1. Centro Nacional de Microelectrónica, IMB-CNM, CSIC, Campus de la Universidad, Autónoma de Barcelona, Esfera UAB, Bellaterra-08193, Spain, Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden, and Department of Materials Chemistry, The Ångström Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden