Excellent thermoelectric performance of open framework Si24 nanowires from density functional based tight-binding calculation

Abstract

Open framework Si24 is a cage-like silicon allotrope with a highly anisotropic character and predicted to possess fascinating optoelectronic and thermoelectric performance. In this paper, we systematically investigated the thermoelectric conversion performance of Si24 nanowires (Si24NWs) through using nonequilibrium Green's function method as implemented in the density functional based tight-binding framework. The calculations reveal that Si24NWs possess superb electronic transport properties, e.g., the Seebeck coefficient could approach 2.67 mV/k at room temperature (several times greater than that of diamond silicon nanowires). Meanwhile, evident anisotropic thermal transport is also observed in Si24NWs, where the room temperature phonon thermal conductance of Si24NW-[100]/[010]/[001] is 1.36/0.27/0.38 nW/k, respectively. Through analyzing the phonon spectra of Si24NWs, we explain the origin of such an anisotropic thermal transport property. By combining the electron and phonon transport properties, the thermoelectric properties of Si24NWs are predicted and the ZT value of Si24NW-[100]/[010]/[001] could, respectively, approach 0.18/1.28/0.85 at room temperature. In order to achieve better thermoelectric performance, the Si/Ge core–shell structures are constructed (based on Si24NW-[010]). The results show that the core–shell structure is indeed a viable and efficient way to improve the thermoelectric conversion efficiency (the figure of merit could be boosted to 2.2 at room temperature). The findings presented in this paper shed light on the thermoelectric performance of Si24NWs and could provide a helpful guideline for designing and fabricating excellent thermoelectric devices based on Si24NWs.