Retraction note
The authors have retracted this article because Figures. 2, 3 and 4, as well as parts of the text have been previously published [1]. This article is therefore redundant. Yu Zhang, Denghui Liu, Jiaqi Song, Huojun Zhang and Jianping Lu agree to this retraction. Xiaofei Zhu has not responded to any correspondence from the publisher about this retraction.
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