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The equilibrium geometric structures, relative stabilities, and electronic properties of AunC- and Aun+1- (n = 1-10) clusters are systematically investigated using density functional theory with hyper-generalized gradient approximation. The optimized geometries show that one Au atom capped on Aun-1C- clusters is a dominant growth pattern for AunC- clusters. In contrast to Aun+1- clusters, AunC- clusters are most stable in a quasi-planar or three-dimensional structure because C doping induces the local non-planarity while the rest of the structure continues to grow in a planar mode, resulting in an overall non-2D configuration. The relative stability calculations show that the impurity C atom can significantly enhance the thermodynamic stability of pure gold clusters. Moreover, the effect of C atom on the Aun- host decreases with the increase of cluster size. The HOMO-LUMO gap curves show that the interaction of the C atom with Aun- clusters improves the chemical stability of pure gold clusters, except for Au3- and Au4- clusters. In addition, a natural population analysis shows that the charges in corresponding AunC- clusters transfer from the Aun- host to the C atom. Meanwhile, a natural electronic configuration analysis also shows that the charges mainly transfer between the 2s and 2p orbitals within the C atom. [ABSTRACT FROM AUTHOR]

We report optical gain enhancements in Si nanocrystals (Si-NCs) via hydrogenation and Ce3+ ion doping. Variable stripe length technique was used to obtain gains. At 0.3 W/cm² pumping power density of pulsed laser, net gains were observed together with gain enhancements after hydrogenation and/or Ce3+ ion doping; gains after loss corrections were between 89.52 and 341.95 cm-1; and the photoluminescence (PL) lifetime was found to decrease with the increasing gain enhancement. At 0.04 W/cm² power density, however, no net gain was found and the PL lifetime increased with the increasing PL enhancement. The results were discussed according to stimulated and spontaneous excitation and de-excitation mechanisms of Si-NCs. [ABSTRACT FROM AUTHOR]