Semimetal to semiconductor transition in Bi/TiO2 core/shell nanowires

M. Kockerta, R. Mitdanka, H. Moonb, J. Kimc, A. Mogilatenkod, S.H. Moosavie, M. Kroenere, P. Woiase, W. Leeb and S.F. Fischera

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Nanoscale Adv., vol. 3, no. 1, pp. 263-271, doi:10.1039/D0NA00658K (2021).

Copyright © 2021 The Author(s). Published by the Royal Society of Chemistry.
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Abstract:

We demonstrate the full thermoelectric and structural characterization of individual bismuth-based (Bi-based) core/shell nanowires. The influence of strain on the temperature dependence of the electrical conductivity, the absolute Seebeck coefficient and the thermal conductivity of bismuth/titanium dioxide (Bi/TiO2) nanowires with different diameters is investigated and compared to bismuth (Bi) and bismuth/tellurium (Bi/Te) nanowires and bismuth bulk. Scattering at surfaces, crystal defects and interfaces between the core and the shell reduces the electrical conductivity to less than 5% and the thermal conductivity to less than 25% to 50% of the bulk value at room temperature. On behalf of a compressive strain, Bi/TiO2 core/shell nanowires show a decreasing electrical conductivity with decreasing temperature opposed to that of Bi and Bi/Te nanowires. Wefind that the compressive strain induced by the TiO2 shell can lead to a band opening of bismuth increasing the absolute Seebeck coefficient by 10% to 30% compared to bulk at room temperature. In the semiconducting state, the activation energy is determined to ❘41.3±0.2❘ meV. We show that if the strain exceeds the elastic limit the semimetallic state is recovered due to the lattice relaxation.

a Novel Materials Group, Humboldt-Universität zu Berlin, 10099 Berlin, Germany
b Department of Material Science and Engineering, Yonsei University, 03722 Seoul, Republic of Korea
c Division of Nanotechnology, DGIST, 42988 Daegu, Republic of Korea
d Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, 12489 Berlin, Germany
e Laboratory for Design of Microsystems, University of Freiburg - IMTEK, 79110 Freiburg, Germany