Topic guide 11.2: Key concepts used to solve ... · PDF file2012-11-02 · Key concepts used to solve ... crystallisation occurring inside a droplet of water ... the surface tension

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Unit 11: Nanotechnology

In this topic guide you will look at the scientific principles that underpin the main fabrication methods used to generate nanoparticles. Such techniques include both top-down and bottom-up approaches. These methods, and the characterisation techniques used in quality control, rely on principles from across the scientific disciplines of chemistry, physics and biology.

You will be introduced to the main types of nanofabrication processes and the scientific principles underlying these techniques are described. Some case studies will then be examined to show how the scientific principles link to specific nanofabrication techniques. Finally, you will look at the characterisation methods used in the quality control of nanofabrication.

On successful completion of this topic you will: • understand key concepts in engineering, physics, chemistry and biology

used to solve nanotechnology problems (LO2).

To achieve a Pass in this unit you need to show that you can: • explain the principles of surface/colloid chemistry (2.1) • discuss thin film deposition and characterisation processes (2.2) • explain chemical templating (2.3) • compare imaging techniques for quality control in nanofabrication (2.4).

Key concepts used to solve nanotechnology problems11.2

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11.2: Key concepts used to solve nanotechnology problems

1 NanofabricationFabricating nanostructures can take place by ‘bottom-up’ or ‘top-down’ approaches, as shown in Figure 11.2.1.

In top-down fabrication, conventional macroscopic processes (such as lithography) are miniaturised down to the nanoscale. The basic materials of top-down nanofabrication are layers (e.g. thin films).

Bottom-up fabrication involves the manipulation of individual atoms into specific arrangements; this may involve the use of nanomachines to automate the process or may rely on spontaneous processes that create ordered crystals and colloids. The basic materials of bottom-up nanofabrication are atoms and molecules.

Table 11.2.1 compares the two methods.

Table 11.2.1: A comparison of top-down and bottom-up fabrication methods.

Top-down fabrication methods Bottom-up fabrication methods

nanolithography chemical vapour growth (VLS growth)

templated synthesis colloidal self-assembly

solution-based and vapour-phase synthesis dynamic self-assembly

additive deposition processes (e.g. sputtering)

manipulating individual atoms (e.g. using an atomic force microscope)

milling biomimetics (e.g. DNA-based nanosynthesis)

Many of these techniques are described in detail in this topic guide or in Topic guide 11.3.

Some techniques combine features of both types of fabrication and are known as hybrid techniques.

Take it furtherIntroductory material: Three helpful PowerPoint® presentations are available through the NACK network at http://nano4me.live.subhub.com/categories/modules (as explained in Topic guide 11.1, these resources require a (free) registration).

A helpful PowerPoint® presentation giving an overview of fabrication techniques can be found at: http://www.matecnetworks.org/webinars/pdf/Fabrication.pdf. Simple animations are used to illustrate some of the techniques discussed.

An academic but approachable discussion of the different meanings of the term self-assembly can be found at http://www.sciencedirect.com/science/article/pii/S1369702109701567.

Top-down fabrication: NanolithographyConventional lithography

Lithography is the process used to produce the patterns of semiconductor materials that make up integrated circuits.

Figure 11.2.1: Nanoparticles can be fabricated using top-down

or bottom-up approaches.

Bulk

Powder

Nanoparticle

Clusters

Atoms

Top down

Bottom up

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The key stages in this process are shown in Figure 11.2.2 and involve the following operations:

• Coating a substrate with a thin film of substance known as a resist. • Transferring a pattern onto the film. This is done by exposing the resist to

electromagnetic radiation using a mask to prevent some of the radiation reaching the resist.

• Removing material from the resist to create the desired structure. The principle is that exposure to radiation may make the resist more soluble in a certain solvent; treatment with the solvent removes the exposed resist – this is positive developing of the pattern. In negative developing, the resist becomes less soluble in the solvent and so the unexposed resist is removed.

Further etching and stripping processes enhance the pattern.

Resist

PositiveNegative

Substrate

Mask

hυ

Coating

Exposure

Develop

Etch

Strip

In nanolithography, these techniques are adapted to enable the deposition of very thin films and to enable patterns with nanoscale dimensions to be transferred to the film. The details of these fabrication processes will be covered in Topic guide 11.3.

ActivityUse research to suggest why making integrated circuits on a nanoscale might be important.

Top-down fabrication: Solution-based synthesisThis is probably the cheapest and easiest method of manufacturing nanoparticles. Essentially this uses the familiar process of crystallisation, but under carefully designed conditions to control the size and shape of the particles formed.

The process involves three stages: • dissolving the reagents (and any additives) in a suitable solvent • forming stable solid particles which will act as nuclei for further crystallisation • adding material from the solution to these nuclei.

Key termsSubstrate: The semiconductor material upon which the resist layer is deposited.

