Characterization of individual Bi2Te3 nanowires electrodeposited in etched ion-track membranes for nano-ARPES and electrical transport studies

Abstract

Promising research fields associated to solid state physics, such as thermoelectrics and spintronics, have been influenced by the unique properties of bismuth-based materials. Bismuth telluride (Bi2Te3), for example, is a semiconductor with a very small band gap and excellent thermoelectric properties which show one of the highest thermal to electrical energy conversion efficiency at room temperature. Moreover, bismuth telluride belongs to a new class of materials called Topological Insulators (TI), which exhibit surface conductivity with spin-momentum-locked electronic surface states while being bulk insulators. Both properties promise many application possibilities in electronic and spintronic devices with regards to energy efficiency and faster computing. The experimental investigation of the special topological insulator characteristics is very challenging, because the bulk conductivity dominates the electrical signal resulting in hardly accessible surface states. In order to overcome this challenge, the present dissertation initially presents the fabrication of Bi2Te3 nanowires with extremely large surface-to-volume ratio and independently controllable geometric, crystallographic and morphologic properties that enable a resolution of the surface states. The Bi2Te3 nanowires were synthesized by electrodeposition in ion-track etched polymer templates. For this purpose, 30-µm thick polycarbonate foils were irradiated by highly energetic heavy ions. Each ion creates an ion track, which can be converted into cylindrical nanopores by selective chemical etching. Subsequent electrochemical deposition within these nanopores resulted in nanowires with diameters between 25 and 100 nm. X-ray diffraction and transmission electron microscopy reveals highly textured nanowires consisting of single crystalline sections that are several hundreds of nanometers long. For both, bulk and surface, the chemical composition was analyzed by energy-dispersive x-ray spectroscopy and x-ray photoemission spectroscopy. The former shows a chemically homogeneous composition close to stoichiometry, while the latter revealed oxide and carbon contaminations, which is attributed to polymer residues from the template. Investigations of the electronic properties of individual Bi2Te3 nanowires were performed using nano-angle-resolved photoemission spectroscopy (nano-ARPES) at the French synchrotron SOLEIL. Sections of single nanowires were analyzed by employing a setup that was especially developed to obtain photoemission signals from individual nanoobjects. Angle-integrated measurements along the length of the nanowire recording the core levels, confirmed the homogeneous chemical composition. Employing the angle-resolved mode, the valence band structure of single nanowires sections was successfully revealed. First principles calculations of the preferably deposited crystallographic orientation are in good agreement with the experimental nano-ARPES results. In order to conduct electrical transport measurements, individual nanowires were contacted by laser and electron beam lithography. The resistivity recorded as a function of temperature exhibits typical metallic behavior and increases with decreasing wire diameter. Magnetotransport investigations for different nanowire diameters were successfully performed by applying pulsed (up to 60 T) or static (up to 12 T) magnetic fields perpendicular and parallel to the wire axis. Generally, the magnetoresistance was found to be positive, increasing either linearly or quadratically with the magnetic field and shows no saturation. At low temperatures and within the zero-magnetic-field regime, the appearance of weak antilocalization effects indicates the presence of quantum interference caused by large spin-orbit coupling. The obtained results provide first signs of quantum phenomena in electrochemically deposited Bi2Te3 nanowires. The presented successful investigation of electrodeposited Bi2Te3 nanowires by nano-ARPES and magnetotransport measurements opens new possibilities that are of great importance for future investigations of the electrical transport characteristics of topological insulators nanostructures. The developed experimental methods lay the groundwork to pursue these studies as a function of various nanowire properties such as diameter, crystallographic structure and grain boundaries. This knowledge combined with the easy and controlled fabrication of the unique nanowire samples employed here, promises many applications as innovative electronic and spintronic devices.