The spontaneous formation of organized nanocrystals in semiconductors has been observed during heteroepitaxial growth and chemical synthesis. The ability to fabricate size-controlled silicon nanocrystals encapsulated by insulating SiO2 would be of significant interest to the microelectronics industry. But reproducible manufacture of such crystals is hampered by the amorphous nature of SiO2 and the differing thermal expansion coefficients of the two materials. Previous attempts to fabricate Si nanocrystals failed to achieve control over their shape and crystallographic orientation, the latter property being important in systems such as Si quantum dots. Here we report the self-organization of Si nanocrystals larger than 80 A into brick-shaped crystallites oriented along the (111) crystallographic direction. The nanocrystals are formed by the solid-phase crystallization of nanometre-thick layers of amorphous Si confined between SiO2 layers. The shape and orientation of the crystallites results in relatively narrow photoluminescence, whereas isotropic particles produce qualitatively different, broad light emission. Our results should aid the development of maskless, reproducible Si nanofabrication techniques.

Soft x-ray photoelectron spectroscopy of the S 2p levels at relatively high spectral resolution has been used to characterize the interaction of methanethiol, , ethanethiol, and dimethyl disulphide, with a Cu(111) surface at temperatures from approximately 130 K to 500 K. The results are consistent with previous reports of the formation of a surface thiolate species at intermediate temperatures, but also provide clear evidence for two distinct surface intermediates in addition to the intact molecules and chemisorbed atomic sulphur reported previously for this surface. These two intermediates appear to be similar to the two thiolate species reported in studies on Ni(111). Prior structural studies of the surface at room temperature show the surface to be reconstructed, and the lower temperature species identified here is assigned to a thiolate species on an unreconstructed surface, reconstruction being hindered at low temperatures. Additional evidence is found for two different atomic sulphur states in a narrow temperature range.

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Low-temperature vertical carrier transport in layered structures comprised of Si nanocrystals separated in the growth direction by angstrom-thick SiO2 layers exhibits entirely unexpected, well-defined resonances in conductivity. An unusual alternating current (ac) conductivity dependence on frequency and low magnetic field, negative differential conductivity, reproducible N-shaped switching and self-oscillations were observed consistently. The modeled conductivity mechanism is associated with resonant hole tunneling via quantized valence band states of Si nanocrystals. Tight-binding calculations of the quantum confinement effect for different Si nanocrystal sizes and shapes strongly support the tunneling model.