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

Abstract

Over the past few years, there has been a growing interest in layered transition metal dichalcogenides (TMD) such as molybdenum disulfide (MoS 2 ). Most studies so far have focused on the electronic and optoelectronic properties of single-layer MoS 2 , whose band structure features a direct bandgap, in sharp contrast to the indirect bandgap of thicker MoS 2 . In this paper, we present a systematic study of the thickness-dependent electrical and thermoelectric properties of few-layer MoS 2 . We observe that the electrical conductivity ( ) increases as we reduce the thickness of MoS 2 and peaks at about two layers, with six-time larger conductivity than our thickest sample (23-layer MoS 2 ). Using a back-gate voltage, we modulate the Fermi energy ( # ) of the sample where an increase in the Seebeck coefficient ( ) is observed with decreasing gate voltage ( # ) towards the subthreshold (OFF state) of the device, reaching as large as 500 µV/K in a four-layer MoS 2 .While previous reports have focused on a single-layer MoS 2 and measured Seebeck coefficient in the OFF state, which has vanishing electrical conductivity and thermoelectric power factor ( = / ), we show that MoS 2 -based devices in their ON state can have as large as > 50 12 345 6 in the two-layer sample. The increases with decreasing thickness then drops abruptly from double-layer to single-layer MoS 2 , 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.3

Thermal Transport in 2D Semiconductors—Considerations for Device Applications

“…Wu et al later measured the Seebeck coefficient of monolayer CVD grown MoS 2 with values ≈30 mV K −1 , even though the conductivity was low and the authors demonstrated that the transport was dominated by variable‐range hopping (VRH). Kayyalha et al reported the thermoelectric transport properties of MoS 2 with different number of layers and it was shown that both electrical conductivity and Seebeck coefficient demonstrate a strong dependence on thickness ( Figure a,b). With reducing thickness, the thermoelectric power factor (PF) keeps increasing until a peak, where a high ≈50 µW cm −1 K − 2 PF was shown for two‐layer MoS 2 sample in its ON state, before dropping greatly for the monolayer, which was theoretically attributed to a difference in the energy dependence of the electron MFP.…”

“…All the PF values are for room temperature. Reproduced with permission . Copyright 2016, American Institute of Physics.…”

Tackling Challenges in Seebeck Coefficient Measurement of Ultra‐High Resistance Samples with an AC Technique

“…Thus, if one could make the temperature gradient oscillates at a given frequency and measure the voltage signal under the same frequency using a lock‐in amplifier, the Seebeck voltage could be measured with much reduced noise and much improved accuracy. AC Seebeck measurement has been used previously to measure small Seebeck coefficients from metallic samples 21,22. With redesign of hardware and metrology, this technique can be applied to extend the Seebeck coefficient measurability in high resistance samples.…”

2D Materials for Large‐Area Flexible Thermoelectric Devices

“…More recently, thickness‐dependent thermoelectric measurements were performed with cleaved MoS 2 flakes . Interestingly, the largest power factor of 8500 µW m −1 K −2 was realized in bilayer samples and is also the largest value among semiconducting thermoelectric materials (Figure f) .…”