Size dependence of energy gaps in small carbon clusters: The origin of broadband luminescence

The visible broadband luminescence from carbon-related films has recently been attributed to the band-tail states caused by the variations in the energy gap of individual sp² carbon clusters due to their difference in size and/or shape. In this paper, these band-tail states are classified into two parts: localized and confined. The localized states result from the structural deviation from graphite-like configuration, and the associated luminescence may be described by using the conventional theory for amorphous materials. The confined states are generated due to the existence of stable graphite-like local structures with various sizes and are the main factor for giving efficient, room-temperature luminescence. Our calculations of a series of small hexagonal carbon clusters with first-principle and semi-empirical methods demonstrate that the energy-gap distribution, due to the difference in size, is considerably broad, which may explain the broadband feature of luminescence. Calculations for some tetrahedral clusters were also made for comparison. © 1998 Elsevier Science S.A. All rights reserved.