Abstract: We report accurate high level calculations of the optical gap and absorption spectrum of small Si nanocrystals, with hydrogen and oxygen at the surface. Our calculations have been performed in the framework of time dependent density functional theory (TDDFT) using the hybrid nonlocal exchange and correlation functional of Becke and Lee, Yang and Parr (B3LYP). The accuracy of these calculations has been verified by the high level multi-reference second order perturbation theory. The effect of oxygen contamination is studied by considering several different bonding configurations of the surface oxygen atoms. We show that for nanocrystals of sizes smaller than 20 A, the widening of the gap due to quantum confinement facilitates the stabilization of Si[double bond, length as m-dash]O double bonds. For this type of bonding, the oxygen related states determine the value of the optical gap and make it significantly lower compared to the corresponding gap of oxygen-free nanocrystals. For diameters larger than 20 A, the double bonds delocalize inside the valence band. We find that for small amounts of oxygen, the size of the optical gap depends strongly on their relative distribution and bonding type, while it is practically insensitive to the exact number of oxygen atoms.
Abstract: Using the Density Functional Theory (DFT) with the hybrid nonlocal exchange correlation functional of Becke and Lee, Yang and Parr (B3LYP), we have calculated the optical gap of small Ge nanocrystals passivated by hydrogen and with diameters between 2 and 20 Å. Our results show that the optical gap exhibits a size dependence (due to quantum confinement) with many similarities as in the case of Si quantum dots. The diameter of the smallest Ge nanocrystal emitting in the visible region of the spectrum, is approximately 19 Å. This critical dimension is smaller than the one found for the case of Si nanocrystals [Phys. Rev Lett. 87 [http://dx.doi.org/10.1103/PhysRevLett.87.276402] 276402 (2001)].
