1. Key Laboratory of Luminescence and Optical Information; Ministry of Education; School of Science; Beijing Jiaotong University; Beijing 100044 P. R. China
2. Department of Electrical and Electronic Engineering; The University of Hong Kong; Pokfulam Road Hong Kong
3. Lab of Printing and Packaging Material and Technology-Beijing Area Major Laboratory; Beijing Institute of Graphic Communication; Beijing 102600 P. R. China
4. Department of Molecular Sciences and Nanosystems; Università Ca' Foscari Venezia; Via Torino 155/B 30172 Mestre Venezia Italy