Insights on properties of 2D materials

20 Sep 2017. NUS researchers have established new insights on the fundamental properties of two-dimensional (2D) molybdenum disulphide (MoS₂) with potential for device applications.

2D materials are a special class of nanomaterials which are ultra-thin, having only a single layer of atoms.  Electrons in these materials can only move freely in the 2D plane.  There has been research interest in 2D materials such as MoS₂ because they exhibit unique optical, mechanical and electrical properties that can be used in a wide range of electronic device applications. A research team led by Prof Andrew WEE and Prof Andrivo RUYSDI, from the Department of Physics, NUS has discovered that the presence of oxygen affects the dielectric properties of MoS₂. In another separate study, the team has developed a technique which can modify the optical band gap in the material.

In the first study, the researchers found that the dielectric function of MoS₂ changes in the presence of oxygen. By controlling the amount of oxygen which the material comes into contact with when subjected to a heat treatment process, the dielectric function of MoS₂ can be made to change accordingly.  MoS₂ is found to have a higher dielectric function when it is exposed to oxygen during this process.

In another study, the researchers discovered that MoS₂ deposited on gold film can create two unusual optical band gaps within the material. Conventional semiconductor material typically has only one optical band gap.  MoS₂ is able to have two optical band gaps due to the electron doping from the gold film which is facilitated by lattice strain. This is due to the formation of the quasi-metallic 1T’ phase, which forms when the MoS₂-gold interface is heated to between 200^oC to 250^oC.

These research findings provide a deeper understanding of the material properties of MoS₂, which can open up new possibilities for advanced industrial applications.

Abundant sulphur vacancies (red arrows) present in MoS₂ suppress exciton formation. In ambient conditions, these sulphur vacancies are covered with oxygen from the atmosphere. However, upon heat treatment in an ultrahigh vacuum environment, these oxygen molecules are released, reducing the excitonic absorption. Upon oxygen exposure, the excitonic absorption is recovered.

MoS₂ exhibits multiple electronic properties associated with different crystal structures. Inverted and fundamental energy band gaps have been observed during a heat treatment process, which induces a semiconductor to a quasi-metallic phase transition in monolayer MoS₂ on gold.

References:

Gogoi PK*; Hu ZL; Wang QX; Carvalho A; Schmidt D; Yin XM; Chang YH; Li LJ; Sow CH; Neto AHC; Breese MBH; Rusydi A*; Wee ATS*, “Oxygen Passivation Mediated Tunability of Trion and Excitons in MoS2” PHYSICAL REVIEW LETTERS Volume: 119 Issue: 7 Article Number: 077402 DOI: 10.1103/PhysRevLett.119.077402 Published: 2017.

Yin X; Wang Q; Cao L; Tang CS; Luo X; Zheng Y; Wong LM; Wang SJ; Quek SY; Zhang W*; Rusydi A*; Wee ATS*, “Tunable inverted gap in monolayer quasi-metallic MoS2 induced by strong charge-lattice coupling” NATURE COMMUNICATIONS Volume: 8 Article number: 486 DOI:10.1038/s41467-017-00640-2 Published: 2017.