Anisotropic tHermoelectric Transport Study in Two-Dimensional Heterostructures and Superlattices

We explored the thermoelectric transport properties along both in-plane and cross-plane directions in two-dimensional (PbSe)₁(VSe₂)₁ heterostructures. Along the in-plane direction, we observed an abrupt 86% increase in the Seebeck coefficient, a 245% increase in the power factor, and a slight decrease in the electrical resistivity. This unusual behavior is mainly due to the charge density wave (CDW) transition of VSe₂ in the monolayer limit, which induces a deviation from the Mott relationship through correlated electron states. We further conducted temperature-dependent Raman and X-ray diffraction to probe the CDW transition in the VSe₂ constituent layer. While the metallic VSe₂ layer dominates in-plane transport, the cross-plane transport is mainly contributed by the semiconducting PbSe layer, leading to drastically different thermoelectric transport behavior. On the other hand, a better understanding of anisotropic thermal and thermoelectric transport properties relies on the accurate estimation of anisotropic electrical resistivity, which is still underexplored, especially in the cross-plane direction, mainly due to the substantial contribution from the contacts. Here, we developed a theoretical framework to estimate the potential and current distribution in the material channel sandwiched between the top and bottom contacts. Experimental approaches are designed to accurately extract the anisotropic electrical transport properties by completely eliminating influences from contacts. (PbSe)₁(VSe₂)₁ heterostructure exhibited a 4 orders of magnitude difference between cross-plane and in-plane resistivities over the 6−300 K temperature range, resulting from alternating metallic and semiconducting layering configurations. In contrast, the MoSe₂ superlattices comprising only semiconducting layers showed an anisotropic resistivity ratio of 22. Furthermore, the developed method can be generalized to study anisotropic transport properties on most thin film materials down to nanometer thickness.