π-bonded clusters in amorphous carbon materials

Abstract

Total energies favour a segregation of the sp² sites in amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C: H) into sp²-bonded clusters embedded in a sp³-bonded matrix. The size of the clusters determines the local bandgap. The presence of clusters forces the mobility edges further into the bands. Clustering also introduces strong symmetrical fluctuations of the band edges. This causes intracluster transitions between localized and localized states to become allowed. The effect of such clustering on the luminescence properties, the resonant Raman spectra, the Urbach tail and the subgap defect optical absorption of a-C: H is discussed. The sp² sites form mainly sixfold rings within the clusters. The presence of a small fraction of adjacent fivefold and sevenfold rings washes out the third-neighbour peak at 2 % 4 A in the radial distribution function of sputtered a-C.