1. Center for Programmable Materials School of Materials Science & Engineering; Nanyang Technological University; Singapore 639798 Singapore
2. CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience; National Center for Nanoscience and Technology; Beijing 100190 P.R. China
3. School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
4. State Key Laboratory of Silicon Materials and School of Materials Science and Engineering; Zhejiang University; Hangzhou 310027 P. R. China