nano materials self-assembly seminar final

8/8/2019 Nano Materials Self-Assembly Seminar Final

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1Self-assembly of nanomaterials

1. Introduction

Before knowing about self-assembly, some basics of nanomaterials and fabrication of

nanostructure is discussed in the following section.

1.1. Classification of nanoparticles

1.1.1. Zero-Dimensional Nanoparticles: Nanoparticles include single crystal, polycrystallineand amorphous particles with all possible morphologies, such as spheres, cubes and platelets are

part of zero-dimensional nanoparticles.

1.1.2. One-Dimensional Nanoparticles: One-dimensional nanostructures include whiskers,

fibers or fibrils, nanowires and nanorods. In many cases, nanotubules and nanocables are also

considered one-dimensional structures.

1.1.3. Two-Dimensional Nanostructures: These are nanostructure with dimensions along twoaxis such as thin films (coatings).

1.2. Classification of nanoparticles fabrication processes based on nucleation

1.2.1. Homogeneous nucleation:

For the formation of nanoparticles by homogeneous nucleation, a supersaturation of

growth species must be created. When the concentration of a solute in a solvent exceeds itsequilibrium solubility or temperature decreases below the phase transformation point, a new

phase appears. Another method is to generate a supersaturation through in-situ chemical

reactions by converting highly soluble chemicals into less soluble chemicals.

E.g.:- Zero-D nanoparticles and 1-D nanoparticles like nanorods, nanotubes, etc.

1.2.2. Heterogeneous nucleation:

When a new phase forms on a surface of another material (substrate), the process is

called heterogeneous nucleation. This method is suitable for two-dimensional nanostructures likethin films.

E.g.:- Evaporation, molecular beam epitaxy (MBE), sputtering, chemical vapor

deposition (CVD), and atomic layer deposition (ALD), self-assembly, electrochemicaldeposition, chemical solution deposition (CSD), Langmuir-Blodgett films.

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1.3. Classification of film growth methods

1.3.1. Vapor-phase growth:

This includes evaporation, molecular beam epitaxy (MBE), sputtering, chemical vapor

deposition (CVD), and atomic layer deposition (ALD).

1.3.2. Liquid-phase growth:

Electrochemical deposition, chemical solution deposition (CSD), Langmuir-Blodgett

films and self–assembly are liquid-phase growth methods.

1.4. Fundamentals of Film Growth

Growth of thin films, as all phase transformation, involves the processes of nucleation

and growth on the substrate or growth surfaces. The nucleation process plays a very important

role in determining the crystallinity and microstructure of the resultant films. For the depositionof thin films with thickness in the nanometer range, the initial nucleation process is even more

important. Nucleation in film formation is a heterogeneous nucleation. The size and the shape of the initial nuclei are assumed to be solely dependent on the change of volume of Gibbs free

energy, due to supersaturation, and the combined effect of surface and interface energies

governed by Young’s equation.

1.4.1. Vacuum Science in film growth

In addition, most film deposition and characterization processes are conducted under a

vacuum. The mean distance traveled by molecules between successive collisions is called the

mean free path and is an important property of the gas that depends on the pressure, given by:

λ mfP = 5 x 10-3 /P

Where λ mfP is the mean free path in centimeter and P is the pressure in torr. When the pressure is below 10-3torr, the gas molecules in typical film deposition and characterization systems virtually

collide only with the walls of the vacuum chamber, i.e. there is no collision among gas

molecules.

1.5. Film growth modes

In practice, the interaction between film and substrate plays a very important role indetermining the initial nucleation and the film growth. Many experimental observations revealed

that there are three basic nucleation modes:

(1) Island or Volmer-Weber growth,

(2) Layer or Frank-van der Merwe growth, and

(3) Island-layer or Stranski-Krastonov growth.

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Island growth occurs when the growth species are more strongly bonded to each other

than to the substrate.

The layer growth is the opposite of the island growth, where growth species are equally

bound more strongly to the substrate than to each other.

The island-layer growth is an intermediate combination of layer growth and island

growth.

Different types of film growth modes discussed in above section is illustrated in

following figure.

