 27.3
wt% p-QD’s and 140kD 20%-crosslinkable PS (15 mg/ml) suspended in toluene was spin
coated on top, thermally annealed at 160°C for 24 hrs in air allowing the segregation of
the p-QD’s, then heated to 250°C for 30 minutes in air to activate the crosslinking
process and the ﬁrst robust layer was generated. Then a thicker layer of a similar p-
QD/PS blend (15.5 wt% p-QD and 30 mg/ml of PS) was spin coated on top and likewise
annealed with the same procedure. Again, the concentrations of p-QD’s in both layers
were calculated so they would provide one monolayer coverage on the substrate. While
the p-QD’s segregated on the top of the ﬁrst layer, it outlined the original air interface.
The second layer ﬁlm conforms around the larger Si02 particle as the p-QD’s assemble
on the silicon substrate as well as the SiOz particle. One can clearly see from the TEM
image that even though the ﬁlm was curved above the SiOz particle but still stayed three-
dirnensionally stable. Thus, the assembly of nanoparticles to the polymer—substrate
interface is so strong that it enables a thin polymer ﬁlm to follow surface features that are

larger than the ﬁlm thickness41 instead of dewetting from the substrate.

29

 

180 nm

 

 

 

 
 
    
  

 

 

 

 

         

.‘~\\“\ ‘a‘\\\\\!\.h“\\“ -\

\‘QV‘YQ: \ ..‘-\ 7‘: {\x'

 

 

 

 

Air interface

Gold layer

 
   

Substrate '_ j _ ._ . if” ‘ ' - , ‘

Sic, particle-‘k ‘ d ,.
100nm ”WW?” '

_ .

\

 

Figure 3.2. A sparse layer of $102 particles 121 i: 14 nm in diameter were deposited onto
a silicon wafer, then various thickness ﬁlms of p-QD and 75kD PS were spin coated onto
this rough substrate. The system was thermally aged for 24 hrs at 160°C. The resulting
AFM images of the ﬁlms (a) show a relatively ﬂat surface for a ﬁlm thickness of 180 nm
which becomes rougher as the nominal ﬁlm thickness rs decreased to 120 nm and then 40
nm A similar ﬁlm was prepared (b) with polystyrene that was crosslinked13 then another
layer of p-QD and polystyrene was deposited on top and thermally annealed causing the
p-QD to deposit on top of the ﬁrst layer outlining the original air interface. It is clear that
the p-QD assemble to the silicon substrate and additionally around the silica particle.

30

One feature of the cross-section TEM image to note is that there appears to be no
or only a very thin polymer layer on top of the silica particle in Figure 3.2b, which
contradicts the previous XPS results indicating a roughly 10 nm polymer layer. However,
we believe that it is due to the diamond knife in the microtomy process likely destroying
the feature on top of the silica particle. The focused ion beam (FIB) technique can
probably give a ﬁner, undestroyed local feature of this kind of cross-sectional
nanostructures if one wishes.

Scanning electron microscopy (SEM) experiments were preformed for more
surface morphology information. Figure 3.3a shows a single Si02 particle spin coated on
the silicon substrate. The stage in the SEM was titled to a certain angle so the sample was
imaged in a slight cross-sectional view. Then a blend of p-QD/PS (similar to the sample
in Figure 3.1c) was spin coated on top and thermally annealed at 160°C for 24 hrs in air,
as shown in Figure 3.3b. We did not clearly observe similar ﬁlm decay proﬁles as the
previous AFM images (Figure 3.10), however there was a signiﬁcant difference in the

surface morphology after the ﬁlms were spin coated onto the silica particles.

31

 

Figure 3.3. Scanning electron microscope (SEM) images: (a) A silicon dioxide (SiOz)

particle spin coated on a silicon wafer substrate. (b) A 67 nm thick polystyrene ﬁlms

containing pyridine-coated cadmium selenide quantum dots (p—QD’s) spin coated on to (a)
and thermally annealed at 160°C for 24 hrs.

It is worthwhile to reemphasize the importance of the nanoparticles added in
polymer ﬁlms since without them the ﬁlms will dewet upon thermal annealing. Figure
3.4a shows an optical micrograph showing a sample similar to Figure 3.1c, i.e. a nominal
85 nm thick PS ﬁlm containing 13.5 wt % p-QD’s spin coated onto a silicon substrate
sparsely decorated with silica particles and thermally annealed. One can clearly observe
that the ﬁhns did not dewet, and the corresponding AFM image (Figure 3%) shows a
smooth ﬁlm covering the silica particle protrusion. The concentration of p-QD’s was
again made to provide a monolayer surface coverage. However, if a similar PS ﬁlm with
no nanoparticles was used, the ﬁlms dewetted as shown in Figure 3.4c; the diameters of
the dewetting holes range from 20 to 40 pm. An AFM scan was preformed near the rim
of the hole giving us a 3D rim proﬁle and we observed that the ﬁlms dewetted from the
substrate (Figure 3.4d); an XPS scan was performed on this sample and the SiOz peak

was detected verifying the dewetted polymer ﬁlms. Thus, it is crucial to have the

32

nanoparticles in the polymer ﬁlms so that the ﬁlms can remain three-dimensionally

smooth and continuous upon thermal annealing.

. .dpm

., 126nm

IOnm

 

 

Figure 3.4. (a) Optical micrograph of a nominal 85 nm thick PS ﬁlm containing 13.5 wt
% p-QD’s spin coated onto a silicon substrate sparsely decorated with silica particles and
thermally annealed and (b) corresponding AF M image. (0) Optical micrograph of a
similar ﬁlm to (a) but without p-QD’s and (d) an AFM scan near the rim of a dewetting
hole.

We performed similar dewetting experiments with oleic acid coated quantum dots
(o-QD’s) that will assemble to the air interface in polystyrene ﬁlms after thermal
annealing due to their lower surface energy (refractive index), in contrast to p-QD’s,
which assemble to the substrate. Again the silicon substrate was sparsely decorated with
8102 particles and a 63 nm thick polystyrene ﬁlm containing 42 wt% of o-QD was then
spin coated on top. The concentration of o-QD’s was determined in order to provide one

monolayer surface coverage on the substrate. After thermal annealing for 24 hrs in air,

33

the ﬁlms dewetted according to the optical micrograph and the AFM image shown in
Figure 3.5. Therefore, assembly of nanoparticles to the air interface is not as robust as

assembly to the substrate.

10 um 116nm

 

Onm

 

Figure 3.5. (a) Optical micrograph of a 63 nm thick polystyrene ﬁlm containing 42 wt%
of o-QD spin coated onto a silicon substrate sparsely decorated with silica particles and
thermally annealed and (b) corresponding AFM image. (c) A cartoon showing the o-
QD’s assemble to the air interface causing thermal ﬂuctuations so the ﬁlms dewet.

Previous researchls’17 has shown that sub-monolayer nanoparticle coverage does
not inhibit the dewetting of polymer ﬁlms on low energy substrates, while one monolayer
and above does. A jammed two-dimensional assembly is created by the entropic pressure
employed by the polymer molecules holding the nanoparticles at the substrate interface
when a monolayer is present, effectively making a solid coating. By contrast,
submonolayer coverage allows the nanoparticles ﬁeedom to move and therefore
dewetting is not inhibited. In the same manner, when the o-QD’s are impelled to the air

interface, there is no such entropic pressure; a more ﬂuid nanoparticle ﬁlm is created

34

which can dewet on account of thermal ﬂuctuations. Thus, we know that assembly of
nanoparticles to the solid substrate is the most robust ﬁlm stabilization protocol, a robust
enough phenomenon to create three-dimensional curved liquid surfaces.

The strength of the nanoparticle assembly to the solid substrate was ﬁrrther
veriﬁed in our published work41 by using polystyrene nanoparticles of similar size.18
The PS nanoparticles assemble to the solid substrate in PS ﬁlms,15 just as the p-QD’s do,
and thus also stabilize polystyrene ﬁlms on three-dimensional substrates (silicon wafers
sparsely decorated with SiOz particles).

