NANOSCIENCE Nanoscience is the study, understanding and control of phenomena and manipulation of material at the nanoscale, so nanoscience is the world of atoms, molecules, macromolecules, quantum dots, and macromolecular assemblies STRUCTURE OF CARBON NANOTUBE Carbon nanotube (CNT) is one form of carbon, with nanometer-sized diameter and micrometer-sized length (where the length to diameter ratio exceeds 1000). The atoms are arranged in hexagons, the same arrangement as in graphite. The structure of CNT consists of enrolled cylindrical graphitic sheet (called graphene) rolled up into a seamless cylinder with diameter of the order of a nanometer. It is understood that CNT is the material lying in between fullerenes and graphite as a quite new member of carbon allotropes Carbon nanotubes are members of the fullerene structural family, which also includes buckyballs. Whereas buckyballs are spherical in shape, a CNT is cylindrical, the ends of some CNTs are open; the others are closed with full fullerene caps. CNTs name is derived from their size, since the diameter of a CNT is on the order of a few nanometers (approximately 50,000 times smaller than the width of a human hair), while they can be up to several micrometer in length. Commercial applications for CNT have been rather slow to develop, however, primarily because of the high production costs of the best quality CNTs. STRUCTURE OF CNTS Carbon nanotubes are sheets of graphite that have been rolled into a tube (Thostenson et al., 2001). A graphene sheet can be in rolled more than one way, producing different types of CNTs, (graphene is an individual graphite layer, Fig. 4). 1. Download full-size image Figure 4. Rolling of a single layer of graphite sheet into SWCNT. CNTs are considered as nearly one-dimensional structures (1D buckytube shape) according to their high length to diameter ratio. Most important structures are SWCNTs and MWCNTs. A SWCNT is considered as a cylinder with only one wrapped graphene sheet while MWCNTs are similar to a collection of concentric SWCNTs. The length and diameter of these structures differ a lot from those of SWCNTs and, of course, their properties are also very different. The bondings in CNTs is sp2 and consist of honeycomb lattices and are seamless structure, with each atom joined to three neighbors, as in graphite. The tubes can therefore be considered as rolled up graphene sheets. The type of CNT depends on how the graphene sheet is oriented on rolling. This can be specified by a vector (called chiral vector), which defines how the graphene sheet is rolled up. Fig. 5 showing how a hexagonal sheet of graphite is rolled to form a CNT in a vector structure classification. Figure 5. The 2D graphene sheet diagram showing a vector structure classification used to define CNT structure (Dresselhaus et al., 1996). The vector is determined by two integers (n,m). Two atoms in a planar graphene sheet are chosen and one is used as origin. The chiral vector C is pointed from the first atom toward the second one and is defined by the relation (Dresselhaus et al., 1995): C=na1+ma2 Where: n and m are integers. a1 and a2 are the unit cell vectors of the two-dimensional lattice formed by the graphene sheets. The direction of the CNT axis is perpendicular to this chiral vector. For example; to produce a CNT with the indices (6,3), say, the sheet is rolled up so that the atom labelled (0,0) is superimposed on the one labelled (6,3). The length of the chiral vector C is the circumference of the CNT and is given by the corresponding relationship: c=|C|=a(n2+nm+m2) Where the value a is the length of the unit cell vector a1 or a2. This length a is related to the carbon–carbon bond length acc by the relation: a=|a1|=|a2|=acc3 For graphite, the carbon–carbon bond length is acc = 0.1421 nm. The same value is often used for CNTs (Wildoer et al., 1998). But due to the curvature of the tube a slightly larger value such as acc = 0.144 nm should be a better approximation (Murakami et al., 2003, Saito et al., 2000, Jorio et al., 2001). Using the circumferential length c, the diameter of the CNT is thus given by the relation: d=c/π The angle between the chiral vector and zig-zag nanotube axis is the chiral angle θ. With the integers n and m already introduced before, this angle can be defined by: θ=tan−1(m3)/(m+2n) Carbon nanotubes are only described by the pair of integers (n,m) which is related to the chiral vector. It can be seen from Figure 5, Figure 6, Figure 7 three types of CNTs are revealed with these values: •m = 0 for all zig-zag tubes and (θ = 30°); •n = m for all armchair tubes and (θ = 0°); •Otherwise, when n ≠ m they are called chiral tube and (0° < θ < 30°). The value of (n,m) determines the chirality of CNT and affects the optical, mechanical and electronic properties. CNTs with |n − m| = 3i are metallic like as in (10,10) tube, and those with |n − m| = 3i ± 1 are semiconducting