1. Materials Science Division and Centre for Molecular Engineering, Argonne National Laboratory, Lemont, Illinois 60439, USA
2. State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
3. Pritzker School of Molecular Engineering, University of Chicago, Chicago, IL, 60637, USA
4. X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, USA