1

Diploma Thesis

Studies on the Self-organization of

Colloidal Nanoparticles at

Interfaces

By Ajiguli Nuermaimaiti

Supervisor: Vassilios Kapakilis

Department of Physics and Astronomy

Uppsala University

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Contents

1. Introduction……………………………………………………….…………….3

2. Self-assembly………………………………………………………...4

2.1 Self-assembly – general scheme……………………………………………..4

2.2 Self-assembly and self-organization…………………………………………7

2.3 Implications of Self-assembly in Nanotechnology…………………………..8

3. Self-organization of colloidal particles at interfaces……………. 10 3.1 Monodispersed Colloidal Polymer Spheres………………………………….10

3.2 Particle interactions…………………………………………………………..13

3.2.1 Van der Waals force …………………………………………………….13

3.2.2 Electric double-layer ……………………………………………………14

3.2.3 Capillary force …………………………………………………………15

3.3 Assisted interfacial self-organization of colloidal particles………………........17

4. Colloidal Crystal Growing Methods and Instrumentation………19

4.1 Spin-coating…………………………………………………………………19

4.2 Dip coating………………………………………………………………….22

4.3 Parallel transferring with water evaporation....………………………............25

4.4 Langmuir-Blogett Deposition…………………………………………..........26

5. Experimental Process ……………………………………………………….28

5.1 Materials …………………………………………………………………….28

5.2 Substrate surface modification and contact angle measurement .....................29

5.3 Sample growing……………………………………………………………….32

5.4 Sample characterization…………………………………………………….33

6. Results and discussions……………………………………………...............37

6.1 Spin coating Results ………………………………………………………….37

6.2 Parallel transferring with water evaporation ……………………………39

6 . 3 La n g m u i r -B l o ge t t d ep o s i t i o n … …… … … … … … … … … … 4 0

6.4 Dip-coating results and discussions…………………………………………..43

6.5 SEM results …………………………………………………………………..49

6.6 Other results and discussions…………………………………………………51

Future work …………………………………………………………………..57 Acknowledgements………………………………………………………58 Bibliography …………………………………………………………………59

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1. Introduction

Two dimensional (2D) periodic nanoscale and microscopic structures can be achieved

using a bottom-up approach – self-organization process of the colloidal particles [1, 2,

3, and 4]. Studies on the self-organization of this colloidal particle system has

profound implications both in understanding the basic physics of the order from

disorder phenomena and understanding of the 2D crystal systems as well as the

existing and potential applications such as the lithographic masks and templates [5,6]

porous materials fabrication [7], photonic crystals [8] and sensors [9].

Therefore, the fabrication of the 2D periodic structures of the colloidal particles in a

controlled way and manipulating it by tuning the growth parameters is very

interesting both in basic science and applications. This project is focused on the

fabrication of well-ordered, hexagonal arrays of latex particles.

In this thesis, investigations on different methods of growing well-ordered crystalline

arrays of polystyrene particles will be presented. Series of experiments which were

performed on each growing method, namely spin-coating, Parallel transferring with

water evaporation, Langmuir-Blogett deposition and dip coating to study the optimal

experimental conditions to let the particles form 2D crystals as well as the effects of

different growing parameters on the self-organization of the particles will be

discussed with all experimental details and results.

A short introduction to the basics of self-assembly and self-organization of the

colloidal particles will be given in the following two chapters. And the brief

description of each growing methods as well as the corresponding instrumentations

will be explained in chapter four. So the last two chapters will be dedicated to the

experimental work have been done in this project together with the results and

discussions.

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2. Self-assembly

2.1 Self-assembly – General Scheme

Put the different parts of a car in a big box, and shake the whole, will you get a car?

This is a classic, impressive metaphor describes what self-assembly can achieve [10].

Self-assembly is an intriguing order-from-disorder phenomenon.

As the term implies, self-assembly is a natural ordering process. According to

Whiteside’s definition, Self-assembly is “autonomous organization of components

into ordered structures or patterns as a result of specific interactions among the pre-

existing components themselves, without human intervention”. Understanding of self-

assembly will help understanding of life since this spontaneous ordering process is

used in all living systems for ages. Besides, self -assembly is one of the few practical

ways of making ensembles of nanostructures. It is therefore, an essential and very

useful part of nanotechnology [11].

Self-assembly has a wide range of building blocks, including atoms, molecules and

even much bigger colloidal particles. Figure 2.1 shows this classification and their

typical range of dimensions.

Figure 2.1. Classification of self-assemblies based on the size/nature of building units

Typical building units and example systems of the atomic, molecular and colloidal

self-assemblies are given in table 2.1. Among all of them, molecular self-assembly

1Å 1nm 10nm 100nmmm

1um 10um 100um 1cm

Atomic Self-Assembly

Molecular Self-assembly

Colloidal (nano and mesoscopic) self-assembly

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(formation of surfactant micelles would be the good example) is most widely studied

and currently best understood as the concepts of self-assembly were developed with

molecules. As a typical one, the surfactant self-assembly has been well formulated in

thermodynamic scheme [12] and successfully been applied to a similar type of self-

assembly for amphiphilic polymers, such as block copolymers [13]. However, self-

assemblies at larger scales (nanometers to micrometers) recently have been getting

more interests because of their important applications on nano or microstructure

generation.

Self-assembly is abundant in all biological systems. As typical examples, amino acids

and lipid molecules as small building units form different types of proteins and

membranes respectively. And the folding of protein is also a typical molecular self-

assembly process.

Table 2.1 Different classes of self-assemblies, typical building units and examples

of self-assembled systems

In the self-assembly process, interactions among the building units ( which are

determined by the characteristics of the individual building units such as shape,

charge, surface properties) are crucial to the ordering. There should be attraction

Classification Building Units Self-assembled systems

Atomic

Metal atoms

Epitaxial films Nanocrystals

Surfactant molecules

Micelles, bilayers Self-assembled monolayers

Polymer molecules Microemulsions

Molecular

Amino acids, lipid molecules

DNA, RNA, proteins Enzyme, membranes

Colloidal

Nanoparticles Nanotubes Fullerene, colloidal objects

Suspensions, dispersions, sol, liquid crystals Colloidal crystals

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forces to let the components aggregate and also repulsive forces so that they do not

stick together irreversibly since they will form disordered glassy aggregates rather

than ordered structures in that case. The components must be able to move and adjust

their positions with respect to each other once they are in an aggregate. So, when they

are in their steady state positions, there is a delicate force balance [14] between the

attraction and repulsive forces among the components, and in some cases external or

directional forces also participate, as the schematic figure 2.2 shows.

Fig2.2. Self-assembly in general can be seen as a delicate force balance process.

Some molecular self-assembly such as micellization of surfactant molecules involves

only the intermolecular, week or noncovalent interactions such as Van der walls,

electrostatic and hydrophobic interactions. While in the nanostructure formation of

metal atoms on solid substrates or self-assembly of bigger colloidal particles, there are

always external forces such as interfacial interaction forces between the

particles/atoms and the substrates. And most of the biological self-assemblies are

directional as well. Self-assembly that is associated with relatively large building

units, which is colloidal self assembly, is sensitive to the external forces such as

electromagnetic field, gravity and capillary forces and this property is very important

in tailoring and designing of self-assembled structures.

One another very important classification of self-assembly is based on the mechanism

of its process, which classifies two types of self-assembly; static and dynamic self-

assembly (commonly referred as “self-organization” instead). This two, self-assembly

and self-organization will be discussed separately in the following text.

Directional/external force

Repulsive force Attractive force

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2.2 Self-assembly and Self-organization

This two terms – self-assembly and self-organization are widely used interchangeably

in current literature. But it is necessary to distinguish them so that we can avoid

confusion.

Based on the thermodynamics of the process, there are two types of self-assembly:

static and dynamic self-assembly. And the dynamic self-assembly is always referred

as self-organization. Self –assembly (or the static self-assembly) is due to the

minimization of free energy in a closed system, is thermodynamically driven. It leads

to equilibrium state. While self-organization (or the dynamic self-assembly ) is a non-

equilibrium process which only occurs far from equilibrium, in open systems, and so

requires external energy input. It is a production of order out of irreversible process

with energy dissipation.

Besides the thermodynamic difference between the two, there is also a difference in

formation. The first difference is what “encodes the global order of the whole” in self-

assembly where as in self-organization these initial encodings are not necessary.

Another slight contrast refers to the minimum number of units needed to make an

order. Self-organization appears to have a minimum number of units where as self-

assembly does not [15].

Most of the molecular self-assembly (especially the molecular self-assembly in

chemistry) are the examples of static self-assembly. While the self-assemblies of

smaller components – atomic self-assembly and the ones of larger components -

colloidal self-assembly always require an input of energy or external forces to direct

them, so they are the examples of self-organization. And most of the self-assemblies

in biological systems are dynamic, or self-organization process. Though the

knowledge on self-assembly is built on static self-assembly so far, the self-

organization process is also very interesting in science and important in technology.

And so, self-organization needs more attention both due to its more complexity and

the fact that less knowledge has been developed about it.

But it is hard to make clear boundary between this two as there are always some

exceptions. For instance, some equilibrium systems are traditionally called self-

organized such as alignment of magnetic domains, crystallization. Besides, these two

processes may coexist or overlap, such as the new terms “Self-assembly assisted self-

organization” and “Self-organization assisted self-assembly” imply, which were given

by Yamaguchi [16] recently.

