1. Department of Materials Science and Engineering, UC Berkeley , Berkeley, CA 94720 , USA
2. National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory , Berkeley, CA 94720 , USA
3. Physical Measurement Laboratory, National Institute of Standards and Technology , Boulder, CO 80305 , USA
4. Carl Zeiss SMT Inc. , Danvers, MA 01923 , USA