Moreover, an even lower energy surface than a silicon wafer17 was used to
examine the capability for the three-dimensional nanoparticle assembly. The silicon
wafer was sparsely decorated with S102 particles similar to previous tests and then
silanized using Sigmacote® ((SiC12C4H9)20) to reduce the surface energy17 from 71 i l
mJ/m2 to 28.5 i 4.9 mJ/mz. Due to the extremely low energy of the rough silanized wafer,
we need to ﬂoat ﬁlms onto its surface since spin coating was not possible. The resulting
rough silanized substrate is so non-wettable that a 63 nm thick PS ﬁlm will completely
dewet within 5 — 10 s at 160°C, over 100 times faster than on a ﬂat, equivalent
substratels’17 on which the ﬁlms also dewet. Yet, when a similar PS ﬁlm with p-QD’s
was ﬂoated on top of the rough silanized substrate, the ﬁlm remained stable at 160°C for
days (as shown in Figure 3.6a). This shows the extraordinary stability provided by an
appropriate layer (one monolayer) of self-assembled nanoparticles. Additionally, when
we performed similar tests with o-QD’s on the rough silanized substrate, the ﬁlms
dewetted as expected (Figure 3.6b). This is due to the assembly to the interface causing

thermal ﬂuctuations, just as in the sample shown in Figure 3.5. Both of the samples in

35

Figure 3.6 have the concentrations of nanoparticles providing one monolayer of surface

coverage.

 

Figure 3.6. Optical micrographs of (a) a 63 nm thick polystyrene ﬁlm containing p-QD’s
spin coated onto the rough silanized substrate and thermally annealed, (b) a similar ﬁlm
to (a) but with o-QD’s.

In order to understand the reason why the nanoparticle assembly energy is strong
enough to overcome surface tension forces induced by the curved features and enables
the ﬁlm to follow the surface protrusions instead of dewetting from the substrate, we can
estimate the pressure P due to surface tension at the particle apex to be P = 2S/R = 0.5
MPa by taking the local radius of curvature as that of the Si02 particle (R z 100 nm) and
the polystyrene surface tension to be 25 mJ/m2 at 160°C.45 The assembled nanoparticle
layer at the solid substrate — polymer interface as well as around the silica particle (Figure
3.2b) counters the surface tension induced pressure as its assembly free energy (see
Chapter 2), arkBT, is quite large. The force to remove a nanoparticle ﬁom the layer is
then around arkBT/a, where a is the nanoparticle radius, suggesting an assembly pressure,

arkBT/a3, that is over 10 MPa for the p-QD’s with a radius53 of ~ 2-3 mm This is clearly

36

much larger than the disassembling pressure due to the liquid surface curvature,
explaining the observed stability of the three-dimensional rough liquid ﬁlm.

In addition to the experimental results, wei also preformed theoretical calculations
to model the detailed structure of the polymer surface proﬁle. In the calculations we
generalized the analysis of Robbins et a1.48 by including the van der Waals forces
missing in the above discussion, and applied a continuum theory for ﬁlm morphology47
in which we minimized an interfacial free energy for completely wetted surfaces. The
nanoparticle assembly forces were also taken into account in the calculations, of which
details can be found in our published work.41

A cartoon of the system, as shown in Figure 3.7a, describes the parameters that
were used in the calculations. Figure 3.7b is an example of the comparison between the
experimental and theoretical results for a nominal 92 nm thick ﬁlm; one can observe that
they correspond nicely as the experimental proﬁle was averaged over more than 10 data
sets measured by AFM. The large silica particle was assumed to be 120 nm in diameter
for the calculation, which is within the measured particle size range (121 i 14 nm). In
Figure 3.7b the polymer ﬁlm thickness at the particle apex (ho), which is h(x) at x = 0, is
25 nm greater than the particle diameter and AF M measurements show apparent errors of
order i 15 nm. This is due to the polydispersity of the silica particles (i 14 nm) in spite
of the measurement accuracy of the AFM and smoothness of the ﬁlms (far from the silica

protrusions) being ~ 1 nm. It is clear since farther from the apex the apparent error is

reduced.

 

lDr. Erin McGarrity performed the calculations described here.

37

 

 

E

 

 

 

h(x) (nm)

5%

 

 

 

 

 

roo—-'

 

 

 

 

 

 

 

C -
oTheory
—Experiment ’
180»
A 160.
E
1:
~—
0
8 140»
120» .
1 A A
so 150 200

 

 

100
h” (nm)
Figure 3.7. Calculated ﬁlm proﬁles, using independently determined parameters, agree
well with the experimental observations. A cartoon of the system describes the
parameters (a) that used to calculate the results presented in b and c. (b) The height
proﬁle as a function of distance from the center of the sphere agrees well with the
experimentally determined values for a nominal 92 nm thick ﬁlm. (0) The ﬁlm height
above the sphere apex (ho) is similar to the ﬁlm thickness far ﬁom the sphere (hog) for
large ﬁlm thicknesses while for thinner ﬁlms it approaches a value of ca. 25 nm The
error bars are larger for thinner ﬁlms as the size dispersity of the silica particles (121 i 14
nm) leads to variations in the measured height at the apex.

38

In Figure 3.7c, ho is plotted as a function of ﬁlm thickness far from the silica
sphere (hoe), which is the nominal ﬁlm thickness; one ﬁnds the two parameters equal, as
expected, for the thickest sample (180 run) since the ﬁlms ﬁrlly cover the silica particles.
For thicker ﬁlms, the value of ho from AF M measurements shows much less variance,
while thinner ﬁlms show deviations due to the size dispersity of the silica particles and ho
becomes larger than h00 reﬂecting the stabilizing potential of the nanoparticles.

Finally, a different size of Si02 particles was used to create three—dimensional
liquid surfaces in similar experiments as described above. The particle height (diameter)
above the substrates was measured by AFM of more than 50 particles to be 39 i 7 nm.
Likewise, the silicon wafer was sparsely decorated with the Si02 particles by spin
coating a dilute solution of them in water. Then, a mixture of 75kD polystyrene with
pyridine-coated quantum dots (p-QD’s) in toluene was spin coated on top to make 22 nm
or 32 nm thick ﬁlms and subsequently annealed at 160°C for 24 hrs in air. As expected,
neither ﬁlm dewetted as a result of the concentrations of p-QD’s being made to provide
one monolayer coverage on the substrate (detailed in Table 3.2). This shows that we can
use the same nanoparticle assembly technique to coat a smaller surface protrusion

creating the three-dimensional liquid surfaces in a different size regime.

Table 3.2. Different concentration of the stock solution is required to manufacture a
given ﬁlm thickness to coat the silica particles (39 i 7 nm) and the substrate with a
monolayer (6 = 1.0007, 2a = 5.3 nm).

 

 

 

Film -QD volume QD Concentration of
thickness pfraction ( ) 210/ the stock solution
h (nm) (p 0 (mg/ml)
22 0.239 65 5
32 0.164 53 7

 

 

 

 

 

 

39

We are interested in the comparison of the polymer ﬁlm proﬁles and the ﬁlm
height above the particle apex (ho) between the 121 nm silica particles and the 39 nm
ones. Figure 3.8a is an example showing the ﬁlm proﬁle of the 32 nm thick ﬁlm coating
the 39 nm silica particle, in which the location of the particle apex was deﬁned as x = 0.
From this plot we know the value of ho, i.e. the value of h(x) at x = 0, and one can again
plot ho versus hog, which is the inﬁnite ﬁlm thickness. It is found that ho decreases with
hog, the same trend as appeared in the previous experiments with 121 nm particles (Figure
3.70).

Another result we would like to compare is the thickness of the ﬁlms located
above the silica particle apex. To obtain that information, we can simply subtract the
silica particle height (d3) from ho, i.e. ho — ds. The results are shown in Figure 3.80,
containing two data sets of the silica particles for different sizes. If neglecting the size
dispersities of silica particles (2t 7 nm or :t 14 nm), for 22 nm or 32 nm thick ﬁlms we
suspect that the ﬁlms located above the silica particle are thicker when coating 39 nm
particles than those coating 121 nm ones. However if one wishes to obtain more accurate
values of (ho — ds) for both sizes of silica particles, the focused ion beam (FIB) for cross-

sectional TEM images may still be an suitable technique to use.