Fig.: Schematic illustrating three basic modes of initial nucleation in the film growth. Island growth occurs when thegrowth species are more strongly bonded to each other than to the substrate.

1.5.1. Requirements of film growth

Whether the deposit is single crystalline, polycrystalline or amorphous, depends on the

growth conditions and the substrate. Deposition temperature and the impinging rate of growth

species are the two most important factors and are briefly summarized below:

1. Growth of single crystal films is most difficult and requires:

• A single crystal substrate with a close lattice match,• A clean substrate surface so as to avoid possible secondary nucleation,

• A high growth temperature so as to ensure sufficient mobility of the growth species and

• Low impinging rate of growth species so as to ensure sufficient time for surface diffusion

and incorporation of growth species into the crystal structure and for structural relaxation

before the arrival of next growth species.

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2. Deposition of amorphous films typically occurs

• When a low growth temperature is applied, there is insufficient surface mobility of

growth species and or

• When the influx of growth species onto the growth surface is very high, growth speciesdoes not have enough time to find the growth sites with the lowest energy.

3. The conditions for the growth of polycrystalline crystalline films fall between the conditions

of single crystal growth and amorphous film deposition. In general, the deposition temperature ismoderate ensuring a reasonable surface mobility of growth species and the impinging flux of

growth species is moderately high.

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2. Self-Assembly

2.1. Introduction

Self-assembly is a generic term used to describe a process by which ordered arrangement

of molecules and small particles occurs spontaneously, without guidance or management from an

outside source, under the influence of certain forces such as chemisorption, electrostatic force,hydrophobicity and hydrophilicity and capillary force. Self-assembly is a ‘bottom-up’

manufacturing technique.

In general, chemical bonds are formed between the assembled molecules and the

substrate surface, as well as between molecules in the adjacent layers. Therefore, the major

driving force here is the reduction of overall chemical potential. A variety of interactions or forces have been explored as driving forces for the self-assembly of nanometer subjects as the

fundamental building blocks.

The driving force for the self-assembly includes: electrostatic force, hydrophobicity and

hydrophilicity, capillary force and chemisorption. In the following discussion, we will focus on

the formation of SA monolayers that chemisorb on the substrates.

Of the diverse approaches possible for Molecular Self-Assembly, two strategies have

received significant research attention – Electrostatic Self-Assembly (or layer- by-layer

assembly) and “Self-Assembled Monolayers (SAMs). Electrostatic self-assembly involves thealternate adsorption of anionic and cationic electrolytes onto a suitable substrate. Typically, only

one of these is the active layer while the other enables the composite multilayered film to be

bound by electrostatic attraction. Self Assembled Monolayers or SAMs based on constituentmolecules, such as thiols and silanes are the ones which are most often used. For SAMs,

synthetic chemistry is used only to construct the basic building blocks (that is, the constituent

molecules), and weaker intermolecular bonds such as Van der Waals bonds are involved in

arranging and binding the blocks together into a structure. This weak bonding makes solution,and hence reversible, processing of SAMs (and in general, MSAs) possible. Thus, solution

processing and manufacturing of SAMs offer the enviable goal of mass production with the

possibility of error correction at any stage of assembly. It is well recognized that this methodcould prove to be the most cost-effective way for the semiconductor electronics industry to

produce functional nanodevices such as nanowires, nanotransistors, and nanosensors in large

numbers.

2.2. Characteristics of self-assembled nanostructures

Self-assembly is successful when a right balance between attractive forces (that tend to

stick the components together) and Brownian motion (thermal agitation that tend to pull them

apart) is reached. In practice, these tendencies generally come into balance on length scales

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Fig.: A typical self-assembling surfactant molecule consisting of three parts: surface group, alkyl or derivatized

alkyl group, and surface-active headgroup.

There are several types of self-assembly methods for the organic monolayers and these

include

• Organosilicon on hydroxylated surfaces, such as Si02 on Si, A1203 on Al and glass, etc.,

• Alkanethiols on gold, silver and copper,

• Dialkyl sulfides on gold,

• Alcohols and amines on platinum and

• Carboxylic acids on aluminium oxide and silver.

