1. Instituto de Microelectrónica de Madrid, IMM-CNM (CSIC), Isaac Newton 8 (PTM), Tres Cantos, 28760 Madrid, Pittsburgh Supercomputing Center, Carnegie Mellon University, 300 S. Craig Street Pittsburgh, Pennsylvania 15213, and The Birck Nanotechnology Center and School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907