40

h (x) (nm)

Figure 3.8. (a) AFM measured values of the 32 nm ﬁlm height proﬁle as a function of
distance from the center of the 39 nm silica particle. (b) The ﬁlm height above the 32 nm
particle apex (ho) plotted versus the inﬁnite ﬁlm thickness (hm). (0) The thickness of the
ﬁlms located above the silica particle apex (ho — d3) plotted versus the inﬁnite ﬁlm

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I I
an —----—-- ’ - E
w ._-_
so Li I A
:1 E
I co m '-.
TI TITTT.
‘° Em 11* 1mm
40
All!) 500 0 500 1000
it (run)
C
60 1 1
+ds = 39 nm
- A -ds = 121 nm
v A ' ’ -
GD 2 5’
O ’Ar
/
1 1
50 1(1) 150
he. ("I“)

h... (M)

thickness (hog) for ﬁlms coating the 39 nm or 121 nm silica particles.

41

3.4 Conclusion

Three—dimensional surfaces which can support curved but continuous polymer
thin ﬁlms even in the liquid state are created through the assembly of nanoparticles to the
solid substrate. This assembly energy is so strong that it can overcome surface
tension/energy forces induced by the curved features and enables the ﬁlm to follow the
surface protrusions instead of dewetting from the substrate, even though the ﬁlm
thicknesses are thinner than the surface protrusion sizes.

In order to create these three-dimensional surfaces, it is necessary for
nanoparticles to assemble on the solid substrate, to which they are pushed by entropy.
The resulting nanoparticle layer on the substrate is a jarmned state that ﬁrnctions as an
effective solid layer and provides a mechanism to inﬂuence surface phenomena of
polymer thin ﬁlms. In contrast, if the nanoparticles assemble to the air interface, the ﬁlms

can still dewet as a result of thermal ﬂuctuations.

42

Chapter 4

N anoparticle Assembly along Single-Walled Carbon Nanotubes

4.1 Introduction

In this work, we use carbon nanotubes as the “templates” for nanoparticle
assembly in one- and two-dimension. Carbon nanotubes have been applied to many
disciplines of research, such as ultra-strong wires, nanoelectronic devices, ﬁeld electron
emitters, nanocomposite materials, bio-medical devices, etc.54 because of their unique
structure dependent electronic, mechanical, optical and magnetic properties,55-56 as well
as high aspect ratios and surface areas. These reasons make nanotubes an interesting
template for nanoparticle assembly.

A variety of research has been conducted for one-dimensional assembly of
nanoparticles on nanotubes.57-59 In one case, Zettl et al.57 used a chemical deposition
method to coat the surface of single-walled carbon nanotubes with crystalline tin oxide
nanoparticles. In another example, the nanotube surface was chemically modiﬁed in
order to attach gold nanoparticles by electrostatic interactions.58 However, for our work
we attached nanoparticles to nanotubes without any particular chemical treatment or
surface modiﬁcation of the nanotubes; attachment was achieved simply through van der
Waals forces.

Furthermore, we were able to assemble nanoparticles two-dimensionally along
nanotubes in polymer thin ﬁlms, creating polymer-ﬁlm/naoparticle/nanotube composites
for potential applications,60-61 e.g. solar cells. In the literature, nanoparticles could be

attached to polymer-wrapped nanotubes by electrostatic forces,62 or polymer/nanotube

43

composites could be synthesized together and nanoparticles were then encapsulated into
the resulting nanotubing to form a free-standing particle assembly on the tubes.63 In our
work, we created polymer thin ﬁhn nanocomposites with nanoparticles and nanotubes
incorporated. A simple nanoparticle assembly method was performed by depositing
nanotubes on substrates, followed by spin coating nanoparticle/polymer solutions on top,
and thermal annealing the resulting polymer composite. The assembly of nanoparitcles

along the nanotubes was achieved driven by dispersion forces.

44

4.2 Experimental Methods

Functionalized single-walled carbon nanotubes (SWNTs) were synthesized using
the HiPco process64'65 in Prof. Matteo Pasquali’s laboratory at Rice University. This
process is a gas phase method using an iron catalyst to catalytically disproportionate high
pressure CO gas to generate a mixture of metallic and semi-conductive SWNTs. The
direction the graphene sheet is rolled determines whether the nanotube will be conductive
or semi-conductive.66 The SWNTs used in our experiments were ﬁrnctionalized with a
butyl group via alkylation.67 This functional group produces a poorly conducting
nanotube, but allows the SWNTs to be soluble in chloroform. Even so, in all experiments,
the solution was sonicated for 30 mins prior to use to ensure proper dissolution. The
nanotube solutions were then spin coated at 5000 rpm for 40 seconds onto a freshly
cleaved mica substrate (Electron Microscopy Sciences Co).

IgG goat antibody nanoparticles (Sigma Aldrich Co) is in 0.01 M phosphate
buffered saline (PBS) and it has a concentration of 11.3 mg/ml. The solution was diluted
down to l ug/ml with phosphate buffered saline (PBS) and then deposited onto the
SWNTs-mica substrate for 30 seconds. Afterwards, the sample was rinsed with PBS,
followed by deionized water and then dried with nitrogen. The average height of the IgG
nanoparticles on the mica substrates was determined via atomic force microscope to be
4.3 d: 1.8 nm.

Cadmium selenide (CdSe) quantum dots (QD’s) with a core radius of 2.2 nm were
prepared according to a well known procedure20 in Prof. Michael Wong’s laboratory at
Rice University and were stabilized with either a pyridine or oleic acid layer, denoted p-

QD’s or o-QD’s respectively. The ligand thickness of either pyridine or oleic acid was

45

determined by dynamic light scattering of a dilute solution, and the CdSe core radius was
measured by transmission electron microscopy averaged over 20 particles.

Polystyrene/nanoparticle solutions were prepared by dissolving 75kD PS and
nanoparticles in toluene, with concentrations ranging from 5 to 15 mg/ml. Solutions were
mixed on a rocking platform for at least 12 hrs before use, sonicated and ﬁltered through
a 200 nm PTFE ﬁlter. Films were deposited by spin coating at 5000 RPM for 40 seconds
and ﬁlm thicknesses were determined through ellipsometry (JA Woolam M-44).

Surface morphology measurements were performed by a Paciﬁc Nanotechnology
Nano-R atomic force microscope in close contact (oscillating or tapping) mode. Silicon
probes (BudgetSensors®) with a spring constant of 25-75 N/m, tip radius <10 nm, and a
resonance frequency of 200-400 kHz were used for all experiments.

For top-view images of transmission electron microscope (TEM), the polymer
ﬁlms were spin coated onto freshly cleaved mica substrates (Electron Microscopy
Sciences, EMS), ﬂoated off onto the surface of deionized water and picked up by 300-
meshed copper grids (EMS). The images were taken with either a JEOL 100CX operated

at 100kV or a JEOL JEM-2200FS operated at 200kV.

46

4.3 Results and Discussion

4.3.1 One-Dimensional Assembly of Nanoparticles on Nanotubes

To create one-dimensional assemblies, we studied assembly along the length of
carbon nanotubes, which has been studied by many research groups68 where speciﬁc (e. g.
electrostatic) and non-speciﬁc (e.g. van der Waals) forces have been used. Previous
reasearch69 preformed in our group has shown that various nanoparticles, ranging ﬁom
polystyrene nanoparticles of different sizes to gold nanoparticles as well as cadmium
selenide quantum dots, were drawn towards the nanotubes by non—speciﬁc van der Waals
forces, while they freely diffused in the solvent.

In this work, we expand previous research by using biological nanoparticles and
show the versatility of the one-dimensional assembly which may be used in potential
biotechnology applications.70'71 First, single-walled carbon nanotubes (SWNTs)
suspended in chloroform were spin coated onto a mica substrate to serve as the nucleation
sites for nanoparticles to assemble (Figure 4.1a). From AFM measurements, the lengths
of nanotubes on the substrate are determined to be several microns, and their height is on
the order of 10 nm, indicating agglomerations of nanotubes were formed.72 Moreover,
the nanotubes were ﬁrmly adhered to the substrates after their deposition, and probably
deformed via van der Waals forces,73 allowing subsequent assembly of nanoparticles

from solution onto them.

