1. Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Korea
2. National Center for Electron Microscopy
3. Materials Sciences Division, Lawrence Berkeley National Laboratory
4. Department of Physics
5. Center of Integrated Nanomechanical Systems, University of California at Berkeley, California 94720
6. National Research Laboratory for Nano Quantum Electronics Laboratory, Department of Electronics and Communications Engineering, Hanyang University, Seoul 133-791, Korea
7. Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea