Jun Peng1,2, G Jeffrey Snyder3. 1. Department of Electrical and Computer Engineering, Northwestern University, Evanston, IL 60208, USA. 2. Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA. 3. Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA. jeff.snyder@northwestern.edu.