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2.3 Implications of Self-assembly in Nanotechnology

The two main approaches to the nanotechnology are the top-down and bottom-up

approaches. The conventional top-down approach starts with the bulk material, then

removes all the excess portion using physical, chemical or mechanical methods, and

finally gets the desired nanodevice, as the figure 1.3 shows. The hard lithographic

techniques like photo lithography, electron, ion beam, or X-ray lithography are typical

examples for this. This is the most precise route which can offer very high control

over the composition and geometry of the fabricated nanostructure; but it uses very

expensive equipments, material and time consuming.

Fig. 2.3 Top-down and bottom up approaches for the preparation of nanostructure

Bottom-up approach, on the other hand, starts with the small building blocks which

are nanometer scale objects, such as atoms, molecules, polymers, and colloids and

achieves the bigger nanostructures by assembling them. Nanomanipulation using

advanced microscopes and the natural assemblies of the small building units – self-

assembly are the two main branches of this. Compared to the conventional

lithographic techniques, this bottom-up approach does not provide very high control,

but at the same time it is much cheaper, convenient, material and time efficient.

A variety of different nanostructures are available using the self-assembly process,

and they are tunable by controlling the properties of the small building units and the

environmental factors. Self-assembly, therefore is a very powerful and useful

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technique in synthesis and fabrication of nanostructured materials with significant

advantages.

Nanosphere lithography (also called natural lithography or colloidal lithography) is

one of the most convenient and low-cost technique among all the lithographic

techniques. There are two steps in this technique. First step is preparation of the

lithography masks or templates via self-assembly of colloidal particles. Then in the

second step, by “nano-machining”, which is etching and metal deposition, various

patterned nanostructures are achieved [5, 6], schematic figure 2.4 and 2.5 are the

examples of the nanofabrication via self-assembled nanosphere lithography. As

mentioned in the introduction, the aim of this project was study the self-organization

of polystyrene spheres into ordered monolayers which can be used as lithography

masks.

Monolayer formation by particle self-organization

Patterned metal structure after removing the particles

Reactive ion etching

Deposition of metal

Self-organized particle array

Nanowell structure after removing the particles

Fig 2.4 Patterned nanostructure of metals fabricated by nanosphere lithography

Fig 2.5 Nanowell structure by nanosphere lithography and reactive ion etching

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3. Self-organization of Colloidal Nanoparticles

at interfaces

Due to some dynamic effects, such as long relaxation times, slow diffusion, which are

influenced by the larger particle size and also the interparticle interactions of the

nanoscale colloidal spheres, they do not assemble themselves into their

thermodynamically lowest energy state naturally. In such systems, thermodynamics

no longer causes the self-ordering process (or does so only after a very long time) as

they are far from equilibrium. Consequently, ordering of the particles need to be

directed or engineered by applying external forces on the systems such as gravity,

electric or magnetic fields, interfacial forces by means of physical confinement.

Self-organization of colloidal nanoparticles depends on various factors, such as the

particle interactions, particle size, size distribution, particle shape, surface charge

density, nature and the dimensions of the external forces, substrate surface properties

as well as the other environmental factors such as temperature, evaporation rate.

Because of this complexity, the science of these systems and self-organization process

are still not well-understood, the advances are also still empirical.

3.1 Monodispersed Colloidal Polymer Spheres

Colloidal science concerns multiphase (two or more phase coexist) systems in which

components have at least one dimension in the range of 1 nm to 1 um, and the

components are always called colloidal particles [17]. As good examples of the

colloidal dispersions, monodispersed spherical particles, mostly the silica colloids and

polymer latex, have being most widely studied for several reasons. The advance of the

convenient and reproducible synthesis techniques of truly monodisperse spheres is the

practical reason. On the other hand, colloidal particles that are uniform in shape and

size are basically required in studying and understanding the kinetic, optical,

magnetic, and aggregation properties of the colloidal systems. While spherical shaped

monodisperse particles are the primary choice as the most theoretical models that

describe the properties of the colloids and the interactions between them always use

the spherical shape as well as the importance of the spheres in crystallization and

“self-assembly” because of their simple and highly symmetric geometry. In this

investigation, polystyrene (PS) particles are used. Fig 3.1 (left) shows the PS

dispersion we used; we can see beautiful color came from the diffracted light by the

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dispersion which indicates that there is some metastable order on the special

arrangement of the particles in this dispersion. Figure 3.1 (right) shows the AFM

image of the particle aggregation formed by slowly drying a droplet of suspension on

solid substrate. Figure 3.2 show the crystalline solid phase of the particles which was

obtained by keeping a small amount of dispersion (3 ml) in the oven until the mass

became stable. And the figure 3.3 shows the individual particles.

Fig 3.1 Polystyrene (PS) particles dispersed in water (left) and AFM image of the PS film of

the dispersion on solid substrate after the evaporation of water (right)

Fig 3.2 PS dispersion is dried in the oven to determine the concentrations, photograph of the

dried PS (left) and the AFM image of this white solid (right).

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(a) (b)

Fig 3.3 (a) SEM image of the PS particles, 400 nm in diameter (b) Schematic drawing of the

individual PS particle, X represents the surface group which determines the surface property

of the particles such as surface charge and charge density.

Uniform polymer spheres of different size ranging from 20 nm to 1 um can be

produced by emulsion polymerization process. The major components of this process

include a monomer (e.g. styrene), a dispersion media (usually water), an emulsifier

(surfactant, e.g. SDS) and an initiator (e.g. potassium persulphate). Monomer is

dispersed in a water-surfactant solution forming relatively large emulsion droplets.

Surfactant molecules form micelles which are swollen by the monomers. Then the

initiator is introduced and it immediately reacts with the monomers so that the

monomers get polymerized inside the micelles. In the mean while, the big monomer

droplets act as a reservoir to supply sufficient building units-monomers to the growing

polymer chains in the micelle through diffusion. The particles stop growing when all

the monomers are used up. So, as shown in the figure 3.3, eventually the swollen

micelles become polymer particles which are surrounded by surfactant. The surface

charge of the particle which came from the surfactant repels other particles

electrostatically therefore particles are stopped from coagulating with each other

which would happen without the surface charges due to the van der Waals attraction

(see section 3.2). At the end most of the surfactant molecules on the particle surfaces

are removed away.

For a polymer particle that is 100 nm in size, there are approximately 1000

macromolecular chains entangled as coils in the sphere; each chain starts and ends

with a functional group formed by the decomposition of the radical initiator [10]. By

fine tuning the polymerization reaction conditions, the properties of latex spheres can

be controlled by changing the parameters such as chemical composition, surface

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group (so that the interfacial free energy and surface charge density), and diameter.

Such monodisperse latex spheres have been found very useful in various scientific

researches on colloidal behavior, as well as in new functional materials fabrication

such as photonic crystals, porous materials.

3.2 Particle Interactions

Interactions between the small building units are the most essential key factor affects

the self-assembly process in all length scales. In the self-organization of colloidal

particles, there are also interfacial forces such as particle-substrate interactions or

external inter-particle forces induced by the confinement at the interfaces. And also,

as one of the fundamentals in colloidal systems, interactions between particles are the

main factor controls the stability of the colloidal dispersions both in bulk and at

interfaces.

3.2.1 Van Der Waals Force

Van Der Walls force is a universal force, like the gravitational force, which exists

between all atoms, molecules and colloidal particles even if they are totally neutral. It

is originated from interactions between the permanent or induced dipoles within

atoms [18]. Thus there can be three different types of Van Der Waals forces:

permanent dipole – permanent dipole interaction (Keesom interaction), permanent

dipole – induced dipole (Debye interaction), and induced dipole – induced dipole

(dispersion or London interaction). These three types of dipole interactions are

collectively called Van Der Waals force. In most cases, the dispersion interaction

takes the largest contribution as it does not require a permanent dipole or charge. Van

Der Waals interactions are always attractive at atomic, molecular scales and between

the identical colloidal particles.

For identical spherical particles with radius a, and a center to center distance r, the van

der Waals interaction depends on a material constant A and the geometry of the

particle; the van der Waals potential energy, after adding all three types of possible

dipole interactions, may be expressed as [19,20]:

−++−

−=2

22

2

2

22

2121 4

ln2

4

2

6 r

ar

r

a

ar

aAV vdw (1)

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The Hamaker constant, A121, depends on the dielectric properties of the spheres and

surrounding medium and can be calculated utilizing the Lifshitz theory [18, 20].

According to the literature, the Hamaker constant of the polystyrene particles in air

and water are respectively 6.37 × 10 -20 J and 0.911 × 10 -20 J [21]. When the particles

are at interfaces, we should consider that the particle is partly immersed in two

different media. Thus the Hamaker constant should be modified accordingly.

3.2.2 Electro-static Force: Electric Double Layer

Surfaces always get charged when contacted with or immersed into aqueous solutions

in a number of ways such as adsorption of ions from solution or as a result of

dissociation, ionization, and surface reaction on surfaces. Counter ions in the solution

are attached too, whereas co-ions are repelled from, the charged surface and the free

ions, they together form an “ion cloud” consisting of a layer of firmly attached ions

followed by a more diffusive layer towards the bulk. The ion concentration gradient

and the associated electrostatic potential can be described by Poisson-Boltzmann

equation [13].

The repulsive force between two surfaces arises from the pressure exerted by the ions

in the double layers and can at large separations be the following expression:

( )KrK

TknV Bplane

r −= exp64 2

00 γ (2)

Where n0 is the ion concentration, kBT is the thermal energy, 0γ

=

Tk

ze

B4tanh 00ψ , z is

the charge of the symmetric electrolyte, e0 is the elementary charge, 0ψ is the

surface potential and K is the inverse Debye length, proportional to the ionic strength

in the solution, and r is the distance between two surfaces.

