Nanocrystalline and Disordered Carbon Materials

17 Nanocrystalline and Disordered Carbon Materials Mainak Roy Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India

17.1

Introduction

Nanocrystalline materials have recently attracted the interest of researchers because of their unique properties and potential for advanced applications. Owing to constraint in size at least in one of their dimensions, these materials exhibit a quantum confinement effect and hence properties grossly different from their bulk counterparts. Nanocrystalline materials with constraints in all the three dimensions are termed quantum dots, those having constraints in at least two of their dimensions are called nanowires or nanotubes, and those that have a constraint in only one dimension are nanosheets. The era of nanocrystalline carbon materials started with the discovery of fullerenes or carbon quantum dots. Then came one-dimensional carbon nanotubes (CNTs) and finally two-dimensional graphene sheets. In this chapter, a brief overview of the these materials is presented, with an emphasis on their properties, synthesis and applications. A schematic flow chart of different forms of carbon as they appear in the chapter is given below for the benefit of the readers. Carbon

Nanocrystalline carbon

Disordered carbon

Fullerene

Composites

Carbon nanotube

Thin films

Graphene Nano-diamond Carbon Nanofoam Functional Materials. DOI: 10.1016/B978-0-12-385142-0.00017-9 © 2012 Elsevier Inc. All rights reserved.

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17.2

Functional Materials

Fullerene

Fullerene was the third crystalline allotropic modification of carbon to be discovered and was the first among those to have a true nano-dimension. It was discovered by H.W. Kroto, R.F. Curl and R.E. Smalley in 1985. The landmark experiment involved shining a pulsed laser onto a graphite target followed by the analysis of the plume by mass-spectrometry [1]. Fullerenes are all-carbon close structures and are composed of pentagons and hexagons fused together in the form of spherical/ellipsoidal balls. C60, having the structure of a truncated icosahedron, is the smallest and most abundantly synthesized fullerene. It consists of 12 pentagons and 20 hexagons with no two pentagons sharing an edge (see Figure 17.1). The pentagon rings in fullerenes impart a positive curvature to the structure, resulting in a strained non-planar buckyball. There are two different bond lengths in C60. The bonds between two successive hexagons are shorter than those between a hexagon and a pentagon, and the average ˚ . The diameter of C60 molecule is B0.7 nm. Each carbon atom bond length is B1.4 A in C60 is covalently bonded to three other carbon atoms, which implies that one nonbonded electron per carbon atom is available for the extended π-electron sea covering both sides of its surface. This accounts for the high electrical conductivity of C60. In fact, C60 is a nano-sized electrical conductor. However, C60 is neither aromatic nor super-aromatic and delocalization of the π-electrons is rather poor. Hence it behaves more like an unsaturated hydrocarbon that is slightly electron deficient and thus preferably reacts with electron-rich species. The next homologue in the series is C70, which is also a commonly occurring fullerene molecule. C70 is made up of 12 pentagons and 25 hexagons (see Figure 17.1). Most importantly, in contrast to diamond and graphite, fullerenes are soluble in organic solvents and can take part in homogeneous organic reactions. Reduction of fullerenes is easy whereas their oxidation requires higher energy. C60 absorbs in the blue
violet region of the spectrum, which imparts the purple colour in the solution, whereas solution of C70 is reddish brown. Recent studies on fullerene toxicity to human beings show that pristine fullerenes have low toxicity [2] and short-term occupational exposure to the same may not

C60

C70

Figure 17.1 Schematic representation of C60 and C70 fullerenes.

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induce risk of handling to the users. Fullerenes are highly incompressible molecules. They do not stick to each other and remain loosely bound together by means of weak van der Waals forces. They can roll smoothly over each other and therefore may find applications as lubricants. Some fullerene derivatives exhibit superconductivity at low temperatures [3
6a] and some charge transfer complexes of fullerene are found to be ferromagnetic.

17.2.1 Synthesis Common fullerenes like C60 and C70 exist in small amounts in naturally produced soots. Laser ablation of graphite target in an inert atmosphere, as originally developed by Kroto et al. [1], produces fullerenes at a small scale. Large-scale production of fullerenes was only realized in 1990, when Kratschmer and Huffmann developed an apparatus in which vaporization of graphite under inert atmosphere could be achieved by striking an arc across the electrodes [6b]. The method mainly produces C60 and C70, although higher fullerenes can also be produced by optimizing the electrode surface and vaporizing conditions. The desired fullerenes may be obtained in high purity by extracting them from the soot by dissolving in organic solvents like toluene followed by liquid column chromatography. Fullerenes have also been produced from benzene flames and by pyrolysis of aromatic compounds. Other techniques of fullerene production include electron beam evaporation and sputtering. These methods primarily result in higher fullerenes.

17.2.2 Mechanism of Fullerene Formation Fullerenes are formed by growth of a carbon cluster, and its growth mechanism follows the pentagon rule which states that carbon networks with a maximum number of pentagons have the lowest energy and thus are favoured. In laser vaporization of graphite, the growing carbon clusters present in the vapour state are annealed at the high-temperature region of the plume to form the most stable structures, which eventually leads to the production of fullerenes. Basically, the process involves self-assembly of graphene sheets followed by their annealing at a rate faster than their growth, while avoiding rearrangement to a closed fullerene before reaching a desired size. Therefore, the temperature should be high enough for annealing of the clusters but not so high that the structure rearranges extensively creating barriers for conversion of open pentagons to closed fullerenes. In the case of a resistively heated graphite rod as the carbon source, carbon radicals that are produced in the vapour state from the surface of graphite condense at a slow rate to form carbon clusters, which eventually form fullerenes. Critical size of the clusters may be coarsely controlled by adjusting the pressure of the buffer helium gas. Arcs (both DC and AC), instead of a resistively heated graphite rod are used in a commercial setup for production of C60 and other fullerenes in good yield. Here the method of vaporization is different from that of resistive heating, but again the pressure of the buffer gas dictates the critical size of the clusters formed.

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17.2.3 Reaction of Fullerenes Fullerenes exhibit wide range of fascinating chemistry. The principal reactions are electrophilic addition reactions, Diels
Alder type cycloadditions, radical additions, substitution reactions, charge and/or electron transfer reactions, ring opening reactions, and forming complexes with transition metals. Often addition reactions are accompanied by a change of hybridization of the carbon atoms from sp2 to sp3, which reduces angular strain in the molecules; thus, such reactions of fullerene are likely to be exothermic in most cases. Sometimes, atoms are deliberately introduced into the spherical cavity of the fullerenes, thus producing so-called endohedral fullerenes. Endohedral fullerenes with reactive atoms entrapped and stabilized within their cavities often exhibit entirely different electronic and magnetic properties. The Diels
Alder reaction forms another very important class of reactions of the fullerenes. Many dienes, including 1,3-dienes, cyclic dienes, heterodynes, and so on, react with C60 to form Diels
Alder products [7]. Often more than one fullerene reacts with a single diene molecule to form a bridged product [8]. Primary, secondary and tertiary aliphatic amines also readily and repeatedly add to C60 to form well-defined aminated fullerene structures in good yields [9]. Subsequently, amination reactions are used for making fullerene-based polymers. Polyfullerenes and fluorinated polyfullerenes are expected to be more stable and easier to manipulate than existing organic polymers. Apart from organic molecules, C60 forms complexes with organo-iridium compounds, ferrocene, rubidium, and so on.