Resist: A thin layer onto which a pattern can be transferred.

Mask: A structure that contains the geometric pattern that is transferred to the resist.

Etching: The process of selectively removing substrate material which is no longer protected by the resist.

Figure 11.2.2: The process of conventional lithography.

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The reaction can be carried out at ambient temperature and does not require sophisticated and specialised equipment. Such fabrication processes are described as soft chemical fabrication. Surfactant molecules are used as stabilisers to prevent redissolving and subsequent reprecipitation, which would create particles that would be larger than desired.

Activity • Find some examples of substances and structures that have been produced using soft chemical

fabrication. • Identify any which could be classified as nanostructures.

Top-down fabrication: Vapour-phase synthesisVapour-phase synthesis can be used to produce both nanoparticles and thin films (for use as coatings or in nanolithography), as shown in Figure 11.2.3.

Substrate

Vapour incarrier gas

(a)

(b)

Vapour incarrier gas

Nanoparticle

HEATER

HEATER

HEATER

Thin film

Nanoparticles

The principles of nucleation and growth of nanoparticles described in the section on solution-based synthesis can also be used to form nanoparticles from a supersaturated vapour.

Solid reagents are vapourised and then passed into a heated chamber. Nanoparticles are collected downstream, as shown in Figure 11.2.3 (b).

Control of particle size is more difficult in the vapour phase as surface stabilisers cannot easily be added, as they are for solution-based synthesis. Instead, particle size is controlled by, for example, controlling the rate of flow of the condensing gas.

Nanochemicals produced by this method include oxides, nitrides and carbides – for example SiO2, TiO 2, ZrN and SiC.

Key termSoft chemical fabrication: A chemical process carried out in an open reaction vessel that uses reactions at ambient temperature; often there are similarities between the reactions involved and those occurring in biological systems.

Figure 11.2.3: Different vapour-phase techniques are used to fabricate (a)

thin films and (b) nanoparticles.

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Thin films and templating

Deposition from a hot vapour can occur on a solid substrate, as shown in Figure 11.2.3 (a). This produces thin films and is an enormously important technique since it is the primary method for creating the thin nanoscale films that are required for nanolithography. The substrate is described as a template, as it plays a part in directing the assembly of the thin film and hence determines its physical structure.

You will read more about the applications of thin film technology in Topic guide 11.3.

Take it furtherInorganic Chemistry (5th edition) (Shriver and Atkins, OUP, 2010) has a useful section on solution-based and vapour-phase synthesis of nanoparticles (p658–662).

Top-down fabrication: Chemical templatingSolution-based synthesis uses reaction conditions to control the size of the resultant particles.

Chemical templating is often used in solution-based synthesis to control and organise the product molecules in some way. This will be discussed further in Topic guide 11.3. A simple example of chemical templating is the use of nanoscale reaction vessels – for example, crystallisation occurring inside a droplet of water encased in a reverse micelle. The small size of the vessel helps to control the size and shape of the crystals that form.

ActivityFind an example of a simple synthetic reaction that makes use of chemical templating, for example, in coordination chemistry, and explain the advantage of using chemical templating in this process.

Take it furtherIntroductory material: A presentation (aimed at US high school students) to introduce a range of top-down fabrication processes is available from NACK at http://nano4me.live.subhub.com/ downloads/20090529_3 (as explained in wTopic guide 11.1, these resources require a (free) registration).

Although not requiring much in the way of high-level science, the presentation goes into some detail about the fabrication processes and will also be helpful for Topic guide 11.3.

Bottom-up processes: Layer-by-layer self-assemblyIn previous sections you have seen how thin layers can be formed by top-down techniques such as vapour-phase deposition.

However, bottom-up processes, known as self-assembly processes, can also be used to form thin layers; two-dimensional nanostructures such as graphene sheets are formed in this way. The process by which layers are formed by self-assembly relies on electrostatic interactions between a surface and the molecules that assemble on it.

Key termsTemplating: The use of a template to direct the formation of a nanostructure.

Template: A substance or platform used to direct the formation of a nanostructure. This can be a solid surface, a nanoscale reaction vessel, a nanoporous membrane or a self-assembled structure such as a functionalised fullerene.

Self-assembly: A process in which disordered components (such as molecules) form an organised structure. This is due to the innate interactions between the components rather than any external factor.

http://nano4me.live.subhub.com/downloads/20090529_3

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Bottom-up fabrication: Colloidal self-assemblySome biological materials, such as fatty acids and phospholipids, are described as amphiphiles – they have both hydrophilic and hydrophobic components (see Figure 11.2.4). Surfactants are common examples of amphiphiles; they reduce the surface tension of a liquid or the interfacial tensions at the boundary of two liquid phases.

Figure 11.2.4: Amphiphiles have both hydrophilic and hydrophobic groups.