This electrostatic repulsive interaction energy can be modified to be valid for curved

surfaces, such as particles, by employing the Derjaguin approximation [18], which for

two equal spheres results in [20]:

( ) ( )Krze

TkaV B

r −= exp32

220

2

0

2

0 γεε (3)

The electric double layer force at interfaces is quite complicated and so hard to model.

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The total force acting on the colloidal objects in solution is always the sum of electric

double-layer force and the van der Waals force, as the DLVO theory addresses. Thus,

the net interaction potential between two spherical particles in solution, using the

mathematical expression of the DLVO theory, is:

( ) ( ) ( )

−++−

−−=2

22

2

2

22

2121

22

0

20

2

0 4ln

24

26

exp32

r

ar

r

a

ar

aAKr

ze

TkarU B γεε (4)

3.2.3 Capillary forces

Different types of capillary forces

Capillary forces are generally exist between any curved interfaces due to the Laplace

pressure and determined by the surface tension at the interface as well as the geometry

of the curvature. For the colloidal particles mediated by fluid interface, as shown in

the schematic figure 3.4, there are three types of capillary forces originated from the

deformation of the liquid surface: immersion, floatation and bridged. For immersion

and floatation capillary forces, the direction of the capillary force is lateral with

respect to the liquid-particle contact line. This is why these forces are called lateral

capillary force. The lateral capillary forces can be attractive or repulsive. For the

bridged type, its direction is normal to the contact line. This is the only type of normal

capillary force.

Figure 3.4 Schematics of three major types of capillary forces

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Immersion capillary force appears when the colloidal particles are partially immersed

in a liquid on a substrate. Wetting of the liquid on the surface of the particles

generates the menisci around them; the magnitude and the direction of the capillary

force exerted by the menisci are determined by the wettability of the liquid and

surface property of the particles. While the floatation capillary force appears when the

particles are floating and so deforming the liquid surface and it is caused by the

particle weight. The bridged capillary force is induced when a liquid or gas phase

connects two solid surfaces: particle – particle or particle – substrate. And it is always

attractive while the lateral capillary forces can be attractive or repulsive.

Lateral capillary Forces

Fig.3.5 Capillary forces of flotation (a, c, e) and immersion (b, d, f) type: (a). attraction between two

similar floating particles; (b). attraction between two similar particles immersed in a liquid film on a

substrate; (c). repulsion between a light and a heavy floating particle; (d). repulsion between a

hydrophilic and a hydrophobic particle; (e). small floating particles do not deform the interface and do

not interact, (f). small particles confined within a liquid film experience capillary interaction because

they deform the film surfaces due to the effects of wetting [22].

As shown in the schematic figure 3.5, lateral capillary forces are different in different

cases both in direction and magnitude. The mathematical modeling of these lateral

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capillary interactions is already well established. Asymptotic expressions for the

lateral capillary forces are given as [22, 23]:

( ) ( )[ ]22

121 12 kRqOqLqKQQF +−= πσ Lrk << (5)

Where L is the center-to center distance of the two particles, σ is the interfacial

tension, R is the sphere radius; Q is the “capillary charge” [(Qi= ri sin iψ ), where i=1,

2], q2 = [∆ σρg ], rk is the radius of the contact line, and K1 is the modified Bessel

function of the second kind and for qL << 1 equation (5) can be simplified as:

LQQF /2 21πσ−= 1−<<<< qLrk (6)

As we can see from the equation (5) and (6), the immersion and flotation capillary

force have the same functional dependence on the interparticle distance. But due to

their different physical origin, they have different magnitudes of “capillary charge”

which renders a big difference on the magnitudes of these two lateral capillary forces.

For identical particles: R1=R2=R when they at a special distance: rk<<L<<q-1 the

lateral capillary forces can be approximated to [22, 23, 24]:

( ) ( )qLKRF 1

6 σ∝ For the flotation force

( )qLKRF 1

2σ∝ For the immersion force (7)

According to the equations above, flotation capillary force increases when the

interfacial tension decreases; while the immersion capillary force decreases together

with the interfacial tension. Besides, the flotation capillary force decreases much

stronger with the decrease of R than the immersion capillary force. For particles with

a radius below 5-10 um, floating on a liquid surface, the lateral capillary force can be

ignored since the small particles are too light to create a significant interfacial

deformation. Whereas the immersion capillary force can be remarkable for much

smaller particles with the size down to R=2 nm as this kind of lateral capillary force is

driven by the wetting effect [22].

3.3 Assisted interfacial self-organization of colloidal particles

As mentioned the colloidal particles are not thermodynamically driven to assemble

into the lowest energy state by themselves. And always need an input of energy or

external/directional force to assist their self-organization. A number of different types

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of forces can be used as the assisting force; as one of them, cases capillary forces are

orders of magnitude stronger than van der Waals force and other colloidal forces. So

the capillary forces can help achieve self-organization of colloidal particles. All three

different types of capillary forces can assist colloidal self-organization process.

Examples include the three dimensional colloidal self-organization with normal

capillary force and two dimensional self-organization with lateral capillary forces.

For the trapping at the interface and ordering mechanism of the particles, according to

Earnshaw [25], there is also an electrostatic contribution except for the surface tension

related capillary forces. This contribution is proportional to the square of the particle

surface charge and significant for the larger, highly charged particles. As the particles

used in this project are quite small (100 nm and 145 nm in diameter) and with low

surface charge density, we assume that the capillary interaction may be one of the

main reasons causing the particle ordering.

Both the flotation and immersion capillary forces have been involved in this

investigation. In the first approach, the polystyrene colloidal particles are floated on

water subphase after they have been spread onto the air-liquid interface with help of a

spreading agent (ethanol is used in this studies), as we know from the equation (7)

low interfacial tension gives stronger flotation capillary attraction. The ordered

polystyrene arrays organized on water-air interface are then transferred onto solid

substrates in different ways; both parallel and vertical deposition methods are used.

In the second method, particles are partially immersed into the dispersion media and

are directly in contact with the substrate. Deformation of the liquid surface is related

to the wetting properties of particles. A complete wetting of colloidal dispersion on

the substrate is crucial in obtaining a uniform monolayer. So the wetting has been

improved by adding a surfactant to the colloidal dispersion (in dip coating method see

section 4.2 and section 6.4).

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4. Colloidal Crystal Growing Methods and

Instrumentation

Highly ordered crystalline arrays of colloidal particles can be achieved via self-

organization of monodisperse colloidal spheres. Studies on the formation of this

interesting microstructure is of great importance both in fundamental questions of

self-organization of soft matter as well as in its potential and present applications such

as nanopatterning, fabrication of photonic devices and chemical, bio-sensors.

As mentioned in the precious section, self-organization of colloidal particles happens

only under the assistance of external forces. Different forces such as gravity, electric-

field and capillary forces can be applied for this purpose. A variety of practical

methods, e.g. electrophoretic deposition [4], crystallization under physical

confinement cell [26], and capillary force assisted deposition via controlled

evaporation [3, 27 ] and spin coating [2] have been developed. However, majority of

the reported achievements of the periodic polystyrene arrays are gained using the

micrometer (diameter) or sub-micrometer sized particles while the ones from smaller

particles have not been so much studied. Self-organized colloidal crystal arrays which

use smaller polystyrene spheres (50-200 nm) as building units are more difficult to

achieve due to their stronger Brownian motion as well as the different particle

interactions because of the smaller size. Therefore we started this project to do further

studies in this topic. All experiments are performed on PS spheres with 100 nm and

140 nm in diameter.

In this project, four different deposition methods are used; spin coating, dip-coating,

parallel transferring with water evaporation and Langmuir-Blodgett method. In the

first two techniques, particle suspension is in direct contact with solid substrates, and

the particles are organized directly on the substrates where immersion capillary force

assists the particle organization. While in the last two methods, Particle arrays are

formed on air-water interface with assistance of floatation capillary force and then

transferred into substrates.

4.1 Spin-coating

Spin coating is a common method to produce uniform thin films of organic materials

on flat substrates. This technique also can be used on fabricating of the colloidal

sphere arrays by spin-coating the colloidal dispersions on substrates. Evaporation of

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the solvent in the dispersion can be accelerated by spin-coating so when the liquid

becomes almost as thick as the diameter of the spheres, a strong immersion capillary

attraction arises between the particles and assists the self-organization of the particles.

In this process, excessive amount of suspension is placed on the wafer and then

rotated at a high speed around the axis perpendicular to the substrate plane in order to

spread the fluid by centrifugal force. Due to the centripetal acceleration, most of the

suspension is flung off the edge of the substrate, leaving a thin film of the particles

after evaporation of the liquid on the wafer surface.

Fig 4.1 Schematics of the spin-coating process Fig 4.2 Spin coating instrument

The whole spin-coating process sketched in Fig 4.1 is divided into four distinct stages

so that the physics of the process can be well modeled. The four stages are

deposition, spin-up, spin-off and evaporation of solvent. As seen from the figure

above, the last two stages overlap. And these last two stages have most impact on

final film thickness.

In deposition stage, an excess amount of suspension is applied on to the center of the

substrate by a micropipette. Then the second stage starts; the substrate is accelerated

up to the desired final spin speed. Large amount of the material is ejected from the

wafer surface due to the strong rotational motion. The third stage is when the

substrate is spinning at a constant rate and fluid is stably out flowing. The viscous

forces dominate fluid thinning behavior. When the spin-off stage ends the film drying

stage begins. In this stage substrate still spins at a constant rate but centrifugal outflow

stops and further thinning of the film is due to solvent evaporation. With the loss of

solvent, the suspended or dissolved solids become highly concentrated and form a

high viscosity, low diffusivity layer or solid film. Coated film is quite uniform in the

21

central area of the substrate but not so perfect at the edges because the fluid flows

outwards and forms droplets at the edges which became thicker film at the end.

