Nucleation and growth of Si nanowires by laser ablation and thermal evaporation of Si powder sources mixed with SiO 2 have been investigated by means of transmission electron microscopy. At the initial nucleation stage, Si oxide vapor condensed on the substrate and formed Si nanoparticles ͑the nuclei of nanowires͒. Each Si nanowire nucleus consisted of a polycrystalline Si core containing a high density of defects and a Si oxide shell. A growth mechanism was proposed based on the microstructure and different morphologies of the Si nanowires observed. ͓S0163-1829͑98͒51348-3͔
Semiconductor wires with nanometer widths have attracted much attention in recent years for their potential applications in mesoscopic research and nanodevices. Since the 1960s, Si whiskers grown from the vapor-liquid-solid (VLS) reaction have been extensively studied. In the VLS reaction, Au particles are generally used as the mediating solvent on a Si substrate since Au and Si form a molten alloy at a relatively low temperature. Si in the vapor phase diffuses into the liquid-alloy droplet and bonds to the solid Si at the liquid-solid interface, which results in the growth of Si whiskers. The diameter of the whisker is determined by the diameter of the liquid-alloy droplet at its tip. Si whiskers generally grow along ⟨111⟩ directions epitaxially on Si(111) substrates in the form of single crystals by the VLS reaction.In different materials systems, however, a variety of whisker forms can be obtained. For example, GaP whiskers display rotational twins around their ⟨111⟩ growth axes, while GaAs whiskers grow in the form of the wurtzite structure.Thus far, the synthesis of one-dimensional nanostructured materials on a large scale remains a challenge. In recent years, many efforts have been made to synthesize Si nanowires by employing different methods such as photolithography and etching techniques and scanning tunneling microscopy. One method of particular interest is a recently developed laser ablation of metal-containing semiconductor targets, by which bulk quantities of semiconductor nanowires can be readily obtained. Our recent studies show that oxides play a dominant role in the nucleation and growth of high-quality semiconductor nanowires in bulk quantities by laser ablation, thermal evaporation, or chemical vapor deposition. A new growth mechanism called oxide-assisted nanowire growth has therefore been established. The ability to synthesize large quantities of high-purity (no contamination), ultra-long (in millimeters), and uniform-sized semiconductor nanowires (a few nanometers to tens of nanometers in diameter) from this new technique offers exciting possibilities in fundamental and applied research.
Si nanowires with uniform size have been synthesized by laser ablation of highly pure Si powder targets mixed with SiO2. A bulk quantity of Si nanowires was successfully obtained by mixing 30%–70% of SiO2 into the Si powder target. SiO2 played a crucial role in enhancing the formation and growth of the Si nanowires. The morphology and microstructure of the Si nanowire tips have been systematically characterized by means of high-resolution transmission electron microscopy. No evidence of metal was found at the tips. The results suggest that Si oxide is more important than metal in catalyzing the formation of Si nanowires.
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