O

O-

Hydrophilic ‘head’

Hydrophobic(or lipophilic) ‘tail’

Figure 11.2.5: Above a certain critical concentration, amphiphiles assemble into micelles.

Above a certain threshold concentration, amphiphiles in a solvent (usually water) will assemble themselves into a micelle, as shown in Figure 11.2.5. This is a familiar process in biological systems where phospholipids form a particular type of micelle in which a bilayer of amphiphiles is present as shown in Figure 11.2.6. These form two-dimensional bilayer sheets used to construct the plasma membrane of cells, or smaller vesicles within cells.

The thickness of a phospholipid bilayer is approximately 5 nm and a typical vesicle diameter is in the range 30–50 nm so these can be safely classified as biological nanostructures.

ActivityArtificial membranes can now be fabricated in a process that mimics biology (a biomimetic process). Use research to find a possible application for such artificial membranes.

Nanoscale micelles containing synthetic polymer molecules, such as polystyrene-b-poly(acrylic acid) (PS-b-PAA), can also be formed spontaneously, as discussed in the next section.

Complex adaptive systemsClearly, as in the previous example of vesicle and membrane formation by living cells, biological systems have the capability to synthesise nanostructures and to interlink them in order to carry out specific functions. These linked structures are responsive – or adaptive – to changes in their environment and can be classified as a complex adaptive system (a term applied to a range of real world systems from macroeconomics to computing systems).

Some writers have suggested that if synthetic nanostructures were linked together, the overall structure would indeed be a complex adaptive system – these might share many of the attributes of a living system, although it would remain fully artificial.

LinkThis section links with ideas about surfactants in Unit 6: Physical chemistry of spectroscopy, surfaces and chemical and phase equilibria, Topic guide 6.4.

Key termsAmphiphile: A chemical compound possessing both hydrophilic and lipophilic properties.

Micelle: An aggregate of amphiphilic molecules dispersed in a liquid, such as water.

Complex adaptive system: A complex system comprising a macroscopic collection of microstructures that are relatively similar and partially connected that is formed in order to adapt to the changing environment, and increase its survivability as a macrostructure.

Figure 11.2.6: Membranes in biological systems consist of two‑dimensional

bilayers of amphiphiles.

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Drug delivery systems

A good example of how a complex adaptive system could be created to be adaptive to its environment might be a ‘smart’ drug delivery system. It might contain the following components:

• a lipid envelope to contain the drug • molecules on its surface to functionalise it in order to be able to recognise

particular biological systems • an imaging system (for example, magnetic nanoparticles as an MRI contrast agent) • a core of gold nanoparticles, which can absorb radiation and re-emit it as heat

to help destroy certain target cells • an ability to react to certain physiological changes, such as changes in pH.

•

ActivityExplain why this drug delivery system can be classified as a complex adaptive system.

Take it furtherIntroductory material: NACK has an introductory-level presentation about bottom-up fabrication at http://nano4me.live.subhub.com/downloads/20090529_4 (as explained in Topic guide 11.1, these resources require a (free) registration).

Some examples of what might be termed ‘complex adaptive systems’ can be found in the appendix of Nanochemistry (Ozin and Arsenault, 2005), for example, chemically driven nanorod motors and muscle-micromechanical hybrid systems.

2 Surface and colloid chemistryIn this section you will be reminded of some key principles of surface and colloid chemistry. Case studies will then be used to show how these are applied in particular examples of nanofabrication processes.

Principles of colloid chemistryColloids

A colloid is defined as a system in which fine particles (of between 1 and 1000 nm in diameter) are dispersed evenly through another substance.

The dispersed particles are known as the dispersed phase, and the substance through which they are dispersed is known as the dispersion medium. In an emulsion, for example, the dispersed phase and dispersion medium are different liquids – such as oil and water.

In Section 1 of this topic guide you saw how micelles play a role in self-assembly and chemical templating. Micelles are an example of an association colloid; they are formed from surfactants that possess hydrophobic and hydrophilic groups.

Examples of other colloids that are significant in nanofabrication include sols and gels.

LinkThis section links back to Unit 6: Physical chemistry of spectroscopy, surfaces and chemical and phase equilibria, Topic guide 6.4.

Key termsSurfactant: A substance that reduces surface tension or interfacial tension (for example, at the boundary between two liquids).

Hydrophobic: Substances that do not form forces of attraction to water molecules (and are therefore repelled by water molecules).

Hydrophilic: Substances that can form forces of attraction to water molecules.

Sol: A colloidal dispersion of solid particles in a liquid.

Gel: A ‘pseudo-solid’ in which a network of filamentous solid particles encloses the dispersed phase of a colloid.

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Case study: Micelles

Micelles in colloidal self-assemblyPolystyrene-b-poly(acrylic) acid (PS-b-PAA) is an example of a block co-polymer; these are polymers consisting of two segments (or blocks) of chain, one formed from one monomer and the other from a different monomer. Its structure is shown in Figure 11.2.7.

O O-

Y

x

PS-block PAA-block(ionised)

y

X

Figure 11.2.7: The structure of PS-PAA showing the two blocks (and the terminating groups X and Y).

The polystyrene (PS) block, consisting of a simple hydrocarbon chain, is hydrophobic, while the poly(acrylic) acid (PAA) block is partially ionised in water and is hydrophilic.

Micelles will be formed when these polymers are added to water; the shape and size of the micelle will vary depending on the ratio of PS:PAA.

Block co-polymer micelles have been suggested as possible delivery systems for drugs; the drug can be trapped within the hydrophobic centre of the micelle and the hydrophilic surface. This surface is designed to biodegrade within the body to release the drug at a controllable rate.

ActivityRepresenting PS-b-PAA by the structure shown in Figure 11.2.8, sketch out the structure of a micelle that will be formed from molecules of this substance in aqueous solution. Indicate where the drug molecule might be encapsulated when the micelles are used for a drug delivery system.

X YPS PAA

Figure 11.2.8: Simplified structure of PS-b-PAA block co-polymer.

Take it furtherYou can read more about this process in Nanochemistry (Ozin and Arsenault, 2005), p451–452. Introduction to Nanoscience (S.M. Lindsay, OUP, 2010), p214–218, includes a rather technical mathematical treatment of the factors affecting micelle formation and micelle shape.

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Reverse micelles in chemical templating

A reverse micelle is where an aqueous phase is dispersed in a non-polar dispersion medium, stabilised by the presence of amphiphilic surfactant molecules.

In the case of reverse micelles, it is the hydrophobic groups that form the external surface of the micelle, as shown in Figure 11.2.9.

Hydrophilic group

Aqueous solution of reactants

Hydrophobic group

This water droplet enclosed by the surfactant layer forms a nanosized reaction vessel, and solution-based synthesis can take place within it. The advantage of this is that the size of the nanoparticles formed is controlled by the volume of the micelle and this in turn is determined by factors such as the concentration of salts in the aqueous phase, and the composition of the micelle (for example, the ratio of different polar groups on the hydrophilic surface).

Take it furtherThe uses of reverse micelles are covered in Inorganic Chemistry (5th edition) (Shriver and Atkins, OUP, 2010), p662.

Principles of surface chemistrySurfaces, in the form of substrates or templates, play a critical role in nanofabrication techniques, such as thin film deposition or self-assembly.

Solid-gas interfaces

A solid surface exposed to a gas is being continually bombarded with gas molecules. These gas molecules will tend to become attached to the surface and form a layer covering the surface, a process described as adsorption.

• If the attachment is a result of the formation of weak forces, such as Van der Waals interactions, this is described as physical adsorption.

• If chemical bonds (usually covalent) are formed between the gas molecule and the surface, then this is described as chemisorption.

Mathematical models

The process of adsorption is a dynamic equilibrium involving gaseous molecules and a surface consisting of a fixed number of adsorption sites. Mathematical models, such as the Langmuir isotherm, are used to calculate how the fraction of the surface covered depends on the partial pressure of the gas.

Figure 11.2.9: In a reverse micelle, the hydrophobic groups form the

external surface of the micelle and the hydrophilic groups enclose a

droplet of aqueous solution.

LinkThis section links to Unit 6: Physical chemistry of spectroscopy, surfaces and chemical and phase equilibria, Topic guide 6.4, where the various mathematical models used to describe adsorption are described in some detail.

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At low partial pressures, monolayers of adsorbed gas tend to be formed, particularly if chemisorption is involved; higher pressures cause the breakdown of the Langmuir isotherm model and result in further adsorption of gas molecules on top of the monolayer surface. In fact, for other reasons, vapour deposition is carried out at very low pressures.

Solid-liquid interfaces

In processes such as layer-by-layer self-assembly, a charged surface is in contact with a solution containing a polyelectrolyte.

At the surface there will exist an electrical double layer, created by the attraction between the charged surface and the counter ions in solution. Various models have been used to describe the interaction between the charged surface and the ions in solution; the most useful is the Stern model (see Figure 11.2.10).

If a solid surface has a positive charge (as in the case study below), then the Stern model suggests that bonding between the surface and negative ions in the solution will create a layer of tightly held negative ions at the surface – creating the ’electrical double layer‘ or Stern layer. Further away from the surface, the concentration of negative ions gradually falls as the distance from the surface increases (the Gouy-Chapman layer).

Layer-by-layer self-assembly makes use of the formation of the electrical double layer to build up alternating layers of positively and negatively charged polyelectrolytes.