Final film thickness and other properties depend on the nature of the solution or

suspension (viscosity, drying rate, concentration of the solid, surface tension, etc.) and

the parameters chosen for the spin process – factors such as amount of the delivered

solution, final rotational speed, acceleration, spin time. The mathematical modeling of

the process is quite challenging because of the complex coupling of the fluid rheology

and solvent evaporation. And numbers of different models have been already

developed. A simple model has been established by Meyerhofer and has been applied

even though it decouples the evaporation and flow.

According to Meyerhofer [28], the final film thickness of a spin-coated layer depends

on the processing and material parameters like spin speed, viscosity and solvent

evaporation rate by the semi-empirical formula shown in equation (8) below.

31

200 2

31

•

−=

ωρηφ

ρρh (8)

Where h is final film thickness, ρ is the mass of the solvent per unit volume, 0ρ is

the initial value ofρ , η is viscosity, ω is the final spin speed and φ is the

evaporation rate of the solvent. Since the parameters: evaporation rateφ , viscosityη ,

and solvent densityρ , change during the coating process, this formula only can guide

the experiments, but the parameters related to the film thickness are determined

empirically in practice. So a more simple formula, given in eq. 9, is always used:

BAh −= ω. (9)

Where A and B are constants to be determined empirically. In the most reported

studies, B was determined to be in the interval between 0.4 and 0.7, which is in good

agreement with eq. (8), where the exponent for ω is 0.67 [29].

However, the present theoretical models were all established for organic solutions not

for colloidal particle dispersions. So the equations (8) and (9) are also not appropriate

for particle films where the film thickness are discreet (decided by the particle

diameter) not like the organic solution films with continuous thickness. Therefore the

equations above are just used as good reference in estimating the experimental

parameters.

Fig 4.2 shows the spin-coating instrument used in this project. There is also a small

vacuum pump connected to this spin-coater which takes away the air inside the small

22

chamber before the coating starts. After the vacuum pump is switched on, a flux of N2

gas is applied from the blue tube connected to the chamber which also helps to

remove the air. Then the coating process explained previously is performed. When the

all coating is finished, first the N2 gas then the vacuum pump is switched off, then the

coated sample can be taken away from the spin stage.

Spin-coating is a mature technique has being used very widely in many fields of

industry. This method has many advantages; it is a relatively simple, low cost and fast

fabrication system. But it also has some disadvantages; first of all its lack of material

efficiency, usually only 2-5% of the material is really used. Then for the preparation

of self-organized structures from colloidal spheres using this method, it has been

observed that the domain size of ordered arrays always decreases with the dimension

of spheres, no ordering occurred when the spheres smaller than 50 nm [8].

4.2 Dip Coating

Dip coating method (or the vertical deposition as referred in some literature) can be

described as a process where a clean, wettable substrate is vertically dipped into the

dilute suspension of colloidal particles and then withdrawn with a slow, precisely

controlled lifting speed, as schematically shown in fig 4.3.

Fig 4.3: Schematic of the set-up in dip coating method. Substrate is lifted by stage and the motion of stage is precisely controlled by the computer.

Fig 4.4: Sketch of the array formation in a wetting film on a substrate which is being withdrawn from suspension. jw and jp are the water and particle fluxes in the vicinity of the particle array. The inset shows the meniscus formed due to water evaporation which causes the capillary attraction between the particles.Vw is the substrate lifting speed, VC is array growth rate, and je is water evaporation flux [3].

23

Array Formation Mechanism

When a substrate is immersed into the particle suspension, a meniscus region is

formed on the substrate due to wetting; evaporation of the water from this meniscus

leads to a constant suspension flux which brings the particles in to the meniscus. Due

to the further drying of the wetting film, lateral immersion capillary force arises

between neighboring particles and organizes them in to hexagonal close packed

arrays. The array thickness is dependent on the flux of particles into the meniscus,

therefore a model is established by Nagayama [3] just using this material mass

balance to derive a formula for array thickness. In this model, the formation of layered

arrays is split into two main stages: (1) convective transfer of particles from the bulk

of the suspension to the thin wetting film due to the water evaporation from the film

surface and (2) capillary interaction between the particles lead to organized structures.

And according to this Nagayama’s model, the theoretical relationship between the

numbers of arrays and the concentration is:

( )ϕϕβ

−=

1605.0 dv

Ljk e (10)

Where k is the number of array layers, φ is the particle concentration in the

suspension, je is the solvent evaporation rate, d is the diameter of the colloidal

spheres, and ν is array growth rate which is a balance between the solvent evaporation

rate and substrate withdrawal rate, and the substrate lifting speed must not be higher

than this growth rate. L is the height of the meniscus; β is the ratio of the mean

velocity of the particles in the suspension to the mean velocity of the water molecules

which approaches to 1 for dilute suspension of weekly interacting particles. Since

some parameters such as evaporation rate je, meniscus height L, and the array growth

rate ν are can only be determined experimentally, parameters in the experiments in

this investigation were chosen semi-empirically using this formula and other

experimental data from literature.

Estimation of the Substrate Lifting Speed

Substrate lifting speed is one of the two most important parameter in this technique

which we can easily change (another one is the particle concentration), and it has

24

strong influence both in film thickness and crystallization quality. If the lifting speed

is too high, there might not be enough time for the particles to reach the substrate

from the bulk suspension, while very low lifting speed will give us thick and

disordered structures (under normal laboratory evaporation rate in room temperature,

~20 °C).

The average displacement of the particles in the suspension due to the Brownian

motion in a given time t is:

(11)

D is the diffusion coefficient given by Einstein-Stokes equation for spherical partials:

(12)

Where R is the radius of the spheres, and η is the viscosity of the suspension. As the

polystyrene particle dispersions we used are very dilute, we can simply use the

viscosity of water here. So for the particles with radius 70 nm, the calculated result of

average displacement in 1s is 1.75um. Therefore, the 70nm particles in the suspension

can move 1.75 um per second in room temperature; this inclines that the substrate

lifting speed should not be higher than 1.75um/s, otherwise not enough (or not quick

enough) particles from the bulk suspension can reach the vicinity of the substrate

causing that the particle influx to the meniscus would not be sufficient.

So the upper limit of the lifting speed is 1.75um/s, and the lower limit is decided by

the dipping instrument we build up in the university workshop since it is difficult to

get extremely low speed.

Instrumentation

The experimental set up of the dip-coating method is schematically shown in fig 4.3,

but the suspension container is exaggerated there. Actually a tiny beaker is used in

our experiments, which is 2.5 cm in diameter and 3.0 cm in height, for avoiding the

material waste since our substrates are small too (1cm × 2.5 cm ). We built up a

dipping stage for the experiments, which is shown in figure 4.5, where a stepper

motor moves the sample stage up and down, and a gear box is also used to get desired

slow speed, finally the lowest speed – 1.3 um/s was achieved, and available highest

speed was about 150 um/s. The stage speed was carefully calibrated and controlled by

Dt=2χ

ηπR

kTD

6=

25

a labview program in computer. For holding the small samples, just a plastic paper

clamp was used as this simple way allowed us to start our experiment earlier.

Fig 4.5 the dipping stage we built up for the experiments (left), and the stage is

controlled by a labview program (right)

4.3 Parallel transferring with water evaporation

Fig 4.6: Schematic illustration of the deposition process. A suspension of the PS particles with a

spreading agent is dispensed onto the water surface. Particles are trapped at the interface by the

surface tension. With the quick evaporation of the spreading agent, strong lateral capillary force

arises between the particles and organizes them into periodic arrays. And then the arrays are

transferred into solid substrate by lifting it upwards parallel to the monolayer.

26

The deposition process is schematically shown in the figure 4.6. A small amount of

suspension with a spreading agent (where ethanol is used in our experiments) is

carefully applied using a syringe or micropipette on to the water surface which is

contained in a petri dish. The particles are trapped at the air-water interface due to the

surface tension effect. With the evaporation of the spreading agent, a strong flotation

capillary force (see section 3.2.3) arises between the particles (see equation 7) and

organizes the particles into the ordered periodic arrays. Then the whole container was

heated mildly (at 50 °C for 30 min) and then left to let the water evaporate, after the

water surface became lower, a clean, hydrophilic silicon substrate was dipped from

the side and then was lifted upwards parallel to the water surface. The colloidal

particle film is so transferred to the solid substrate.

4.4 Langmuir-Blogett Deposition

Figure 4.7 Schematic of the particle film deposition using a Langmuir-Blogett trough, where the

barriers anneal the monolayer and the Wilhelmy plate measures the surface tension. The monolayer

is transferred on to the substrate by lifting it vertically at a controlled speed.

In this method, ethanolic suspension of the PS particles is slowly applied on to the

water surface in a Langmuir-Blogett trough, as shown in the figure 4.7, the barriers in

the trough anneal the monolayer (compress and expand several times) at a constant

rate, and the surface tension at the interface is measured by the Wilhelmy plate so that

a surface – area isotherm can be achieved telling the average particle density in the

monolayer. Due to the floatation capillary force between the particles as well as the

27

mechanical force from the barriers, a well-organized particle monolayer forms at the

air-water interface. Checking from the surface – area isotherm shown in the NIMA

labview program in the computer, when the particles are relatively densely packed or

the monolayer reaches to the solid phase, the barriers were stopped from compressing

and the corresponding surface tension was set as the target pressure which is the

pressure kept constant during the monolayer transferring process by the barriers. The

substrate was quickly dipped into the clean water (strictly checked by the surface

tension measured by the plate) prior to the spreading of the particles, and after the

annealing and isotherm measurements, when the monolayer is ready, the substrate is

lifted up at a slow speed under a monolayer-controlling target pressure.

