Large-scale MoS2 thin films with a chemically formed holey structure for enhanced Seebeck thermopower and their anisotropic properties

Abstract

The thermoelectric (TE) effect in nanoscale materials is of great interest as an ideal platform for small-scale energy harvesting and micro-cooling technologies. In this regard, two-dimensional (2D) metal dichalcogenides (TMDCs) can be considered as a promising material for TE applications owing to their superior electronic and phonon transport properties provided by a favorable large energy band gap and an atomically thin layer, which serves as an ideal quantum well structure. We investigate and conduct a direct comparison of the cross-plane Seebeck coefficients of various TMDC films prepared by using the chemical vapor deposition method. In particular, these coefficients for MoS₂, WSe₂, and WS₂ thin films are determined to be approximately 115, 129, and 211 μV K⁻¹ at 300 K, respectively. The chemically formed holey structure of MoS₂ thin films with a thickness of ∼7 nm exhibits superior in-plane Seebeck coefficients of ∼742 μV K⁻¹, showing strong anisotropic behavior with a ratio of ∼6.5 along in- and cross-plane directions at 300 K. Such behavior can be explained by the energy filtering effect in the holey MoS₂ film in the in-plane direction. Moreover, an extremely high anisotropic ratio (∼2.4 × 10⁸) of the power factor was observed owing to the large in-plane Seebeck coefficients and the electrical conductivity of the MoS₂ films across the samples. This study is the first to assess the cross-plane Seebeck coefficients of large-scale MoS₂, WS₂, and WSe₂ thin films at 300 K and analyze the anisotropic behavior of the MoS₂ film. The results reported here confirm the importance of providing reliable Seebeck coefficient information on films formed by using TMDC materials for further application on TE devices.