Case study: Layer-by-layer self-assembly (LbL)Surfaces, such as silicon or silica, can easily be primed (or functionalised) to give them a positive charge. For example, molecules such as alkoxysilanes can attach to the hydroxyl groups on the surface of silica, or aminoalkylthiol can chemisorb onto gold, as shown in Figure 11.2.11.

S

+NH3

Gold atom

S

+NH3

S

+NH3

S

+NH3

Figure 11.2.11: Gold can be primed to give its surface a positive charge.

If this surface is then alternately exposed to aqueous solutions containing first anionic polyelectrolytes and then cationic polyelectrolytes, a polyelectrolyte film consisting of layers with alternating positive and negative charge will be progressively built up (see Figure 11.2.12).

This process is a soft-chemical fabrication as it can be achieved without any specialised equipment and occurs at ambient conditions. It is also easily automated and can produce layers of any desired thickness, so is both cheap and highly controllable.

Each layer will have a thickness of less than 1 nm, so it is easy to control the process to produce nanoscale films.

Continued

Key termPolyelectrolyte: A solution containing ions with a high molecular mass (i.e. long chain organic molecules rather than simple inorganic ions).

Figure 11.2.10: The Stern model of the electrical double layer.

+

+

+

+

+

+

Sternlayer

Chargedsurface

Gouy-Chapmanlayer

+

+

+

+

+

+

+

+

LinkThis section links to Unit 6: Physical chemistry of spectroscopy, surfaces and chemical and phase equilibria, Topic guide 6.4, where the various models of the electrical double layer are described in more detail.

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++++

+++

+++

+

Gold surface

Anionic polyelectrolyte

Cationic polyelectrolyte

Figure 11.2.12: Layer-by-layer self-assembly builds up structures consisting of alternating charged layers.

ActivityUse research to find one application of a nanoparticle formed by layer-by-layer self-assembly on a gold surface.

ActivityResearch a layer-by-layer self-assembly process that starts with a silicon or silica surface. Identify the substance (if any) used to prime the silica/silicon surface and the polyelectrolytes used in the layer-by-layer assembly.

Take it furtherNanochemistry (Ozin and Arsenault, 2005) discusses LbL self-assembly in Chapter 3. Many specific examples are given to show how this approach is used to create a range of different materials.

3 Thin film applicationsThin film assemblyAs explained in Section 1 of this topic guide, lithography at both the micro and nanoscale relies on the ability to be able to deposit thin films on surfaces.

Take it furtherA detailed description of several thin film processes can be found in Introduction to Nanoscience (S.M. Lindsay, OUP, 2010), Chapter 5.

Inorganic Chemistry (5th edition) (Shriver and Atkins, OUP, 2010) explains the meaning of some of the terms used to describe these processes on p664–665.

Key termThin film: A layer of material that may range in thickness from a monolayer (less than 1 nm) up to several micrometres.

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These methods make use of high or ultra-high vacuum technology. The deposited material is evaporated and introduced into a high vacuum chamber that contains the surface that is to be coated with the thin film.

In order to achieve a completely pure layer, uncontaminated with molecules from air, a pressure of below 10–12 bar is required, but, in practice, higher pressures than this are used.

Epitaxial films

In many cases of thin film assembly, a monolayer will form at the start of the process (according to the Langmuir isotherm model). When subsequent layers of atoms are adsorbed onto this monolayer, the arrangement is somewhat random and there is no ordered three-dimensional structure in the resultant thin film. However, in other cases, the tendency to form a three-dimensional structure is much stronger, and the structure tends to grow outwards rather than forming a monolayer first, forming an epitaxial crystal, as shown in Figure 11.2.13.

(a) (b)

Different techniques are used depending on whether an epitaxial crystalline structure is required. Techniques include:

• Molecular beam epitaxy: A beam of atoms is fired at a substrate, creating a region of monolayer on the substrate. Subsequently, different atoms may be used to build up layers with different compositions. Pressure is kept very low to prevent contamination and low temperatures are used, which minimises the tendency of atoms to bounce back from the surface.

• Sputtering: This differs from other techniques in the method used to generate the vapour that is to be deposited. A beam of high-energy ions bombards a solid sample of the required material. Atoms of the material are then dispersed and will eventually coat the substrate to form a thin layer. It can be used for materials that are difficult to evaporate by heating, such as SiO2 (which has a very high melting point).

• Plasma deposition: In this technique, the vapour to be deposited is in the form of a plasma (a vapour containing ionised atoms or molecules). As a result, when the plasma is deposited, the surface will be positively charged, affecting the surface properties – this is useful in helping ink or adhesive to bond to the surface.

Take it furtherAn animation of epitaxial growth is available from the University of Cambridge Materials Science department at http://www.doitpoms.ac.uk/tlplib/epitaxial-growth/index.php.

ActivityResearch these processes in order to find some of the applications of each process.

Figure 11.2.13: (a) Epitaxial and (b) non-epitaxial growth.

