A comparison study of the structural, electronic, elastic, dielectric and dynamical properties of Zr-based monolayer dioxides (ZrO2) and dichalcogenides (ZrX2; X = S, Se or Te) as well as their Janus structures (ZrXY; X, Y = O, S, Se or Te, Y ≠ X )

From first-principles calculations, we make a comparison study of the structural, electronic, elastic, dielectric and dynamical properties of Zr-based monolayer dioxides (ZrO2) and dichalcogenides (ZrX2; X = S, Se or Te) as well as their Janus structures (ZrXY; X, Y = O, S, Se or Te, Y≠X). Regardless the constituents, the bond length between Zr+4 cation and its nearest-neighbor X−2 anions dZr-X equals the sum of ionic radii of Zr+4 and X−2 ions (X = O, S, Se or Te) reflects a p-d hybridized ionic bond instead of a sp2 hybridized covalent bond in graphene. For six O-free monolayers ZrS2, ZrSe2, ZrTe2, ZrSSe, ZrSTe, ZrSeTe, the conduction band minimum (CBM) and valence band maximum (VBM) are located at the M and Γ points, respectively, while for four O-containing monolayers ZrO2, ZrOS, ZrOSe, ZrOTe, the CBM and VBM are located at K (except ZrOSe between MΓ) and Γ (except ZrO2 between ΓK) points. The decreasing indirect band gaps of semiconductor 4.41, 1.88, 1.06 eV and 1.19, 0.76, 0.50 eV are obtained for three Te-free and O-containing monolayers ZrO2, ZrOS, ZrOSe and three Te-free and O-free monolayers ZrS2, ZrSSe, ZrSe2 respectively due to increasing the sum of dZr-X and dZr-Y, while the metal characteristics are obtained for four Te-containing monolayers ZrTe2, ZrOTe, ZrSTe, ZrSeTe due to the larger radius of 2.21 Å for Te−2 anion. The large dielectric permittivity, mechanical stability and dynamical stability show three Te-free and O-free monolayers ZrSSe, ZrS2 and ZrSe2 are the promising dielectric materials.