Nanomaterials are materials with structural units on a nanometer scale in at least one dimension. They are metals, ceramics, polymeric materials, or composite materials with very small dimensions in the range 1- 100 nanometers (nm). According to Publicly Available Specification (PAS), published by the British Standards Institute, a nanomaterial is defined as a material having one or more external dimensions in the nanoscale or which is nanostructured. Nanomaterials exhibit unique physical and chemical properties such as, electrical, magnetic, mechanical, catalytic, electonic and thermal properies. These novel properties make them suitable for applications in medical, military, scientific, electronic, environmental and commercial sectors. The development of nanotechnology has been stimulated by enhancement of tools like electron microscopy and scanning tunnelling microscopy, to see the nanoworld. This chapter gives an overview of various types of nanoscale materials, their properties and significant foreseeable applications.
Nanomaterials are broadly classified into two; nanostructured materials and nanophase materials (or nanoparticles). Nanostructured materials are condensed bulk materials, which are made of nanoscale grains. However, nanophase materials are usually nanoparticles distributed uniformly in a medium. It is interesting that there are other terms commonly used in describing different nanoparticles. Nanocrystals are specifically denoted to single crystal nanoparticles. Quantum dots are used to describe small particles that exhibit quantum size effects. Similarly, quantum wires are referred to as nanowires when exhibiting quantum effects. To distinguish nanomaterials from bulk, it is important to demonstrate the unique properties of nanomaterials and their prospective impacts in science and technology.
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Nanomaterials exhibit novel or superior size dependent properties as compared to their bulk counter part. For instance, nanoscale TiO2 and ZnO are transparent and they can absorb and reflect ultraviolet light. There are various types of man made nanomaterials, such as carbon based nanomaterials, inorganic nanotubes, nanowires, and quantum dots, and many others are expected to investigate in future. Moreover, a large number of nanomaterials with an array of useful properties make them ideal for variety promising applications.
2.2 Carbon Based Materials
Carbon based nanomaterials mainly consist of carbon, and are generally in the form of hollow spheres, ellipsoids or tubes. Carbon nanomaterials with spherical or ellipsoidal structures are called fullerenes, and cylindrical structures are referred to as nanotubes. These materials have a range of properties and a large number of possible applications.
Fullerenes consist of hexagonal and pentagonal rings of carbon atoms with spherical or ellipsoidal and tube shapes. In the mid 1980s a new allotropic form of carbon was invented by Kroto and Smalley, carbon 60 (C60). It is called "buckminsterfullerene", in respect of the renowned architect Buckminster Fuller who made geodesic domes. Any closed carbon cage is then named as fullerene.
Fig. 2.1: Fullerenes C60 and C540.
The fullerene C60 has spherical molecular configuration with a diameter of about 1 nm. It consists of 60 carbon atoms, which are arranged in 20 hexagons and 12 pentagons, similar to the soccer ball configuration. Spherical fullerenes are called buckyballs. The smallest buckyball is C20 and it consists of 12 pentagons only. Bigger buckyballs are denoted as C2n, where n = 12; 13; 14â€¦, and have non-isomorphic shapes. Models of C60 and C540 are shown in figure 2.1.
Fullerenes generally exhibit properties such as superconductivity, heat resistance, chemically unreactive, soluble in many solvents and so on. Fullerenes have many potential applications which include, drug delivery vehicle and miniature ball bearings to lubricate surfaces. Structure, properties and applications of fullerenes are presented in chapter 6.
2.2.2 Carbon Nanotubes (CNTs)
CNTs were discovered by Iijima (1991). They are cylindrical fullerenes. Single walled nanotube (SWNT) and multi walled nanotube (MWNT) are the two types of CNTs. SWNT consists of one cylindrical tube with a diameter of about 1-2 nm, while MWNT consists of several concentric tubes having a diameter of about 2-25 nm. The interlayer distance of MWNT is around 3.3 Çº. Both the CNTs have a length range from several micrometres to centimeters and their length-to-diameter ratio can go beyond 10,000.
Fig. 2.2: Models of carbon nanotube: (a) SWNT and (b) MWNT.
