Low loss EELS and EFTEM study of Bi 2 Te 3 based bulk and nanomaterials
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Low loss EELS and EFTEM study of Bi2Te3 based bulk and nanomaterials N. Peranio1, Z. Aabdin1, W. Töllner2, M. Winkler3, J. König3, O. Eibl1, K. Nielsch2, H. Böttner3 1 Institut für Angewandte Physik, Universität Tübingen, Auf der Morgenstelle 10, D-72076 Tübingen, Germany 2 Institut für Angewandte Physik, Universität Hamburg, Jungiusstrasse 11, D- 20355 Hamburg, Germany 3 Fraunhofer Institut Physikalische Messtechnik, Heidenhofstrasse 8, D-79110 Freiburg, Germany ABSTRACT Energy-filtered transmission electron microscopy (EFTEM) yields new possibilities for the investigation of Bi2Te3 based nanomaterials. Combined low-loss electron energy-loss spectroscopy (EELS) and energy-dispersive x-ray microanalysis (EDS) and energy-filtered TEM were applied on a Zeiss 912Ω TEM to investigate nanowires, thin films, and bulk materials. Multilayered Bi-Sb-Te nanowires with a diameter of 65 nm and a period of 200 nm and stoichiometric Bi2Te3 nanowires were grown by potential-pulsed electrochemical deposition. Tellurium elemental maps of the multilayered nanowires were obtained by two-window edgejump ratio images (EJI). EDS chemical analysis showed that small Te fluctuations of 3 at.% yielded significant contrast in EJI. Energy-filtered TEM applied on nano-alloyed Bi2Te3 thin films grown by molecular beam epitaxy (MBE) revealed 10-20 nm thick Bi-rich blocking layers at grain boundaries. Plasmon spectroscopy by EELS was applied on Bi2(Te0.91Se0.09)3 bulk and yielded a plasmon energy of 16.9 eV. Finally, plasmon dispersion was measured for Bi2(Te0.91Se0.09)3 bulk by angle-resolved EELS, which yields a fingerprint of the anisotropy and the dimensionality of the electronic structure of the materials. INTRODUCTION Peltier materials based on Bi2Te3, are widely used for room temperature applications due to their high thermoelectric figure of merit ZT ≈ 1 at T = 300 K. In 1993, Hicks and Dresselhaus predicted for low-dimensional systems an enhancement of the thermoelectric figure of merit ZT due to quantum confinement effects and lattice phonon scattering [1]. Thereafter, fabrication of Bi2Te3 based thin films, nanowires, and nanostructured bulk have become a main topic in research on thermoelectric materials. Particularly, Venkatasubramanian et al. obtained for Bi2Te3/Sb2Te3 superlattice structures an outstanding ZT value of 2.4 at 300 K [2]. There were also increased efforts in understanding the anisotropy and dimensionality of the band structure of these materials [3]. A project was initiated and is funded by the German Research Council (DFG) to study the role of dimensionality and nanostructure on thermoelectric properties in Bi2Te3 based nanomaterials and to establish a road map for reaching higher ZT [4]. Materials synthesis technology includes nanowires, thin films and superlattices, and nanostructured bulk. Structure, chemical composition, as well as electronic and phononic excitations have been investigated by x-ray, neutron, and electron diffraction and by analytical energy-filtered transmission electron microscopy (EFTEM). Ab-ini
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