Another way to group the self-assembly methods could be based on the types of chemical bonds formed between the head groups and substrates. They are

•Covalent Si-0 bond between organosilicon on hydroxylated substrates that include metalsand oxides,

• Polar covalent S-Me bond between alkanethiols, sulfides and noble metals such as gold,

silver, platinum and copper, and

• Ionic bond between carboxylic acids, amines, alcohols on metal or ionic compoundsubstrates.

One of the important applications of self-assembly that has been extensively studied isthe introduction of various desired functionalities and surface chemistry to the inorganic

materials. In the synthesis and fabrication of nanomaterials and nanostructures, particularly the

core-shell structures, self-assembled organic monolayers are widely used to link different

materials together.

2.4. Production methods of self-assembled monolayers

2.4.1. Self-assembled monolayers of organosilicon or alkylsilane derivatives

Typical formulas of alkylsilanes are RSiX3, R2SiX2 or R 3SiX, where X is chloride or alkoxy group (carbon and hydrogen chain group singular bonded to oxygen: R—O) and R is a

carbon chain that can bear different functionalities, such as amine or pyridyl. The formation of

monolayers is simply by reacting alkylsilane derivatives with hydroxylated surfaces such as Si02,Ti02.

In a typical procedure, a hydroxylated surface is introduced into a solution (e.g. ≈ 5×10 -3

M) of alkyltrichlorosilane (C3H5Cl3Si) in an organic solvent (e.g. a mixture of 80/20 Isoparafin-

G/CCl4) for a few minutes (e.g. 2-3 min.). A longer immersion time is required for surfactants

with long alkyl chains. A reduction in surfactant concentration in solution takes longer time to

form a complete monolayer.

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The ability to form a complete monolayer is obviously dependent on the substrate, or the

interactions between the monolayer molecules and the substrate surface. After immersion, the

substrate is rinsed with methanol, DI water and then dried. Organic solvent is in general requiredfor the self-assembly for the alkylsilane derivatives, since silane groups undergo hydrolysis and

condensation reaction when in contact with water, resulting in aggregation. For alkylsilanes with

more than one chloride or alkoxy groups, surface polymerization is commonly invokeddeliberately by the addition of moisture, so as to form silicon-oxygen-silicon bonds between

adjacent molecules as sketched in Figure.

Fig. : Alkylsilanes with more than one chloride or alkoxy groups, surface polymerization capable of forming silicon-

oxygen-silicon bonds between adjacent molecules as commonly invoked deliberately by the addition of moisture.

The fabrication of oxide-metal core-shell nanostructures is heavily relied on the

formation of an organic monolayer linking core and shell materials. For example, in a typicalapproach to the fabrication of silica-gold core-shell nanostructures, organosilicon with amine as

a functional group is used to form a monolayer on the surface of silica nanoparticles by self-assembly. The surface amine groups then attract gold nanoclusters in the solution, which result inthe formation of a gold shell.

One of the ultimate goals of using SA films is the construction of multilayer films thatcontain functional groups that possess useful physical properties in a layer-by-layer fashion.

Examples of those functional groups include electron donor or electron acceptor groups,

nonlinear optical chromophores, moieties with unpaired spins. The construction of an SA

multilayer requires that the monolayer surface be modified to be a hydroxylated surface, so thatanother SA monolayer can be formed through surface condensation. Such hydroxylated surfaces

can be prepared by a chemical reaction and the conversion of a non-polar terminal group to a

hydroxyl group. Examples include a reduction of a surface ester group, a hydrolysis of a protected surface hydroxy group, and a hydroboration-oxidation of a terminal double bond.

Oxygen plasma etching followed with immersion in DI-water also effectively makes the surface

hydroxylated. A subsequent monolayer is added onto the activated or hydroxylated monolayer through the same self-assembly procedure and multilayers can be built just by repetition of this

process. Figure below shows such a SA multilayer structure. However, it should be noted that in

the construction of multilayers, the quality of monolayers formed by self-assembly may rapidlydegrade as the thickness of the film increases.

