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

The successful synthesis of carbon nanotubes has stimulated much research in nanomaterials, in particular, in semiconducting nanostructures, which are interesting for their novel sizeand dimensionality-dependent physical properties, and their potential applications in nanoscale optoelectronics. Silicon is the most important semiconducting material, with its contemporary microelectronic technology being one of the greatest successes of the twentieth century. Bulk silicon does not emit visible light, because it is an indirect-bandgap material with a small exciton-binding energy (15 meV). Low-dimensional Si nanostructures, such as quantum dots, nanocrystals, porous silicon, and nanowires can have a direct bandgap, and emit visible light according to the quantum-confinement effect. For this reason, a great deal of effort has been invested in preparing these low-dimensional Si nanostructures; their potential applications include the fabrication of Si-based optoelectronic devices. One-dimensional (1D) silicon nanowires (SiNWs) in particular have been intensely investigated. Much effort has been made to prepare SiNWs by different methods, such as chemical vapor deposition, laser ablation, thermal evaporation decomposition, supercritical-fluid–liquid–solid (SFLS) synthesis, and other methods. These methods are quite accessible and well controlled. For example, single-crystalline SiNWs with controlled diameters have been prepared by laser-assisted catalytic growth. The axial orientation of SiNWs can be tuned when using the SFLS method. The importance of electrochemistry in Si microelectronic technology has spurred intense research activity. In particular, silicon shows novel electrochemical properties in solutions containing hydrofluoric acid: the complex electrochemical etching behavior has raised considerable research interest. Electroless metal deposition (EMD) and electroless etching have been widely investigated, owing to their important applications in the microelectronics industry. EMD is often described as a galvanic displacement process that involves the spontaneous oxidation of Si atoms, and the reduction of metal ions to metallic particles and films in the absence of an external source of electric current. The galvanic displacement process can be described using mixed potential theory. Now the EMD technique has been extended to the fabrication of various nanostructures. Herein, we report the room-temperature fabrication of highly oriented SiNW arrays, and also porous silicon, using simple chemical etching of Si wafers in an aqueous HF solution containing Fe ions: the process is based on the metal-seed-induced excessive local oxidation and dissolution of Si substrates. This direct approach allows the rapid fabrication of high-quality, well-aligned SiNW arrays with large-area homogeneity and tunable depths. By utilizing this method, single-crystalline SiNWs with desirable crystallographic orientations can be readily and controllably created by the selection of Si wafers with the corresponding crystallographic orientations. According to the quantum-confinement effect, accurate axial-ori-