1. State Key Laboratory of Tribology; Tsinghua University; Beijing 100084 P. R. China
2. Department of Physics and Astronomy; University of Waterloo; Waterloo Ontario N2L 3G1 Canada
3. Waterloo Institute of Nanotechnology; University of Waterloo; Waterloo Ontario N2L 3G1 Canada
4. Centre for Advanced Materials Joining; University of Waterloo; Waterloo Ontario N2L 3G1 Canada