Tunable Photoluminescent and Cathodoluminescent Properties of ZnO and ZnO:Zn Phosphors

Light Emission from Single Oxygen Vacancies in Cu2O Films Probed with Scanning Tunneling Microscopy

Global photoluminescence (PL) and spatially resolved scanning tunneling microscopy (STM) luminescence are compared for thick Cu₂O films grown on Au(111). While the PL data reveal two peaks at 750 and 850 nm, assigned to radiative electron decays via localized gap states induced by O vacancies, a wide-band emission between 700 and 950 nm is observed in STM luminescence. The latter is compatible with cavity plasmons stimulated by inelastic electron tunneling and contains no spectral signature of the Cu₂O defects. The STM luminescence is nonetheless controlled by O vacancies that provide inelastic excitation channels for the cavity plasmons. In fact, the emission yield sharply peaks at 2.2 V sample bias, when tip electrons are resonantly injected into O defect states and recombine with holes at the valence-band top via plasmon stimulation. The spatially confined emission centers detected in photon maps of the Cu₂O films are therefore assigned to excitation channels mediated by single or few O vacancies in the oxide matrix.

Preparation of Polycarbonate-ZnO Nanocomposite Films: Surface Investigation after UV Irradiation

Polycarbonate (PC)-ZnO films with different percentages of ZnO were prepared by a solution stirring technique and subjected to ultraviolet (UV; λ = 254 nm) irradiation. Structural parameters of the samples and the effects of UV irradiation on the surface properties of the PC and PC-ZnO nanocomposites were evaluated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), water contact angle (WCA) measurements, and a Vickers microhardness (H_V) tester. The XRD patterns of the nanocomposite films were found to show an increase in crystallinity with the increasing ZnO nanoparticles percentage. The WCA was found to be reduced from 90° to 17° after 15 h of UV irradiation, which could be ascribed to the oxidation of the surface of the samples during the irradiation and exposure of the ZnO nanoparticles, a result that is also supported by the obtained XPS data. The microhardness value of the PC-ZnO films including 30 wt.% ZnO enhanced considerably after UV radiation, which can also be attributed to the exposition of the ZnO nanoparticles after photodegradation of the PC superficial layer of the nanocomposite films.

Research of Fluorescent Properties of a New Type of Phosphor with Mn2+-Doped Ca2SiO4

Fluorescent optical fiber temperature sensors have attracted extensive attention due to their strong anti-electromagnetic interference ability, good high-voltage insulation performance, and fast response speed. The fluorescent material of the sensor probe directly determines the temperature measurement effect. In this paper, a new type of fluorescent material with a Mn2+-doped Ca2SiO4 phosphor (CSO:Mn2+) is synthesized via the solid-state reaction method at 1450 °C. The X-ray diffraction spectrum shows that the sintered sample has a pure phase structure, although the diffraction peaks show a slight shift when dopants are added. The temperature dependence of the fluorescence intensity and lifetime in the range from 290 to 450 K is explored with the help of a fluorescence spectrometer. Green emission bands peaking at 475 and 550 nm from Mn2+ are observed in the fluorescence spectra, and the intensity of emitted light decreases as the temperature rises. The average lifetime of CSO:Mn2+ is 17 ms, which is much higher than the commonly used fluorescent materials on the market. The fluorescence lifetime decreases with increasing temperature and shows a good linear relationship within a certain temperature range. The research results are of great significance to the development of a new generation of fluorescence sensors.

Giant defect emission enhancement from ZnO nanowires through desulfurization process

Zinc oxide (ZnO) is a stable, direct bandgap semiconductor emitting in the UV with a multitude of technical applications. It is well known that ZnO emission can be shifted into the green for visible light applications through the introduction of defects. However, generating consistent and efficient green emission through this process is challenging, particularly given that the chemical or atomic origin of the green emission in ZnO is still under debate. In this work we present a new method, for which we coin term desulfurization, for creating green emitting ZnO with significantly enhanced quantum efficiency. Solution grown ZnO nanowires are partially converted to ZnS, then desulfurized back to ZnO, resulting in a highly controlled concentration of oxygen defects as determined by X-ray photoelectron spectroscopy and electron paramagnetic resonance. Using this controlled placement of oxygen vacancies we observe a greater than 40-fold enhancement of integrated emission intensity and explore the nature of this enhancement through low temperature photoluminescence experiments.

Oxygen Vacancy Defects Boosted High Performance p-Type Delafossite CuCrO2 Gas Sensors

p-type ternary oxides can be extensively explored as alternative sensing channels to binary oxides with diverse structural and compositional versatilities. Seeking a novel approach to magnify their sensitivities toward gas molecules, e.g., volatile organic compounds (VOCs), will definitely expand their applications in the frontier area of healthcare and air-quality monitoring. In this work, delafossite CuCrO₂ (CCO) nanoparticles with different grain sizes have been utilized as p-type ternary oxide sensors. It was found that singly ionized oxygen vacancies (V_o^•) defects, compared with the grain size of CCO nanoparticles, play an important role in enhancing the charge exchange at the VOCs molecules/CCO interface. In addition to suppressing the hole concentration of the sensor channel, the unpaired electron trapped in V_o^• provides an active site for chemisorptions of environmental oxygen and VOCs molecules. The synergetic effect is responsible for the observed increase of sensitivity. Furthermore, the sensitive (V_o^• defect-rich) CCO sensor exhibits good reproducibility and stability under a moderate operation temperature (<325 °C). Our work highlights that V_o^• defects, created via either in situ synthesis or postannealing treatment, could be explored to rationally boost the performance of p-type ternary oxide sensors.

