Single layers of WS2 and MoS2 form a type II heterojunction, in which electrons and holes are spatially separated. These heterostructures have attracted attention due to the good rectifying character of their intrinsic p-n junction and ultrafast charge transfer, allowing them to be used in high-mobility field-effect transistors. They also shows evidence of a strong photovoltaic effect with a band gap in the near-infrared to visible region, giving them potential use in light detection and solar cells.
While MoS2/WS2 heterostructures have shown promise in the construction of 2D electronics, one of the largest challenges has been the reliable synthesis of consistent and high-quality heterostructures, particularly at the interfacial junction. The best current methods are epitaxial growth methods which grow one of the monolayers (usually MoS2) directly onto the other through chemical vapor deposition (CVD)
The primary goal of this project is to study the effect of varying growth conditions on the structural and compositional properties of the MoS2/WS2 heterostructures. It has been well documented in previous experiments that variation in CVD growth conditions (flow rate, temperature), can lead to greatly different structural properties (e.g defects, terminations) in isolated monolayers like MoS2, and that these structural variations can greatly affect relevant electronic and optical properties. The effect of growth conditions on CVD grown MoS2/WS2 heterostructures have not yet been studied, and they are of particular interest due to the added complexity of the heterostructures interface.
My study attempted a two-step epitaxial CVD synthesis due to greater control over individual precursor flow and concentration. We began by spincoating a cleaned SiO2 substrate with PTAS seeds to promote the formation of WS2 monolayers. A second substrate was cleaned and placed downstream of this substrate. WO3 powder was in a ceramic boat, and the substrate was placed either on top or downstream of the boat inside a quartz tube. The furnace was then heated with a specified Ar gas flow rate at a specific pressure. At a certain temperature, a heat belt was used to vaporize S powder, before letting the system cool to room temperature. We then repeated this procedure using MoO3 rather than WO3 powder to epitaxially grow MoS2 onto WS2, and vary our growth conditions over a wide range.
The result of our experiment were very useful for identifying ideal conditions for XS2 monolayer growth. It was discovered that the degree of growth in our experiments was highly localized due to variations in the growth substrate surface that originated from the cleaning process. Thus one of the largest and most difficult challenges of the project was to minimize these variations as much as possible. Further work will include expanding this procedure to a variety of other 2D materials, such as WS2.