We report the first experimental observation of a striking convergence of Auger recombination rates in nanocrystals of both direct- (InAs, PbSe, CdSe) and indirect-gap (Ge) semiconductors, which is in contrast to a dramatic difference (by up to 4-5 orders of magnitude) in the Auger decay rates in respective bulk solids. To rationalize this finding, we invoke the effect of confinement-induced mixing between states with different translational momenta, which diminishes the impact of the bulk-semiconductor band structure on multiexciton interactions in nanocrystalline materials.

Gas-phase plasma-synthesized silicon nanocrystals (Si-NCs) are doped with boron (B) or phosphorous (P) during synthesis. The doping efficiency of B is smaller than that of P, consistent with the theoretical prediction of impurity formation energies. Despite vastly different synthesis conditions, the effect of doping on the photoluminescence (PL) of gas-phase-synthesized Si-NCs is qualitatively similar to that of Si-NCs doped during solid phase nucleation. Studies of oxidation-induced changes in PL and etching-induced changes in dopant concentration show that P resides at or near the Si-NC surface, while B is in the Si-NC cores. The oxidation of Si-NCs follows the Cabrera–Mott mechanism [N. Cabrera and N. F. Mott, Rep. Prog. Phys. 12, 163 (1948)].

44 wileyonlinelibrary.com www.particle-journal.com www.MaterialsViews.com Optimizing the light-emitting effi ciency of silicon quantum dots (Si QDs) has been recently intensifi ed by the demand of the practical use of Si QDs in a variety of fi elds such as optoelectronics, photovoltaics, and bioimaging. It is imperative that an understanding of the optimum light-emitting effi ciency of Si QDs should be obtained to guide the design of the synthesis and processing of Si QDs. Here an investigation is presented on the characteristics of the photoluminescence (PL) from hydrosilylated Si QDs in a rather broad size region (≈2-10 nm), which enables an effective mass approximation model to be developed, which can very well describe the dependence of the PL energy on the QD size for Si QDs in the whole quantum-confi nement regime, and demonstrates that an optimum PL quantum yield (QY) appears at a specifi c QD size for Si QDs. The optimum PL QY results from the interplay between quantum-confi nement effect and surface effect. The current work has important implications for the surface engineering of Si QDs. To optimize the light-emission effi ciency of Si QDs, the surface of Si QDs must be engineered to minimize the formation of defects such as dangling bonds at the QD surface and build an energy barrier that can effectively prevent carriers in Si QDs from tunneling out.tuning of the optical properties of Si QDs by means of surface engineering. [24][25][26] Freestanding Si QDs that are either produced in liquid/gas phases [27][28][29][30] or released from solid matrices [31][32][33][34] have recently become popular largely because of their easily accessible surface. A standard surface modifi cation schemehydrosilylation [35][36][37][38][39] -has been developed to effectively disperse freestanding Si QDs in common solvents. It is found that hydrosilylation not only enables the formation of colloidal Si QDs, but also improves the effi ciency and stability of the light emission from Si QDs. Given the growing concern on the potential environmental impact of archetypal colloidal II/IV-VI QDs (e.g., CdSe and PbS QDs), [ 40 ] colloidal Si QDs hold great promise for the development of a variety of QD-based structures and devices.Since the Bohr radius of an exciton in Si is ≈5 nm, [ 17 ] the size regions of ≈5 and 5-10 nm approximately correspond to strong quantum confi nement and weak quantum confi nement for Si QDs, respectively. It has been observed that the photoluminescence (PL) energy of hydrosilylated Si QDs increases with the decrease of the QD size in both the strong and weak quantum-confi nement regimes, [ 35,41,42 ] consistent with the well-known quantum-confi nement effect. [ 43,44 ] Hessel et al. [ 41 ] have recently demonstrated that the PL quantum yield (QY) of hydrosilylated Si QDs increases with the decrease of the QD size in the weak quantum-confi nement regime. However, Jurbergs et al. [ 35 ] and Mastronardi et al. [ 42 ] previously showed that the PL QY of hydrosilylated Si QDs decreases with the decrease of the...

Silicon nanocrystals have recently attracted significant attention for applications in electronics, optoelectronics, and biological imaging due to their size-dependent optical and electronic properties. Here a method for synthesizing luminescent silicon nanocrystals from silicon tetrachloride with a nonthermal plasma is described. Silicon nanocrystals with mean diameters of 3-15 nm are synthesized and have a narrow size distribution with the standard deviation being less than 20% of the mean size. Control over crystallinity is achieved for plasma pressures of 1-12 Torr and hydrogen gas concentrations of 5-70% through adjustment of the plasma power. The size of nanocrystals, and resulting optical properties, is mainly dependent on the gas residence time in the plasma region. Additionally the surface of the nanocrystals is covered by both hydrogen and chlorine. Oxidation of the nanocrystals, which is found to follow the Cabrera-Mott mechanism under ambient conditions, is significantly faster than hydrogen terminated silicon due to partial termination of the nanocrystal surface by chlorine.