1. Department of Physics & Astronomy and California NanoSystems Institute, University of California , Los Angeles, CA , USA
2. School of Physical Science and Technology, ShanghaiTech University , Shanghai , China
3. Department of Physics, University of California at Berkeley , Berkeley, CA , USA
4. Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, CA , USA
5. Kavli Energy NanoSciences Institute at the University of California , Berkeley, CA , USA
6. National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory , Berkeley, CA , USA