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Fig.: Schematic showing the process of formation of self-assembled multilayer structure

2.4.2. Self-assembled monolayers of alkanethiols and sulfides

Monolayers of alkanethiols (R-S-H group) on gold surfaces are an extensively studied SA

system. Sulfur compounds (-S-H group) can form strong chemical bonds to gold, silver, copper

and platinum surfaces. When a fresh, clean, hydrophilic gold substrate is immersed into a dilutesolution (e.g. 10-3M) of the organosulfur compound in an organic solvent, a closely packed and

oriented monolayer would form. However, immersion times vary from a few minutes to a few

hours for alkanethiols, or as long as several days for sulfides and disulfides. For the self-assembly of alkanethiol monolayers, 10-3 M is a convenient concentration widely used for most

experimental work, a higher concentration such as 10-2M can be used for simple alkanethiols.

Although ethanol has been used in most experiments as the preferred solvent, other solvents may

be used. One important consideration in choosing a solvent is the solubility of alkanethiolderivatives. It is recommended to use a solvent that does not show a tendency to incorporate into

the two-dimensional system and examples include ethanol, THF, acetonitrile, etc.

It is found that sulfur, phosphorus strongly interact with the gold surface, resulting in the

formation of a closely packed, ordered monolayer. It is also found that thiol group forms the

strongest interaction with the gold surface over all the head groups studied.

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2.4.2.1. Self-assembly of alkanethiol on gold

Much of the recent work on self-assembling organic monolayers at metal surface hasfocused on the adsorption kinetics of alkanethiolate/gold system, where alkanethiols adsorb

spontaneously onto the metal surface to form highly ordered array.

a) Preparation of Self assembled monolayer films

Gold films are prepared by immersion into alkanethiol solution. The gold substrates haveto be handled with preventive measures to keep them clean and without contaminations.

The dimensions of substrate pieces have to be 0.5×0.5cm. the immersion time depends on

the thiol solution but normally for a good quality of SAMs, the small pieces of gold areimmersed into the solution for 24h. The concentration of these solutions, used varies from 0-

20μM to 0.1 mM depending on the solubility of thiol substances. 24 h after immersion, the gold

substrate are removed from incubation solution and rinsed carefully with absolute ethanol or

dichloromethane and again with ethanol to remove the physisorbed overlayers. Afterwards thesubstrates were dried in nitrogen stream.

b) Self-assembly kinetics and mechanism

Figure below shows that the thiol molecules will adsorb on gold surface and they first

will form so called striped-phase with their molecular axis parallel to the surface. The adsorption process of thiols onto gold can be divided into two steps. First step takes less than 10s to finish

(sulfur adsorption) and 10h for completion of second step (orientation ordering). The orientation

ordering is governed by the interchain interactions.

Fig.: Schematic mechanism diagram for the self-assembly of thiols on Au: a) Initial adsorption, b) Striped phase or

lying-down phase, c) 2D phase with transition from lying-down to standing phase and d) Formation of a complete

SAM.

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The formation of SAMs may be considered formally as an oxidative addition of the S-H

bond to the gold surface, followed by a reductive elimination of hydrogen. X-ray photoelectron

spectroscopy experiments suggest that chemisorption of alkanethiols on gold (o) surface yieldsthe gold (I) thiolate (R-S) species. The adsorption chemistry is

R-SH + Aun

0

R-S-

- Au+

+ 1/2H2 + Aun-1

0

The bonding of the thiolate group to the gold surface is very strong (the hemolytic

strength is approximately 160 KJ/mol)

Fig.: Constant-current STM topograph of an octanethiol monolayer on Au.

2.4.3. Monolayers of carboxylic acids, amines and alcohols

Spontaneous adsorption and self-arrangement of long chain alkanoic acids on oxide andmetal substrates have been another widely studied self-assembly system. The most commonly

used head groups include -COOH, -OH and -NH2, which ionize in the solution first and then

form ionic bond with substrates. Although the interaction between head groups and substrate plays the most important role in self- assembly and thus determines the quality of the resultant

SA monolayers, the alkyl chains also play an important role. In addition to the interchain van der

Waals and electrostatic interactions, alkyl chains may provide space for better arrangement of

head groups resulting in the formation of closely packed SA monolayers or restrict packing andordering in the self-assembly, depending on the molecular structures of alkyl chains.

SA monolayers have been exploited for applications of surface chemistry modification,introduction of functional groups on the surface, construction of multilayer structures. SA

monolayers have also been used to enhance the adhesion at the interface. Various functional

groups can also be incorporated into or partially substitute alkyl chains in surfactant molecules.SA monolayers have also be used in the synthesis and fabrication of core-shell nanostructures

with silane groups linking to oxides and amines linking to metals. Self-assembly is a wet

chemical route to the synthesis of thin films, mostly organic or inorganic-organic hybrid films.

This method is often used for the surface modification by formation of a single layer of

molecules, which is commonly referred to as self-assembled monolayer (SAM). This method hasalso been explored to assemble nano-structured materials, such as nanoparticles into an ordered

macroscale structures, such as arrays or photonic band-gap crystals. Arguably, all spontaneousgrowth processes of formation of materials such as single crystal growth or thin film deposition

can be considered as self-assembly process. In those processes, growth species self-assemble at

low energy sites. Growth species here for self-assembly are commonly atoms. For moreconventional definition of self-assembly, the growth species are commonly molecules. However,

nanoparticles or even micron-sized particles are also used as growth species for self-assembly.

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2.5. Core-Shell Structures

Core-shell structures of novel metal-oxide, novel metal-polymer, and oxide-polymer systems mostly fabricated by self-assembly methods. Discussion on metal-oxide structure is

done in the following section.

2.5.1. Metal-oxide structures (Gold-silica)

We shall take gold-silica core-shell structure as an example to illustrate the typicalexperimental approaches. Gold surface has very little electrostatic affinity for silica, since gold

does not form a passivation oxide layer in solution, and thus no silica layer will form directly on

the particle surface. Furthermore, there are usually adsorbed organic monolayers on the surfaceto stabilize the particles against coagulation. These stabilizers also render the gold surface

vitreophobic. A variety of thioalkane and aminoalkane derivatives may be used to stabilize gold

nanoparticles. However, for the formation of core-shell structures, the stabilizers are not only

needed to stabilize the gold nanoparticle by forming a monolayer on the surface, but also

required to interact with silica shell. One approach is to use organic stabilizers with twofunctionalities at two ends. One would link to gold particle surface and the other to silica shell.

The simplest way to link to silica is to use silane coupling agents. (3-aminopropyl)trimethoxysilane (APS) has been the most widely used complexing agent to link gold core with

silica shell.

Figure sketched the principal procedures of fabricating gold-silica core-shell structures.

There are typically three steps. The first step is to form the gold cores with desired particle size

and size distribution. The second step is to modify the surface of gold particle from vitreophobic

to vitreophilic through introducing an organic monolayer. The third step involves the depositionof oxide shell. In the first developed fabrication process, gold colloidal dispersion is first

prepared using the sodium citrate reduction method, resulting in the formation of a stablecolloidal solution with gold nanoparticles ≈15nm and 10% dispersity.

Fig.: Principal procedures for the formation of gold-silica core-shell structures. (a) Formation of monosized gold

particles, (b) modifying the surface of gold nanoparticles by introducing a monolayer of organic molecules through

self-assembly, and (c) deposition of silica shell.

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In the second step, a freshly prepared aqueous solution of APS (2.5 mL, 1 mM) is added

to 500mL of gold colloidal solution under vigorous stirring for 15 min. A complete coverage of

one monolayer of APS is formed on the gold particle surface. During this process, the previouslyadsorbed, negatively charged citrate groups are displaced by APS molecules, with the silanol

groups pointing into solution. The process is driven by the large complexation constant for gold

amines. The silane groups in APS molecules in aqueous solution undergo rapid hydrolysis andconvert to silanol groups, which may react with one another through condensation reactions to

form three-dimensional network. However, the rate of condensation reaction is rather slow at

low concentration. It should also be noted that during the self-assembly of APS on the surface of gold particles, the pH needs to be maintained above the isoelectric point of silica, which is 2-3,

so that the silanol groups is negatively charged. In addition, the pH is required to ensure the

adequate negative surface charge on the gold nanoparticles, so that the positively charged amino

groups are attracted to the gold surface.