CNTs have a significant role in nanotechnology, because they exhibit unique chemical and physical properties. They show very high mechanical properties. For example, Young's modulus is above 1 TPa and tensile strength is over 126 GPa. Besides these, CNTs are flexible along their axis and are very good electrical conductors. Furthermore, all these properties make them ideal for a verity of potential applications, such as in microelectromechanical devices, AFM tips and display devices. A detailed discussion of carbon based nanomaterials and carbon nanotubes is given in chapter 6.
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Carbon nanobuds are fabricated by combining carbon nanotubes and fullerenes. In nanobuds fabrication, fullerene buds are covalently bonded to the outer sidewalls of the CNT, as shown schematically in figure 2.3. They possess amazing properties of both fullerenes and CNTs. They have excellent properties like huge capacity for hydrogen or lithium storage, catalytic reactivity and good field emitters. In composite nanobud materials, the bonded fullerene molecules may act as molecular anchors and thereby preventing slipping of the nanotubes, which may improve the composite's mechanical properties.
Fig. 2.3: Model of a nanobud.
2.3 Inorganic Nanotubes
Inorganic nanotubes and fullerene materials were discovered just after the discovery of carbon nanotubes. These materials have outstanding properties like effective resistance against shockwave impact, better catalytic activity and capacity for hydrogen and lithium storage. Inorganic nanotubes have number of potential applications due to their noval properties. For example, oxide based nanotubes (TiO2) can be used in catalysis, photo-catalysis and energy storage.
Nanoshells are a new type of nanoparticles and are ball shaped. They are made up of a core of nonconducting substance such as silica (glass), which is coated with an extremely thin metallic layer (shell) such as gold or silver. Nanoshells are about the size of a virus or about 100 nm wide. The physical properties of gold nanoshells are similar to gold colloid. The absorption of light by gold nanoshells creates a red colour which can be used in medical products. It is worth mention that the optical properties of nanoshells change as the relative size of the core and the shell changes. Hence, it is possible to change the colour of nanoshells by varying the dimensions of core and shell, and can tune across a wide range of spectrum. Metal nanoshells were invented by the researchers at Rice Quantum Institute. The synthesis of nanoshells generally consists of following simple steps:
Grow dielectric nanoparticles (e.g., silica) dispersed in solutions.
Bond small metal seed (~1-2 nm) colloid into the surface of the dielectric nanoparticles by molecular linkages. A discontinuous metal colloid layer is formed over the surface of the dielectric core.
Breed additional metal on the seed metal colloid adsorbates through chemical reduction in solution.
This approach is successful for the growth of both gold and silver nanoshells on silica nanoparticles. Figure 2.4 shows growth stages of gold shells over silica nanoparticles.
Fig. 2.4: Growth stages of gold shells around silica nanoparticles
Nanoshells have found potential application in cancer treatment without any toxic side effects as in chemotherapy. These nanoshells can be injected safely into the body. When the nanoshells in the body are illuminated with laser beam, intense heat will be produced and thus destroys the tumor cells. Nanoshells combined with lasers can also be used to eradicate cancer cells. Nanoshells are already being developed for other medical applications such as drug delivery and testing for proteins associated with Alzheimer's disease.
2.5 Quantum Well
A quantum well is a thin layer in which particles (electrons or holes) are confined in the dimension perpendicular to the layer surface, while movement is possible in other dimensions. The confinement of particles is a quantum effect and it will occur as thickness of the quantum well approaches the deBroglie wavelength of charge carriers, resulting discrete energy levels to carriers. Quantum well structures in semiconductors are formed by sandwitching a material between other layers of another material having a wider bandgap. For example, GaAs quantum well can be sandwitched between the layers of aluminium arsenide having a large bandwidth. The thickness of the quantum well is typically about 5-20 nm. These quantum well structures with monolayer thickness can be synthesised by MBE or metal organic CVD process. In laser diodes, semiconductor quantum wells are used in the active region. These quantum wells are embedded between two wider layers with a large band gap. The cladding layers act as a waveguide. But particles are captured by the quantum well as the difference in band gap energy is sufficiently large. A 5 nm GaAs quantum well embedded between AlAS layers is shown in figure 2.5.
Fig. 2.5: A 5 nm GaAS quantum well.
