1. Department
of Chemistry, Dongguk University—Seoul, Jung-gu, Seoul 100715, South Korea
2. Electronics and Telecommunications Research Institute, Synapse Device Creative Research Centre, 218 Gajeong-ro, Yuseong-gu,
Daejeon 305-700, South Korea
3. Department of Ceramic Engineering, Gangneung-Wonju National University, Gangneung 210702, South Korea