Functionalization of Fullerenes When specific molecular groups are attached to the fullerene surface, the process is called functionalization. Functionalization is done for better manipulation of the molecules for specific optical and biological applications. For example, molecular shuttlecocks formed by functionalization of fullerenes are expected to be used in nonlinear optics, photonics, molecular electronics and liquid crystal applications [10]. Fullerenes functionalized with inorganic compounds also have tremendous potential in the field of electronic devices.

17.2.4 Applications Fullerenes have tremendous potential for application in storage devices, solar cells, X-ray photo-detectors and fuel cells. They are excellent electron acceptor molecules with very low reorganization energy. They are n-type semiconductor materials with intermediate band-gap (B2.3 eV) and high electron mobility (0.1 cm2/Vs). Hence, they find applications in making transistors and in organic photovoltaics. They may also find applications in drug delivery and act as a potential neuroprotector, an antibacterial agent and in the inhibition of certain diseases like AIDS and Parkinson’s. Apart from therapeutics, they find applications in diagnostics as well. A special type of fullerene is being tried in magnetic resonance imaging (MRI) technique as a contrast agent. Recently, fullerene-like materials have been

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used in the catalytic conversion of ethylbenzene to styrene, resulting in increased yield of the products [11].

17.3

CNTs

17.3.1 Different Types of CNT CNTs form a class of one-dimensional nano-carbon structure formed by graphitic planes rolled up in the form of tubes, often closed at both ends. CNT was first discovered by Ijima in 1991 [12]. CNTs are low-density materials with very high specific strength. They exhibit high anisotropic thermal conductivity along the tube axis. Absorption spectrum of CNTs shows sharp peaks owing to Van Hove singularities. CNTs can be single-walled as well as multi-walled. Although single-walled tubes consist of an isolated graphite layer, multi-walled tubes are formed by successive layers of single-walled tubes rolled on top of the other. Normally diameter of a single-walled CNT is only a few nanometres (2
5 nm), whereas their length can extend up to several millimetres. Depending on the axis about which the graphite layer is folded to form a single-walled tube, the circumference of the tube is denoted by r 5 m a 1 1 n a 2 , where a 1 and a 2 are the unit vectors on the graphite sheet, as shown in Figure 17.2, and m and n are the indices. There are three common types of CNT. If either of the vectors m or n is zero, the CNT is called zigzag. If both the vectors m and n are equal, then the CNT is called armchair. All other types of CNT are termed chiral. Different types of CNT are shown in Figure 17.3. The diameter of an ideal nanotube can be calculated from its (n, m) pffiffiffiffiffi ffi indices using the formula d 5 ða=πÞð m2 1 n2 1 mnÞ, where a 5 0.246 nm. Again, depending on the electrical transport properties, CNTs can be either [m, n]

2

(0,0) 1

3 → a1

→

a2

→

→

→

r = ma1 + na2

6 4

Folding of the tube takes → place along r 5

Figure 17.2 Chiral vector in a CNT.

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→

ma1

→

a1 a2

na2

→ r m=n

→ Armchair single-walled carbon nanotube ma1 → r

a1 a2

n=0

→ ma1

Zigzag single-walled carbon nanotube

a1 a2

→ na2 → r

m =/n

Chiral single-walled carbon nanotube

Figure 17.3 Armchair, zigzag and chiral nanotubes.

semiconducting or metallic in nature. Usually, a single-walled CNT is metallic if the value of the difference of their indices (m 2 n) is divisible by three. Otherwise, the nanotube is semiconducting. In a mixture of semiconducting and metallic nanotubes, where values of m and n are random, semiconducting tubes are expected to be in greater populations than the metallic nanotubes. Excitonic photoluminescence emission is observed in semiconducting nanotubes but not in metallic nanotubes. Raman spectroscopy is a versatile technique that may be used to differentiate between single-walled and MWNTs, and between metallic and semiconducting tubes.

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17.3.2 Raman Spectroscopy of CNTs CNTs in general exhibit two distinct Raman peaks at B1350 cm21 and 1580 cm21, respectively, assigned as the D- and G-bands. The G-band originates from the tangential in-plane vibration of carbon atoms in the graphite sheet whereas the D-band is activated by lowering of crystal symmetry in a disordered sp2 carbon network induced by defects in the curved walls of the nanotube and/or tube ends [13]. Often the G-band also exhibits the signature of A1g mode at B1610 cm21 in the form of high-energy shoulders. Additionally, single-walled nanotubes (SWNTs) exhibit a strong Raman band in the range 100
500 cm21 owing to radial breathing mode (RBM) of the aromatic rings. It involves out of the plane bond stretching of all the carbon atoms in the tube, during which they coherently move in and out in the radial direction. The RBM is observed only for SWNTs and is not exhibited by MWNTs. Thus, in a CNT sample, the presence of SWNT is characterized by the presence of the RBM. The diameter of the SWNT can be roughly estimated from the peak position of the RBM because its frequency approximately varies inversely with the number (mass) of the carbon atoms along the perimeter of the tube. Therefore, the frequency of the RBM may be given by the following equation: ν RBM ðcm21 Þ 5

K ðnmÞ d

ð17:1Þ

where d is the diameter of the tube and K is a constant that depends on the tube
substrate and tube
tube interaction. For an isolated SWNT on SiO2 substrate, a typical value for K is taken to be B248 cm21 nm [14]. Apart from first-order Raman scattering, the second-order bands appear in the spectrum of CNTs. The G0 -band appears at B2700 cm21 as an overtone of the D-band, owing to second-order two-phonon Raman scattering processes. The corresponding one-phonon Raman mode appears at B1350 cm21 and has been discussed earlier as the D-band. Peak positions, of both the D-band and G0 -band, shift slightly with the energy of the incident laser. Similarly, overtone of the G-band appears at B3200 cm21 and is called the 2G-band. Neither the G-band nor the 2G-band show any dispersion of peak position with incident laser light [15]. Often, the G-band of metallic CNT shows a Breit
Wigner
Fano-like asymmetric line profile whereas profiles of semiconducting tubes are more symmetrical in nature. Apart from Raman spectroscopy, ultraviolet
visible spectroscopy may also be used to distinguish between the two types of CNTs. Note that metallic and semiconducting CNTs may be separated from each other by selectively functionalizing the metallic tubes using fluorous chemistry [16] or by shining laser in resonance with the electronic level of a particular tube.

17.3.3 Synthesis CNT is primarily produced by three techniques. They are (i) arc discharge, (ii) laser vaporization and (iii) chemical vapour deposition. In all these methods a suitable carbon source is atomized wherefrom CNT is formed. Additionally,

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transition-metal nanoparticles are used which serve as a catalyst. CNT can also be synthesized without using any metal catalyst by the use of proper templates.