The density of states of electrons in quantum wells as a function of energy, and possess distinct steps due to quasi-two dimensional nature. Additionally, the reduced amount of active materials in quantum well leads to better performance in optical devices, such as laser diodes for DVDs and laser pointers, infra-red lasers in fibre optic transmitters and IR photodetectors for IR imaging. Quantum wells can also be used to develop High Electron Mobility Transistors (HEMTs), which are found applications in low-noise electronic devices.
2.6 Quantum Wires
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Quantum wires are ultrathin wires or linear arrays of dots with a diameter of about1-10 nm. They are defined as nanostructures with a diameter or thickness constrained to a few nanometers, but no restriction to length. Quantum mechanical effects are significant at these scales and hence, they are named quantum wires. They are formed by self assembly of atoms on a solid surface. Quantum wires are made from metals (e.g., Ni, Pt, Au), semiconductors (e.g., Si, InP, GaN) and insulators (e.g., SiO2, TiO2). Molecular quantum wires are formed by repeated (organic or inorganic) molecular units (e.g., DNA). Quantum wires are also known as nanorod or nanowires. Quantum wires are electrically conducting wires, their transport properties are governed by quantum size effects. Because of the quantum confinement of free electrons across the wire, transverse energy is quantized into discrete energy values E0, E1, etc. It is important to note that the classical equation for electrical resistivity of a wire, ï² = RA/l, is not valid for quantum wires. Where l is the length of the wire and A its cross-sectional area.
The preparation of nanowires is based on growth techniques, such as self assembly processes, CVD and MBE. These fabrication techniques are discussed in chapter 4. The materials used for the growth of nanowires should be soluble in the catalyst nanoparticles. For example, gold catalyst nanoparticles are used to create silicon nanowires because silicon is soluble in gold. The nanowires, which are made from CNTs exhibit unique properties, such as high electrical conductivity and large tensile strength. Semiconductor nanowires have shown important optical, electronic and magnetic properties.
Fig. 2.6: Memory device using a crossbar array of nanowires.
Nanowires have an array of potential applications, which include high density data storage, various nanodevices and metallic interconnect. Researchers have reported that memory devices based on indium oxide nanowires can store 40 gigabits per square centimeter. For data stotage, each memory unit is formed at the intersection of two nanowires. Sorage device using crossbar array of nanowires is shown in figure 2.6. Nanowire biosensors can be used to detect diseases in blood samples. In such biosensors, nanowires are the working parts and they are attached to some nucleic acid molecules. These molecules are attached to a cystic fibrosis gene in the blood sample, if it is present. As a consequence, there is a change in the conductance of the quantum wire and may result a current flow. These types of sensors give immediate analysis of blood samples. Nanowires also have tremendous applications in optoelectronics, microelectronics, photonics and field emission devices.
2.7 Quantum Dots
Semiconductor nanoparticles and quantum dots were predicted in the 1970s, however, their experimental investigations were reported in the early 1980s. Quantum dots are semiconductors whose excitons are confined in three dimensions. Hence, their properties lie between those of bulk semiconductors and those of discrete molecules. Quantum dots were invented by Louis E. Brus, and the term "Quantum Dot" was called by Mark Reed. The conductivity of quantum dots changes with the size and shape of individual crystals. The band gap is large for small crystals and hence, more energy is liberated when the crystal is deexcited. Since wavelength or colour of light depends on the energy liberated, the optical properties of the particles can be tuned by controlling their size. Moreover, quantum dots are made to absorb or emit specific colours by manipulating their size. It is important that high level control over the conductivity of the quantum dots is possible because the size of the crystals produced can be controlled. Monodispersed crystalline quantum dots with a dimension of about 2 nm were reported in literature. Atomic force microscopic image of InAs quantum dots is shown in figure 2.7.
Fig. 2.7: A typical AFM image of InAs quantum dots.
The flow of electrons through quantum dots can be controlled by applying suitable small voltages to the leads, and thereby spin and other properties therein can be measured. Quantum dots have an array of promising applications in solar cells, lasers, medical imaging, biological labels, composites and quantum computation. Figure 2.8 illustrates density of states in different semiconductor structures, such as bulk, quantum well, quantum wire and quantum dot.
Fig. 2.8: Density of states in semiconductor structures depending on dimensionality.
