Doping of RE ions in the 2D ZnO layered system to achieve low-dimensional upconverted persistent luminescence based on asymmetric doping in ZnO systems
Herein, we dope a low-dimensional ZnO system with a wide range of rare earth (RE) ions. Through systematic calculations, the dopable range of all ZnO systems was found to be asymmetrical, which accounts for the difficulty in achieving p-type doping. Low-dimensional ZnO systems, similar to 2D graphene-like nanosheets, have a wider doping limit. Thus, 2D ZnO is a promising candidate to achieve a wider doping range in ZnO. To further examine energy transfer in upconversion luminescence, the excited states of all lanthanide (Ln) elements in both the Ln2+ and Ln3+ ionic state in the bulk ZnO lattice were extensively studied. The probability of mixed valences of the Ln dopant ions occurring in ZnO was discussed, along with the analysis of the relative oscillator strengths. At the Ln2+ states, the heavy lanthanide elements usually dominated the energy transmission channel at high energy, the medium lanthanide elements mostly occupied the middle range of the optical fundamental gap, and the light lanthanide elements were widely spread over the optical band gap, as well as the conduction band range. However, Ln3+ ions, as the sensitizing dopant, have reduced energy barriers for excited state absorption, showing wider energy transfer channels that are evenly distributed within 3.0 eV, which is lower than the conduction band edge absorption in Ln2+. Meanwhile, each energy level has an obviously stronger oscillator strength, indicating a larger probability for excitation and energy transport between the inter-levels. Thus, in physicochemical and biological terms, trivalent Ln doping follows the removal of apical dominance concept, contributing more flexible energy transfer within the biological window for in vivo imaging or other related optoelectronic devices.