47

12.31nm 26.24nm

 

 

 

 

       

0.00pm 1.05pm 2.10pm 0.00pm 0.70pm 1.40pm

Figure 4.]. AF M height images of (a) single-walled carbon nanotubes spin coated onto a
mica substrate. (b) IgG nanoparticles deposited onto (a) showing the one-dimensional
assembly of nanoparticles on the nanotubes.

Next, a dilute solution (1 rig/ml) of IgG goat antibody nanoparticles in phosphate
buffered saline (PBS) was deposited onto the SWNTs-mica substrate for 30 seconds. The
sample was then rinsed with PBS, followed by deionzied water and then dried with
nitrogen. AF M measurement of the sample shows clearly that IgG nanoparticles
assembled on the nanotubes, as presented in Figure 4.1b. It should be noted that the
nanotubes along with the IgG nanoparticles were adhered to the substrate even after
rinsing with the buffer solution.

This result is unique since only speciﬁc chemical binding of antibody to
nanotubes was reported in the literature,68 while in this work we believe that simple non-
speciﬁc van der Waals forces were the driving force for assembly. It has be found that69
the van der Waals strength between a particle and cylinder greatly increases as the
particle approaches the substrate, driving the nanoparticles to assemble onto the

nanotubes, which explains the observed behavior. Moreover, it is believed that the

48

assembly is not due to the “coffee ring effect”,74’75 which is driven by the loss of solvent
by evaporation. Since the coffee ring effect on the substrate would force agglomerates of
nanoparticles toward the edge of the droplet, it is clear from Figure 4.1 that this does not

apply in our work here.

4.3.2 Two-Dimensional Assembly of Nanoparticles along Nanotubes

In this section, instead of depositing the nanoparticle solution onto the SWNTs-
mica substrate, we increase the viscous or hydrodynamic drag by spin coating the
nanoparticles with polymer solutions in order to observe the two-dimensional assembly
along the nanotube contour in the much more viscous polymer liquids. In the literature,76
it has been found that self-assembly of semiconductor nanoparticles (e.g. quantum dots)
onto SWNT surfaces was accomplished by simple van der Waals forces without chemical
functionalization of the surfaces of SWNTs. However, the assembly was only in one-
dirnension since quantum dots and SWNT were co-suspended in the same solvent and the
assembly was observed after the solvent was evaporated.

In this work, we spin coated CdSe quantum dots in a 75 kD polystyrene solution
onto a mica substrate with a sparse layer of nanotube bundles, and thermally annealed the
resulting ﬁlm above the polymer’s glass transition temperature. Top-view transmission
electron microscopy (TEM) was performed in order to provide the information of
nanoparticle assembly in such polymer ﬁlms both before and after annealing. All the
ﬁlms were spin coated onto mica substrates, ﬂoated off onto water, and then picked up

with TEM grids for viewing.

49

First, a dilute solution of nanotubes was spin coated onto mica substrates and their
distribution on the substrate is shown in the AF M image in Figure 4.1a. Dilute solutions
were used so that the nanotubes would be more sparsely distributed on the surface and
then more easily observed under subsequent TEM analysis. Nanotubes tended to
agglomerate more if solutions of higher concentrations were used. Next, a ~ 70 nm thick
75kD polystyrene ﬁlm containing 0.74 wt% pyridine-coated CdSe quantum dots (p-QD’s)
dissolved in toluene was spin coated on top of the mica/nanotube substrate. A low
concentration of p-QD’s was used since we would like to observe the assembly of them
on the nanotubes more clearly. From the TEM image in Figure 4.2a, it is clear that the p-
QD’s were unifome distributed in the polymer ﬁlm, which corresponds with our
previous results15 of polystyrene nanoparticles spin coated along with polystyrene. After
annealing at 160°C for 24hrs, well above the polymer’s glass transition temperature of
105°C, one can observe that most of the p-QD’s assembled along the nanotubes, as
shown in Figure 4.2b. However, we also noted that some p-QD’s were left in the ﬁlm and
not assembled on the nanotubes in some other area of the same sample (Figure 4.20).

The reason for p-QD’s assembling on the nanotubes is due to the ordering of
refractive index (dispersion forces) to minimize the surface energy, as discussed in
chapters 2 and 3. According to Equation 2.2, the trilayer makeup required to provide a
negative Hamaker constant and thus a stable system based on the refractive index of each
component will be PS (1.59) — p-QD (2.0) —- nanotubes (3), explaining that the p-QD’s
assembled along the nanotubes in polystyrene ﬁlms. Table 4.1 lists the reﬁ'active index of
each material used in this work. The nanotubes we used were ﬁrnctionalized with a butyl

group via alkylation which ultimately produces a poorly conducting nanotube67 yet were

50

soluble in organic solvents such as chloroform; nevertheless the Hamaker constant or

reﬁ'active index of the nanotubes is still large which is necessary to induce assembly.

 

Figure 4.2. Top-view TEM images: (a) A 70 nm thick polystyrene ﬁlm containing 0.74
wt% pyridine-coated CdSe quantum dots was spin coated onto nanotubes adhered to a
mica substrate showing randomly oriented nanotube bundles and distrrbuted
nanoparticles. (b) After annealing a similar ﬁlm to (a) at 160°C for 24 hrs, it is found that
the nanoparticles assemble along the nanotube bundles; (inset) with higher magniﬁcation.
(c) The same sample as (b) but imaged at a different area; some nanoparticles were left in
the ﬁlm and not assembling on the nanotubes.

51

Table 4.1. The refractive index of the materials used in chapter 4

 

 

 

 

 

 

Material Refractive Index (11)
Air 1 .0
o-QD 1 .54
PS 1 .59
p-QD 2.0
Nanotubes 3

 

 

 

 

Remember that entropy also plays a role in the assembly of nanoparticles; thus,
when p-QD’s are located near the substrate, they may stick there since nanotubes were
only sparsely distributed on the substrate. The reason for nanoparticles sticking on the
substrate is that they lose less conﬁgurational entropy than polymer chains so the
polymers are pushed away from the substrate.19 This explains why some p-QD’s did not
assemble on the nanotubes in Figure 4.20.

Moreover, how nanoparticles diffuse in the polymer may inﬂuence their assembly
in our system. Tuteja et al. have found53 that nanoparticles diffuse approximately 200
times faster in a polymeric liquid than predicted by the Stokes-Einstein relation. The
diffusion coefﬁcient (D) of nanoparticles in 393 kD polystyrene melt at 160°C was
determined to be 1.64 nmz/s, based on the measured viscosity,

D = kBT/67r,ua (4.1)
in which k3 is the Boltzmann constant, p is the macroscopic polymer melt viscosity, T is
the temperature, and a is the radius of the nanOparticle. One can then calculate the
diffusion coefﬁcient of nanoparticles in 75 kD polystyrene according to the relationship

77

between melt viscosity p and molecular weight M for entangled linear polymers

,u ~ M“ (4.2)

52

to be 458 nmz/s and then the mean square diffusion length of a particle "RMS for one hour
annealing time t

rRMS = 6Dt (4.3)
to be 3145 nm, which is much larger than the ﬁlm thickness used in our experiments.
However, one can imagine there will be many ﬂuctuations during the assembly procedure
so the nanoparticle will not statically diffuse to the substrate (or nanotube). Additionally,
we do not know how nanoparticles diffuse or their diffusion coefﬁcient if they are
already stuck on the substrate. Thus these are other reasons for some p-QD’s not
assembling on the nanotubes.

Furthermore, in order to investigate the inﬂuence of ﬁlm thickness on the
nanoparticle assembly in the polymer ﬁlms, we reduced the ﬁlm thickness to about 20
nm and performed similar experiments, as shown Figure 4.3. Before annealing, the p-
QD’s were randomly distributed in the ﬁlm although some agglomerations were observed.
After thermal annealing, some p-QD’s assembled on the nanotubes but some did not.
This may be because the ﬁlm is so thin (the heights of nanotubes on the substrate are ~ 10
nm) and nanoparticles may simply diffuse to the substrate, sticking there and not diffuse

to the nanotubes.

