Wolfgang Jäger

Institute of Materials Science, Christian-Albrechts-University of Kiel, 24143 Kiel, Germany EU

Advanced high-resolution imaging and spectroscopic techniques of transmission electron microscopy (TEM) play a crucial role in characterizing the structure-property relationships of inorganic functional materials and interfaces. The microstructure, the elemental composition, and the physical properties of nanomaterials can be characterized quantitatively and with spatial resolutions in the nanometer regime or even on the atomic level. This report describes two applications of in situ studies to semiconductor nanowire (NW) materials.

The first example illustrates phenomena that occur under thermal load of ZnO NWs (dimensions ≤ 100nm) with core materials of low melting temperature 1-3. ZnO NWs fabricated by thermal methods from Sn-based precursor materials under certain conditions possess nanotube-like core-shell morphologies with partially or completely filled core regions. Scanning transmission electron microscopy (STEM) combined with energy-dispersive x-ray spectroscopy (EDXS) confirm the presence of a Sn or a Sn-rich core material 2. Whenexposing the NWs alternatingly to high and low electron beam fluxes, melting, re-solidification, and the reversible thermal expansion and contraction of the molten core material can be observed by in situ TEM. These methods enable monitoring the behaviour of the core and the modification of its chemical composition during extended thermal cycling.

The second example illustrates the effects of elastic straining on electronic and on electrical transport properties of individual GaAs and InAs semiconductor NWs (diameters ≈ 100nm, lengths ≥ 5 µm) 4. Using a TEM holder equipped with a piezo-controlled scanning tunnelling tip 5 allows the application of controlled values of elastic strain to individual NWs while simultaneously monitoring their electronic properties by valence electron energy loss spectroscopy (VEELS), measuring their change in electrical resistance by taking I-V curves, and imaging their morphology and microstructure. Elastic bending experiments reveal that the GaAs NWs are highly flexible. I-V plots show an increasingly non-linear behavior with increasing strain. NW morphology and the electrical properties are restored upon complete strain release. TheVEELS measurements show changes of the electronic band structure with bending for the GaAs and the InAs NWs. The impact of strain and other possible influences on the electronic and transport properties will be discussed.

It is my pleasure to acknowledge the collaborations with my colleagues and co-authors mentioned in the references.

1. Authors: Yanicet Ortega^a,b, Dietrich Häussler^a, Paloma Fernández^b, Javier Piqueras^b, and Wolfgang Jaeger^a (^a Institute for Materials Science, Christian-Albrechts-Universität zu Kiel, Kiel, Germany, ^b Dept. Materials Physics, University Complutense of Madrid, Madrid, Spain)
2. Y. Ortega Villafuerte, Ch. Dieker, W. Jäger J. Piqueras, P. Fernández: Voids, nanochannels and formation of nanotubes with mobile Sn fillings in Sn-doped ZnO nanorods. Nanotechnology 21, 225604 (2010).
3. Authors: Lunjie Zeng^a, Thomas Kanne Nordqvist^b, Peter Krogstrup^b, Wolfgang Jäger^c,a and Eva Olsson^a (^a Department of Physics, Chalmers University of Technology, Gothenburg, Sweden; ^b Center for Quantum Devices, Nano-Science Center, Niels Bohr Institute, University of Copenhagen, Denmark; ^c Institute of Materials Science, Christian-Albrechts-University of Kiel, Germany).
4. Y. Jompol, H. Olin and E. Olsson: Compact design of a transmission electron microscope-scanning tunnelling microscope holder with three-dimensional coarse motion. Rev. Sci. Instr. 74, 4945 (2003).