Controlling the Structure of Two-dimensional Nanoparticle
Supracrystals from Long-range Order to Anisotropy by
Tailoring Ligand Interactions
by
Jin Young Kim
B.S., Korea Advanced Institute of Science and Technology (KAIST), Korea (1998)
M.S., Korea Advanced Institute of Science and Technology (KAIST), Korea (2000)
Submitted to the Department of Materials Science and Engineering
In Partial Fulfillment of the Requirements of the Degree of
DOCTOR OF PHILOSOPHY
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
JUNE 2012
© Massachusetts Institute of Technology 2012. All Rights Reserved
Author………………………………………………………………………………………
Department of Materials Science and Engineering
May 22, 2012
Certified by…………………………………........................................................................
Francesco Stellacci
Professor of Materials Science and Engineering
Thesis Supervisor
Certified by…………………………………........................................................................
Caroline A. Ross
Professor of Materials Science and Engineering
Thesis Supervisor
Accepted by…………………………………........................................................................
Gerbrand Ceder
Chair, Departmental Committee on Graduate Students
Controlling the Structure of Two-dimensional Nanoparticle
Supracrystals from Long-range Order to Anisotropy by
Tailoring Ligand Interactions
by
Jin Young Kim
Submitted to the Department of Materials Science and Engineering
on May 22, 2012 in partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Materials Science and Engineering
Abstract
Ligand-stabilized nanoparticles (NPs) assembled into long-range ordered arrays,
also known as “nanoparticle supracrystals (NPSCs)”, are expected to provide a powerful
general platform for designing new types of solids. In particular, the NPs are themselves
self-assembled structures consisting of a core and a self-assembled monolayer of ligand
molecules surrounding it. The self-assembled structure of the NPs themselves determines
the structure of the self-assembled supracrystals. Ligands are of special interest in this
respect, because it is an important component for the NP system which play a major role in
the design of self-assembly of the complex matter and also provide a powerful entry into
the supracrystal engineering. The increasing ability to control the way in which ligand
molecules associate gives means for the designed generation of supraparticle architectures
in the self-assembly. In spite of this, elucidation of how the ligands play a role in affecting
the structural behavior of NPSCs remains largely unrevealed.
In this thesis, the effect of ligands for the two dimensional (2D) self-assembled
NPSCs structure was investigated. The key materials advancement that enables this work is
that we have been able to synthesize monodisperse gold NPs of same core size but different
ligand molecules. Additionally, a new method for monolayer film processing has been
developed to prepare the 2D NPSCs, based on a Langmuir assembly through successive
compression cycles. Importantly, as there is little effect exhibited by solvent interactions in
the NPs structure obtained from this approach, the corresponding NPs structural variation
in this work is truly driven by the different ligand interactions in NPSCs.
Specifically, we show that such ligand interactions have direct consequences on the
ordering and symmetry of the assembled NPSCs structures. Here, we report on a set of
NPSC arrays in which small changes in either the NP ligand environment or the ligand
2
configuration geometry induce significant variations in the order parameters of the crystal.
First, we show that the packing organization of a 2D NPSC array of hydrophobic
alkanethiol ligands varies with subtle chemical changes in the system, leading to a
transition between long-range to short-range (almost glassy) ordered phases. The balance
between long and short-range order is driven by small differences in intermolecular
interpenetration of the ligand molecules, that can be related to ligand conformational and
that can be rigorously the experimentally measured. Second, we show the first 2D NPSC
structures to have unique anisotropic symmetry due to the interaction between amphiphilic
NP ligand shells. It is understood that the ligand interactions on NPs through their unique
molecular configuration of amphiphilic ligands may provide the anisotropic feature in the
orientational alignment of NPSC symmetry.
Thesis advisor:
Francesco Stellacci, Professor of Materials Science and Engineering at EPFL
Caroline A. Ross, Toyota Professor of Materials Science and Engineering at MIT
3
Acknowledgements
This thesis would not have been completed without people who support and inspire
me during the course of Ph.D. work at MIT.
First of all, I thank my two advisors, Professors Francesco Stellacci and Caroline A.
Ross. I thank Francesco for his guidance, support, and inspiration throughout my studies. I
thank Prof. Ross for her generous nature and constant encouragement. Both of them has
taught me much and have always been reliable mentors who have guided me into right
direction during Ph.D. study at MIT. It was great pleasure to work with them.
I also thank my thesis committee members, Professors. Carl V. Thompson and Karl
K. Berggren, for constructive and insightful comments.
I am grateful to my former advisor Prof. Duk Young Jeon at KAIST for his sincere
support, guidance, and advice for my studies and career development.
The members of SuNMaG and SuNMIL have always been friendly and supportive.
In particular, I would like to thank Benjamin Wunsch, Jeffrey Kuna, Kislon Voitchovsky,
and Osman Bakr whom I have greatly learned from and enjoyed interacting with.
Additionally, I would like thank Korean friends at MIT: Byungwoo Kang, Dong
Kyun Ko, Dong Sup Lee, Hyewon Kim, Jae Chul Kim, Jae Jin Kim, Jihun Oh, Jiye Lee,
Jongnam Park, Seok Joon Kwon, Seung Woo Lee, Sung Keun Lim, Yeon Sik Jung, and
Yong Chul Shin to share their valuable time with me.
I would like to express my respect, love, and thanks to my family – parents, and my
brother and my sister-in-law’s family. Lastly, I thank my lovely wife, Hae Jung for her
understanding and encouragements.
4
Table of Contents
List of Figures
7
List of Tables
12
Chapter 1. Introduction and Background……………………………… 13
1.1 Introduction………………..……………….…………………………………... 13
1.2 Nanoparticle Supracrystal (NPSC).………………………………….……….… 16
1.3 Preparation of NPSCs………………….…………………………….………...
17
1.3.1 Assembly Components……………………….. …………….…….…. 17
1.3.2 Assembly Methods………………………….……………….……….. 25
1.4 Ligand-stabilized AuNPs….…………………………………………….….….. 32
1.5 Preparation of AuNPs………………….…………………………………..…... 39
1.6 Referecnes……..…………………………………………………………..…… 46
Chapter 2. Preparation of 2D NPSCs through Langmuir Assembly via
Successive Compression Cycles…………………………………............ 51
2.1 Introduction……………………………………………………………………
51
2.2 Experimental.………………………………………………………………….. 53
2.3 Effect of Compression in Langmuir DDT-AuNPs Monolayer Film………….. 54
2.4 Stress Relaxation Behavior in Langmuir DDT-AuNPs Monolayer Film……... 56
2.5 Effect of Compression/Relaxation Cycles in Langmuir DDT-AuNPs Monolayer
Film……………………………………………………………………………. 59
2.6 Role of the DDT Ligands in NPs Assembly……………………………….….. 66
2.7 Summary…….………………………………………………………………… 67
2.8 References………..…………………………………………………………… 68
Chapter 3. Ordering Behavior in the 2D Alkanethiol-Au NPSCs by
Ligand Interactions………………..…………………………………… 72
3.1 Introduction…………………………………………………………....……… 72
3.2 Experimental..………………………………………………………….....…..
5
74
3.3 Structure Phase Behavior for 2D Supracrystals of Alkanethiol-AuNPs…….... 76
3.4 Interparticle Spacing Results for 2D Supracrystal of Alkanethiol-AuNPs….... 80
3.5 IR Study for Ligand Conformational Behavior in 2D Alkanethiol-AuNPSCs.. 84
3.6 Summary…...………………………………………………………………….. 88
3.7 References….………………………………………………………………….. 88
Chapter 4. Self-assembly of Amphiphilic NPs into 2D NPSCs by
Langmuir Method………………………………………………………. 92
4.1 Introduction………………………………………………………………….
92
4.2 Experimental.…………………………………………………………………
94
4.3 Self-assembly of Amphiphilic NPs into 2D NPSCs by Langmuir Method…. .
95
4.4 Optical Characteristics Induced by Anisotropic NP Packing into NPSCs…… 100
4.5 Free Energy Contribution of Mixed Ligands Effects for Self-assembly of NPs into
2D NPSCs by Langmuir Method…………………………………………..…. 101
4.6 Summary……………………………………………………………………… 103
4.7 References………………………………………………………………..…… 103
Chapter 5. Conclusions and Outlook…..……………………………... 105
5.1 Conclusions..………………………………………………………………….. 105
5.2 Outlook……..…………………………………………………………………. 106
5.3 References…..…………………………………………………………..…….. 108
6
List of Figures
Figure 1-1.
(a) Highly ordered monolayer of pentadecylamine-stabilized AuNPs. The
average mean diameter of NPs size is 8.3±0.7 nm. (b) Selected area diffraction patterns
(high and low angles) for the monolayer. (25)……….…………………….……………. 19
Figure 1-2.
Transmission electron micrographs (TEM) of the different structures
according to the size ratio in the decanethiol-stabilized AuNPs mixtures; (left) random
mixture at the ratio of greater than 0.85, (middle) binary ordered structure at the ratio of
~0.58, and (right) phase separation at the ratio of ~0.47. (27)……….…………………... 20
Figure 1-3.
TEM images of various self-assembled supracrystal structures of
nonspherical NPs; (a) nanocubes, (b) nanoctahedra, (c) nanoplates, and (d) nanorods, (3134)……………………………………………………………………………..…..…….. 21
Figure 1-4.
Magnified TEM images of (a) dodecanethiol- and (b) dodecylamine
stabilized AuNP assemblies. (40)……………………………………..……..…….……. 23
Figure 1-5.
Schematic model of fcc and bcc structure of NPSC formed by controlling
symmetry and shape-anisotropic interactions through variable ligand surface coverage.
(42)……………………………………………………………………………………..... 25
Figure 1-6.
NP patterns formed by drop-casting of 5.5 nm sized-DDT-stabilized AuNPs
solution. NP concentrations were as follows: (a) (a) 1.0×1012, (b) 4.8×1012, (c) 1.2×1013, (d)
1.2×1013 with excess ligand volume fraction of 6.3×10-3. (45)...……………………….. 27
Figure 1-7.
TEM images of NP arrays produced by micro-contact printing. The NP
arrays appear darker in these imgaes. (a) Parallel
lines. (b) Square pads.
(48)……………………………………………………………...……………………….. 29
Figure 1-8.
Schematic of the interfacial configurations of the Janus NPs in two
Langmuir techniques, up-stroke and down-stroke modes; in the up-stroke deposition, the
hydrophilic face was in direct contact with the substrate whereas the hydrophilic face was
exposed and in the down-stroke deposition, an opposite configuration was obtained.
(50)………………………………………………………………………………………. 30
7
Figure 1-9.
Scheme of DNA-programmable NP crystallization. AuNP-DNA conjugates
can be programmed to assemble into different crystallographic arrangements by changing
the sequence of the DNA linkers. (57)……………..……………….…………………... 32
Figure 1-10. A plot of the relative lowering of the melting for AuNPs vs the inverse NP
radius. (62)………………………………………………………….…………………… 34
Figure 1-11. Single electron tunneling on a single ligand-stablized Au55 cluster at 90K.
The junction capacity was calculated to be 3×10-19 F by fitting. Coulomb staircase
behavior is visible. (65)………………………………………………………………….
35
Figure 1-12. X-ray crystal structure determination of Au102(p-MBA)44 NP. Electron
density map (red mesh) and atomic structure (gold atoms depicted as yellow spheres, pMBA shown as framework and small spheres sulfur in cyan, carbon in gray and oxygen in
red). (69)…………………………….…………………………………………………... 36
Figure 1-13. Schematic diagram of an ideal, single-crystalline SAM of alkanethiolates
supported on a gold surface with a (111) texture. (72)…………………………………. 38
Figure 1-14. Schematic diagram depicting the arrangement of decanethiol on Au(111)
lattice. (a) Structural model of the commensurate adlayer formed by thiols on the gold
lattice. The arrangement shown is a (√ ×√ )R30°structure where the sulfur atoms (dark
gray circles) are positioned in the 3-fold hollows of the gold lattice. (b) Cross-section of the
SAM formed from decanethiol.(72)..……………………………………………………. 38
Figure 1-15. Various thiols used for the generation of monolayer-protected AuNPs.
(75)………………………………………………………………………………………. 41
Figure 1-16. Formation of AuNPs protected with alkanethiolates by reduction of Au(III)
compounds in the presence of alkanethiols. (75)……..…………………………………. 42
Figure 1-17. TEM images of different-sized AuNPs produced by varying reaction solvent
(a-b) and temperature (c-f): (a) 2.1 ± 0.3 nm; (b) 3.5 ± 0.3 nm; (c) 5.3 ± 0.4 nm; (d) 6.2 ±
0.3 nm; (e) 7.1 ±0.5 nm; (f) 8.3 ±0.5 nm. All scale bars are 20 nm. (78)..….…………. 42
Figure 1-18. General scheme and examples of functionalized thiols for the ligand
exchange reactions. (75)…………………………………………………………………. 44
8
Figure 2-1. (a) Surface pressure-area compression isotherm curve for the DDT-AuNPs
monolayer. (b) TEM images of the NPs monolayer film structures transferred to the
substrates at various surface pressure. Scale bar : 100 nm.……….……………..……… 55
Figure 2-2. (a) Surface pressure-time relaxation isotherm curve for the DDT-AuNPs
monolayer compressed to 18 mNm-1. (b) TEM images of the close-packed Langmuir NPs
monolayered film taken before and after relaxation process. Scale bar : 100nm.…….… 57
Figure 2-3. (a) Relaxation isotherm curves for the DDT-AuNPs film achieved at the
various surface pressure. Each segment on the time trace corresponds to the relaxation in
various surface pressures after a compression step. The four regions (I-IV) identified are
discussed in the text. (b) TEM images of the NPs films corresponding to each region. The
film structures were obtained from after reaching the plateau in each regions of the
isotherm curve. Scale bar : 100 nm.………………………….………………………….. 58
Figure 2-4. Schematic drawing illustrating the preparation of the Langmuir NP monolayer
through cyclic compression cycles at 12 mNm-1 (top) and surface pressure-time & area
change-time curves for each step (bottom): (1) monolayer spreading – the NPs monolayer
was spread by careful casting of a 0.2 mg/ml solution in mixed solvents of
dichlorofmethane and hexane (vol 50:50) onto water subphase, (2) compression – the area
of the monolayer was decreased by moving a barrier with a rate of 10 cm 2min-1. (3)
relaxation – the compressed monolayer in (2) step is kept under constant area. In this
process, surface pressure decays over time, followed by plateau, (4) cyclic compression
processes in Langmuir NPs monolayer, accompanied by relaxation step.………….…... 60
Figure 2-5. (a) TEM images of NPs monolayer by compression cycles at 12 mNm -1. (b)
Langmuir relaxation isotherm curves during cyclic compression cycles for DDT-AuNPs at
12 mNm-1. (c) High-magnification TEM images of DDT-AuNPs monolayer obtained from
3rd compression cycles. Scale bar : 100 nm….…………………………………………. 61
Figure 2-6.