53

 

Figure 4.3. Top-view TEM images: (a) A 20 nm thick polystyrene ﬁlm containing 0.74
wt% pyridine-coated CdSe quantum dots was spin coated onto nanotubes adhered to a
mica substrate. (b) After annealing a similar ﬁlm to (a) at 160°C for 24 hrs, it is found
that some of the nanoparticles assemble along the nanotubes.

Moreover, to challenge the versatility of the nanoparticle assembly, we changed
the nanoparticles to oleic-acid coated quantum dots (o-QD’s), which have lower surface
energy and thus assemble to the air interface in polystyrene ﬁlms.13 Similar tests were
performed: nanotubes were spin coated onto mica substrates sparsely and a ~ 70 nm thick
polystyrene ﬁlm containing 0.74 wt% o-QD’s was spin coated on top (Figure 4.4a). Note
that the agglomerations of o-QD’s might be due to the degradation of the oleic acid
ligand since the QD’s had been in the solution for months. The sarrrple was thermally
annealed at 160°C for 24 hrs and viewed under TEM (Figure 4.4b). One can observe that
most of the o-QD’s did not assemble on the nanotubes. This is due to the ordering of
refractive index providing a negative Hamaker constant, giving the trilayer system as o-

QD (1.54) — PS (1.59) — nanotubes (3). Thus the nanotubes do not attract o-QD’s as

54

much as they attract p-QD’s as shown previously. In the same experiments with o-QD’s

but with 20 nm thick ﬁlms, similar results were obtained (Figure 4.5).

 

Figure 4.4. Top-view TEM images: (a) A 70 nm thick polystyrene ﬁlm containing 0.74
wt% oleic acid coated CdSe quantum dots was spin coated onto nanotubes adhered to a
mica substrate. (b) A similar ﬁlm to (a) and was annealed at 160°C for 24 hrs.

55

 

Figure 4.5. Top-view TEM images: (a) A 20 nm thick polystyrene ﬁlm containing 0.74
wt% oleic acid coated CdSe quantum dots was spin coated onto nanotubes adhered to a
mica substrate. (b) A similar ﬁlm to (a) and was annealed at 160°C for 24 hrs.

For the ordering of reﬁactive index as discussed above, when considering the
trilayer Hamaker constant A132 for media 1 and 2 interacting across medium 3, we are in
fact calculating the amplitude of the dispersion forces between interface (13) and
interface (32). If these two interfaces have an attractive van der Waals interaction making
A132 positive,38 then the more stable conﬁguration is the interface (12), with medium 3
expelled. On the other hand, if A132 < 0, it means interface (13) and interface (32) repel
so the ﬁnal order is 1-3-2. Thus, when calculating Hamaker constants we are comparing
the relative van der Waals attractions between each component in the system. For this
work, the p-QD’s are attracted to nantotubes in polystyrene ﬁlms, while the o-QD’s are

not.

56

4.4 Conclusion

One-dimensional assembly of nanoparticles on nanotubes was demonstrated by
depositing solutions of IgG goat antibody nanoparticles onto a mica substrate sparsely
decorated with nanotubes. The IgG nanoparticles were found to assemble on the
nanotubes due to non—speciﬁc van der Waals forces.

Two-dimensional assembly of nanoparticle along the nanotubes was achieved by
spin coating quantum dots along with polystyrene onto the nanotubes-mica substrate,
followed by subsequent thermal annealing. Pyridine-coated quantum dots were found to
assemble along the nanotubes in polystyrene ﬁlms, while oleic acid stabilized ones were
not. Based on the Hamaker constant analysis according to the refractive index of each
component, it is believed that nanotubes attract p-QD’s in polystyrene ﬁlms, but not 0-

QD’s.

57

Chapter 5

Nanoparticle Assembly on Silica Particles in Polymer Melts

5.1 Introduction

In previous chapters, we investigated the assembly of nanoparticles in polymer
thin ﬁlms, especially their assembly around large objects ($102 particles, etc.) to form
three-dimensional surfaces. Here, we extend our research interest by studying
nanoparticle assembly with the presence of Si02 particles in polymer melts forming
polymer-based nanocomposite, which may be very attractive for many applications”.80
A large amount of research has been performed on nanoparticle/polymer composites

9

because of their property improvements,2 - 3 such as physical and mechanical

properties. Especially, recently we found that the addition of nanoparticles in the polymer
reduced the viscosity and made other multifunctional performance enhancements.“-86
We then are very interested in the morphology of the nanoparticle/polymer composites
and would like to discern the structure-property relationship.

The nanoparticles used in this work are cadmium selenide quantum dots (QD’s)
stabilized with either a pyridine or oleic acid layer. In the literature, the addition of QD’s
to polymers has opened a gateway for a variety of applications ranging from electronic
materials”.88 to biosensors.89-90 It has been found to be challenging to disperse QD’s
into many polymers, because of the tri—n-octylphosphine oxide (TOPO) ligands covering
their surface that have been commonly used. However, we were able to disperse QD’s in

polystyrene using a rapid precipitation processing technique,” which will be used to

manufacture the nanoparticle/polymer composite.

58

The main focus of this work is the assembly of nanoparticles onto the SiOz
particles in polymer melts upon different annealing durations. Since we have found53 that
nanoparticles diffuse approximately 200 times faster in a polymeric liquid than predicted
by the Stokes-Einstein relation, we would like to study the relation between the
annealing time and the numbers of quantum dots assembling on the SiOz particles. This
can beneﬁt the understanding of how nanoparticles diffuse within the polymer melt, i.e.
the nanoparticle dynamics, which may be important for some experimental techniques

. . 91-92 . . . . .
like rrrrcrorhelogy that use Stokes-Ernstem relation for data interpretatron.

59

5.2 Experimental Methods

Cadmium selenide (CdSe) quantum dots (QD’s) with a core radius of 2.2 nm were
prepared according to a well known procedure20 in Prof. Michael Wong’s laboratory at
Rice University, and were stabilized with either a pyridine or oleic acid layer, denoted p-
QD’s or o—QD’s respectively. The ligand thickness of either pyridine or oleic acid was
determined by dynamic light scattering of a dilute solution, and the CdSe core radius was
measured by transmission electron microscopy averaged over 20 particles.

The silicon dioxide (SiOz) particles were purchased from Bangs Laboratories, Inc.
and their size in deionized water was 125 nm in diameter as determined by dynamic light
scattering. In order to co-suspend the quantum dots and polystyrene in pyridine, the as
received Si02 particle solutions were diluted to 10 mg/ml and then placed in dialysis
tubing, which had been washed in stirred deionized water for 6 hrs prior to use. The
dialysis tubing containing SiOz particles was placed in stirred deionized water, which
was changed every hr in the ﬁrst four hrs and then every 24 hr for three days. On the
fourth day, the deionized water was changed to pyridine and the stirring was stopped. The
pyridine was changed every 24 hr for three days. Finally, the SiOz solution was taken out
ﬁom the tubing.

Linear 393kD polystyrene (PS) (Scientiﬁc Polymer Products Inc.) was used in all
experiments for manufacturing the nanoparticle/polymer composites. The concentration
of PS in the solvent was made to be about 8 mg/ml (i.e. 250 mg polystyrene in 30 ml
pyridine) and was then placed on a rocking platform for at least 24 hrs to ensure complete
dissolving. Next, the Si02 particle solution and p-QD’s solution were placed into the PS

solution to achieve the desired volume concentrations. The resulting solution was placed

60

on the rocking platform for at least 24 hrs and was sonicated for 60 mins prior to the
rapid precipitation.

For the “rapid precipitation”,93-94 the solution was dripped into a mutual non-
solvent, methanol, where the polymer and nanoparticles rapidly precipitated, forming an
intimately mixed, homogeneous dispersion of nanoparticles in the linear polymer. By
decanting and vacuum ﬁltration with copious methanol rinsing, most of the liquid was
removed ﬁom the sample, which was then dried for a week in an vacuum oven at 40°C to
ensure complete solvent removal.

For annealing experiments, the dried powder was molded by compression under
vacuum, and annealed at 170°C for the desired time duration in a pellet press (8 mm
diameter), to ensure that no trapped air remained in the sample.

