Optimizing Thermal Energy Harvesting in Few-Layer MoS2 with Measurements of Electron's Effective Mass in Two-Dimensional Semiconductors

Two-dimensional transition metal dichalcogenides (2D TMDs) have emerged as promising candidates for next-generation electronic devices in the post-Moore's Law era. However, the high-power densities of these devices lead to substantial heat generation, highlighting the importance of thermoelectric generators for waste heat recovery. Among 2D TMDs, ultrathin MoS2 has shown considerable potential for applications in both nanoscale transistors and thermoelectric generators. Nevertheless, the correlation between its electrical transport and thermoelectric properties remains insufficiently understood and warrants further investigation. To address this, we fabricated thermoelectric field-effect transistors using MoS2 flakes (1 to 39 layers). We measured electrical and thermoelectric properties from 80 K to 600 K, analyzing transport behaviors using Mott's hopping transport, thermal activation, and phonon scattering theories. We especially studied their dependence on the carriers' concentration. This analysis allowed us to extract the universal Boltzmann constant and the electron's effective mass . Our findings indicate that the Seebeck coefficient and power factor achieve the optimum state for MoS2 flakes with thicknesses less than 20 layers, where the high-voltage output and high-power density output occur at 15-20 and 1-5 layers, respectively. Notably, we present the electron's effective mass as a function of flake thickness. We identified an extrinsic effect of memory steps, caused by trapped charge release above 450 K, which significantly enhances electrical and thermoelectric properties.