探討小角度堆疊之雙層二硫化鎢的晶格重構現象、電子結構及鐵電效應

Abstract

透過控制堆疊條件，扭轉雙層過渡金屬二硫化物提供了額外的自由度能夠操控其原子結構和電子性質，為其增添更多元的應用潛能，因而引起各方高度的研究興趣。當上下兩層材料之間因晶格常數或堆疊角度差異而產生晶格錯位時，就會產生莫列超晶格(moiré superlattice)。當兩層材料之間的相對角度小於3.5度時，扭轉雙層過渡金屬二硫化物的組成原子會產生微小位移以達到更低能量的堆疊狀態，並在材料上產生應變，此過程稱為晶格重構。晶格重構可能包含水平和垂直方向之位移，並且能夠改變材料的能帶結構。因此，探討晶格重構和材料電子性質之間的關聯對於調控雙層 TMD 裝置十分重要。 本研究利用掃描穿隧顯微鏡（STM）和非接觸式原子力顯微鏡（nc-AFM），探討高定向熱解石墨（HOPG）基板上之小角度扭轉雙層二硫化鎢(WS2)原子結構。實驗結果展示了樣品的晶格重構現象，以及重構所形成的菱面堆疊 (XM與MX 堆疊) 之三角形區域、域壁 (Domain wall, DW) 和縮小的XX堆疊區域。此外，藉由分析底層二硫化鎢與基板所形成之莫列圖紋，我們觀測到集中於域壁的剪應變 (shear strain)，和XX堆疊附近產生約5度之額外旋轉 (rotation) 角度。 我們利用掃描穿隧能譜（STS）量測不同堆疊區域的電子結構，觀測到價帶的Γ谷能量呈現高低變化，其能量主要受到層間間距影響。然而，理論計算顯示XM與MX堆疊具有相同的層間間距，與量測結果矛盾。為了解此問題的原因，我們藉由量測穿隧電流隨針尖-樣品距離的衰減常數來獲取樣品功函數資訊，並發現XM與MX區域之功函數相差21.3毫電子伏特(meV)，此為造成兩區域的Γ谷能量差之主因。功函數的變化可歸因於扭轉雙層二硫化鎢中固有的局部偏極化，而此偏極化之方向是由兩層二硫化鎢中原子的相對位置所決定。偏極化現象也代表著樣品具有鐵電性質，而我們藉由觀測偏極化區域隨電場而擴張及縮小之鐵電反應，再次驗證MX與XM區域具有局部偏極化之特徵。 本研究為扭轉雙層過度金屬二硫化物之應變行為提供更全面的理解，並展示了三維晶格重構行為與材料電子性質之間的相關性。

Twisted bilayer transition metal dichalcogenides (tb-TMD) have recently attracted tremendous interest because they offer new opportunities for modulating their atomic structures and electronic properties via controlling the stacking conditions, providing a more diverse application on semiconductor devices. The lattice misalignment, caused by the difference of lattice constant or aligned angle between two layers, results in the moiré superlattice. When the relative angle between two layers is small (<3.5°), the structure of tb-TMD is expected to undergo a reconstruction process, in which the atoms of the TMD lattice slightly adjust their arrangement to attain lower stacking energy and induce strain on the constituent layers. This rearrangement can involve in-plane and out-of-plane directions. The lattice reconstruction can alter the band structure of tb-TMD. Hence, exploring the correlation between lattice reconstruction and electronic properties is important for engineering bilayer TMD devices. In this study, we investigate the atomic structure of marginally twisted bilayer WS2 (tb-WS2) on the highly ordered pyrolytic graphite (HOPG) substrate by utilizing scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). The results reveal lattice reconstruction, which leads to the triangular domains exhibiting rhombohedral stacking (i.e., XM and MX stackings), the domain wall (DW), and the shrunk XX stacking regions. Additionally, by analyzing the displacement of the bottom WS2/HOPG moiré pattern, we identify localized shear strain at the domain walls and an additional 5° rotation at XX staking sites. The local electronic structures of different stacking probed by scanning tunneling spectroscopy (STS) present the undulating Γ peak energy, which results mainly from the fluctuating interlayer separation (ILS). However, theoretical calculations suggest the same ILS in XM and MX domains, which contradicts our STS result. To explore this contradiction, we investigate the work function of tb-WS2 by measuring the decay constants of the tunneling current. Our results show a 21.3 meV difference in the work functions between XM and MX domains, which results in the Γ peaks difference between them. The work function change is attributed to the local polarizations in tb-TMD, determined by the relative positions of atoms in the two WS2 layers. We additionally observe the ferroelectric response of tb-WS2, reconfirming the polarizations in XM and MX domains. Our work provides a more complete understanding of strain behavior in the tb-TMD and demonstrates the correlation between 3-dimensional lattice reconstruction and the electronic properties of tb-TMD.