DDT-AuNPs monolayer obtained from 3 compression cycles at surface
pressures of 10, 11, and 12 mNm-1; (a) Langmuir relaxation isotherm curves during
9
compression cycles, (b) TEM images of DDT-AuNPs monolayer obtained from 3
compression cycles. Scale bar : 100 nm……………..………………………..………… 64
Figure 2-7. Structural order in NPs monolayer structures by the radial distribution
functions (RDFs); (a) as-compressed to 18 mNm-1, (b to d) 3 compression cycles at 10, 11,
and 12 mNm-1. The RDFs was calculated from the TEM data. The correlation distances of
the NPs structures are listed in Table 1. Within correlation distance, the NPs are closepacked with a six-fold symmetry………………..……………………………………… 65
Figure 2-8. Structural order in NPs monolayer structures by spatial distribution of the
extent of interdigitation (E.I.) (top) and the histograms for the distribution of the ligand E.I.
values; (a) as-compressed to 18 mNm-1, (b to d) 3 compression cycles at 10, 11, and 12
mNm-1. Here, the E.I. is averaged over the first nearest neighbors and shown as the particle
color. See the color bars for meaning of the particle color. The average values for the
spatial distribution of the E.I. values are listed in Table 1. Scale bar : 100 nm.……….. 65
Figure 2-9. A plot representing the average inter-edge spacings of a NP in NPs monolayer
structures as a function of NPs size, which is averaged over its first nearest neighbors; (a)
before and (b) after compression…………………….…………………………………. 67
Figure 3-1. Langmuir relaxation isotherm curves during compression cycles. The area
(black curve, left axis) and surface pressure (blue curve, right axis) versus time during
Langmuir film processing through successive compression cycles.…………………… 77
Figure 3-2. Structural phase behavior 2D alkanethiol-Au NPSCs. (a) TEM images, (b)
grain size analysis, (c) FFT patterns, and (d) radial distribution function (RDF) results for
2D alkanethiol-Au NPSCs. Scale bar : 100 nm….……………………………………. . 79
Figure 3-3. Interparticle spacing results for 2D alkanethiol-Au NPSCs. (a) Interparticle
spacing distribution for 2D alkanethiol- Au NPSCs. The red lines in the diagram represent
one (left) and two (right) ligand span lengths, respectively. (b) Plots representing the
average inter-edge spacings of a NP in 2D NPSC structures as a function of NP size, which
is averaged over its first nearest neighbors…..………………………………………....
81
Figure 3-4. IR spectra in (a) ν(C–H) regions and (b) average peak position of d- and d+
CH2 stretching vibrations of alkanethiol- Au NPSCs…………...……………………… 85
10
Figure 3-5. IR spectra in (a) ν(C–C) and (b) ν(C–S) regions of alkanethiol- Au NPSCs. (c)
Plot of the ratio of intensity of the band at ~1342 cm-1 (square) and ~1366 cm-1 (circle),
ascribable to end-gauche defect and internal kink defect respectively, to the sum with the
band at ~1377 cm-1 as a function of chain lengths for alkanethiol……………………… 87
Figure 4-1. TEM results of 2D NPSCs structures of AuNPs coated with HDT and MUOL
with the varied ratio of ligand compositions; (top) TEM images, (bottom) FFT patterns.
Scale bar : 100 nm.……………………………………….……………………………… 97
Figure 4-2.
Plot of interrelationship between interparticle spacing, eccentricity, and
correlation distance measured in the NPSCs structures……………………………........
99
Figure 4-3. Normalized absorbance intensity at 572 nm as a function of the rotation angle
of NPSCs to the polarized light beam...……………………………………………........ 100
11
List of Tables
Table 2-1. Summary of structural order parameters for the NPs monolayered structures
studied…………………………………………………………………………..……….. 66
Table 3-1. Summary of particle size distribution results for the synthesized alkanethiolAuNPs with different chain lengths…………………………………..….…………..….. 75
Table 4-1. Summary of particle size distribution results for the synthesized AuNPs used in
this study….……………………….…………………………………..….……………... 94
Table 4-2. Axes values and eccentricity obtained by the fitted FFT patterns of NPSCs
structures……………………………..………………………………..….……………... 97
12
1. Introduction and Background
1.1. Introduction
Self-organization of nanoparticles (NPs) opens a new and challenging area in
nanoscience. It is established that NPs present a new building block for the fabrication of
advanced materials that are expected to exhibit novel chemical and physical properties.
Furthermore, as with understanding of how atoms or molecules organize into condensed
matter systems, thus producing new, collective phenomena investigation of novel types of
condensed matter built from NPs is now drawing interest in the scientific community.
Inspired by the complex and functional systems formed extensively in Nature
through self-organization, the concept of self-assembly is a critically enabling “bottom-up”
approach to organizing NPs into well-defined lattice structures or geometries. Much like
atoms or molecules which can crystallize into solids, NPs can also assemble into larger
scale structures which can demonstrate various level of hierarchy. In terms of technological
applications it is expected that the distinct properties of the nanometer-scale building blocks
in combination with the novel collective properties of their assemblies, will provide a
versatile approach to materials for a wide range of potential applications, including
thermoelectric, photovoltaic, memory storage, and chemical sensing aspects (1-6). With the
first report of the self-assembly of a NP supracrystal (NPSC) (also known as superlattice)
by Benton et al. in 1989 (7), the study and prospects for such materials have advanced
rapidly (8-10).
13
However, despite its enormous potential, self-assembly of NPs still poses a number
of substantial challenges. The central challenge to all NP self-assembly is understanding
how to rationally design and fabricate systems. Many of the ideas that are crucial to the
development of this area have been pursued from various vantage points: NPs synthesis, the
interplay between enthalpy and entropy, the nature of non-covalent forces in self-assembled
NPs structures, thermodynamic approach of self-assembling process,
macroscopic/microscopic packing arrangement in self-assembled structure (11-19).
Here, we mainly explore the concept of ligand molecules in NPSCs engineering.
We believe that self-assembled ligand configurational structures and their interactions in
close proximity during NP assembly process could become an important and useful design
principle in these and related areas. In particular, in the sense that the ligand molecules
affect not only the thermodynamic interaction potential but also dynamic assembling
process, the detailed understanding of the ligand interactions becomes crucial to control the
order in NPs assemblies. Yet, elucidation of how the ligands play a role in affecting the
ordering behavior of self-assembled NPs structures remains largely unknown. One of the
major limiting factors for the study is the scope of the available analytical tools for further
characterization. Rigorous identification of the structures themselves can be difficult
because of their lability and complex connectivity. Therefore, description of the behavior of
their assembly is considered to be a formidable challenge.
The focus of this thesis project is to study and understand the two dimensional
NPSC of gold nanoparticles (AuNPs) with ligand molecules and how the ligand
interactions affect the structural behavior of NPSCs. Specifically, we show that interactions
14
between ligand molecules on the NPs has direct consequences on the ordering and
symmetry of the assembled NPSCs structures. We show that the degree of ordering of a 2D
NPSC array of hydrophobic alkanethiol ligands varies with subtle chemical changes in the
system, leading to a transition between ordered and glassy phases. In addition, we show the
2D NPSC structures to have unique anisotropic symmetry via the interaction between
amphiphilic NP ligand shells. We show that the structural behavior is a direct consequence
of the ligand interaction in 2D NPSCs.
The key materials advancement that enables this work is that we have been able to
prepare monodisperse AuNPs of same core size but with different ligand molecules from
the synthesis, not ligand exchange: the latter case inevitably introduces a number of
additional complications including changes in surface chemistry, solubility, and NP
morphology. Additionally, for film processing in this study, we use an experimental
approach to prepare the 2D NPSCs through a Langmuir assembly and then annealing them
through multiple compression cycles. Importantly, because NPs are driven into assembled
structures by mechanical compression and/or relaxation in the process, there is little
interaction with the solvent. Thus, the corresponding NPs structural variation in this work is
truly driven by the different ligand interactions in NPSCs.
In the following chapter 1, I concentrate mainly on a discussion of NPSCs based on
observed phenomena reported in the literature. Additionally, an overview of AuNPs is also
presented.
15
1.2. Nanoparticle Supracrystal (NPSC)
The properties of most materials are intimately connected to the way in which they
are ordered on the atomic scale. A new study suggests that in materials made from the
regular arrangement of discrete NPs, control of order and periodicity could be exploited at a
whole new level. NPSCs are a relatively new class of materials that have become possible
through recent advances in the chemical preparation of ligand-stabilized metal and
semiconductor NPs with very narrow size distributions. These particles can self-organize to
form crystalline materials with both atomic and nanometer-scale order via self-assembly.
Self-assembly refers to the process by which NPs or other discrete components
spontaneously organize, either directly through specific interactions and/or indirectly,
through their environment (20). Self-assembly is typically associated with thermodynamic
equilibrium, the organized structures being characterized by a minimum in the system’s
free energy, although this definition is too broad (21). Essential in self-assembly is that the
building blocks organize into ordered, macroscopic structures, either through direct or
indirect interactions.
NP self-assembly is of growing interest as one has the possibility of exploiting
spatial or temporal attributes of NPs to tailor assembly formation. By altering the
substituents of the assembling building blocks, desirable changes in the assembly process
are achieved. The self-assembling interactions between the NPs are generally dominated by
the attractive van der Waals (vdW) interactions, which are in turn balanced by
steric/electrostatic repulsion. The interparticle interactions are further influenced by the size
and shape of the NPs and the length and molecular arrangement of the ligand shells capping
16
NPs. Moreover, the medium (solvent) also plays a role in the interaction between the NPs
in complex manners. Such complex behavior results in kinetically-trapped NPs structures,
which makes it difficult to bring them into an equilibrium state (18). In addition, desired
properties and structural effects in the assembled materials can be accomplished using
confined space or surface. This strongly affects the force balance for the self-assembled
structures, and therefore the packing geometry. Self-assembly of NPs can be achieved using
simple methods such as evaporation-based assembly, assembly at an interface, etc., or by
more advanced methods of patterned self-assembly using lithography (22) and microcontact printing (23), where only the patterned areas with suitable chemical functionalities
create organized NP networks on substrates.
1.3. Preparation of NPSCs
NPSCs are generally prepared from the “bottom-up”, where the film structure is
controlled via both the assembly components themselves and the method for inducing selforganization. In the following section, we discuss the effect of NPs components in the
ordering behavior and review a number of methods that have been commonly employed in
NPSCs assembly studies.
1.3.1. Assembly Components
NP size effects - Commonly, small particles have been shown to have a less wellordered structure compared to larger particles. One of the reasons for this phenomenon is
17
due to the larger atom per particle distribution for the given size deviation at the smaller
NPs size. For example, a 1.4 nm gold particle requires only 100 atoms to increase in core
diameter by 0.5 nm, whereas the equivalent change in a 10 nm particle is 5000 atoms. The
net result is that small deviations in core size easily disrupt the assembly of small particles
but have less effect on the assembly of large particles.
Another interpretation can come from the examination of the interaction forces
operating in the self-assembly process. In particular, the amplitude of the vdW potential
needs to be sufficient to counterbalance the thermal energy, but only when the AuNPs are
sufficiently large as the amplitude of the vdW potential is size dependence. For example,
Abécassis et al. (24) calculated the vdW potential for AuNPs of 3 different radii as a
function of the inter-NP separation distance. When the particles drops below a critical
radius, the thermal energy becomes strong enough to influence the particles interactions
and the self-assembling process will not be possible.
Brown et al. (25) reported the size effect in the pentadecylamine-AuNP system for
the formation of NPSCs. They have shown that AuNPs of different sizes varying between
1.8 and 8 nm demonstrate different structures of NPSCs. They demonstrated from the
analysis of electron diffraction that bigger NPs self-assemble into the better ordered
structures (Figure 1-1). Additionally, Whetten et al. (26) have produced a phase diagram of
NPSCs structure based on free energy calculations of the formed phase. They argued that
for the ligand they employed, as the NPs size becomes smaller, the structural phase formed
tends toward a lower degree of symmetry.
18
Figure 1-1. (a) Highly ordered monolayer of pentadecylamine-stabilized AuNPs. The
average mean diameter of NPs size is 8.3±0.7 nm, (b) Selected area diffraction patterns
(high and low angles) for the monolayer. (25)
When NPs that differ in the core size, the ligand molecules, or the core material, are
co-assembled, the interaction force is more complicated, with a number of distinct pairwise
interactions combining with hard-sphere space-filling rules to drive the self-assembly of
multicomponent NPs. Indeed, mixtures of AuNPs of different sizes have very different
assembly behaviors depending on the ratio between the core diameters. Considerable
theoretical and experimental work has been done to clarify the causes of these different
regimes but many questions remain (27-29). It is accepted that a combination of enthalpy
and entropy factors leads to the formation of these aggregates. When the former are
dominant, phase separation prevails; when the latter are dominant, homogenous mixtures
are formed, and supracrystals exist in an intermediate region (Figure 1-2). Additionally, it
has been found that NPs have a net charge, leading to additional coulombic attraction and
19
repulsion forces that need to be taken in account when studying the formation of these
particle crystals (31).
Figure 1-2. Transmission electron micrographs (TEM) of the different structures according
to the size ratio in the decanethiol-stabilized AuNPs mixtures; (left) random mixture at the
ratio of greater than 0.85, (middle) binary ordered structure at the ratio of ~0.58, and (right)
phase separation at the ratio of ~0.47. (27)
NP shape effects - Recent synthetic advances enable a possibility of generating nonspherical (faceted) NPs with precise control of the size, shape, and surface chemistry,
allowing a rational assembly of these non-spherical NPs into close-packed supracrystals
(31-34). For spherical NPSCs, the crystal orientations of building blocks are random and
the whole pattern should be isotropic with a translational order. In contrast, non-spherical
NPs can produce a richer variety of assembled structures due to their unique geometrical
features with their anisotropic assemblies. Various structures of supracrystals assembled
from non-spherical NPs including nanocubes, nanooctahedra, nanoplates, nanorods, and
other morphologies are displayed in Fig. 1-3.
20
Figure 1-3. TEM images of various self-assembled supracrystal structures of nonspherical
NPs; (a) nanocubes, (b) nanooctahedra, (c) nanoplates, and (d) nanorods. (31-34)
Ligand effects - Only a few studies have been reported on the effect of the
interaction between ligand molecules for the NPSCs. Of particular interest is the question
of how the ligand interactions affect the ordering behavior of NPSCs. Given the difficulties
and complexities involved in getting NPs to form high-quality supracrystals, and the fact
that they are often limited to ligand interactions, investigating such behavior has not been
easy.
Hostetler et al. (35) reported a comprehensive characterization with infrared
transmission spectroscopy of the structures of alkanethiols bound to AuNPs, with chain
21
lengths varying from 3 carbon atoms (propanethiol) to 24 (tetracosanethiol). They found
that the shorter chain length thiols were substantially disordered compared to the longer
ones and most resembled the free alkanes in the liquid state. Crystalline packing of the
alkanethiol groups was observed from the region of chain lengths higher than six-carbon
atom (hexanethiol). This same issue was addressed theoretically by Luedtke & Landman
(12), who performed molecular dynamics simulations that corroborated this picture for 3D
NPSCs of AuNPs. The longer-chain alkanethiols such as dodecanethiol (12 carbon atoms)
preferred to organize into bundles. This alkanethiol bundle thus governed the organization
of the particles in the NPSCs.
It is known that when alkanethiol-stabilized metal NPs assemble, their ligand shells
interpenetrate in a process known as interdigitation (36, 37). This phenomenon is one of the
driving forces for the formation of NPs lattices and can lead to close-packed supracrystal
structures. For octanethiol-stabilized silver particles of 5 nm, the interdigitation enthalpy is
~20 kJ/(mol of ligand), a sizeable energy. This energy shows strong dependence on the
ligand chain lengths and ligand composition (38). The supracrystals generated by drying
these particles under similar conditions are very different in size and quality even though
the size and dispersity of the metallic cores are kept identical, so it is clear that the
interdigitation energy plays a complex role that needs to be better understood. Kim et al.
(39) reported that the extent of ligand interdigitation is estimated to be ~ 0.5 in the DDTcoated AuNPs monolayer. Also, they have shown the relationship between both the NPs
diameter and the ligand interdigiation within the NPs assembly to the formation of ordered
NPs structures. Another example in terms of interdigiation is shown in the work of Prasad
22
et al (40). They have shown differences in their assemblies from dodecylamine- and DDTstabilized AuNPs. It should be noted that as both these molecules contain 12 carbon atoms
the ligand shell on the surface is expected to be of the same thickness. It is also shown that
while the ligand shells from neighboring AuNPs in the thiol case participate in full
interdigitation, ligands with amine head group do not do so (Figure 1-4). This has been
partially attributed to the weaker binding of the amine head group to the NP surface. Such
weak binding may result in a higher disorder in the alkane chain hence preventing
interdigitation.
Figure 1-4. Magnified TEM images of (a) dodecanethiol- and (b) dodecylamine-stabilized
AuNP assemblies. (40)
Fink et al. (41) have prepared particles capped by quaternary ammonium bromides
of varying chain lengths of six carbon atoms to eighteen carbon atoms. It was shown that
23
these particles self-assemble into 2D and 3D assemblies on substrates and that the
interparticle separation gradually increases as the chain length is increased. An interesting
formation of multilayers where the particles occupy 2 fold saddle sites instead of 3 fold
hollow sites is observed and attributed to the interplay between the electrostatic repulsion
and vdW attractive forces. In the thiol chain length variation experiment, it was observed
that as thiol chain length increases, the gap between the particles increases by 1.2 Å
gap/carbon atom.
Recently, Hanrath and coworkers (42) studied the control of ligand attachment
and/or coverage on NPs as a tunable parameter to direct the self-assembly of supracrystlas
with predefined symmetry (Figure 1-5). They show that PbS NPs with dense ligand
coverage assemble into supracrystal of face-centered cubic (fcc) structures whereas NPs
with sparse ligand coverage assemble into supracrystal of body-centered cubic (bcc)
structures. They suggest that the loss of ligands occurs preferentially on {100} than on
{111} PbS NP facets by different surface chemistry on crystallographic plane. Additionally,
they reported that solvent vapor annealing can be an efficient route to tune the selfassembled NPs structure through the control of ligand-ligand and ligand-solvent
interactions in the self-assembly process (43). They studied the NPs assembly of various
polymorphs, including fcc, bcc, and body centered tetragonal (bct) structures, using in situ
grazing incidence small angle X-ray scattering under controlled solvent vapor exposure.
24
Figure 1-5. Schematic model of fcc and bcc structure of NPSC formed by controlling
symmetry and shape-anisotropic interactions through variable ligand surface coverage. (42)
1.3.2. Assembly Methods
Drop-casting - Drop-casting is the relatively simple method of assembling NPs on a
surface. When a solution of NPs is spread on a solid substrate by drop-casting, the
relatively weak attraction forces between NPs or between the NPs and the substrate gain
importance during the slow solvent evaporation, forcing NPs to self-organize into mono- or
multilayer structures. The drop-cast films of NPs typically exhibit well-ordered, selfassembled NP patterns on the surface with domain sizes ranging from a few hundred
nanometers to several hundred microns. The NPs film structures assembled through this
method have been shown to depend on three factors: NP size distribution, NP concentration,
and solvent volatility. However, due to the highly non-equilibrium conditions of an
evaporating drop, the thermodynamic control over the structure still remains unclear (17).
Technically, the most important variable for controlling ordered self-assembled NPs
structures is to prepare uniform-sized NPs with a standard size deviation (σ) of typically
25
less than 10 %, these particles being termed monodisperse. In the sense that core-core
interactions such as vdW or dipolar interactions show size-dependence in NPs assembly,
the use of monodisperse NPs is expected to be necessary for the NPSCs even if the extent
of required monodispersity of NPs can be different depending on the materials. Korgel et al.
(44) reported that NPs assemblies were not observed to form into ordered structures upon
solvent evaporation in the case of polydisperse dodecanethiol (DDT)-stabilized silver NPs
with σ of larger than 10 %. On the other hand, the same NPs with better size dispersity led
to the formation of supracrystals with the face-centered cubic (fcc) structure.
Lin et al. (45) have studied the effect of NP concentration on the formation of 2D
NPSC. When a solution of monodisperse DDT-stabilized gold NPs of ~ 5.5 nm was spread
on silicon nitride substrates, the sizes of NP domains strongly depended on the NP
concentration (Figure 1-6). At a low NP concentration, NPs structures are formed into
isolated island structures. At a higher NPs concentration, the structures get more dense NPs
structures at the monolayer level, and thus form into close-packed NPs monolayer
structures with an average domain size of a few hundreds μm2 at a concentration of 1.2 ×
1013 mL-1. However, the domain increase at the monolayer was inhibited above the NP
concentration level, so that film formed multilayer regions rather than increasing the
monolayer size. The growth behavior of the island areas was studied under an optical
microscope; the island areas growing linearly and exponentially in time by NP diffusion
and aggregation along the island perimeter, and the growth rate varies from island to island.
These islands eventually merge to form a continuous monolayer (41).
26
Figure 1-6. NP patterns formed by drop-casting of 5.5 nm sized-DDT-stabilized AuNPs
solution. NP concentrations were as follows: (a) 1.0×1012, (b) 4.8×1012, (c) 1.2×1013, (d)
1.2×1013 with excess ligand volume fraction of 6.3×10-3. (45)
Evaporation kinetics is examined to show the strong influence on the selfassembling process of NPs. As one example, the evaporation rate of the solvent can be
modified by adding a second solvent. Murray et al. (5) have reported that structures of NP
films prepared by drop-casting solutions of ~8 nm CdSe NPs coated with trioctylphosphine
oxide/trioctylphosphine mixture strongly depended on the solvent composition. In
particular, by tuning the mixing ratio of hexane/octane solvents with different solvent
volatility, NP assemblies exhibited either supracrystals with a high degree of order, or glass
structures of with a large amount of disorder. The presence of excess ligands in NPs
solution is reported to strongly influence the solvent evaporation kinetics as well. In the
27
presence of excess ligands, DDT-AuNPs did form supracrystals with an extraordinary
degree of monolayer perfection extending over several microns (Fig. 1-6d). The authors
proposed that the excess surfactant reduced the evaporation rate of the solvent, thereby
preventing the solvent from dewetting the surface (45,46).
Langmuir methods - Langmuir methods can be used to prepare a monolayer of
ordered NPs over macroscopic dimensions at an air/water interface. Typically, a solution of
NPs of ligands with hydrophobic functional groups is spread on the water surface. After
solvent evaporation, the NPs are formed into insoluble NPs monolayers first, which are
slowly compressed along the water surface using a movable barrier on the Langmuir trough.
The structural phase of NPs monolayer formed during the compression is monitored by
measuring the net decrease in the surface tension of water, called surface pressure. The
formed monolayer is then transferred to a substrate for further characterization, using either
horizontal or vertical lift-off. Of interest, this method is simple, robust, scalable, insensitive
to the substrate material and topography, and has a strong preference for forming
monolayer films. As such, it stands out as an excellent candidate for the fabrication of
technologically important ultrathin film materials (47).
A benefit of the Langmuir approach is that it offers efficient control over inter-NP
separation. We have shown a new Langmuir approach of getting NPs monolayers to move
toward an equilibrium phase by employing successive compression and relaxation cycles
(see chapter 2). They have investigated the structural evolution of NPs assemblies
depending on the compression cycles. Successive compression cycles rendered the
28
interparticle spacing distribution more homogeneous and significantly increased the long
range order in the assembled structure. Also inter-NP separations can be controlled by
tuning surface pressure in the compression process or incorporating new ligand molecules
directly into the Langmuir monolayers for ligand exchange.
Due to the robustness in the processing, the Langmuir NPs structures can be
efficiently engineered in various ways. For example, after transferring the Langmuir
monolayered structure to a patterned polydimethylsiloxane (PDMS) stamp, the NPs
assembly could be printed onto a substrate by bringing the PDMS stamp into contact with
the surface, allowing the patterning of mono- or multilayered arrays of close-packed NPs
(Figure 1-7) (48).
Figure 1-7. TEM images of NP arrays produced by micro-contact printing. The NP arrays
appear darker in these images. (a) Parallel lines. (b) Square pads. (48)
29
In addition, through interfacial ligand exchange process, the types of ligands on the
Langmuir NPs monolayer can be engineered to a high degree. Perro et al. (49) showed that
Janus NPs, with one half of hydrophilic phase and the other half of hydrophilic phase,
could be designed through this approach. The presence of the Janus NPs has been verified
by different contact angle test results of the particle monolayers deposited onto a substrate
surface (50). Here two interfacial configurations were achieved by different modes of
transfer (Figure 1-8). For enhanced stability of NPs film, a number of groups have
demonstrated that robust NPs monolayer could be produced by incorporating bifunctional
linkers into the Langmuir monolayer, thus leading to chemically cross linking of the NPs or
ligands within the film (51).
Figure 1-8. Schematic of the interfacial configurations of the Janus NPs in two Langmuir
techniques, up-stroke and down-stroke modes; in the up-stroke deposition, the hydrophilic
face was in direct contact with the substrate whereas the hydrophilic face was exposed and
in the down-stroke deposition, an opposite configuration was obtained. (50)
30
Layer-by-layer (LbL) method - Layer-by-layer (LbL) assemblies using electrostatic
interaction offer simple and flexible pathway for creating well-organized NP assemblies
(52-54). The principle of the LbL assembly is quite simple; it consists of repeating cycles
with sequential dipping of the substrate in oppositely charged materials. Most often,
negatively charged NPs and positively charged polymers are used, having electrostatic
attraction to each other are used. Additionally, other intermolecular forces such as
coordination, covalent or hydrogen bonding can also lead to sequential layering. The LbL
method is an effective technique to tailor the thickness of NP layers as well as to control the
interactions between the layers. In addition, it is applicable to curved surfaces, for example
core–shell architectures (55). However, despite the above-mentioned advantages, the LbL
method remains to be challenging in the preparation of the self-assembled NPs structures
with long range order due to the strong electrostatic interaction.
DNA method - DNA-mediated assembly of NPs is an attractive way to organize NPs
into periodic structures through the programmable DNA Watson-Crick base-pairing
interactions and the ability to construct branched DNA nanostructures of various
geometries. Recent success in using DNA as molecular glue to direct AuNPs into periodic
crystalline lattices further demonstrates the power of DNA as a building block for selfassembly.
Sharma et al. (56) reported that by the attachment of single-stranded DNA to
AuNPs of various 3D architectures can form, ranging in shape from stacked rings to single
spirals, double spirals, and nested spirals. Mirkin and coworkers (57,58) have demonstrated
31
the ability to control AuNPs rationally by directing the formation of large single- and
binary NPSCs through programmable base-pairing interactions (Figure 1-9).
Figure 1-9. Scheme of DNA-programmable NP crystallization. AuNP-DNA conjugates can
be programmed to assemble into different crystallographic arrangements by changing the
sequence of the DNA linkers. (57)
1.4. Ligand-stabilized AuNPs
Ligand-stabilized AuNPs of a few nanometers in size are of fundamental interest for
their chemical and quantum electronic properties, and of practical interest for many
potential applications. In particular, AuNPs exhibit high chemical stability. In principle,
they can be functionalized with almost any type of electron-donating molecule, including
biomolecules (59). Beyond that a variety of protocols have been steadily developed to
allow their assembly into multiple types in a controlled manner. Taken together, these facts
have led to the development of design concepts for novel materials based on ligandstabilized AuNPs.
