Correction to: NPG Asia Materials (2014) 6, e130; doi:10.1038/am.2014.79; published online 26 September 2014.

After online publication of this article, the authors noticed an error in the acknowledgement section.

The correct acknowledgement statement of this article should have read as below.

‘We acknowledge GRF of Hong Kong (PolyU 153041/14P), The Hong Kong Polytechnic University (Project A-SA78) and NSFC (21125316, 21434009) for financial support of this work.’

The authors apologize for any inconvenience caused.