like as in (10,0) tube, (i is an integer). The armchair and zig-zag tubes structures have a high degree of symmetry. These terms refer to the arrangement of hexagons around the circumference. While the chiral tube structure, which in practice is the most common, meaning that it can exist in two mirror-related forms .SYNTHESIS OF CNTS The way in which CNTs are formed is not exactly known. The growth mechanism is still a subject of study, and more than one mechanism might be operative during the formation of CNTs. One of the mechanisms consists out of three steps. First a precursor to the formation of CNTs and fullerenes, C2, is formed on the surface of the metal catalyst particle. From this metastable carbide particle, a rodlike carbon is formed rapidly. Secondly there is a slow graphitisation of its wall. This mechanism is based on in situ TEM observations (Ayumu et al., 2002). The exact atmospheric conditions depend on the technique used. The actual growth of the CNT seems to be the same for all techniques mentioned.
TYPES OF CNTS The two main types of CNT are the single and multi-walled, but there are some other rare types such as fullerite, torus, and nanoknot. 3.1. SINGLE-WALLED A single-walled carbon nanotubes (SWCNTs) can be considered to be formed by the rolling of a single layer of graphite (called a graphene layer) into a seamless cylinder (long wrapped graphene sheets). As stated before, CNTs generally have a length to diameter ratio of about 1000 and more so they can be considered as nearly onedimensional structure. Most SWCNTs have a diameter of close to 1 nm. More detailed, a SWCNT consists of two separate regions with different physical and chemical properties. The first is the sidewall of the tube and the second is the end cap of the tube (Iijima and Ichihashi, 1993). SWCNTs are a very important variety of a CNT because they exhibit important electric properties that are not shared by the MWCNT variants. The most basic building block of these systems is the electric wire, and SWCNTs can be excellent conductors. SWCNTs are still very expensive to produce, and the development of more affordable synthesis techniques is vital to the future of carbon nanotechnology. If cheaper means of synthesis cannot be discovered, it would make it financially impossible to apply this technology to commercial scale applications (Collins and Avouris, 2000). 3.2. MULTI-WALLED Multi-walled carbon nanotubes (MWCNTs) can be considered as a collection of concentric SWCNTs (consist of multiple layers of graphite rolled in on themselves to form a tube shape) with different diameters. The length and diameter of these structures differ a lot from those of SWCNTs and, of course, their properties are also very different (Iijima and Ichihashi, 1993). The interlayer distance in MWCNTs is close to the distance between graphene layers in graphite, approximately 3.3 Å. The special case of MWCNTs (double-walled carbon nanotubes DWCNTs) must be emphasised here because they combine very similar morphology and properties as compared to SWCNT. DWCNT synthesis on the gram scale was first proposed in 2003 (Flahaut et al., 2003) by the chemical vapour deposition (CVD) technique, from the selective reduction of oxides solid solutions in methane and hydrogen CHARACTERISATION OF CNTS There are many production methods for CNTs, each producing material that is slightly different: different in diameter, length, chirality, purity, catalysts, impurity species, and defects. And, although purification methods increase the fraction of CNTs in the sample, it also modifies the CNT themselves: it may open both or one on CNT ends, reduced length, modify functional groups, and sometimes it cause defects Carbon nanotubes are nanometric carbon particles with a graphitic structure, but it also contains many of the impurities. Characterisation of CNTs to determine the quantity, quality, and properties of the CNTs in the sample is very important, because its applications will require certification of properties and function Production of CNTs in a controlled way and in large amount encounters problems, which remains to be solved. It is needed to identify all properties of these tubes. In order to investigate the morphological and structural characterisations of CNTs, a reduced number of techniques could be used. However, to fully characterise CNTs, there are not so many techniques available at the individual level such as scanning tunneling microscopy and transmission electronic microscopy. X-ray photoelectron spectroscopy is helpful in order to determine the chemical structure of nanotubes while neutron and X-ray diffraction, infrared and Raman spectroscopy are mostly global characterisation techniques. CARBON NANOTUBES PROPERTIES Electrical Conductivity There has been considerable practical interest in the conductivity of CNTs. CNTs with particular combinations of N and M (structural