Significant Enhancement in Thermoelectric Performance of S-shaped Germanene Nanoribbon Devices

Due to the advantages including high efficiency and environmental friendliness, the two-dimensional thermoelectric materials have received huge attention. However, the energy conversion efficiency is not high enough for application. In this work, we have theoretically investigated the thermoelectric performance for both zigzag (ZGeNRs) and armchair (AGeNRs) germanene nanoribbons with edge defects by nonequilibrium Green's function method. For perfect germanene nanoribbons (GeNRs), it is shown that with its width increasing, the maximum of ZT values (ZTmax) decreases monotonously while the phononic thermal conductance increases linearly. For both ZGeNRs and AGeNRs, with increasing defect numbers in longitudinal direction, ZTmax increases monotonously while the phononic thermal conductance decreases. In particular, by introducing periodic defects, the ZTmax of both ZGeNRs and AGeNRs has been raised to above 2, which is due to higher Seebeck coefficient and lower thermal conductance. Our findings provide an effective way to enhance thermoelectric performance, which is beneficial for the design of ultra-thin thermoelectric devices.