We present density-functional theoretical results of structural, electronic and optical properties of bare and surface passivated (-H, -OH, -NH2) stoichiometric SinCn (n = 10, 16, 28, 37, 43, 55, 68) quantum dots (QDs) as a function of the size of the dots. We have studied both the effect of size and surface passivation on the band edge electronic and optical properties of SiC nanoparticles. We show that the HOMO-LUMO gap of SiC nanoparticles can be tuned by using different functionalizing agents. Thus -H passivated QDs have the largest HOMO-LUMO gap followed by -OH and -NH2 passivated QDs. We also present few results on non-stoichiometric SimCn clusters. (C) 2012 Elsevier B. V. All rights reserved.