Notable Thermoelectric Performance of Laterally Arranged Si Nanowires: Constriction Engineering as a Promising Pathway

Abstract

The conversion of thermal energy to electricity using the thermoelectric effect presents a promising and environmentally friendly approach for power generation and the efficient recovery of waste heat. In this study, a new class of nanostructures with nanoscale constrictions is introduced as a prospective approach to increase the thermoelectric conversion efficiency of materials. To demonstrate this idea, a laterally arranged nanowire-based structure called a Nanowire Chain (NWC) with one-dimensional nano-constrictions is studied within this work. A combination of classical molecular dynamics and ab-initio calculations is used to evaluate the lattice thermal conductivity and the electronic properties of the structures. A notable order of magnitude reduction of thermal conductivity with respect to the precursor non-sintered nanowires was observed and is attributed to the variation in phonon vibrational density of states along the heat transfer direction caused by the shape of the structure. This was found to be a unique quantum-confinement based effect present in NWC structures. Through first principles calculations, it is revealed that a maximum thermoelectric figure of merit (\(ZT\)) of 1.9 was obtained for Si NWCs at a carrier concentration of 3.8 × 10²⁰ cm⁻³ at room temperature. Increasing the temperature to 600 K, the maximum \(ZT\) increases to 5.4 at a carrier concentration of 2.6 × 10²⁰ cm⁻³. This two-order improvement in thermoelectric \(ZT\) over doped bulk Si is achieved using the NWC structures. Consequently, the present study demonstrates that engineering crystalline nano-constrictions could be a promising technique for developing high \(ZT\) thermoelectric materials.