Monolayer alkali and transition-metal monoxides: MgO, CaO, MnO, and NiO

Two-dimensional crystals with strong interactions between layers has attracted increasing attention in recent years in a variety of fields. In particular, the growth of a single layer of oxide materials (e.g., MgO, CaO, NiO, and MnO) over metallic substrates were found to display different physical properties than their bulk. In this study, we report on the physical properties of a single layer of metallic oxide materials and compare their properties with their bulk and other two-dimensional (2D) crystals. We found that the planar structure of metallic monoxides are unstable whereas the buckled structures are thermodynamically stable. Also, the 2D-MnO and NiO exhibit different magnetic (ferromagnetic) and optical properties than their bulk, whereas band-gap energy and linear stiffness are found to be decreasing from NiO to MgO. Our findings provide insight into oxide thin-film technology applications.

Figure 1

The variation of energy with lattice constant of monolayer MgO(100) calculated by using LDA, for (a) the planar and the buckled structures. Phonon dispersion spectra of the planar (b) and the buckled (c) structures. The red-dashed lines indicate the imaginary frequency region for planar MgO(100).

Figure 2

The electronic band structure of spin up, total DOS, and PDOS of [(a),(b)] the buckled monolayer MgO(100) and [(c),(d)] bulk MgO, as obtained from DFT-PBE.

Figure 3

(a) The electronic band structure of spin up, (b) total DOS, and (c) PDOS of the buckled monolayer CaO(100). (d) The side and top views of the buckled structure of monolayer CaO(100), from DFT-PBE.

Figure 4

(a) The electronic band structure of spin up, (b) total DOS, and (c) PDOS of the buckled monolayer MnO(100). (d) The side and top views of the buckled structure of monolayer MnO(100), from DFT-PBE.

Figure 5

(a) The electronic band structure of spin up, (b) total DOS, and (c) PDOS of the buckled monolayer NiO(100). (d) The side and top views of the buckled structure of monolayer NiO(100), from DFT-PBE.

Figure 6

Comparison of the total density of states of monolayer MnO(100) [(a),(c),(e),(g)] and monolayer NiO(100) [(b),(d),(f),(h)] for different values of $U_{eff}$ obtained with the LDA+$U$ approach.

Figure 7

Simulated STM images from top, for MnO(100) (a) and for NiO(100) (b). STM images were calculated with $I=0.1$ nA and $V_b=-0.5$ V.

Figure 8

Mulliken charge distribution for (a) Mn, O and (b) Ni, O atoms. Black and gray bars indicate the charge of spin up and spin down of Mn and Ni atoms, and the empty and filled blue bars indicate the charge of spin up and spin down of the O atoms. The spatial spin-density distribution for Mn and Ni $\left(\rho \uparrow -\rho \downarrow \right)$ is also plotted (Mn: spin up, blue; spin down, green; Ni: spin up, violet; spin down, pink) (c), (d) Splitting of PDOS of $3d$ orbital for spin up and spin down of MnO(100) and NiO(100).

Figure 9

(a) Comparison of the optical absorption spectrum computed for the $X\mathrm{O}\left(100\right)$ monolayers $(X=\text{Mg}$, Ca, Mn, and Ni) and their bulk in two different phases; for interacting (solid line) and for noninteracting electron-hole case (dashed line), i.e., (a) and (b) are the results for monolayer MgO(100) and (b) and (c) are for bulk MgO, etc.

Figure 10

The variation of the total energy (a) and electronic band gap with applied strain (b).