Arc Discharge This is the most primitive of all methods used for the production of CNT. In fact, the first discovery of CNT took place when Ijima was looking through a transmission electron microscope (TEM) on an arc discharge CNT. The technique is similar to that used for fullerene described earlier. In this method, an arc is struck between graphite electrodes in an inert atmosphere or under liquid nitrogen. MWCNTs are obtained along with other products as deposits on the cathode. However, by using graphite cathode with catalytic metals, SWCNTs are obtained in the soot. In this method, usually a complex mixture of materials is produced and CNT has to be purified from the soot and other crude products. By this method, nanotubes up to a length of B50 μm can be synthesized with a moderate yield of 30%. The method is, however, difficult to scale up for commercial applications.

Laser Vaporization Laser vaporization is a technique for producing CNTs in high yield. Usually a pulsed laser beam is focused under inert atmosphere on a composite of graphite and transition-metal catalysts (Ni and Co for example) maintained at a high temperature. Carbon nanotubes formed is deposited on the cooler part of the chamber. In this method, single-walled CNTs are primarily produced with a yield of B70%. Thereafter, the samples are given a second round of heat treatment in vacuum to remove the fullerenes that are also formed along with CNT. Often multiple pulses are used instead of a single pulse for better yield. The nanotube diameter and size distribution can be roughly controlled by optimizing the growth parameters, catalyst composition and deposition temperature. However, the method is highly expensive and purification of samples produced by laser vaporization is a problem owing to the presence of other forms of carbon.

Chemical Vapour Deposition Chemical vapour deposition (CVD) is the most widely used technique for synthesizing CNTs. It is used for their commercial production because it is an economically viable process and the yield can be easily scaled up in this process for practical application. In this process, nanoparticles of transition-metal catalyst in the form of Ni, Co, Fe or their mixture are kept in a ceramic boat or coated onto a suitable substrate. The size of the nanoparticle is crucial in controlling the tube diameter and nature of the tube (multi-walled or single-walled). The catalyst is kept in a furnace maintained at a high temperature. Usually, a gaseous hydrocarbon is used as a source for carbon. The hydrocarbon gas is diluted suitably with either hydrogen or ammonia and introduced into the furnace. Thermal decomposition of the carbon source produces CNT on the catalyst surface. Microwave plasmaenhanced CVD has also been used for synthesizing CNTs [17
19]. Here, instead

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of thermal activation, the carbon source is excited by microwaves to form stable plasma. The precursor species generated in the plasma then react on the catalyst-coated substrate to form CNT. Both MWCNTs and SWCNTs can be synthesized by selecting appropriate catalyst [20] and growth conditions like chamber pressure, substrate temperature and gas flow. Application of electric field during the growth process helps align the nanotubes along the field. A recent modification of the CVD technique for continuous and bulk production of CNT and related materials includes a fluidized bed reactor wherein the catalyst particles, often dispersed on suitable supports, are suspended at high temperature in a pressurized flow of hydrocarbon for the growth of CNT [21,22]. However, agglomerated CNTs are sometimes formed by this process [23]. Yet another modification of the technique is water-assisted CVD [24]. Here water is added into the reaction chamber to grow a dense forest of millimetre-tall vertically aligned SWNTs. The nanotubes produced by this process could be easily separated from the catalyst and material with a carbon purity of more than 99.8% obtained.

CNT from Renewable Plant Products/Waste Recently, a method has been developed based on spray pyrolysis for bulk synthesis of CNTs and related carbon materials using cheap carbon sources like kerosene [25] and renewable plant resources such as camphor [26], betel nut and turpentine oil [27,28]. In this method, the carbon precursor in the atomized form is introduced along with a carrier gas into a furnace chamber containing transition-metal catalysts. The furnace is maintained at a high temperature wherein the carbon precursors react and are deposited on the metal catalysts in the form of CNTs or porous carbon, depending on growth conditions and the nature of the catalyst. These carbon materials exhibit excellent properties and are being proposed for applications including lithium batteries, uses in the chloro-alkali industries, electrochemical capacitors and hydrogen storage.

Non-Conventional Methods Ball milling of graphite powder followed by thermal annealing sometimes produces CNTs, especially MWCNTs. Combustion of a suitable hydrocarbon gas at high temperature also produces CNTs. The method, often termed flame synthesis, shows promise for scaling up.

17.3.4 Mechanism of CNT Deposition The mechanism of CNT formation is not well understood yet. It has been observed that presence of a transition-metal nanoparticle catalyst facilitates the formation of CNT. First, the metal nanoparticle dehydrogenates the precursor hydrocarbon molecule/radicals on its surface. The carbon then diffuses into the nanoparticle, which then gets saturated with carbon wherefrom it emanates in the form of a tube in a root growth mechanism. The growth model imposes a restriction on the outer

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diameter of the tube that can be produced with nanoparticles of a given size. Single-walled nanotubes are produced from very small particles, bigger particles form thicker multi-walled tubes.

Catalyst-Free Synthesis Catalyst-free synthesis of CNTs involves use of tubular preformed templates made of materials like porous anodized alumina, silicon nanowires [29], and so on. The pyrolyzed carbon precursors become entrapped within the channels of the tubular templates, then react on the inner surface of the channels and finally CNTs are formed in a root growth mechanism, even in absence of a transition-metal catalyst, owing to shape and size constraints.

17.3.5 Purification of CNT As-grown CNT invariably contains impurities in the form of amorphous carbon and remains of metal catalysts. The metals are removed by digesting the tubes in dilute acids and sometimes in baths of concentrated acid. However, such acid treatment may also oxidize the surface, induce carboxylic group functionalization of it and even introduce defects in the tubes. Amorphous carbon is usually removed from CNTs by heating the sample above 450 C in air. CNTs usually remain stable up to 650
700 C under dynamic conditions.

17.3.6 Application Because of their unique mechanical, electrical and thermal properties, CNTs (both single-walled and multi-walled) are being considered as potential materials in the field of advanced electronics, catalysis, hydrogen storage and biological applications. Some prospective applications of CNTs are given in the next sections. i. Advanced electronics

CNTs have remarkable mechanical properties and extremely high thermal conductivity. Because of these interesting properties, CNTs have strong potential for applications in the field of advanced electronic devices and reinforced nanocomposites. CNTs may be semiconducting as well as metallic. The band gap of the semiconducting nanotube varies inversely with the diameter of the tube [30]. Prototype field-effect transistors have been fabricated on semiconducting CNT that is capable of operating at room temperature [30,31]. Metallic nanotubes are very robust and hence are prospective interconnects of the future [30]. CNTs are also being used for field-emission devices because of their excellent emission properties from the sharp wire tips. A fully sealed, high-brightness CNT field-emission display has been developed by Samsung Advanced Institute of Technology. Besides, carbon nanotubes can also work as biosensors, gas sensors, actuators and memory devices. Owing to the high thermal conductivity of CNTs, they are potential heat sinks for

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cooling chips. The possibility of fabricating atomic force microscopy (AFM) tips made out of CNT is also being explored. ii. Hydrogen storage

