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.

The surface and materials science of tin oxide

Hydrothermal growth of ZnO nanostructures

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

Nano-scale GeO2 wires synthesized by physical evaporation

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.

Synthesis and photoluminescence properties of semiconductor nanowires

Field emission characteristics of single-walled carbon nanotubes were studied by using a simple method in a field emission microscope. The nanotube emission gun works effectively at the room temperature under a threshold field as low as 3.9 mV/μm. A typical I-V relationship of field emission was obtained with a high current density. The observed stable bright spots on the fluorescent screen originate from an ensemble emission from micro-ropes of the single-walled carbon nanotubes.

A Simple Method to Make Field Emitter Using Ropes of Single-Walled Carbon Nanotubes

The methods for synthesizing one-dimensional Si nanowires with controlled diameter are introduced. The mechanism for the growth of Si nanowires and the growth model for different morphologies of Si nanowires are described, and the quantum confinement effect of the Si nanowires is presented.

Z. G. Bai

Papers

Nano-scale GeO2 wires synthesized by physical evaporation

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

Synthesis and photoluminescence properties of semiconductor nanowires

A Simple Method to Make Field Emitter Using Ropes of Single-Walled Carbon Nanotubes

Growth mechanism and quantum confinement effect of silicon nanowires