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. Development of nanotechnology has been spurred by refinement of tools to see the nanoworld, such as electron microscopy and scanning tunnelling microscopy. This chapter gives an overview of various types of nanoscale materials, their properties and significant foreseeable applications.
Nanomaterials are classified into nanostructured materials and nanophase materials or nanoparticles. Nanostructured materials refer to condensed bulk materials that are made of grains with sizes in the nanometer range. However, nanophase materials are usually dispersive nanoparticles. 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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Nanoscale materials exhibit new or enhanced size dependent properties as compared to their bulk materials. For instance, nanoscale TiO2 and ZnO are transparent and are able to absorb and reflect UV light. There are various types of man made nanomaterials and many others are expected to investigate in future. Nanomaterials can be classified into different types such as, carbon based materials, inorganic nanotubes, nanowires, quantum dots, biopolymers, dendrimers, nanocomposites and diamondoids. Moreover, the variety of nanomaterials is vast, and their array of properties and promising applications are enormous.
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 range of properties and a large number of possible applications.
Fullerenes consist of hexagonal and pentagonal rings of carbon atoms. They come up in shapes of spheres, ellipsoids and tubes. While graphite is a planar structure containing only hexagonal faces, the pentagons make the fullerenes 3D-molecules. Models of C60 and C540 are shown in figure 2.1.
Fig. 2.1: Fullerenes C60 and C540.
In the mid 1980s a new class of carbon material was discovered by Kroto and Smalley, carbon 60 (C60). It is called "buckminsterfullerene", in recognition of the great architect Buckminster Fuller who built geodesic domes. The name fullerene was then given to any closed carbon cage. The C60 has spherical molecular configuration with a diameter of about 1 nm. It consists of 60 carbon atoms arranged as 20 hexagons and 12 pentagons, similar to the configuration of a football. 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. 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)
Carbon nanotubes were discovered by Iijima in 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 assumed an important role in the context of nanomaterials, because they exhibit peculiar 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 about their axis and can conduct electricity well. 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 inorganic fullerene like materials based on layered compounds (e.g., molybdenum disulphide) were discovered soon after CNTs. They have excellent properties like resistance against shockwave impact, catalytic reactivity and large capacity for hydrogen and lithium storage. Inorganic nanotubes extend a range of promising applications due to their useful properties. Oxide based nanotubes (e.g., 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 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 optical absorption of gold nanoshells creates a red colour which can be used in medical products. It is worth mention that the optical properties of nanoshells depend on relative size of the core material and the shell. It is possible to change the colour of nanoshells by varying the core and shell dimensions. The colour of nanoshells can be tuned across a broad range of optical spectrum. Metal nanoshells were invented by the researchers at Rice Quantum Institute. The synthesis of gold nanoshells consists of following simple steps:
Grow or obtain silica nanoparticles dispersed in solutions.
Attach small metal seed (~1-2 nm) colloid to the surface of the nanoparticles via molecular linkages. These seed colloids cover the dielectric nanoparticles surfaces with a discontinuous metal colloid layer.
Grow additional metal on the seed metal colloid adsorbates via chemical reduction in solution.
This approach is successful for the growth of gold and silver metallic shells on the 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 tumour 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 potential well in which particles are confined to two dimensions and thereby occupy a planar region. The effects of quantum confinement 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 two layers of another material having a wider bandgap. For example, GaAs can be sandwitched between the layers of aluminium arsenide with a large bandwidth. These quantum well structures with monolayer thickness can be synthezied by MBE or CVD process.
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 electronics.
2.6 Nanowires (Quantum Wires)
Nanowires are ultrathin wires or linear arrays of dots with a diameter of about1 nm. They are formed by self assembly of atoms on a solid surface. Nanowires can be built from a wide range of materials such as silver, gold, iron or semiconductors. Nanowires are also known as nanorod or quantum wires. 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 formula for electrical resistivity of a wire, ï² = RA/l, is not valid for quantum wires. Where Ï is the resistivity, l is the length, and A is the cross-sectional area of the wire. Usually, semiconductors exhibit conductance quantization for large wires (diameters about 100 nm), since the electronic modes due to confinement are spatially extended. This may result a large Fermi wavelength and hence, they will have low energy gaps. It is intersting that these energy levels can be resolved at cryogenic temperature, where the thermal excitation energy is smaller than the inter-mode energy separation. For metal wires, quantization is only observed for atomic wires and their corresponding wavelength will be small. Furthermore, their energy gap is large and hence, resistance quantization can be seen at room temperature.