Today, hydrogen is being considered as a clean alternative to conventional fossil fuels. CNT is one of the potential materials that is being explored for hydrogen storage. It is believed that H2 storage takes place in CNT by both physisorption and chemisorption. Hydrogen storage by chemisorption takes place at higher pressures compared with physisorption because they tend to form chemical bonds. Obviously, for complete desorption, it is required to heat the chemisorbed samples at higher temperatures. In chemisorption, hydrogen is adsorbed either on the outer or inner surface of the tube and subsequently induces sp3-like hybridization in the tube, whereas in physisorption it is primarily stored inside the empty space of the tube by Van der Waals interaction. Theoretical calculation predicted an uptake capacity of B1.3 wt.% for CNT with a diameter of B1 nm at room temperature and at a storage pressure of B10 MPa [32]. The storage capacity increases with increasing tube diameter of CNT. It has also been found to depend strongly on the helicity of the tube [31]. Optimum hydrogen storage is predicted in the temperature range 150
200 K [32] and it increases with the gas pressure. However, pressure-induced excessive hydrogen storage can also lead to rupturing of the CNT walls. Metal doping of CNTs also enhances the uptake of hydrogen. Remains of transition-metal catalysts such as Fe, Co and Ni dispersed over large surface area of CNT increase H2 storage capacity of CNT. Similarly alkali metals like K and Li are also known to improve H2 storage in CNT [33,34]. Formation of metallic hydrides is believed to have facilitated hydrogen storage in such metal-doped CNTs. iii. Catalytic support

CNT is a wonderful support for catalysts. Because of its very high specific surface area, the catalysts can be dispersed easily on CNT. Besides, being a pool of electrons, CNT can interact electronically with the catalysts, thereby enhancing their catalytic activity. Rapid preferential oxidation of Co at room temperature in the presence of H2 has been reported for platinum supported on CNT [35], which improved markedly upon doping with Ni
MgO or Fe
Al2O3. Further, CNT has been used as a support for Pt for electro-oxidation of ethanol at room temperature [36]. Aerobic oxidation of cyclohexane into cyclohexanol, cyclohexanone and adipic acid in the liquid phase by nitrogen-doped CNT with excellent activity and controlled selectivity has been reported by Yu et al. [37]. An electronic effect of CNT in catalyzing ammonia synthesis by ruthenium nanoparticles has been demonstrated by Guo et al. [38]. Hydrogen production on both SWCNT and MWCNT supported Pt catalyst has been studied by Wang et al. [39]. They also showed that although functionalization of CNT improves its specific surface area and hence dispersibility of the Pt catalyst, its catalytic activity, especially in terms of the turnover frequency, actually decreased upon functionalization. TiO2
CNT nanocomposites are also found to serve as excellent photo-catalysts. Yu et al. have shown enhanced oxidation of phenol using TiO2
CNT heterojunction arrays on Ti substrate [40]. Similarly, methylene blue was photo-degraded successfully in the presence of the Cr2O3
CNT/TiO2 composite

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under visible light irradiation by Chen et al. [41]. Xu et al. studied the gas-phase degradation of benzene and the dye degradation reaction of methyl orange using TiO2
CNT nanocomposite as a photo-catalyst [42]. iv. Biological application

Y and T junctions made of CNT are postulated to mimic biological neural networks for possible transmission and switching of signals across them. CNTs are also being tested for targeted drug delivery. The biomedical application includes tagging of CNT surfaces with the drug molecule and its application to biological systems [42a]. It has been shown that apoptotic cell death decreases when the drug was delivered along with CNT. CNT also acts as biosensor for glucose [43] and DNA [44] detection. They are being tested for tumour targeting [45] and selective cancer-cell destruction [46]. v. CNT as a nano-reaction vessel

Carbon nanotubes are composed of channels of the order of nanometres, often covered at both ends with caps. Once the caps are removed, either by mechanical or chemical means, CNTs may be subjected to capillarity-induced filling of the channels with molten metals, metal salt solution and/or precursors to different nanomaterials. Subsequent workup produces nanoparticles within the tubes. In such cases, the shape and size of the encapsulated nanoparticles is dictated by the diameter and morphology of the tube. Thus, CNT provides with a natural template for synthesizing nanocrystalline materials with an aspect ratio greater than one. Moreover, growth of nanoparticles within the CNT channels takes place under a protective environment and hence the products exhibit properties that are markedly different if cooked outside the nano-reactor. Probably the first report of capillarity-induced filling of CNT was by Ajayan and Iijima [47], who filled up the CNT channels with lead metal. They also synthesized V2O5-filled CNT [48]. In a similar fashion, nanorods of GaN were synthesized by Han et al. [49]. Encapsulated NiO was produced by Tsang et al. using the wet chemical technique [50]. Recently, Roy et al. have developed a novel ultrasonic technique for producing encapsulated β-In2S3 nanorods in CNT nanovessels [51a]. The nanorods exhibited better thermal stability and fewer optically active defects than In2S3 nanoparticles synthesized without the CNT template. A significant change in material property was also observed for metal catalysts entrapped inside CNT. Coercivity of Fe-filled CNT was studied as a function of the aspect ratio of the entrapped Fe nanoparticles by Shi et al. [51b]. Filling of CNT with Ni and Ni96Pt4 was observed by Singh et al. [17]. Properties of encapsulated Ni single crystal have been studied by Tyagi et al. [18]. Removal of the CNT templates may be achieved by heating CNT above 650 C.

17.4

Graphene: The Slimmest Carbon

Graphene is a monolayer of graphite consisting of a one-atom-thick planar sheet of sp2-bonded carbon atoms densely packed in a honeycomb crystal lattice. It may be viewed as an SWCNT cut open on one side. Alternatively, it may be thought of as

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an indefinitely extended fused array of fully condensed polycyclic aromatic hydrocarbon (PAH) molecules. Much against the theoretical prediction and mindset of the past century, a single layer of graphene was first isolated from graphite in 2004 by Novoselov et al. [52] It eventually helped scientists to probe many exciting properties of graphene. The extremely high electron mobility of graphene could be measured experimentally [53], and its ambipolar field-effect [53] and room-temperature quantum Hall effect [54] verified. Soon graphene became the most promising material for today’s technology, because of its exceptional mechanical, electronic and optoelectronic properties. Graphene is a zero band-gap material with a band gap ranging between 0 and 0.25 eV. Hence, it may be considered as a semi-metal with room temperature resistivity of the order of micro-ohm centimetres. Electron mobility in graphene is very high, B15,000 cm2/V/s. Graphene is semi-transparent in the visible region and exhibits very high thermal conductivity (B4.8
5.3 3 103 W/m/K1 T), which is even higher than that of CNT and diamond. It is an extremely strong material, with a spring constant of 1
5 N/m and Young’s modulus of 0.5 TPa. Its possible application as an active element in gas sensors [55
57] and as transparent electrodes for display applications [58] and solar cells [59] has been contemplated. Unfortunately, large-scale production of processable graphene is still a major challenge. Today, graphene is produced primarily in a top-down fashion by mechanical exfoliation of graphite [52]. However, the method lacks the merit of being able to be scaled up and at times lacks reproducibility in terms of the number of exfoliated graphene layers. Alternatively, graphene may be produced by reduction of graphene oxide synthesized by Hummer’s method [60] or its modified version. Graphene oxide flakes, chemically exfoliated from graphite, are dispersed either in water or organic solvents and deposited on substrates by spin coating and/or drop casting. Often it is required to functionalize the graphene oxide surface for complete dispersion of the same in aqueous/organic solvents. The oxide films are then reduced by a suitable reducing agent such as H2, hydrazine, etc. to produce single or multi-layers of graphene. Although these layers exhibit high electrical conductivity and optical transparency [61], the presence of structural defects continues to be the major drawback of this synthetic procedure. A bottom-up approach to synthesizing graphene from organic molecular species is a rather new concept. In this approach, large, fully condensed PAH molecules, often termed molecular graphenes or nanographenes because of their structural similarity (in the nanometre regime) to an infinite graphene lattice, are first deposited on suitable substrates. It is hypothesized that wellorganized molecular domains of these large PAHs, upon heating, might undergo fusion by dehydrogenation to produce extended graphene layers. The process is schematically represented in Figure 17.4. Obviously, this approach is expected to provide better control over film thickness, i.e. the number of graphene layers eventually formed, but suffers from a similar drawback of unintentionally produced structural defects due to incomplete fusion of the organic moiety. So, the bigger the starting molecule, the lesser will be the density of structural defects in the final graphene layer. However, with gradually increasing molecular weight, these molecules become less soluble in organic solvents; finally, molecules bigger than hexabenzocoronene (C42H18) are completely insoluble in any kind of solvent.

