Enhanced thermoelectric properties in phosphorene nanorings

Using the tight-binding approach, we investigate the thermoelectric (TE) properties of rectangular phosphorene nanorings for both symmetrically and asymmetrically attaching to phosphorene nanoribbon leads. We design our phosphorene-based nanostructures to enhance the TE performance in the absence and the presence of perpendicular magnetic fields. Our results show that when zigzag phosphorene nanoribbons (ZPNRs) are coupled symmetrically to rectangular rings, a comparatively large band gap is induced in the electronic conductance due to the suppression of the contribution of edge states. This gives rise to a remarkable increase in the thermopower response compared to the case of pristine ZPNRs. More intriguingly, we found that though the maximum power factor in this system is about the same as the one for its ZPNR counterpart, the much smaller electronic thermal conductance of this phosphorene-based nanostructure can remarkably contribute to the improvement of the figure of merit. Also, we found that the symmetry/asymmetry of our designed nanostructures, the geometrical characteristics of the ring, and the magnetic flux are three important factors that control the thermoelectric properties of phosphorene quantum rings. Our numerical calculations show that by changing the magnetic flux through the nanoring, a drastic increase in the thermopower is observed near an antiresonance point. We demonstrate the tunability of the thermopower and the possibility to switch on and off the TE response of phosphorene nanorings with the magnetic flux. Moreover, for asymmetric connection configurations with armchair-edged leads, we found that though the thermopower is almost intact, a remarkable reduction of the electronic thermal conductance can lead to a notable improvement in the figure of merit. Our results suggest phosphorene nanorings as promising candidate nanostructures for TE applications.