Electronic and electron emission properties of X/ZnO monolayer (X=Li, Na, K, Rb, or Cs): A first-principles study

Two-dimensional (2D) materials have attracted increasing research interest due to their unique physical and chemical properties. As a typical 2D material, ZnO monolayers not only generate scientific interest but also may have important technological applications. However, it is still a great challenge for ZnO monolayers to be used in electron emission devices. Here, the first-principles calculations based on DFT + D2 have been used to investigate the structural stabilities, electronic structure, electron emission properties and diffusion behaviors of X/ZnO (X = Li, Na, K, Rb, or Cs) monolayers. The results demonstrate that the top of the center of the hexagonal ring of Zn-O is the most stable adsorption site for alkali metal X adsorbed onto the ZnO monolayers. The band gaps of X/ZnO monolayers are reduced to zero, and exhibit metallic characteristics. Furthermore, after alkali metal X adsorption, the electron emission properties of the ZnO monolayer, including the work function, ionization potential and Fermi levels, are rapidly improved due to the amount of charges transferred from the alkali metal X to the substrate. In addition, the diffusion behaviors of alkali metal X on the ZnO monolayer suggest that X adatoms favor the formation of a metal nanotemplate on the ZnO monolayer. These findings may provide guidance to extend the potential applications of ZnO in low-dimensional electron emission devices.