1. Department of Physics, Durham University, South Road, Durham DH1 3LE, U.K.
2. Low Dimensional Materials Research Centre, Department of Physics, University of Malaya, Kuala Lumpur 50603, Malaysia
3. Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
4. Department of Electrical Engineering, Graduate Institute of Electronics Engineering, and Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan
5. Institute of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan