Nanodiamond

Apart from naturally occurring diamond there is by now a variety of artificial carbon materials that feature diamond structure as well. These include the synthetic diamond generated by high pressure and temperature, but also films, polycrystalline materials resembling the carbonados (Section 1.3.2) and the so-called 

The small particle size gives rise to a number of interesting effects. Especially the large portion of surface atoms contributing to the total mass makes itself felt in the physical and chemical properties. 

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Recently Butler et al. [4] reported the deposition of nanocrystalline diamond films with the conventional deposition conditions for micrometer-size polycrystalline diamond films. The substrate pretreatment by the deposition of a thin H-terminated a-C film, followed by the seeding of nanodiamond powder, increased the nucleation densities to more than 10 /cm on a Si substrate. The resultant films were grown to thicknesses ranging from 100 nm to 5 fim, and the thermal conductivity ranged from 2.5 to 12 W/cm K. 

A pure form of sp3 hybridized carbon is known as diamond and this may also be synthesized at the nanoscale via detonation processing. Depending on their sizes, these are classified as nanocrystalline diamond (10 nm 100 nm), ultrananocrystalline diamond (< 10 nm) and diamondoids (hydrogenated molecules, 1 nm-2 nm). Nanodiamond exhibits low electron mobility, high thermal conductivity and its transparency allows spectro-electrochemistry [20,21]. However, ultrananocrystalline diamond exhibits poor electron mobility, poor thermal conductivity and redox activity [21,22]. 

Pure elements Graphite, carbon nanotubes, fullerene, nanodiamond, etc. 

The next point to realize is that the best emitter is a metal. Many forms of carbon initially studied are semiconductors or even insulators, including nanodiamond [8-11] and diamond-like carbon (DLC) [12-13,4]. Combine this with local field enhancement means that there is never uniform emission from a flat carbon surface, it emits from local regions of field enhancement, such as grain boundaries [8-11] or conductive tracks burnt across the film in a forming process akin to electrical breakdown [13]. Any conductive track is near-metallic and is able to form an internal tip, which provides the field enhancement within the solid state [4]. Figure 13.2 shows the equipoten-tials around an internal tip due to grain boundaries or tracks inside a less conductive region. 

Thus, emission can be increased by using nanodiamond grain boundaries or DLC with conductive tracks. However, these systems suffer from irreproducibility and instability. Eventually, sp2 carbon systems such as carbon nanotubes (CNTs), nanowalls and nanocarbons were studied, which are the best systems, because they are metallic and have the desired shape for field enhancement. 

Other meteorite classes like C2, CO and ordinary chondrites contain much smaller inclusions less than 1 mm (MacPherson et al. 1988) and require ion microprobe techniques to evaluate the isotopic compositions. On the least metamorphosed side. Cl have very few inclusions or oxide grains, but are the carrier of the greatest amounts of stellar nanodiamond and other carbides (Anders and Zirmer 1993). As will be shown for Cr anomalies in carbonaceous chondrites, the survival of the mineral carriers of the anomalies also depends on the metamorphic grade of the meteorites. Nevertheless, isotopic anomalies have also been formd in higher metamorphic grade from other classes, especially in the reduced enstatite chondrites. 

Dai ZR, Bradley JP, Joswiak DJ, Brownlee DE, Hill HGM, Genge MJ (2002) Possible in-situ formation of meteoritic nanodiamonds in the early Solar System. Nature 418 157-159 Dauphas N, Marty B, Reisberg L (2002) Molybdenum nucleosynthetic dichotomy revealed in primitive meteorites. Astrophys J 569 L139-L142... 

Fig. 4.3 (a) Crystal structure of diamond and (b) the smallest nanodiamond adamantine... 

Diamondoids are hydrogen-terminated nanodiamonds with sp hybrid bonding between the carbon atoms. The smallest of these hydrogenated nanodiamonds is adamantane (Figs. 4.3b and 4.5). 

Fig. 4.5 The sp hybridized orbitals on a cluster of ten carbon atoms terminated by hydrogen atoms form molecule of the smallest of nanodiamonds adamantane, ...

The nanostructured surfaces resemble, at least to a certain degree, the architecture of physiological adhesion substrates, such as extracellular matrix, which is composed from nanoscale proteins, and in the case of bone, also hydroxyapatite and other inorganic nanocrystals [16,17,24-27]. From this point of view, carbon nanoparticles, such as fullerenes, nanotubes and nanodiamonds, may serve as important novel building blocks for creating artificial bioinspired nanostructured surfaces for bone tissue engineering. 

Fig. 27 Supercapacitor study using graphene electrodes (5 mg each), (a) Cyclic voltammogram of chemical vapor deposited graphene (CG) nanodiamond derived graphene (NG) and exfoliated graphene (EG), (b) Evolution of specific capacitance vs scan rate. (Reprinted with permission Ifom [243])...

Schranda AM, Daia L, Schlager JJ et al (2007) Differential biocompatibUity of carbon nanotubes and nanodiamonds. Diamond Relat Mater 16(12) 2118-2123... 

Carbonaceous material (Fig. 12.8b) is intimately mixed with silicates and is very abundant (carbon abundance averages 13% and varies up to 50%) in CP IDPs. Some carbon is elemental (graphite), but C-H stretching resonances in infrared spectra show that aliphatic hydrocarbons are also present. Polycyclic aromatic hydrocarbons (PAHs) also occur. Nanodiamonds have been identified in cluster IDPs, but not in smaller CP IDPs. Enormous D/H and 15N/14N anomalies have been measured in bulk IDPs, and the hydrogen isotopic anomalies are correlated with organic-rich domains. Ratios of D/H as high as 25 times the solar ratio suggest the presence of molecular cloud materials. 

Another environment that has been successfully searched for nanodiamonds is the Cretaceous-Tertiary boundary layer. Carlisle Braman (1991) carried out the now time honoured acid dissolution procedure on samples from Knudsen s Farm, Alberta, Canada. They found 45 ppb of a white fraction 97 % carbon almost entirely 3-5 nm... 

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