Shi-Hsin Lin

We theoretically explored new two-dimensional materials near the ionic instability (three-dimensional structures are favored), with covalent bonded systems (graphene) sitting at the opposite end of the spectrum. Accordingly, monolayer alkaline earth and transition metal halides, many of their bulk forms being layered structures, were investigated with density functional calculations. We thus predicted a new class of two-dimensional materials by performing structure relaxations, cohesive/formation energies and full phonon dispersion calculations. These materials exhibit strong ionic bonding characters, as revealed by significant charge transfers. The superior charge donating/accepting abilities and the large specific area make these new materials promising for adsorption and catalytic reactions. We demonstrated adsorptions and diffusions of Li on these materials, which are relevant for Li ion battery electrodes and hydrogen storage. Also the new materials with varied charge donating abilities and their nanostructures can enhance and tune catalytic reactions, such as Ziegler-Natta catalysts. Moreover, they exhibit diverse electronic properties that can be of great application interests, ranging from insulators, metals, and even spin-polarized semiconductors. 

Tuning Electronic Properties of Two-Dimensional Materials

Two-dimensional (2D) materials have attractted a lot of interest since the synthesis of the monolayer graphene. They offer excellent electronic properties that can revolutionise modern electronics. Among them, monolayer MoS2, MoSe2, WS2, and WSe2 were found especially interesting, thanks to their direct band gaps that are relevant to optoelectronics devices. Moreover, 2D materials are known, both experimentally and theoretically, to be strong and able to sustain large strains that are much beyond the limits of their bulk counterparts. Therefore, we computationally investigated how the material properties of MoS2, MoSe2, WS2, and WSe2 can be tuned via applied strains. We performed extensive calculations on structures, band structures, and phonon dispersions of the four 2D materials under various type of strains. We found that the band gaps can be tuned with applied strains, and turn from direct to indirect gaps. By analyzing the orbitals near band gaps, we were able to correlate the electronic structures with the X-M-X angles and X-X distances (M = Mo, W; X = S, Se). Accordingly, it was found that the band gaps are more sensitive to biaxial strains. This study also suggests a possible band gaps widenning window by applying compressive strains. We also revealed that, when applying uniaxial strains, the induced X-M-X angle anisotropy leads to to splitting in some of the phonon modes, as observed in recent Raman spectroscopy results. The phonon calculations also addressed the stability of these strained MX2.