Fig 4.8 L-B trough with the Wilhelmy plate Fig 4.9 a typical pressure-area isotherm of the ps particle film

Fig 4.10 the experimental set up for this vertical deposition process. The dipping apparatus we

built up was used to transfer the monolayer onto substrate.

28

5. Experimental Process During this project, all the four different particle film growing methods explained in

the previous chapter were investigated. And the samples were characterized using

several different techniques. Therefore, plenty of experimental work has been done. In

this chapter, some important experimental process will be discussed in detail.

5.1 Materials

Common used 10 × 10 mm silicon (111) wafers and glass slides (about 25×10 mm)

cut from ordinary microscope cover glass (76×26×1 mm) were used as substrates.

Home-made monodisperse latex dispersions of polystyrene particles of 140nm

(marked as PS3) and 100nm (marked as PS4) in diameter with (HSO4) - functional

group were used for particle film growing. 99.7% pure ethanol is used as spreading

agent for spreading the particles on water surface and was also used in the cleaning of

the L-B trough. 95% - 98% sulfuric acid (HSO4) and 30% hydrogen peroxide (H2O2)

were used in substrate cleaning. 98% chlorotrimethylsilane [(CH3)3SiCl] was used in

substrate surface modification. A surfactant – sodium dodecyl sulfate (SDS) was

added in to the dilute dispersions to improve the wetting in the dip-coating

experiments. During the all experiments, deionized water was used.

In order to measure the concentration of the polystyrene dispersions, 3ml of

dispersion was dried by keeping it at 60 °C in the oven until the mass became stable,

and the dried PS solid particles were weighed. Finally, the concentrations were

determined as: PS3 is 7.5% and PS4 is 10% by volume fraction.

Substrate cleaning

Substrates were cleaned using piranha solution. The process is: substrates were rinsed

in a piranha solution (H2O:H2O2:H2SO4 in ratio 5:1:4) and then quenched in water

bath keeping the temperature around 70-80 c° for 10 minutes. As the piranha solution

is a strong oxidizer, it removes most organic matter from the substrate and it will also

hydroxylate (add OH groups) it, so the substrate will become hydrophilic after this

treatment. After the quenching, silicon substrates were taken out from the piranha

solution and rinsed in deionized water several times, then dried in N2 gas. The glass

29

substrates we cut from microscope slides were kept in the piranha solution at least 24

hours before they were dried. Some of the substrates were further cleaned by UV-

ozone cleaning process. Using this dry cleaning technique after the piranha wet

cleaning, organic contaminants remained on the substrate can be removed by the UV

radiation and it also improves the hydrophilicity of the substrate surface. Substrate

cleanliness and hydrophilicity were checked by AFM and contact angle

measurements, and assured that after these treatments substrates become very clean

and hydrophilic.

5.2 Substrate Surface Modification and Contact Angle

Measurements

The wetting property of the substrate is very important in self-organization process as

it influences the interfacial forces between particles and substrate. And this surface

property of the substrates can be changed by a relatively simple chemical treatment.

As a quantitative measure of the wettability of a solid surface, contact angles were

measured before and after the treatment.

Contact angle

The contact angle is a result of the interface/surface tensions (surface free energies)

between liquid and solid surrounded by vapor. It is defined geometrically as the angle

formed by a liquid at the three phase boundary where a liquid, gas and solid intersect

as shown below:

Fig 5.1 the contact angle of the liquid drop sitting on a solid

θc = contact angle

Tangent to the drop profile

30

Where θc is the contact angle, γSG is the surface tension at the solid-gas interface, γLG

is surface tension at liquid-gas interface, and γSL is the tension at solid-liquid contact

line. When the liquid drop is at equilibrium, it should satisfy the thermodynamic

relations between these parameters given by Young’s equation [17]:

LGSLSG γγγ +=cθcos (13)

Contact angle measurements using static sessile drop method

In this sessile drop technique, contact angle of the liquid drop sitting on a solid

substrate is measured directly using a goniometer. As shown in figure 5.2, a certain

volume of droplet (drop shape and volume is calibrated before the measurement) is

deposited by a syringe pointed vertically down onto the sample surface, and a high

resolution camera captures the image, which then is analyzed using image analysis

software. Due to the gravity, the droplet shape is not stable. Contact angle tends to

recede. Therefore the data which were measured just at moment when the droplet

touches the substrate should be used. But when the droplet comes down from the

syringe, there are always some fluctuations as well, so a set of data is acquired to get a

good average, as shown in figure 5.4.

Fig 5.2 experimental set up for the contact angle measurement

Fig 5.3 the PG3 Pocket goniometer

31

Fig 5.4 a drop image and the measurement data, when the droplet comes down from the syringe, there

are some fluctuations in the contact angle data, so a set of data is measured to get an average.

Silanization of the silicon substrates

Silane is a molecule with chemical formula SiH4. A Silane that contains at least one

silicon-carbon bond is called organosilane. Organosilanes can be used to modify the

surface chemistry and wetting properties of some solid substrates such as mica, glass

and metal oxide. As shown in figure, organosilanes can react with the hydroxy (OH)

group on the substrate surface and form Si – o – Si covalent bond bringing the Si – C

bond together onto the substrate surface; the Si – C bond is very stable and nonpolar,

so it gives rise to low surface energy and hydrophobic effects.

Fig 5.5 Silicon substrates were silanized using chlorotrimethylsilane. Stable, non- polar Si-C

bonds covered the substrate surface making it hydrophobic.

32

In this study, clean silicon substrates were silanized using chlorotrimethylsilane

[(CH3)3SiCl] by a self-assembly process simply keeping the substrates under the

vapor of chlorotrimethylsilane for 30 minutes. According to the contact angle

measurements, the wetting property of the substrates were changed by this

Silanization process, as shown in the drop images in figure 5.6; the contact angle of

the clean water droplet on clean silicon substrates were around 10°, and after the

Silanization treatment, contact angle of the same water droplet on the substrates

became about 70°~ 85° (data differs somehow according to the amount of the exposed

silane vapor and exposing time).

Fig. 5.6 the water droplet sitting on clean silicon substrate (left) and the same droplet on the

silanized substrate (right). The substrate became hydrophobic after the silanization.

5.3 Sample Growing

Large number of samples has been grown using four different techniques; spin

coating, parallel transferring with water evaporation, and dip-coating method (for the

basic theory and instrumentation for each method, see chapter 4). Series of

experiments were performed in order to determine the optimum conditions for the PS

particles to self-organize into ordered structure and also to study the effects of

different growing parameters on the final structure formation.

In spin coating method, (a) samples were grown on different substrates: hydrophilic

glass and hydrophilic silicon substrates to check the effects of different substrates.

(b) Samples with different PS4 concentrations of 1.7%, 2.5%, 3% and 5% (volume

fraction) were grown in order to investigate the effect of different particle

concentrations on film thickness and crystallization quality.

33

In L-B method, (a) effect of different ethanol volume fraction within the PS/ ethanol

suspension on the floating property of the particles was investigated. (b) Surfactant

SDS with different concentrations was added into the suspension instead of the

ethanol, in order to investigate if the SDS can help floating the particles on water

surface. SDS was also applied on the water surface after the spreading of PS/ethanol

suspension to see its effect on surface tension and floating of the particles. (c) In order

to investigate the effect of the substrate surface property, different samples were

grown on silanized hydrophobic silicon and clean hydrophilic silicon substrates. (d)

Samples were grown with different lifting speed to see effect of that.

In dip-coating method, (a) samples with different PS3 concentrations were grown with

out adding surfactant into the suspension. (b) To study the effect of surfactant SDS,

samples with different SDS concentrations were grown at constant PS concentration.

(c) Samples with different PS volume fraction were grown at constant SDS

concentration.

The results of the experiments listed above, will be discussed in chapter 6 together

with experimental data.

5.4 Sample Characterization

Polystyrene particle films were characterized using optical microscopy, atomic force

microscopy (AFM), and scanning electron microscopy (SEM) techniques.

Optical Microscopy

Optical microscopy is a common and easy technique. Using the optical microscopy,

the domains of the ordered region can be distinguished, which can not be achieved

from atomic force microscopy because of its limitation on scan area. And from

difference of color, the sample quality such as defects, contaminants also can be

judged quantitatively. We used a reflective light microcopy .Figure 5.7 is an optical

microscopy image of PS4 (100 nm big in diameter) particle film grown by L-B

deposition method, image is taken under 100 times magnification. The majority light

green part represents the particle film and the white road in the middle is the empty

substrate which shows that the film is not continues but has different domains. The

34

darker green spots may reveal the thickness variation in the domains and the black

dots might come from the contaminants.

But because of the resolution limit, the 100 nm big polystyrene particles can not be

distinguishably imaged by optical microscopy which can be done by atomic force

microscopy.

Fig 5.7: Optical microscopy image (×100 magnification) of PS4 particle film grown by L-B deposition method.

Atomic Force Microscopy

Atomic Force Microscopy (AFM) is a powerful tool which is very popular in surface

characterization in nanometer and submicrometer scale. Compared to other

microscopy techniques, AFM has lots of advantages; Compared to the optical

microscopy, it has much higher resolution. The lateral resolution is normally 1nm

(can reach the atomic resolution using extremely sharp tip and clean surface) and very

sensitive to the topography having 1 Å vertical resolution. As shown in figure 5.9, we

can clearly see the individual particles, the ordered arrays, and defects. Compared to

SEM or STM, AFM does not require special sample preparation, does not require

sample conductivity, even can scan soft materials (even liquids) and also can work in

ambient conditions (no vacuum requirement). Because of these reasons, AFM has

become a foremost technique in nanoscience and technology.