In the third step, a silica sol prepared by slowly reducing the pH of a 0.54wt% sodium

silicate solution to 10-11 is added to the gold colloidal solution (with a resulting pH of -8.5)

under vigorous stirring for at least 24 hours. A layer of silica of 2-4nm thick is formed on themodified surface of the gold nanoparticles. In this step, slow condensation or polymerization

reaction is promoted by controlling the pH, so that the formation of a thin, dense and relativelyhomogeneous silica layer around the gold particle can be produced. Further growth of the silica

layer was achieved by transferring the core-shell nanostructures to ethanol solution and by

controlling the growth condition such that further growth of silica layer would be diffusion

predominant, which is often referred to as Stober method. Figure shows TEM images of gold-silica core-shell nanostructure.

Fig.: TEM Images of silica-coated gold particles produced during the extensive growth of the silica shell around 15

nm Au particles with TES in 4: 1 ethanol/water mixtures. The shell thicknesses are (a, top left) 10 nm, (b, top right)

23 nm, (c, bottom left) 58nm, and (4 bottom right) 83nm.

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3. Electrostatic self-assembly

3.1. Introduction

Self-assembly has emerged as a useful strategy for assembling molecular components

into complex aggregates. The basic principle of this technique is straightforward: Two oppositely

charged particles, suspended in a fluid, will attract. Electrostatic interactions have a longer rangethan hydrophobic or hydrogen-bonding interactions,

3.2. Fabrication method

Self-assembly through electrostatic interactions consisted of the preparation of a charged

surface, the fabrication of charged microstructures, and the assembly of these microstructures

onto the surface (Figure below).

3.2.1. Preparation of Charged Surfaces

Substrates with a uniform surface charge were made by immersing clean gold surfaces in

a 1mM ethanolic solution of alkanethiol for 10 min. Derivatization with HS(CH2)11 NH3+Cl-,

HS(CH2)11 NMe2 yielded positively-charged surfaces, with HS(CH2)15COOH andHS(CH2)11PO3H2 yielding negatively charged surfaces. Gold substrates with zero surface charge

were made by immersion of clean gold films in 1mM ethanolic solution of HS(CH 2)15CH3 or

HS(CH2)2(CF2)5-CF3.

A clean silicon surface with native oxide as a negatively-charged surface (the surface Si-

OH groups have p K a ≈2-4 and are extensively ionized in water at pH 7) and an HF-treatedhydrogen-terminated silicon surface as a neutral surface (with 1-5% residual Si-OH groups) areused.

Surfaces with patterned charge were fabricated by microcontact printing, liftoff, or etching (Figure a). Substrates patterned by microcontact printing were made by stamping a gold

film for 1 min with a poly-(dimethylsiloxane) stamp inked with HS(CH2)15CH3 or

HS(CH2)15COOH, washing for 30 sec with a 1mM ethanolic solution of charged thiol, and

washing for 15 s with 1% HCl/EtOH to remove any electrostatically bound material.

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Fig.: Diagram of experimental methods: (a) Fabrication of patterned surfaces; (b) Fabrication of charged goldmicrostructures; (c) Assembly of particles.

3.2.2. Fabrication of Charged Gold Microstructures

Gold disks were fabricated by electro-deposition in a photoresist mold. Our strategy was

to electroplate gold directly on a silicon surface and to rely on weak adhesion between gold andsilicon to enable subsequent removal of the gold particles. This approach required that the

cathode consist of a metallic surface sandwiched between an insulator and the silicon surface

(Figure b). The silicon wafers used had a sufficient high resistivity that plating withoutembedded electrodes resulted in non-uniform electro-deposition. Cathodes were made by

evaporating 200 nm of gold onto the rough backside of hydrogen terminated silicon wafers andcuring a thin film of polyurethane (Norland Optical Adhesive) onto the gold or by evaporating200 nm of gold, followed by 50-100 nm of silicon, onto the smooth side of an oxidized silicon

wafer. The cathodes were patterned with photoresist, washed with 1% aqueous HF (with a few

drops of Triton X-100 to enhance wetting) to remove any native silicon oxide, and plated to

deposit gold in regions not masked by photoresist.

