Carbon vacancies

Figure 7.5 (a) Artificial quantum dot architecture showing the confined electron spins, (b) A diamond unit cell showing a NV centre - a nitrogen defect and a carbon vacancy - with an S = 1 electronic spin... 

The color of diamond due to nitrogen impurities has been described in Section 9.6.3 It has been found that nitrogen impurities that are located next to a carbon vacancy in diamond thin films endow the solid with quite new properties, somewhat similar to the properties of a solid containing FLi centers compared with ordinary F centers. The diamond structure is built up of carbon atoms each surrounded by four... 

Under electron irradiation (or by other mechanisms) it is possible to generate carbon vacancies leading to the formation of extended defect domains (with the presence of pentagonal and heptagonal, and even four-membered carbon rings) showing semiconductor character. This is the mechanism of formation of semiconductor properties in quantum-dot carbon nanoparticles or graphene nanoribbon. The mechanism... 

The selectivity of alcohol depends on the carbide preparation. A maximum in alcohol is achieved for the sample WC/Ti02 (T3) for which the preparation of the carbide combines reduction and carburization steps at moderate temperatures (respectively, 873 K and 1073 K). In this case, anionic vacancies stabilized by mixed oxides are formed, associated with carbon vacancies in mixed carbides resulting in a better interaction of carbidic and oxidic phases. On silica, ceria and zirconia, the extent of carburization is too high and the interaction of the carbide phase with the oxide support is suppressed giving larger isolated particles of tungsten carbide with low dispersion. 

The intrinsic nature of tungsten carbide catalyst in CO-H2 reactions is to form hydrocarbons. This property can be modified by oxidic promoters as for the case of noble metals like Pt or Rh or by the presence of carbon vacancies at the surface. To increase the production of alcohols in the Fischer-Tropsch reaction, the catalyst should be bifunctional, with oxidic and carbidic components as in the case of WC on Ti02. Overcarburization of WC on supports like Si02 or Zr02 where the W-O-metal interaction is weak leads to C/W ratios close to unity and does not result in alcohol formation. 

Naturally, also the very core of the interfaces might store Li at potentials still higher than metallic lithium. The same can happen at dislocation cores, pores, or frozen-in point defects (cf. Li-storage in carbon vacancies [52]). 

Bratus VY, Petrenko TT, Okulov SM, Petrenko TL. Positively charged carbon, vacancy in three inequivalent lattice sites of 6H-SiC Combined EPR and density functional theory study. Phys Rev B. 2005 71 1-23. 

In order to account for this, further calculations [84] have been carried out envisaging various possible defect structures, such as monovacancies at carbon site, Y site, and clusters of carbon vacancies, etc. Based on these calculations, it is inferred [84] that the measured lifetime at room temperature in YM2B2C is best accounted for in terms of positron trapping at clusters of carbon vacancies... 

Figure 8.15 Positron lifetime vs temperature in YNi2B2C. The arrow indicates Tc. The continuous line is the fit based on a model incorporating the detrapping of positrons from shallow traps at carbon vacancies. From Sundar et al. [84].

CR Hexagonal Bh, WC-type a = 2.906-2.9066 A, c=2.8364—2.8374 A. The trigonal prismatic interstices of the parent metal lattice are completely filled HR WC exhibits a very narrow homogen >ty range and is practically stoichiometric. This means that there are neither tungsten nor carbon vacancies or interstitials other than those created by thermal activation. 

When using supported catalysts, the formation of alcohols is, on the one hand, related to the surface WC stoichiometry. Only layers with carbon deficiency (vacancies) produce alcohols while high carbon concentrations produce alcanes only. On the other hand, by reaction with special supports, anionic vacancies are formed, stabilized by mixed oxides, and associated with the carbon vacancies in the mixed carbide. The optimal support for alcohol production is Ti02. The bifunctionality is due to carbidic and oxidic phases. 

Wang et al [30], Li and Lin-Chung [22], and Talwar and Feng [23] have calculated the formation energy and the bound electronic states of native defects. Li and Lin-Chung [22] showed that isolated silicon and carbon vacancies and a divacancy complex of Si and C sites induce gap states, while no defect-induced state is found in the energy gap for either the isolated Si or C antisite defects or for the pair of antisite defects in SiC. Kohyama et al [31],... 

All SiC poly types have the valence band maximum at the zone centre but the conduction band minimum is polytype dependent. The indirect bandgaps increase with polytype hexagonality from 2.39 eV for 3C-SiC to 3.33 eV for 2H-SiC. In 3C-SiC, isolated silicon and carbon vacancies and a divacancy complex of carbon and silicon sites produce states in the forbidden gap but antisite defects do not. Theoretical values for the energy gap in 2H-SiC do not correspond to the experimental values. The valence band offset between 3C- and 2H-SiC is found to be 0.13 eV. Relatively little work has been carried out on the other poly types. Theory and experiment are also at variance for the energy gap of 4H- and 6H-SiC polytypes. 

Over the last two decades, two-dimensional (2-D) and 3-D structures of composition have been proposed. However, in the absence of single crystal structure data, the atomic connectivity and composition of these materials remain controversial. Three important graphitic C3N4 structures in which triazine is linked by nitrogen atoms have been proposed. Conceptually, such structures are derived from the hypothetical CN graphitic structure by creating either carbon vacancies (cf. 17 and 18) or triazine vacancies (cf. 19 Figure 12). 

Alternative structures have been proposed by Liu and Wentzcovitch [180], Wentz-covitch and Martins [181] and Wentzcovitch [182], proposing a cubic zinc blende structure with one carbon vacancy per unit cell, and a structure resembling graphitic CN with one carbon vacancy per four nitrogen sites. This study was based on an ab initio molecular dynamics scheme with a variable cell shape algorithm. 

Ghavamian and Ochsner [43] were analyzed the effect of defects on the buckling behavior of single and multiwalled CNT based on FE method considering three most likely atomic defects including impurities, vacancies (carbon vacancy) and introduced disturbance. The results demonstrate that the existence of any type kinks in CNTs structure, conducts to lower critical load and lower buckling properties. 

Defects in G can be carbon vacancies on the layer. Where many carbon atoms are missing, a hole on the G sheet appears. These holes contain at the periphery carbon atoms that have incomplete valence generally denoted as... 

Clean tungsten carbides, a-WC and a-W C, form essentially only hydrocarbons from CO—H2 reactions. At 673 K and atmospheric pressure, the main products on WC, W2C, and W are methane, CO2, and H2O (121). Ethane and propane are also formed at lower temperatures. WC was substantially more active than W2C and W. The nature of the products can be modified by oxide promoters, as for the case of Rh or Pt, or by the carbon vacancies at the surface (122). At 573 K and 5 MPa with 2H2/CO, turnover rates (based on sites titrated by CO chemisorption) of 0.25-0.85 s were reported for hydrocarbon synthesis over bulk and Ti02-supported tungsten carbides. In addition, WC and WC/Ti02 produced alcohols and other oxygenates with 20-50% selectivity. However, W2C of more metallic character did not produce any oxygenates. Coexistence of carbidic and oxidic components on the catalyst surface appeared to be responsible for alcohol formation. 

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