Pure ZnO with cool white, warm white, orange and turquoise color emissions: a purely experimental approach

Abstract

Pure ZnO microstructured phosphor of cool white, warm white, orange and turquoise color was prepared by chemical bath deposition (CBD). The color variation was solely a consequence of controlled variation in the relative contents of the native crystalline defect. Only hexagonal wurtzite phase of ZnO with good stoichiometry without additional phases and without crystalline parameters variation was observed. The band gap energy was successfully maintained between 3.05 and 3.22 eV, with an additional high optical absorption edge at 551 nm, associated with the excess of Zn ions in crystalline structure of ZnO. The absorption centers were categorized into thirteen bands and associated with energy levels related to native defects. The photoluminescence (PL) and cathodoluminescence (CL) emission spectra coincidentally exhibited four main peaks around 465 nm, 504 nm, 596 nm and 646 nm. The deconvolution of these spectra revealed emission bands that were categorized into 15 energy ranges. The energy associated with these bands closely matches the energy of the absorption bands identified by transmittance and reflectance spectroscopy without the need to consider the existence of additional acceptor levels close to the valence band that have not been observed experimentally, which allowed to propose a purely experimental energy diagram for ZnO. HR-TEM analysis revealed that for the cool white emitting ZnO, an irregular and incomplete hexagonal microstructure could be responsible for its native defect content that gives rise to its particular cool white emission.