High quality TINR synthesis and band gap engineering Recent developments in catalyst-free synthesis methods at UL have opened up new horizons for device technologies based on TINRs where high purity large scale nanoribbons and nanorods of Bi2Te3 and Bi2Se3 can be grown with high reproducibility and quality (demonstrated charge carrier mobility 8000 cm2 V-1s-1). Previous growth of both TI films and TINRs is plagued by the presence of contaminants and defects introduced during growth. This greatly inhibits the detection of surface transport properties and makes device fabrication challenging and irreproducible. In samples where the surface to volume ratio is small, the surface transport is overshadowed by bulk transport since the Fermi level is always shifted into the bulk conduction band, partly due to non-ideal growth and partly because of the presence of remaining charge dopants. A novel growth method developed at University of Latvia forms the foundation for this project and will be used to develop devices. Concurrently, using feedback from theoretical studies, available crystallographic characterisation techniques (such as SEM, AFM, HRTEM, SAED and EDX), and measurements on devices we seek to tailor the TINR properties to achieve accurate control of desired properties during growth. This includes controlling size, shape, charge doping and band gap engineering (magnetic doping) while maintaining the high quality of the material. High frequency scanning probe techniques Figure 2 The main stage of the NPL 4-probe SPM. Each tip is capable of AFM and STM and can be used transport measurements. Several approaches are used to develop the underlying technology and establish a good understanding of the high frequency properties of TIs. Specifically, scanning probe techniques are used to study TI transport properties and provide a baseline of knowledge for the development of devices and for rapid feedback in materials synthesis. Scanning tunnelling spectroscopy is a powerful tool for the exploration of TI surface states which allows for efficient separation of bulk and surface properties, and manipulation using an AFM tip can remove individual van-der-Waals bound layers of TINRs thereby manipulating the surface-to-surface tunnel coupling. NPL is developing a unique platform for characterisation of TINRs that includes two–port nanoscale microwave imaging combined with in situ four probe STM, AFM and SGM for accurate correlations of surface morphology, electronic density of states and high frequency properties. Together with in situ materials deposition and etching capabilities this route provides a versatile platform for rapid engineering and validation of TINR properties. In addition, we use state-of-the art nanoscale THz imaging to access surface band transitions which are sensitive to stoichiometry and doping. Our aim is to use these techniques in the development of materials as well as to understand the influence of lithographically defined gates and electrodes on the local TINR properties. TINR device fabrication Figure 3 3-terminal device fabricated using a TINR One cornerstone of this project is the bottom-up nanofabrication facilities and expertise available at Chalmers University of Technology we are applying this expertise to build functional devices. In previous work on TINRs basic methods have been developed to contact them and to make basic devices. Using state-of-the-art fabrication facilities we are now developing the needed processes for fabricating tunnel barriers and quantum dots in TINRs. Technology development starts with contacting individual TINRs with metallic leads and verify charge transport. Techniques to create tunnel barriers (by thinning down the TINR and/or depositing magnetic gates) are being developed using input from scanning probe measurements. This will help to narrow down the choice of possible materials and geometries. Theoretical device modelling The theoretical work in this project is focusing on metal-insulator phase transitions, light-matter interactions and non-equilibrium transport and in topological Dirac materials. The electron localization properties of the surface states of the TINRs are being explored in a variety of geometries and situations including in the presence of gating and magnetic impurities in applied static magnetic fields. In this context the effect of electron-electron and electron-phonon interactions needs to be considered. The electromagnetic response of TINRs coupled to resonators will also be modelled. Measurable signals of the topological properties of the material and its unique spectrum are being devised in line with experimental capabilities. We will also use Floquet methods to model periodic charge pumping in high frequency situations that extend beyond the adiabatic pump. The roles of electron-electron and electron-phonon interactions on the properties of the pump is also being explored.