32
Some of the unique characteristics of metal NPs in general are melting point
depression, which melt at lower temperatures than the same bulk material (Figure 1-10).
For example, bulk gold has a melting point of around 1064 ◦C (60), and it is defined as an
elemental specific property (61). Upon reducing the size, the melting point drops
continuously, with AuNPs of ~ 2 nm exhibit a melting point of around 200 ◦C (62). It is
understood that changes may occur to the thermodynamic and thermal properties of
nanoscale materials due to the much larger surface-to-volume ratio compared to bulk
materials.
Another characteristic of AuNPs is optical property, which is responsible for the
brilliant red color seen in stained glass windows. This effect is governed primarily by
coherent oscillations of conduction band electrons at the NPs surface, known as surface
plasmons (SPs) (63). SPs localized in NPs have the ability to strongly scatter and absorb
light and to squeeze light into nanometer dimensions, producing large local enhancements
of electromagnetic fields. The property is strongly influenced by the size, shape and the
surrounding media of the NP. The SPs properties of AuNPs have been widely investigated
in many potential applications including optical energy transport, chemical and biological
sensors, surface-enhanced Raman scattering, near field scanning optical microscopy, and
nanoscale optical devices (64).
33
Figure 1-10. A plot of the relative lowering of the melting for AuNPs vs the inverse NP
radius. (62)
An additional fundamental aspect of NPs is revealed in their electronic properties.
Single NPs with sizes in the range of a few nanometers exhibit an electronic structure that
corresponds to an intermediate between the continuous band structure of the bulk metal and
the discrete energy levels of molecules. Individual metal NPs are allowed to handle
individual charge carriers due to their small capacitance (which scales as the inverse of the
NP radius) (65). As a result, electron flux onto and off of a NP is quantized in one-electron
increments, leading to Coulomb blockade and Coulomb staircase phenomena in which
current through a NP increases in a stepwise manner (Figure 1-11). As with optical
properties, the conductivity profile of metal NPs is affected by the size, shape and the
surrounding medium.
34
Structural determination of AuNPs has been complicated by the problem that
particles are typically heterogeneous as synthesized. Electron microscopy (66), powder xray diffraction (67) and theoretical studies (68) have led to the idea that AuNPs would
adopt closed geometric shells with crystalline packing. This would lead to defined core
sizes such that the gold clusters would have a discrete number of atoms representing closed
geometric shells. Recently, Zadzinsky et al. (69) reported the structure of pmercaptobenzoic acids (p-MBAs)-stabilized gold nanoparticle through X-ray analysis of
single crystals. The resulting electron density map revealed a particle of 102 gold atoms
and 44 p-MBAs (Figure 1-12).
Figure 1-11. Single electron tunneling on a single ligand-stabilized Au55 cluster at 90K.
The junction capacity was calculated to be 3×10-19 F by fitting. Coulomb staircase behavior
is visible. (65)
35
Figure 1-12. X-ray crystal structure determination of Au102(p-MBA)44 NP. Electron density
map (red mesh) and atomic structure (gold atoms depicted as yellow spheres, p-MBA
shown as framework and small spheres sulfur in cyan, carbon in gray and oxygen in red).
(69)
From the above mentioned attractive characteristics, AuNPs have been a subject of
intensive research because of the relative ease in the synthesis and chemical modification
for many potential applications (70). Thiols have been the most popular choice of capping
reagents for AuNPs because of a higher stability over alternative capping reagents, such as
amines and carboxylates, which are weakly anchored to the Au particle core. It is clear
from the above discussion that thiol chemistry as a means of surface functionalization of
AuNPs has received considerable attention and is the method of choice in most studies.
Indeed, the nature of the AuNP thiolate bond and the mechanism of ligand exchange
36
involving replacement of one thiolate species on the AuNP surface with another thiolate is
fairly well understood (64).
In particular, the stabilization of AuNPs with a 3D self-assembled alkanethiol
monolayer was a direct result of advancement in the passivation of 2D gold surfaces using
alkanethiols (71). In order to better appreciate and understand the packing, conformation,
and composition of the surrounding ligand shell (3D SAM) of NPs, it is illustrative to
consider 2D SAM's of thiol ligands on flat gold. On flat Au(111) surfaces, alkanethiols
ligands tend to form fully-extended and highly ordered monolayers (Figures 1-13 and 1-14).
The packing structure for 2D self-assembled alkanethiol on extended flat Au(111)
surfaces is determined through competing electronic interactions between the Au substrate
atoms and sulfur atoms as well as interactions between the ligand chains, with shorter chain
packing structure more influenced by the substrate and longer chain by their interchain
interactions (72). The tilt angle (~30 º) and twist of the molecules are optimized to allow for
interlocking of the chain backbones, thereby maximizing vdW interactions between the
chains while under the constraint of surface bonding at the Au lattice 3-fold hollow sites.
37
Figure 1-13. Schematic diagram of an ideal, single-crystalline SAM of alkanethiolates
supported on a gold surface with a (111) texture. (72)
Figure 1-14. Schematic diagram depicting the arrangement of decanethiol on Au(111)
lattice. (a) Structural model of the commensurate adlayer formed by thiols on the gold
lattice. The arrangement shown is a (√ ×√ )R30°structure where the sulfur atoms (dark
gray circles) are positioned in the 3-fold hollows of the gold lattice. (b) Cross-section of the
SAM formed from decanethiol. (72)
38
With respect to ligand conformation of alkanethiol ligands on AuNP, it is reported
that the structure of a 3D alkanethiol SAM on AuNP is very similar to the well established
models of analogous monolayers on the flat gold surfaces (73,74). The majority of the
alkanethiol ligands on AuNP are observed in an all-trans zigzag conformation. Therein lies
promise for a wide range of opportunities for study and use of the 3D SAMs by extending
the lines of investigations used for 2D SAMs. Evidence has been found for defects at all
alkyl chain lengths, in particular near the surface of the gold cluster and at the outer surface
of the ligand skin.
1.5. Preparation of AuNPs
Many synthetic protocols have been developed for the preparation of ligandstabilized AuNPs, allowing variation of size, shape and ligand shell composition. However,
the need to control multiple parameters in current synthesis methods still represents a
challenge to the preparation of uniform AuNP. Very comprehensive and extended
overviews are given by Daniel & Astruc (64).
Ligand-stabilized metal NPs are supramolecular assemblies consisting of a metallic
core (often a polycrystal) coated with a 3D self-assembled monolayer of ligand molecules.
The principle synthesis method is based on the reduction of metal salts by suitable reducing
agents in the presence of ligand molecules, which form self-assembled monolayers on the
NP surface and thus stabilize the NPs. The NP morphology often emerges from a result of a
competitive growth of different crystallographic surfaces. This is typically achieved by
39
altering the relative growth rates of different facets by the selective localization of ligand
molecules adsorbed at the NPs surface, but also by the modulation of nucleation and
reaction parameters. The ligand molecules for AuNPs synthesis, besides the well-known
citrate, are typically thiolates, phosphines and amines, among which the thiolates are the
most intensively investigated owing to the strong binding character of thiol to gold
compared with other capping agents (64,75). Figure 1-15 shows the various thiol-based
ligand molecules used for the synthesis of gold NPs.
Typical reducing agents used in the direct reduction route are citrate and sodium
borohydride. The latter is the most frequently applied since Brust et al. (76) introduced a
synthetic method in which a gold salt (hydrogen tetrachloroaturate) is reduced by sodium
borohydride in the presence of an alkanethiol, yielding particles having average core
diameters in the range of 2-8 nm. This reaction was first developed within a biphasic
context, taking advantage of phase transfer compounds to shuttle ionic reagents in aqueous
phase to an organic phase (Figure 1-16).
Subsequent variations of this procedure demonstrated the full scope of this reaction,
substituting a wide range of thiols and varying the ratio of reagents in order to control the
average diameter of the products. In this case, better particle size control is achieved by
varying the thiol/gold salt ratio and the addition rate of sodium borohydride. An one-phase
synthesis route was also developed by Brust et al. (77), which allows working without a
phase transfer agent and thus avoids the phase transfer step and further purification from
the residual impurities of phase transfer compounds.
40
There are several issues to be addressed in the use of sodium borohydride as
reducing agent for AuNPs synthesis. One is the difficulty in the controlling size dispersity,
thereby there have been several studies aimed at getting more monodisperse particles from
milder reducing agents. Zheng et al. (78) employed various amine-borane complexes of
mild reducing ability in the AuNPs synthesis, which demonstrated better control of size
dispersity. Furthermore, this approach allows the size of the NPs to be tuned within the
range 1.9–5.2 nm, depending on the reaction temperature (Figure 1-17). Another
disadvantage of sodium borohydride is its strong reducing capacity, which may cause it to
undergo reaction with fuctionalized ligands. Thus, an alternative much milder reducing
agent needs to be allowed for the synthesis of AuNP stabilized with the reactive functional
group.
Figure 1-15. Various thiols used for the generation of monolayer-protected AuNPs. (75)
41
Figure 1-16. Formation of AuNPs protected with alkanethiolates by reduction of Au(III)
compounds in the presence of alkanethiols. (75)
Figure 1-17. TEM images of different-sized AuNPs produced by varying reaction solvent
(a-b) and temperature (c-f): (a) 2.1 ±0.3 nm; (b) 3.5 ± 0.3 nm; (c) 5.3 ± 0.4 nm; (d) 6.2 ±
0.3 nm; (e) 7.1 ±0.5 nm; (f) 8.3 ±0.5 nm. All scale bars are 20 nm. (78)
42
The modification of NPs by the incorporation of a functional group is of
significance to potential applications of NPs (Figure 1-18) (64,79,80). The incorporation of
multiple functional groups and moieties onto the surface of AuNPs can most readily be
achieved by the ligand exchange method. This concept is promising, as it can allow control
of the number and topology of functional groups on the particle surface. In the exchange
reaction, the incoming ligands replace the pre-existing ligands on NPs by an associative
reaction. The rate depends on the chain length and/or steric bulk the ligands, as well as the
charge on the NPs. Various functional groups ranging from simple organic functional
groups to functional complexes (electroactive or photochemical groups) can be
incorporated into the NPs using ligand exchange reactions. Reactions through AuNPs were
also frequently used for the incorporation of more specific organic groups and moieties
having electroactive, photoresponsive, catalytic, mechanical, and biological properties.
Besides the choice of the ligand and the reactions condition, there remains the
challenge of gaining reproducible control over the particle size distribution. Much progress
has been made over the past ten years on the synthesis of monodisperse NPs (5,14).
However, although several chemical methods have been developed to synthesize
monodisperse NPs of various materials, gaining a comprehensive understanding of how
monodisperse NPs form is still very challenging. Some mechanistic studies have shown
that monodisperse NPs are produced when the burst of nucleation that enables separation of
the nucleation and growth processes is combined with the subsequent diffusion-controlled
growth process through which the particle size is determined.
43
Figure 1-18. General scheme and examples of functionalized thiols for the ligand exchange
reactions. (75)
It is often desirable to reduce the NP size distribution because polydispersity can
increase disorder in assemblies and broaden ensemble behavior. In size-selective
precipitation, a post-synthetic method is used to narrow NP size distributions, in which a
nonsolvent (miscible with the original dispersing solvent) is gradually added to a NP
solution (81,82). At certain average solvent polarities, larger NPs are no longer soluble in
44
the solvent mixture and precipitate, narrowing the size distributions of both precipitate and
supernatant. Monodisperse NPs can be obtained by redissolving the precipitate and
repeatedly using the size-selective precipitation/redissolution steps. The size distribution of
NPs during size-selective precipitation has been investigated by optical absorption and
mass spectroscopy. Other techniques for narrowing the dispersity of NP size include
heating (83), etching (84), annealing (85), and chromatography (86).
The utilization of nontoxic chemicals, environmentally benign solvents, and
renewable materials are emerging issues that merit important consideration in the
development of synthetic strategies (87). For the synthesis of metallic NPs, an obvious
approach is to take a metal salt in water and reduce it to the elemental metal. While one
may not have many choices of different salts, there is an abundance of reducing agents that
are available in biology—including sugars, glutathione, etc. Several groups are beginning
to use plant and algal extracts to reduce metal ions to metal NPs in water.
45
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50
2. Preparation of 2D NPSCs through Langmuir Assembly via Successive
Compression Cycles
Reproduced in part with permission from Jin Young Kim, Shilpa Raja, and Francesco
Stellacci, Evolution of Langmuir Film of Nanoparticles Through Successive Compression
Cycles, Small 7, 2526-2532, (2011).
2.1. Introduction
A Langmuir film is a thin layer of molecules adsorbed at a liquid-vapor interface
that cannot be solubilized into either phase. Typically the liquid is water (or an aqueous
solution) and the gas air. As the film is compressed, it may form quasi two-dimensional
analogues of a gas, liquid, liquid-crystal, solid, or other phases depending on the surface
density (1-8). A thermodynamic equation of state (compression curve) for the Langmuir
layer relates the surface pressure to the area that the molecules occupy within the film, at
equilibrium (9-12). It is clear that the Langmuir isotherm is an ideal situation, asuming an
infinitely slow compression; in real films at finite compression rates, molecules tend to
occupy larger areas than the equilibrium structure due to the development of space filling
defects such as vacancies, dislocations, domain boundaries, etc (1-8). A common approach
to anneal these defects and approach the idealized equilibrium curve is to apply pressures
considerably higher than the equilibrium spreading pressure. The over-compressed
monolayers can remain in a transient metastable state for extended periods of time, but
ultimately relax towards the minimum energy configuration of the domain by annealing the
defects (1-8,13). There has been a growing recognition for the functional significance of
51
equilibrium structures in the wide variety of materials for which Langmuir films are
adopted (14-22). Here we focus on the case of ligand coated gold NPs self-assembled into
ordered monolayer at the air-water interface and show that it is possible to devise annealing
procedures to achieve an ordered structure closer to the equilibrium state.
