Abstract
Layered van der Waals materials have recently attracted attention owing to their exceptional electrical and optical properties in thin layer form. One way to extend their utility is to form a heterostructure which combines various properties of layered materials to reveal intriguing behavior. Conventional heterostructure synthesis methods are difficult to develop and the heterostructure formed can be limited to a small area. Here, we investigate the phase transformation of SnS2 to SnS by removing sulfur atoms at the top surface using Ar plasma. By varying the plasma power and exposure time, we observed that SnS is subsequently formed on top of the mogul-like structure of SnS2. Since SnS is a p-type semiconductor and SnS2 is an n-type semiconductor, we naturally formed a vertical p-n junction. By using graphene at the top and bottom as transparent electrodes, a vertical p-n diode device is constructed. The device demonstrates good rectifying behavior and large photocurrent generation under white light. This method can be applied to large-area heterostructure synthesis using plasma via phase transformation of various metal dichalcogenides to metal monochalcogenides.
Highlights
Motivated by graphene with extremely high electrical mobility but poor on/off current ratio, which is a serious drawback for switching device applications, other types of two-dimensional (2D) layered materials such as layered metal dichalcogenides and h-BN have emerged recently as promising candidates for devices with both high mobility and high on/off current ratio
Ar plasma treatment was conducted by a reactive ion etch (RIE) system (AFS-R4T, All For System)
The sample was subsequently put into a reactive ion etch (RIE) chamber for plasma treatment (See Methods section)
Summary
Motivated by graphene with extremely high electrical mobility but poor on/off current ratio, which is a serious drawback for switching device applications, other types of two-dimensional (2D) layered materials such as layered metal dichalcogenides and h-BN have emerged recently as promising candidates for devices with both high mobility and high on/off current ratio. The junction area is too narrow and large optical gain cannot be expected To overcome this obstacle, CVD-grown vertical heterostructures could be an ideal platform for practical optoelectronics. The realization of CVD-grown vertical heterostructures over large areas has been sparse and remains elusive Another approach to form heterostructures is to take advantage of structural phase transformation[25,26]. These methods have been restricted to the modification of phases in the lateral direction Another interesting method to achieve vertical heterostructures is to use electron beam or laser irradiation on a SnS2 (tin disulfide) thin film and to convert the top layers into SnS (tin monosulfide) to form a SnS2-SnS heterostructure[30,31]. While this method yields an in situ vertical heterostructure at the irradiated area, which is advantageous for micro-patterning, phase conversion of the whole area is still not achievable
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