1. IMEC, SPDT/ADRT Division, Kapeldreef 75, B-3001 Leuven, Belgium, Department of Electrical Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 1, B-3001 Heverlee, Belgium, and Department of Chemical Engineering, Kyoto University-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan