Funding for this research was provided by:
National Science Foundation of China (21575113)
Natural Science Foundation of Shaanxi Province in China (2017JM2036, 2018JQ2029)
Fostering Foundation of Northwest University for the Excellent Ph.D. Dissertation (YYB17012)
Northwest University Graduate Innovation and Creativity Funds (YZZ17125)
Article History
Received: 31 January 2019
Accepted: 24 May 2019
First Online: 27 June 2019
Compliance with ethical standards
: The author(s) declare that they have no competing interests.