Theoretical study of diamond-like carbons and nucleation of diamond.

摘要

Different forms of amorphous carbon and hydrocarbons with varying elastic and optical properties, hardness, density and hydrogen content exist depending on the preparation technique. The structure can vary from graphitic to diamond-like, i.e., from mainly threefold coordinated to mainly four-fold coordinated. In order to study the properties of such materials, microscopic models must be developed. These studies include the modelling of crosslinked defective graphite, diamond nucleation along the graphite edges, and diamond-like carbons.; Tamor's proposed structure for diamondlike carbon consists of crosslinked graphitic regions. We studied a concrete realization of this model in which the cross-links are produced by shortening the interplanar bond lengths. The model study was accomplished with a pure rhombohedral graphite cell. For this study we used a semi-empirical potential based on Tersoff's environment-dependent potential which contains angular terms. It is enhanced by a long-range potential which describes the interplanar interactions. We found a configuration corresponding to a local minimum. More general features such as the randomness of the distribution of cross-links are needed for a realistic model.; A model study of diamond/graphite interfaces was motivated by recent observations by Li and Angus. They observed a significant enhancement of diamond nucleation on the graphite edge planes with the preferential orientation relationship: {dollar}rm{lcub}0001{rcub}sb{lcub}g{rcub} Vert {lcub}111{rcub}sb{lcub}d{rcub}, langle 1120ranglesb{lcub}g{rcub} Vert langle 101ranglesb{lcub}d{rcub}.{dollar} Two possible interface structures were studied using the Tersoff potential. We found that the models have comparable low interface energies even if they contain some dangling bonds. Moreover, lower interface energies were found when the dangling bonds of the non-bonded diamond layer were satisfied with hydrogen. We have proposed a growth mechanism based on this study.; Finally, we constructed realistic models of dense amorphous carbon. The WWW (introduced earlier for a-Si by Wooten, Winer and Weaire) model was the starting structure. The effects of clustering of the threefold coordinated atoms in pairs, chains, or graphitic (planar hexagonal clusters) were studied. The resulting models were relaxed using the Tersoff potential. Their electronic structures were studied using an empirical tight-binding scheme with parameters adjusted to reproduce the diamond and graphite band-structures. The models were found to have densities of {dollar}sim{dollar}3 g/cm{dollar}sp3{dollar} and bulk moduli of {dollar}sim{dollar}3.1 Mbar. Localized dangling bonds and {dollar}pi - pisp*{dollar} states were found within the wide gap of the WWW model consistent with optical gaps of the order of 0.5-2 eV. Hydrogen atoms were introduced to remove some of the dangling bonds. The models were found to account for the essential features of ion-beam deposited amorphous carbon and hydrogenated amorphous carbon.