35

In this project, a Nanosurf Mobile S 2.1 AFM was used. Polystyrene particle films

were scanned in dynamic force tapping mode using a non-contact tip (it has a large

spring constant, which is 48 N/m in our case). In this tapping mode, the tip oscillates

over the sample surface and touches it periodically, which greatly reduces the tip-

sample contact consequently reducing the tip contamination, and so enables us to scan

soft materials. As already discussed, for the 100nm particles, optical microscopy is

not a really good technique due to its resolution limit, and the SEM is a much more

complicated and not always available technique, AFM is used as the main

characterization technique in this project. All of the samples were scanned by AFM in

the first step after growing.

Scanning Electron Microscopy

Scanning electron microscopy (SEM) is used as a complementary technique together

with AFM to get a more complete profile of the sample. Using AFM, we only can see

the surface (or the top most layer) of the film. While, the SEM technique reveals the

part under the surface if we do a cross section image as it has a large depth of field

which is not possible by AFM due to the surface roughness. Another advantage of the

SEM is the larger lateral field of view (see figure 5.10) and quick scan speed

compared with AFM.

SEM technique requires conductive sample surface and vacuum environment. In this

study, the polystyrene particle films were coated with a thin gold layer by sputtering

technique. A LEO 440 type scanning electron microscopy was used. Figure 5.10 is a

SEM image of 140 nm PS particles grown by dip-coating method. The image size is

around 25um, which is apparently much larger than AFM image size. From this

image we can distinguish the substrate, the thickness variations (larger area is a

monolayer and some part is two-layer), the large domain of the arrays and the grain

boundaries.

36

Fig 5.9 AFM image of PS3 latex spheres grown by dip-coating, PS3 volume fraction is 0.375%, with 20 mM of SDS, substrate lifting speed 1.5 um/s.

Fig 5.10 SEM image of PS3 latex spheres grown by dip-coating, PS3 volume fraction

is 0.375%, with 1o Mm of SDS, substrate lifting speed 1.5 um/s.

37

6. Results and discussions In the first three sections of this chapter, samples grown by different methods will be

discussed separately as the different growing methods give different conditions to the

particles letting them self-organize in some different way. The SEM images are

discussed in another subsection. As mentioned in previous text, much more

experiments were carried on in this project; a few of them will be discussed in the last

section of this chapter.

6.1 Results from Spin-Coated Samples

A series of experiments have been carried out with different polystyrene

concentrations of 5%, 3.0%, 2.5%, and 1.7% (volume fraction). AFM images of these

samples are given below, in figure 6.1 – 6.4. As shown in figure 6.1, the 5% film is

quite thick and not well-ordered (in long range). When the particle concentration was

reduced, as shown in 6.2 and 6.3, the film became thinner as expected according to

the theoretical formula given by equ.8. (See section 4.1). But at the mean time, as

shown in the zoomed in given in figure 6.3, the size of particle aggregates were also

reduced. By the most dilute suspension, 1.7% one, we got a film which only covered

very small area of the substrate as shown in figure 6.5.

Fig 6.1 AFM image of 100nm PS4 latex spheres grown by spin-coating.100ul of 5% PS was spin coated in 8000rpm for 25sec.

Fig 6.2 AFM image of 100nm PS4 latex spheres grown by spin-coating.100ul of 3% PS was spin coated in 8000rpm for 25sec.

38

Using this spin-coating method, we did not get a well-ordered monolayer film as we

expected. The possible reasons for this result are as following: As discussed in the

section 4.1, the experimental parameters in this method are empirically determined.

And so, the parameters we set for the experiments might not the preferable one for the

particles to self-organize into long-range ordered monolayer arrays. Besides, as

described above, by higher particle concentrations, from 5% - 2.5%, we got

multilayered small aggregates of particles; while by the less concentrated suspension,

as shown in figure 6.4, a film which is not fully covered was achieved. From this, we

can speculate that the aqueous suspension of polystyrene particles do not completely

wet the substrate. If this is the main reason, adding some surfactant in to the

suspension or making the silicon substrate more hydrophilic by some chemical

treatment will improve the wetting so that better quality samples might be achieved.

But regarding the inherent drawback of this spin-coating method, which is the lack of

uniformity and the surface roughness of the film at the edges of the substrates, no

further studies were done in this method and some other methods were studied whose

results will be discuss in the following sections.

Fig 6.3 AFM image of 100nm PS4 latex spheres grown by spin-coating.100ul of 2.5% PS was spin coated at 8000rpm for 25sec.

Fig 6.4 AFM image of 263 nm PS latex spheres grown by spin-coating.100ul of 1.7% PS was spin coated at 2000rpm for 25sec.

39

6.2 Parallel transferring with water evaporation As discussed in section 4.3, this is a relatively simple method. Where by mixing the

equal amount of 10 % PS suspension and pure ethanol, an ethanolic PS suspension

was prepared and then very gently applied on to the water subphase. Big areas of

colorful islands of monolayer film were formed on water surface. Part of the subphase

water is evaporated by heating the whole petri dish at 50c° for 30 minutes. Then the

water level is decreased and the film is transferred into clean silicon substrate by

lifting it parallel to the water surface just by hand using a twissors.

Even though this process is simple and not delicately controlled (for example the

lifting speed), the sample is nice, as shown in the AFM image in figure 6.5; a large

area of fully covered, clean, flat and ordered monolayer film is transferred into the

substrate. The image shows that the defects and the dislocations were mainly caused

by the polydispersity of the particles which implies that this is a promising and easy

technique where the film quality can be further improved by increasing the

monodispersity of the particles.

Figure 6.5 AFM image of 100nm PS4 latex spheres parallel transferred with water evaporation; the floated monolayer film was horizontally transferred in to silicon substrate.

40

6.3 Langmuir-Blodgett method

In this process, monolayer is formed in a L-B trough instead of the petri dish so that

the film area can be controlled by the barriers, and the film quality also can be

improved by annealing it using the barriers, the surface tension measured by the

Wilhelmy plate and an electrobalance checks the cleanliness of the surface and a

pressure –area isotherm is measured telling more information about the floated

monolayer film. The film is transferred by dipping and lifting the substrate vertical to

the monolayer surface in a delicately controlled speed using the dipping apparatus.

Two kinds of different substrates, namely hydrophilic and hydrophobic substrates,

were used to study the effect of the substrate surface property.

With Clean - Hydrophilic Substrate

Here the silicon substrates which cleaned with Piranha solution were used. The

contact angle measured after the cleaning and drying process was around 10°. The

figure 6.6 shows the sample surface scanned by AFM; the image is not bad

concerning the size of the aggregates, surface roughness and the ordering even though

it has the typical defects and contaminants. But the problem is, samples grown in this

way has very small film coverage, where are only several dots (less then 1 mm2) of

polystyrene film leaving the majority of the substrate clean as before. So we modified

the surface of the silicon substrate to improve the film coverage.

Fig 6.6 AFM image of the 100nm PS4 latex spheres grown using L-B trough and vertical deposition on hydrophilic

silicon substrate.

41

Silanized – Hydrophobic Substrate

As described in the previous section, vertical deposition of the L-B film onto the

hydrophilic clean substrate gave a bad coverage. To improve this, the clean silicon

substrate was silanized (see section 5.2) and became hydrophobic which is proved by

the contact angle measurements. The contact angles after the Silanization were ranged

from 7o°~ 85°. After this Silanization treatment, much larger areas of PS particle film

were transferred into the substrate. The size of the covered area is about 10 mm2-

40mm2 and the size of the continuous part of the film is also large which is examined

by reflective optical microscopy, figure 6.7 is one of the examples. Figure 6.8 is an

AFM image of a sample grown by this L-B vertical deposition method on

hydrophobic substrate; the PS concentration is 5% (where are 50% of pure ethanol in

the aqueous suspension), the substrate lifting speed is 5um/s, the target pressure set on

the barriers during the transferring is 13.7 mN/m. As shown in the figure 6.8,

relatively large area of closely packed monolayer particle film was transferred and as

shown in the zoomed in image given by figure 6.9, the film surface is flat and

relatively ordered with typical defects and dislocations.

Fig 6.7 Optical microscopy image (×100 magnification) of PS4 particle film grown by L-B deposition method on silanized hydrophobic substrate.

42

Discussion, horizontal and vertical deposition????

Fig 6.8: AFM image of 100nm PS4 latex spheres grown by L-B deposition method on hydrophobic substrate.

Fig 6.9 Amplified image of 100nm PS4 latex spheres grown by L-B deposition method on hydrophobic substrate.

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Discussion on two growing methods:

Parallel transferring with water evaporation and L-B deposition

Both in this two methods, parallel transferring and L-B deposition, particle monolayer

was formed on water surface, but then the monolayer was transferred into substrates

in different ways. According to our experience with the sample growing and

characterization process, it is more difficult to achieve good quality samples from

Langmuir-Blodgett method compared to the parallel transferring with water

evaporation which is a simple technique we easily got nice sample using it. The

possible reasons for this are as following; in the parallel transferring method, substrate

is placed just under the floated monolayer and then lifted upwards parallel to the

water surface fishing the monolayer up together; While in the L-B deposition process,

substrate moves upwards perpendicular to the water surface tearing and pulling up the

monolayer. Therefore, during the L-B deposition process, the floated monolayer

might be collapsed due to the hydrodynamic forces, gravity or due to the surface

roughness. Besides, monolayer may also be collapsed because of the compression

given by the trough barriers, but this can be recognized and avoided looking at the

pressure-area isotherm of the monolayer. There are also some other parameters which

can influence the sample quality in the L-B deposition process, such as the substrate

lifting speed and target pressure set on the barriers during the deposition.

