Electro-optic properties and applications of thermally-assisted conversion based few-layer MoS2 devices

Abstract

The conventional methods of MoS2 preparation are challenged in this work by integrating MoS2 synthesis using a modified version of CVD technique in different stages of developing a multilayer stack. The modified technique used in this work is referred to as thermally-assisted conversion (TAC) that directly sulphurises a transition metal. This gets rid of the exfoliation and transferring process which can be very time consuming and incompatible with industrial processes and reduces the market value of scientifically known extraordinary materials like TMDs. This technique is also photolithographically compatible paving way for large scale synthesis of devices that aim to utilize the extraordinary properties of TMDs like MoS2 but are limited by tricky transferring processes, thin film degradation, difficulty in patterning TMDs to a desired shape and contacting issues. Additionally, the conventionally utilized monolayers of MoS2 exhibiting high absorption and hosting strongly bound room temperature excitons in the visible range of the electromagnetic spectrum were used as a reference to obtain similar performance from few-layer MoS2 thin films. Given the direct correlation of MoS2 excitons with its absorption, this work aimed at establishing active control on exciton properties and investigating the tunable optical properties of MoS2 as a response to applied electrical bias. Depending upon the application, few-layer thin films of MoS2 produced using the TAC process perform similar to monolayer MoS2 for instance in the case of Fabry-P�rot based active modulation of refractive index. For photodetector applications, thicker films serve the purpose better especially when backed with metal reflector. This makes MoS2 more desirable in the field of integrated photonics since much of the monolayer properties were shown to be retained even in few-layer MoS2 films and thicker MoS2 films were shown to be suitable for many sensor applications based on pyroelectricity, photodetection and bolometric detectors. This work reported photo-pyroelectricity in mixed/3R phase of MoS2. A thorough analysis following this observation concluded that this mechanism too can be controlled which is dependent on laser frequency, modulation frequency, external applied bias and load. This work also presents a thorough electrical and optical characterization of TAC based few-layer MoS2 films to understand light-matter interaction better in films beyond the monolayer limit.