The self-organization of NPs opens a new and challenging area in nanoscience
(23,24). We already know that NPs are a new building block of advanced materials that are
expected to exhibit unusual chemical and physical properties (25,26). In particular, the
collective properties of such mesoscopic assemblies differ from isolated NPs and the bulk
phase (27). NPs self-assembly is a process governed by an interplay between inter-particle
interactions of various natures such are core-core van der Waals and dipolar interactions as
well as ligand shell interdigitation (28-33). Often, the result of this delicate balance
between strong interactions is kinetically trapped self-assembled NPs structures (34-36). A
distinctive feature of capping ligand (or polymer) interactions is the extreme sluggishness
with which equilibrium is attained once the molecules are confined within a narrow space
(37,38). Furthermore, it has been recently shown that as complexity in NPs system
increases (e.g. binary particles supracrystals), the dynamics of self-assembly slow down,
and becomes more heterogeneous (39-43). For these reasons, often NPs assemblies are
kinetically trapped or blocked in local minima. Hence the need for annealing strategies such
as thermal or vapor annealing approaches which have been developed recently (44-46).
Here, we report an experimental approach for preparing self-assembled NPs monolayered
structures through a Langmuir approach and then annealing them through over compression
cycles. We show that monolayer defects disappear (as shown by increased long range order)
52
but also that ligand shell contribute to increased short-range order by (i) coverging towards
and homogeneous interdigitation fraction and by (ii) compensating for NPs core size
inhomogeneities. We conclude that to understand order in NP film one must consider the
ligand shell as a dynamic object.
2.2. Experimental
NPs synthesis – Dodecanethiol (DDT) -stabilized AuNPs were synthesized by
modifying a method reported by Zheng et al to control the injection speed of reducing
agents (47). The rationale for this modification is to separate nucleation from growth during
particle formation (23,24). 0.25 mmol chlorotriphenylphosphine gold (AuPPh3Cl) was
mixed with 0.125 mL DDT in 40 mL of benzene to form a clear solution to which 2.5
mmol of tert-butyl amine borane complex were then added in one portion. The mixture was
stirred at 90 °C for one hour, then cooled to room temperature. 100 mL of ethanol was then
added to precipitate the NPs. The precipitated NPs were collected by centrifugation and
washed 3 times with a mixture of benzene and ethanol. The synthesized DDT-AuNPs
exhibit particle size distributions with standard deviations (σ) of less than 7% of the mean
particle diameter of 5.3 nm.
Formation of NPs monolayer film - Langmuir NPs monolayers were prepared using
a Langmuir trough (type 601M, NIMA Technology). The monolayer was spread by careful
casting of a 0.2 mg/ml solution in mixed solvents of dichloromethane and hexane (vol
50:50) onto milli-Q water. The solvent was allowed to evaporate for at least 10 min prior to
compressing the surface. The area of the monolayer was varied by moving a barrier with a
53
rate of 10 cm2min-1 across the water surface. The surface pressure was measured with the
Wilhelmy plate method. After compressing Langmuir monolayer films to the desired
surface pressure, they were transferred to solid substrates such as carbon-coated copper
grids or silicon wafers by bringing the two into contact in the horizontal direction, a process
known as the Langmuir-Schaefer method.
2.3. Effect of Compression in Langmuir DDT-AuNPs Monolayer Film
Figure 2-1a shows a surface pressure versus area compression isotherm curve for
the DDT-AuNPs monolayer during compression. As the area of the monolayer decreases, a
unique phase transition identified with the slope change in the isotherm can be observed.
Figure 2-1b shows representative TEM images of the Langmuir NPs films transferred to the
substrates at various surface pressures. At low surface pressure, the NPs are packed in
patches connected randomly with large void spaces. The NPs monolayer becomes denser
with diminishing void spaces as surface pressure is increased. Monolayers at 18 mNm-1
exhibit an extraordinary degree of order over the centimeter scale. When the twodimensional film is compressed beyond this stability limit, its structure becomes so
unstable, such that it gives rise to a transition from a monolayer to a multilayer, with
(usually) an abrupt change of slope in the compression curves indicating collapse behavior,
the process by which the monolayer transforms to a bulk phase (1-8). It is reported that the
collapse phenomena formed in the compression process is highly irreversible and depends
on various experimental parameters such as the nature of the molecules, compression rate,
temperature, and pre-history of the monolayer itself (48,49).
54
(a)
(b)
Figure 2-4. (a) Surface pressure-area compression isotherm curve for the DDT-AuNPs
monolayer. (b) TEM images of the NPs monolayer film structures transferred to the
substrates at various surface pressure. Scale bar : 100 nm.
55
2.4. Stress Relaxation Behavior in Langmuir DDT-AuNPs Monolayer Film
At times, uniform monolayer structures in the Langmuir method appear to be
formed from one simple compression step under proper operation conditions, yet these
structures are not stable at the air water interfaces, and can remain stable only when trapped
on ‘harder’ interfaces where mobility is limited. The intrinsic instability of these films is
typically revealed as the film is kept under constant area. A plot of pressure over time
shows sudden drops followed by plateaus. This phenomenon is called surface pressure
relaxation (50-54). The elucidation of the relaxation mechanisms is important for
understanding not only molecular interactions occurring in the system but also the
instability of the Langmuir monolayer, which is an essential parameter in this film
processing (1-8). Figure 2-2a shows a surface pressure-time relaxation isotherm curve or a
close-packed DDT-AuNPs monolayer compressed to 18 mNm-1 where this phenomenon
can be clearly observed suggesting that the as-compressed monolayer is not in an
equilibrium state. Fig. 2-2b shows that during relaxation process, the close-packed NPs
domains undergo a transition resulting in a collapse phenomenon which may be due to a
buckling instability, wrinkling or the formation and growth of cracks through various
microscopic processes (15).
56
Figure 2-2. (a) Surface pressure-time relaxation isotherm curve for the DDT-AuNPs
monolayer compressed to 18 mNm-1. (b) TEM images of the close-packed Langmuir NPs
monolayered film taken before and after relaxation process. Scale bar : 100nm.
We investigated relaxation behaviors of film achieved at the various surface
pressures. Figure 2-3a shows the surface pressure versus time trace for a relaxation
isotherm at room temperature. Three distinct relaxation regimes, prior to collapse, were
observed. At very low surface pressures below 2 mNm-1, in region I on Fig. 2-3a, the
relaxations are very quick-only a few seconds long. Little stress relaxation was observed in
this low surface pressure regime I. This points out that no slow collective motions exist
when the NPs remain isolated. In the pressure range from 4 to 10 mNm-1 ~ region II,
relaxations become slow with decay times of a few hundred seconds. Overall changes in
the surface pressure can be as large as 2-3 mNm-1 within each relaxation in this
intermediate pressure regime. At higher pressure, in region III, the overall change of
surface pressure in the decay process increases but the timescale remains mostly the same.
In most cases, the decay process could not be described by a simple exponential or
57
algebraic expression. Finally, in region IV the monolayer collapses (see Fig. 2-3b) and
relaxation times increase drastically.
Figure 2-3. (a) Relaxation isotherm curves for the DDT-AuNPs film achieved at the various
surface pressure. Each segment on the time trace corresponds to the relaxation in various
surface pressures after a compression step. The four regions (I-IV) identified are discussed
in the text. (b) TEM images of the NPs films corresponding to each region. The film
structures were obtained from after reaching the plateau in each regions of the isotherm
curve. Scale bar : 100 nm.
58
2.5. Effect of Compression/Relaxation Cycles in Langmuir DDT-AuNPs Monolayer
Film
As the scope of this work was to investigate NPs monolayer structures as close as
possible to their true equilibrium, we excluded films that exhibited instabilities over time at
the air-water interface, mostly those that would result in spontaneous collapse towards
multilayers. Hence we achieved highly order structures through cyclic compression cycles
of NPs monolayer at a relatively low surface pressure. We used constant surface pressure
versus time as an indication of a close-to-equilibrium phase. An example of our approach is
shown in Fig. 2-4. We first prepared Langmuir films by spreading NPs solution onto a
water surface, then applied a certain surface pressure for the compression, here 12 mNm-1.
Films were held at a constant surface area until they relaxed to the plateau region, followed
by subsequent compression cycles at the same surface pressure. The result is that when
going from first to second, and then third compression cycle, the NPs coverage goes from
84.7 to 99.8% (Fig. 2-5a). We believe that this is due to a progressive annealing of the
structural defects, which is also indirectly confirmed by the increased surface pressure
values of the plateau region observed in the further trials (Fig. 2-5b). Thus, Langmuir NPs
structures obtained from the 3rd cycle of compression and relaxation at 12 mNm-1 exhibit an
extraordinary degree of monolayer order without the accompanying collapse phenomena
observed when compressing at 18 mNm-1. We should point out that even in this case
particle vacancies, dislocation, and multiple grains were still observed (Fig. 2-5c), hence
this process does not lead to an arbitrarily large single crystal.
59
Figure 2-4. Schematic drawing illustrating the preparation of the Langmuir NP monolayer
through cyclic compression cycles at 12 mNm-1 (top) and surface pressure-time & area
change-time curves for each step (bottom): (1) monolayer spreading – the NPs monolayer
was spread by careful casting of a 0.2 mg/ml solution in mixed solvents of
dichlorofmethane and hexane (vol 50:50) onto water subphase, (2) compression – the area
of the monolayer was decreased by moving a barrier with a rate of 10 cm 2min-1. (3)
relaxation – the compressed monolayer in (2) step is kept under constant area. In this
process, surface pressure decays over time, followed by plateau, (4) cyclic compression
processes in Langmuir NPs monolayer, accompanied by relaxation step.
60
Figure 2-5. (a) TEM images of NPs monolayer by compression cycles at 12 mNm -1. (b)
Langmuir relaxation isotherm curves during cyclic compression cycles for DDT-AuNPs at
12 mNm-1. (c) High-magnification TEM images of DDT-AuNPs monolayer obtained from
3rd compression cycles. Scale bar : 100 nm.
Motivated by the cyclic process in the Langmuir monolayer described above, we
carried out structural characterization for DDT-AuNPs monolayers prepared by cyclic
compression and relaxation under various applied surface pressures. The results are shown
in Fig. 2-6. Here we show results prepared from the 3rd cycle at 10, 11, and 12 mNm-1 as
61
typical examples. 12 mNm-1 is the maximum surface pressure for the monolayer to exist
stably through the 3 compression cycles.
First, we emphasize that the spatial correlation of structural order on a macroscopic
level is described by the radial distribution function (RDF) of its structure. We use the
correlation distance ξ as a relevant order parameter for the structure’s long-range order.
Figure 2-7 shows the RDF curves of the NPs structures shown in Fig. 2-6. One obvious
trend is that NPs structures with equilibrium phases often tend to have longer correlation
distances than as-compressed ones (see Table 1). We also note that, by the 3rd cycle the
structure obtained at 12 mNm-1 has a much higher ξ implying significantly better longrange order.
Another important structural feature in NPs assembly is that ligand shells
interpenetrate in a process known as interdigitation (64-66). This phenomenon is one of the
driving forces for the formation of supraparticle lattices leading to close-packed structures.
Nothing in the particle assembly process imposes a spatially homogeneous distribution of
interdigitation distances aside from the symmetry of the resulting lattice. Molecules in the
ligand shell can encounter a molecule in another ligand shell in many different ways
depending upon the NPs size/shape distribution and can easily become trapped in nonequilibrium configurations. To quantitatively evaluate the spatial and temporal distribution
of the particles’ interdigitation we define the extent of interdigitation (E.I.) of ligand shell
as
where
E.I. of i th NP = [
∑
}⁄
{
]
(1)
is the center-to-center distance between pairs of NPs in the first nearest neighbors,
for particles of diameter
coated with ligands of length , and
62
is the number of the first
nearest neighbors. Hence, once the inter-edge spacing between NPs is known, the amount
of interdigitation can be defined. We have developed image analysis software capable of
calculating and plotting the E.I. between any two particles. An example is shown in Figure
2-8, where the spatial distribution of the E.I. averaged over the first nearest neighbors is
shown through a color coding of the original TEM images. The histograms for the
distribution of the ligand E.I. values show a gradual shift as the film undergoes
compression cycles. Analysis of the variance indicate that the variation in average values of
the E.I. are statistically significant (α = 99%). These results indicate that compression
cycles not only anneal defects in the crystal structure but also affect the pairwise particle
interaction inducing a larger extent of interdigitation. Hence the compression cycle affects
short-range order as well as long-range one. We believe that, upon relaxation, ligand
molecules reorganize to a more stable arrangement that maximizes their E.I. and hence the
enthalpy interaction with their nearest neighbors.
63
Figure 2-6. DDT-AuNPs monolayer obtained from 3 compression cycles at surface
pressures of 10, 11, and 12 mNm-1; (a) Langmuir relaxation isotherm curves during
compression cycles, (b) TEM images of DDT-AuNPs monolayer obtained from 3
compression cycles. Scale bar : 100 nm.
64
Figure 2-7. Structural order in NPs monolayer structures by the radial distribution functions
(RDFs); (a) as-compressed to 18 mNm-1, (b to d) 3 compression cycles at 10, 11, and 12
mNm-1. The RDFs was calculated from the TEM data. The correlation distances of the NPs
structures are listed in Table 1. Within correlation distance, the NPs are close-packed with a
six-fold symmetry.
Figure 2-8. Structural order in NPs monolayer structures by spatial distribution of the
extent of interdigitation (E.I.) (top) and the histograms for the distribution of the ligand E.I.
values; (a) as-compressed to 18 mNm-1, (b to d) 3 compression cycles at 10, 11, and 12
mNm-1. Here, the E.I. is averaged over the first nearest neighbors and shown as the particle
color. See the color bars for meaning of the particle color. The average values for the
spatial distribution of the E.I. values are listed in Table 1. Scale bar : 100 nm.
65
Table 2-1. Summary of structural order parameters for the NPs monolayered structures
studied.
Film
Correlation distance (nm)
Extent of interdigitation (E.I.)
