Gate-tunable and thickness-dependent electronic and thermoelectric transport in few-layer MoS₂

Over the past few years, there has been a growing interest in layered transition metal dichalcogenides such as molybdenum disulfide (MoS₂). Most studies so far have focused on the electronic and optoelectronic properties of single-layer MoS₂, whose band structure features a direct bandgap, in sharp contrast to the indirect bandgap of thicker MoS₂. In this paper, we present a systematic study of the thickness-dependent electrical and thermoelectric properties of few-layer MoS₂. We observe that the electrical conductivity (⁠$σ$⁠) increases as we reduce the thickness of MoS₂ and peaks at about two layers, with six-times larger conductivity than our thickest sample (23-layer MoS₂). Using a back-gate voltage, we modulate the Fermi energy (⁠$EF$⁠) of the sample where an increase in the Seebeck coefficient (⁠$S$⁠) is observed with decreasing gate voltage (⁠$EF$⁠) towards the subthreshold (OFF state) of the device, reaching as large as $500 μV/K$ in a four-layer MoS₂. While previous reports have focused on a single-layer MoS₂ and measured Seebeck coefficient in the OFF state, which has vanishing electrical conductivity and thermoelectric power factor (⁠$PF=S2σ$⁠), we show that MoS₂-based devices in their ON state can have $PF$ as large as $>50μWcm K2$ in the two-layer sample. The $PF$ increases with decreasing thickness and then drops abruptly from double-layer to single-layer MoS₂, a feature we suggest as due to a change in the energy dependence of the electron mean-free-path according to our theoretical calculation. Moreover, we show that care must be taken in thermoelectric measurements in the OFF state to avoid obtaining erroneously large Seebeck coefficients when the channel resistance is very high. Our study paves the way towards a more comprehensive examination of the thermoelectric performance of two-dimensional (2D) semiconductors.