  In most of the realistic ab initio and model calculations which have appeared
on the emission of light from Si nanocrystals, the role of surface oxygen has
been usually ignored, underestimated or completely ruled out. We investigate
theoretically, by density functional theory (DFT/B3LYP) possible modes of
oxygen bonding in hydrogen terminated silicon quantum dots using as a
representative case of the Si29 nanocrystal. We have considered Bridge-bonded
oxygen (BBO), Doubly-bonded oxygen (DBO), hydroxyl (OH) and Mix of these
oxidizing agents. Due to stoichiometry, all comparisons performed are unbiased
with respect to composition whereas spatial distribution of oxygen species
pointed out drastic change in electronic and cohesive characteristics of
nanocrytals. From an overall perspective of this study, it is shown that bridge
bonded oxygenated nanocrystals accompanied by Mix have higher binding energies
and large electronic gap compared to nanocrystals with doubly bonded oxygen
atoms. In addition, it is observed that the presence of OH along with BBO, DBO
and mixed configurations further lowers electronic gaps and binding energies
and trends. It is also demonstrated that oxidizing constituent besides their
spatial distribution significantly alters binding energy and highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gap
(HOMO-LUMO gap up to 1.48 eV) within same composition.
