High thermoelectric figure of merit in devices made of vertically stacked graphene layers

We study the thermoelectric properties of devices made of two partially overlapped graphene sheets. As a consequence of the weak van der Waals interactions between graphene layers, it is shown that the phonon conductance in these junctions is strongly reduced compared to that of single graphene layer structures. In contrast, their electrical conductance is more weakly affected. We hence demonstrate that the thermoelectric figure of merit can reach values higher than 1 at room temperature in graphene materials having the advantage of offering either a bandgap or a conduction gap. We consider in particular the cases of stacks of armchair graphene nanoribbons, misoriented graphene nanoribbons and graphene nanomeshes. For the latter device, a figure of merit of 1.8 is obtained at room temperature and reaches even 3.2 at 600K. The vertical design of graphene layers thus appears as an efficient way to achieve high thermoelectric efficiency in graphene devices.