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

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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.