探討扭轉重構之莫列超晶格中的原子級空間尺寸量子侷限效應

Abstract

過渡金屬二硫族化物因其可調的電子特性和原子級薄層結構而備受關注。通過控制堆疊條件，扭轉堆疊的雙層過渡金屬二硫族化物（twisted bilayer transition metal dichalcogenides, tb-TMD）可以產生多元的光電性質及元件應用。在微小的扭轉角度（< 3°）下，莫列超晶格會發生重構，形成相稱堆疊區域和狹窄域壁。重構後的相稱堆疊區域中，兩層材料之間不再有晶格錯位，其電子特性應相似於完美堆疊的雙層系統。 然而，晶格重構會使材料的能帶結構具有顯著的空間分布，形成莫列位能。此奈米尺度的位能變化會限制載子的水平移動，可能引起能帶修正。因此，若要清楚解析tb-TMD上各區域的電性，除了考慮堆疊方式外，也需要考慮環境中的位能對重費米子的侷限效應。侷限效應對能帶的修正與區域尺寸有關，但tb-TMD中的各區域能帶位置與區域尺寸之間的關係卻很少被討論。 我們使用掃描穿隧顯微鏡和掃描穿隧能譜（STM/STS）在高定向熱解析石墨（HOPG）基板上測量小角度扭轉堆疊之雙層二硒化鎢的電性分布，觀察不同莫列週期下導帶邊緣和價帶邊緣的能量位置，並依各種堆疊區域進行分析。結果顯示，能隙大小與莫列週期之間存在相關性，這為尺寸相關的量子侷限效應提供證據。 我們的研究展示了莫列週期與各區域尺寸及電性之間的關聯，從而對二維材料中量子侷限效應有了更全面的理解，為扭轉堆疊二維材料的設計提供新的指引。

Transition metal dichalcogenides (TMD) have garnered massive attention due to their tunable electronic properties and atomically thin layer structure. By controlling the stacking conditions, diverse optoelectronic properties and device applications can be achieved in twisted bilayer TMD (tb-TMD). At a small twist angle (<3°), structural reconstruction leads to the formation of commensurate domains and domain walls (DW) in tb-TMD. In these commensurate regions, the lattices in the two layers align, resembling the properties of perfectly aligned bilayer systems. However, lattice reconstruction enhances a nanoscale moiré potential, affecting carrier transport and causing band modifications. To thoroughly analyze the electronic structure in various tb-TMD regions, it is essential to consider both the stacking configuration and the confinement effects on heavy fermions. While the size effect of confinement is well-recognized, the specific relationship between band position and domain size in tb-TMD has rarely been discussed. We used scanning tunneling microscopy/spectroscopy (STM/STS) to measure current curves under varying moiré periods of marginally twisted bilayer WSe2 on a highly ordered pyrolytic graphite (HOPG) substrate. Our observations of the conduction and valence band edges, analyzed for each domain category, reveal that band gaps correlate with their moiré periods, evidencing size-dependent confinement. This study demonstrates the correlation between moiré period, domain size, and electronic properties in moiré superlattices. It enhances the understanding of quantum confinement effects in two-dimensional materials, offering prospective guidance for the engineering of twisted bilayer two-dimensional materials.