So the L-B deposition is a more complicated and more controlled technique which

also needs more studies. We almost did not see any detailed studies on this method

used in colloidal particle film deposition (this technique is most commonly used in

transferring the amphiphilic molecular film, where the gravity on much smaller

molecules are also small). Therefore our results may help the further studies on this

topic.

6.4 Dip Coating

Among all the four different growing methods, this dip-coating is the most successful

one from our experiments. Highly ordered particle arrays were achieved using this

method. The mechanism of particle array formation and the sample growing process

is discussed in detail in section 4.2. Here the results will be given in detail discussing

44

the influence of different parameters. At last, a brief summary and a discussion on the

dip-coating and L-B method will be given.

Samples without surfactant

Our experiments were started without surfactant. A series of experiments have been

carried out with dipping the hydrophilic glass substrate dipping into suspensions with

PS concentrations 0.75%, 0.375% and 0.075% (volume fraction). Figure 6.10 and

6.11 shows the AFM images of the 0.75% and 0.375% samples; while with the most

dilute 0.075% suspension, no film was achieved, substrate was left clean after the

dipping process. As seen in the AFM images, the 0.75% sample is a multilayer film

with lots of voids on the surface, the 0.375% one is also voids, when the concentration

goes down to 0.075% no film was formed. From this, decided to add surfactant into

the suspension to improve the wetting.

Effect of Surfactant SDS

As described above, dipping the substrate into 0.075% dilute PS suspension did not

give a particle film, but after adding 10mM of surfactant – sodium dodecyl sulfate

(SDS) a highly ordered particle arrays were achieved, which is shown in figure 6.12.

By adding the same 10mM of SDS into the 0.375% suspension, even larger area

(10×10 um) of ordered film was scanned as shown in figure 6.13. The figure 6.14

Fig 6.10: AFM image of 140 nm PS3 latex spheres grown by dip-coating. PS concentration: 0.75%, substrate lifting speed: 1.5um/s.

Fig 6.11: AFM image of 140 nm PS3 latex spheres grown by dip-coating. PS concentration: 0.375%, substrate lifting speed: 1.5um/s.

45

shows the higher SDS concentration film with 0.375% PS concentration. It is clear

that this sample is also highly ordered and denser than the lower SDS concentration

one shown in figure 6.13.All the surfactant containing suspension were ultrasonicated

for 10 min just before the dipping; and this makes the particles more evenly dispersed

and better mixed with the surfactant molecules.

Fig 6.12: AFM image of 140 nm PS3 latex spheres grown by dip-coating. PS concentration: 0.075%, SDS concentration: 10 mM, substrate lifting speed: 1.5um/s.

Fig 6.13: AFM image of 140 nm PS3 latex spheres grown by dip-coating. PS concentration: 0.375%, SDS concentration: 10 mM, substrate lifting speed: 1.5um/s.

46

From the results given above, it is obvious that the surfactant SDS helps much on the

particle ordering. We suppose that there are several reasons for this. First, adding

surfactant lowers the surface tension and, hence, improves the wetting of the substrate

by the suspension which subsequently changes height and the shape of the wetting

meniscus. According to the formula given in equ.10 in section 4.2, film thickness K is

directly proportional to the meniscus height L; therefore the larger L due to the

surfactant gives an increased film thickness K. This is proven by the experiment on

0.075% dilute suspension, where without the surfactant the number of layers k equals

to 0 and after adding 10mM of SDS, k equals to 1.

Fig 6.14: AFM image of 140 nm PS3 latex spheres grown by dip-coating. PS concentration: 0.375%, SDS concentration: 20 mM, substrate lifting speed: 1.5um/s.

Fig 6.15 Schematic of the meniscus without (left) and with SDS (right); SDS changes the meniscus height and shape.

Fig 6.16 Schematic of the equilibrium distance between particles without (left) and with SDS (right); due to the screening effect comes from the counterions introduced by SDS, the equilibrium distance become shorter

47

On the other hand, as the SDS is an anionic surfactant, adding surfactant also

increases the ion strength in the suspension. Due to the screening effect caused by the

counterions introduced by the dissociation of the surfactant molecules, the

equilibrium distance between the particles become shorter. And this decreased

equilibrium distance increases the number of particles which reach to the substrate

from the bulk suspension in a certain time; that is why the film thickness and the

domain size are increased with surfactant. At the same time, the repulsive

electrostatic interaction between the particles is also reduced because of this screening

effect, and this is another reason why the ordering is improved.

Influence of different PS volume fraction

The particle concentration is another important experimental parameter which we can

change easily. From the figure 6.10 and 6.11, and the fact that no film was achieved at

0.075% PS concentration, it is obvious that when there is no surfactant, the film

thickness decreases with the decrease of PS concentration. By comparing the AFM

images given in figure 6.12 and figure 6.17, at a constant SDS concentration, the film

thickness also increase with the increase of PS concentration since the image 6.12 is a

Fig 6.17: AFM image of 140 nm PS3 latex spheres grown by dip-coating. PS concentration: 0.375%, SDS concentration: 10 mM, substrate lifting speed: 1.5um/s.

48

monolayer and 6.17 shows a thickness variation in the sample where larger part of

the image is a monolayer and other small part is a multilayer. Therefore, the

conclusion can be made that the film thickness is in direct promotion to the PS

concentration in our experiments which is in good agreement with the theoretical

equation (10) given in section 4.2.

Summary in dip-coating method

According to the experimental results presented previously, it can be concluded that

for the 140 nm PS3 particles, highly ordered crystalline particle arrays can be achieved

under the following experimental conditions:

PS concentration: 0.075% ~ 0.75%, SDS concentration 10~20mM, substrate lifting speed 1.5 um/s.

Discussion on two growing methods: Dip-coating and L-B deposition

As shown in the experimental results, samples grown by dip-coating method have

better sample quality such as better crystalline long range ordering compared to the

ones from the L-B method. Comparing the images from these two methods, we can

easily know that L-B method is more sensitive to the particle polydispersity. This is

because, the flotation capillary force assisting the particle self-organization in L-B

method is more sensitive to the particle size as shown in the figure 3.5 and the equ.(7)

(in section 3.2.3) than the immersion capillary force. As shown in equ. (7), the

flotation capillary force is proportional to the particle radius to the power six while the

immersion capillary force is proportional to the radius squared. Another reason for the

better ordering in dip-coated samples is that the attractive and repulsive interactions

between the particles are better tuned by adding proper amount of surfactant. Besides,

in the L-B method, the monolayer is formed in water surface. And it is not easy to let

the particles float and form a uniform, stable monolayer on the water surface since

this has some special requirements to the suspension. Usually in the experiments,

some amount of the particles (20% - 50%) goes down to the water subphase instead of

floating at the interface making it hard to decide the amount of suspension which was

required to form a monolayer. Regarding this, dipping directly into the particle

suspension is much easy.

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6.5 SEM Images

One of the best samples grown by dip-coating was scanned with scanning electron

microscopy. As a complementary technique, SEM can fast scan a large and rough

surface area and tell us some more information which can not be achieved by only

using the atomic AFM. SEM image showed in figure 6.18 reveals that the film in the

image is a monolayer by the color contrast from the substrate (black) and the particle

film (grey). From the image in figure 6.19, we know that large area (which is more

than 200 um 2 and large regarding the particle size – 140 nm) of highly ordered

crystalline particle films was achieved. We also can see the typical defects and

dislocations which are common in all two dimensional crystals. One of the main

reasons of these defects is the polydispersity of the particles. Figure 6.20 confirms the

thickness variation which we also found in the AFM image of the same sample shown

in figure 6.15.

Fig 6.18 SEM image of PS3 latex spheres grown by dip-coating, PS3 volume

fraction is 0.375%, with 10mM of SDS, substrate lifting speed 1.5 um/s.

50

Fig 6.19 SEM image of PS3 latex spheres grown by dip-coating, PS3 volume

fraction is 0.375%, with 10 mM of SDS, substrate lifting speed 1.5 um/s.

Fig 6.20 SEM image of PS3 latex spheres grown by dip-coating, PS3 volume

fraction is 0.375%, with 10 mM of SDS, substrate lifting speed 1.5 um/s.

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6.6 Other Results and Discussions

Influence of the UV-Ozone Cleaning Process

The figure 6.21 below shows a dip-coated film on a glass substrate which is just

cleaned by piranha solution, while the figure 6.22 and 6.23 show the film grown

under all the same conditions except for the UV-ozone cleaning of the substrate after

the piranha cleaning process. As the images tell (image 6.22 is the best image we got

as the other spots of the piranha cleaned sample were even more rough; while almost

all surface of the UV cleaned sample was uniform and smooth, the two images given

are the examples), the UV-cleaned one is much better regarding the cleanliness and

surface roughness. In the first sample, there might still be some organic contaminants

remained after the piranha cleaning or got contaminated during the washing or drying

process and this kind of contaminants have been removed by the UV-ozone radiation

in the second sample. Or, we may suppose that the UV-cleaned surface is more

favorable for the PS particles rather than the piranha cleaned one as the different

cleaning process gives different surface property to the substrate.

Fig 6.21: AFM image of 140 nm PS3 latex spheres grown by dip-coating. PS concentration: 0.375%, SDS concentration: 20 mM, substrate lifting speed: 1.5um/s. A substrate cleaned by piranha solution was used.

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Fig 6.22 AFM image of 140 nm PS3 latex spheres grown by dip-coating. PS concentration: 0.375%, SDS concentration: 20 mM, substrate lifting speed: 1.5um/s. Substrate is cleaned by UV-ozone cleaning process after the normal piranha cleaning.

