Collaborative Research: Understanding Cross-plane and In-plane Transport in 2D Layered Heterostructures

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The goal of this collaborative effort is to develop an understanding of thermal and thermoelectric transport in an important new class of two-dimensional (2D) crystalline materials. This class of materials has the lowest thermal conductivities ever observed in a fully dense material. Highly efficient thermoelectrics can provide efficient solid-state cooling that rivals conventional refrigeration systems. This effort leverages measurement techniques recently developed in the Cronin lab, which enable the cross-plane (i.e. in the direction perpendicular to the layers) thermal and thermoelectric transport of extremely thin films to be measured accurately for the first time, and the unique materials synthesis capabilities developed in the Johnson lab, which enable specific sequences of 2D layers to be prepared and structurally characterized. Together, the labs will address several open questions regarding the thermal and thermoelectric transport properties of this interesting materials system, such as how the arrangement of layers and density of interfaces impact the difference between in plane and cross plane transport properties. This proposal will explore thermal and thermoelectric phenomena in a unique class of materials poised between the amorphous and crystalline states and test potential device structures using novel measurement techniques. In addition to thermoelectric energy conversion, the proposed scheme of studying cross-plane transport can be applied to a wide range of other device systems, including LEDs, FETs, and RTDs, currently being investigated by other groups. The proposed heterostructure geometries open up new degrees of freedom in the cross-plane transport with independent control of electrons and phonons, which is essential for achieving efficient thermoelectric energy conversion devices. The proposed layered heterostructures will enable many parameters to be varied such as inter-material barrier height, band gap (across the semiconductor-semimetal spectrum), and charge density wave transitions over a wide range of compositions to optimize thermoelectric phenomena. These structures enable investigation of systematic structural changes that are not possible with traditional approaches, for example, atomically-sharp interfaces that are not lattice matched and metal/semiconductor superlattices with atomically abrupt interfaces.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.