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Characterisation processesAs you learned in Topic guide 11.1, one of the key events in the development of nanotechnology was the ability to be able to image individual atoms and other particles on the nanoscale, by the use of the atomic force microscope.

‘Seeing’ nanoscale systems, such as thin films, is known as characterisation and is a critical step in the control of the fabrication techniques discussed above. The following will be discussed in Section 5 of this topic guide:

• profilometry • ellipsometry • spectrophotometry • atomic force microscopy.

•

Take it furtherIntroductory material: A PowerPoint®–based lecture on characterisation methods is available from NACK at http://nano4me.live.subhub.com/downloads/20090506_3 (as explained in Topic guide 11.1, these resources require a (free) registration).

ActivityUse Topic guide 11.2, section 5 and any other suitable sources to read about these techniques. Which techniques would be most suitable for characterising the thickness of a thin film on a nanodevice?

Portfolio activity (2.2)Write a short report to compare the different techniques that exist to deposit thin films. In your answer you should:

• list the techniques • explain how they work and describe the differences between them • outline some typical applications of thin films produced by these techniques • comment on how the thin films produced can be characterised.

4 Chemical templatingTemplate-directed synthesis

ActivityLook back at Section 1 of this topic guide and find the reference to chemical templating. Explain why the process described in this section can be regarded as chemical templating.

Chemical templating, or template-directed synthesis, is a process by which a system is pre-organised to enable a specific chemical reaction to take place. In the context of nanoscience, this is done in order to create specific molecular structures.

The template interacts with the reactants by forming interactions such as Van der Waals interactions, hydrogen bonds and metal-ligand interaction to hold the

Key termCharacterisation: Producing images and obtaining data about the physical dimensions of nanoscale structures.

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reactive sites of the reactants close together to enable the desired reaction to occur. It can be regarded as analogous to the biological ‘lock and key’ mechanism by which enzymes facilitate the reaction of substrate molecules.

The template could be a simple metal ion, a polymer or a complex molecular structure.

Sol-gel synthesis

Templates are used in the process of sol-gel synthesis. In this technique, the nanoparticles are deposited from a sol as a gel – networks of polymers enclosing a solvent liquid.

Sol-gel techniques are common in macro- and micro-scale chemistry, for example, in the preparation of synthetic ceramics.

A solution (sol) of a precursor reactant, usually a metal alkoxide, is deposited on a substrate and condensation reactions occur to form the gel. Controlled evaporation of the enclosed solvent then allows the formation of solid nanoparticles.

The case study below shows how this is done in order to form titanium (IV) oxide nanoparticles.

Case study: Titanium oxide formationNanoparticles of titanium (IV) oxide can be formed in a sol-gel process on an alkoxide template (for example, titanium isopropoxide, as shown in Figure 11.2.14).

OTi

OO

O

Figure 11.2.14: Titanium isopropoxide.

A sol of titanium isopropoxide is obtained by using an ethanol-water solvent, and this is converted into a TiO

2 gel by hydrolysis of the alkoxide, followed by condensation reactions to form a gel

based on a TiO 2 network. Gradual evaporation of the water/ethanol solvent then produces TiO

2

(titanium (IV) oxide) nanoparticles. Altering the temperature affects the crystalline structure of the nanoparticles. An example of a section of the crystal structure in a TiO

2 nanoparticle is shown in

Figure 11.2.15.

Oxygen

Titanium

Figure 11.2.15: The crystalline structure of titanium (IV) oxide, which can be obtained by a sol-gel technique.

Key termAlkoxide: An organic functional group containing an oxygen atom bonded to an aliphatic hydrocarbon chain (for example, ethoxide –O–CH

2CH

3).

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Photonic crystals

As described in Topic guide 11.1, structures that are constrained in one or more dimensions display interesting behaviour when they interact with light. One such type of substance is described as a photonic crystal. The band gap between the valence and conducting band (see Topic guide 11.1) is such that certain frequencies of light are not transmitted through the crystal but interact with the electrons in the crystal.

Such crystals may have considerable applications in future computing technologies. The case study below describes one familiar example of a photonic material and its fabrication.

Case study: Opals and photonic crystalsThe mineral opal is an example of a naturally occurring photonic material; it is a hydrated form of silica (silicon dioxide), SiO

2.xH

2 O, without any large-scale crystalline structure.

However, its structure has been shown to consist of spheres of silica with nanoscale dimensions, and it is the arrangement of these spheres that causes diffraction of light and creates the photonic properties, as well as the coloured appearance of samples of opal, as shown in Figure 11.2.16.

Artificial opals can be manufactured using colloidal techniques. One such technique, electrophoretic deposition, uses electrophoresis, where charged colloidal particles move through a liquid medium towards an oppositely charged electrode, where they are deposited.

Figure 11.2.16: The colours of opal crystals are due to photonic effects.

