Funding for this research was provided by:
National Natural Science Foundation of China (11471089)
the Fundamental Research Funds for the Central Universities (HIT. IBRSEM. A. 201401)
National Natural Science Foundation of China (11371048)
National Natural Science Foundation of China (11301207)
Article History
Received: 21 June 2015
Accepted: 1 September 2015
First Online: 14 September 2015
Free to read: This content has been made available to all.