Surface passivation of semiconductor nanocrystals (NCs) is critical in enabling their utilization in novel optoelectronic devices, solar cells, and biological and chemical sensors. Compared to the extensively used liquid-phase NC synthesis and passivation techniques, gas-phase routes provide the unique opportunity for in situ passivation of semiconductor NCs. Herein, we present a method for in situ gas-phase organic functionalization of plasma-synthesized, H-terminated silicon (Si) NCs. Using real-time in situ attenuated total reflection Fourier transform IR spectroscopy, we have studied the surface reactions during hydrosilylation of Si NCs at 160 °C. First, we show that, during gas-phase hydrosilylation of Si NCs using styrene (1-alkene) and acetylene (alkyne), the reaction pathways of the alkenes and alkynes chemisorbing onto surface SiH(x) (x = 1-3) species are different. Second, utilizing this difference in reactivity, we demonstrate a novel pathway to enhance the surface ligand passivation of Si NCs via in situ gas-phase hydrosilylation using the combination of a short-chain alkyne (acetylene) and a long-chain 1-alkene (styrene). The quality of surface passivation is further validated through IR and photoluminescence measurements of Si NCs exposed to air.

A systematic study of tin-catalyzed vapor−liquid−solid (VLS) growth of silicon nanowires by plasma-enhanced chemical vapor deposition at temperatures ranging from 300 to 400 °C is presented. Wire structure, morphology, and growth rate are characterized as a function of process variables. The nanowires are observed to have a crystalline core with a polycrystalline shell due to simultaneous VLS axial growth and vapor−solid radial growth. Axial and radial growth rates are controllable through hydrogen dilution of the plasma which affects the concentration of silane radicals in the plasma. In addition, wire length is observed to saturate with increasing growth time. Post growth chemical analysis suggests this is due to etching and disappearance of tin seeds in the hydrogen plasma which occur in parallel with wire growth. This opens up the possibility of a unique in situ approach to fabricating metal-free nanowire arrays for device applications.

We have determined the surface hydride composition of amorphous and crystalline Si nanoparticles (NPs) (3À5 nm) synthesized in a low-temperature SiH 4 /Ar plasma using in situ attenuated total reflection Fourier-transform infrared spectroscopy and H 2 thermal effusion measurements. With increasing power to the plasma source, the particles transition from amorphous to crystalline with a corresponding increase in the fraction of SiH species on the surface. The surface hydride composition indicates that Si NPs synthesized at higher plasma powers crystallize in the gas-phase due to a greater degree of plasma-induced heating, which enhances the desorption rates for SiH 2 and SiH 3 . Furthermore, these Si NPs do not contain any detectable H in the bulk.

The atomic-scale interactions of H atoms with hydrogenated amorphous carbon ͑a-C:H͒ films were identified using molecular dynamics ͑MD͒ simulations and experiments based on surface characterization tools. Realistic a-C:H films developed using MD simulations were impinged with H atoms with a kinetic energy corresponding to a temperature of 700 K. The specific chemical reactions of the H atoms with the a-C:H surface were identified through a detailed analysis of the MD trajectories. The MD simulations showed that hydrogenation occurs primarily at the sp 2 sites and converts them to sp 3-hybridized C atoms. Depending on the hybridization of the next-nearest neighbor, a dangling bond may or may not be created. The hydrogenation reaction is highly exothermic, Ͼ2.5 eV, and proceeds with a negligible activation energy barrier via a mechanism similar to Eley-Rideal. In certain cases hydrogenation may also cleave a CC bond. The reaction events observed through MD simulations are consistent with the surface characterization of D-exposed a-C:H films using Raman spectroscopy, spectroscopic ellipsometry, and in situ attenuated total reflection Fourier-transform infrared spectroscopy.

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