What You Will Learn
- 0.1 NANOMATERIALS
- 0.2 Gecko-inspired adhesive (or bio-rubber)
- 0.3 Self-healing adhesives
- 0.4 Biomimetic membranes, capsules and bioreactors
- 0.5 Biomimetic energy nanomaterials
- 1 Self-assembled nanomaterials
- 2 Polymers
- 3 Semiconductors
- 4 Ceramic and glassy materials
- 5 Carbon-based materials
- 6 Composites
- 7 Nanocoatings
- 8 Top-down
- 9 Bottom-up
Nanoscience will impact the design and fabrication of new materials with innovative properties and functions. Contributions to this field are very widespread, and include enhancing the properties of plas- tics, ceramics, coatings, composites, fibres and many more. Nanoscience also introduces an entirely new concept in material design, the bottom-up approach of material self-assembly, which is directly inspired from how organic and inorganic materials are created in nature. As a matter of fact, as shown in the first section, nature is a great source of inspiration to materials engineers, since many natural nanomaterials perform extraordinary functions as a result of their inner nanostructure.
This chapter provides an overview of nanomaterials, their properties and functions. The application areas of the cited nanomaterials are also mentioned. The second module of this Teachers Training Kit is devoted to applications of nanotechnologies.
The question: What is a‘nanostructured material’? must be considered before discussing examples of various functional nanomaterials. Nanostructured materials are solids or semi-solids (e.g. hydro- gels, liquid crystals) characterised by a nano-sized inner structure. They vary differ from crystalline, microstructured and amorphous solids because of the scale order. In crystalline solids, the atoms are neatly arranged in a grid where the distance between neighbouring atoms is well defined, and this order extends to macroscopic dimensions. In contrast, microstructured materials show structural variation only on a micron scale, whereas amorphous materials exhibit short-range order only. In nanostructured materials, the spatial order is at the nanoscale, which lies between the microscopic and the atomic scale.
The size of the nanostructures and the scale order within them in the solid impacts the properties of a material. Nanostructured materials differ from conventional polycrystalline mater- ials in the size of the structural units of which they are composed. They can exhibit properties that are drastically different from those of conventional materials, and this is often a direct consequence of the large fraction of grain boundaries (i.e. the space between the nanostructures) in the bulk material. This means that in a nanostructured material there is a large proportion of surface atoms (i.e. atoms that are located at or near a surface). Due to the large surface area, bulk properties become governed by surface properties. This surface — also called an interface — can form a border with the embedding matrix, a nanoparticle, air or vacuum in the case of a pore or defect.
Examples of nanostructured materials are nanoporous, nanocrystalline, nanocomposite and hybrid materials: nanoporous materials have nano-sized pores; a nanocrystalline material consists of many nano-sized crystalline domains; a nanocomposite material contains two or more phase- separated components with morphology of spheres, cylinders or networks with nano-sized dimensions (further divided into inorganic and polymer nanocomposites); and hybrid materials are made of a combination of organic and inorganic components interconnected at a molecular level (e.g. block copolymers). The materials within this chapter were categorized based on the function they serve, and for this reason, throughout the text, they are often cross-referenced: a nanocoating can be a nanocomposite but, because of its specific function, is described in a separate group.
One of the distinguishing features of nanostructured materials is that they can have properties that differ significantly from those displayed in bulk. This means that scientists have the opportunity to design new materials with specific functions by exploiting the intrinsic properties of nanomaterials. As a result, coatings, plastics and metals with new properties can be made to fulfill specific functions. As discussed in this chapter, numerous new materials are being developed with exciting properties. Although much research is still needed in this field, numerous commercial realities already exist and exciting new materials should be expected in the future. These materials have numerous applications that extend from the medical sector (e.g. antibacterial coatings) to improved cutting tools.
The first class of engineered nanomaterials to be reviewed is biomimetic materials. As mentioned in Module 1, Chapter 2: Nanoscience in nature, nature is the best nanotechnology platform. Over thousands of years of evolution, nature has developed an enormous array of materials, ranging from feathers to shells, wood, bone and many more that have intricate hierarchical structures at the nano, micron and macro levels which confer specific properties on the material, such as strength, lightweight, permeability, colour. Natural materials provide an amazing platform and inspire materials engineers to fabricate advanced materials that possess specific functions. As a matter of fact, numerous ‘macro’ materials we use today were developed after inspiration from natural materials. One example is Velcro, which was developed in 1948 by a Swiss engineer named George de Mistral who was inspired by the mechanism that enables cockleburs to cling to dog hair and fabric. Nanostructures in natural materials often play a crucial role, inspiring scientists to mimic them starting from a molecular level (molecular biomimetics). This is also called biomimetic nanotechnology. Now, some examples of these biomimetic nanomaterials are provided.
Gecko-inspired adhesive (or bio-rubber)
In Chapter 2, the adhesive properties of the gecko foot were discussed and, specifically, how these are not related to a glue in the foot, but rather to van der Waals and capillary forces exerted by millions of nanostructures (called setae) that make up the foot. This allows the animal to walk upside down, against gravity and on many different surfaces, including those which are wet. Moreover, a gecko can walk on a dirty surface without losing adherence, since its feet are also self-cleaning. A truly amazing material! Scientists have been inspired by this animal to design and fabricate adhesives for numerous applications. For instance, a group of researchers at the University of California, Berkeley, has developed adhesive gecko-foot-like surfaces for use in climbing robots. The adhesive is made of patches of microfibre arrays with 42 million polypropylene microfibre per square centimeter. The patches can support up to 9 N cm-2: a 2 cm2 patch can support a load of 400 g. This result is very close to the loads supported by a gecko, which is about 10 N cm-2. This gecko-like adhesive is very similar in functionality to the natural gecko foot, but not as good—yet. Researchers still have to make it topography-independent (capable of attaching to any surface) and self-cleaning. At another university, a biomimetic gecko tape has been produced using polymer surfaces covered with carbon nanotube hairs, which can stick and unstick on a variety of surfaces, including Teflon®.
Diatoms are a type of algae with nanostructured amorphous silica surfaces. Some diatom species have evolved strong self-healing underwater adhesives. Some are free-floating, others have adhesive properties in water: for instance, diatoms in the Antarctic seas can attach to ice. Others secrete viscous mucilage which binds colonies together while protecting the silica shells from wear as they rub against each other. Mollusks are another species that have adhesive properties underwater. Both diatoms and mollusks have strong underwater glues that can also resist stress and self-heal if necessary. For these reasons, they serve as a biomimetic model for self-healing materials. Researchers have studied these natural adhesives and found that their self-healing properties are due to the properties of the proteins contained in them. These proteins have ‘sacrificial’ bonds that allow the molecule to be reversibly stretched by breaking and re-bonding. This sacrificial bond behavior has been observed in many other materials, such as wool. The detailed analysis of these natural materials is inspiring new nano- adhesives with self-healing properties.
Biomimetic membranes, capsules and bioreactors
The bilipid membrane has served as a biomimetic model for decades. A simple example is liposomes (lipid vesicles) which are easily formed by shaking oil vigorously in water. Planar-supported bilayers are also inspired by the lipid membrane and are formed by simply ‘dipping’ a suitable substrate inside an organic aqueous phase.
Biomimetic energy nanomaterials
Many challenges that we are now facing in the energy area (improvements needed in solar panels, hydrogen fuel cells, rechargeable batteries, etc.) can be solved through the use of nano-engineered materials. Some of these materials have been developed following direct inspiration from nature, such as the new types of solar photovoltaic cells, which try to imitate the natural nanomachinery of photosynthesis. Another interesting example is that of using battery electrodes with self-assembling nanostructures grown by genetically engineered viruses.
The concept of self-assembly derives from the observation that, in natural biological processes, molecules self-assemble to create complex structures with nanoscale precision. Examples are the formation of the DNA double helix or the formation of the membrane cell from phospholipids. In self- assembly, sub-units spontaneously organize and aggregate into stable, well-defined structures through non-covalent interaction. This process is guided by information that is coded into the characteristics of the sub-units and the final structure is reached by equilibrating to the form of the lowest free energy. An external factor, such as a change in temperature or a change in pH, can disrupt this organization. For instance, a protein self-assembles into a specific structure but, if exposed to conditions such as high heat or high acidity, it can denature, which means that its structure is damaged, and the protein unfolds. This means that the protein loses its function as its structure is damaged. So, in nature, self-organized structures have specific functions.
Molecules in nature change conformation and move from one self-organized structure into another as they bind to certain ions or atoms. Many examples are available, such as hemoglobin (which captures and releases an iron ion), or the potassium-sodium pump, chlorophyll, etc.
The use of self-assembly to create new materials is a bottom-up approach to nanofabrication and is thus an essential tool in nanotechnology (see Module 1, Chapter 7: Fabrication methods). Instead of carving nanostructures out of larger materials (which is the typical top-down approach, such as micromachining and microlithography, used to fabricate integrated electronic circuits), nanostructures are created from the bottom, from atomic building blocks that self-assemble into larger structures. In the laboratory, scientists can make use of this self-organization of matter as a way of programming the building of novel structures with specific functions. Examples of self-assembled nanomaterials include dendrimers, DNA nanostructures, cyclodextrins, self-assembled monolayers (SAMs), liquid crystals.
Self-assembled monolayers (SAMs)
Some organic molecules, when exposed from solution or vapor to a suitable substrate, self-assemble to produce homogeneous, densely packed layers of monomolecular thickness. These organic molecules have long chains with two different end groups. The monolayer is formed when one of the two end groups of the organic molecule reacts with a particular surface forming a chemical bond. The surface properties of the substrate are then defined by the exposed functional groups of the monolayer. For example, alkyl-silane or alkanethiol molecules exposed to a silica or metal surface assemble into organized layers. SAM-forming materials may be physisorbed layers, such as Langmuir-Blodgett films, or chemisorbed layers, such as organosilanes bonded to silica or organ thiols bonded to gold. Films of mixed SAMs with tailored surface properties can be fabricated by mixing two (or more) precursor molecules and photosensitive SAM layers can be produced by molecularly engineering the precursor to include a photoreactive species. Whitesides and his co-workers from Harvard University have introduced the use of mixed SAMs of alkanethiols on gold surfaces to control protein and cell adhesion. SAMs of alkanethiol on gold is formed when a gold surface is exposed to a solution or to the vapor of an alkanethiol. The sulfur atoms of the alkanethiols coordinate with the gold surface, while the alkyl chains are closely packed and tilted to 30° from the surface normal. The terminal end group of a ω-substituted alkanethiol determines the surface properties of the monolayer.
Surfaces with micron and nanopatterns of SAMs are interesting since they allow the creation of metallic ‘hybrid’ circuits where biomolecules can be selectively attached (and released) to patterns of gold. Different methods exist for creating micro and nanopatterns of SAMs, such as microcontact printing (µCP), nanocontact printing (nCP), conventional lithography and Dip Pen Nanolithography®. The details of these fabrication methods are discussed in Module 1, Chapter 7: Fabrication methods.
A liquid crystal is the fourth state of matter: it has properties between those of a conventional liquid and those of a solid crystal. Like a liquid, it flows, and, like a crystal, it can display long-range molecular order (Figure 2). In terms of classifications, liquid crystals (together with polymers and colloids), are often classified as ‘soft matter’ (Figure 3) and treated under the branch of physical chemistry of condensed matter.
A fascinating and characteristic feature of liquid-crystalline systems is that they change their molecular and supermolecular organization drastically as a result of very small external disturbances: the molecules in liquid crystal displays, for instance, are reoriented by relatively weak electrical fields. If a small number of chiral molecules are dissolved in an achiral liquid-crystalline host phase, remarkable macroscopic chirality effects occur, ranging from helical superstructures to the appearance of ferroelectricity.
Liquid crystal self-assembly
Liquid crystals are partly ordered materials, somewhere between their solid and liquid phases. This means that liquid crystals combine the fluidity of ordinary liquids with the interesting electrical and optical properties of crystalline solids.
Molecules of liquid crystals are often shaped like rods or plates or some other form that encourages them to align collectively along a certain direction (Figure 4).
Liquid crystals are temperature sensitive since they turn to solid if it is too cold and to liquid if it is too hot. This phenomenon can, for example, be observed on laptop screens when it is very hot or very cold.
Phases of liquid crystals and their properties
A liquid crystal is formed by the self-assembly of molecules into ordered structures, or phases. An external disturbance, such as a change in temperature or magnetic field, even very small, can induce the liquid crystal to assume a different phase. Different phases can be distinguished by their different optical properties (Figure 5).
Liquid crystals are divided into three groups:
- thermotropic liquid crystals consist of organic molecules, typically having coupled double bonds, and exhibit a phase transition as the temperature is changed (Figure 5, left);
- lyotropic liquid crystals consist of organic molecules, typically amphiphilic (water-loving) and exhibit a phase transition as a function of both temperature and concentration of the liquid crystal molecules in a solvent (typical water) (Figure 5, right, and Figure 6);
- metal lyotropic liquid crystals are composed of both organic and inorganic molecules, and their liquid crystal transition depends not only on temperature and concentration but also on the organic-inorganic composition ratio.
Lyotropic liquid-crystalline phases are abundant in living systems, such as biological membranes, cell membranes, many proteins (such as the protein solution extruded by a spider to generate silk), as well as tobacco mosaic virus. Soap is another well-known material that is in fact a lyotropic liquid crystal. The thickness of the soap bubbles determines the colors of light they reflect.
Liquid crystal applications
The order of liquid crystals can be manipulated with mechanical, magnetic or electric forces. What is interesting is that this change of order can be obtained with very small variations of these forces. The properties of liquid crystals are useful in many applications. The color of some liquid crystals depends on the orientation of its molecules, so any influence that disturbs this orientation (e.g. a difference in the magnetic or electric field, temperature, or the presence of certain chemicals) can be detected with a color change.
Liquid crystals are routinely used in displays for cell phones, cameras, laptop computers and other electronics. In these displays, an electric field changes the orientation of the molecules in the liquid crystal, and affects the polarisation of light passing through them. Because of their sensitivity to temperature, and the property of changing color, they are also used in thermometers. In miniaturized sensors, liquid crystals can detect certain chemicals, electric fields and changes in temperature.
In the future, liquid crystals might have very interesting applications. Recently, it has been found that the inclusion of specific molecules, often in the order of nanometres, in the liquid crystal can lead to inherent new electrical and optical properties. For example, liquid crystal materials have been modified to induce new photonic functions and for application in waveguides and optical storage (ICT application area). Another class of liquid crystals under development is photoconductive liquid crystals, which aim to use new materials in photovoltaic cells and light-emitting diodes (Energy application area). Another area of study is the modification of liquid crystals in such ways that allow them to be used in stimuli-responsive materials and as templates for nanoporous polymers (potential nanomedicine application area).
Nanostructured metals and alloys
Metal nanoparticles are a clear example of how the properties of matter can change at the nanometre scale. For instance, metal gold is notably yellow in color and used for jewelry. As the noblest of all metals, gold is very stable (e.g. it does not react with oxygen or sulfur). However, if gold is shrunk to a nanoparticle, it changes color, becoming red if it is spherical (Figure 7) and even colorless if it is shaped in a ring. Moreover, gold nanoparticles become very reactive and can be used as new catalysts (this is discussed in Module 2, Chapter 2: Environment).
Noble metal nanoparticles, meaning gold, silver, platinum and palladium nanoparticles, show localized surface plasmon-resonances (LSPR), an effect that was described in Module 2, Chapter 4: Fundamental ‘nano-effects’. The energy of the LSPR depends on the particle shape, size, composition, inter-particle spacing and dielectric environment. The surface of the nanoparticles can be functionalized with numerous chemical and biochemical molecules enabling specific binding of organic molecules such as antibodies, making them useful in sensors.
For this reason, they are of special interest for optical detection and sensing in analytical chemistry and molecular biology. The refractive index can be used as the sensing parameter: changes in the local dielectric environment, induced by the sensing process, are used to detect the binding of molecules in the particle nano-environment. The change in aggregation between the nanoparticles as a result of analyte attachment affects the LSPR energy, so this effect can be used for highly miniaturized sensors. Figure 8 schematically illustrates these two LSPR-based detection methods.
Localized surface plasmons have been investigated in a range of nanoparticle shapes such as disks, triangles, spheres and stars. More complex structures have also been studied, such as holes in thin metal films, nanoshells and nanorings. Based on nanostructured metallic surfaces, a variety of optical applications are possible. An important feature of plasmonic structures is that they allow label-free detection, which is important in optical sensing. Currently, plasmonic components are being investigated with respect to future applications in cancer therapy, solar cells, waveguides, optical interconnection, camera LEDs, OLEDs and more.
Metal nanoparticles are used as reinforcement in alloys for applications in lightweight con- struction within the aerospace sector and, increasingly, the automotive sector. The method is used, for example, to harden steel. For instance, titanium nanoparticles are used as an alloy compound in steel, and the resulting material shows improved properties with respect to robustness, ductility, corrosion and temperature resistance. Particles of iron carbide are also precipitated in steel to make it harder. The nanoparticles block the movement of the dislocations in the crystalline material (this effect was described in detail in Module 1, Chapter 4: Fundamental ‘nano-effects’) increasing the hardness. The trade-off between steel strength and ductility is an important issue, since modern construction requires high strength whereas safety and stress redistribution require high ductility. The presence of hard nanoparticles in the steel matrix could lead to a material with a combination of these properties, effectively matching high strength with exceptional ductility.
Some forms of metal nanoparticles have important environmental applications.
- Zero-valent (Fe0) iron nanoparticles are under investigation for the remediation of contaminated groundwater and Iron, when exposed to air, oxidizes easily to rust; however, when it oxidizes around contaminants such as trichloroethylene (TCE), carbon tetrachloride, dioxins, or PCBs, these organic molecules are broken down into simple, far less toxic carbon compounds. Iron nanoparticles are more effective than conventional ‘iron powder’, which is already used to clean up industrial wastes. Iron nanoparticles are 10 to 1 000 times more reactive than commonly used iron powders.
- Silver nanoparticles have a strong antibacterial They are used in numerous products to prevent or reduce the adherence of bacteria to surfaces.
Nanocrystalline metals are classic metals and alloys that have an ultra-fine crystalline structure below 100 nm. They exhibit extraordinary mechanical and physical properties which make them interesting for many applications. Examples of nanocrystalline metal materials are aluminum, magnesium, and Al-Mg alloys which offer high strength and are lightweight. Other examples are titanium and Ti-Al alloys.
Some crystalline metals offer exceptional magnetic properties. An example is the Finemet® nanocrystalline soft magnetic alloys, which consist of melt-spun Fe-Si-B alloys containing small amounts of niobium and copper.
The application of nanomaterials in the field of magnetic materials is very important and promising, with applications in magnetic recording, giant magnetoresistance, magnetic refrigeration and magnetic sensing. These novel materials are composed of magnetic nanoparticles dispersed in a magnetic or non-magnetic matrix (particle-dispersed nanocomposites) or of stacked magnetic thin films (magnetic multilayer nanocomposites).
Ferrofluids are colloidal mixtures composed of ferromagnetic or ferrimagnetic nanoparticles (such as magnetite) suspended in a carrier fluid, usually an organic solvent or water. The ferromagnetic nanoparticles are coated with a surfactant to prevent agglomeration (due to van der Waals and magnetic forces). Although the name may suggest otherwise, ferrofluids do not display ferromagnetism, since they do not retain magnetization in the absence of an externally applied field. Rather, they display bulk-like paramagnetism and are often referred to as ‘superparamagnetic’ materials due to their large magnetic susceptibility. When a paramagnetic fluid is subjected to a sufficiently strong vertical magnetic field, the surface spontaneously forms a regular pattern of corrugations; this effect is known as the normal-field instability (Figure 9).
Ferrofluids have numerous applications: in mechanical engineering, particularly for vehicle suspension and braking systems, due to their low-friction properties; in the ICT area, as liquid seals (ferrofluidic seals) around the spinning drive shafts in hard disks; and in the biomedical sector as drug carriers.
Most metals are very hard and take a lot of effort to deform, but once they have been molded into shape, they will stay like that until another force changes them. Memory metals or ‘shape-memory alloys’ (SMAs) are different. They can be ‘programmed’ to remember a specific shape and if the metal is bent or deformed, it quickly returns to its original configuration. This is because memory metal has two distinct crystal structures at the nanoscale and can be made to flip between them. Both are regular lattices. The so-called parent phase (or austenite phase) occurs when the metal is at higher temperatures (Figure 10). When shaped at high temperatures the metal will ‘remember’ this shape. As the metal cools, its crystal structure changes to the second (martensite) phase. The gentle heating of the metal makes it return to its original parent shape.
TIP FOR TEACHERS: An analogy can be made between thermotropic liquid crystals (LCs) and shape- memory alloys: in both materials, there is a change in nanostructure as its temperature is changed, with a ‘macroscopic’ effect visible (i.e. a change in color in the case of the liquid crystals, and a physical deformation in the shape-memory). The two materials differ dramatically in the type of nanostructure involved: a ‘soft’ structure in the case of the liquid crystals (see ‘Liquid crystals’ in this chapter) and a crystalline structure in the case of the memory-alloy.
Glasses made of memory metal take advantage of the phenomenon ‘pseudo elasticity’. In this instance, the metal is in its austenite phase at room temperature and the martensitic phase is brought about by applying a stress, rather than cooling. When the stress is removed, the metal reverts to its austenite phase and its associated shape. Nitinol is used in orthodontics for braces. Once the Nitinol is placed in the mouth, its temperature rises to ambient body temperature causing it to contract back to its original shape. This results in a constant force being applied to the teeth. Nitinol wires do not need retightening as often as they can contract as the teeth move, unlike conventional stainless steel wires.
A polymer is a large molecule made of a chain of individual basic units called monomers joined together in sequence. A copolymer is a macromolecule containing two or more types of monomers. When the polymer is a good conductor of electricity, it is referred to as a conductive polymer (or organic metal). In this section, nanostructuring of polymers and the effect this can have on the properties of polymers are described. The focus is on copolymers since these are extremely useful in nanotechnologies.
Polymers that are good conductors of electricity are called conductive polymers and include polyacetylene, polyaniline, polypyrrole, polythiophene many more have also been synthesized. These polymers are characterized by their alternating double-single chemical bonds, so they are π-conjugated. The π-conjugation of the carbon bonds along the oriented polymer chains provides a pathway for the flow of conduction electrons and is thus responsible for the good electrical conduction properties of the material. A detailed SEM analysis of conductive polymers has revealed that these are made of a sequence of metallic nanoparticles about 10 nm in diameter. The high conductivity of polymers such as polyacetylene and polyaniline is related to the nanostructure of the polymer. Polyaniline and its analogs change color when a suitable voltage is applied, or when reacting
with specific chemicals (electrochromic and thermochromic). For this reason, they are promising for use in light-emitting diodes (LEDs). Other applications are the surface finish of printed-circuit boards, corrosion protection of metal surfaces, semi-transparent antistatic coatings for electronic products, polymeric batteries and electromagnetic shielding.
A copolymer is a macromolecule containing two or more types of monomers and a block copolymer comprises these basic units or monomer types joined together in long individual sequences called blocks. An example is the diblock polymer (A)m(B)n, which is made of a linear sequence of m monomers of type A joined together to a linear sequence of n monomers of type B. A transition section joints the two blocks:
[end group]-[polyA]m-[transition member]-[polyB]n-[end group]
Often, block copolymers are made of a hydrophilic (water-attractive) block and a hydrophobic (water-repellent) block. In general, macromolecules having hydrophilic and hydrophobic regions, such as lipids, self-assemble in ordered structures when in water: the hydrophobic region packs together, avoiding the water molecules, leaving the hydrophilic molecules to the exterior of the structure. In the same way, block copolymers made of hydrophilic and hydrophobic blocks when mixed in a selective solvent, such as water, can self-assemble into ordered architectures at the nanoscale level.
The geometry and degree of order of these structures depend on the concentration and the volume ratio between insoluble and soluble blocks. Depending on these parameters, the block copolymer can form spherical micelles (nanospheres), cylindrical micelles and membranes. Both cylindrical and spherical micelles consist of a non-soluble (hydrophobic) core surrounded by a soluble corona. Membranes are made up of two monolayers of block copolymer aligned to form a sandwich-like membrane: soluble block-insoluble block-soluble block. Molecules that, at low concentration, form spherical aggregates will assemble into cylindrical and eventually membrane-like structures as the concentration is increased (Figure 11).
Among spherical micelles, if the lengths of the projections formed by the hydrophilic corona are short compared to the sphere diameter, the nanostructure is called a ‘hairy nanosphere’, whereas if the sphere is small and the projections long, it is called a ‘star polymer’ (Figure 12).
Responsive ‘smart’ polymers
Block copolymers form nanostructures that are very sensitive to external fields. For example, moderate electrical fields or shear stimulation can trigger macroscopic rearrangements in specific directions. This is a versatile property for making materials that respond and change on demand. The intrinsic macromolecular nature of these copolymers leads to very slow and kinetically controlled phase transitions. Thus, metastable or intermediate phases have longer lifetimes, which is desirable in applications that want to exploit the phase transition properties of these materials.
The ability of block copolymers to form nanoparticles and nanostructures in aqueous solutions makes them particularly useful for biomedical applications, such as therapeutics delivery, tissue engineering and medical imaging. In the field of therapeutic delivery, materials that can encapsulate and release drugs are needed. Hydrogels are very useful for the controlled release of drugs and block copolymer hydrogels are particularly advantageous for the possibility of conferring some stimuli-activated properties, such as temperature-sensitivity. Block copolymers form nanostructures with both hydrophilic and hydrophobic areas, so they can form vesicles that can encapsulate and carry both hydrophobic and hydrophilic therapeutic agents. Micelles formed using block copolymers have a hydrophilic corona that makes them more resistant to the interaction of proteins, in particular, plasma proteins; therefore, these types of micelles exhibit long circulation times in vivo. Insoluble domains can also be engineered to exploit the sensitivity of specific hydrophobic polymers to external stimuli such as pH, oxidative species, temperature and hydrolytic degradation.
Block copolymers are also of interest for preparing scaffolds in tissue engineering — for instance, very long micelles that mimic the natural extracellular have recently been prepared to exploit the self- assembly properties of a peptide copolymer.
The ability to generate compartmentalized volumes at the nanometre level is one of the fundamental mechanisms used by cells in synthesizing biomolecules and performing the biochemical processes necessary for their function. This motif is now reassembled using block copolymer micelles and vesicles as nanoreactors. This approach has been employed in carrying enzymatic reactions in nano-sized compartments.
It has been shown that this approach can also be extended into a non-aqueous solvent. For instance, a recent work reports block polymer nanostructures in ionic liquid and the ability of micelles to shuttle from an aqueous solvent to an ionic liquid as a function of temperature: this opens the way to using block copolymer technology in more sophisticated synthesis routes.
Artificial moving parts
Block copolymer structures can also be used to mimic the ability of biomolecules to convert chemical energy into mechanical energy, for instance by using a pH-sensitive block copolymer whose micelles swell according to pH variation. Thus, these materials are being investigated to create artificial muscles or moving nanostructures.
The application of block copolymers is not limited to the biomedical field. They can be used in conjunction with other materials to form block copolymer nanocomposites. For instance, star polymers are used in industry to improve mental strength.
Finally, block copolymers can form nanoporous membranes for applications in filtering systems and fuel cell technology.
Nanostructured fibrous materials, or nanofibres, are an important class of nanomaterials, now readily available due to recent developments in electrospinning and related fabrication technologies. In contrast to conventional woven fabrics, they have the typical structure illustrated in Figure 13.
Nanofibres have some unique properties: they are highly porous (i.e. they have a large interconnected void volume in the range of 50 % or even greater than 90 % and possess a very high surface-to-volume ratio). It is possible to increase the mechanical stability of nanofibrous structures by anneal- ing the fabric so to join together the crossing points of those fibres. These properties make nanofibrous scaffolds useful for many biomedical and industrial applications.
In addition, researchers have succeeded in making coaxial nanofibres composed on two different polymers or composite coaxial fibres. Researchers are also trying to make aligned nanofibres. These types of materials, particularly if made of conducting polymers, could have important applications for electronic and medical devices.
Nanofibres have a broad spectrum of applications as schematized in Figure 14.
Semiconductors, unlike metals, have a bandgap. The bandgap is between the valence band and the conduction band. In intrinsic semiconductors which possess no impurities (e.g. boron, germanium, indium, silicon), there are no electronic states in the band gap. The properties of semiconductors, in par- ticular the band gap, are manipulated by the addition of dopants — impurities able to donate charge carriers in the form of electrons (n-type) or holes (p-type).
As with metals, the reduction in the size of the semiconductor triggers the insurgence of novel physical properties. The most evident example is that of quantum dots.
Quantum dots are made of semiconductor materials, such as CdSe, ZnSe and CdTe, about 10 nm in size. As in the case of metal nanoparticles, electrons in quantum dots are localized in space.
A quantum dot (QD) has a discrete quantized energy spectrum, so it can absorb a specific wavelength and emit a monochromatic color. Depending on their size, QDs emit different colors, as shown in Figure 15. The reason for this was explained in Module 2, Chapter 4: Fundamental ‘nano-effects’: the width of the bandgap is related to the size of the semiconductor, smaller sizes lead to a blue shift in the emission spectra.
Semiconducting oxides like TiO2 and ZnO in bulk (macro) form are widely used in industry in many products. When they are in a nanoscale form, they display interesting physical properties that allow the design of new materials and the improvement of old. A short description of these properties follows.
Titanium dioxide (TiO2) is a mineral mainly found in two forms: rutile and anatase. Titanium dioxide is the most widely used white pigment because of its brightness (white color) and very high refractive index (n = 2.4). It is used in paints, plastics, toothpastes, papers, inks, foods and medicines. In sunscreens with a physical blocker, titanium dioxide is used both because of its high refractive index and its resistance to discoloration under ultraviolet light. This is because TiO2 is a UV filter: it absorbs UV light. Titanium dioxide, particularly in the anatase form, can be employed also as a photocatalyst under UV light. It oxidises water to create hydroxyl (OH) radicals and it can also oxidise oxygen or organic materials directly. For this reason, TiO2 is added to confer sterilising, deodorising and antifouling properties to paints, cement, windows and tiles.
Titanium nanoparticles (30–50 nm, often referred to as nano-TiO2) are at the center of much attention due to their optical and catalytic properties: they retain the ability to absorb UV light but light scattering is dramatically reduced so that TiO2 goes from appearing white to transparent (a detailed explanation of this effect is given in Module 1, Chapter 4: Fundamental ‘nano-effects’. Nano-TiO2 is thus suitable for transparent coatings, and for new-generation sunscreens, which are characterized by a high protective factor but transparent appearance. The catalytic properties of TiO2 when nano-sized are also greatly enhanced by the large surface-to-volume ratio. This property is increasingly used for chemical catalysis applications such as photocatalytic purification of water and air to decompose organic pollutants (solar photocatalytic remediation). Thin films of TiO2 are used on windows to confer self-cleaning properties on the glass (this application of nano-TiO2 nanoparticles is reviewed in the section on nanocoatings).
One limitation of using TiO2 as a photocatalyst is that this material only absorbs UV light, which represents about 5 % of the solar spectrum. In this context, nanotechnology could bring an improvement in the form of nanoparticles with surfaces modified with organic or inorganic dyes to expand the photoresponse window of TiO2 from UV to visible light.
Zinc oxide (ZnO) has some similar properties to TiO2 (i.e. its nanoparticles scatter light so it can be used for transparent UV filters, in creams or coatings). Like TiO2, it is used for solar photocatalytic remediation but, compared to TiO2, it has a weaker photocatalytic effect. Zinc oxide also suffers from the same limitation of absorbing only a fraction of the solar spectrum so research is underway to increase its photoresponse.
A peculiarity of ZnO is that it has a tendency to grow in self-organized nanostructures. By controlling crystal growth conditions, a variety of crystal shapes are possible. Researchers have been able to grow nanoscale wires, rods, rings, etc. (Figure 16). Zinc oxide nanocolumns are of particular interest since low-temperature photoluminescence measurements have revealed intense and detailed ultraviolet light emission near the optical band gap of ZnO at 3.37 eV. Thus, ZnO can act as an optical amplification medium and as a laser resonator.
Zinc oxide wires arrayed on a surface are also being investigated as piezoelectric elements for miniaturised power sources. This would allow the creation of flexible, portable power sources that could be included in textiles so that energy from body motion, light wind, airflow, etc., could be scavenged.
Indium tin oxide
The use of ZnO for the development of miniaturized power sources is described in detail in Module 2, Chapter 3: Energy.
Indium tin oxide (ITO) is a semiconducting material whose main feature is the combination of electrical conductivity and optical transparency. ITO is typically around 90 % indium(III)-oxide (In2O3) and 10 % tin(V)-oxide (SnO2). It is widely used in its thin-film form as transparent electrodes in liquid crystal displays, touch screens, LEDs, thin-film solar cells, semiconducting sensors, etc. ITO is an infrared absorber and is currently used as a thermal insulation coating on the window glass. Its anti-static properties make it additionally useful in applications such as the packaging and storage of electronic equipment. Since the material is very expensive, alternative materials, such as fluorescent tin oxides and aluminum zinc oxides are being considered. The use of ITO in ‘smart coatings’ is further discussed in the section on nanocoatings.
A photonic crystal consists of a lattice of periodic dielectric or metal-dielectric nanostructures that affects the propagation of electromagnetic waves. Essentially, photonic crystals contain regularly repeating internal regions of high and low dielectric constant. Photons (behaving as waves) propagate through this structure — or not — depending on their wavelength. The periodicity of the photonic crystal structure has to be of the same length-scale as half the wavelength of the electromagnetic waves (i.e. approximately 200 nm (blue) to 350 nm (red) for photonic crystals operating in the visible part of the spectrum). Such crystals have to be artificially fabricated by methods such as electron-beam lithography and X-ray lithography.
Photonic crystals exist in nature. For instance, in Module 1, Chapter 2: Nanoscience in nature it was shown how the beautiful blue wings of some butterflies owe their color to their internal nanostructure, which is in fact a photonic crystal structure.
Ceramic and glassy materials
Ceramic materials by definition are ionically bound; they are hard materials, both electrically and thermally very stable. Included in this category are, for example, Al2O3, Si3N4, MgO, SiO2, Na2O, CaO and ZrO2. Ceramics are characterized by being hard yet brittle, therefore, in many cases, they are used as composites where they are mixed with other materials (e.g. metals) to increase their mechanical performance. Composite hard nanocoatings are particularly important for cutting tools (see the section on inorganic composites).
Ceramics exist both in a crystalline and non-crystalline form (e.g. the broad family of ‘glass’). Nanostructures within the material have important consequences for their properties, as discussed in the next sections.
Porous alumina membranes produced by anodic aluminum oxidation are characterised by having hexagonally close-packed channels with diameters ranging from 10 to 250 nm or greater. This material is often used as a template for the synthesis of other materials.
Zeolites are natural crystalline materials with pores having regular arrangements (Figure 17). They are also often used in the template synthesis of nanomaterial. They can also be used to prepare organised structures of a certain material to confer new optical properties on it. For instance, selenium can be incorporated into the channels of mordenite, a natural zeolite. The difference between a mordenite- selenium crystal and a natural selenium crystal is noteworthy: the optical absorption spectrum is considerably shifted to higher energies for the mordenite-selenium crystal.
Silicon dioxide (SiO2) is the main component of quartz. It is chemically robust and finds widespread applications. In commercial products, it appears as an additive in rubber products for vehicle tires but it is also the component of new types of aerogels. Generally, an aerogel is considered as a solid with up to 95 % of the volume consisting of nanoscale pores. Aerogels are manufactured with a sol-gel technique and can be made of carbon, metal oxide, polymers or silicates.
Due to their high porosity, aerogels have an extremely high surface area and very low thermal conductivity. Thus they are suitable for thermal insulation and as filter materials (Figure 18). Another prominent property is their low specific weight, making them interesting for lightweight construction. Aerogels are also interesting for their optical properties, namely high optical transparencies. Silica aerogels are made of pores of about 10 nm arranged in distances between 10 and 100 nm. They are resistant and chemically inert to liquid metals, heat resistant up to 1 200 °C and non-toxic, thus they have also biomedical applications, such as substrates for cell growth and analysis. One of the problems of SiO2 aerogels that needs to be addressed is that this material has to be expensively protected against humidity since it is not water-resistant and suffers from a loss of stability and thermal conductivity once it gets wet.
In nature, there are some pure materials that have strikingly different properties even though they are made up of the same atoms. For instance, graphite and diamond (Figure 19): two very popular materials, one used conventionally in pencils and the other in jewelry. These two materials could not be more different: graphite is soft, light, flexible, and conducts electricity while diamond is extremely strong, hard and does not conduct electricity. Both materials are made of carbon atoms linked through strong bonds (covalent): in graphite, each carbon atom uses three out of its four electrons to form single bonds with its neighbors, forming a linear sheet, whereas, in diamond, each carbon atom uses all its four electrons to form four single bonds, resulting in a 3D structure. The different properties of graphite and diamond are a consequence of the different ways the carbon atoms in the materials are bonded together.
Graphite and diamond are two pure forms (allotropes) of carbon. In 1985, a new allotrope of carbon was discovered formed of 60 atoms of carbons linked together through single covalent bonds arranged in a highly symmetrical, closed-shell that resembles a football. This material was officially named Buckminsterfullerene and is often referred to as buckyball, fullerene or simply C60. Since its discovery, fullerenes with 70, 80 and even more carbon atoms have been discovered.
In the early 1990s, an incredible new carbon form was discovered: carbon nanotubes. These appear to be like graphite sheets rolled up with fullerene-type end caps, but have totally different properties compared to graphite. Figure 20 shows different forms of carbon allotropes (images (d) and (h) are structures of C60 and a nanotube, respectively).
It is now known that fullerenes and carbon nanotubes form naturally in common places like flames (produced by burning methane, ethylene and benzene) and in soot. Scientists have now developed methods to synthesize these nanomaterials with control over their final properties.
Another exciting carbon-based nanomaterial is graphene which is the basic constituent of graphite (the material used in the lead of a pencil). Graphene is a layer of carbon atoms, just one-atom-thick; although the thinnest material ever made, graphene is the strongest material ever measured, some 200 times stronger than steel and it is also the most conductive. This material was isolated in 2004 by Andre Geim and Konstantin Novoselov (University of Manchester) who used adhesive tape to repeatedly peel apart shavings of graphite until the pieces were just one atom thick. The scientists extensively investigated the amazing properties of graphene and were awarded the Nobel Prize in Physics in 2010 for their studies. Scientists believe in the future graphene could be used to make super-fast transistors and incorporated into plastics to make transparent electronics for flat panels and mobile phones.
Carbon nanotubes can appear as single-wall nanotubes (SWNTs), with a diameter of approximately
- nm, or multi-wall nanotubes (MWNTs), consisting of 2–30 concentric tubes with an outer diameter of 30–50 The structure of an SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite sheet (grapheme) into a cylinder. To complete the nanotube, imagine adding two half fullerenes on each end of the nanotube.
Carbon nanotubes can range in length from a few tens of nanometres to several micrometers and can have metallic properties (comparable to, or even better than, copper) or can be semiconductors (such as silicon in transistors), depending on their structure. These nanomaterials are truly amazing and have great potential in numerous fields as illustrated in Figure 21.
The chemical bonding of nanotubes is composed entirely of sp2 bonds (carbon double bonds) similar to those of graphite (whereas, in diamonds, all bonds are sp3). This bonding structure, which is stronger than the sp3 bonds found in diamonds, provides the molecules with their unique strength. Nanotubes naturally align themselves into ‘ropes’ held together by van der Waals forces.
The mechanical properties of carbon nanotubes are summarised in Table 1. Young’s modulus is a measure of how stiff, or elastic, a material is. The higher the value, the less a material deforms when a force is applied. Tensile strength describes the maximum force that can be applied per unit area before the material snaps or breaks. A third interesting measure of a material is its density, which gives an idea of how light it is. From Table 1, it can be seen that wood is very light but weak, whereas nanotubes are many times stronger than steel and yet much lighter.
Electrical and thermal properties
The electrical properties of a material are based on the movement of electrons and the spaces or ‘holes’ they leave behind. These properties are based on the chemical and physical structure of the material. In nanoscale materials, some interesting electrical properties appear. Carbon nanotubes are the best example of this effect at the nanoscale. If one considers ‘building’ a nanotube by rolling up a graphene sheet, the resulting nanotube can be conductive (and, indeed, very conductive!) or semiconductive with relatively large band gaps. The electrical properties of the nanotube depend on the way it was ‘rolled up’ (technically known as chirality). If it is rolled up so that its hexagons line up straight along the tube’s axis, the nanotube acts as a metal (conductive). If it is rolled up on the diagonal, so the hexagons spiral along the axis, it acts as a semiconductor.
Why is this so? Graphene (that is, one layer of graphite) is not an insulator, but also neither a metal nor a semiconductor; it has electrical properties somewhere in between and is called a semi-metal. When rolled up, it leads to a structure that is either metallic or semiconducting. On the other hand, diamond has a tetrahedral structure (derived from the fact that carbons are hybridized sp3 rather than sp2 as in graphene) and is an insulator.
One interesting property found in single-walled carbon nanotubes (SWCNT) is that electric conductance within them is ballistic (which means that all electrons that go into one end of the conductor come out the other end without scattering, regardless of how far they need to travel).
Researchers are also investigating whether nanotubes can be superconductors near room temperature, meaning ballistic conductors that also exhibit a resistance of zero. A superconductor can transport an enormous amount of current flow at tiny voltages. At present, known superconductors work at very low temperatures. This field of research is very important since if the material were superconductive at room temperature, it would carry current with no resistance, with no energy lost as heat. This could lead to faster, lower-power electronics and the ability to carry electricity long distances with 100 % efficiency.
Carbon nanotubes are very stable: they can withstand the attack of numerous chemicals and resist exposure to a large temperature range. However, their chemical structure can be changed by the addition of specific ligands with functional groups that allow interaction with different chemicals. This allows them to be used in sensors.
Carbon nanotubes are very promising materials for numerous applications (these will be described in more detail in Module 2). Applications include nanomedicine (drug delivery), environment (chemical sensors), energy (supercapacitors, hydrogen storage materials, solar cells), ICT (integrated circuits, electronic paper), advanced materials for construction, transport, sports equipment and more.
Scientists have now developed methods to control the synthesis of carbon nanotubes to obtain regular structures with specific properties. To date, though, the synthesis methods lead to moderate amount of CNTs and mostly very limited length (of the order of millimeters). Often, these processes lead to CNTs which are not totally pure (traces of the catalyst used in the synthesis are present in the product) and this has been associated with toxicity issues of CNTs.
The cost associated with the production of carbon nanotubes is extremely high. In the future, this cost must be considerably reduced to allow large-scale production and use of CNTs.
The idea behind nanocomposites is to use building blocks with dimensions in the nanometre range to design and create new materials with unprecedented flexibility and improvements in their physical properties. This concept is exemplified in many naturally occurring materials, such as bone, which is a hierarchical nanocomposite built from ceramic tablets and organic binders (see Module 1, Chapter 2: Nanoscience in nature). When designing the nanocomposite, scientists can choose constituents with different structures and composition and, hence, properties, so that materials built from them can be multifunctional.
As a general definition, a nanocomposite is a conventional material reinforced by nanoscale particles or nanostructures which are dispersed throughout the bulk material. The base material itself generally consists of non-nanoscale matrices. Nanocomposites typically consist of an inorganic (host) solid containing an organic component or vice versa or two (or more) inorganic/organic phases in some combinatorial form. At least one component must be nano-sized. In general, nanocomposite materials can demonstrate different mechanical, electrical, optical, electrochemical, catalytic and structural properties that are different from those of the individual components. Apart from the properties of the individual components, interfaces in a nanocomposite play an important role in determining the overall properties of the material. Due to the high surface area of nanostructures, nanocomposites present many interfaces between the intermixed phases and, often, the special properties of the nanocomposite are a consequence of the interaction of its phases at interfaces. In comparison, the interfaces in conventional composites constitute a much smaller volume fraction of the bulk material.
In this chapter, nanocomposites are subdivided in two main groups: inorganic nanocomposites, which are characterized by an inorganic matrix (e.g. ceramic) reinforced by nanoscale particles or nanostructures of inorganic (e.g. metal) or organic (e.g. carbon-based) nature; and polymer nanocomposites, which are characterized by an organic matrix (e.g. polymer) reinforced by nanoscale particles or nanostructures by inorganic (e.g. clay) or organic nature.
High-performance ceramics are sought in many applications, such as highly efficient gas turbines, aerospace materials, cars, etc. The field of ceramics that focuses on improving their mechanical properties is referred to as structural ceramics.
Nanocomposite technology is also applicable to functional ceramics such as ferroelectric, piezoelectric, varistor and ion-conducting materials. In this case, the properties of these nanocomposites relate to the dynamic behavior of ionic and electronic species in electro-ceramic materials. Among these materials,in this document, the review is limited to nanocomposite with enhanced magnetic properties.
Presently, even the best-processed ceramics pose many unsolved problems such as poor resistance to creep, fatigue and thermal shock; degradation of mechanical properties at high temperatures; and low fracture toughness and strength. To solve these problems, one approach has been to incorporate a second phase such as particulates, platelets, whiskers or fibers in the micron-size range at the matrix grain boundaries. However, the results obtained with these methods have been generally disappointing. Recently, the concept of nanocomposites has been considered, where nanometre-size second-phase dispersions are inserted into ceramic matrices. Large improvements in both the fracture toughness and the strength of ceramic can often be achieved with nanometre-range particles embedded in a matrix of larger grains at their grain boundaries. These can involve the incorporation of a nano-ceramic in a bulk ceramic, a nano-metal in ceramic, or a nano-ceramic in a metal. Another possibility is the incorporation of a polymer in a ceramic. Without going into too much detail, the following are examples of inorganic nanocomposites that have improved structural properties.
- The incorporation of fine SiC and Si3N4nanoparticles in an alumina matrix (Al2O3, a structural ceramic material) first demonstrated the concept of structural nanocomposites. The dispersion of these particles has been shown to improve the fracture toughness from 3 to 4.8 MPa m1/2 and the strength from 350 to 1 050 MPa at only 5 vol. % additions of SiC.
- One possibility for the fabrication of advanced structural ceramics is the dispersion of metallic second-phase particles into ceramics which improves their mechanical properties, such as fracture toughness, and influences other properties including magnetic and optical properties. Nanocomposites of this type are Al2O3/W or MgO/Fe. Granular films can also be made with a ceramic phase embedded with nano-size metal granules (such as Fe/Al2O3, Fe/SiO2). Such films often show unusual or enhanced transport, optical and magnetic properties. The inclusion of nano-sized metals in a thin ceramic film can transform it from an insulator to a conductive film.
- Another possibility is to add fine, rigid ceramic reinforcements to a ductile metal or alloy matrix (metal matrix composites or MMCs). The reinforcement can be either in the form of particles (e.g. silicon carbide, aluminum oxide), fibers (e.g. silicon oxide, carbon), or a mixture of both (hybrid reinforcement). Materials produced by this method are particularly useful in the aerospace, automotive and aircraft The advantage of MMCs is that they combine metallic properties (ductility and toughness) with ceramic characteristics (high strength and modulus), leading to materials having greater strength to shear and compression and higher working temperature capabilities. The properties of MMCs are controlled by the size and volume fraction of the reinforcements as well as by the nature of the matrix/ reinforcement interface. The attention is now oriented towards the incorporation of nanoparticles and nanotubes for structural applications, since these materials exhibit even greater improvements in the physical, mechanical, and tribological properties compared to MMCs with micron-size reinforcements.
- Nanoscale ceramic powders with carbon nanotubes provide another method for creating dense ceramic-matrix composites with enhanced mechanical properties. For instance, hot-pressed alumina with mixed carbon nanotubes results in lightweight composites with enhanced strength and fracture toughness compared to polycrystalline alumina. The processing conditions greatly influence the properties of this material, though. In metal matrix composites (MMCs), the incorporation of carbon nanotubes is considered very promising since these materials have higher strength, stiffness, and electrical conductivity compared to conventional metals.
Nanocomposites with enhanced magnetic properties
Materials with outstanding magnetic properties are in high demand as these are employed in nearly all important technical fields including electrical power, mechanical power, high-power electromotors, miniature motors, computer elements, magnetic high-density recording, telecommunications, navigation, aviation and space operations, medicine, sensor techniques, magnetic refrigeration, materials testing and household applications. Recent developments in the field of magnetic materials have involved materials with a nanocrystalline structure or, in the case of thin films, layers of nanometre thickness.
Nanostructuring of bulk magnetic materials leads to soft or hard magnets with improved properties. One example is the Finemet® nanocrystalline soft magnetic alloys, which consist of melt-spun Fe-Si-B alloys containing small amounts of niobium and copper. When annealing at temperatures above the crystallization temperature, the Fe-Si-B-Nb-Cu amorphous phase transforms into a crystalline solid with a grain size of about 10 nm. These alloys have excellent magnetic induction and large permeability, and a very small coercive field.
Nano-sized magnetic powders can have extreme properties and have no hysteresis at any temperature. These materials are called superparamagnetic and one example is nano-sized powders of a Ni-Fe-Co alloy.
Nanostructuring has also been studied in the context of hard magnets (permanent magnets). The strongest known magnets are made of neodymium (Nd), iron, and boron (e.g. Nd2Fe14B). In these materials, it is has been found that the coercive field decreases significantly below approximately 40 nm, and the remnant magnetization increases. Another approach to improving the magnetization curve of permanent magnets has been to fabricate nanocomposites made of hard magnetic phases, such as Nd2Fe14B and Sm2Fe17N3, within soft magnetic matrices (e.g. soft phase of iron). The effect of the inclusion of a soft iron mixed in with a hard material is to increase the remnant field.
The grain size of the material also influences the magnetization saturation point. For instance, the magnetization of ferrite increases significantly below a grain size of 20 nm. Thus, reducing the size of the grains in the magnet increases the energy product (which is the product of magnetization and coercivity). The coercivity also increases with decreasing grain sizes. In the case of nanocomposite magnetic films, this is true if the grains are isolated (no interaction), but when the grains are in contact and exchange interaction kicks in, the coercivity falls rapidly with grain size. The coercivity is thus highest at percolation when the grains just start touching each other. This effect is important in the context of designing thin-film nanocomposites (magnetic multilayer nanocomposites), for instance for high magnetic density recording.
Polymer composites are materials where a polymer is filled with an inorganic synthetic and/or natural compound in order to increase several properties, such as heat resistance or mechanical strength, or to decrease other properties, such as electrical conductivity or permeability for gases like oxygen or water vapor. Materials with synergistic properties are used to prepare composites with tailored characteristics; for instance, high-modulus but brittle carbon fibers are added to low-modulus polymers to create a stiff, lightweight polymer composite with some degree of toughness. Current polymer composites are really filled polymers since these materials lack an intense interaction at the interface between the two mixed partners. Progress in this field has involved moving from macroscale fillers to micron-scale fillers, to even smaller fillers.
In recent years, scientists have started to explore a new approach to the production of polymer composites with the use of nanoscale fillers, in which the filler is below 100 nm in at least one dimension. Nanoscale fillers include nanoparticles; nanotubes; and layered (also called ‘plate-like’) inorganic materials such as clays (Figure 22).
Although some nano-filled composites have been used for more than a century, such as carbon-black and fumed-silica-filled polymers, researchers have only recently started to systematically produce and study these materials. The motivation has been the realization of the exceptional combined properties that have been observed in some polymer nanocomposites. This, together with substantial development in the chemical processing of nanoparticles and in the in situ processing of nanocomposites, has led to unprecedented control over the morphology of these materials.
One of the most common reasons for adding fillers to polymers is to improve their mechanical performance. In traditional composites, this often compromises the ductility of the polymer and, in some cases, negatively impacts its strength because of stress concentration caused by the fillers. Well-dispersed nano-fillers, such as nanoparticles or nanotubes, can improve the modulus and the strength and maintain (or even improve) ductility because their small size does not create large stress concentration. For all nano-fillers, a key requirement is the homogeneous dispersion of the filler within the polymer matrix. As discussed in the following sections, this is a challenge in many cases and a topic of intense research.
Although the scientific community has made remarkable progress in this field in the last years, polymer nanocomposites have just started to be explored, and many research questions still need to be addressed. What is clear so far is that the use of nanoscale fillers opens the way for the development of materials with exceptional properties. For instance, nanoparticles do not scatter light significantly, thus it is possible to make polymer composites with altered electrical or mechanical properties that remain optically clear. Nanoparticles are also of interest not just for their small size, but for their inherent unique properties. Carbon nanotubes, for instance, display the highest values so far seen of elastic modulus, as high as 1 TPA, and strengths that can be as high as 500 GPa. This could allow, for example, the fabrication of polymeric composites with exceptional strength and flexibility.
Another outstanding property that the use of nanoscale fillers confers on nanocomposites is an exceptionally large interfacial area. The increase of surface area below 100 nm is dramatic. The interface controls the degree of interaction between the filler and the polymer and is thus responsible for the composite properties. Thus, the largest challenge in nanocomposite science is learning to control the interface.
Nanoscale fillers can potentially allow the creation of a vast range of different polymeric materials with advanced properties. In general, macroscopic reinforcement elements have the limitation of always containing imperfections, but as the reinforcements become smaller and smaller, structural perfection could be reached. The ideal reinforcements would have atomic or molecular dimensions and be intimately connected with the polymer. The use of nanoscale fillers, however, also introduces a series of fabrication challenges. Because of their small size and high surface area, nanoscale fillers such as nanoparticles have a strong tendency to agglomerate rather disperse homogeneously in a matrix. This leads to particle-matrix mixtures with high viscosities, which can make the processing of those materials quite challenging. The result is that even the most exciting polymer nanocomposites have very low fractions of particle content, and relatively weak mechanical properties when compared to those predicted in theory. Therefore, polymer nanocomposites are an exciting type of advanced materials that hold great promise in many application, but which are still mainly in a development stage. The intense research efforts in this area suggest, however, that these materials will become more readily accessible in the near future.
Nanoparticles are a type of nano-filler that offers the opportunity of developing polymers with new or advanced properties. As already discussed, the size of a nanoparticle affects its properties: for instance, gold nanoparticles have different optical absorption spectra depending on the particle size. One of the advantages of using nanoparticles in polymer composites is that the particle size and distribution can be tuned. Materials that cannot be grown easily as single crystals can be used at the nanoscale and dispersed in a polymer to take advantage of the single-crystal properties.
In general, nanoparticle-filled polymers display better mechanical properties, at least at low volume fractions, and if well dispersed in the polymer. The reason is that nanoparticles are much smaller than the critical crack size for polymers and thus do not initiate failure. Thus, nanoparticles provide a way for simultaneously toughening and strengthening polymers. Proper dispersion is critical, however, in achieving this.
The size scale of nanoparticle-polymer composites ranges from hybrid nanocomposites, in which the polymer matrix and the filler are so intimately mixed that they are no longer truly distinct, to discrete particles in a continuous matrix. Hybrid nanocomposites often arise from the use of block copolymers, and this is discussed in the section on polymers in this chapter).
In terms of properties, the use of nanoparticles in filling polymers can influence not only the polymers’ mechanical properties but also the polymers’ mobility and relaxation behavior, which, in turn, are connected to the glass transition temperature of the polymer (Tg) (i.e. the temperature at which a polymer becomes brittle on cooling or soft on heating). In general, adding a well-dispersed, exfoliated nano-filler increases the Tg of the polymer. Nanoparticle-filled polymers also show an increase in the abrasion resistance of the composite.
One of the most exciting prospects of using nanoparticles in polymer composites is to create composites with combined functionalities, such as electrically conducting composites with good wear properties that are optically clear: this is possible because nanoparticles do not scatter light.
Carbon nanotubes in polymer composites
Carbon nanotubes have very distinct properties compared to graphite, as summarised in Table 1 in the section on mechanical properties. In the context of nanocomposites, SWNTs are the most promising nanotube fillers. Some properties are particularly interesting: in particular their flexibility under mechanical stress, behavior under high-temperature conditions and electrical properties. As with other applications that make use of carbon nanotubes, it has been observed that, in composites, the processing conditions ultimately affect the properties of the nanotubes and, as a consequence, the purity of the composite, as well as the purity of the nanotubes. Carbon nanotubes can also be doped, for example with nitrogen and boron, which changes their surface reactivity. For instance, nitrogen atoms inserted into the lattice of nanotubes makes them more dispersed in solution (carbon nanotubes are insoluble in water). Modified nanotubes present different electrical and optical properties and hence their use could lead to composites with novel properties.
It should be noted that the inclusion of carbon nanotubes in a polymer does not necessarily improve its mechanical properties. Although, in theory, the modulus of nanotubes is much higher than any graphite fiber, and hence the composite should have outstanding mechanical properties, it has been demonstrated that a number of variables influence this outcome. For SWNT composites, the SWNTs are in a bundle; until SWNTs are isolated from the bundles or the bundles are cross-linked, the modulus of composites made of these materials will be limited. For this reason, researchers are concentrating on developing processing methods that make it possible to obtain significant volume fractions of exfoliated nanotubes. As will be discussed later concerning clay-polymer nanocomposites, the structural arrangement of the nano-filler within the polymer is also important: if the nanotubes are not straight when placed in the composite, the modulus of the composite significantly decreases.
Apart from the mechanical enhancement of polymers, nanotubes are also of interest for their electrical properties. Carbon nanotubes are inherently more conductive than graphite. It has been found, for instance, that nanotube-PPV composites show a large increase in electrical conductivity, compared to simple PPV (poly(p-phenylene vinylene), of nearly eight orders of magnitude. Recently, an improvement of 4.5 orders of magnitude in the electrical conductivity of nanotube-PVA nanocomposites has been reported.
Finally, nanotube-polymer composites are promising in the context of light-emitting devices. These devices were developed after the discovery of electroluminescence from conjugated polymer materials (such as PPV). The practical advantages of polymer-based LEDs are their low cost, low operating voltage, ease of fabrication and flexibility. Small loadings of nanotubes in these polymer systems are used to tune the color of emitted light from organic LEDs.
In the late 1980s, it was discovered that adding 5 % by weight of nano-sized clays to Nylon 6, a synthetic polymer, greatly increased its mechanical and thermal properties. Since then, polymer-clay nanocomposites have been widely studied and many commercial products are available. These hybrid materials are made of organic polymer matrices and clay fillers. Clays are a type of layered silicates that are characterized by a fine 2D crystal structure; among these, mica has been the most studied.
Mica is made up of large sheets of silicate held together by relatively strong bonds. Smectic clays, such as montmorillonite, have relatively weak bonds between layers. Each layer consists of two sheets of silica held together by cations such as Li+, Na+, K+ and Ca2+. The presence of the cations is necessary to compensate for the overall negative charge of the single layers. The layers are 20–200 nm in diameter laterally and come into aggregates called tactoids, which can be 1 nm or thicker. Naturally occurring clays include montmorillonite (MMT) and hecrite, and their synthetic equivalents are saponite and laponite respectively.
For these layered silicates to be useful as fillers in nanocomposites, the layers must be separated, and the clay mixed thoroughly in the polymer matrix. This is not trivial as silicate clays are inherently hydrophilic, whereas polymers tend to be hydrophobic. The solution is to exchange the cations that keep the layers in the silicate together with larger inorganic ions that can thus open the galleries between the layers (intercalation). When the silicate layers are completely separated, the material is said to be exfoliated. In the case of intercalation, extended polymer chains are present between clay layers, resulting in a multilayer structure with alternating polymer/clay phases at repeated distances of a few nanometres: in the exfoliated state, the silicate layers are totally separated and dispersed in a continuous polymer matrix.
As already mentioned, the fabrication of a polymer-clay nanocomposite requires mixing two components that are intrinsically non-compatible. Surfactants are ionic, and thus interact well with the clay, but not with the polymer. An ideal solution is the use of ‘macro-surfactants’ such as block copolymers combining hydrophilic and hydrophobic blocks that can interact with the clay and the polymer respectively. For instance, poly(ethylene oxide) (PEO) is an excellent intercalation material.
Although homogeneous dispersion of the filler in the polymer is an important parameter, another important aspect is the packing (or alignment) of the filler in the polymer. To understand this concept, it is useful to consider a natural nanocomposite, bone. The unique properties of bone are a list of apparent contradictions: rigid, but flexible; lightweight, but solid enough to support tissue; mechanically strong, but porous. In order to meet these different demands, bone has a hierarchical structure that extends from the nanoscale to the macroscopic length scale. The hierarchical structure of bone is responsible for its load transferability (see Module 1, Chapter 2: Nanoscience in nature for details on the structure of bone). Nanoparticle-filled polymer composites have mechanical properties that are actually disappointing compared to theoretical predictions, and this is due to the difficulty in obtaining well-dispersed large volume fractions of the reinforcing nanomaterial and a lack of structural control. Taking this into consideration, some scientists have recently reported the fabrication of ultra-strong and stiff layered polymer nanocomposites. In this work, the bottom-up assembly of polymer-clay nanocomposite allowed the preparation of a homogeneous, transparent material, where the clay nanosheets have a planar orientation. It was found that the stiffness and tensile strength of these multilayer nanocomposites are one order of magnitude greater than those of analogous nanocomposites.
Nanoclays are also used as fillers in polymers to increase the thermal stability of polymers. This property was first demonstrated in the late 1960s for montmorillonite-PMMA composites. The dispersion of the clays is critical to increasing the thermal stability (that is, raising the degradation temperature) of the polymer.
In addition to thermal stability, the flammability properties of many polymer-clay nanocomposites are also improved. Combining traditional flame retardants with intercalated or, better, exfoliated clays can result in further improvements in flame retardancy.
Finally, polysaccharide-clay nanocomposites are a class of materials that are important especially to the food industry. These composites make use of naturally occurring polymers, such as starch, mixed with clay to make a biopolymer film with enhanced properties, in particular, permeability to water vapor.
Nanocoatings are a type of nanocomposite. The layer thickness of a nanocoating is usually 1–100 nm. Nanocomposite films include multilayer thin-films, in which the phases are separated along with the thickness of the film, or granular films, in which the different phases are distributed within each plane of the film (Figure 23).
Nanocoatings make it possible to change the properties of some materials, for example to change the transmission of visible and IR radiation in glass, or to introduce new properties such as ‘self-cleaning’ effects. Here, this first type of nanocoating will be discussed under the umbrella term ‘responsive nanocoatings’.
Another important area of application of nanocoatings is tribological coatings. Tribology is the science and technology of interacting surfaces in relative motion. Tribological properties include friction, lubrication and wear. Tribological coatings are those coatings that are applied to the surface of a component in order to control its friction and wear. In this area, the term ‘thin films’ is often used as an alternative to nanocoatings — due to the fact that this is an area of innovation that has existed for many years and has now reached the nanoscale.
Tribological coatings play a key role in the performance of the internal mechanical components of a vehicle, such as an engine and powertrain. They are also key elements in cutting tools and machinery in general. By reducing wear and friction, these coatings increase the lifetime of the working material while also reducing the dissipation of energy as heat, thus increasing the efficiency of the moving part. When applied to machinery and tools, tribological coatings can reduce (or eliminate) the need for lubricants, increase cutting speed, increase the rate of material removal, reduce maintenance costs or reduce processing cycle times.
Traditional materials used in coatings for tribological applications are carbides, cemented carbides, metal-ceramic oxides, nitrides and carbon-based coatings. Since the microstructure controls many of the physical properties of the coating, having a nanoscaled microstructure may lead to significant improvements in the mechanical properties of the coating (e.g. hardness), the chemical properties (e.g. corrosion resistance) and the electrical properties. Thus, nanocomposite coatings are now being investigated as alternatives to the traditional approach of using specific alloying elements in single-phase coating materials to improve, for instance, properties such as hardness. One type of nanocomposite coatings is multilayered thin films made of different layers in the order of nanometres. These films are mostly used for their enhanced hardness and elastic moduli, which is higher in multilayered films than inhomogeneous thin films of either component, and for their wear properties. Commercial multilayer coatings made with multilayer periods in the nanoscale range already exist, such as WC/C coatings used in the cutting tool industry. Other examples are films of alternating layers of TiN and NbN, or TiAlN/CrN multilayers, which are more efficient than TiAlN films.
Responsive nanocoatings are those where the properties of the material in the coating react to environmental conditions, such as light or heat, either in a passive or an active way. These coatings allow the properties of some materials to change, such as glass, by conferring new or improved properties.
The use of glass is very common in modern buildings since it allows the construction of transparent and seemingly lightweight structures. However, the relative high transmittance of visible and infrared (IR) light is a major disadvantage, since this leads to a large heat transfer which is particularly undesirable in summer. The problem is reversed in winter when the heat is dispersed through the glass. In order to address these problems, various types of nanocoatings that modulate light transmission in glass are under investigation and being commercialized. The aim is to reduce indoor heating in summer, so less air conditioning is required to keep the atmosphere cool, with consequent energy saving. One type of coating referred to as ‘Low-E’ (meaning ‘low emissivity’), is based on a thin silver film, about 10 nm thick, surrounded by dielectric layers. Metallic layers have been widely used to increase the reflectivity of light (and reduce transmittance) for years, but they have the disadvantage of giving a mirror-like appearance. Silver loses its metallic appearance when scaled to a nano-film, thus eliminating this problem. Such a coating is commercialized by Von Ardenne.
‘Low-e’ coatings are a type of passive nanocoatings since the properties of the layers are undisturbed during its operation. Another class of coatings used in glasses is those often described as dynamic or ‘smart coatings’. In this case, the environmental conditions, such as radiation intensity or temperature, induce a change in the properties of the coating (e.g. darkening of windows). When the effect is a change in the color (including a change to transparency), they are called chromogenic smart materials. The change can be induced actively by pressing a button. This is the case of electrochromic coatings where applying a small voltage induces a change in transmittance and in the case of gas chromic coatings which changes their transmittance in the presence of specific gases. Gaschromic glazing makes use of the properties of tungsten oxide thin films (WO3) which go from colorless to blue in the presence of hydrogen with a suitable catalyst. Gaschromic windows follow a double pane model: on one pane a film of WO3 is deposited with a thin layer of catalysts on top.
Hydrogen gas is fed into the gap producing coloration (windows color with 1.1–10 % hydrogen, which is below the flammability concentration). To switch the color of the window back, another gas is purged (oxygen). Smart coatings can also be passive in the sense of changing their optical properties due to a change of external temperature (thermochromic) or light incidence (photochromic). Another example of nanotechnology applied to smart coatings is the use of a family of wavelength-selective films for manufacturing ‘heat mirrors’. One of these materials is indium tin oxide (ITO), an infrared absorber. A 300 nm ITO coating on glass provides more than 80 % transmission for the wavelengths predominant in sunlight. The transmission properties of the window can be varied by changing the thickness and material composition of the coating so that a combination of materials could be used to produce smart windows that reflect solar energy in summer but transmit solar energy in winter.
Another example of functional nanocoatings is photocatalytic coatings (commercialized as ‘self- cleaning’ glass) which use the catalytic properties of titanium dioxide (TiO2). When irradiated with UV light, the coating becomes super hydrophilic: therefore, rainwater adheres to the glass providing ‘self- cleaning’. Pilkington Activ™ Self-cleaning Glass is a commercial example of glass with a photocatalytic coating that renders the material easier to clean. Details of this product are given in Module 2, Chapter 2: Environment.
Superhydrophilic coatings are also useful for rendering a surface fog-resistant. These coatings are produced using a solution or a colloid that reduces the surface tension of the material (e.g. glass). Substances that can be used as anti-fog agents include surfactants (e.g. soap), hydrogels, hydrophilic nanoparticles and colloids. The anti-fog agent creates a thin film that does not allow the formation of water droplets but rather ‘forces’ the water molecules to spread on the surface. This film reduces the surface tension of the liquid (the surface tension is the result of the cohesive forces between the molecules that are responsible for the formation of spherical droplets). In the case of water, its surface tension is very high (and, for this reason, liquid water tends to form droplets). The anti-fog agent creates a super hydrophilic surface (very low contact angle) so that water sprayed on the surface creates a thin layer instead of forming round droplets. The layer is so thin that it does not scatter light — meaning that it appears transparent to our eyes. This is explained by the fact that the water molecules are subjected to two different forces: inherent surface tension that tends to round off the droplets, and bonding to the anti-fog nanoparticles that flattens them. Bonding to the surface occurs through hydrogen bonds, weak forces that can become dominant when numerous. The result is a very thin layer of water on the surface: the layer is so thin that it does not scatter light so that it appears transparent to our eyes and so we can see through it well. This type of technology helps prevent goggles used for skiing or swimming steaming up.