As-compressed to 18 mNm-1
81
0.46 ±0.04
3 compression cycles at 10 mNm-1
72
0.48 ±0.05
3 compression cycles at 11 mNm-1
90
0.49 ±0.05
3 compression cycles at 12 mNm-1
115
0.50 ±0.04
2.6. Role of the DDT Ligands in NPs Assembly
Finally, it is worthwhile to critically analyze the role of the ligands in the NPs
assembly. Figure 2-9 shows plots representing the averaged inter-edge spacings (inversely
proportional to the E.I. according to equation (1)) between a NP and its six nearest
neighbors as a function of NPs size in Langmuir NPs monolayer structure before and after
compression. This plot gives a sense as to how strongly an approaching NP can
differentiate one end of the NP from the other or, alternatively, how favorable inter-edge
spacings can be. For example, the inter-edge spacings of the NPs monolayer structure
before compression does not show their size dependence (Fig. 2-9a), whereas after
compression we note that the smaller the NPs, the larger the inter-edge spacing, hence the
larger the volume occupied by the ligands, which means less ligand interdigitation in this
region (Fig. 2-9b). This finding suggests that in the NPs self-assembly process, ligand’
movements are actually correlated so as to keep the particles centers of mass of a lattice.
Hence the soft corona that surrounds the particles plays a key role in forming an ordered
66
array. This result is in agreement with what was recently observed by Bockstaller for
polymer coated particles (67). We suggest that it might be entropic in nature, as by keeping
the particles centers of mass on a lattice, one avoids other particles, and especially ligand
jamming, and hence maximizes the entropic freedom.
Figure 2-9. A plot representing the average inter-edge spacings of a NP in NPs monolayer
structures as a function of NPs size, which is averaged over its first nearest neighbors; (a)
before and (b) after compression.
2.7. Summary
In chapter 2, we have shown that it is possible to form ordered NPs monolayers
through compression cycles using the Langmuir assembly method. These cycles affect both
the long-range order in the crystal, by annealing out part of the defect, and the short range
order by increasing the E.I. and making the array more homogeneous. Finally we have
shown that the ligands play a correcting role, helping to repair and homogenize the NP
array, by modulating their E.I. so as to act a buffer to the size polydispersity in the particle
67
cores. We believe that the approach shown here is a general one and should allow for indepth studies of 2D particle crystals.
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3. Ordering Behavior in the 2D Alkanethiol-Au NPSCs by Ligand
Interactions
3.1. Introduction
2D ligand-stabilized NPSC networks generated via self-assembly provide an
attractive and highly flexible route to the formation of surfaces with specific structural
arrangements and functionalities (1-6). In particular, development of NPs synthetic
chemistry and versatility of ligand molecules has long attracted the considerable attention
of researchers, not only because it is intriguing intrinsically, but also as a way to control the
broad range of configuration and properties of the NPSCs structures. However, although
self-assembly represents a simple and efficient route to the construction of large and
complex structures, understanding how this process works is not straightforward (7,8).
Design of assemblies for 2D NPSCs has been pursued in two broad methodologies.
One is control of the solution dynamic behavior exhibited by these systems (9-13) and the
other reflects the integration of constituent components (14-20). Recently, as the selfassembling processes have been well understood, the design strategy has been increasingly
focused on NP components and concerned with manipulating the structure of matter at the
molecular scale. In fact, the crux of NP self-assembly ultimately may lie in the preparation
of appropriate components.
In particular, in designing components for NPs self-assembly, by considering the
ligand shell as a dynamic object, the focus is importantly on application of these ligand
molecules to the self-assembling problems. Which one will be formed (and which geometry
72
will it have) depends on the nature of the ligands and the geometry around the NP center.
NPs can have unique binding sites; this, together with the geometry of the NP centers, will
largely determine the discrete nature of the supraparticle assemblies. Therefore, detailed
understanding of the complex role of ligands in NPs assembly becomes illuminating and
crucial to control the order in NPSC assemblies. However, elucidation of how the ligands
play a role in affecting the structural behavior of NPSCs remains largely incomplete.
In this chapter, to simplify the question, we have studied the effect of the
alkanethiol ligand chain lengths on the phase behavior of 2D Au NPSCs. In particular, the
packing structure and conformation for the self-assembled monolayers (SAM) of
alkanethiol on Au NP surface has been determined by various surface analysis techniques
(21-24). With respect to ligand structures of alkanethiol ligands on AuNP, it is reported that
the alkanethiol ligands on AuNP exhibit a very similar structure to the well-established
models of analogous monolayers on the flat gold surfaces, in which the majority are
observed in an all-trans zigzag conformation.
Here we show that the packing organization of a 2D Au NPSCs array of alkanethiol
ligands with different chain lengths varies with the subtle chemical changes in the system,
leading to a transition between long-range ordered and short-range ordered (hereafter
glassy) phases. We reveal this phase behavior using image analysis and measure the
variation using a radial distribution function which can provide the correlation length,
allowing a quantification of the degree of ordering in the array. The balance between order
and glassiness is driven by small differences in intermolecular interpenetration, which can
be related by a ligand conformational study to the experimentally measured interparticle
73
spacing results. Significant phase variations occur with very small ligand conformation
differences that stabilize the network, which highlights the delicate balance between
entropic and energetic effects in complex NPs self-assembly processes.
3.2. Experimental
NPs synthesis - Alkanethiol-stabilized AuNPs were synthesized in the same way
described in chapter 2.2 (25). 0.25 mmol chlorotriphenylphosphine gold (AuPPh3Cl) was
mixed with 0.75 mmol alkanethiol in 40 mL of benzene to form a clear solution to which
2.5 mmol of tert-butyl amine borane complex were then added in one portion. The mixture
was stirred at 90 °C for one hour, then cooled to room temperature. 100 mL of ethanol was
then added to precipitate the NPs. The precipitated NPs were collected by centrifugation
and washed 3 times with a mixture of benzene and ethanol. The synthesized alkanethiolstabilized AuNPs exhibit the same average mean diameter of ~ 5nm with standard
deviations of less than 10%; All of the corresponding particle size distribution data shown
in Table 3-1.
Formation of NPs monolayer film - Langmuir NPs monolayers were prepared in a
similar way as to that described in chapter 2.2. After spreading the 0.2 mg/ml NPs solution
in mixed solvents of dichloromethane and hexane (vol 50:50) onto milli-Q water, the
formed NPs monolayer was compressed by moving a barrier with a rate of 3 cm2min-1
across the water surface. The surface pressure was measured with the Wilhelmy plate
method. After compressing Langmuir monolayer films to the desired surface pressure, they
74
were transferred to the solid substrates such as carbon-coated copper grids or silicon wafers
simply by bringing the two into contact in horizontal direction, a process known as the
Langmuir-Schaefer method.
Table 3-1. Summary of particle size distribution results for the synthesized alkanethiolAuNPs with different chain lengths.
Chain length
Particle size distribution (nm)
10
5.1±0.4
11
5.0±0.4
12
5.6±0.4
15
5.5±0.4
16
5.3±0.4
18
5.0±0.4
20
5.0±0.3
Infrared spectroscopy – NPSCs prepared on gold-coated mica substrate were used
for the measurement. Infrared (IR) spectra were acquired using a Nicolet Continuum FT-IR
Microscope (MCT detector) equipped with movable stages for spatial measurement.
Spectra were collected in the reflection mode with an unpolarized beam, at a resolution of 2
cm-1 with 128 scans, a size of 100 × 100 μm2 and a spectral window of 4000 – 600 cm-1.
75
The IR sample was purged continuously with dry nitrogen and maintained at room
temperature for 20 min before a spectrum was acquired. Background spectra of the clean
substrate were collected and subtracted from the sample spectra at the room temperature.
3.3. Structural Phase Behavior for 2D Supracrystal of Alkanethiol-AuNPs
To achieve 2D alkanethiol Au NPSCs, we used the Langmuir assembly method. In
chapter 2, we showed that successive compression/relaxation cycles through Langmuir
processing support the uniformly close-packed monolayer structure and renders the whole
assembly more homogeneous. An example of our approach is shown in Figure 3-1. We first
prepared Langmuir films by spreading NPs solution onto a water surface, then applied the
required surface pressure for the compression (here 13 mNm-1) after which the films were
held to a constant surface area until they relax to the plateau regions, followed by
compression cycles at the same surface pressure.
It is noted that the effective area decrease was followed (continuously) over the
course of cycles (black curve in Fig. 3-1). The transition proceeds rapidly at first (possibly
through transient structures), but then it becomes gradually inhibited, showing less
macroscopic structural evolution due to interactions between ligand molecules in selfassembled NPs structures. This is also indirectly confirmed by the increased surface
pressure values of the plateau region that we find in the further trial (blue curve in Fig. 3-1).
It was found that Langmuir NPs monolayer structures were able to be much uniformly
corrected by annealing them through successive multiple compression cycles, from
76
structures with kinetically trapped or blocked local minima to those having increased long
range order and homogeneous interdigitation fraction (26). During each cycle, the NPs
structures overcome the remaining potential barriers at the local trapped sites, and the
structure evolves toward the lower configuration energy sites. We believe that this is due to
a progressive annealing of the structural defects via ligand reorganization through the
successive stress relaxation processes in self-assembled NPs structures.
Figure 3-1. Langmuir relaxation isotherm curves during compression cycles. The area
(black curve, left axis) and surface pressure (blue curve, right axis) versus time during
Langmuir film processing through successive compression cycles.
77
For the investigation of the structural behavior for 2D Au NPSCs according to the
chain length of alkanethiol ligand molecule, we obtained TEM images of the NPSCs, and
analyzed them through in-house software in order to visualize the corresponding
representations of color map (spatial variation) (Figures 3-2a and 3-2b). Here, each NPs are
represented by a sphere colored (different color) according to its orientation. All TEM
image measurements were conducted at the same magnification scale (×27,000). As
illustrated in Fig. 3-2b, it is clear that there is an identified continuous variation in the NP
structures with increased chain lengths, and demonstrating the corresponding decrease in
the domain sizes of ordered phases.
Further analysis of our images corroborates explicitly the variation between the
corresponding structural phases. We converted the experimentally determined large-area
NPSCs structures shown from TEM images (Figure 3-2a) into fast Fourier transform (FFT).
The result shows a clear difference of the resulting characteristic patterns (Figure 3-2c),
which varies from discrete dot patterns on the left to diffuse halo pattern on the right
through the continuous ring patterns in the midway. As discussed above, the representation
indicates the spatial variation in a less ordered fashion in the NPSCs structures with the
increased number of chain lengths, which is associated with the translational and rotational
symmetry breaking in structures with the transition to the less ordered phase.
Lastly, to represent the quantitative degree of order for the structural determination,
we have done the radial distribution function tests for those structures, which also illustrate
the spatial variation to the less ordered phases by showing the smaller correlation distances
as chain length increases, consistent with the structural analysis results (Figure 3-2d).
78
Figure 3-2. Structural phase behavior 2D alkanethiol-Au NPSCs. (a) TEM images, (b) grain
size analysis, (c) FFT patterns, and (d) radial distribution function (RDF) results for 2D
alkanethiol-Au NPSCs. Scale bar : 100 nm.
79
3.4. Interparticle Spacing Results for 2D Supracrystal of Alkanethiol-AuNPs
It is clear from Figure 3-2 that different chain lengths of alkanethiol on AuNPs give
rise to NPSC arrays with varying degrees of order. These results show that phase variation
from small modifications of chain lengths lead to the exploration of the space of
intermolecular regions between NPs. In particular, it is known that when alkanethiolstabilized AuNPs assemble, their ligand shells interpenetrate (27-30). This phenomenon is
one of the driving forces for the formation of NPs lattices leading to close-packed
supracrystal structures.
For the alkanethiol ligands considered here, we note that the structural integrity of
the alkanethiol retains the conformation form in the predominantly fully-extended state in
intermolecular penetration regions during the NPs assembly process; details of the
experimental ligand structure are provided and further discussed in the chapter 3.5. If the
ligand molecules are interpenetrated as fully-extended, the interparticle spacing values are
considered to have the ranges of between one and two ligand span lengths. Thus, in this
case, if the value of interparticle spacing and ligand layer are given by d and l respectively,
their difference, given by ∆ (∆ = d – l), would have the positive value in the ranges of
between 0 and l. However, the NPs regions of negative ∆ would be present in the case if the
particles approach to less one ligand span layer. Conversely, in this case the ligand
molecules cannot have fully-extended conformation, leading to being folded in some extent
within NPs regions.
To characterize the parameter ∆ in NPSCs structure with respect to the chain length,
first, the interparticle spacing data are mapped from the observed structures. Figure 3-3a
80
shows the histogram of interparticle spacing distribution data from the mapping results
plotted against ligand chain length. Additionally, to determine the degree of ligand
interpenetration depending on the chain length, the ligand chain lengths are placed on the
histogram (indicated by vertical red dot lines); for the ligand lengths, the values for fullyextended structures were used with tilt angle of between 36 ºand 42 ºrelative to the gold
NP surface normal depending on the chain length (31).
Figure 3-3. Interparticle spacing results for 2D alkanethiol-Au NPSCs. (a) Interparticle
spacing distribution for 2D alkanethiol- Au NPSCs. The red lines in the diagram represent
one (left) and two (right) ligand span lengths, respectively. (b) Plots representing the
average inter-edge spacings of a NP in 2D NPSC structures as a function of NP size, which
is averaged over its first nearest neighbors.
The following noticeable results were obtained from Fig. 3-3a: (i) in all NP samples
investigated, the shift of ∆ values to more negative occurs with increasing chain length.
This finding clearly demonstrates that ligand conformational behavior in intermolecular
81
interpenetration regions varies continuously with chain lengths, thereby creating a different
local environment around NP core; (ii) for n=10 and 11, all interparticle spacing results are
distributed within l and 2l (i.e. positive ∆ in every NP regions). Here, fully-extended
network is favored, which indicates that structural integrity of the alkanethiol is maintained
intact within the interpenetration regions during formation of the structures, and (iii) in
sharp contrast, from n=12, we note that at the longer chain length the fraction of negative ∆
is getting more pronounced in interparticle regions.
To investigate the relationship between the interparticle spacing and NPs size
depending on the chain lengths is of particular interest. The numerical results in Fig. 3-3b
provide a relationship between the averaged interparticle spacings between an NP and its
first nearest neighbors as a function of NP size in the monolayer structures. This plot gives
a sense as to how strongly an approaching NP can differentiate one end of the NP from the
other or, alternatively, how favorable interparticle spacings can be. For example, we
typically observe that the NP size is inversely proportional to the interparticle spacing. This
finding suggests that in the NP self-assembly process, ligands’ movements are actually
correlated so as to keep the particles centers of mass to a lattice. Hence the soft corona that
surrounds the particles plays a key role in forming an ordered array. We suggest that it
might be entropic in nature as discussed in chapter 2, as by keeping the particles centers of
mass on a lattice, one avoids particles and especially ligands jamming and hence maximizes
their entropic freedom.
