Every peer-reviewed research article appearing in this journal will be published open access. This means that the article is universally and freely accessible via the internet in perpetuity, in an easily readable format immediately after publication. The author does not have any publication charges for open access. Tianjin University will pay to make the article open access. Authors who publish in NPE retain the copyright to their work under a Creative Commons license. Under this license, users are free to share and adapt the material in any format, provided appropriate credit is given. Visit the AIP Publishing Open Access Policy for more information about our policies on open access.