Take it furtherThe process of manufacturing colloidal photonic crystals is described in a paper at http://pdeis.at.tut.by/cm2721.pdf.

5 Quality control in nanofabricationQuality control and characterisationMany of the key features of nanostructures are highly dependent on structural quantities – the thickness of films, the degree of order in a crystal, and so on. The development of characterisation methods, such as those listed in Section 3 of this topic guide, allow these quantities to be measured and therefore allow quality control in nanofabrication.

In-situ characterisation

Many of the characterisation techniques can be used in a non-invasive way to follow the progress of a manufacturing process and hence allow it to be precisely controlled in order to produce product of the desired quality.

Measurements could include: • substrate temperature • film thickness • growth rate

Key termPhotonic crystal: A crystalline substance which responds to, or can manipulate, specific frequencies of light.

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• optical qualities (e.g. refractive index) • residual gas content.

Failure analysis (ex-situ characterisation)

In some cases, where product quality has not been achieved, failure analysis is carried out at the end of the manufacturing process.

Unlike in-situ characterisation, failure analysis may involve invasive or destructive techniques, often requiring special, time-consuming sample preparation.

Analysis and measurement techniquesSome examples of different techniques are described below:

• Optical profilometry (see Figure 11.2.17): A beam of light is split into two beams; one beam reflects off the surface to be tested and the other is reflected off a reference surface. From the interference patterns obtained from the two beams, the distance between the reference surface and the surface being tested can be calculated. This provides information about the surface topography (‘bumpiness’) or the thickness of a thin film.

Beam splitter

Interferometer

Reflected beam

Reference beam

• Ellipsometry (see Figure 11.2.18): Polarised light from a laser is reflected off the surface of the sample. The polarisation of the light changes depending on factors such as refractive index and film thickness. By analysing the reflected light, these properties of the sample can be deduced.

Analyser

Photoelasticmodulator

Detector

Polariser

Sample

Light source

• Spectrophotometry: As an alternative to ellipsometry, the spectrum of a reflected beam of light can be analysed. When compared with the spectrum obtained with a reference film, the film thickness and refractive index can be deduced.

Key termsFailure analysis: The process of systematically collecting and analysing data in order to determine the cause of a failure.

Polarised light: Light in which the oscillation of the electromagnetic field is in a specific plane.

Refractive index: A measure of the angle by which a light beam is refracted as it passes into a substance.

Figure 11.2.17: The principles of optical profilometry.

Figure 11.2.18: The principles of ellipsometry.

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Imaging techniquesScanning electron microscope

Scanning electron microscopes are widely used to study nanostructures such as nanocrystallites and nanotubes, and also quantum wells and other sub-micrometre devices. In scanning electron microscopy (SEM) a beam of low energy electrons is focused by electromagnets onto the surface of a sample. When the beam hits the surface, the electrons that are scattered back are detected. An image of the surface is then built up from the pattern of scattering.

Using SEM, remarkable images can be produced showing the fine detail of nanostructures, especially of the nanomachines known as MEMS (microelectromechanical systems – see Topic guide 11.3). Figure 11.2.19 is a good example of such an image. It is evident that any defects in the nanostructure show up very clearly on such an image.

Scanning probe microscopy (SPM)

In this type of microscopy, a probe with an extremely thin tip is scanned over the surface of a sample. The probe may interact with the surface in different ways, generating tiny electrical currents in the probe. As the interaction depends on the distance between the probe tip and the surface, an electronic feedback system can be used to maintain a constant distance between the probe tip and the surface, and this allows the surface to be mapped as the probe passes over it. The resolution possible from these techniques is remarkably small and allows individual atoms to be imaged.

Take it furtherThe link http://www.nanoscience.com/education/tech-overview.html has some good background on SPM, including some computer animations of surfaces which have been studied using SPM techniques.

A gallery of images created by SPM techniques is available at http://researcher.watson.ibm.com/ researcher/view_project.php?id=4245.

IBM researchers have now used SPM techniques to create the world’s smallest animated film, by manipulating atoms to create a series of frames that shows a moving figure constructed of individual atoms. A report and a link to the animation is available at http://www.bbc.co.uk/news/science-environment-22364761.

Scanning tunnelling microscopy (STM)

This was the first SPM technique to be developed, in 1981. It makes use of the quantum mechanical effect of ‘tunnelling’ whereby an electron can pass through a barrier that would normally constrain it (for example, the gap between probe and surface). If there is a potential difference between the probe and surface, then the tiny current generated by the tunnelling effect can be used as the basis of the feedback loop which maintains a constant distance between probe and surface.

Because it relies on setting up a potential difference between probe and surface, it can only be used to image conductors or semiconductors.

Figure 11.2.19: A scanning electron micrograph of a MEMS.

http://researcher.watson.ibm.com/researcher/view_project.php?id=4245

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Atomic force microscope

This is the main technique used in characterising the surface of nanomaterials, as it can be used with almost any type of surface, including biological materials.

The principle of the atomic force microscope is shown in Figure 11.2.20.

A diamond tip (with a radius of a few nm) is attached to a cantilever that will deflect or twist depending on the movement of the tip over the surface. This deflection is measured by reflecting a laser beam from the back of the cantilever.

The movement of the tip is a result of repulsive or attractive forces between the atoms in the tip and those on the surface.

In one mode of operation, the tip is in ‘hard’ contact with the surface and experiences a repulsive force. In another mode, the tip is not in contact with the surface but is made to oscillate; changes to the amplitude or frequency indicate changes in distance between the tip and the surface.

Laser diode

CantileverTip

Sample

Position-sensitivephotodetector

Atomic force microscopy (AFM) can also be used to manipulate atoms and molecules by using the system described above to exert a force of known size on a surface atom. This enables the atom to dissociate from the surface or simply to be moved across the surface, and thus surface details can be carved, atom-by-atom, in a ‘bottom-up’ process.

Surface and bulk materials analysisAs well as the physical investigation of the surface topology and film thicknesses, quality control of nanofabrication will also need to use analytical tools to investigate such aspects as:

• the elemental composition of the nanochemical (for example, the exact stoichiometry of the compound(s) present

• the presence of any contaminants, on the surface or in the bulk material.

A range of techniques are used, which may be invasive or non-invasive.

X-ray photoelectron spectroscopy (XPS)

This non-invasive technique makes use of the fact that when X-rays are absorbed by an atom of an element, electrons are emitted with an energy that is characteristic of that element.

The non-invasive technique of XPS involves focusing a beam of electrons on a sample (in a vacuum chamber). The energies of the electrons emitted are measured and the

Figure 11.2.20: The principle of atomic force microscopy.

Key termStoichiometry: The ratio of atoms of different elements present in a compound.

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elements present, and their relative quantities, can be deduced from the recorded energies. Hydrogen and helium cannot be detected using this method, however.

The technique is essentially a surface analysis, as only the top 10 nm of the sample can be investigated.

SIMS (Secondary ion mass spectrometry)

This is a more sensitive technique, allowing measurement of composition as low as 1 ppb (parts per billion). It involves accelerating a stream of ions (such as Cs+, O2

+ or Ar+) onto the surface. When these (primary) ions hit the surface they cause the emission of secondary ions from the surface itself (a process known as sputtering). These secondary ions are then analysed by mass spectrometry.

The primary ions will, of course, contaminate the surface, so this is an invasive (destructive) technique. The beam of primary ions can be adjusted so that they ‘excavate’ secondary ions from below the surface, allowing composition at different depths to be investigated.

ChecklistAt the end of this topic guide you should be familiar with the following ideas: there is a wide range of nanofabrication routes, which can be classified as top-down (such as

lithography) or bottom-up (self-assembly) nanolithography relies on the formation of thin films, produced from the deposition of

vapour onto a surface self-assembly may make use of colloid chemistry or chemical templating techniques to direct

the process scanning probe microscopic techniques enable the imaging of the surface of nanostructures spectroscopic techniques may be used for analysis of nanomaterials.

Further readingMany texts on fabrication are highly complex and specialised. However, some helpful PowerPoint® presentations, often with good embedded graphics and animations, are available from the NACK network (as explained in Topic guide 11.1, these resources require a (free) registration). Try the general introduction at http://www.matecnetworks.org/webinars/pdf/Fabrication.pdf and then the introductory presentations on fabrication available for registered users at http://nano4me.live.subhub.com/categories/modules – modules 6, 7 and 8 cover fabrication and module 5 covers characterisation.

Nanochemistry (Ozin and Arsenault, 2005) covers micelle formation (p451–2), layer-by-layer self-assembly (Chapter 3), and some thoughts on complex adaptive systems are found in the appendix. Nanolithography is covered in several chapters in very thorough detail, although the depth and level of detail may render the material too difficult to be useful.

Processes relevant to thin film formation are covered in Inorganic Chemistry (5th edition) (Shriver and Atkins, OUP, 2010), p658–662 and, in some detail, in Introduction to Nanoscience (S.M. Lindsay, OUP, 2010), Chapter 5.

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AcknowledgementsThe publisher would like to thank the following for their kind permission to reproduce their photographs:

Shutterstock.com: imredesiuk; Science Photo Library Ltd: Pasieka 6, Paul Biddle 15, David Parker 17

All other images © Pearson Education

We are grateful to the following for permission to reproduce copyright material:

The process of conventional lithography, from the Henderson Research Group. Used with permission.

Every effort has been made to trace the copyright holders and we apologise in advance for any unintentional omissions. We would be pleased to insert the appropriate acknowledgement in any subsequent edition of this publication.

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