Janus Zn3CdC2 and ZnCd3C2 monolayers in carbides: A first-principles study

Abstract

In this work, two Janus carbides, Zn3CdC2 and ZnCd3C2 monolayers, are designed. The stability, electronic, mechanical, piezoelectric, and light absorption properties are investigated using the first-principles calculations. The results demonstrate that both structures are chemically, dynamically, and thermodynamically stable. A direct bandgap semiconductor, the ZnCd3C2 monolayer has a bandgap of 1.82 eV, while the indirect bandgap semiconductor Zn3CdC2 monolayer has a slightly larger gap of 2.03 eV. However, Zn3CdC2 monolayer can be transformed into a direct bandgap semiconductor by applying biaxial compression at 2%. The mechanical properties indicate that both materials possess significant mechanical anisotropy, high flexibility, and distinctive deformation pattern. More strikingly, two Janus monolayers display out-of-plane piezoelectric properties due to their noninversion symmetry and out-of-plane asymmetry structures. At the same time, the different charge transfer amounts along the x and y directions give them unequal out-of-plane piezoelectric coefficients d31 and d32. For Zn3CdC2 monolayer, the value of d31 is −0.52 pm/V, and the value of d32 is 0.25 pm/V; The values of d31 and d32 for ZnCd3C2 monolayer reach −0.32 pm/V and 0.27 pm/V, respectively. In addition, Zn3CdC2 and ZnCd3C2 monolayers have strong light absorption coefficients as high as 105 cm−1 in visible and ultraviolet bands. Our works reveal that the two Janus monolayers have great potential in piezoelectric, photovoltaic, and other sustainable energy applications.