However, such dependence becomes gradually diminished in the NPs structures of
an increased chain length with exhibiting less correlation in the behavior. As illustrated
82
above, the inverse relationship between the interparticle spacing and NPs size is the key
feature that underlines the performance of the entropic nature by ligand interactions for the
formation of NPSCs structure. This point is further discussed below.
The NPs, unfettered by other NPs, are free to sample many more conformations,
however in a supracrystal, NPs are constrained to close proximity because of the presence
of other nearby their architectures. In NPs assembly, the ligands of the NPs drive the
translational and rotational order of the NPs in the supracrystal. However, this constrained
environment leads to the ligand’s encounters with ligand molecules of other neighboring
NPs, as a result the degrees of freedom of the ligand movements are severely restricted. It
seems that the ligands conformational behavior resulted in this process (here, whether chain
is fully-extended or folded) may be an important way to bring the extent of ordering in
phase to assembled NPs structures. Especially, in the sense that the NPs structures
configuration is determined solely by maximizing the configurational entropy, upon chainfolding, the number of ligand chain configuration that can be adopted become limited and
lead to a loss of configurational entropy.
It is noted that ligand conformation differences may be associated with the ordering
behavior of NPSCs structure formed with interactions of the corresponding ligand
molecules. For example, at the chain lengths of n=10 and 11, all NPs regions have positive
∆, indicating that the NPs structure is dominated by fully-extended ligand networks,
exhibiting a high ordered phase. As the chain lengths increases, the fraction of negative ∆
in NPs structure becomes more pronounced, which leads the system to undergo a transition
to a less ordered phase due to larger amounts of folded chain bonds in NPs structure. We
83
reiterate the analogy in which variations from ordered to less ordered phases occur when
the absolute value of interparticle spacing is decreased below one ligand span length (i.e.
chain-folded).
Accordingly, we measured the extent of ligand interpenetration and found excellent
matching with the ordering behavior in the phase of the NPSCs structure. This result
provides strong support for our analysis and confirmation that what range of NPSCs
structures is formed can be understood in terms of what the extent and perfection of ligand
structures are. From our experiments it is clear that phase variations in the alkanethiol Au
NPSCs structure is attributed to the conformational changes of ligand molecules in the
network. In addition, this result highlights the role of entropy in ligand molecular networks
in the balance between order and disorder in NPSCs structures.
Additionally, other possible mechanisms to affect the structural behavior of NPSCs
were taken into consideration for the analysis: 1) Change of tilt angle – It is known that the
tilt angle is determined by the competition between the potential energy and entropy
contributions to the free energy. The potential energy contribution favors the tilting of the
molecule (i.e. lying down) whereas the entropy contribution favors zero tilting (i.e. standing
up) due to larger available volume for conformational changes. Especially, because the
entropic freedom by ligands movement to correct NPs dispersity in the self-assembly
process of alkanethiol-AuNPs is observed to play a crucial role, the increase of tilt angle of
ligand molecules in the process is unlikely to happen. Additionally, that the isotropic
symmetry of the particle through ligand interpenetration with neighboring particles is
expected to lead to a long-range ordered NPSCs structures for assembly supports that
84
ligand interpenetration process is more likely to happen in the NPs assembly, 2) Ligand
desorption – Thiol self-assembled monolayers (SAMs) on surfaces of NPs present features
different to those of planar Au substrates due to curvature effects and to the large density of
substrate defects. Especially, we speculate that the large number of non-coordinated sites of
the defective surfaces allows a higher surface coverage of ligands and stronger
chemisorption of the S head to the Au surface than them for planar Au substrate.
Additionally, from differential scanning calorimetry (DSC) experiment, thiol-thiol
interactions show the corresponding increase with the increase of chain length of
alkanethiol molecules. Based on these two experimental results, the ligand desorption is
unlikely to happen in the NPs assembly process. 3) Monolayer crumpling – To check
monolayer crumpling by compression of monolayer in Langmuir processing, the mapping
result of NP size distribution at many different positions was investigated, which shows
very homogeneous NPs size distribution over the NPs surface, indicating no crumpling in
NPs monolayer structure.
3.5. IR Study for Ligand Conformational Behavior in 2D Alkanethiol-Au NPSCs
Lastly, for a complete microscopic description of the ligand conformation in the
NPSCs, we undertook Fourier transmission infrared (IR) spectroscopy. In particular, the
very detailed study on the conformational behavior of straight chain alkane hydrocarbons
and 2D self-assembled alkanethiol monolayers on flat Au surfaces allowed a fairly
complete set of mode assignment to be found in the IR spectra of 3D alkanethiol on AuNP
85
surface (32-38). The details of the spectrum analysis obtained from NPSCs are provided in
the following.
First, symmetric (d+) and antisymmetric (d-) CH2 stretching vibrations in the ν(C–H)
region between 2800 and 3000 cm-1 have been used as a sensitive indicator of the degree of
alkyl chain ordering (39-44). In particular, a crystalline micro-environment of alkyl chain in
alkanethiol self-assembled monolayer on AuNPs is reported to be determined by the dpeak lying between 2918 and 2920 cm-1 and d+ peak between 2848 and 2850 cm-1. We have
examined the C–H stretching regions of the alkanethiol- AuNPs with chain lengths of n=10,
12, 16, and 20 in which both the antisymmetric d- and symmetric d+ peaks are located
within the regions for our 3D SAM system (Figure 3-4), identifying that the alkanethiol
ligands studied here predominantly exist in an all trans zigzag conformation. However, it is
found that for n=20 d- value slightly deviates from the border, presumably with a higher
density of gauche defects.
Figure 3-4. IR spectra in (a) ν(C–H) regions and (b) average peak position of d- and d+ CH2
stretching vibrations of alkanethiol- Au NPSCs.
86
The ν(C–C) region contains additional information about the conformational
behavior of the alkane chain. The spectra for alkanethiol- AuNPSCs in the region are
shown in Figure 3-5a. The band at ~1065 cm-1 is assigned as the νa(C–C)t, and the band at
~1125 cm-1 is assigned to the νs(C–C)t (45,46). Another spectral feature in this region is the
band indicated by • in Fig. 3-5a. The band appears at ~1074 cm-1 for n=10, 1080 cm-1 for
n=12, 1097 cm-1 for n=16, and 1105 cm-1 for n=20. Even though this band is clearly not
identified yet, this band is not known as the gauche defect band (46). We note that for n=20
the longest chains studied, it is observed that there are pronounced detectable amounts of
gauche defects.
To further examine the defect structure in ligand chain, we have investigated the
relevant defect-related bands in the whole IR spectral region. Figure 3-5b shows the spectra
of ν(C–S) region in which no detectable intensity of bands ascribable to gauche defect,
ν(C–S)g, at ~ 655 cm-1 is observed, suggesting that there is no detectable defects at surface
regions. The literature describes several important sets of bands in the spectral region
between 1200 and 1400 cm-1. Especially, there are several bands which can be assigned
either end-gauche defect or internal kink defects in the regions. Snyder reported that there
are four bands between 1300 and 1400cm-1 which can be assigned to defect structures and
whose peak values are approximately independent of alkyl chain length to within 5 cm-1;
these bands are found at ~1345 cm-1 for a chain end-gauche defect, 1366 and 1306 cm-1 for
an internal kink defect, and ~1353 cm-1 for a double gauche defect (38). In our samples
studied here, the presence of an end-gauche defect band, found at ~1342 cm-1, and internal
kink defect band, found at ~1367 cm-1 were observed. Qualitative defect amounts are
87
obtained from the relative intensity ratio of the ~1377 cm-1 used as an internal yardstick.
Interestingly, whereas the intensity of internal kink defect band (~1367cm-1) gradually
increases with chain length, that of end-gauche defect band (~1342 cm-1) increases with
chain length up to n=16, whereupon it decreases at n=20 (Figure 3-5c). No evidence was
obtained of any other defect types.
These IR results plausibly rationalize the chain folding behavior observed for
alkanethiol- Au NPSCs: (i) the majority of alkanethiol ligands are in all-trans zigzag
conformation, (ii) considering that the alkanethiol ligands are bonded to a highly curved
surface, the outermost groups will have more freedoms than those near the core, resulting in
higher concentrations of end-gauche defects and increased chain folding at the end regions,
and (iii) with increasing chain length, the extent of chain folding progresses in a ligand
chain, so that the defect regions appear to propagate from the end regions to the inner
regions of the chains.
Figure 3-5. IR spectra in (a) ν(C–C) and (b) ν(C–S) regions of alkanethiol- Au NPSCs. (c)
Plot of the ratio of intensity of the band at ~1342 cm-1 (square) and ~1366 cm-1 (circle),
ascribable to end-gauche defect and internal kink defect respectively, to the sum with the
band at ~1377 cm-1 as a function of chain lengths for alkanethiol.
88
3.6. Summary
Our study demonstrates that the ligands play a crucial role in affecting the long
range ordering behavior of self-assembled 2D NPSCs structures. We have shown that the
organization of a 2D NPSC array of alkanethiols varies with subtle chemical changes in the
system between ordered and glassy phases, which is driven by small differences in
intermolecular interpenetration. Especially, our experiments give clear limits on the
tolerance of the ordered phase to symmetry breaking: the more chain folding in ligand
conformation induced from the interparticle spacing of less than ligand one span length will
drive the NPSC structures into a less-ordered phase. In addition, our work highlights the
role of entropy in the balance between order and disorder in self-assembly process of
NPSCs structures.
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4. Self-assembly of Amphiphilic NPs into 2D NPSCs by Langmuir
Method
4.1. Introduction
An amphiphile, a molecule that contains both hydrophilic and hydrophobic parts,
can self-assemble in solution or at an interface to form diversified molecular assemblies
such as micelles, monolayers, and vesicles by the repelling and coordinating action between
the hydrophilic and hydrophobic parts to the surrounding environment (1,2). Since
amphiphilicity is the chemical basis of self-assembly, if the amphiphilicity of building
blocks can be tuned, the process of the self-assembly can be controlled to some extent.
Recently, this self-assembly principle has been anticipated to be applied to ligandcoated NPs system to provide a new and simple route for a controlled self-assembly of such
particles into unique hierarchical self-assembly structures (3-5). In particular, NPamphiphiles are a new class of nanomaterials that, along with other NPs-based selfassembling nanomaterials, are finding potential applications in many fields. This is due to
their ability to self-assemble into a unique set of well-defined structures, the chemical
diversity which can be tolerated within this nanostructure, and their ease of synthesis.
In chapter 4, we have studied the self-assembly of amphiphic AuNPs into 2D
NPSCs by Langmuir method at air-water (A/W) interface. Here, amphiphilic NPs are made
by directly introducing hydrophobic and hydrophilic ligands, 1-hexadecanethiol (HDT) and
11-mercaptoundecano-1-ol (MUOL) respectively, to the NPs synthesis. This reaction yields
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AuNPs coated with a mixture of HDT and MUOL, and the ratio of the two species is
determined by the ratio of the reactants and the solvent media present in the reaction
mixture. As for ligand configuration, our previous work reported that the combination of
these ligand systems tends to phase separate into stripe-like domains on the AuNP surface
(6-8).
Here we find that the amphiphilic NPs exhibit cooperative, coupled self-assembly at
A/W interface. Notably, under self-assembly into 2D NPSCs, the amphiphilic mixed
ligands of HDT and MUOL-coated NPs are self-organized into distorted hexagonal
structures with anisotropic symmetry, whereas purely hydrophobic homoligand HDTcoated NPs into close-to-isotropic crystal with high symmetry. The relationships and
parameters for structural analysis of NPSCs by Fourier transform images of Transmission
Electron Micrographs (TEM) are summarized, and then used to explain certain changes in
the self-organization.
This work demonstrated for the first time that anisotropic 2D NPSCs from closely
spherical NPs can be formed through the cooperative self-assembly via ligand engineering
of NPs. Furthermore, we demonstrated that the anisotropic morphology of NPSCs shows
unique tunability for optical absorbance response, emphasizing the importance of the selfassembly structure and NP arrangement.
93
4.2. Experimental
NPs synthesis: NPs were synthesized using a slightly modified procedure described in the
chapter 3.2. Briefly, to 20 ml of mixed solvents of benzene and pentanol, 6.25 mM of
AuPPh3Cl were dissolved with 18.75 mM of total thiolated molecules in the desired
stoichiometric ratio. 62.5 mM tert-butyl amine borane complex were added, and the
mixture was stirred at 90 °C for one hour, then cooled to room temperature. Particles were
precipitated with acetone and centrifuged for three times. The synthesized AuNPs exhibit
average mean diameter of ~ 4.3 nm with standard deviations of less than 10% (Table 4-1).
Table 4-1. Summary of particle size distribution results for the synthesized AuNPs used in
this study.
Ligand
Particle size distribution nm)
HDT
5.3±0.5
HDT2MUOL1
4.3±0.5
HDT1MUOL1
4.3±0.5
HDT1MUOL2
4.1±0.5
MUOL
4.2±0.5
Formation of NPs monolayer film: Langmuir NPs monolayers were fabricated in the same
way described in the chapter 3.2.
94
Optical absorbance measurement: Measurement of the optical absorbance has been
performed on a conventional spectrophotometer (Agilent 8453) equipped with rotational
stages for angular measurement in the wavelength interval between 400 and 800 nm. The
linear-polarized light was perpendicular to the plane of incidence and the sample was
mounted on a mechanically rotational stage (Thorlabs). For the measurement, the NPs
samples were deposited on a quartz microscope slide substrate (Ted Pella) and the bare
substrate was used as the blank.
4.3. Self-assembly of Amphiphilic NPs into 2D NPSCs by Langmuir Method
To carry out our studies we synthesized monodisperse AuNPs of ~4.3 nm in size,
closely spherical in shape, and coated with binary mixtures of HDT and MUOL of varying
ratios and then assembled NPs into 2D NPSCs structures at A/W interface using Langmuir
assembly method through multiple compression cycles. Across compositions, we found
that in films the NPs form compact hexagonal 2D arrays and are had very similar coverage
on the substrate.
