On the mechanism ruling the morphology of silicon nanowires obtained by one-pot metal-assisted chemical etching.

One-pot Ag-assisted chemical etching (SACE) of silicon provides an effective, simple way to obtain Si nanowires (NWs) of potential interest for technological applications ranging from photovoltaics to thermoelectricity. The detailed mechanism ruling the process has not been yet fully elucidated, however. In this paper we report the results of an extended analysis of the interplay among doping level and type of silicon, nanowire nanomorphology and the parameters controlling the chemistry of the etching process. We provide evidence that the SACE mechanism entirely occurs at the interface between the etching solution and the Si substrate as a result of Si extrusion by sinking self-propelled Ag particles. Also, a rationale is advanced to explain the reported formation of (partially) porous NWs at high doping levels in both p- and n-type Si. A model not relying on the asserted formation of potential barriers enables to recover full consistency between SACE electrochemistry and the mechanism of formation of porous silicon in electrochemical cells.

High Power Thermoelectric Generator Based on Vertical Silicon Nanowires.

Thermoelectric generators, which convert heat directly into electrical power, have great potentialities in the energy harvesting field. The exploitation of these potentialities is limited by the materials currently used, characterized by good thermoelectric properties, but also by several drawbacks. This work presents a silicon-based thermoelectric generator, made of a large collection of heavily p-doped silicon nanostructures. This macroscopic device (area of several mm2) collects together the good thermoelectric features of silicon, in terms of high power factor, and a very reduced thermal conductivity, which resulted in being exceptionally low (1.8 W/(m K), close to the amorphous limit). The generated electrical power density is remarkably high for a Si-based thermoelectric generator, and it is suitable for scavenging applications which can exploit small temperature differences. A full characterization of the device (Seebeck coefficient, thermal conductivity, maximum power output) is reported and discussed.