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A 5 cm×5 cm platinum sheet served as the anode. Following plating, the cathodes were

rinsed with acetone to remove the photoresist and sonicated in a beaker of charged thiol solution:

Sonication separated the plated gold disks and allowed a SAM to form as their surfaces. Theresulting suspension of SAM-covered disks was filtered through a 20-ím nylon mesh and left

under thiol solution for several hours. Once the charged SAM had formed, these gold disks could

be further coated with an oppositely charged polymer (hexadimethrine bromide for negatively-charged gold, sodium poly (vinylsulfonate) for positively-charged gold) by immersion in a

20mM aqueous solution of the polymer for 10-30 min.

3.2.3. Assembly of Charged Particles onto Surfaces

Gold particles covered with an ionized SAM were placed in a glass test tube with

approximately 3 ml of solvent; the ionized SAM stabilized the gold particles against aggregation(Figure c). Wafers with patterned surface charge were cut into 0.5-cm × 3-cm pieces and placed

in the same test tube. The test tube was then agitated to suspend the gold particles and reagitated

whenever the particles settled. After a few minutes, the wafer, along with any attached gold

particles, was carefully removed from the test tube, placed in a dish of heptane, and removedfrom heptane and allowed to dry. (The treatment with heptane reduces capillary forces after

removal from solvent.)

Fig.: SEMs of patterns of disks (or stars) after electrostatic self-assembly in ethanol: (a) NMe3+-terminated gold

disks, assembled on gold surface patterned with HS(CH2)15COOH and HS(CH2)11 NMe3+; (b)PO3H

--terminated gold

disks, assembled on a gold-covered capillary patterned with HS(CH2)15COOH and HS(CH2)11 NMe3+; (c)

hexadimethrine bromide-coated gold stars, assembled in silicon grooves. The gold mesas were covered with NMe3+-

terminated SAMs.

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4. Microcontact and microdisplacement printing

4.1. Microcontact printing

Microcontact printing is a technique that forms patterned SAMs with geometrically well

defined regions of different chemical functionality and thus different physical and chemical

properties. This technique uses an elastomeric ‘stamp’ and alkanethiol ‘ink’ to form patterned

SAMs of alkanethiolates on gold films with dimensions ranging from 200 nm to several cm.Patterned SAMs formed have many applications, including microfabrication, studies of wetting

and nucleation phenomena, protein and cellular adhesion; and in analytical studies involving

scanning electron microscopy, and scanning probe microscopies. In this paper, we focus onapplications related to the fabrication of structures with micrometer and sub-micrometer

dimensions.

Microcontact printing transfers by contact alkanethiol ‘ink’ from an elastomeric ‘stamp’

to a gold surface: if the stamp is patterned, a patterned SAM forms

The stamp is fabricated by casting polydimethylsiloxane (PDMS) on a master having thedesired pattern. Masters are prepared using standard photolithographic techniques, or constructed

from existing materials having microscale surface features.

Fig.: Schematic of the procedure for microcontact printing of SAMs.

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We ‘inked’ the elastomeric stamp by exposing the stamp to a 0.1–1.0 mM solution of

alkanethiol in anhydrous ethanol, usually by rubbing the stamp gently with a Q tip saturated with

inking solution. The stamp dried until no liquid was visible by eye on the surface of the stamp(typically about 60 s), either under ambient conditions, or by exposure to a gentle stream of

nitrogen gas. Following inking, the stamp was applied by hand to a gold surface. We used 50–

2000 A°-thick gold films, prepared by electron-beam evaporation on Si wafers, with 10–100 A°of Ti as an adhesion promoter (between the gold and the Si). Very light hand pressure aided in

complete contact between the stamp and the surface. The stamp was then peeled gently from the

surface

Fig.: (a) Scanning electron micrographs of masters used to cast elastomeric stamps. (d) Scanning electron

micrographs of patterned SAMs formed by μCP with stamps cast from masters in (b)

4.2. Microdisplacement printing of self-assembled monolayer

Microdisplacement printing uses a preassembled monolayer that is well ordered to protect the surface, but is sufficiently labile such that other molecules can displace it through

competitive adsorption. Stamping on such a surface directs the molecules to adsorb where their local concentration is highest and holds the ink molecules in place once adsorbed during the

stamping procedure. This procedure also limits the need for solution processing because the

unpatterned regions are prefilled.