parameters indicating how much the nanotube is twisted) can be highly conducting, and hence can be said to be metallic. Their conductivity has been shown to be a function of their chirality (degree of twist), as well as their diameter. CNTs can be either metallic or semi-conducting in their electrical behavior. Strength And Elasticity The carbon atoms of a single (graphene) sheet of graphite form a planar honeycomb lattice, in which each atom is connected via a strong chemical bond to three neighboring atoms. Because of these strong bonds, the basal-plane elastic modulus of graphite is one of the largest of any known material. Thermal Conductivity And Expansion CNTs may be the best heat-conducting material man has ever known. Ultra-small SWNTs have even been shown to exhibit superconductivity below 20oK. Research suggests that these exotic strands, already heralded for their unparalleled strength and unique ability to adopt the electrical properties of either semiconductors or perfect metals, may someday also find applications as miniature heat conduits in a host of devices and materials. The strong in-plane graphitic C-C bonds make them exceptionally strong and stiff against axial strains. The almost zero in-plane thermal expansion but large inter-plane expansion of SWNTs implies strong in-plane coupling and high flexibility against nonaxial strains. Many applications of CNTs, such as in nanoscale molecular electronics, sensing and actuating devices, or as reinforcing additive fibers in functional composite materials, have been proposed Electron Emission Field emission results from the tunneling of electrons from a metal tip into vacuum, under application of a strong electric field. The small diameter and high aspect ratio of CNTs is very favorable for field emission. Even for moderate voltages, a strong electric field develops at the free end of supported CNTs because of their sharpness. HIGH ASPECT RATIO CNTs represent a very small, high aspect ratio conductive additive for plastics of all types. Their high aspect ratio means that a lower loading (concentration) of CNTs is needed compared to other conductive additives to achieve the same electrical conductivity. This low loading preserves more of the polymer resins’ toughness, especially at low temperatures, as well as maintaining other key performance properties of the matrix resin. CNTs have proven to be an excellent additive to impart electrical conductivity in plastics. Their high aspect ratio (about 1000:1) imparts electrical conductivity at lower loadings, compared to conventional additive materials such as carbon black, chopped carbon fiber, or stainless steel fiber. GRAPHENE Graphene is the basic structural element of carbon nanotubes. It is one-atom thick planar sheet of (sp 2 bonded) carbon atoms that are densely packed in a honeycomb crystal lattice, The definition of graphene is “a flat monolayer of carbon atoms tightly packed into a two-dimensional honeycomb lattice, which is a basic building block for graphitic materials of all other dimensionalities. It can be wrapped up into fullerenes, rolled into
nanotubes or stacked into graphite”. Graphene is stronger and stiffer than diamond. It, however, can be stretched like rubber. Its surface area is the largest known for its weight. The C–C bond length in graphene is ~0.142 nm. The graphene sheets stack to form graphite with an interplanar spacing of 0.335 nm, which means that a stack sheets will be ~1 mm thick.of 6 3 10 Graphene is an allotrope of carbon that exists as a two-dimensional planar sheet. One way to think of graphene is as a single atomic graphite layer. PROPERTIES OF GRAPHENE Graphene is technically a non-metal but is often referred to as a quasi-metal due to its properties being like that of a semiconducting metal. As such, it has many unique properties that you don’t find with other non-metallic materials. Each carbon atom is covalently bonded (sp2 hybridized) to three other carbon atoms in a hexagonal array, leaving one free electron per each carbon atom. This free electron exists in a p-orbital that sits above the plane of the material. Each hexagon in the graphene sheet exhibits two pielectrons, which are delocalized, allowing for an efficient conduction of electricity. The holes in the structure also allow phonons to pass through unimpeded, which gives rise to a high thermal conductivity. Graphene has many unique properties, making it an ideal material for use in electronic applications when compared to conventional materials. Electrical conductivity the most prevalent and important property of graphene. Graphene doesn’t have an electronic band-gap (meaning that it can’t be switched on or off) as the valence and conduction bands have a small overlap and the electrons act as massless relativistic particles. At room temperature, graphene can exhibit a concentration of charge carriers up to 1013 cm-2, with a mobility of 1 X 104cm2 V-1 s-1. At low temperatures, this can increase to 2 X 105 cm2 V-1 s-1. The QHE is the relationship of the charge, density and velocity of the charge carriers. It occurs when a magnetic field is applied along the axis perpendicular to the plane of the conducting material. Under these conditions, the path of the carriers becomes curved, leading to an accumulation of opposite charges at either end of the material. Due to the two-dimensional nature of graphene, the electron confinement produces discrete band levels known as Landau levels, which are filled by the charge carriers. Unlike other materials, the charge carriers in graphene only half-fill these levels, leading to a quantization of the Landau levels, and in effect the energy levels of graphene. Graphene also has great optical, thermal and mechanical properties. Single sheet graphene is a highly transparent material but each layer in thickness absorbs up to 2.3% of white light, with less than 0.1% reflectance. There is also a linear absorbance increase with respect to the number of layers stacked on top of each other. A suspended graphene sheet can exhibit a thermal conductivity of 3000-5000 W m-1 K-1 at room temperature. However, this can drop to as low as 600 W m-1 K-1 when it is attached to another substrate. The drop is caused by a scattering of phonons at the interface which impedes their movement, whereas in free standing graphene the phonon path is uninterrupted. Even at this lower conductivity, the thermal conductivity is still twice as high as copper. Graphene is also known to be one of the strongest materials ever made and a single-layer graphene sheet can withstand up to 42 N m1 of stress, with a Young’s modulus of 1.0 Tpa. TYPES OF GRAPHENE Graphene-molecular-structure There are many types of graphene. True Graphene is only one atomic layer thick (often called a monolayer) and it typically exists as a film but it can be floated off the substrate and can be redeposited onto another substrate or used in it’s isolated form. There are, however, several types of graphene containing powder form materials such as graphene oxide, graphene nanoplatelets, graphene nanoribbons, and graphene quantum dots as well as graphene enabled products such as graphene ink or graphene masterbatches. There are 3 main ways to synthesize graphene, they are: • Chemical Vapor Deposition • Chemical or Plasma Exfoliation from natural Graphite • Mechanical cleavage from natural Graphite Graphene can also be fully synthetic but those methods haven’t proven to be commercially viable. Chemical Vapor Deposition (CVD) Graphene Films Monolayer Graphene Film On Wafer Graphene films can be produced by varying methods, which include mechanical and thermal exfoliation, chemical reduction and epitaxial growth; but the most common method used in production today is by chemical vapor deposition (CVD). CVD works by combining and depositing volatile gas molecules onto a substrate. The process takes place in a reaction chamber, where the material is formed on the surface of the substrate and the waste gases are pumped out. Temperature dependence plays a vital role and can affect the type of reaction that occurs. CVD produces graphene films of high quality and purity, but the by-products produced during the reaction can be toxic due to the volatile nature of the precursor gases. The graphene film is created by CVD in two steps. The first involves the pyrolysis of a precursor material to form carbon atoms on a substrate material. By pyrolyzing the material on the substrate, carbon clusters are prevented from forming. Due to the amount of energy required to break the carbon bonds (C-C = 347 kjmol-1, C=C = 614 kjmol-1, C≡C = 839 kjmol-1, C-H = 413 kjmol-1), a high heat is required, and therefore a metal catalyst is required during the process. The second step is a heat intensive step which assembles the dissociated carbon atoms onto a substrate (in the presence of a catalyst), which forms a single layer structure. CVD graphene films are predicted to have strong chemical, electronic, mechanical and magnetic properties depending on the arrangement of the atoms in the film. Due to the five-order-ofmagnitude difference between the grains and atoms at grain boundaries, only a few experiments have been produced to study these interactions. One substrate that is known to produce high-quality graphene is copper. Copper acts as both a catalyst and the substrate. The copper bonds to the carbon atoms, which provides strong carbon-substrate interactions, allowing for a single graphene layer to be easily formed on the surface. Copper oxide can also be inserted between layers of graphene, making it an easy process to remove a single layer. Treating the copper substrate can also rearrange the surface morphology of the substrate-catalyst and is known to produce graphene with fewer defects. GRAPHENE OXIDE (GO) is most commonly produced by the oxidation of graphite oxide. The oxidation process is beneficial, as it functionalizes the surface of the graphene layers with multiple species of oxygenated functional groups. The multiple functional groups provide an enhanced layer separation and improved hydrophilicity. The hydrophilicity allows the graphene oxide to undergo ultrasonic irradiation, which
