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

Krieg, Janina (2017)
Characterization of individual Bi2Te3 nanowires electrodeposited in etched ion-track membranes for nano-ARPES and electrical transport studies.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication

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.

Item Type: Ph.D. Thesis
Erschienen: 2017
Creators: Krieg, Janina
Type of entry: Primary publication
Title: Characterization of individual Bi2Te3 nanowires electrodeposited in etched ion-track membranes for nano-ARPES and electrical transport studies
Language: English
Referees: Trautmann, Prof. Dr. Christina ; Krupke, Prof. Dr. Ralph
Date: 2017
Place of Publication: Darmstadt
Refereed: 19 July 2017
URL / URN: http://tuprints.ulb.tu-darmstadt.de/6750
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.

Alternative Abstract:
Alternative abstract Language

Vielversprechende Forschungsfelder aus dem Bereich der Festkörperphysik, wie zum Beispiel Thermoelektrik und Spintronik, werden durch die einzigartigen Eigenschaften von Bismut-basierten Materialien geprägt. Bismuttellurid (Bi2Te3) ist beispielsweise ein Halbleiter mit kleiner Bandlücke und exzellenten thermoelektrischen Eigenschaften, die für die Umwandlung von thermischer in elektrische Energie bei Raumtemperatur eine der höchsten Effizienzen aufweist. Außerdem wird Bismuttellurid einer neuen Klasse von Quantenmaterialen, den Topologischen Isolatoren (TI) zugeordnet, die Spin-Bahn-gekoppelte Oberflächenleitfähigkeit bei gleichzeitig isolierendem Verhalten im Volumen besitzen. Beide Eigenschaften verheißen Anwendungen in der Elektronik und Spintronik, die durch Energieeffizienz und hohe Rechenleistung bestechen. Die experimentelle Bestimmung der speziellen Charakteristika des TIs stellt jedoch eine besondere Herausforderung dar, weil die Leitfähigkeit des Volumens das elektrische Signal dominiert und die Oberflächenzustände nur schwer zugänglich sind. Um diese Hürde zu überwinden beschäftigt sich diese Dissertation zunächst mit der Herstellung von Bi2Te3 Nanodrähten, deren Oberflächen-zu-Volumen Verhältnis extrem groß ist und deren geometrische, kristallographische und morphologische Eigenschaften unabhängig von einander kontrolliert eingestellt werden können. Dadurch können die Anteile der Oberflächenzustände aufgelöst werden. Bi2Te3 Nanodrähte wurden durch elektrochemische Abscheidung in ionenstrahlgeätzten Polymermembranen synthetisiert. Hierfür wurden zuerst 30 µm dicke Polycarbonatfolien mit hochenergetischen Schwerionen bestrahlt. Jedes einzelne Ion erzeugt eine Ionenspur, die durch einen selektiven chemischen Ätzprozess in zylindrische Nanoporen umgewandelt wird. In diesen Poren wurden anschließend Nanodrähte mit Durchmessern zwischen 25 und 100 nm abgeschieden. Röntgendiffraktometrie und Transmissionselektronenmikroskopie zeigen texturierte Nanodrähte, die aus mehreren einkristallinen und bis zu mehreren hundert Nanometern langen Segmenten bestehen. Die chemische Zusammensetzung wurde durch energiedispersive Röntgenspektroskopie und Röntgenphotoemissionsspektroskopie, sowohl für das Volumen als auch für die Oberfläche, bestimmt. Erstere zeigt dabei eine homogene, beinahe stöchiometrische Zusammensetzung, während letztere das Vorhandensein von Sauerstoff- und Kohlenstoffkontaminationen bestätigt. Diese Kontamination wird Polymerresten zugeschrieben, die von der Auflösung des Polymertemplates herrühren. Untersuchungen der elektronischen Eigenschaften einzelner Bi2Te3 Nanodrähte wurde mit Hilfe von nano-winkelaufgelöster Photoemissionsspektroskopie (nano-ARPES) am französischen Synchrotron SOLEIL durchgeführt. In einem speziellen Aufbau, der für das Detektieren von Photoemissionssignalen von Nanoobjekten entwickelt wurde, wurden einzelne Abschnitte der Nanodrähte analysiert. Winkelintegrierte Messungen von Rumpfelektronenniveaus entlang der Drahtoberfläche bestätigen die homogene chemische Zusammensetzung. Mit winkelaufgelösten Messungen wurde die Valenzbandstruktur einzelner Drahtabschnitte erfolgreich aufgenommen. First-Prinzipal Rechnungen für die bevorzugt abgeschiedenen Kristallorientierungen stimmen gut mit den experimentellen nano-ARPES Ergebnissen überein. Um elektrische Transportmessungen durchzuführen, wurden einzelne Nanodrähte mittels Laser- und Elektronenstrahllithographie kontaktiert. Der spezifische Widerstand weist die für Metalle typische Temperaturabhängigkeit auf und steigt mit abnehmendem Drahtdurchmesser an. Des Weiteren wurden Magnetotranportmessungen in gepulsten (bis 60 T) und statischen (bis 12 T) Magnetfeldern mit paralleler und senkrechter Orientierung bezüglich der Drahtachse, durchgeführt. Im Allgemeinen ist der Magnetwiderstand positiv und steigt mit größer werdendem Magnetfeld linear oder quadratisch an ohne zu saturieren. Schwache Antilokalisierungseffekte, die bei kleinen Temperaturen und in schwachen Magnetfeldern auftreten, weisen auf das Vorhandensein von Quanteninterferenzphänomenen aufgrund der hohen Spin-Bahn-Wechselwirkung hin. Die erhaltenen Ergebnisse sind erste Hinweise auf Quantenphänomene an elektrochemisch in Polymermembranen abgeschiedenen, zylindrischen Bi2Te3 Nanodrähten.

Die in dieser Arbeit dargelegte erfolgreiche Vermessung von elektrochemisch abgeschiedenen, einzelnen Bi2Te3 Nanodrähten mittels Nano-ARPES und Magnetotransportmessungen eröffnen neue Möglichkeiten, die für weiterreichende Untersuchungen von topologischen Isolator-Nanostrukturen von großer Bedeutung sind. Die entwickelten experimentellen Methoden legen den Grundstein für künftige Untersuchungen in Abhängigkeit des Drahtdurchmessers, der Kristallorientierung und der Korngröße. Dieses Wissen, in Kombination mit der hier gebrauchten einfachen und kontrollierbaren Bi2Te3 Nanodrahtherstellung, verspricht ein großes Anwendungsportfolio als innovative Elektronik- und Spintronikbauteile.

German
URN: urn:nbn:de:tuda-tuprints-67504
Classification DDC: 500 Science and mathematics > 500 Science
Divisions: 11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences > Material Science > Ion-Beam-Modified Materials
11 Department of Materials and Earth Sciences
Date Deposited: 17 Sep 2017 19:55
Last Modified: 17 Sep 2017 19:55
PPN:
Referees: Trautmann, Prof. Dr. Christina ; Krupke, Prof. Dr. Ralph
Refereed / Verteidigung / mdl. Prüfung: 19 July 2017
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