Electron nanodiamonds

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]. 

The spectral features d d transform into broad peculiarity tf after diamond nanopowder Alit treatment in hydrogen. As a result, after diamond nanopowder Alit treatment, its CXa-emission band becomes similar to reference refined diamond powder (Fig. le). So far as nanodiamond powders CKa-bands investigation was carry out at minimum anode current densities (1 mA), the unrefined diamond powder surface remained chemisorbed atoms and molecules. However, owing to electron bombardment in sample emission focus, some of chemisorbed atoms disappeared. Therefore it was important to obtain CA -spectra... 

Graphene has been prepared by different methods pyrolysis of camphor under reducing conditions (CG), exfoliation of graphitic oxide (EG), conversion of nanodiamond (DG) and arc evaporation of SiC (SG). The samples were examined by X-ray diffraction (XRD), transmission electron microscopy, atomic force microscopy, Raman spectroscopy and magnetic measurements. Raman spectroscopy shows EG and DG to exhibit smaller in-plane crystallite sizes, but in combination with XRD results EG comes out to be better. The CG, EG and DG samples prepared by us have BET surface areas of 46,... 

The ability to synthesize carbon nanostmctures, such as fullerenes, carbon nanotubes, nanodiamond, and mesoporous carbon functionalize their surface or assemble them into three-dimensional networks has opened new avenues for material design. Carbon nanostructures possess tunable optical, electrical, or mechanical properties, making them ideal candidates for numerous applications ranging from composite structures and chemical sensors to electronic devices and medical implants. 

C MAS-NMR spectroscopy was used to quantify the different carbon species in synthetically produced nanodiamonds.430 A 13C study has been reported to study the modification of the electronic properties of SWCNT by alkali intercalation.431,432 13C MAS-NMR data were used to study the structure of 13C-enriched SWCNT, prepared by catalytic decomposition of CH4.433 H, 13C and 15N MAS-NMR spectra of amorphous carbon nitride (a-CNx) films were consistent with sp2 hybridised nitrogen atoms in an aromatic carbon... 

Yet another HRTEM study of a Murchison acid-resistant residue (MIL) showed, for the first time, carbyne in a petrographic context. An amorphous carbon matrix contained scattered 1. single-crystal cubic nanodiamonds (3-4 nm) identified by the 0.2 nm (111) /-spacing and in polycrystalline ("ring ) electron diffraction patterns ( 5% error of measurement), and... 

The generation of carbon onions in space has not yet been fuUy elucidated. However, it seems reasonable to assume that they originate from nanoscopic diamond particles. These may be converted into carbon onions upon heating, electron bombardment, or intensive irradiation (Section 4.3.5.4). The existence of nanodiamonds in extraterrestrial material could be confirmed by analyses on different meteorites. Especially the AUende meteorite contains significant amounts of tiny diamond particles (Section 5.1.2). 

Appreciable amounts of perfectly spherical carbon onions are hard to obtain, and so only few experimental data on their electronic properties are available. For the irregular onion-hke carbon that may for instance be prepared from nanodiamond, on the other hand, the conductivity and other parameters have been studied much more extensively. 

Further defects include, for example, the directed doping with boron, nitrogen or nickel. These confer certain electronic or optical properties to the nanodiamond particles (Section 6.2.3). Experimental as well as theoretical results show that only few elements Hke boron, nitrogen, silicon, oxygen, or phosphorus can be incorpo-... 

In addition to the electron microscopic examination, valuable information about the material s structure can also be obtained from the evaluation of spectroscopic data. Analytical methods chiefly reflecting the properties of either the surface or the bulk phase will be considered in this section. This distinction is of particular relevance for nanodiamond as surface properties and bulk characteristics differ in parts significantly here. 

Defects and impurities, in general, play a comparably important role for the luminescence properties of nanodiamond like they do for the bulk material. Owing to their existence, there are electronic states situated within the bandgap, which allow for inducing luminescence in nanodiamond samples also with longer wave radiation. Upon excitation with wavelengths between 300 and 365 nm, fluorescence bands are observed at more than 400 nm. They arise from various nitrogen defects. In comparison to bulk diamond, the Ufetime of the excited states is rather short, which possibly is due to the effect of surface states and to the increased density of excitons on the surface. 

ESR-Spectroscopy The ESR-spectroscopy provides information on the existence of unpaired electrons. As mentioned in Section 5.2 they play an important role both for the surface properties and in the crystal lattice of nanodiamond. 

Contrary to bulk diamond, the spin density is increased by sample purification because the treatment, for example, with concentrated oxidizing mineral acids, removes the graphitic layer from the surface of the nanoparticles (Section 5.3.4). In this way new, unsaturated bonding sites are generated. A part of the spin density, however, is localized in the crystal lattice for nanodiamond as well, and again nitrogen centers and other defects give rise to the unpaired electrons. 

Examining the electronic properties of nanodiamond turns out to be rather complicated by the variable sample quality and the associated differences in the electronic structure, so in this chapter, just some important aspects of electronic properties and structure will be discussed. For a more detailed presentation refer to the respective original literature given in the appendix. 

Particularities like unpaired electrons at unsaturated bonding sites play a role for the electronic properties too, of course. However, a spin density of only 10 -10 spins per gram is determined from respective measurements (Section 5.4.1.5), which corresponds to a one-digit number of spins per nanodiamond particle. It results from a strong tendency toward saturation by the formation of re-bonds. In doing so, surface states rather graphitic in character are formed that cause, among other effects, also an electric conductivity (see below). 

The electronic structure of a nanodiamond sample may be examined by various spectroscopic techniques. Depending on the choice of method, the information obtained from doing so corresponds to different depths of the lattice. The Auger spectroscopy, for instance, has a low penetration depth, so it may serve to determine the situation of n-electrons on the surface. In the Cls-loss spectrum, on the other hand (Section 5.4.1.3), no 7i-transitions are observed as this method chiefly yields information on the second to seventh layer of atoms below the surface. 

From the hydrogenation or fluorination of a diamond material, a very hydro-phobic surface results that may then enter into an exchange with rather nonpolar compounds. A connection via Jt-stadting, however, plays just a minor role because graphitic fragments are only found in small domains on the particle surface. In the case of thermally graphitized nanodiamond particles, on the other hand, the conditions largely resemble those observed for multiwalled nanotubes. The interaction of the 7t-electrons with the polymer molecule causes a stable noncovalent incorporation into the composite. 

Daulton TL, Eisenhour DD, Bematowicz TJ, Lewis RS, Buseck PR (1996) Genesis of presolar diamonds comparative high-resolution transmission electron microscopy study of meteoritic and terrestrial nanodiamonds. Geochim Cosmochim Acta 60 4853-4872... 

---