For transmission electron microscope (TEM) viewing, the completely dried
sample or pressed disc was cut into sections, ~70 nm in thickness, with a diamond knife
using an ultramicrotome (Power Tome XL, RMC). Sections were picked up with a 300
mesh copper grid and placed on ﬁlter paper overnight to ensure the sections ﬂattened out.
The images were taken with a Tecnai GZ 12 Twin TEM operated at 120kV or a JEOL

JEM-2200FS TEM operated at 200kV.

61

5.3 Results and Discussion

Before investigating the assembly of nanoparticles with the presence of SiOz
particles in bulk polystyrene, we would like to study how the nanoparticles themselves
behave in the bulk polymer without SiOz particles. To achieve that, we manufactured
nanoparticle/polymer composites by using the “rapid precipitation” method to blend 0.3
vol% of pyridine-coated CdSe quantum dots (p-QD’s) with linear 393kD polystyrene, co-
suspended in pyridine”).94 The completely dried sample was ultramicrotomed to ~70
nm thick sections and viewed under transmission electron microscopy (TEM). The
resulting image in Figure 513 shows that the p-QD’s were well dispersed in the
polystyrene matrix; we believe the rapid precipitation method provided an intimately
mixed, homogeneous dispersion of nanoparticles in the linear polymer.

Figure 5.1b shows the same sample as 5.1a, but thermally annealed in a pellet
press at 170 °C under vacuum for 1.75 hrs. One can observe that the p-QD’s were
dispersed in the polystyrene, corresponding with our previous results using different
nanoparticle/polymer combinations (ﬁrllerene/polystyrene,94 oleic acid coated quantum
dot/polystyrene53). Interestingly, if we annealed the sample longer to 24 hrs,
agglomerates of p-QD’s were formed (Figure 5.10). Zhang et a1.95 performed Monte
Carlo simulation and observed particle aggregation under thermodynamic equilibrium,
which provides a rough explanation for our result here; for 24 hr annealing time, the

mixture is approaching its equilibrium state.

62

 

Figure 5.1. TEM images of 0.3vol% pyridine-coated CdSe QDs in bulk polystyrene
matrix. The samples were thermally annealed at 170°C for different durations: (a) 0 hr (b)
1.75 hr and (c) 24 hr.

63

The processing conditions in manufacturing these nanoparticle/polymer
composites are crucial to disperse nanoparticles well in the polymer melt. Previous
research performed in our group has shown that85 traditional solvent evaporation method
made large agglomerations of nanoparticles in various nanoparticle/polymer
combinations, while the rapid precipitation technique provided dispersed nanoparticles in
the polymer. Other research groups96 also found similar results of nanoparticle
agglomerates using solvent evaporation method.

Moreover, another important factor for well-dispersed system is that the radius of
gyration (Rg) of the linear polymer needs to be larger than the radius of the
nanoparticle,94 which applies to both inorganic and organic nanoparticles. The reason is
that the radius of gyration of the polymer increases when the nanoparticles are
homogeneously dispersed in the polymer.97 We also proposed that due to increasing
molecular contacts at the surfaces of dispersed nanoparticles compared to that of phase-
separated nanoparticles, this entropically unfavorable process is offset by an enthalpy
gain.94 The proposed statement applies to our experimental results here, in which the
radius of the p-QD’s is about 3 nm, smaller than the radius of gyration of 393kD PS (17

nm).

64

393k-PS
(Rg ~ 17 nm)

O \

, Pyridine-

' Oielc acid-

 

coated QD’s (r ~ 3 nm)

Figure 5.2. A cartoon showing the materials used for this work: QD’s with the presence
of SiOz particles in polystyrene.

In the next experiments, we blended 0.23 vol% SiOz particles into the previously
used 0.3 vol% p-QD/polystyrene composite and studied how the nanoparticles (p-QD’s)
behave in the resulting composite upon various annealing durations, as illustrated in
Figure 5.2. The polystyrene was well-dissolved in pyridine ﬁrst, followed by mixing in
measured amounts of the 8102 particle solution and p-QD’s solution to achieve the
desired volume concentrations. Figure 5.3a is the TEM image showing the “rapid
precipitated” sample; one can observe that the p-QD’s are randomly distributed in the
bulk polymer matrix and some of them were found around the surface of the SiOz
particle. After annealing for 1.75 hrs at 170 °C, well above the polymer’s glass transition
temperature, it is clearer that the p-QD’s were on the Si02 particle surface, as shown in

Figure 5.3b. Furthermore, we annealed the samples for 4 hrs and 24 hrs; the

65

corresponding TEM images are shown in Figure 5.30 and 5.3d. As the annealing time
increases, it is found that more p-QD’s assemble on the Si02 particle surface, and again

form more agglomerations in the polystyrene.

' "9'; -

' )r‘xp\.;f:\ ..‘A'v..‘1'. ._

é. '~-‘.- .f'kk.‘<"".
’ _

.
a ; '~,.,

.5
1 .-

595$».
gig}? ‘,

;

     

Figure 5.3. EM images of 0.3vol°o p-QD’s blended with 0.vol% SrOz particles in
bulk PS matrix. The samples were thermally annealed at 170°C for different durations: (a)
0 hr(b) 1.75 hr (0) 4hrand (d) 24 hr.

66

In order to study how different annealing durations affect the nanoparticle
assembly, we counted the numbers of nanoparticles that were “not” on the surface of the
Si02 particle in a (150 nm * 150 nm) area, since it was difﬁcult to count the
nanoparticles on the SiOz surface based on the TEM images that were taken. In this way,
we know how many nanoparticles there were assembling on the Si02 particle, assuming
the same numbers of nanoparticles in a certain area. For each annealing time, three
different areas centered upon three different Si02 particles were chosen when we counted
the nanoparticles.

Moreover, we performed a theory calculation based on the well-known Einstein
diffusion equation98 to calculate out the numbers of nanoparticles assembling on the
SiOz particle upon different annealing times. Firstly we used Stokes-Einstein relation
(Equation 4.1) for the diffusion coefficient (D) of the nanoparticles to be 0.13 nmz/s, in
which the viscosity of the PS/p-QD/SiOz composite was ~106 Pa-s ﬁom the rheology
measurement. In the calculation, we considered particles (QD’s) freely diffusing around a
spherical target (Si02 particle, radius a) in a liquid without forces acting on them other
than forces due to random collisions with liquid molecules. It is assumed that the
diffusing particles are initially distributed at a distance r0 from the center of the target
with all directions being equally likely, and that the particles were completely absorbed
by the spherical target. By deriving from the Einstein diffusion equation98 (detailed in the
Appendix section), we can calculate the numbers of particles absorbed on the spherical

target through the following equation:

-1 “"0
N(t|r0,t0)— r0 [1+elfl: m) t_t0 D

67

in which t is the annealing time, to is the initial time, and D is the diffusion coefficient.
Note that for the calculation, we may overestimate the numbers of nanoparticles that
assembled on the Si02 particles since the agglomerations of nanoparticles might slow
their diffusion in the polymer melt. Moreover, we also used the diffusion coefficient of
the oleic acid coated quantum dots,53 i.e. D = 1.64 nmz/s, to ﬁt the experimental data.

The ﬁt was not great so we tried D = 0.4 and 0.8 nmz/s for comparison.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1 1 1 1 1
0 Experimental
a ......... 0:013 n-
z ----- o=o.4 z
_ - - - o=o.e _
é 1" ..... D=1.64 E
C 0.5 — .1 _1 C
3 3
“5 ‘- \\ “5
C . s“. T" C
o :3:::TTT:; ttttt . ........ o
a 0.0 _ ------- . M.~— _ a
E L“
u. u.
I L I J 1 I
o 5 10 15 20 25 0.01 0.1 1 10 100
Annealing Time (hr) Annealing Time (hr)

Figure 5.4. (a) Annealing time (t) plotted versus the “fraction of un-assembled

nanoparticles (Nun); solid data points are ﬁom TEM measurements and the dashed line
represents the theory calculation. (b) The same plot as (a) in log scale.

