1. Royal Institute of Technology, Department of Chemistry, Teknikringen 30, S-10044 Stockholm, Sweden, Attana AB, Björnnäsvägen 21, S-11347 Stockholm, Sweden, Uppsala University, Ångström Laboratory, Solid State Electronics, P.O. Box 534, S-75121 Uppsala, Sweden, Linköping University, Department of Physics, Chemistry and Biology, S-58183 Linköping, Sweden, and Uppsala University, Ångström Laboratory, Physics and Materials Science, P.O. Box 530, S-751 21 Uppsala, Sweden