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，其特点是具有像绝缘体一样的块状带隙，但在其边缘或表面表现出受保护的导电状态。这些状态可能是由于自旋轨道相互作用和时间反转对称性的结合。拓扑绝缘体在无耗散电子学和室温自旋电子学应用的道路上是非常有趣的候选者。三维拓扑绝缘体面临的一个主要挑战是减少在电子传输测量过程中隐藏表面态特性的大量残余散体载体的浓度。以纳米线形式的拓扑绝缘体材料的研究，具有可控的几何形状和减小的尺寸，有望为这些新型固体材料的物理学提供新的启示。随着线材直径的减小，例如从500 nm减小到10 nm，表面体积比增大，可以系统地研究线材的表面态表现和电子光学性质。此外，通过在一定尺寸下改变纳米结构的几何形状（圆柱形和菱形截面），我们可以研究表面几何形状对拓扑表面状态和纳米结构性质的影响。在这个提议中，我们的目标是通过电沉积在蚀刻离子轨道膜上合成拓扑绝缘体Bi2Se3， Bi2Te3和Bi1-xSbx的单晶纳米线。对其进行了详细的电化学生长研究，并用HRSEM、XRD、TEM、EDX和XPS对其晶体结构和化学成分进行了分析。将探索掺杂和封盖策略，以降低纳米线的整体电导率，并减少导致额外载流子产生的外部污染。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作为线径和温度的函数。样品还将进行低温磁阻测试。在法国的SOLEIL同步加速器（ANTARES光束线）上，我们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依赖性进行详尽的研究。此外，将开发适当的NW接触技术，为SPP1666内的合作者准备特定的样品，以进行进一步的表征：A. Holleitner教授小组（Technische Universität m<e:1> nchen）进行的时间分辨光诱导输移电流测量，V. Kataev博士（IFW Dresden）进行的电子自旋共振光谱测量，以及太赫兹电导率测量和太赫兹发射（T. Kampfrath博士，Fritz Haber研究所，柏林）。拓扑绝缘体纳米结构是研究外来表面态和制造功能器件的理想候选者。

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.