1. Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan
2. National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
3. Center for Nanotechnology Innovation@NEST, Istituto Italiano di Tecnologia, Pisa 56127, Italy
4. NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, 56127 Pisa, Italy