中央研究院 Logo

Abstract

Two-dimensional (2D) materials with atomic-scale thickness, possessing unique electrical, magnetic, and mechanical properties. Represented by graphene and transition metal dichalcogenide (TMD), they exhibit characteristics such as high carrier mobility and tunable bandgaps [1].
In this talk, I will first present moiré superlattice of MoS2 on highly ordered pyrolytic graphite (HOPG) at 4.5 K under magnetic field by scanning tunneling microscopy (STM). As shown in Figure 1, the results clearly showed that the evolving moiré superlattices changed, which origin from the expension of MoS2 lattice when different magnetic fields were applied. Our study is the very first discovery on this fascinated phenomenon, it not only reveals a new prospect of structural manipulation in two-dimensional materials, but also give rise to considerable approaches for future applications on multiple devices [2].
Second, charge density wave (CDW) resulting from a periodic distortion in the lattice creates new orders beyond the original lattice. In 2H-NbSe2, one of the layered TMD, the 3×3 charge order appears in 2D layers [3]. Although CDW can be observed by different techniques, the spatial distribution within a 2D layer has never been systematically visualized. Here, by using STM and density functional theory (DFT), we monitored the evolution of CDW along the c-axis and realized a nearly tomography scan of CDW of the topmost monolayer. The results show that the appearance of the CDW varies while tuning the tunneling current and undergoes a transition from the outermost Se level to Nb level. The calculation of orbital charge distributions shows that both CDW intensity modulation and the transition are strongly correlated with the distribution of Se 4p orbitals and Nb 4d orbitals.
Reference:
[1] Lin, M. K. et al. “Tip-mediated Bandgap Tuning for Monolayer Transition Metal Dichalcogenides.,” ACS Nano 16, 14918–14924 (2022).
[2] Hao, D. et al. “Magnetic Field‐Induced Polar Order in Monolayer Molybdenum Disulfide Transistors” Adv. Mater. 36, 2411393 (2024).
[3] Lee, Y. T. et al. “Revealing the Charge Density Wave Caused by Peierls Instability in Two-Dimensional NbSe2” ACS Materials Lett. 6, 2941 (2024)