能帶對齊作為半導體研究的核心概念，對理解晶體管和量子阱激光器等各類半導體器件的原理起著關鍵作用。2020 年，科研人員使用化學氣相沉積在單壁碳納米管和氮化硼納米管上生長出直徑小至 3.9 納米的單壁單晶二硫化鉬納米管。這些化學組成不同的同軸納米管形成一類全新的異質結構，即一維范德華異質結。

Fig. 1 One-dimensional (1D) van der Waals (vdW) heterostructures and diameter-dependent band-edge levels of MoS2 and WS2?nanotubes.

闡明同軸納米管之間的能帶對齊是一維范德華異質結物理性質與器件研究的關鍵一步。然而，納米管的曲率如何影響一維范德華異質結中的能帶對齊，在此之前尚不清楚。

Fig. 2 Diameter-dependent band-edge levels of MoSe2, WSe2 and MoTe2 nanotubes.

來自中國西湖大學工學院的李文彬教授團隊通過系統的第一性原理計算表明，對于TMDC一維范德華異質結，曲率可以導致能帶對齊類型發生轉變。

Fig. 3 Effect of curvature-induced flexoelectricity on the band-edge levels of MoSe2 nanotubes.

他們發現，隨著納米管管徑的減小和曲率的上升，單個TMDC 納米管的半導體導帶底能級呈現出快速且單調的降低，而價帶頂能級先下降再上升。

Fig. 4 Effect of circumferential tensile strain on the band-edge levels of MoSe2 nanotubes.

這些特性可以根據曲率誘導的撓曲電和相應的靜電勢效應，以及 TMDC 納米管中固有的周向拉伸應變和彎曲應變來充分解釋。當TMDC納米管形成同軸范德華異質結時，撓曲電電勢差導致的管間耦合效應，協同受管徑調控的帶邊能級變化，可使具有較大管徑的MoSe₂@WS₂、MoTe₂@MoSe₂ 和 MoTe₂@WS₂ 一維范德華異質結發生從 II 型到 I 型能帶對齊類型的轉變。

Fig. 5 Band alignment in 1D vdW heterostructures consisting of WS2 nanotubes nested in MoS2 nanotubes.

李文彬教授團隊的這一成果為理解一維范德華異質結中的能帶對齊奠定了重要基礎，并為合理設計TMDC一維范德華異質結鋪平了道路。相關論文近期發布于npj?Computational Materials?9:?92 (2023)。手機閱讀原文，請點擊本文底部左下角“閱讀原文”，進入后亦可下載全文PDF文件。

Fig. 6 1D vdW heterostructures that exhibit Type I band alignment at large tube diameters.?

Editorial Summary

1D van der Waals heterostuctures: Band alignment controlled by the curvature

Band alignment is a core concept in the study of semiconductors, explaining a wide range of phenomena underlying applications such as transistors and quantum well lasers. In 2020, researchers used chemical vapor deposition to successfully grow single-walled, single-crystal MoS₂ nanotubes with diameters as small as 3.9 nm on single-walled carbon nanotubes and boron-nitride nanotubes. These coaxial nanotubes with different chemical compositions form a new class of heterostructures, namely one-dimensional (1D) van der Waals (vdW) heterostructures. Elucidating the band alignment between coaxial nanotubes is a key step in exploring the physical properties and device applications of such 1D vdW heterostructures. However, the effect of curvature on the band alignment of 1D vdW heterostructures remained unclear. This study provides a comprehensive theoretical framework to understand the curvature-dependent band alignment in 1D vdW heterostructures of transition metal dichalcogenides (TMDCs).

Through comprehensive first-principles calculations, a team led by Prof. Wenbin Li from the School of Engineering, Westlake University, China, reveals that the effect of curvature can lead to a transition in the band-alignment type in 1D vdW heterostructures of TMDCs. They find that, as the diameter of a TMDC nanotube decreases, the conduction band minimum exhibits a rapid and monotonic decrease, whereas the valence band maximum exhibits an initial decrease before increasing. These properties can be fully explained in terms of curvature-induced flexoelectricity and the associated electrostatic potential effect, as well as the intrinsic circumferential tensile strain and bending strain within the TMDC nanotubes. They predict that in 1D vdW heterostructures of coaxial TMDC nanotubes, the concerted effect of diameter-dependent band-edge levels and intertube coupling via flexovoltage can lead to a transition of intertube band alignment from Type II to Type I in large-diameter MoSe₂@WS₂, MoTe₂@MoSe₂, and MoTe₂@WS₂heterostructures. This study lays down an important foundation for understanding the band alignment in 1D vdW heterostructures and paves the way for rational design of TMDC-based 1D vdW heterostructures. The?article was recently?published in?npj?Computational Materials?9,:?92?(2023).

原文Abstract及其翻譯

Curvature-controlled band alignment transition in 1D van der Waals heterostructures (一維范德華異質結中曲率控制的能帶對齊類型轉變)

Shu Zhao, Chunxia Yang, Ziye Zhu, Xiaoping Yao & Wenbin Li

Abstract?The effect of curvature on the band alignment of one-dimensional (1D) van der Waals (vdW) transition metal dichalcogenide (TMDC) heterostructures is studied by comprehensive first-principles calculations. We find that, as the diameter of a TMDC nanotube decreases, the combined effect of curvature-induced flexoelectricity and circumferential tensile strain causes a rapid lowering of the conduction band minimum, whereas the valence band maximum exhibits an initial lowering before rising. As individual TMDC nanotubes form coaxial heterostructures, the concerted effect of diameter-dependent band-edge levels and intertube coupling via flexovoltage can result in a transition of intertube band alignment from Type II to Type I in multiple heterostructural systems, including large-diameter MoSe₂@WS₂, MoTe₂@MoSe₂, and MoTe₂@WS₂?heterostructures. These results lay down a foundation for the rational design of 1D vdW heterostructures.

摘要通過第一性原理計算，本文研究了曲率對過渡金屬二硫化物 (TMDC) 一維范德華異質結中能帶對齊的影響。我們發現，隨著納米管管徑的減小，在曲率誘導的撓曲電效應和周向拉伸應變的共同作用下，TMDC納米管的導帶底能級快速下降，而價帶頂能級則先降低再上升。當不同TMDC納米管形成嵌套的同軸異質結時，與管徑相關的帶邊能級變化，疊加通過撓曲電電勢差進行的管間耦合，可協同導致管間能帶對齊類型在多個異質結系統中從 II 型轉變為 I 型，包括大管徑的MoSe₂@WS₂、MoTe₂@MoSe₂和 MoTe₂@WS₂ 異質結。這些結果為一維范德華異質結的理性設計奠定了基礎。