Intrinsic piezoelectricity in monolayer MSi2N4 (M = Mo, W, Cr, Ti, Zr and Hf) San-Dong Guo, Yu-Tong Zhu, Wen-Qi Mu and Wen-Cai Ren EPL, 132 5 (2020) 57002 Published online: 30 December 2020 DOI: 10.1209/0295-5075/132/57002