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Semiconductor nanowires made of silicon, gallium nitride and indium phosphide have shown significant optical, electronic and magnetic properties. The preparation of nanowires based on growth techniques, such as self assembly processes at which atoms arrange themselves naturally on surfaces, chemical vapour deposition onto patterned substrates or molecular beam epitaxy. 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, light weight, small diameter and high tensile strength.
Fig. 2.5: 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 cell will be formed at the intersection of two nanowires. Sorage device using crossbar array of nanowires is shown in figure 2.5. Nanowire biosensors can be used to detect diseases in blood samples. In such biosensors, nanowires are the working parts and they are attached to certain nucleic acid molecules. These nucleic acid molecules bond to a cystic fibrosis gene in the blood sample, if it is present. This may result a change in the conductance of the nanowire and causes a current to flow. These types of sensors give immediate analysis of blood samples.
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, theire 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 depends on the size and shape of the individual crystal. The band gap is large for small crystals and hence, more energy is liberated when the crystal is deexcited. This energy is related to wavelength or colour of light, and 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. An AFM image of InAs quantum dots is shown in figure 2.6.
Fig. 2.6: AFM image of InAs quantum dots.
Quantum dots have found applications in composites, solar cells, medical imaging, fluorescent biological labels and quantum computation. Quantum dot lasers with ultralow-threshold current densities and low sensitivity to temperature variations have been reported. The flow 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. Figure 2.7 illustrates density of states in different semiconductor structures, such as bulk, quantum well, quantum wire and quantum dot.
Fig. 2.7: 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. 2.8: 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 2.8.
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 encapsulating drugs within their dendritic structure, or by interacting with drugs via electrostatic or covalent bonds. They also help to promote the bioavailability of drugs that are poorly soluble. 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 high degree of branching, multi-valency, globular architecture and well defined molecular weight make them ideally suited 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.
Nanocomposites are multiphase solid materials or structures with nanoscale dimensions. Usually nanocomposites are solid combination of a bulk matrix and nano-dimensional phase(s) differing in properties due to dissimilarities in structure and chemistry. The physical and chemical properties of the nanocomposite differ greatly from that of the component materials. The size limits for, catalytic activity <5Â nm, making a hard magnetic material <20Â nm, refractive index changes <50Â nm and achieving superparamagnetism <100Â nm. Out of the large verities of polymer nanocomposites, dispersion of small amounts of nanoparticles in a polymer matrix is the commercially advanced one. Addition of nanoparticles enhances the mechanical properties and thermal stability, and reduces material flammability. For example, addition of around 2% of silicate nanoparticles to a polyimide resin increases the strength by 100%. Clay/polymer nanocomposites have been considered as matrix materials for fibre based composites destined for aerospace components. Aircraft and spacecraft components require lightweight materials with high strength and stiffness, among other qualities. Nanocomposites, with their superior thermal resistance, are also attractive for such applications. Polymer based nanocomposites exhibit photoluminescence and other remarkable optical properties. has been expanded to anti-corrosion coatings on metals, and thin film sensors. Besides these, polymer-matrix nanocomposites have potential applications in package films, anti-corrosion coatings on metals and thin films sensors. Spacecraft and aircraft components require materials with light weight, high strength and stiffness, and large thermal resistances. Nanocomposites like clay or polymer nanocomposites are best suited for such applications. They are also widely used in automobile applications to construct structural units of vehicles.
2.10 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. 2.9: Schematic diagram of ferritin.
The ferritin molecule consists of a spherical protein shell, whose inner diameter is ~ 8 nm and outer diameter ~12 nm, is shown in figure 2.9. 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 favor 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, also known as nanodiamonds, are organic compounds with unique structures and properties. They are investigated as molecular building blocks for nanotechnology. Some derivatives of diamondoids have been used as antiviral drugs for many years. Nanotechnology molecular building blocks (MBBs) are distinguished for their unique properties. They include graphite, fullerene molecules made of variety number of carbon atoms (C60, C70, etc.), carbon nanotubes, diamondoids, nanowires, nanocrystals and amino acids. Diamondoid family of compounds is one of the best candidates for molecular building blocks to construct nanostructures compared to other MBBs known so far.
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. The diamondoids are valuable molecular building blocks because of their properties like rigidity, strength and assortment of their 3D shapes. The simplest of these polycyclic diamondoids is adamantane, followed by its homologous adamantane, tria-, tetra-, penta- and hexamantane.
