1. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province College of Optoelectronic Engineering Shenzhen University Shenzhen 518060 China
2. Department of Mechanical and Materials Engineering and Nebraska Center for Materials and Nanoscience University of Nebraska‐Lincoln Lincoln NE 68588‐0656 USA
3. Department of Applied Physical Sciences University of North Carolina at Chapel Hill Chapel Hill NC 27599 USA