The opposite to super hydrophilic coatings are superhydrophobic coatings, which totally repel water. Droplets of water on these surfaces have very high contact angles and ‘bead-up’ forming nearly spherical droplets. Superhydrophobic coatings have surfaces that mimic the surface found in the lotus leaf and are being developed for many applications that require resistance to dirt and ease of cleaning. The details of the Lotus effect® are given in Module 1, Chapter 2: Nanoscience in nature while examples of applications of these coatings are given in Module 2, Chapter 2: Environment.
This chapter summarises some of the methods used in the fabrication of nanomaterials, meaning materials with at least one dimension at the nanoscale level (1–100 nm). These include nanostructured surfaces, nanoparticles, nanoporous materials, etc. The aim of this chapter is to answer the questions: How are nanomaterials made? What fabrication tools are used in nanoscience and nanotechnologies?
Methods for fabricating nanomaterials can be generally subdivided into two groups: top-down methods, and bottom-up methods. In the first case, nanomaterials are derived from a bulk substrate and obtained by the progressive removal of material, until the desired nanomaterial is obtained. A simple way to illustrate a top-down method is to think of carving a statue out of a large block of marble. Printing methods also belong to this category. Bottom-up methods work in the opposite direction: the nanomaterial, such as a nanocoating, is obtained starting from the atomic or molecular precursors and gradually assembling it until the desired structure is formed. The method resembles a building with Lego® bricks.
In both methods, two requisites are fundamental: control of the fabrication conditions (e.g. energy of the electron beam) and control of the environment conditions (presence of dust, contaminants, etc.). For these reasons, nanotechnologies use highly sophisticated fabrication tools that are mostly operated in a vacuum in clean-room laboratories.
Numerous top-down fabrication methods used in nanotechnologies are derived from the fabrication methods used in the semiconductor industry to fabricate the various elements of computer chips (integrated circuits). These methods are collectively called lithography and use a light or electron beam to selectively remove micron-scale structures from a precursor material called resist. In recent years, there has been a tremendous push to reduce the size of electronic devices and integrate more functions into them, which has been possible thanks to the advances in lithographic fabrication methods. Nowadays, it is possible to obtain single features below 100 nm (the transistors in latest generation processors are about 45 nm). Therefore, in the semiconductor industry, nanostructures are routinely fabricated. Lithographic methods that are capable of producing nanoscale features are reviewed in the next section.
There are many more methods to fabricate nanostructures top-down; here, the discussion is limited to the most common methods.
Lithography includes a series of fabrication techniques that share the principle of transferring an image from a mask to a receiving substrate. A typical lithographic process consists of three successive steps: (i) coating a substrate (Si wafer or glass) with a sensitive polymer layer (called a resist);
- exposing the resist to light, electrons or ion beams; (iii) developing the resist image with a suitable chemical (developer), which reveals a positive or negative image on the substrate depending on the type of resist used (i.e. positive tone or negative tone resist). In conventional micro-fabrication used in the semiconductor industry, the next step after lithography is pattern transfer from the resist to the underlying This is achieved through a number of transfer techniques, such as chemical etching and dry plasma etching.
Lithographic techniques can be broadly divided into two groups.
- Methods that use a physical mask, where the resist is irradiated through the mask which is in contact or in proximity with the resist surface. These methods are collectively called mask lithography, among which photolithography is the most
- Methods that use a software mask, where a scanning beam irradiates the surface of the resist sequentially, point by point, through a computer-controlled program where the mask pattern is These methods are collectively called scanning lithography.
The main difference between mask and scanning lithography is speed: whereas mask lithography is a parallel, fast technique, scanning lithography is a slow, serial technique. Another important difference is resolution which, in general terms, is higher for scanning methods. The price paid for higher resolution is the use of more energetic radiation sources, which entails expensive equipment.
Photolithography uses light (UV, deep-UV, extreme-UV or X-ray) to expose a layer of radiation-sensitive polymer (photoresist) through a mask. The mask is a nearly optically flat glass (or quartz, depend- ing on the light used) plate which contains the desired pattern: opaque areas (the pattern, made of an absorber metal) on a UV-transparent background. The image on the mask can be either replicated as it is, placing the mask in physical contact with the resist (contact mode photolithography) or reduced, usually by a factor of 5 or 10, and projected to the resist layer through an optical system (projection mode photolithography) (Figure 1).
The resolution of contact mode lithography is typically 0.5–0.8 μm when UV light (360– 460 nm) is used. Higher resolutions cannot be achieved due to the inability to reduce the gap between the mask and the flat substrate below approximately 1 µm, even when elaborate vacuum systems are used to hold the two parts together. To produce patterns with higher resolution, projection photolithography or ‘next-generation photolithography’ techniques (i.e. extreme UV and X-ray photolithography) need to be employed. These technologies use very expensive equipment and, therefore, their use is limited to selected applications (such as photomask fabrication). The equipment needed is available only in specialized laboratories.
Energetic particles such as electrons and ions can be used to pattern appropriate resist films leading to features with nanometre resolution. When using electrons, the technology is called electron beam lithography (e-beam), whereas using ions, the technology is called focused ion beam lithography. Finally, a recently established technology uses nanometre scanning probes for patterning resist films and is therefore referred to as Scanning Probe Lithography (SPL). This technology has been extended to the deposition of a nano quantity of material (Dip Pen Nanolithography® or DPN®).
In a typical e-beam lithography process, a tightly focused beam of electrons scans across the surface of an electron-sensitive resist film, such as poly(methyl methacrylate) (PMMA). The main advantage of e-beam lithography over photolithography is its high resolution: patterns with features as small as 50 nm can be routinely generated. The resolution of this technology is mainly determined by the scattering of the electrons in the resist film and the substrate. When using particles with a mass higher than electrons, however, this effect is largely reduced. Focused ion beam lithography works on the same principle of e-beam lithography, but ions such as H+, He++, Li+ and Be++ are used. Both techniques provide a resolution much higher than photolithography but share the main disadvantage: both are serial techniques, very slow in the process, so their use is mostly limited to producing photomasks in optical lithography.
Soft lithography is the name for a number of techniques that fabricate and use a soft mold prepared by casting a liquid polymer precursor against a rigid master. These methods have been developed specifically for making large-scale micro and nanostructures with equipment that is easier to operate compared to those used in ‘conventional’ lithography, cheaper and also available in biological laboratories. Figure 2 shows the general principle of soft lithography.
The resolution of soft lithography is mainly determined by van der Waals interactions, wetting and kinetic factors such as filling the capillaries on the surface of the master, but not by optical diffraction. This is an important advantage over ‘conventional’ lithographic techniques. The master is normally fabricated via a conventional lithographic method.
Various polymers (e.g. polyurethanes, epoxides and polyimides) can be used for molding: most commonly, the elastomer poly(dimethylsiloxane) (PDMS) is used. PDMS is non-toxic so it can be used safely with biological materials, including live cells. This is a big advantage in devices that aim to integrate nanostructures with biological systems.
A PDMS mold is fabricated by pouring its liquid precursor over a lithographically-made master (e.g. a photoresistor silicon master), cured to induce cross-linking, and then peeled off. The stamp can then be used either for printing the desired material (the ‘ink’) from the stamp to a suitable surface (microcontact printing, µCP) or, when in contact with a flat or curved surface, to define physical constraints where a liquid can be confined.
Microcontact printing is useful for patterning features with a lateral dimension of 500 nm or larger. One of the major challenges for µCP has been to achieve the capability to print with high resolution (i.e. with lateral dimension lower than 100 nm). This has recently been achieved by improving the stability of the PDMS, which, being soft and highly compressible, has a tendency to deform and collapse. One way to improve the stability of the patterns is to fix a stiff backplane to the stamp or to change the chemical formulation of the stamp itself, in order to obtain a harder polymer. With these modifications, it is now possible to print features as small as 50 nm. This printing method, which uses harder stamps, is called nanocontact printing (NCP).
The concept of nano-imprint lithography is to use a hard master with a 3D nanostructure to mold another material, which assumes its reverse 3D structure. Imagine taking a Lego® block and pressing hard on a piece of Play-Doh. Since the master has a fine nano-structure, to be successful, the process must be done under pressure, a coating must first be placed on the master to avoid catastrophic adhesion to the mould, and the mould must be heated (above its Tg temperature) in order to be soft enough to completely enter the fine master nanostructure and be effectively replicated. The method is the equivalent of embossing at the nanoscale and requires specialized equipment.
In nanosphere lithography, an ensemble of nanospheres ordered on a surface is used as a mask (Figure 4). The nanospheres are dispersed in a liquid (i.e. a colloid) and a droplet placed on a surface and left to dry. Depending on the surface properties (e.g. charge) and media used in the colloid (e.g. presence of electrolytes) the nanosphere will self-assemble in an ordered pattern. In some conditions, a colloidal crystal is obtained: each nanoparticle is surrounded by six other nanospheres. This regular arrangement (which is a 2D colloidal crystal) can be used to create ordered structures on surfaces.
TIP FOR TEACHERS: To illustrate this in class, take a number of spherical beads and pour them onto a shallow plate. The beads will self-organize in the same way.
In the regular arrangement of nanospheres, there will be an empty space between them, which is regularly repeated over the entire surface. In the simplest method, this space is employed to create relatively flat nanopatterns on the surface. The nanosphere pattern is used as a mask, and a material (e.g. gold, silver) sputtered on top of it. Once the nanospheres are removed, a regular pattern of ‘dots’ is left, each shaped like a triangle but with concave sides.
The gold pattern (dots) can also act as growth sites, for example for the growth of carbon nanotubes or ZnO. The result is a regular array of nanotubes or nanowires, as shown in Figure 5.
Nanosphere lithography has now evolved into a method that allows the fabrication of very complex arrays of nanostructures, including 3D features with small holes in them.
Colloidal lithography shares the same principle of nanosphere lithography by using a colloid as a mask for the fabrication of nanostructures on surfaces (Figure 6). In this method, electrostatic forces are employed to obtain short-range ordered arrays of nanospheres on the surface.
The array can then be used to create a number of different nanostructures, through various processes such as etching, lift-off, etc.
Without going into details of the method, what is interesting to note is the different types of nanostructures that can be formed: holes, cones, rings, ‘sandwiches’ made of different materials, etc. (Figure 7).
If made of metals, these nanostructures present a localized surface plasmonic resonance effect (LSPR) which can be used for sensing. These materials are therefore understudy for various sensing applications (e.g. for medical devices).
Scanning probe lithography
Scanning probe microscopy (i.e. STM, AFM, etc.) uses small (< 50 nm) tips to image surfaces with atomic resolution (these methods are described in Module 1, Chapter 6: Characterisation methods). This ability suggests opportunities for their use in generating nanostructures and nanodevices. In this form, they are referred to as Scanning Probe Lithography (SPL), which uses the tip of an AFM to selectively remove certain areas on a surface and Dip Pen Nanolithography® (DPN®), which, similarly, uses the AFM tip to deposit material on a surface with nanometre resolution (Figure 8).
Both are direct writing techniques and their main advantages are high resolution and the ability to generate patterns with arbitrary geometries. Like e-beam and ion-beam lithography, SPL and DPN are serial techniques whose main limitation is speed.
Writing ‘atom by atom’
A particular feature of an STM is that it can be used for more than just to visualize atoms. Twenty years ago, researchers at IBM were able to demonstrate that they could use the STM tip to carefully move atoms on a surface and write the company logo with atoms, as shown in Figure 9.
If one were to write using atoms, letters would be around 1 nm each. With letters of this size, the whole of Encyclopaedia Britannica could be reproduced in an area as small as the tip of a human hair (10-4 m2). Indeed, with letters of this size all the world’s books would fit on a single A4 sheet, but it would take incredibly long to write, and in addition they could only be read with the STM.
Nevertheless, the ability to move individual atoms using an STM has great potential for the future generation of data storage devices. Today, data is stored on CD-ROMs using minute ‘bits’ of semiconductors around 0.1 µm (10-7 m) in size. If these bits were written with atoms instead, far greater data capacity would be achieved. One of these ‘nano-CDs’ with ‘atomic bits’ could contain as much information as one million current CD-ROMs.
The STM allows a material to be built atom by atom independently of its chemistry and physics, as shown in Figure 10. This can lead to new materials that most likely have completely new properties. The process is still very slow, since the atoms can be moved only manually, and this must be one atom at a time. Mass production of new nano-materials using this method is thus not yet possible.
Bottom-up methods can be divided into gas-phase and liquid-phase methods. In both cases, the nanomaterial is fabricated through a controlled fabrication route that starts from the single atom or molecules:
- gas-phase methods: these include plasma arcing and chemical vapor deposition;
- liquid phase: the most established method is sol-gel synthesis; molecular self-assembly is emerging as a new
This is the most common method for fabricating nanotubes. The method uses a plasma which is an ionized gas. A potential difference is placed between two electrodes and the gas in between ionizes. A typical arcing device is made of two electrodes, and an arc passes from one electrode to the other. The first electrode (anode) vaporizes as electrons are taken from it by the potential difference. For instance, a carbon electrode is used to produce carbon nanotubes and this is consumed during the reaction, producing carbon cations. These positively charged ions pass to the other electrode, pick up electrons,and are deposited to form nanotubes.
Plasma arcing can also be used to deposit nanolayers on surfaces rather than making new structures. The deposit can be as little as a few atoms in-depth (and must be at least 1 nm thick to be considered a nanomaterial). In this sense, plasma arcing is complementary to chemical vapor deposition (described next).
Chemical vapor deposition
In this method, the material to be deposited is first heated to its gas form and then allowed to deposit as a solid on a surface. This method is normally performed under a vacuum. The deposition can be direct or through a chemical reaction so that the material deposited is different from the one volatilized. This process is routinely used to make nanopowders of oxides and carbides of metals if carbon or oxygen are present with the metal. The method can also be used to generate nanopowders of pure metals, although not so easy to do.
Chemical vapor deposition is often used to deposit a material on a flat surface. When a surface is exposed to a chemical vapor, the first layer of atoms or molecules that deposit on the surface can act as a template on which material can grow. The structures of these materials are often aligned. During the deposition, a site for crystallization may form in the depositional axis (the axis perpendicular to the surface to be coated). As a result, aligned structures start to grow vertically. This is therefore an example of self-assembly.
Molecular beam epitaxy
This is essentially a very sophisticated evaporation method in which molecular beams interact on a heated crystalline substrate under ultra-high vacuum (UHV) conditions to produce a single crystal film. Molecular Beam Epitaxy (MEB) makes it possible to fabricate crystals one atomic layer at a time. The growth process is highly controlled to avoid contaminants being introduced during crystal growth. A range of surface analysis techniques is used to monitor the growth process and ensure the ty of the crystal. MBE is presently used in the semiconductor industry, where the performance of the device (e.g. computer chip) depends on precise control of dopants and on the production of extremely thin crystal layers with hyper-abrupt interfaces. MBE is used for the fabrication of numerous important devices such as light-emitting diodes, laser diodes, field-effect transistors, read-write heads for computer drives and more.
This method is carried out in the liquid phase. It is a useful self-assembly process for fabricating nanoparticles as well as nanostructured surfaces and three-dimensional nanostructured materials such as aerogels.
A ‘sol’ is a type of colloid (7) in which a dispersed solid phase is mixed into a homogeneous liquid medium. An example of a naturally occurring sol is blood. As the name suggests, the sol-gel process involves the evolution of networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel).
The first stage in the sol-gel process is the synthesis of the colloid. The precursors are normally ions of a metal. Metal alkoxides and alkoxysilanes are the most popular since they react readily with water (hydrolysis). The most widely used alkoxysilanes are tetrameter alkoxysilane (TMOS) and tetraethoxysilane (TEOS) which form silica gels. Alkoxides such as aluminates, titanate, and borates are also used, often mixed with TMOS or TEOS. In addition, since alkoxides and water are immiscible, a mutual solvent is used, such as alcohol.
The sol-gel process involves four steps. First, the hydrolysis reaction, in which the -OR group is replaced with an -OH group. The hydrolysis reaction can occur without a catalyst but is more rapid and complete when catalysts are used. As in any hydrolysis reaction, the catalyst can be a base (NaOH or NH3) or an acid (HF or CH3COOH).
After hydrolysis, the sol starts to condense and polymerize. This leads to a growth of particles which, depending on various conditions such as pH, reach dimensions of a few nanometres. The condensation/polymerization reaction is quite complex and involves many intermediate products, including cyclic structures. The particles then agglomerate: a network starts to form throughout the liquid medium, resulting in thickening, which forms a gel.
All four steps described are affected by the initial conditions for the hydrolysis reaction and the condensation/polymerization. These conditions include pH, temperature and time of reaction, nature of the catalyst, etc.
The sol-gel process is very commonly used to make silica gels. Other types of gels can also be formed: aluminosilicate gels are special because they form tubular structures. One such product is imogolite which has an external diameter of about 2.5 nm and an eternal tube diameter of 1.5 nm. These types of nanostructures are known to be good adsorbents of anions such as chloride, chlorate, sulfate and phosphates. The imogolite structure can be dissolved away with hydrofluoric acid (HF). Therefore, these nanostructures can be used for template synthesis: the tube can be filled with atoms and then dissolved away, leaving rows of atoms (2.5 atoms of gold in a row measuring 1 nm).
Sol-gel process for making nanostructured surfaces
Figure 11 summarises the different sol-gel processes. To make the most of the large surface area of nanoparticles, the gel can be placed on a surface. This way a greater bulk-area ratio is obtained. Another strategy is to form an aerogel. These are three-dimensional continuous networks of particles with air (or any other gas) trapped at their interstices. Aerogels are characterized by being porous and very light yet able to withstand 100 times their own weight.
A versatile way to create ordered surface nanostructures is to perform the sol-gel synthesis in a liquid which is itself ordered. Liquid crystals are precisely this: they have a crystalline structure but exist in a liquid (rather than solid) phase. Nanostructured silica with controlled pore size, shape and order can be made in this way.
The liquid crystalline casting method just described can also be used to produce nanostructured metals. This development is very useful for making nanostructured catalytic surfaces, such as platinum or palladium surfaces. Since these metals are very rare and expensive, it is highly advantageous to have surfaces where nearly all metal atoms can take part in the catalytic reaction (being on the surface), and not just surface atoms as in conventional solids.
Functionalised silica glass surfaces
The sol-gel method also allows the incorporation of organic, inorganic and bio-organic molecules within the silica glass structure. Most organic and inorganic molecules cannot be incorporated (doped) in glass because this is prepared using very high temperatures. The sol-gel process occurs at relatively low temperatures (in some cases at room temperature), so these molecules can be incorporated in the process. This makes it possible, for example, to incorporate molecules such as enzymes inside the silica glass. The result is a material that has the advantages of plastics (the product can be made in any form, it can be attached to other materials, etc.) but also many improvements: the glasses are inert, more stable to heat and chemical attack, the entrapped molecules do not leach out, and are protected in their reactivity, and the glasses are transparent (8).
Self-assembly is the ‘fabrication tool’ of nature: all-natural materials, organic and inorganic, are produced through a self-assembly route. In natural biological processes, molecules self-assemble to create complex structures with nanoscale precision. Examples are the formation of the DNA double helix or the formation of the membrane cell from phospholipids. In self-assembly, sub-units spontaneously organize and aggregate into stable, well-defined structures through non-covalent interaction. This process is guided by information that is coded into the characteristics of the sub-units and the final structure is reached by equilibrating to the form of the lowest free energy
TIP FOR TEACHERS: Self-assembly is a concept that can easily be integrated into conventional lessons on genetics or biology. There are various size scales of self-assembly, from the molecular level (from proteins to DNA) to the ‘macro’ level (evolution of a fetus into a baby). Basically, all-natural processes are examples of self-assembly.
To date, devices have been fabricated starting from the top (a large piece of metal) and carving down to small pieces; now scientists are considering and studying ways to truly build materials from the bottom up, mimicking nature’s own fabrication strategy. Instead of carving nanostructures out of larger materials (which is the typical top-down approach used to fabricate integrated electronic circuits such as micromachining and microlithography), nanostructures could be created bottom-up, from atomic building blocks that self-assemble into larger structures.
In the laboratory, scientists can make use of this self-organization of matter as a way of programming the construction of novel structures with specific functions. Thus, the fabrication process is molecular recognition-directed self-organization. To build bottom-up, specific patterns are inserted into molecules to form organized supermolecules — nanostructures making use of supermolecular chemistry. This can be understood as lock-and-key self-organization of matter: scientists can pre-organize a ‘key’ in a molecule to fit a ‘lock’ in another or vice versa. Once in proximity, the two (or more) molecules assemble together: the lock-and-key mechanism serves to bind the two molecules together in a specific geometry. In supermolecular chemistry, there are no chemical bonds formed during the self- assembly process: the molecules are held together through metal-ion coordination, hydrogen bonding, donor-acceptor interactions, van der Waals forces and mediation effects (e.g. solvent). Transition- metal-mediated structures are a type of supermolecular structure. In the structure, metal cations are the cement that holds the molecules (the ‘bricks’) together. Interest in supermolecular structures derives from the fact that they can have properties that are dramatically different from those of their components (e.g. change in electric properties). Another method uses the structural motifs and self- recognition properties of DNA to self-assemble pre-designed nanostructures in a bottom-up approach. This field is called DNA nanotechnology.
Two other important types of supermolecular structures that are created through a self-assembly process are dendrimers and cyclodextrins (Figures 12 and 13).
DNA nanotechnology exploits the structural motifs and self-recognition properties of DNA to self- assemble pre-designed nanostructures in a bottom-up approach. Two and three-dimensional structures have been fabricated using this self-assembly method. Recently, the revolutionary DNA origami method was developed to build two-dimensional addressable DNA structures of arbitrary shape that can be used as a platform to arrange nanomaterials with high precision and specificity. Researchers at the Centre for DNA Nanotechnology (Aarhus University) have developed a software package to facilitate the design of DNA origami structures and it was initially applied in the design of the dolphins in the former logo of Aarhus University (Figure 14).
The design program was further applied in the design of a three-dimensional DNA box with dimensions 42 × 36 × 36 nm3 that can be opened by external ‘key’ signals (Figure 15). The controlled access to the interior compartment of the DNA container opens the way for several interesting applications of the DNA box, for example as a logic sensor for multiple sequence signals and for the controlled release of nano-cargos with potential applications in the emerging area of nanomedicine.
DNA nanotechnology represents one of the latest developments in nanotechnology. It has applications for the fabrication of nano-guides (e.g. waveguides), sensors (for diagnostic and imaging), logic gates, drug release, nano-motors and electronics (wires, transistors). It could lead to bottom-up electronics and DNA computing, which could become the computing of the future.