原子級探討單層非對稱過渡金屬二硫化物的電子結構

Abstract

本研究運用掃描穿隧式顯微鏡和掃描穿隧式光譜，對單層硫硒化鉬的電子結構進行了全面性探討。我們著重分析不完全硒化、機械應變以及負電荷缺陷所帶來的影響，並深入討論奈米尺度組成變化和結構擾動如何影響材料的局部電子性質。

我們的研究結果顯示，相較於完全硒化區域，不完全硒化的區域展現出更強的能隙內峰值強度。此觀察結果與理論預測相符，表明在這些區域中，硫相關態在價帶最大值附近佔據主導地位，其效應類似於硫摻雜。我們還發現，能隙內峰的能量位置隨著硒化程度的不同而移動，硒化程度低的區域的能隙內峰位於較負的能量位置。此外，能隙內峰的半高寬在不同組成變化下仍保持一致，進一步支持這些能隙態的本徵性質，並證實這些變化來自於受控的硒化過程而非缺陷。

此外，我們還研究了機械應變對電子結構的影響。掃描穿隧式顯微鏡影像展示了表面不規則性與複雜性。經由快速傅立葉變換與實空間間距量測，證實了晶格常數的縮小，並觀察到這些受應變區域的能帶間隙顯著變窄。此現象與理論計算受應變的二維材料特性一致，而此應變則歸因於製備過程以及材料固有的物理性質。 最後，我們對負電荷缺陷的研究揭示了其獨特的電子特徵。依偏壓而異的形貌證實了負電荷缺陷隨電荷變化的行為：在負偏壓下，它們由於陷阱態中的電子積累而呈現為亮點；而在正偏壓下則變暗。光譜分析識別出帶隙內空間分佈不均勻的局域缺陷態，這些缺陷態也與觀察到的形貌特徵相關。我們進一步區分了兩種 負電荷缺陷類型：A 型負電荷缺陷呈現出「核-環」結構，伴隨顯著的能帶彎曲和帶隙縮窄，這與硒空位一致。相反地，B 型負電荷缺陷是一種更局限於「核心」的構型，引起價帶的顯著擾動並表現出類絕緣體行為。

本研究強調了單層硫硒化鉬的電子結構對奈米尺度組成和結構變化的高度敏感性，並展示了掃描穿隧式顯微鏡和掃描穿隧式光譜在原子尺度解析這些效應中的應用潛力。這些發現為理解二維詹努斯過渡金屬硫族化合物及其在電子學和光電子學中的潛在應用提供了重要的貢獻。

This study employs Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy (STS) to comprehensively investigate the electronic structure of monolayer (ML) MoSSe. We focus on analyzing the impacts of incomplete selenization, mechanical strain, and Negative Charge Defects (NCDs), providing a discussion on how nanoscale compositional variations and structural perturbations influence the material's local electronic properties. Our findings reveal that regions with incomplete selenization exhibit stronger in-gap peak intensities compared to fully selenized areas. This observation aligns with theoretical predictions, indicating that sulfur-related states dominate near the Valence Band Maximum (VBM) in these regions, mimicking the effects of sulfur doping. We also demonstrate that the energy position of the in-gap peak shifts with the degree of selenization, with lower selenization regions showing the peak at more negative energies. Furthermore, the Full Width at Half Maximum (FWHM) of the in-gap peak remains consistent across compositional variations, further supporting the intrinsic nature of these in-gap states and the hypothesis that these changes stem from controlled selenization rather than defects. Additionally, we investigated the influence of mechanical strain on the electronic structure. STM images reveal surface irregularities and complexities. Through Fast Fourier Transform (FFT) and real-space profile measurements, a reduction in the lattice constant was confirmed, and a significant band gap narrowing in these strained regions was observed. This phenomenon is consistent with theoretical predictions for strained Two-Dimensional (2D) materials, with the strain attributed to fabrication processes and the inherent physical properties of the material. Finally, our investigation into NCDs revealed their distinct electronic signatures. Bias-dependent STM topographies confirmed the charge-dependent behavior of NCDs: they appear as bright spots under negative bias due to electron accumulation in localized trap states, and diminish under positive bias. Spectroscopic analyses identified spatially inhomogeneous localized defect states within the band gap, which also correlated with observed topographical features. We further differentiated two types of NCDs: Type A NCDs exhibited a "core-ring" structure with significant band bending and a narrowed band gap, consistent with selenium vacancies (VSe). Conversely, Type B NCDs were a more localized "core"-dominated configuration, inducing significant perturbations in the Valence Band (VB) and exhibiting insulating-like behavior. This research highlights the high sensitivity of MoSSe's electronic structure to nanoscale compositional and structural variations and demonstrates the potential application of STM and STS in resolving these effects at the atomic scale. These findings provide significant contributions to the understanding of Janus Transition Metal Dichalcogenides (TMDs) and their potential applications in electronics and optoelectronics.