Computational discovery of two-dimensional copper chalcogenidesCuX(X= S, Se, Te)

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We investigate the energy landscape of 2D CuS, CuSe, and CuTe compounds using a global evolutionary algorithm in combination with density functional theory. Four low-lying energy ${\mathrm{Cu}}_{2}{X}_{2}$ (X=S, Se, Te) slabs with $P\overline{3}m1$, $P4/mmm$, $P4/nmm$, and $Pmmn$ symmetries have been identified on their respective potential energy surfaces. Three structures present tetrahedral ${\mathrm{Cu}X}_{4}$ motifs which pave the 2D slabs, while the latter is based on shared ${\mathrm{Cu}}_{4}{X}_{2}$ octahedra. The viability of each phase was examined by looking at dynamical, thermodynamic, and thermal stability criteria. We find that 2D copper monosulfide crystallizes in the $P\overline{3}m1$ phase, reminiscent of the covalent slab found in the $\mathrm{Ca}{\mathrm{Al}}_{2}{\mathrm{Si}}_{2}$ prototype. Two polymorphs of 2D copper selenide with symmetries $P\overline{3}m1$ and $P4/mmm$ are close in energy. The latter ${\mathrm{Cu}}_{2}{\mathrm{Se}}_{2}$ slab is made of fused ${\mathrm{Cu}}_{4}{\mathrm{Se}}_{2}$ square bipyramids with electronegative Se atoms in apical positions. Finally, the ground-state 2D CuTe has a $Pmmn$ space group containing distorted ${\mathrm{CuTe}}_{4}$ tetrahedra with some Te--Te bonding. The electronic and bond analyses show that all of these predicted 2D CuX phases are metallic with ionocovalent bonds.