Various configurations of oxygen adsorption with corresponding energies. (a) Sulphur, molybdenum, and oxygen are represented by yellow, blue, and red spheres, respectively. The sulphur vacancy site is represented by the cross. (b) Nudged elastic band calculation of the energy barrier for migration of an oxygen molecule towards a sulphur vacancy and respective trapping. The energy barrier, measured from the starting point, is 56 meV. The calculation was performed in the spin-averaged state. (c) Calculated ionization levels for relevant defects (all energies are in eV). VBM and CBM refer to the valence band maximum and conduction band minima, respectively. (d) Representation of the charge density of the trapped electrons at sulfur vacancies.
National University of Singapore (NUS) researchers have established new findings on the properties of 2D molybdenum disulfide (MoS2), a widely studied semiconductor of the future. In two separate studies, the researchers uncovered the role of oxygen in MoS2, and a novel technique to create multiple tunable, inverted optical band gaps in the material. These novel insights deepen the understanding of the intrinsic properties of MoS2 which could potentially transform its applications in the semiconductor industry.
MoS2 is a semiconductor-like material that exhibits desirable electronic and optical properties for the development and enhancement of transistors, photodetectors and solar cells. Prof Wee explained, “MoS2 holds great industrial importance. With an atomically thin two-dimensional structure and the presence of a 1.8eV energy band gap, MoS2 is a semiconductor that can offer broader applications than graphene which lacks a band gap.” Presence of oxygen alters the electronic and optical properties of MoS2
NUS researchers conducted an in-depth analysis which revealed that the energy storage capacity or dielectric function of MoS2 can be altered using oxygen. MoS2 displayed a higher dielectric function when exposed to oxygen. This new knowledge shed light on how adsorption and desorption of oxygen by MoS2 can be employed to modify its electronic and optical properties to suit different applications. The study also highlights the need for adequate consideration of extrinsic factors that may affect the properties of the material in future research.
In the second study published in Nature Communications on 7 September 2017, the team of NUS researchers discovered that as opposed to conventional semiconductors which typically have only one optical band gap, electron doping of MoS2 on gold can create two unusual optical band gaps in the material. In addition, the two optical bandgaps in MoS2 are tunable via a simple, straight forward annealing process. The research team also identified that the tunable optical band gaps are induced by strong-charge lattice coupling as a result of the electron doping.
The research findings from the two studies lend insights to other materials that possess similar structure with MoS2. “MoS2 falls under a group of material known as the two-dimensional transitional metal dihalcogenides (2D-TMDs) which are of great research interest because of their potential industrial applications. The new knowledge from our studies will assist us in unlocking the possibilities of 2D-TMD-based applications such as the fabrication of 2D-TMD-based field effect transistors,” said Asst Prof Rusydi.
Leveraging the findings of these studies, the researchers will apply similar studies to other 2D-TMDs and to explore different possibilities of generating new, valuable properties in 2D-TMDs that do not exist in nature. http://news.nus.edu.sg/press-releases/new-insights-promising-semiconductor-material
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.077402
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