Single-layer Cu2X (X = O, S, Se): auxetic materials modulated by strain engineering

Auxetic materials have shown significant potential for information, medicine, and aerospace applications. This work predicts a series of single-layer (SL) Cu2X (X = O, S, Se) with negative Poisson's ratio (single) using first-principles calculations. It is found that SL Cu2O has an intrinsic ultrawide range of large NPR, which is less discovered in auxetic materials. By strain engineering, we not only reveal the maintainable in-plane NPR of SL Cu2O over a large strain range, but also modulate out non-intrinsic in-plane and out-of-plane NPRs of SL Cu2X. These exotic properties of SL Cu2X could lead to their favorable mechanical potential.