Thermophysical characterization of single-layer MoS2

Abstract

Thermoelectric (TE) devices enable direct power generation from a thermal gradient, and vice versa. They could therefore play a major role in the re-utilization of waste heat and additionally exhibit some unique advantages as they can be well scaled to low power levels, are small in weight and size, have no moving parts and therefore require no maintenance. The efficiency of TE materials can be linked to the dimensionless figure of merit zT, which is proportional to the Seebeck coefficient, electrical conductivity and absolute temperature, but inversely proportional to the thermal conductivity. Recent publications suggest that single-layer (two dimensional) molybdenum-disulfide (MoS2) could be a promising material for TE applications as extraordinary large Seebeck coefficients were predicted and the thermal conductivity was found to be significantly reduced with respect to bulk MoS2. Two dimensional materials attracted a lot of interest since the discovery of graphene in 2004, which has triggered a huge field of research. Such two dimensional materials often show very different, but admirable properties compared to bulk material. In the case of MoS2 for example, such a change comprises the transition from an indirect bandgap semiconductor (Egap = 1.2eV) to a direct semiconductor (Egap = 1.8eV). The aim of this diploma thesis was to investigate the TE properties, in particular the Seebeck coefficient of single-layer MoS2. Therefore, single-layer MoS2 flakes were integrated into backgated field effect transistor (FET) structures, equipped with resistive heating elements. These structures enable measurements of FET transfer and output characteristics, which are being presented and discussed. Thereby, a remarkable high ION/IOFF ratio of 10 6 and clear n-type behaviour was observed. As degradation due to adsorbed water molecules and the existence of relatively large contact resistances turned out to have appreciable infl