Modulation Of Thermoelectric Transport Properties In 2d-mos₂ Using Molecular Surface Charge Transfer Doping

Transition metal dichalcogenides (TMDCs) are atomically thin, with adjacent layers bonded by weak van der Waals interactions. These materials generally exhibit a bandgap which can be tuned by adjusting the layer thickness. Due to these unique properties, TMDCs have been extensively studied for achieving a high thermoelectric power factor. In this study, we focus on the enhanced thermoelectric performance of an individual MoS₂ nanosheet by bandgap modulation technique. We applied benzyl viologen (BV) molecules doping via surface charge-transfer doping (SCTD) since the BV molecules are expected to be suitable for tuning the thermoelectric properties of n-type MoS₂ by the high reduction potential of BV molecules.
MoS₂ nanosheets were obtained from bulk MoS₂ crystals through mechanical exfoliation. The electrodes such as one heater, two thermometers, and two Hall electrodes were patterned on the individual exfoliated MoS₂ nanosheets using an e-beam lithography technique. Cr/Au (10 nm/100 nm) was deposited using a DC magnetron sputtering system for Ohmic contact. Electrical conductivities were measured using the four-point probe method, while the Seebeck coefficient was determined by applying Joule heating to the microheater without applying any external voltage to the circuit. The temperature gradient across the two thermometers was used to calculate the Seebeck coefficient. Signal variations were obtained using an AC lock-in amplifier.
The size and thickness of the MoS₂ nanosheets were controlled by adjusting the number of exfoliation cycles. The electrical properties of a 70 nm thick MoS₂ nanosheet were measured at a gate voltage of 0 V with the resistance of the pristine nanosheet and BV-doped nanosheet measured at 2.88 × 10⁶ Ω and 5.08 × 10⁵ Ω, respectively. The BV-doped nanosheet had a lower resistance compared to the pristine nanosheet confirming that the BV molecules act as n-type dopants by electrons donation to the MoS₂ nanosheet. Additionally, the Seebeck coefficient of BV-doped nanosheet was measured in the range of -300 to -600 μV/K. The negative sign of the Seebeck coefficient clearly demonstrated that the BV-doped MoS₂ nanosheet exhibits n-type behavior.