大带隙衬底上二维锗烯电子结构与新奇量子现象的探测

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.

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。

锗烯是类似于石墨烯的单层锗结构，理论研究表明锗烯可以具有石墨烯所有的优异性质，而且锗烯因其大的自旋轨道耦合作用可以实现量子自旋霍尔效应，这为拓扑量子器件提供了可能。本项目从实验角度出发，利用扫隧道显微镜和扫描隧道谱为主要实验探测手段，探究了经过超高真空分子束外延可控合成的锗烯并探测其结构与电子学性质等得到了序列重要研究成果，主要为（1）理清楚了锗烯在MoS2生长的机制，并通过生长调控实现了锗烯在MoS2的插层生长，理清楚了锗在MoS2的生长机制。而对锗烯在金属衬底担载的单层WSe2上成功制备，并对WSe2构筑的近自由状态无衬底担载的锗烯调控，发现因对称性破缺可以在锗烯打开0.17eV的带隙。这是在实验上对半金属性锗烯带隙打开的首次报道，对在未来锗烯在电子器件上上应用奠定了基础；（2）关注半导体型TMDs与金属衬底的界面相互作用以及对二维材料电子性质的调控，发展了一种简易方法预处理金箔实现了单晶Au(100)衬底，研究了二维半导体材料MoSe2与衬底电子脱耦合的生长，证实MoSe2与衬底的弱耦合，这一发现打破了金属衬底会与外延薄膜电子态杂化的传统认识，证明通过对衬底的调控也能实现准自由的二维原子晶体的生长。此外，通过调控WSe2和Au(100)衬底的相对转角，实现WSe2掺杂状态的改变，在原子尺度构筑npn型同质结。通过本项目发表SCIE收录的学术论文8篇，已毕业硕士生1名。