Figure 5.4a plots the annealing time (t) versus the “fraction of un-assembled
nanoparticles (Nun)” based on TEM experimental results (solid data point) and theory
calculation results (dashed line). Note it was assumed that after 24 hr annealing (t = 24)
all nanoparticles in a certain area assembled to the $102 particles, although from the

TEM results it was not the case. However, since we would like to compare the

68

experimental results with the theory calculations, in which we had completely absorption
of nanoparticles to the SiOz particles, we normalized the numbers of nanoparticles in
both results to be Nun = O at t = 24, and Nun = l at t = O. From both experimental and
theory results, it is found that fewer nanoparticles were “not” assembling on the SiOz
particles as annealing time increased, indicating that more nanoparticles assemble on
them. Figure 5.4b is the same plot but in log scale and one can see more clearly that at t =
1.75, Nun = 0.2; this means that ~80% of the nanoparticles assembled on the SiOz
particles aﬁer 1.75 hr annealing, in a certain area.

We explain this from an entropic point of view. Upon annealing, nanoparticles
will diffuse in the polymer; however, once they diffuse and reach the surfaces of Si02
particles, the nanoparticles get “stuck” on the curved surfaces, as they lose less
conﬁgurational entropy than the polymer chains. We have seen similar results for

polymer thin ﬁlms in previous chapters.

69

Furthermore, to explore the effect of nanoparticle concentration on its assembly to
the SiOz surface, we lowered the concentration of p-QD to be 0.03 vol%. They were
blended with the same 0.23 vol% SiOz/polystyrene composite. Similar trends were
observed, that is longer annealing time (4 hr) made more p-QD’s assemble on the SiOz
particle surface, as shown in Figure 5.5, although this result was not as signiﬁcant as the

previous example due to the low concentration of nanoparticles.

 

  

Figure 5.5. TEM images of 0.03vol% p-QD’s blended with 0.23vol% SiOz particles in
bulk PS matrix. The samples were thermally annealed at 170°C for different durations: (a)
1.75 hr (b) 4 hr.

70

In addition to p-QD’s that were used in previous tests, another kind of
nanoparticles — oleic acid coated QD’s (o-QD’s) — were employed in similar experiments,
to challenge the versatility of this nanoparticle assembly. It is found that they also
assembled on the Si02 particle surface aﬂer thermal annealing, as shown in the TEM
images in Figure 5.6. We believe this is because of entropy, the same phenomenon we
studied for the p-QD’s previously. Interestingly, for longer annealing time (24 hr) we

found again that agglomerations of o-QD’s were formed in the polystyrene.

 

Figure 5.6. TEM images of 0.3vol% oleic acid coated CdSe QDs blenedd with 0.23vol%
SiOz particles in bulk PS matrix. The samples were thermally annealed at 170°C for
different durations: (a) 1.75 hr (b) 24 hr.

71

To further test the effect of the concentrations of both p-QD and SiOz particles on
their assembly, we manufactured a 1 vol% p-QD/l vol% Si02/PS nanocomposite. Note
that the volume concentrations of both p—QD’s and SiOz are much higher than previous
samples. Also this is the only sample in which we used hexanes as non-solvent during the
rapid precipitation process, whereas methanol was used for all experiments discussed
above. The TEM image in Figure 5.7 shows the sample after thermal annealing for 1.75
hrs at 170°C. We note that there were more agglomerations of p-QD’s in polystyrene and

also more of them assembly on the Si02 particle surface.

 

Figure 5.8. TEM image of a 1% p-QD/ 1% SiOZ/PS composite thermally annealed for
1.75 hrs at 170°C.

72

Finally, in order to study the effect of the addition of nanoparticles to a
SiOz/polystyrene composite, we manufactured a 0.23 vol% SiOz/polystyrene composite
thermally annealed for 1.75 hrs at 170°C and a similar one but with 0.3 vol% p-QD’s, as
shown in the TEM image in Figure 5.8. One can see that there are agglomerations of
Si02 particles in the SiOz/polystyrene composite; however, Si02 particles are more
dispersed in the p-QD/SiOz/polystyrene. We suggest that the addition of nanoparticles
may beneﬁt the dispersion of large particles (SiOz particles) in the polymer-based
composite; however, more research might be needed for overall information of the

sample.

 

.-

Figure 5.8. TEM images of samples thermallyannealedfor 1.75 hrs at 170°C: (a) 0.23%
SiOZ/PS composite. (b) 0.3% p-QD/0.23% Si02/PS composite.

I'"<'_ "~' 5
I

73

5.4 Conclusion

In this work, we investigated the assembly of nanoparticles with the presence of
SiOz particles in polystyrene melts upon different annealing durations. It is found that
longer annealing time made more nanoparticles assemble around the SiOz particles,
which were proven by TEM experimental results as well as theoretical calculations based
on the Einstein diffusion equation. This is because upon annealing, nanoparticles diffuse
in the polymer, but once they reach the surfaces of Si02 particles, they adhere on the
curved surfaces, as the nanoparticles lose less conﬁgurational entropy than the polymer

chains would.

74

Chapter 6

Summary and Conclusions

The assembly ofnanoparticles to interfaces in polymer thin ﬁlms was investigated.
Pyridine-coated CdSe quantum dots (p-QD’s) assembled on the substrate in polystyrene
ﬁlms after thermal annealing, in which both entropic and enthalpic forces favored the
assembly. If the same blend was spin coated onto a substrate containing large silica
particles and thermally annealed, the p-QD’s assembled around the silica particles.
Furthermore, if polystyrene, p-QD’s and large silica or polystyrene particles were
simultaneously spin coated onto a substrate and thermally annealed, p-QD’s were found
to assemble around the large particles, since the entropic terms dominated the assembly.

The same nanoparticle assembly technique was employed to create three-
dimensional surfaces, which could support curved but continuous polymer thin ﬁlms
covering surface protrusions (silica particles), even though the ﬁlm thicknesses were
thinner than the protrusion sizes. This indicates very strong assembly energy which can
overcome surface tension forces induced by the curved features instead of dewetting of
the ﬁlms ﬁ'om the substrate. For this effect, it is necessary for nanoparticles to assemble
on the solid substrate, where they form a jammed state that functions as an effective solid
layer to inhibit the dewetting of the ﬁlms.

Carbon nanotubes were used also as the “templates” for nanoparticle assembly in
one- and two- dimension. We showed one-dimensional assembly of nanoparticles on
nanotubes by depositing solutions of IgG nanoparticles onto a substrate containing

nanotubes. Their assembly was driven by non-speciﬁc van der Waals forces. Two-

75

dimensional assembly of nanoparticle along the nanotubes was achieved by spin coating
p-QD’s along with polystyrene onto a nanotubes-mica substrate, followed by subsequent
thermal annealing. The oleic acid stabilized QD’s did not assemble along the nanotues
due to their lower refractive index, thus demonstrating that nanotubes attract p—QD’s in
polystyrene ﬁlms, but not o-QD’s.

Finally, nanoparticle assembly in polymer melts with the presence of silica
particles was explored. Upon different annealing durations, we found nanoparticle
assembly around the silica particles increased with longer annealing time. It is believed
that longer annealing time made the nanoparticles diffuse more in the polymer; once they
reached the surfaces of silica particles, they adhered on the curved surfaces due to
entropic forces.

Using the studied nanoparticle assembly technique, we can create a wide variety
of nano-scale polymeric surfaces or structures. One possible application for these
structures will be constructing an optimal active layer of nanoparticle-polymer hybrid

solar cells.

76

Appendix A
Derivation of Einstein Diffusion Equation for Free Diffusion around a

Spherical Object

In order to calculate the numbers of nanoparticle absorbed on a spherical object
(e. g. the silica particle used in our work),99 we consider a situation that a particle ﬁ'eely
diffuse around a spherical target (radius a) in a liquid without forces acting on them other
than forces due to random collisions with liquid molecules. It is assumed that the
diffusing particles are initially distributed at a distance r0 from the center of the target
with all directions being equally likely, and that the particles were completely absorbed
by the spherical target (a) —» 00), where the reactivity is measured by the parameter (0.

