1. Department of Memory Business, Samsung Electronics 1 , Samsungjeonja-ro, Hwasung-si 18448, Republic of Korea
2. School of Mechanical Engineering, Sungkyunkwan University 2 , Seobu-ro, Suwon 16419, Republic of Korea
3. School of Mathematics and Computing, Yonsei University 3 , Seoul 03722, Republic of Korea
4. Department of NanoEngineering, University of California San Diego 4 , La Jolla, California 92093, USA