A dendrimer is referred to as a macromolecule, which is characterized by its 3D structure with a high degree of surface functionality and versatility. The term "dendrimer" originated from two Greek words: dendron, meaning tree and meros, meaning part. There are different types of dendrimers, and their multivalent and monodisperse properties make them suitable for various significant applications, such as drug delivery, gene therapy, chemotherapy and environmental clean-up.
Fig. 2.9: The Dendritic Structure
Dendrimers are built from a starting atom and other elements are bonded to it by a repeating series of chemical reactions, generating a spherical branching structure. By repeating the process, successive layers will be added and the size of the dendrimer can be expanded to the desired level. Dendrimers consist of mainly three components: (i) an initiator core, (ii) interior layers (generations) composed of repeating units, attached to the interior core and (iii) exterior attached to the outermost interior generations. The dendritic structure is shown in figure 2.9.
Dendrimers have many potential applications, which are based on their properties like unparalled molecular uniformity, multifunctional surface and internal cavities. They can be used as drug carriers by holding drugs within their structure, or by interacting with medicines via electrostatic or covalent bonds. The modification of poly(lysine) dendrimers with sulfonated naphthyl groups help to use as antiviral drugs against the herpes simplex virus. The excellent properties of dendrimers like branching, multi-valency and globular structure make them ideally fit for various pharmaceutical and diagnostic applications. Large numbers of drugs, which are being developed today, are facing problems like poor solubility, bioavailability and permeability. In such problematic drugs, dendrimers can be used as a tool for optimizing drug delivery.
2.9 Biological Nanomaterials
The study of biological nanomaterials can help to improve the production of synthetic nanomaterials with similar characteristics. Biological systems contain many nanophase materials. For example, most of the living systems produce mineral materials (e.g., bone) with nanoscale particle structures. The biomineralization process consists of delicate biological control mechanisms that produce well defined nanomaterials. The study of these processes helps to construct novel materials. These materials can be modified by changing the biological situation in which they are produced. The biological materials can also be subjected to in vitro manipulation following extraction. Moreover, biological materials are used as the starting material for many of the standard procedures for the synthesis and processing of nanomaterials, such as vapour techniques, mechanical attrition, etc. For example, ferritins (a class of proteins) provide a biological system in which living organisms can synthesize and interact with nanosized particles of iron in the organism, such as oxyhydroxides and oxyphosphates. These proteins have been found in many types of living organism, from bacteria to man. The important biological functions of ferritins include the storage, transport and detoxification of nanosize iron in the organism.
Fig. 2.10: Schematic diagram of a ferritin molecule.
The ferritin molecule consists of a spherical protein shell, whose inner diameter is ~ 8 nm and outer diameter ~12 nm, is shown in figure 2.10. The spherical protein shell consists of 24 protein subunits and these subunits self assemble to form a structure with channels, which permit iron ions to pass in and out. Ferritins provide a range of possibilities in the context of the synthesis and study of nanomaterials. Native ferritin consists of well separated nanometer sized magnetic particles, produced by biologically controlled mineralization within the cavity of the protein shell. The native material can provide a range of magnetic small particle model systems for the investigation of nanomaterial behaviour by a variety of techniques. The interpretation of this behaviour can of course assist in our understanding of the extracted material and, in turn, the system from which it has come.
"The constraints of predictability, stability and positional control in the face of thermal vibration all favour the use of diamondoid covalent structures in future nanomachines. Present-day experience with design and computational modeling of such devices has generated useful design heuristics, including reasons for favoring diamondoid materials with many noncarbon atoms."
K. Eric Drexler
Diamondoids are organic compounds with unique structures and unusual properties. Diamondoids are usually known as cage hydrocarbons. In 1933, Diamondoids were discovered and liberated from Czechoslovakian petroleum, and then were found in many different crude oils, with varying concentrations and compositions. They possess diamond like fused ring structures, and are highly symmetrical and strain free. They are the one of the best candidates for molecular building blocks (MBBs) to construct nanostructures because of their properties like rigidity, strength and assortment of their 3D shapes. Members of this polycyclic diamondoid family are adamantane, homologous adamantane, tria-, tetra-, penta- and hexamantane.