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Void

5

Figure 17.4 Schematic representation of nanographenes thermally fused under heat treatment.

Moreover, these molecules cannot be sublimed intact by conventional evaporation techniques because they tend to decompose well below their sublimation temperature [62]. Although these molecules can be made soluble by attaching long aliphatic side chains to their aromatic cores [63], and thereafter processed using solution processing techniques like spin coating, drop-casting and inkjet printing [64], the aliphatic chains invariably induce steric hindrance, resulting in poor packing of molecules on the surface and hence low charge-carrier mobility in test devices. Therefore, a new technique was required for making the unprocessable molecules processable without changing their molecular structure. Pulsed laser deposition (PLD), a non-equilibrium process, has recently been used successfully in depositing intact molecules of significantly large PAHs that were considered unprocessable by any equilibrium technique [65]. In this process, a pulsed laser beam carefully tuned to its optimized irradiation density is made to impinge on a target of precursor molecules kept under vacuum, thereby generating a plume of that material, which expands with approximately supersonic speed towards a substrate kept in close vicinity and is subsequently deposited on it (Figure 17.5). The deposited layers may be viewed using a scanning tunnelling microscope (STM). STM micrographs of PLD-deposited intact layers of three different PAHs are given in Figure 17.6A
C. More than 100 nm of almost defect-free two-dimensional lattice features (honeycomb packing of molecular domains) are clearly observed in the micrographs. Evidence of molecules remaining intact even after deposition comes from their respective matrix-assisted laser desorption/ionization
time of flight (MALDITOF) mass spectrum [65]. These molecular layers deposited on suitable substrates are then pyrolyzed under an inert atmosphere at temperatures of 500
800 C. Figure 17.7 shows the AFM topography of a PLD layer deposited on an SiO2/Si substrate and pyrolyzed at 850 C under a flowing N2 atmosphere. Although short-range ordering of the hexagonal lattice seems to have taken place, long-range order of an extended lattice is largely missing. Such pyrolyzed

Nanocrystalline and Disordered Carbon Materials

Attenuator

689

Lens

Mirror

Pulsed N2 laser Quartz window Substrate

Manipulator

Target Vacuum chamber

Molecules in gas phase

Rotation

Turbo pump

Rotary pump

Manipulator

Figure 17.5 Schematic representation of a PLD setup. Reproduced with permission from ‘Subliming the Unsublimable: How to Deposit Nanographenes’ by Ali Rouhanipour, Mainak Roy, Xinliang Feng, Hans Joachim Ra¨der and Klaus Mu¨llen, Angew. Chem. Int. Ed. 2009, 48, 4602
4604. Copyright Wiley-VCH Verlag GmbH & Co. KGaA

layers have tremendous potential both as active layers and electrodes for organic field-effect transistors. Another popular approach to bottom-up synthesis of graphene is by graphitization of silicon carbide single crystals [66,67]. In this process, the SiC surface is first etched by hydrogen and then graphitized at very high temperature (.1100 C) under argon pressure. Sublimation of Si from SiC surface facilitates formation of mono/multi-layers of graphene. Reconstruction of the SiC surface and its terminal bonding greatly influences the properties of epitaxial graphene layers. A few layers of graphene can also be prepared by CVD. Here, a gaseous carbon source is made to decompose on the surface of metal substrates like nickel, copper, iridium, ruthenium, and so on, at high temperatures; eventually, graphene layers get deposited [68]. Then the epitaxial graphene layers may be transferred to arbitrary substrates by dry transfer [69] for suitable applications. There are also reports of converting nano-diamond clusters into graphene. Nano-diamond particles are thermally graphitized upon heating at 1600 C [70,71]. Recently, it has been claimed that graphenebased electronics may be fabricated simply by tracing with pencils. Graphitic remains from pencil tracks are believed to produce devices that will work much faster than silicon-based devices [72]. Moreover, CNT that is a rolled on sheet of graphene may be opened up by oxidizing it with mild oxidizing agents like KMnO4. Kosynkin et al. [73] obtained oxidized nanoribbons by this technique.

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(A)

(B) 50 nm

25 nm

(C)

50 nm

Figure 17.6 (A
C) STM images of intact layers of giant PAHs deposited by PLD. Reproduced with permission from ‘Subliming the Unsublimable: How to Deposit Nanographenes’ by Ali Rouhanipour, Mainak Roy, Xinliang Feng, Hans Joachim Ra¨der and Klaus Mu¨llen, Angew. Chem. Int. Ed. 2009, 48, 4602
4604. Copyright Wiley-VCH Verlag GmbH & Co. KGaA

The resulting nanoribbons were highly soluble in water, ethanol and other polar organic solvents. Carbon nanotubes can be etched as well by using ionized argon gas and opened up to form graphene.

17.4.1 Characterization of Graphene Graphene layers may be characterized by one or a combination of the following techniques.

Microscopy Conventional optical microscopy may be used to view graphene layers even as thin as monolayers deposited on SiO2/Si substrates. Owing to differential contrast

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30.0 nm

0.0

1: Height

745.0 nm

Figure 17.7 AFM topography of a PLD layer pyrolyzed at high temperature.

provided by the extra carbon layer on the top, they can be clearly observed on the substrate. Apart from SiO2/Si, other substrates like Al2O3/Si and Si3N4 have also been explored for viewing graphene sheets deposited on them. AFM is obviously the most popular microscopic technique that has been widely used for characterizing graphene layers. In fact, AFM was first used by Novoselov et al. for identifying a monolayer graphene on SiO2/Si substrate [52] (see supplementary section in the reference [54]). The step height of a single layer graphene strongly adhered to a sil˚ for single fold and approximately 8 A ˚ for douica layer was measured to be B4 A ble folds. Similar values were also obtained for nanographene grown on highly oriented pyrolytic graphite crystals. TEM is another microscopic technique that has been used to characterize free-standing graphene sheets. In principle, it is possible to differentiate between single- and multi-layer graphenes by studying their electron diffraction patterns [74].

