以第一原理研究二維材料之電子結構的調控

Abstract

Graphene is one of the most widely studied materials nowadays because of its unique electronic properties, for example, high electron mobility, extreme large conductivity, and room temperature quantum Hall effect. However, the pristine graphene is gapless which is an essential problem for applications, such as low on-off-ratio making it difficult for logic circuits. Aiming at tuning electronic properties, in particular opening band gaps, we sys- tematically studied the effects of the adsorption of simple aromatic molecules on the electronic structures of graphene by first-principles calculations. In the first part of this thesis, we show that adsorptions of different aromatic molecules, Borazine (B3N3H6), Triazine (C3N3H3), and Benzene (C6H6), on graphene strongly perturb surface charges and often lead to band gap openings. Moreover, “micromechanical cleavage method”, developed for graphene, has been applied to other layered materials to fabricate two dimensional (2D) materials, for ex- ample BN, MoS2, and other complex oxides. Among those 2D materials, the transition metal dichalcogenides (TMDCs) semiconductors arose more interests because of their intrinsic semiconductor properties. The TMDCs semiconductors have been studied ex- tensively and shown great potential in applications, for instance, lubrication, catalysis, photoelectrochemical cells, and photodetection. A single layer of MX2 (M stands for the transition metal, such as Mo and W; X stands for the chalcogen atom, such as S and Se) has attracted more attentions and been studied widely theoretically and experi- mentally mainly because of its variety properties that different from the bulk state. For the purpose of applications on making electronic and optical devices, it is essential to be able to tune the band gap. One promising route to manipulate band gaps is through the elastic strain engineering. Therefore, in the second part of this thesis, the effects of bi-axial (both compressive and tensile) and uni-axial strains along different directions were examined and we found that the band gap of monolayer MX2 is more sensitive to the bi-axial strains. This notion can be attributed to the fact that, under bi-axial strains, lattice structures of MX2 tend to relax more significantly along the vertical direction and result in noteworthy changes in the bond angles. While most theoretical reports suggested systematic reduction of band gaps under mechanical strains, we found that the direct band gap can be robustly widened by applying compressive bi-axial strain. Hence, our findings in the gap widening mechanism have great potential for future applications in nanoelectronics and photoelectronics.