Adamantanes have unusual physical and chemical properties due to their unique structures, which can have many applications in nanotechnology. The carbon skeleton structure of adamantane results a cage like structure that may be used for encapsulation of other compounds, i.e., in drug delivery. 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 of the higher diamondoid family exhibits more structural complexity and varieties of molecular geometries.
The chemical structures of adamantane, diamantane and trimantane are shown in figure 2.10. The lower adamantologous, adamantane, diamantine and triamantane, contain only one isomer. Depending upon the configuration of the adamantane units, higher polymantanes contain number of isomers and non-isomeric equivalents. The possible tetramantanes are iso-, anti- and skew-tetramantane. All these tetramantanes are isomeric. The iso-tetramantane has three carbon atoms, but two quaternary carbon atoms in anti- and skew-tetramantanes. Out of the seven possible pentamantane structures, six being isomeric (C26H32) obeying the molecular formula of the homologous series, however, the other non-isomeric (C25H30). There are 24 possible hexamantane structures, 17 are regular cata-condensed isomers with the chemical formula (C30H36), six are irregular cata-condensed isomers with the chemical formula (C29H34), and one is peri-condensed (C26H30).
Fig. 2.10: Chemical structures of diamondoid organic nanostructures.
It has been found that adamantane crystallizes in a face centred cubic lattice and thus the molecule is completely free from angular and torsional strain, making it an excellent candidate for various nanotechnology applications. Adamantane shows only cubic and octahedral faces at the beginning of cryatal growth. The effects of this unusual structure upon physical properties are remarkable. Melting point of adamantane is 269 °C and it is the highest melting hydrocarbons known. But it sublimes easily, even at normal temperature and pressure. Because of this, it can have more interesting applications in nanotechnology.
Diamondoids have the potential to produce different types of nanostructural shapes including molecular-scale components of machinery such as rotors, propellers, ratchets, gears, toothed cogs, etc. They also have applications in moulding and cavity formation characteristics because of their organic nature and sublimation properties. Higher diamondoids have different geometries and large number of attachment sites, and hence they can be used for the production of shape derivatives. Some of the potential applications of diamondoids are listed below:
Cages for drug delivery
Designing molecular capsules
In designing an artificial red blood cell, called Respirocyte
Organic molecular building blocks in formation of nanostructures
Pharmacophore-based drug design
Preparation of fluorescent molecular probes
Rational design of multifunctional drug systems and drug carriers
Self-assembly: DNA directed self-assembly
Synthesis of supramolecules with manipulated architecture
Semiconductors which show a negative electron affinity
2.12 Smart Nanomaterials
Smart materials are materials with properties engineered to modify in a controlled way under the influence of external stimuli, such as temperature, force, moisture, electric charge, magnetic fields and pH. 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 smart materials and taking them to the higher level. Smart nanomaterials are materials made at the nanoscale level, which performs the functions of smart materials. These materials possess strange structures and in that they contain 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 integrate nanosensors, nanocomputers and nanomachines into their structure and hence, enable them to react directly to their environment. 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 will be able to shape shift to make comfortable. For instance, flexible clothing for motorcyclists can work as rock hard if it detects an impact or similar smart materials worn by a police officer is comfortable because these materials can detect an approaching projectile and turn itself bullet proof.
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 smart materials are able to memorize their original shape and return to it when heated. For example, shape memory stents-tubes attached into arteries that expand on heating to body temperature to allow increased blood flow. Thermochromic materials can change colour when temperature changes, but photochromic smart materials would change colour in response to colour variations. Magnetorhelogical fluid materials become solid when magnetic influence is felt. These materials can be fixed to buildings or bridges to suppress the damaging effects of earthquakes.
Applications of Smart Nanomaterials
Healthcare, with smart materials that respond to injuries by delivering drugs and antibiotics or by hardening to produce a cast on a broken limb.
Implants and prostheses made from materials that modify surfaces and biofunctionality to increase biocompatibility.
Energy generation and conservation with highly efficient batteries and energy generating materials.
Security and terrorism defence with smart materials that can detect toxins and either render them neutral, warn people nearby or protect them from it.
Smart textiles that can change colour, such as camouflage materials that change colour and pattern depending upon the appearance of the surrounding environment. These materials may even project an image of what is behind the person in order to render them invisible.
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