Fan, H
Chen, K
Duan, L
Wang, Y-Z
Ju, G
Article History
Received: 9 September 2015
Revised: 16 March 2016
Accepted: 19 March 2016
First Online: 3 May 2016
Change Date: 4 November 2016
Change Type: Correction
Change Details: Updated online 21 June 2016: This article was originally published under NPG's License to Publish, but has now been made available under a CC BY-NC-SA 4.0 license. The PDF and HTML versions of the paper have been modified accordingly.
Competing interests
: The authors declare no conflict of interest.