Adamantanes have unusual physical and chemical properties due to their unique structures. The cage like structure of adamantane can be used for the encapsulation of drugs or other compounds. Usually diamondoids are considered as saturated, polycyclic and cage-like hydrocarbons. The general molecular formula of homologous polymantane series is C4n+6H4n+12, where n = 1, 2, 3,â€¦ (n = 1 for adamantane). Each higher diamondoids shows more structural complexity and different molecular geometries.
The chemical structures of adamantane, diamantane and trimantane are shown in figure 2.11. The lower adamantologous, such as adamantane, diamantine and triamantane, have only one isomer. Higher polymantanes contain number of isomers and non-isomeric equivalents, depending upon the configuration of the adamantane units. The possible tetramantanes are iso-, anti- and skew-tetramantane. All these tetramantanes are isomeric. The iso-tetramantane contains three carbon atoms, but two quaternary carbon atoms in anti- and skew-tetramantanes. Six out of seven possible pentamantane structures are isomeric (C26H32), following the molecular formula of the homologous series. But the last pentamantane structure is non-isomeric (C25H30). There are twenty four possible hexamantane structures. Seventeen of these structures are regular cata-condensed isomers with the chemical formula (C30H36). But six hexamantane structures are irregular cata-condensed isomers (C29H34), and the remaining one is peri-condensed (C26H30).
Fig. 2.11: Chemical structures of diamondoid organic nanostructures.
It is important that adamantane crystallizes in f.c.c lattice and thus the molecule is free from both angular and torsional strain. Adamantane shows only cubic and octahedral structures at the beginning of cryatal growth and this unusual structure make remarkable changes in physical properties. Melting point of adamantane is 269 °C and it is the highest melting hydrocarbons known. Adamantane sublimes even at normal temperature and pressure.
Diamondoids have the potential to produce different types of nanostructural shapes. For example, molecular-scale components of machinery like rotors, propellers, ratchets, gears, etc. Their organic nature and sublimation properties make them capable for moulding and cavity formation applications.
Higher diamondoids have different geometries and large number of attachment sites, and hence they are able to create shape derivatives. A few derivatives of diamondoids can be used as antiviral drugs. Some of the potential applications of diamondoids include drug targeting, cages for drug delivery, gene delivery and molecular machines.
2.11 Smart Nanomaterials
Smart materials are materials with one or more properties can be modified in a controlled manner by external stimuli, such as temperature, force, moisture, pH, electric and magnetic fields. They are key materials to offer structures with smart functions and can be used as advanced functional materials. Smart structures show the skill to study their instantaneous environment and execute control actions in reliable way. They found potential applications in various areas like aerospace, transportation, medical hardware, manufacturing technology and so on.
The influence of nanotechnology enhances the potential of these materials. Smart nanomaterials are materials made at the nanoscale level, which performs the functions of smart materials. These materials possess strange structures with mobile electronic charge carriers. These mobile charge carriers can move to new positions in the structure. This change can be controlled by shining light on them or applying an electric field, and the change is like a code. These smart nanomaterials can incorporate nanosensors, nanocomputers and nanomachines into their structure. As a result, they may be enabled to respond directly to their surroundings. Self-tinting automotive glass is an example of a smart material. This glass is clear most of the time, but when the sunlight reaches certain intensity level, the glass darkens to avoid the driver from being blinded. These smart materials are able to shape shift to enable a comfortable environment. For instance, smart flexible clothing for motorcyclists can turn it to hard, as it senses an impact.
There are various types of smart materials, such as piezoelectric, shape memory, thermochromic, photochromic and magnetorheological. The working of the piezoelectric smart material is based on piezoelectric effect. The shape memory materials are able to memorize their initial shape and recover it while heating. For example, if shape memory stents-tubes are attached into arteries, which expand on heating to body temperature and allow smooth blood flow. The colour of thermochromic materials changes as temperature changes, but photochromic smart materials would change colour in response to colour variations. Magnetorhelogical fluid materials become solid when magnetic influence is felt. Thus, they can be used in buildings or bridges to reduce the shock impact of earthquakes. Smart nanomaterials offfer an array of promising applications in a variety of fields, which include healthcare, energy generation and conservation, security and terrorism defence, and implants and prostheses.