The structural saturation and stability, the energy gap, and the density of states of a series of small, siliconbased clusters have been studied by means of the PM3 and some ab initio ͑HF/6-31G * and 6-311ϩϩG**, CIS/6-31G * and MP2/6-31G*͒ calculations. It is shown that in order to maintain a stable nanometric and tetrahedral silicon crystallite and remove the gap states, the saturation atom or species such as H, F, Cl, OH, O, or N is necessary, and that both the cluster size and the surface species affect the energetic distribution of the density of states. This research suggests that the visible luminescence in the silicon-based nanostructured material essentially arises from the nanometric and crystalline silicon domains but is affected and protected by the surface species, and we have thus linked most of the proposed mechanisms of luminescence for the porous silicon, e.g., the quantum confinement effect due to the cluster size and the effect of Si-based surface complexes.The visible luminescence in porous silicon 1 is widely believed to arise from nanometric and crystalline silicon domains which show a quantum confinement effect. 1-5 However, other studies support different mechanisms of luminescence: several experimental and theoretical papers claimed 6-8 that both the quantum confinement and the surface ͑hydrogen saturation͒ effects are responsible; Brandt et al. 9 suggested that it may be attributable to a Si-backbone polymer, such as Si 6 O 3 H 6 ͑some later studies 10,11 also supported a similar idea͒; Xu, Gal, and Gross 12 assigned luminescence to molecules attached to the Si surface; a few authors attributed it to hydrogen-related surface species such as SiH 2 and polysilanes; 13,14 the formation of amorphous silicon has also been considered as a possible luminescent mechanism. 15 Some recent papers 16 -18 are very helpful in understanding the origin of the visible luminescence.The visible luminescence has also been found in other nanoscale silicon-based materials, such as ultrafine silicon particles, 19 crystallized amorphous Si:H/SiN x :H multiquantum-well structures, 20 and the laser-annealed hydrogenated silicon powder produced in plasma-enhanced chemical-vapor deposition processes, 21 which has recently been shown to be crystallized, 22 although their photoluminescence ͑PL͒ dynamics and energetic distribution are very different. This strongly suggests that the visible luminescence is associated with nanosize silicon-based materials, whose structures can be quite different but must possess local ordering domains. [1][2][3][4][5][6][7][8][17][18][19][20][21][22] In this paper, we performed first-principles calculations in order to understand the luminescent mechanism through the study of the atomic and electronic structures of a series of clusters: All the ab initio ͑HF/6-31G * and 6-311ϩϩ G **, CIS and MP2/6-31G *) and semiempirical PM3 calculations were carried out with the GAUSSIAN 92 package 23 on the IBM-sp2 computer at the University of Barcelona. All the structures in this study were optimi...

ABSTRACTa-Si1-x Cx:H powders have been produced in radiofrequency glow discharges of SiH4 and CH4. Samples produced in discharges with different gas flow ratios (R=[CH4] / [SiH4]+[CH4], R ∼ 0.2 to 0.95) were analyzed by several techniques: Infrared spectroscopy, transmission electron microscopy and thermal desorption spectrometry of hydrogen. The microstructural properties of the a-Si1-x Cx:H powders obtained ranged between those previously reported in silicon particles and those of amorphous carbon particles. As a general rule, the particles, whose diameter ranged between 5 and 400 nm in diameter, showed an amorphous microstructure and high hydrogen content. Infrared spectroscopy showed that the hydrogen was mainly located on the particle surface and in microvoids forming CHi and/or SiHi (i=2,3) groups.The hydrogen was removed from the powder by thermal annealing (< 700°C) under vacuum. Anlysis of the a-Si1-x Cx:H powders after this thermal treatment revealed new microstructural and optical features.