produces a single/a few graphene layers that are highly stable when dispersed in DI Water and other solvents. GO has many desirable properties. It disperses very easily in various mediums including aqueous solvents, organic solvents and various matrices. The presence of both electron rich oxygen species and an electron rich graphene backbone allow for further surface functionalization, which gives rise to an adaptable material for multiple applications. Graphene oxide does however suffer from a low electrical conductivity and is an electrical insulator. Graphene oxide is also soluble in many solvents, both aqueous and organic. PROPERTIES OF GRAPHENE The properties of graphene are unique due to its all carbon structure and nanoscale geometry. ELECTRONIC PROPERTIES Flexible Graphene Transistors Because graphene has a delocalized pi-electron system across the entirety of its surface, the movement of electrons is very fluid. The graphene system also exhibits no band gap, due to overlapped pi-electrons, allowing for an easy movement of electrons without the need to input energy into the system. The electronic mobility of graphene is very high and the electrons act like photons, with respect to their movement capabilities. The electrons are also able to move sub-micrometer distances without scattering. From tests done to date the electron mobility has found to be in excess of 15,000 cm2V-1s-1, with the potential of producing up to 200,000 cm2V-1s-1. THERMAL PROPERTIES Graphene Thermal Properties The repeating structure of graphene makes it an ideal material to conduct heat in plane. Interplane conductivity is problematic and typically other nanomaterials such as CNTs are added to boost interplane conductivity. The regular structure allows the movement of phonons through the material without impediment at any point along the surface. Graphene can exhibit two types of thermal conductivity- in-plane and inter-plane. The in-plane conductivity of a single-layered sheet is 3000-5000 W m-1 K-1, but the cross-plane conductivity can be as low as 6 W m-1 K-1, due to the weak inter-plane van der Waals forces. The specific heat capacity for graphene has never been directly measured, but the specific heat of the electronic gas in graphene has been estimated to be around 2.6 μ J g-1 K-1 at 5 K. Mechanical Strength Graphene Composites Applications Graphene is one of the strongest materials ever discovered with a tensile strength of 1.3 x 1011 Pa. In addition to having an unrivaled strength, it is also very lightweight (0.77 mgm-2). The mechanical strength of graphene is unmatched and as such can significantly enhance strength in many composite materials. Flexibility/Elasticity The repeating sp2 hybridized backbone of graphene molecules allow for flexibility, as there is rotation around some of the bonds, whilst still providing enough rigidity and stability that the molecule can withstand changes in conformation and support other ions. This is a very desirable property as there are not many molecules that can be flexible and supportive at the same time. In terms of its elasticity, graphene has found to have a spring constant between 15 Nm-1, with a Young’s modulus of 0.5 TPa. Nanophotonics refers to the use of light in nanoscale projects. This field is associated with some specific breakthroughs in using light in new technologies, including silicon-based semiconductors, where nanophotonics improve speed and performance. Nanophotonics is also known as nano-optics. In this case, nanophotonics involves silicon chips that use light instead of, or in addition to, the types of traditional electrical signals common to semiconductor design. Companies like IBM have pioneered advancements in a chip that uses photodetectors and emitted light to send signals in an integrated circuit environment. The nanophotonics concept further contributes to a more general category of nanotechnology that is revolutionizing how some of the tiniest projects are treated by the research and development (R&D) departments of various fields. While nanotechnology has quite a bit of promise, concerns about the uses of nanoscale technologies include the potential rearrangement or disturbance of molecular structures and the effect of nanoscale materials on larger scale environments The demands imposed on the nanophotonics industry In the present scenario, the nanophotonics industry has to satisfy three major demandsnamely 1. Enormous data transmission rate; 2. Novel optical fabrication technology and 3. High optical memory storage density.