Microdisplacement printing ( μDP) is done using 1-adamantanethiolate (AD) as weak and

labile preassembled monolayer that are selectively replaced with more strongly bound molecules

to create patterned SAMs.

Microcontact printing ( μCP) has been proposed for applications such as microelectronics,chemical and biological sensing biologically compatible surfaces and microfabrication becauseof the rapid and straightforward methodology that produces high-resolution patterns over large

printing areas. However, as discussed in previous reports one limitation of μCP is the lateral

spreading of the molecules used for patterning (“ink”) across the substrate surface during thetime the stamp is in contact with the substrate.

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Fig.: Schematic depicting microdisplacement printing on a 1-adamantanethiolate self-assembled monolayer with an11- mercaptoundecanoic acid-inked stamp. A 1-adamantanethiolate self-assembled monolayer is first formed on

gold by solution deposition for 24 h. Then, the molecularly inked stamp is contacted directly onto the 1-

adamantanethiolate, resulting in patterned regions of both 1-adamantanethiolate and 11-mercaptoundecanoic acid

that mirror the relief pattern on the stamp.

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5. Applications of Self-assembly

• Self assembly in manufacturing of ICs, transistors, chips and other electronic circuit

components.

• Self-assembly of semiconducting oxide nanorods, and nanoribbons.

• Current research in nanoscale semiconducting nanowires for devices applications span

from field-effect transistors, bio/chemical sensors, ultra-violet lasers, light

emittingdiodes, and photo-detectors.

• Nanoscale magnets, in the size regime ranging from a few nm to ~100 nm, have great

technological importance in magnetic data storage and sensor technology as well as for

biomedical applications.

• Nanoscale self-assembly of thin-film molecular materials for electro-optic switching.

• Improved Self-Assembly of Nanomaterials May Enhance Solar Cells

• Self-assembly in magnetic nanowire arrays and microarrays production.

Other applications:

1. Fabrication of Arrayed LEDs by Self-Assembly: Applicable in near eye micro-displays,

video camera, automotive display units, etc.

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2. Arrayed polymer micro lenses: Used in imaging like in endoscope, camera lens, etc.

3. Organic thin film transistors (OTFT): Used in flexible electronic devices.

4. Lipid molecules and cell membranes: Applicable in drug release, chemical sensor,

nanoreactors, chemical sensors and microelectronics.

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Tobacco mosaic virus: Applicable in electrodes of micro-batteries for MEMS.

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Conclusions

Of the diverse approaches possible for Molecular Self-Assembly, two strategies have

received significant research attention – “Self-Assembled Monolayers (SAMs) and Electrostatic

Self-Assembly (or layer- by-layer assembly) due to the easiness in processing.

For SAMs, synthetic chemistry is used only to construct the basic building blocks (that is,

the constituent molecules), and weaker intermolecular bonds such as Van der Waals bonds areinvolved in arranging and binding the blocks together into a structure. This weak bonding makes

solution, and hence reversible, processing of SAMs (and in general, MSAs) possible. Thus,

solution processing and manufacturing of SAMs offer the enviable goal of mass production withthe possibility of error correction at any stage of assembly.

It is well recognized that Self-assembly method could prove to be the most cost-effective

way in production of nanostructures applicable in various fields especially for the semiconductor

electronics industry to produce functional nanodevices such as nanowires, nanotransistors, andnanosensors in large numbers.

Self-assembly has big role to play in future in synthesis of many perfect and cost-

effective nanostructures relating to various fields.

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nano materials self-assembly seminar final

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