Ultrahigh carrier mobility and multidirectional piezoelectricity in 2D Janus copper-containing chalcogenide monolayers

Two-dimensional (2D) materials have attracted enormous research attention due to their remarkable properties and potential applications in electronic and optoelectronic devices. In this work, Janus 2D copper-containing chalcogenides, CuP₂Se_0.5S_0.5 and CuP₂Te_0.5Se_0.5 monolayers, are proposed and studied systematically based on first-principles calculations. These two Janus-structured materials possess the same thermal and dynamic stability as the perfect CuP₂Se structure. Remarkably, we observe multiple VBM and CBM points with negligible energy differences in the band structures of perfect CuP₂Se and Janus CuP₂Se_0.5S_0.5 and CuP₂Te_0.5Se_0.5 monolayers. This will significantly impact the electronic and transport properties of the material. The calculated anisotropic carrier mobilities can reach 10⁴-10⁵ cm² V^-1 s^-1 orders of magnitude, which are higher than those of most reported materials. Meanwhile, the two Janus derivatives, CuP₂Se_0.5S_0.5 and CuP₂Te_0.5Se_0.5 monolayers, exhibit outstanding multidirectional piezoelectricity, which are comparable with those of traditional piezoelectric materials. The combination of ultrahigh carrier mobility and multidirectional piezoelectricity indicates that these novel 2D Janus materials could be promising for applications in electronic and piezoelectric devices under special conditions.