Fig 6.23 AFM image of 140 nm PS3 latex spheres grown by dip-coating. PS concentration: 0.375%, SDS concentration: 20 mM, substrate lifting speed: 1.5um/s. Substrate is cleaned by UV-ozone cleaning process after the normal piranha cleaning.

53

Particle Film Stability Test Result

To check the stability of the particle film, an L-B sample which has been kept in a

sample box under ambient conditions for five months since grown was examined; and

the AFM images revealed that the polystyrene film is stable under the dry ambient

conditions.

Further tests were done in order to check the sample stability after rinsing it in water.

Sample is gently rinsed in deionized water and took out, then kept in clean lab table

covered by a big beaker. After the sample is dried, sample surface is scanned by

AFM. The most part of the surface still had the same film structure as before, as

shown in the AFM image given in figure 6.24; while very small area of the sample

was damaged such as the image in figure 6.25. The most possible reason for this is

that a small water droplet sat on the film surface and damaged the film during its

drying process due to some hydrodynamic forces. However, the majority part of the

surface area (more than 90%) is stable so that the samples can be chemically etched in

an aqueous solution.

Fig 6.24 AFM image of an L-B sample after rinsed in water to test the sample stability, most part of the film was kept stable like this.

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Recognizing and Avoiding Typical Artifacts in AFM imaging

Surface imaging by AFM is an essential, convenient and useful characterization

technique very commonly used in all nanoscience nowadays. Therefore taking good

quality surface image using AFM is of great practical importance, but this is not so

easy especially when the sample is a soft film. And recognizing and avoiding the

typical AFM artifacts help improving the image quality. Except for the artifacts

introduced by a bad tip shape, the most common typical artifacts I have encountered

during this project are given in the images below. The piezoelectric material inside the

AFM is very sensitive to the temperature; using the instrument after a long while (it

cools down after being unused and begins to worm up when it is restarted) or just

exposing the scan head with a new air flux may cause a drift. The figure 6.26 shows a

drift caused by this; the spherical particles became rod-like because of the drift. In

order to avoid this kind of artifact, the very first several scans after the restarting of

the instrument should not be used. And it is also very helpful to keep the scan head

inside a thermal-isolating box.

Fig 6.25 AFM image of an L-B sample after rinsed in water to test the sample stability, very small area of the sample was damaged like this.

55

Another main reason for the artifact is the contaminants and surface roughness, as

shown in figure 6.27. In order to avoid this, one should be very careful during the all

steps of the experiment, certain cleaning process such as piranha cleaning and UV-

ozone cleaning should be taken. Man-made artifacts as shown in the figure 6.28 are

also common. The feedback parameters should be set properly so that this sort of

artifacts can be easily avoid, and this is always explained with detail in the AFM

instrument manuals.

Fig 6.27 AFM artifact caused by the contaminants and surface roughness

Fig 6.26 AFM artifact, a thermal drift

56

Fig 6.28 A drift caused by improper feedback parameter setting

57

Future Work As shown in the results, relatively large areas of highly ordered crystalline arrays of

polystyrene particles were successfully achieved using dip-coating method under

some specific conditions. However, as shown in the SEM image below, the sample

surface was covered by small patches of particle monolayers; we haven’t got a

uniform, continuous film which fully covered the whole substrate. So some future

work should be done on optimizing the experimental parameters to get a continuous

film. Increasing the PS concentration or decreasing the substrate withdrawal speed

might help.

In this project, only very thin films (monolayer or bilayers) of PS particles were

studied so far. So the further studies should be done on the growth parameters to

adjust them in order to get 3D, ordered, thick multilayer arrays which may help

another very interesting topic – photonic band gap materials’ development.

This sort of studies can be extended to the similar particles with even smaller particle

size; this can be challenging and very interesting.

SEM image of the PS monolayer film; the monolayer is not a continuous film but small patches

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Acknowledgements

In the end of my thesis, I would like to thank all of my colleges in this group. First of all, I am very grateful to Professor Björgvin Hjörvarsson for giving me the opportunity to come to Uppsala and work in this excellent group on this fantastic topic! I am extremely thankful to Professor Per Stahle in Malmo University who made it possible for me to come to Sweden to pursue my dream, and really appreciate all his support and help on my study in Sweden. I want to express my enormous gratitude to my supervisor Dr. Vassilios Kapaklis who always patiently answered my questions and supported and encouraged me. I am also very thankful to Professor Adrian Rennie for the discussions and guidance, I literally learnt a lot from them. I am truly grateful to Maja for supplying materials; to Panos and Evanglis for preparing Samples for SEM and am also very thankful to Anders for all the engineering work he did for this project. Thank you Gunnar for being a good listener and a good friend who is ready to help anytime I need! Thanks to Atiyeh for being a good sister; Thank you Junaid for the nice conversations! Ata-anamning we Ailemdiki barliq tuqqanlarning mini yiqindin qollap we ilhamlandurup, yolumni yourutup bergenlikige we men uchun toxtimay qilghan du’alirigha koptin kop rexmet! Mining hemme ishim silerning qollishinglarsiz, silerning du’ayinglarsiz ishqa ashmayti, Allah umringlarni uzun tininglarni salamet qilsun! (Acknowledgments to my parents in Uyghur (My mother tongue)) My very special thanks to my boy friend Mehmudjan for everything he did for me; for being with me in all tough and good times, without him supporting and helping me all the way down, this thesis would not be possible for me to finish, Thank you!

59

Bibliography [1] K.Nagayama. Colloids and Surfaces A 109, 363 (1996).

[2] Hulteen. J, Van. Duyne. R. J. Vac. Sci. Techno A 13 (3), 1553 (1995).

[3] A.S.Dimitrov and K. Nagayama. Langmuir 12, 1303 (1996).

[4] M. Trau, D. A. Saville, and I. A. Aksay. Langmuir 13, 6375 (1997).

[5] A. Kosiorek, W. Kandulski, P. Chudzinski, K. Kempa, and M. Giersig.

Shadow nanosphere lithography, Nano Lett. 4, 1359 ( 2004)

[6] C. L. Haynes, A. D. McFarland, M. T. Smith, J. C. Hulteen, and R. P. Van

Duyne. Angle-resolved nanosphere lithography. Journal of Physical

Chemistry B 106,1898 (2002).

[7] Orlin D. Velev and Erik W. Kaler. Structured porous materials via colloidal

crystal templating, Adv.Mater. 12, 531 (2000).

[8] Xia YN, Gates B, Yin YD, Lu Y. "Monodispersed colloidal spheres: Old

materials with new applications”. Adv Mater 12, 693 (2000).

[9] A. J. Haes, L. Chang, W. L. Klein, and R. P. Van Duyne. Journal of the

American Chemical Society 127, 2264 (2005).

[10] Drexler EK (1986) Engines of Creation, 2nd edn. Anchor Books, New York

(1990).

[11] G. M. Whitesides, B. Grzybowski, “Self-assembly at all scales”, Science 295,

2418(2002).

[12] Clint, J. H. Surfactant Aggregation (Blackie: 1992).

[13] Alexandrids, P., Lindman, B., eds. “Amphiphilic Block Copolymers: Self-

assembly and Application” (Elsevier. 2000).

[14] Yoon S. Lee, “Self-assembly and nanotechnology”, (Jone Willy press, 2008).

[15] Halley, J. D. and Winkler, D.A. "Consistent Concepts of Self-organization and

Self-assembly", Wiley Periodicals, Inc 14, 10 (2008).

[16] Yamaguchi, T.; Suematsu, N.; Mahara, H. “Self-organization of hierarchy:

Dissipative-structure assisted self-assembly of metal nanoparticles in polymer

matrices". Nonlin Dyn Polym Syst, 869, 16, (2004).

[17] Duncan J. Shaw, “Introduction to Colloid and Surface Chemistry”, Fourth

edition, an imprint of Elsevier Science (2003).

[18] J.Israelachvili, Intermolecular and surface forces, Academic press, San

Diego,ed. 2nd, (1992).

[19] J. Gregory, J. Colloidal Interface. Sci 83,183 (1981).

60

[20] Sandra Rodnar, “Interfacial colloidal particle films and their structure

formation”, PhD thesis, KTH University, Sweden (2002).

[21] Hough, D. B. , White , L. R. “ The Calculation of Hamaker Constants from

Lifshitz Theory With Applications to Wetting Phenomena,” Adv. Colloid

Interface Sci. 14, 3 (1980).

[22] P. A. Kralchevsky, K. Nagayama, Adv. Coll. Int. Sci 85,145 (2000).

[23] V.N.Paunov, P.A.Kralchevsky, N.D.Denkov, K.Nagayama, J. Colloid

Interface Sci. 157, 100 (1993).

[24] Chan, D. Y.C, Henry, J, D, and White, L, R. , J. Colloid Interface Sci. 79,

410 (1981).

[25] J C Earnshaw J. Phys. D: Appl. Phys. 19, 1863 (1986).

[26] B. Gates, D. Qin, Y. Xia , Adv. Matter. 11, 466 (1999).

[27] Denkov. N, Velve. O, Karalchevsky. P, K. Nagayama, Langmuir 8, 3183

(1992).

[28] D. Meyerhofer, J. Appl. Phys. 49, 3993 (1978).

M. Mennig, A. Kalleder, T. Koch, S. Mohr, C. Fink-Straube, S. Heusing, B.

Munro, P. Zapp, H. Schmidt, proc. 2nd ICCG, Saarbrücken (1998).

[29] J. H. Lai, Polymer Engineering and Science, 19, 1117 (1979).