Single crystalline Bi2Te3 und Bi2Se3 Nanowires as topological insulator materials: synthesis and properties

拓扑绝缘体是一类新兴的电子材料，其特征是具有类似绝缘体的体带隙，但在其边缘或表面显示受保护的导电态。由于自旋-轨道相互作用和时间反转对称性的结合，这些状态是可能的。在无耗散电子学和室温自旋电子学应用的道路上，拓扑绝缘体是非常有趣的候选材料。3D拓扑绝缘体的一个主要挑战是减少在电子输运测量过程中负责隐藏表面态性质的大量残留体载流子。具有可控几何形状和降维的纳米线形式的拓扑绝缘体材料的研究有望为这些新型固体材料的物理提供新的曙光。随着导线的直径减小，例如从500 nm减小到10 nm，表面体积比增加，并且可以系统地研究导线的表面态的表现以及电子和光学性质。此外，通过改变一定尺寸的纳米结构几何结构(柱面和菱面体截面)，我们可以研究表面几何结构对拓扑表面态和纳米结构性质的影响。在这个方案中，我们的目标是通过在刻蚀的离子径迹膜中电沉积来合成拓扑绝缘体Bi2Se3、Bi2Te3和Bi1-xSbx的单晶纳米线。通过高分辨扫描电子显微镜、X射线衍射仪、透射电子显微镜、能谱分析和X射线光电子能谱分析，对电化学生长过程进行了详细的研究，并对其晶体结构和化学成分进行了分析。将探索掺杂和封顶策略，以降低纳米线的体电导率，并减少导致额外载流子产生的外部污染。对拓扑绝缘体纳米线场效应晶体管的电输运测量将使我们能够阐明纳米线的迁移率和载流子密度随导线直径和温度的变化。还将对样品进行低温磁阻测试。法国Soleil同步加速器(Antares Beamline)的Nano-ARPES将以非常高的空间分辨率(几十纳米)对单个纳米线的几何结构和电子结构之间的相关性进行详尽的研究。此外，还将开发足够的NW接触技术来为我们在SPP1666中的合作者准备特定的样品以进行进一步的表征：A.Holleitner教授(München工业大学)的时间分辨光诱导传输电流测量、V.Kataev博士(IFW德累斯顿)的电子自旋共振光谱以及THz电导率测量和THz发射(T.Kampfrath博士，柏林弗里茨哈伯研究所)。拓扑绝缘体纳米结构是研究奇异表面态和制造功能器件的极佳候选者。

Topological insulators are an emergent new class of electronic materials characterized for having a bulk bandgap like an insulator but exhibiting protected conducting states on their edge or surface. These states are possible due to the combination of spin-orbit interactions and time-reversal symmetry. Topological insulators are very interesting candidates on the road towards dissipationless electronics and room temperature spintronic applications. A major challenge for 3D topological insulators is to reduce the large concentration of residual bulk carriers responsible for hiding the properties of surface states during electron transport measurements. Investigation of topological insulator materials in the form of nanowires with controlled geometry and reduced dimensions is expected to shed new light on the physics of these novel solid materials. As the diameter of the wires decreases, e.g. from 500 down to 10 nm, the surface-to-volume ratio increases, and the manifestation of surface states and the electronic and optical properties of the wire can be studied in a systematic manner. Moreover, by varying the nanostructure geometry (cylindrical vs. rhombohedral cross section) for a certain size, we can study the influence of surface geometry on topological surface states and nanostructure properties. Within this proposal we aim at synthesizing single-crystalline nanowires of the topological insulators Bi2Se3, Bi2Te3, and Bi1-xSbx by electro-deposition in etched ion track membranes. The electrochemical growth will be investigated in detail and their crystallographic structure and chemical composition will be analyzed by HRSEM, XRD, TEM, EDX, and XPS. Doping and capping strategies will be explored to reduce bulk conductivity of the nanowires and to reduce extrinsic contamination leading to additional carrier generation. Electrical transport measurements on topological insulator nanowire field effect transistors will enable us to elucidate the mobility and carrier density of the nanowires as a function of wire diameter and temperature. The samples will be also investigated by low-temperature magneto-resistance. An exhaustive study of the dependence between geometrical and electronic structure of individual nanowires will be investigated with very high spatial resolution (few tens of nm) by nano-ARPES at the SOLEIL synchrotron (ANTARES beamline) in France. Furthermore, adequate NW contacting techniques will be developed to prepare specific samples to our collaborators within the SPP1666 for further characterization: time-resolved photoinduced transport current measurements by the group of Prof. A. Holleitner (Technische Universität München), electron spin resonance spectroscopy by Dr. V. Kataev (IFW Dresden), and THz conductivity measurements and THz emission (Dr. T. Kampfrath, Fritz Haber Institute, Berlin). Topological insulator nanostructures are excellent candidates to study exotic surface states and towards making functional devices.