The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

A novel strategy for preparing large-area, oriented silicon nanowire (SiNW) arrays on silicon substrates at near room temperature by localized chemical etching is presented. The strategy is based on metal-induced (either by Ag or Au) excessive local oxidation and dissolution of a silicon substrate in an aqueous fluoride solution. The density and size of the as-prepared SiNWs depend on the distribution of the patterned metal particles on the silicon surface. High-density metal particles facilitate the formation of silicon nanowires. Well-separated, straight nanoholes are dug along the Si block when metal particles are well dispersed with a large space between them. The etching technique is weakly dependent on the orientation and doping type of the silicon wafer. Therefore, SiNWs with desired axial crystallographic orientations and doping characteristics are readily obtained. Detailed scanning electron microscopy observations reveal the formation process of the silicon nanowires, and a reasonable mechanism is proposed on the basis of the electrochemistry of silicon and the experimental results.

Fabrication of Single-Crystalline Silicon Nanowires by Scratching a Silicon Surface with Catalytic Metal Particles†

A large quantity of nanosized ZnO tubular structures was prepared using a very simple thermal evaporation of mixed Zn–ZnO powders under a wet oxidation condition. The ZnO nanotubes have a hollow core with crystalline wall of 8–20 nm in thickness. Optical properties of ZnO nanotubes were studied at room temperature. Raman peaks arising from the ZnO nanotubes were analyzed, which correspond well to that of the bulk ZnO sample. The photoluminescence measurements of ZnO nanotubes revealed an intensive UV peak at 377 nm corresponding to the free exciton emission, and a broad peak at about 500 nm arising from defect-related emission.

Optical properties of the ZnO nanotubes synthesized via vapor phase growth

The autonomous motion behavior of metal particles in Si, and the consequential anisotropic etching of silicon and production of Si nanostructures, in particular, Si nanowire arrays in oxidizing hydrofluoric acid solution, has been systematically investigated. It is found that the autonomous motion of metal particles (Ag and Au) in Si is highly uniform, yet directional and preferential along the [100] crystallographic orientation of Si, rather than always being normal to the silicon surface. An electrokinetic model has been formulated, which, for the first time, satisfactorily explains the microscopic dynamic origin of motility of metal particles in Si. According to this model, the power generated in the bipolar electrochemical reaction at a metal particle's surface can be directly converted into mechanical work to propel the tunneling motion of metal particles in Si. The mechanism of pore and wire formation and their dependence on the crystal orientation are discussed. These models not only provide fundamental interpretation of metal-induced formation of pits, porous silicon, and silicon nanowires and nanopores, they also reveal that metal particles in the metal/Si system could work as a self-propelled nanomotor. Significantly, it provides a facile approach to produce various Si nanostructures, especially ordered Si nanowire arrays from Si wafers of desired properties.

Motility of metal nanoparticles in silicon and induced anisotropic silicon etching

Silicon nanowires will find applications in nanoscale electronics and optoelectronics both as active and passive components. Here, we demonstrate a low-temperature vapor–liquid–solid synthesis method that uses liquid-metal solvents with low solubility for silicon and other elemental semiconductor materials. This method eliminates the usual requirement of quantum-sized droplets in order to obtain quantum-scale one-dimensional structures. Specifically, we synthesized silicon nanowires with uniform diameters distributed around 6 nm using gallium as the molten solvent, at temperatures less than 400 °C in hydrogen plasma. The potential exists for bulk synthesis of silicon nanowires at temperatures significantly lower than 400 °C. Gallium forms a eutectic with silicon near room temperature and offers a wide temperature range for bulk synthesis of nanowires. These properties are important for creating monodispersed one-dimensional structures capable of yielding sharp hetero- or homointerfaces.

Bulk synthesis of silicon nanowires using a low-temperature vapor–liquid–solid method

The recent success of bulk synthesis of pure nanoscale silicon quantum wires (SiQW's) enables us to evaluate their photoluminescence (PL) characteristics under ultraviolet photoexcitation. Intensive multiple light emissions ranging from dark red to blue regions were revealed for as-grown and partially oxidized SiQW samples. The red light emission was ascribed to a quantum confinement effect originating from the crystalline core of the SiQW's that is closely mediated by the interface. However, the PL emission from green to blue is found to be definitely unrelated to quantum confinement; instead they are attributed to the radiative recombination from defect centers in the overcoating layer of the amorphous silicon oxide outside the SiQW's.

Direct evidence of quantum confinement from the size dependence of the photoluminescence of silicon quantum wires

GaSe and Si nanowires (SiNWs) were synthesized by physical evaporation. The photoluminescence (PL) properties of the Si nanowires were investigated in correlation with the diameter of the crystalline core. Intensive light emission was observed peaking at dark red, green and blue regions for the as grown and partially oxidized SiNW samples. The red PL emission is ascribed to the quantum confinement effect of the crystalline core of the SiNWs, while the green and blue ones are attributed to the radiative recombination of the defect centers in the amorphous silicon oxide layer surrounding the SiNWs.

J. S. Fu

Papers

Direct evidence of quantum confinement from the size dependence of the photoluminescence of silicon quantum wires

Synthesis and photoluminescence properties of semiconductor nanowires