ABSTRACT
One-dimensional nanostructure has attracted significant attention due to its unique chemical and physical properties associated with the geometric feature of one-dimension. Among various one dimensional nanomaterials, silicon nanowires have been widely explored for many applications including nanoscale electronics,1,2 flexible large area electronics,3-6 thermoelectrics,7 solar energy conversion,8-10 battery electrodes,11 and sensors.12-14 However, silicon nanowires are rarely investigated as an optically active material because of the indirect band gap of silicon. On the other hand, silicon nanostructures with critical dimension below 10 nm, such as
silicon quantum dots and nanoporous silicon, show strong photo-and electro-luminescence in visible light range and thus have the potential applications in the fields of silicon-based optoelectronics, bioimaging and biosensors.15-19 Recently, single crystalline porous silicon nanowires have been reported with both electrical and optical activities.20,21 The visible photoluminescence properties of porous silicon nanowires are attributed to the deep quantum confinement effect and/or complex surface states.20,21 Combining other fantastic properties of porous silicon nanowire, including large surface area, rich surface chemistry, high chemical reactivity, biodegradability and biocompatibility, it has been considered as an interesting material for potential applications in a wide range of fields, such as optics, electronics, optoelectronics, chemical sensors, energy production, and biology. In this chapter, we summarize the recent progress on the synthesis, characterization, pore formation mechanism, and properties of porous silicon nanowires. The applications of porous silicon nanowires as the novel photocatalysts and drug delivery and anode materials for lithium-ion battery are also discussed. 9.2 Synthesis of Porous Silicon Nanowires Metal-assisted electroless chemical etching method is employed to synthesize porous silicon nanowires. Originally, this method was developed to produce porous silicon with excellent and repeatable optical properties.22,23 Lately, it was also employed to synthesize vertical silicon nanowire arrays in an etchant solution composed of HF and a selected oxidant such as H2O2, AgNO3, KAuCl4, and K2PtCl6.24-32 This method has been widely studied for the synthesis of silicon nanowires starting from various types of silicon wafers with different doping levels and orientations.24-32 Recent investigations demonstrate it can be adopted to synthesize porous silicon nanowires by finely adjusting reaction conditions including the doping level of starting silicon wafers, concentration of oxidant and etching duration.20,21,33 In general, the metal-assisted chemical etching reactions can be classified into two types: one-step reaction and two-step reaction. Both approaches are found to effectively produce porous silicon nanowires.
