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

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.