We first focus on the TEM results. A series of TEM images and the corresponding
fast Fourier transform (FFT) patterns are shown in Figure 4-1 for the NPSCs structure. To
assess the effect of the ligand shell composition and/or pattern on the structure of the 2D
crystal we studied a series of NPs as shown in Table 1 including the homoligands (HDT or
MUOL, respectively) ones. Interestingly, although based on TEM images the overall NPs
structure seemed to look similar under all conditions, it is found in FFT patterns that
moving along a compositional axis leads to a discrete tuning of the corresponding Fourier
95
transform images, suggesting that the hexagonal packing arrangements of NPs are distorted.
In particular, the degree of distortion in particle lattice structure was determined by the
eccentricity (e) values obtained from the fitted FFT patterns, which are computed through
the evaluation of the spacing between axes in a given image. The eccentricity of the fitted
FFT patterns are between 0 and 1, and as the value approaches to zero, the FFT pattern
shape becomes close to circular, indicating the NPs structure shows close-to-isotropic
crystal with high symmetry of an equilateral hexagonal lattice. The axis lengths and
eccentricity values analyzed from the FFT patterns are summarized in Table 4-2.
To begin with, of particular interest is comparing e for NPSCs of each homoligand
NPs system in spite that the value for homoligand NPs system is rather small. For example,
while the e of HDT NPs system is close to zero, for those of MUOL NPs system they
happen to have a little bit higher e values of FFT patterns with becoming slightly
asymmetric NPs lattice structure. For MUOL NPs system, there are a few possible reasons
to lead to NPs phase of asymmetric attributes in the assembly: (1) introducing hydroxyl (–
OH) group in the alkyl chain makes the MUOL ligand structure to have non-uniform
configuration compared to the HDT ligand structure of all alkyl chains system; (2)
considering that water-insoluble particle core together with ligand shells of hydrophilic end
group, MUOL NP itself may be considered as amphiphilic NP whereas HDT NP is purely
hydrophobic NP; (3) the hydroxyl end group has high affinity with water surface to enable
to interact strongly with water molecules via hydrogen bonding while the alkyl chain
interacts weakly with the water molecules, and (4) the repulsion from the end hydroxyl
groups of ligand is maybe considered to perturb isotropic interactions in NPs self-assembly.
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Figure 4-1. TEM results of 2D NPSCs structures of AuNPs coated with HDT and MUOL
with the varied ratio of ligand compositions; (top) TEM images, (bottom) FFT patterns.
Scale bar : 100 nm.
Table 4-2. Axes values and eccentricity obtained by the fitted FFT patterns of NPSCs
structures.
Major Axis
Minor Axis
Length (a)
Length (b)
(pixel)
(pixel)
HDT
205.496±0.292
HDT2MUOL1
b/a
Eccentricity* (e)
205.104±0.451
0.998±0.001
0.061±0.013
229.829±0.113
218.737±1.677
0.952±0.008
0.306±0.024
HDT1MUOL1
230.737±0.634
221.907±0.010
0.962±0.003
0.274±0.009
HDT1MUOL2
231.309±1.129
226.991±0.401
0.981±0.007
0.191±0.034
MUOL
242.030±3.173
239.881±2.517
0.991±0.003
0.132±0.019
Ligand
*
e=√
( )
97
Typically, we find higher e values for mixed ligands NPs system than their
constituting homoligand NPs ones. Indeed, the HDT2MUOL1 NP system shows the FFT
patterns elliptical to a significant extent and the corresponding e value greatly increases by
up to a factor of 5 compared to HDT ones, leading to unique anisotropic NPs crystal
structure. In addition, it must be stressed that the relative ratio in the mixed ligand shells
appears to have an intriguing effect on the e’s; the range of e varies from 0.191 to 0.306
depending on the mixed ratio used in this study. Importantly, the fact that in mixed ligand
shells the relative amounts of hydrophilic parts higher the e values become lower
counterbalances the previous consideration that hydrophilic ligand MUOL NPs systems
have higher e values than hydrophobic HDT ones. This implies that there would be more
complexation associated with the mixed ligands NPs in addition to the amiphiphlicity in the
process to lead to unique NPs phase.
As noted above, the mixed ligands NPs produce non-ideal phase behavior of 2D
particle assembly which behaves in different way observed for homoligand NPs system. It
is expected that not only such a subtle balance between hydrophobic and hydrophilic parts
of the ligand molecule but also association with water surface gives the NPs assembly
unique phase behavior.
We have studied from our previous work that ligand interpenetration play a major
role in the formation of 2D NPSCs of alkanethiol-coated AuNPs and to investigate on the
interrelationship of interparticle spacing with other structural parameters (e.g. particle size,
ligand length) is a characteristic way to understand the structural characteristics of 2D
NPSCs. On the basis of these findings, in Figure 4-2 we plotted interparticle spacing along
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with eccentricity as well as correlation distance as a function of mixing ratio for HDT and
MUOL. Even if all the properties show a non-ideal change at a mixing concentration
between two ligands, notably in every case they show remarkably similar trends,
confirming that the unique anisotropic NPs crystal structure is driven by a cooperative way
combined by various competing interactions operating in the self-assembly process.
Figure 4-2. Plot of interrelationship between interparticle spacing, eccentricity, and
correlation distance measured in the NPSCs structures.
99
4.4. Optical Characteristics Induced by Anisotropic NP Packing into NPSCs
To demonstrate the practical usefulness of anisotropic supraparticle structure, we
have measured the interactions of surface plasmons with light in the NPSCs structures. The
interaction of light with AuNPs assembly structures leads to novel phenomena mediated by
surface plasmon excitations (9-11). In particular, it is expected that anisotropic supracrystal
structures of AuNPs may possess many unique features such as both tunable frequency and
strength of plasmonic response due to the anisotropic packings of particles. Interestingly,
we observe the strong modulation in the angular dependence of the plasmon response in
NPSCs of HDT2MUOL1 NPs, the case of an anisotropic NPs lattice structure. Clearly, the
HDT2MUOL1 NPs breaks the isotropic symmetry in the NPSCs, generating such an
angular modulation in plasmon response. Indeed, the intensity pattern becomes oscillating
for NPSCs of HDT2MUOL1 NPs while no dependence is observed for them of HDT NPs
(Figure 4-3).
Figure 4-3. Normalized absorbance intensity at 572 nm as a function of the rotation angle
of NPSCs to the polarized light beam.
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4.5. Free Energy Contribution of Mixed Ligands Effects for Self-assembly of NPs into
2D NPSCs by Langmuir Method
As mentioned above, the self-assembly of amphiphilic NPs coated with mixed
hydrophic and hydrophilic ligands by Langmuir method induce the supracrystal growth
with unique phase behavior, which provide new opportunities to engineer NPSCs
possessing specific functionality and physical properties dictated by the unique packing of
these particles. Therefore, a thermodynamic understanding of phenomena involved in the
self-assembly into 2D NPSCs, such as transfer of NPs to A/W interface, interfacial energy,
chain packing, and sterics etc., is of fundamental importance in arriving at the right
composition and control of the system involved. In particular, in this section, free energy
contribution of mixed ligands effects of NPs in the assembly into 2D NPSCs by Langmuir
method is discussed: (1) chain packing free energy, and (2) interfacial free energy with
water contact.
(1) Chain packing free energy – The chain packing free energy is mainly governed
by the enthalpic interactions between NP ligands and the entropy associated with the ligand
chain conformation: First, the enthalpic interactions of self-assembly of the amphiphilic
NPs are determined by the interfacial energy of the hydrophilic-hydrophobic ligand
interface and the repulsive energy of the hydrophilic groups. Thus, for NPs coated mixed
ligands of HDT and MUOL, the hydrophobic HDT and hydrophilic MUOL parts are
balanced by interfacial and repulsive energy. Thus, in particular, if the formation of NPSCs
is regarded primarily as an enthalpy increase (i.e. highly exothermic process), the particles
can have some structural flexibility so that they can adjust to the different geometries of the
101
supracrystal structures depending on the configuration and the nature of the constituting
ligands.
Second, the entropy increase on the particle assembly into NPSCs formation may
arise from the increase in conformational flexibility of the ligand chains. When ligands
interpenetrate in NPs self-assembly, the conformational behavior such as orientations and
bendings of ligand chains are restricted in intermolecular regions under close proximity of
NPs and even further chain folds. Notably, due to the non-favorable interactions with
hydrophilic and hydrophobic ligands, it is energetically costly for the NP ligands to
interpenetrate in intermolecular regions when self-assembly occurs, thus relieving the
ligand strain from chain folding that accounts for entropy increase. These effects are
reflected in the increased interparticle spacing from the mixed ligand NPs system by
repulsive parts of hydrophilic group (Figure 4-2).
2) Interfacial free energy with water contact – The driving force for NP adsorption
at A/W interface is the lowering of the free energy of the interface. For mixed ligands NPs
system, the interfacial free energy of NP adsorption at the interface depends on the ligand
structure and the nature of the constituting ligands that meet the interface. Especially, as the
contact between hydrophobic parts and water reduces, the interfacial free energy of the
system reduces. For example, in the micelle, the surfactant hydrophobic groups are directed
towards the interior of the aggregate and the polar head groups are directed towards the
solvent. However, in the case of mixed ligand NPs the equilibrium conformation of ligand
structures on NPs at A/W interface becomes rather complicated due to geometric
102
constraints of ligands on the NPs surface, interaction between hydrophobic and hydrophilic
ligands, and nature of functional group, etc.
4.6. Summary
In chapter 4, we have studied the self-assembly of amphiphilic NPs, AuNPs coated
with mixed ligand shells of hydrophobic and hydrophilic nature, into 2D NPSCs by
Langmuir method. We show that in such NPs, the system resolve the fact that components
with unfavorable and anisotropic interactions between hydrophobic and hydrophilic ligands
are constrained to close proximity through their building block architecture and hence lead
to new phase of anisotropic NPs packing arrangement. It is expected that self-assembly
based on such NPs enables a rational design of complex anisotropic nanostructures if
nanoscale structural ligand configuration is sensibly introduced to competing assembly
mechanisms.
4.7. References
1. J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, New York (1985).
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5. Conclusions and Outlook
5.1. Conclusions
In conclusion, we show how NP ligands interactions influence the assembly of NPs
into ordered supraparticle structures. Specifically, we show that such interactions have
direct consequences on the ordering and symmetry of the assembled NPSCs structures. In
this thesis, we report on a set of NPSC arrays in which small changes in either the NP
ligand environment or the molecular geometry induce significant variations in the order
parameters of the crystal. The underlying theme of this work is the elucidation in
understanding the critical role the NP ligands play in the structural behavior of NPSCs. It is
expected that this will lead to the generation of new engineering principles for the design of
NPSC assemblies.
In the first study, we show that the packing organization of a 2D NPSC array of
hydrophobic alkanethiol ligands varies with subtle chemical changes in the system between
ordered and random phases, which is driven by small differences in intermolecular
interpenetration. Significant phase variations occur with very small ligand conformation
differences within the interpenetration region, which highlights the delicate balance
between entropic and energetic effects in complex NPs self-assembly processes.
In the second study, we show the 2D NPSC structures to have unique anisotropic
symmetry via the interaction between amphiphilic NP ligand shells composed of
hydrophobic/hydrophilic ligands. It is understood that the ligand interactions on NPs
through their unique molecular configuration of amphiphilic ligands may provide the
105
anisotropic feature in the orientational alignment of NPSC symmetry. We show that if
nanoscale structural ligand configuration is sensibly introduced to NPs assembly process,
the system resolve the fact that NP components with anisotropic interactions between the
amphiphilic ligand shells are constrained to close proximity through their building-block
architecture and hence lead to new phase of anisotropic NPs packing arrangement.
5.2. Outlook
Given that the study of NPSCs is a new field, its exploration is potential for new
discoveries and applications are quite fruitful. I focus here on the opportunities available for
improved understanding and application of the NPSC arrays.
i) Investigating NP self-assembly by spatially constraining or templating, as a
means of gaining structural control of NPSCs. Important, and as yet still unanswered,
questions in NP self-assembly are what range of structures can be formed, what are the
extent and perfection of these structures, and what is the nature of their defects? Recently, it
is suggested that templating (that is, providing constraints topologically or chemically) may
be an important way to bring order and asymmetric structure to self-assembled aggregates
(1-5). Finally, assembling NPs into desired, precise patterns with reliability comparable to
that of the current lithography techniques may be the real basis for future technological
developments. ii) Using the multi-component NPSCs studies to understand further the
causes and mechanisms of phase behavior on complex NPs assemblies. When multicomponent NPs are co-assembled, a variety of structures are predicted to be formed in
106
contrast to the growth of single component NPSCs (6,7). In particular, the assembly
process for multi-component NPs is more complicated, with a number of distinct pairwise
interactions combining with hard-sphere space-filling rules to drive the self-assembly of
multi-component NPs. One of the key questions here is how the ligand molecules affect the
structural diversity of the NPs arrays and/or phases.
Applications - One area that has become particularly fruitful is that of understanding
how the properties of NPSCs are developed, in particular how their relationship to structure
and properties are coupled. Recently, there has been a rapidly growing study of the
properties of NP assemblies, improving our understanding of the chemical environment and
of interparticle coupling through ligand networks in NPs structures, which govern the
properties of these materials and provide the entry point to engineering NPSCs for future
applications (8-14). Potential emerging application for NPSCs are focused on several areas;
(i) Development of NPSCs for unique filtration properties. Different particle/ligand
combinations could be used to efficiently control pore size, selectivity, and sensitivity
within the assembled array. This makes them promising candidates for applications not
only for nanofiltration and separation but also for controlled delivery. (ii) Properties and
applications using charge transport through NPs assemblies have been extensively studied
in detail and many examples exist where the conductivities of the assemblies crucially
depend on the length and chemical functionality of the ligand materials. For this, NPSCs
are characterized that make them not only unique materials in their own right, but also
model systems for the investigation of energy transfer, electron transport, electronic phase
transitions, and most certainly much more.
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5.3. References
1. Y. Yin, Y. Lu, B. Gates, Y. Xia, J. Am. Chem. Soc. 123, 8718 (2001).
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