Spectroscopy Raman spectroscopy is by far the most effective technique for characterization of carbon allotropes because each carbon allotrope displays a clearly identifiable Raman signature. Moreover, the technique is fast, non-destructive and requires practically no sample preparation. The Raman spectrum of graphene, like all other crystalline graphitic carbons, exhibits a G-band at B1580 cm21 owing to first-order E2g zone-centre phonon mode resulting from in-plane vibration of the graphite plane. Besides the G-band, a peak designated as D-band also appears at B1350 cm21 and

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has been attributed to the presence of defects in the graphene layer especially close to its edges. Reportedly it is due to the in-plane A1g zone-edge mode [75] of crystalline graphite. It may be noted that this D-band is not observed in very high quality samples such as mechanically exfoliated graphene layers. Apart from the D- and G-bands, the 2D- or D0 -band that appears at B2700 cm21 has been explained based on a double resonance model [76,77] and is a very important feature in graphene. Its intensity relative to that of the G-band provides information about the number of graphene layers. The G-band of a few layers of graphene on SiC was found to get significantly blue shifted with respect to that of the highly oriented pyrolytic graphite substrate. It also shifted to low wave numbers with increasing graphene layers. Careful analysis of Raman spectra of graphitic discs of varying thickness showed that the 2D-band was composed of two distinct features. Faugeras et al. [78] fitted the spectrum (in the wave number region 2550
2700 cm21) with two Lorentzian line shapes: the centre of the high-energy component was fixed at 2686 cm21 whereas that of the low-energy component freely varied from 2636 to 2653 cm21 depending on the thickness of the disc. It was observed that the low-energy feature gained in intensity relative to the high-energy component as the height of the graphitic discs was reduced significantly from 20 to 2 nm [78]. The intensity ratio of the two peaks gives a measure of the thickness of the graphitic discs. However, one should also account for the temperature effect on Raman spectrum of graphene caused by the incident laser beam. It has been observed that the G-band at B1580 cm21 gradually shifts to a low wave number region with increasing temperature of the sample [75]. The slope differs widely depending on the number of graphene layers. Substrates also have significant influence on the Raman profile of graphene layers. Raman spectra of graphitic discs are markedly different from those of graphene layers grown on GaAs, sapphire or glass substrates owing to different interactions of the substrates with the graphene layer(s) [79a]. For example, the G-band is red-shifted by B5 cm21 in the case of graphene on sapphire, whereas it appears almost at the same position, albeit with different line-shapes, for graphene on GaAs and glass substrates [79a]. The 2D peak of a few layers of graphene deposited on SiO2/Si [79a] gradually shifts to higher wave numbers with increasing numbers of layers. Here also, the low-energy component of the 2D peak is more dominant for samples having fewer graphene layers and gradually decreases in intensity as the sample thickness increases. In fact, a graphene monolayer exhibits only the low-energy component of the 2D peak.

Diffraction Technique X-ray diffraction (XRD) patterns of microcrystalline graphite show an intense 0 0 2 peak at B2θ 5 26.2 that is not observed for graphene samples with fewer than three layers. However, thicker samples do exhibit a small feature at B26 . Graphene oxide films, on the other hand, show a weak feature at B2θ 5 8 owing to reflection from the 0 0 1 planes, instead of the 0 0 2 peak of graphite. A detailed account of characterization of graphene by different techniques is provided by Caterina Soldano and Mahmood [79b].

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17.5

693

Nano-Diamond

Nano-diamond is a nanocrystalline form of carbon consisting of short-range order formed by sp3-bonded carbon atoms. It is a very hard and practically incompressible form of carbon. Bulk modulus of nano-diamond is higher than that of bulk diamond. Moreover, compared with bulk diamond, nanocrystalline diamond has larger surface to volume ratio and superior wear resistance properties. Nano-diamond is produced from a carbon source by the detonation method [80]. The detonation wave produces plasma containing precursors that react under high pressure to form Nano-diamond. So far, the mechanism of formation of Nano-diamond by detonation method has not been clear. Nano-diamond is also produced from graphite and fullerene at high pressure and high temperature. It is also obtained in the form of interconnected rods nearly a micrometre long with diameters ranging between 5 and 20 nm. Nano-diamond is also synthesized by modified laser ablation in liquids [81]. In this method, a pulsed laser with very high fluence is made to shine on a small spot size on a graphitic target kept immersed under a liquid medium. The target is constantly rotated to preserve homogeneity of composition of the target surface. The impinging laser beam produces plasma of carbon species that in turn forms a stable colloidal suspension of nanoparticles. The technique has an edge over conventional PLD because it also includes the advantages of the soft chemical route. The impact of the laser produces a state of non-equilibrium expulsion of the target material and the resulting recoil effect on the target produces a high-pressure zone near the surface. The liquid medium restricts the plasma within a small reaction zone and thus facilitates the formation of nano-diamond that is otherwise formed at high pressure. Microwave synthesis is yet another technique that has been used to synthesize nano-diamond. In a typical experimental condition, a 2.45 GHz microwave source is used to excite the plasma. A hydrocarbon feed is introduced into a vacuum chamber maintained at a base pressure of approximately 1026 mbar. The substrate, which may be a transition metal like nickel, is heated by a resistive heater at B500 C. The deposited films exhibit features corresponding to that of nano-diamond. Apart from microwave plasma, films of nanocrystalline diamond have been deposited using radio frequency (RF) magnetron sputtering using Ar/CH4 [82], arc discharge and the DC discharge CVD process [83]. We had earlier reported formation of nano-diamond phase on the surface of polycrystalline diamond films deposited by hot filament CVD [84]. It was inferred, using surfaceenhanced resonance Raman spectroscopy, that diamond films grown with a methane/hydrogen mixture invariably produced an ultra-thin layer of nano-diamond at the surface. The nanophase increased with the increase in methane/hydrogen ratio and decreased upon adding a small amount of oxygen in the feed gas [85]. The surface layers on top of hot filament CVD diamond films are constantly exposed to the hostile atmosphere of reactive gases and hence the different facets get etched out preferentially, thereby modifying the grain morphology. Lack of coalescence of the crystals restricts grain growth at the surface, which leads to the formation of the nano-diamond phase.

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17.5.1 Characterization of Nano-Diamond Raman spectroscopy is by far the most versatile technique for studying nano-diamond. The Raman spectra of nanocrystalline diamond usually exhibit three features near 1150, 1350 and 1580 cm21 [86]. The last two bands are popularly known as D- and G-bands of graphitic carbon and have been discussed at length in connection with CNTs. The band at B1150 cm21 is related to the phonon density of states of diamond and has been assigned to the presence of a nanocrystalline diamond phase. Raman spectra of nano-diamond samples can also exhibit other peaks corresponding to the phonon density of states of diamond, depending on the nature of phonon confinement in them [84,85]. XRD patterns of nano-diamond exhibit peaks at B2θ 5 44 and 75 , which correspond to diffraction from the (1 1 1) and (2 2 0) planes of cubic diamond. However, full-width at half maximum of the peaks is usually higher than that of the microcrystalline diamond.

17.5.2 Functionalization of Nano-Diamond for Biological Application Like CNTs, the nano-diamond surface is also modified with different organic functional groups like halides, amines, cyanide, azide, hydroxyl and thiols for attachment to bio-molecules, drugs and chiral ligands for potential applications like targeted drug delivery and diagnostics [87]. Again, nano-diamond has very high surface area to volume ratio. Moreover, owing to the high strength of C
C bonds, proven biocompatibility and unique balance between sp2 and sp3 carbon they are being considered as potential electrode material for biosensor applications. In a typical example, glucose oxidase was immobilized on a layer of nano-diamond. Linear response of the enzyme electrode to glucose in the concentration range 1 3 1026 M to 8 mM with a response time of less than 2 min has been reported [88a].

17.6

Carbon Nanofoam

Carbon nanofoam, a newly discovered allotrope of carbon, is a three-dimensional structure that consists of several carbon tendrils loosely bound together to form a mist-like arrangement similar to that observed in the case of an aerogel. The tendrils are made of randomly interconnected nano-sized clusters of carbon with a regular hexagonal pattern, just like in a graphite sheet, but given a negative curvature owing to the inclusion of heptagons. Carbon nanofoam was first discovered by Andrei V. Rode et al. [88b]. It is usually produced by shining a high-intensity pulsed laser on a carbon target in the presence of an inert atmosphere. Carbon nanofoam is one of the lightest solid materials known today, having a density of B2 mg/cm3. It has an extremely high surface area and is a good electrical insulator. It is fairly transparent, quite brittle and can withstand very high temperature. More interestingly, carbon nanofoam contains unpaired electrons that give rise to ferromagnetism in a freshly prepared material at room temperature that tends to vanish with time. However, the ferromagnetic property of carbon nanofoam is retained at

Nanocrystalline and Disordered Carbon Materials

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lower temperatures, for example below 90 K [88c]. Such an unusual intrinsic magnetic property of an all-carbon material is expected to have tremendous applications in the field of spintronics-based devices. It may also find applications in biomedicine for imaging.

17.7

Amorphous Carbon

Amorphous carbon may be regarded as a non-crystalline, glassy form of carbon essentially made up of a graphitic network (with occasional sp3
C bonding) with very short-range order. It may exist both in the form of powder and films deposited on suitable substrates. Historically, the term was used to designate carbonaceous materials found in soot and coal. However, in the true crystallographic sense, those materials may not be designated as amorphous because they are actually bits of poly- and nanocrystalline graphite and diamond embedded within an amorphous carbon matrix. Depending on the amount of hydrogen content of the material, it is termed amorphous carbon (a-C), hydrogenated amorphous carbon (a-C:H) or tetrahedral amorphous carbon (ta-C). Structurally, amorphous carbon differs from any other crystalline allotrope of carbon with inconsistent carbon
carbon bond length and bond angles. Moreover, the π-electron cloud in amorphous carbon is highly localized and contains many dangling bonds that make the material far more reactive than its crystalline counterparts. Amorphous carbon has many applications, including in nuclear reactors and electronic devices, some of which are discussed in the following sections.

17.7.1 Amorphous Carbon for Nuclear Applications In certain high-temperature nuclear power reactors, graphite is used as a moderator because of its excellent neutron-scattering properties [89
91]. However, using graphite moderators always involves a risk of a sudden release of Wigner energy, especially when the reactors are operated at a temperature below 250 C. Owing to exposure to radiation, carbon atoms in the graphite moderator often get dislodged from their normal lattice points, which are then released in the form of Wigner energy and may cause serious accidents [92]. Moreover, radiation-induced changes in physical and mechanical properties of graphite may also lead to subsequent structural failures. Amorphous carbon is being proposed as a moderator material in low-temperature reactors in place of graphite. Obviously, the prerequisite is that the material should be dense, have a high degree of isotropicity, should not graphitize or undergo changes in thermo-mechanical properties during high-temperature processing. Conventionally prepared amorphous carbon from petroleum coke unfortunately lacks some of these properties. Reinforced carbon composites, on the other hand, serve the purpose better because they remain isotropic over a wide range of temperatures. Preparation of amorphous carbon composites from two different starting materials, namely carbon fibre and carbon black, is discussed briefly in the following section. Detailed studies are given elsewhere [93
96].

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Composites from PAN Fibres The process involves dispersion of chopped and carbonized polyacrylonitrile (PAN) fibres in phenol formaldehyde resin matrix. The resin and the fibre are then cast into a dye and heat treated at 1000 C in an argon atmosphere.

Composites from Carbon Black First, phenol formaldehyde resin is cured at B200 C, crushed into small pieces and then carbonized at 1000 C in an inert atmosphere. The carbon powder is then milled and sieved into different size fractions. Metal impurities (if present) are removed by acid leaching. Powder with appropriate particle size is then mixed with carbon black and phenol formaldehyde binder and pelletized. Finally, the pellets are heated at 1000 C to remove volatile organic impurities from the binder. Carbon composite powder is further densified by the impregnation technique. Residual open pores in the powder samples are filled in with liquid phenol formaldehyde resin by repeated impregnation under pressure and subsequent carbonization at B1000 C under an inert atmosphere. The process is performed in cycles until there is no further increase in density of the composites. Eventually, nucleargrade materials containing impurities at a level of less than parts per million are obtained. The carbon-black-based composites show lower percentage porosities and hence higher densities than the composites produced from carbonized PAN fibre over a wide range of compositions [94]. Compressive strength is also higher for composites prepared from carbon black than from carbonized PAN fibres. Moreover, both kinds of composite with optimized compositions exhibit higher compressive strength than the conventional petroleum coke-based materials. They also display a lower coefficient of friction and a negligible content of anisotropic graphitic phase than coke-based samples. It has been shown from XRD, Raman and polarized optical microscopy [94] that composites based on carbon black and PAN fibre (with low fibre content) are 100% amorphous and exhibit practically no anisotropic graphitic phase, even after high-temperature processing, which is a prerequisite for low-temperature reactor applications. Thus composites from carbon black and PAN fibres exhibit a better prospect of being used in moderator channels than conventional coke-based carbon, which shows B25% graphitization at 1000 C.

17.7.2 Thin Films of Amorphous Carbon It is possible to deposit thin films of amorphous carbon on different substrates. Depending on the structural properties and chemical compositions, amorphous carbon films may be either of polymeric nature, mainly comprising an sp2-hybridized carbon
carbon network, or very hard and highly resistive in nature, properties resembling closely those of diamond, which is known as diamond-like carbon (DLC). DLC is a class of amorphous material with short-range order comprising different fractions of stable sp2-hybridized (π-bonded) graphitic structure, meta-stable

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sp3-hybridized (σ-bonded) diamond structure and hydrogen-containing polymeric structures. By controlling the amount of graphite, diamond and polymeric phase in the material, different types of diamond-like carbon films may be formed, differing widely in their properties. Incorporation of a small amount of metal such as tungsten, tantalum, ruthenium, iron, and titanium in the carbon network may largely modify the mechanical properties of DLC and is termed M-C: H. Similarly, dilution of diamond-like carbon films with nitrogen alters their electronic properties and are termed a-C:H:N.

Properties of DLC Films Properties of the DLC films vary largely depending on relative content of sp2- and sp3-hybridized carbon and their growth process. Depending on the amount of bound and unbound hydrogen content of the film, it may be soft, polymeric and transparent in nature or an extremely hard material. DLC thin films are quite transparent, both in the visible and near-infrared region. The optical band gap of the material varies from 0.8 to 3.0 eV. They have large refractive index and mass densities, typically varying between 1
3 g/cm3. DLC films with low hydrogen content exhibit very high hardness values, typically in the range 10
30 GPa and very high resistivity values (B1010 Ω cm). They are resistant to acids and alkalis. Upon heating at high temperatures, DLC films gradually convert to soft, opaque and conducting graphitic materials. Two important methods of deposition of diamond-like amorphous carbon films are given below.

CVD of Diamond-Like Amorphous Carbon Films CVD of diamond-like amorphous carbon films consists of excitation of a gaseous hydrocarbon source either by DC, RF or microwave plasma [97]. Accordingly, the techniques are known as DC-, RF- or microwave plasma-enhanced CVD. The precursor radicals/ions generated in the plasma are made to impinge on the substrates with high energy. The radicals/ions then penetrate into the sub-surface region and enter the interstitial positions below the surface layer. Then, by repeated dehydrogenation of C
H bonds and C
C bond formation, smooth films of amorphous carbon are formed. Owing to high compressive stress, the films then tend to protrude out of the substrate forming thin and dense layers of DLC. Because of this inherent compressive stress, it is sometimes difficult to grow thicker films because of adhesion failure. In principle, any hydrocarbon such as acetylene, benzene, butane, cyclohexane, ethane, ethylene, hexane, isopropane, methane, pentane, propane and propylene with sufficient vapour pressure can be used for deposition of DLC films. Sometimes, hydrogen is added in the feed gas with these hydrocarbons. The nature of the films formed differs widely, depending on the nature of hydrocarbon used and method of excitation of the carbon source.

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Electro-Deposition of Diamond-Like Amorphous Carbon Films This technique was first reported by Namba [98]. It involves room-temperature deposition of amorphous carbon films by high-voltage dissociation of common organic solvents like alcohols, N-N-dimethyl formamide, and so on. By this technique, it is possible to deposit carbon films on a wide variety of substrates, including metals having low-melting points and those having exotic shapes that are otherwise difficult to coat using the high-temperature vapour deposition process. A typical electro-deposition setup consists of an anode, usually in the form of graphite or metal wire, and a conducting/semiconducting cathode that simultaneously acts as the substrate for deposition (see Figure 17.8). A very high voltage of the order of kilovolts is applied to dissociate electrolytes, and the desired carbon coating is formed on the cathode. Many organic solvents including methanol, ethanol, 2-propanol, water
ethylene glycol solution, acetone, tetrahydrofuran, N-N-dimethyl formamide, acetonitrile, acetic acid, formic acid,, etc. have been tried so far. In principle, any organic solvent containing carbon and hydrogen may be used for electro-deposition of amorphous carbon films, although coating properties and deposition rates may vary widely, depending on the dielectric properties of the electrolytes used. Hydrogen-free deposits [99,100] are usually obtained on silicon and stainless steel cathodes whereas hydrogenated films are formed on conducting glasses and on Pd inter-layers [101]. With the formation of resistive carbon coating, current density decreases initially with time and then becomes steady after some time, when the resistance at the interface increases to a very high value owing to the formation of a continuous layer. Deposition rates are typically very low, B1
2 nm/min [100]. Deposition rate in general increases at higher voltages and at higher temperatures, and pulsed sources are usually favoured over continuous DC sources to deposit amorphous carbon films [102]. Deposited layers often exhibit very high resistivity. For example, resistivity of electro-deposited DLC films is B109 Ω cm. Such coatings have very low electron affinity and hence +

Source

A V Metal rod

Water outlet Graphite anode

Thermometer

Water jacket Cathode substrate Water inlet

Electrolyte

Figure 17.8 Schematic diagram of a typical electro-deposition setup.

Nanocrystalline and Disordered Carbon Materials

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exhibit field-emission properties [103]. They also exhibit high band-gap and high refractive index, depending on the voltage applied during deposition [104].

Mechanism of Electro-Deposition of Carbon Films The mechanism of electro-deposition of amorphous carbon films is rather complex. In principle, many precursor species may be formed when an extremely high voltage is applied to the electrolytes. Hence, identification of individual species formed under the deposition process is not easy. Roy et al. [104] put forward a rather simple and concise mechanism of depositing DLC films from ionic electrolytes like 1 acetic acid in water, wherein acetic acid initially dissociated into CH3 and 1 2 1 (COOH) . The CH3 ions then migrate to the cathode along with H generated from the hydrolysis of water present in the electrolyte. Then, through the process of making and breaking of bonds, DLC films are formed on the cathode. A generalized mechanism for electrolytes containing COOH functional groups may therefore be represented as below: R 2 COOH -R1 1 COOH 2 ðR 5 alkyl=aryl groupÞ

ð17:2Þ

At the cathode: R1 1 e2-R:

ð17:3Þ

R: 1 R: -R 2 R ðfilmÞ

ð17:4Þ

At the anode: COOH 2 22e2-CO2 1 H1

ð17:5Þ

Alternatively, electrolytes having OH functional groups will also undergo dissociation under high voltage, producing R1 according to the equation given below: ROH 1 H2 O-R1 1 H1 1 2OH2

ð17:6Þ

Electro-Deposition of Nitrogenated Amorphous Carbon Films Nitrogen doping of amorphous carbon (a-C) films produces a wide band-gap n-type semiconducting material. Because of its excellent and unique properties, such as low or negative electron affinity (NEA), low work function, high thermal conductivity and chemical inertness, nitrogen-doped amorphous carbon (a-C:N) films are being considered as potential materials for cold cathodes in microelectronic devices and flat panel display applications. Electro-deposition provides an easy method of depositing nitrogenated thin films of amorphous carbon at room temperature. Here, along with a carbon source, a suitable nitrogen-containing molecule such as urea is added to the electrolyte for electro-deposition. Figure 17.9 shows an SEM image of nitrogenated amorphous carbon film electro-deposited on a silicon substrate.

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Functional Materials

20 μm

Figure 17.9 SEM image of a nitrogenated amorphous carbon film electro-deposited on silicon substrate (unpublished data).

17.7.3 Characterization of Amorphous Carbon Different types of amorphous carbon may be characterized by a host of spectroscopic and microscopic techniques. XRD is usually inefficient in characterizing amorphous carbon owing to the lack of long-range order. Highly disordered carbon exhibits only a broad hump at low 2θ values. However, carbon
carbon composites (discussed earlier) exhibit a broad d002 XRD peak of graphite at 2θB27 owing to short-range ordering in them. For such materials, turbostratic modelling of XRD data [94] provides an easy means of estimating the degree of graphitization in them. According to the model, limiting values for inter-layer distance in turbostratic graphite is taken to be 0.344 nm, and that in pure 100% graphite is taken to be 0.3354 nm. Percentage graphitization in the samples is then defined by the following equation. 

 0:344 2 d002 3 100 % graphitization 5 0:344 20:33354

ð17:7Þ

where d002 is the distance between two adjacent 0 0 2 planes in the sample given in nanometres and is estimated from the analysis of the XRD peak at 2θB27 . Any sample that exhibits d002 spacing higher than 0.344 nm may be considered 100% amorphous.

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