1. Laboratory of Semiconductor Materials, Institute of Materials, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
2. Center for Analysis and Synthesis and NanoLund, Lund University, Box 124, 221 00 Lund, Sweden
3. Institute of Physics, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland