1. Department of Micro and Nanosciences; Aalto University; Tietotie 3 Espoo 02150 Finland
2. Beneq Oy; Olarinluoma 9 Espoo 02200 Finland
3. Department of Physics; Accelerator Laboratory; University of Jyvaskylä; P.O. Box 35 Jyväskylä 40014 Finland
4. VTT Technical Research Centre of Finland; P.O. Box 1000, 02044 VTT Espoo Finland