Vapor Phase Conversion Synthesis of Higher Manganese Silicide (MnSi_1.75) Nanowire Arrays for Thermoelectric Applications

Abstract

If nanostructured thermoelectric materials are to be used for future energy harvesting and power generation applications, scalable production of thermoelectric nanostructures must be developed. Herein we report a vapor phase conversion method to synthesize nanowire (NW) arrays of semiconducting higher manganese silicides (HMS, or MnSi 1.75 ) for enhanced thermoelectric applications. Dense arrays of silicon NWs obtained by metal-assisted chemical etching were converted to single-crystalline HMS NW arrays with the original nanoscale morphology preserved by reacting with Mn vapor in a sealed stainless steel reactor at 950 °C. Structural characterization by X-ray and electron diffraction and high-resolution transmission electron microscopy confirm that the converted NWs are single-crystalline NWs of HMS phases such as Mn 7 Si 12, Mn 27 Si 47, and Mn 39 Si 68 . Investigations of the conversion process using in situ high resolution powder X-ray diffraction (HRPXRD) and mechanistic experiments reveal that the presence of excess Si substrate underneath the Si NWs, careful control of Mn precursor, and high reaction temperature are crucial to the selective formation of HMS phase. The electrical resistivity of these HMS NWs are similar to that of the bulk HMS.