Graphene-like monolayer monoxides and monochlorides

Abstract

Two-dimensional monolayer materials, with thicknesses of up to several atoms, can be obtained from almost every layer-structured material. It is believed that the catalogs of known 2D materials are almost complete, with fewer new graphene-like materials being discovered. Here, we report 2D graphene-like monolayers from monoxides such as BeO, MgO, CaO, SrO, BaO, and rock-salt structured monochlorides such as LiCl, and NaCl using first-principle calculations. Two-dimensional materials containing d-orbital atoms such as HfO, CdO, and AgCl are predicted. Adopting the same strategy, 2D graphene-like monolayers from mononitrides such as scandium nitride (ScN) and monoselenides such as cadmium selenide (CdSe) are discovered. Stress engineering is found to help stabilize 2D monolayers, through canceling the imaginary frequency of phonon dispersion relation. These 2D monolayers show high dynamic, thermal, kinetic, and mechanic stabilities due to atomic hybridization, and electronic delocalization.

Keywords: 2-dimensional materials; beyond graphene; first-principles calculations; monolayer.

Fig. 1.

(A) Rock-salt lattice structure of bulk NaCl. (B) Planar view of a 2D NaCl monolayer. (C) Planar and side views of 2D MgO, BeO, SrO, CdO, and AgCl monolayers. (D) Lattice constants and bond lengths of 2D materials, as compared to graphene. (E) The simulated STM images of NaCl, MgO, LiCl, and SrO 2D monolayers.

Fig. 2.

(A) Binding energies of predicted 2D materials compared to graphene, germanene, MoS₂, and silicone. (B) Graphene-like honeycomb lattice from 2 interpenetrating triangular lattices (a1 and a2 are the primitive vectors). (C) Brillouin zone corresponding to a planar hexagonal lattice with high symmetry k points. The Dirac cones are located at the K and K′ points for graphene. (D–I) Phonon dispersion relations of (D) 2D MgO monolayers, (E) 2D CaO monolayer, (F) 2D CdO monolayer, (G) 2D NaCl monolayer, (H) 2D LiCl monolayer, and (I) 2D AgCl monolayer.

Fig. 3.

Fluctuation of the total potential energy of (A) 2D NaCl monolayer and (B) 2D MgO monolayer; (C) forces and (D) velocity autocorrelation function of a 2D MgO monolayer for 10 ps at different temperatures during AIMD simulation. Snapshots of (E) 2D MgO monolayers and (F) 2D NaCl monolayers at different temperatures, by top view (Top) and side view (Bottom).

Fig. 4.

(A) Band gaps calculated using norm-conserving PBE, PAW-PBE, MetaGGA-SCAN, and HSE06 exchange functionals. (B–E) Band structure of graphene, 2D MgO, 2D NaCl, and 2D CdO monolayers calculated using PAW-PBE, MetaGGA-SCAN, and HSE06 exchange functionals. PDOS of (F) 2D NaCl monolayer and (G) 2D MgO monolayer. (Inset) Enlarged PDOS. Isosurface plots of deformation electronic density of (H) 2D MgO monolayer, (I) 2D NaCl monolayer, and (J) 2D CdO monolayer. Charge accumulation and depletion regions are shown in red and blue, respectively.