Thus, we can describe the probability of ﬁnding the particle at a distance r from
the center of the target at time t, by a spherically symmetric distribution p(r, tlro, to),
since neither the initial condition nor the reaction-diffusion condition has orientation
preference. We can then write the ensemble of reacting (or absorbing) particles by the

well-known Einstein diffusion equation

6,p(r, tlro, to) = DV2p(r, tIrO, to) (A. l)

and the initial condition is

l
17(r9t0lr09t0): 2 5(r-ro) (A-Z)
472'7'0

 

. Additionally, we assume that the distribution of the particles vanishes at r0 —* 00 so the
boundary condition is imposed at
lim p(r,t|r0,t0)= 0 (A.3)

r—)<X)

77

. Also, the radiation boundary condition describes the reaction at the target, with a

spherical boundary, can be written as

D 6, p(r, t|r0,t0)= a) p(r, tlr0,t0), for r = a (A.4)

. Using (A3) and (A.4), (A.1) becomes
6, r p(r,t|r0,t0)= D 63 r p(r,t|r0,t0) (A.5)

. Using the Fourier transformation and the inverse Laplace transformation, one solves the

ﬁnal result to be

1 1 (r—r )2 (r+r —2c1)2
p(r’tlr0’t0)= 47: r r0 J4” D (t—t0)[exp [—2ﬁ705]+expl— 4DO(“’0) D

aexp[a2D (t—t0)+a(r+r0—2a)] (A.6)

 

 

 

47rrr0

r+r —2a
,/Dlt—t ) 0
xetfc[a 0 +J4D(t_t0)]

 

with
wa+D
a :
Da

 

(A7).

Next, we need to ﬁnd the absorptive boundary at r = a, thus setting a) —’ 00 and
hence a —> 00. We use the well-known asymptotic behavior to rewrite the last summand

of equation and consequently in the limit co —» 00 solution (A.6) becomes

_ l l (r—r)2 (r+r -2a)2
p(r,t|r0,t0)— 47! r r0 J47: D (t — to) [exp I} m] - exp[— 4 Do(t -to) D (At-8)-

 

 

78

Furthermore, the reaction rate for arbitrary a) —> 00 needs to be deﬁned. The rate

of reaction at r = a is given through the general solution (A.6)

K(t|r0,t0)= 472' (12 D 6,. p(r,t|r0,t0)| (A.9)

r=a

. One can conclude from the asymptotic behavior (A8) with r = a and employ for the

limit (a), a -> co) to equation (A.7). Then the reaction rate for a completely absorptive

1 ’0’05 [M] (A.10).

D(t—t0) t—tO xp _4D(t-t0)

boundary is obtained

 

K(t|r0,t0) =E10J477

Finally, we can calculate the fraction of particles reacting at the boundary r = (1

according to

Nreact(tlr0’t0):J:o dt'K(t'lr0’t0) (A'll)

For the general case with the rate deﬁned by (A. 10) and after the integration, we obtain
Nreact (ter a t0)

:aa-l a—ro _(m-a)a+D(t-to)a2 rO-a+2D(t—t0)a (A-12)
roa [1+e'fl4D(t—t0)l e 90%|: ,/4D(t-t0)

 

. For our case, since the particle is completely absorbed at the target, we derive the limit

for a —i 00 for a completely absorptive boundary at x = a with the aid of equation (A.8)

lim Nreact (ter’IO)
a—mo

_ a a-r l 2 1 (r -a)2
‘al‘+e”’lmﬁ—t.5l7l¢4xDo—tnwlzzllemlu—Bmll (“3)

 

 

 

79

. Since a —+ co, equation (A.13) is leﬁ with

. a a—ro
hm N tr ,t =——- 1+erf —— (A.15)
a—>oo react(l0 0) r0[ [ 4Dt—t0 :l]

, which we used in chapter 5 calculating the numbers of nanoparticles assembling on the

SiOz particle upon different annealing durations.

80

Appendix B

Adhesion of N anoparticle-filled Polymer Films to Substrates

We studied the adhesion of nanoparticle-ﬁlled polymer ﬁlms to silicon substates,
using nanoparticles with different surface coverage. The adhesion between the ﬁlm and
substrate was evaluated through a “peel test” using RSA 3 (TA Instruments), in terms of
the peel energy (GIC)- GIC is determined through100

GIC = P(1 — cos (9)/b = 2P/b (for 0 = 180°)
where P is the peel force, obtained from RSA 3; b is the width of the ﬁlm peeled off from
the Scotch tape. We obtained GIC ﬁom the average of at least 3 peel tests per
ﬁlm/substrate system.

About 200 nm thick of 393 kD polystyrene ﬁlms ﬁlled with pyridine coated
cadmium selenide quantum dots (p-QD’s) were made. The ﬁlms were spin coated onto
freshly cleaved mica substrates, ﬂoated off onto deionized water, picked up by silanized
silicon substrate using using Sigmacote®, and ﬁnally dried under vacuum for several
hours. They were thermally annealed at 170°C under vacuum, so we believe all the p-
QD’s assembled onto the substrate interface and formed different surface coverage. A
scotch tape was placed on top of the ﬁlms and a cotton swab was used to ensure no air

bubbles between the tape and the ﬁlm.

81

 

01
l
J

4:.
I
l

 

on
1
l

Peel Energy (J/mz)
l-O-l

N
l
I

 

 

1— L L I 1 J I J
0.0 0.2 0.4 0.6 0.8 1.0

 

Surface Coverage

Figure 8.1. Peel energy plotted versus the surface coverage of nanoparticles on the
substrate, for nanoparticle-ﬁlled polymer ﬁlms.

If we plotted the peel energy versus the surface coverage of nanoparticles on the
substrate, we found that the peel energy increases with the surface coverage. This
indicates that the adhesion of polymer ﬁlms to the substrate may be stronger for those

with more nanoparticles on the substrate.

82

Appendix C

X-ray Photoelectron Spectroscopy Spectrum

18 X 104 08)<PS1188.spe
I I I I I i

 

16~

-Cd3d5-Cd3d3_

I

14 ~

12~ -

10% —

 

c/s
-Cd3p3
-C1s

-O1s
l

’-
,3
_’;::::='— -Cd M N N

:9... -Cd3p1

 

 

 

s;
'51
L
‘i
{z
5.

 

 

 

 

 

0 l l l l l l 7 .l l l l
1 100 1000 900 800 700 600 500 400 300 200 100 0
Binding Energy (eV)

Figure C.1. XPS spectra of a nominal 63 nm thick polystyrene ﬁlms containing pyridine-
coated cadmium selenide quantum dots (p-QD’s) spin coated onto a silicon substrate

sparsely decorated with 120 nm-sized Si02 particles.

83

x105 08)<PSl111.spe

 

 

 

 

 

 

 

 

 

 

 

2 i I I I I I _l I i
0')
8
5
1.8” m ..
8
U
9
1.6e
1.4 . a
1.2K .4
.1 Z I
in Z I'.
5 1 i 2 a ll! I
‘O CO ‘I
a) O P ‘O In '3
,z' i 8 o 5 z} "9
03W” i" I 3 5 ' «ll 9 “
Nah. . 3 ‘ '4’!
\v' I Q .l l l; " l1?
0 6 — ‘l A l") f": :V‘ ill, ii Fix, I l -—<
. i, ﬁf/‘f" V :. ‘w’ ‘Y 3,. f./\, Lil! ‘ l!
W” .., l
: 3 Q
. ; (V)
04>- I I «r «i 8. i
I , ll (I) U.) '
‘w-val".‘ . ' Q
‘JI A Q (
02* wwrl "9 ll
k ,vrw (J
"I
___.l,- __l,_.,._L___,J _ I_ _L_#__; I I I I J
1000 900 800 700 600 500 400 300 200 100 0

Binding Energy (eV)

Figure C.2. XPS spectra of a nominal 63 nm thick polystyrene ﬁlms containing pyridine-
coated cadmium selenide quantum dots (p-QD’s) spin coated onto a silicon substrate

sparsely decorated with 120 nm-sized Si02 particles, after being thermally annealed at
160°C for 24 hrs.

84

Appendix D

Additional Data

 

 

 

 

 

Figure D.l. Cross-sectional transmission electron microscopy (TEM) images: (a) 26 wt%
of polystyrene-coated ferromagnetic cobalt nanoparticles (CoNP) were spin coated with
393 kD polystyrene and (b) thermally annealed under vacuum at 160°C for 24 hrs, under
a magnetic ﬁeld of 26 MGOe applied in the vertical direction of the ﬁlms (the disk
magnets were placed on the top and bottom of the sample).

85

10

ll

12

l3

14

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93

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