[1] Univ Munich, Ctr NanoSci, D-80539 Munich, Germany
[2] Univ Munich, Sekt Phys, D-80539 Munich, Germany
[3] Heriot Watt Univ, Sch Engn & Phys Sci, Edinburgh EH14 4AS, Midlothian, Scotland
[4] Univ Calif Santa Barbara, Dept Mat, Santa Barbara, CA 93106 USA
[5] CSIC, CNM, Inst Microelect Madrid, Madrid 28760, Spain