量子自旋霍尔效应(QSHE)是一种新型非平庸拓扑量子态，理论预言可存在于石墨烯中，然而由于其自旋轨道耦合(SOC)带隙太小（~24μeV），实验上难以实现。类石墨烯二维锗烯的SOC带隙比石墨烯大三个数量级，可在接近室温（~277K）观测到QSHE。六方氮化硼(h-BN)晶格常数与锗烯匹配，且有近6eV的带隙使锗烯费米能级处的相关电子态不与衬底杂化。最近，申请人成功在带隙材料MoS2上制备了锗烯，基于此，我们将分别以MoS2和hBN为衬底制备锗烯，探索其电子结构及新奇量子现象，利用低温扫描隧道显微镜/谱(STM/STS)对锗烯的结构和体相及边缘电子态进行表征，通过STS期望得到锗烯内部有带隙、边缘呈导电性的作为QSHE特征的谱图信息。此外，通过外加电场对锗烯能带进行调控以打开可观的带隙。通过本项目的研究进一步认识锗烯物性与新奇量子效应，并为锗烯基低耗散自旋电子学器件应用提供科学依据。

The quantum spin Hall effect, a novel nontrivial topological quantum state of matter, has been proposed in 2005 by Kane and Mele for graphene. In marked contrast to the conventional quantum Hall effect, the quantum spin Hall effect (QSHE) does not require an external magnetic field. The spin-orbit coupling in graphene opens a band gap at the K and K’ points in the surface Brillouin zone. Unfortunately, the spin-orbit gap in graphene is very small (~24 μeV) and therefore the QSHE in graphene only occurs at extremely low temperatures (< 0.3 K). Germanene (the Ge counterpart of graphene) has a spin-orbit gap that is ~3 orders of magnitude larger than that of graphene. This means that the QSHE for this two-dimensional material would be observable at experimentally accessible temperatures! The QSHE is characterized by topological protected gapless helical edges states. These topological protected edges states are spin-polarized and have a vanishing charge Hall conductance and a quantized spin Hall conductance. Recently we have successfully synthesized germanene on several substrates. Scanning tunneling spectroscopy of germanene grown on molybdenum disulfide (a band gap material) revealed a well-defined V-shaped density of states, which is reminiscent for a 2D Dirac system. Inspired by these encouraging results we also plan to synthesize germanene on hexagonal boron nitride (h-BN) substrates. h-BN is an ideal template for germanene because of (1) its nearly perfect lattice match with germanene and (2) the relevant electronic states of germanene near the Fermi level are decoupled from the substrate due to the large h-BN band gap of ~6 eV. The structural and electronic properties of the interior, as well as the edges of germanene (synthesized on MoS2 and h-BN), will be studied with variable-temperature scanning tunneling microscopy and spectroscopy. A sizeable band gap will be opened in germanene via the application of an external electric field. The opening of the band gap allows us to alter the topological state of germanene. We hope the proposed investigation to help to understand the properties and novel quantum phenomena of germanene and provide opportunities for the next generation of low dissipation spintronics device applications.