First-principles Study Of Field Emission Properties For One-dimensional ZnO Nanostructure

ZnO is a semiconducting material with a direct wide band gap of3.37eV and alarge exciton binding energy of60meV at room temperature.As an important functionmaterials of photoelectric conversion received great attention because of in many fieldssuch as ultraviolet(UV) lasers, light-emitting diodes, field emission devices,high-performance nano-sensors, solar cells, piezoelectric nano-generators exhibitpotential applications. And with advancement in synthesis of one-dimensional (1D)ZnO nanostructure enabled discovery of their unique physical and chemical propertiesgiving rise to become a new research hotspot. Field emission (FE) performance is oneof the most attractive properties and various1DZnO nanostructures have been studied.Moreover,1DZnO nanostructures make excellent candidates for field emissionapplications due to the1DZnO nanostructures exhibit negative electron affinity, highthermal stability, low cost and oxidation resistance under harsh environments, and moreimportantly in addition to the factors of structure and morphology but also prolong thelife of equipment.In this paper, the structural stability and field emission properties of1DZnO nanostructures have been investigated by DMol3module of the first-principlesmethod software named Materials Studio based on density functional theory. Theprincipal elements are listed as follows:1. The first-principles density functional theoretical calculations are performed toinvestigate the effects of N doped and N-M(Cd,Mg) codoped on the geometricalstructures and field emission properties of capped (9,0) zinc oxide nanotubes (ZnONT).The results show that the N could improve the stability of the structure of capped side.With the increase of the applied electric field, the densityof states (DOS) shifts towards the low energy position, the highest occupied molecular orbital (HOMO)-lowestunoccupied molecularorbital (LUMO) gap and the effective work function decreasedrastically, and the electrons congregate to the capped side. The analysesof DOS/localDOS, HOMO/LUMO, and Mulliken population indicate that the field emissionproperties of N-Cd codoped ZnONT are improved, but those of N-Mg codoped ZnONTare worsened.2. The structural stability and field emission properties of N-M(In,Ga,Al)co-doped capped (6,6) zinc oxide(ZnO) nanotubes have been investigated by ab initiocalculations of the pseudopotential density functional method. It is found that:Co-doped could enhanced the stability of the capped side of systems; With the increaseof applied electric field, the density of states(DOS) distributed to the low energyposition, energy gap and effective work function significantly reduced, and the electronscongregate to the capped side. The results of DOS, highest occupied molecular orbital(HOMO)/lowest unoccupied molecular orbital(LUMO), Mulliken population andeffective work function demonstrate that the N-In co-doped systems is more promisingto the electron field emission than others.3. The structural stability and field emission properties of zinc oxide nanowires(ZnONW)with different cross section have been investigated via first-principles methodbased on density functional theory. The results show that ZnONW with large diameter ismore stable, and decreases the variation of the length of Zn-O bond after atomicrelaxation. The density of states (DOS) and partial densities of states (PDOS) ofNWwith large aspect ratio shifts towards the low energy position with the increase ofthe applied electric field,the highest occupied molecular orbital(HOMO)-lowestunoccupied molecular orbital (LUMO) gap decreases drastically, and the electronscongregate to the up side of ZnONW. The analyses of DOS/PDOS, HOMO/LUMO,Effective work function indicate that the NW-1with the smallest cross section is morepropitious to the electron field emission than other systems.