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

Zinc oxide (ZnO) is a semiconducting material with a direct wide band gap of 3.37eV, and the exciton binding energy of 60meV is larger than other semiconducting materials at room temperature. The exciton is so stable that easily to be stimulated excitonic emission at normal temperature or even higher temperature. ZnO have attracted much attention because of the potential research value in many practical application, such as liquid crystal display, thin-film transistor, light emitting diodes, field emission devices, etal. With the growing maturity of preparation and study of nano materials, ZnO nanostructures become one of the hot spots of the research due to its excellent physical and chemical properties, especially one-dimensional ZnO nanostructures have got widely research in field emission performance because of good oxidation resistance, higher thermal stability, negative electron affinity, low cost, and is expected to become one of the most ideal field emission cathode materials. But electron field emission is affected by many factors, such as geometric structure, morphology features, external condition, looking for a way to enhance and improve one-dimensional ZnO nanostructures field emission performance is very important and necessary. However, experimental study has low efficiency and high cost, the simulation calculation is greatly shorten the research cycle from the theoretical research, and play a guiding role in the experimental study to accelerate the process of research. In this paper, the geometrical configuration and field emission properties of one-dimensional ZnO nanostructures have been investigated by DMol3 module of the first-principles method software named Materials Studio based on density functional theory. The principal elements are listed as follows:(1) The different geometrical structures of ZnO-NC (zinc oxide nanocones) were studied based on the first-principles and got five kinds of structural stability of ZnO-NC system (on the basis of the characters of cutting-edge atomic configurations):Zn-Zn(4P), Zn-O(4P), O-O(4P), Zn-O(2Q-2H) and Zn-O(2Q-4H). According to the analysis of the electronic density of states, pseudo-gap, HOMO/LUMO (highest occupied molecular orbital or lowest unoccupied molecular orbital), energy gap, results show that the Zn-Zn(4P) has more potential of emission electron in the external electric field due to excellent electronic structure features.(2) The first-principles density functional theoretical calculations are performed to investigate the field emission properties of In/Mg doped on Zn-Zn(4P) system. Results show that doping enhanced structural stability, in comparison to Magnesium doped and non-doped system, doped with Indium atoms system could improve LDOS (local density of states) with the peak position closer to  (Fermi level), and higher electron density at the tip. The analyses of Mulliken charge, HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) gap and the effective work function indicate that In-In(4P) has more excellent field emission properties.(3) By establishing ZnO-NC (zinc oxide nanocones) mathematical model, numerical calculation of ZnO-NC sophisticated potential and electric field in electrostatic field, the formula of ZnO-NC tip field emission effect factor is β=H/8πε·h/d(among them, h and d were ZnO-NC height and tip diameter respectively). The results show that, the ratio of the height and tip diameter has a profound effect on field emission effect factor, in the case of structural stability and the constant value d, the higher of the ZnO-NC, the field emission effect factor and field intensity will be bigger, therefore the field emission performance is outstanding.(4) Using the first-principles calculation method to study the field emission properties of different height of ZnO-NC. The results show that, in the case of structural stability and the tip diameter d is constant, Without considering the impact on the height of ZnO-NC stability, the tip field emission effect factor β increased along with the height, according to the calculation of electronic density, Mulliken charges, energy gap and effective work function, h had a significant effect on the tip field emission effect factor, the field emission properties improved effectively by controlling the ZnO-NC height.