Radiative mechanism and surface modification of four visible deep level defect states in ZnO nanorods

Visible luminescence from ZnO nanorods (NRs) is attracting large scientific interest for light emission and sensing applications. We study visible luminescent defects in ZnO NRs as a function of post growth thermal treatments, and find four distinct visible deep level defect states (VDLSs): blue (2.52 eV), green (2.23 eV), orange (2.03 eV), and red (1.92 eV). Photoluminescence (PL) studies reveal a distinct modification in the UV (3.25 eV) emission intensity and a shift in the visible spectra after annealing. Annealing at 600 °C in Ar (Ar600) and O2 (O600) causes a blue and red-shift in the visible emission band, respectively. All samples demonstrate orange emission from the core of the NR, with an additional surface related green, blue, and red emission in the As-Prep, Ar600, and O600 samples, respectively. From PL excitation (PLE) measurements we determine the onset energy for population of the various VDLSs, and relate it to the presence of an Urbach tail below the conduction band due to a presence of ionized Zni or Zni complexes. We measured an onset energy of 3.25 eV for the as prepared sample. The onset energy red-shifts in the annealed samples by about 0.05 to 0.1 eV indicating a change in the defect structure, which we relate to the shift in the visible emission. We then used X-ray photoemission spectroscopy (XPS), and elastic recoil detection analysis (ERDA) to understand changes in the surface structure, and H content, respectively. The results of the XPS and ERDA analysis explain how the chemical states are modified due to annealing. We summarize our results by correlating our VDLSs with specific intrinsic defect states to build a model for PL emission in ZnO NRs. These results are important for understanding how to control defect related visible emission for sensing and electroluminescence applications.

Nanoplate-Built ZnO Hollow Microspheres Decorated with Gold Nanoparticles and Their Enhanced Photocatalytic and Gas-Sensing Properties

Hierarchical porous ZnO microspheres decorated with gold nanoparticles (AuNPs) were successfully synthesized by a facile solvothermal route. The hierarchical ZnO superstructure was constructed of interconnected nanoplates with numerous voids. Photoluminescence, X-ray photoelectron spectroscopy, and electron paramagnetic resonance measurements demonstrated that the main defects were oxygen vacancies (V(O)(•)) with minor interstitial oxygen (O(i)(-)) in the hierarchical ZnO hollow microspheres. The as-prepared hierarchical ZnO hollow microspheres and the AuNPs used to decorate them were examined for their photocatalytic degradation ability and as gas sensors. The photodegradation results demonstrated that the degradation rate constant on rhodamine B for undecorated ZnO microspheres was 0.43 min(-1), which increased to 1.76 min(-1) for AuNP-decorated ZnO microspheres. The AuNP-functionalized ZnO microspheres displayed superior sensing properties, with a 3-fold enhancement in their gas response to 1 ppb of dibutyl phthalate.

Nature of defects in blue light emitting CaZrO3: spectroscopic and theoretical study

Defects responsible for intense blue emission in CaZrO₃ are investigated using PL and EPR spectroscopy and complimented by DFT calculations.

Additive-free 1,4-butanediol mediated synthesis: a suitable route to obtain nanostructured, mesoporous spherical zinc oxide materials with multifunctional properties

A family of mesoporous, self-aggregated zinc oxide materials with spherical morphologies of high crystalline quality, is obtained through a facile, additive-free polyol procedure.

Energy-transfer efficiency in Eu-doped ZnO thin films: the effects of oxidative annealing on the dynamics and the intermediate defect states

We have studied ultrafast dynamics in thin films of Eu-doped zinc oxide (ZnO), prepared by radio-frequency sputtering onto sapphire substrates. Following UV excitation of ZnO, a red emission is observed. Postdeposition annealing in an oxygen atmosphere improves the crystallinity and emission intensity of the films, which are highly sensitive to the dopant concentration. Transient-absorption spectroscopy shows that the excited semiconductor host transfers energy to rare-earth ions on a time scale of only a few picoseconds. The dynamics as a function of the probe wavelength change dramatically after annealing, with annealed films showing the fastest dynamics at much lower wavelengths. Our results show that annealing greatly affects the defect energy levels of the films and the dynamics of the trapped carriers. Unannealed films show dynamics consistent with energy transfer from O vacancies to the dopant, while energy transfer in annealed samples involves acceptor-type defects such as Zn vacancies as intermediates.

Introducing dual excitation and tunable dual emission in ZnO through selective lanthanide (Er3+/Ho3+) doping

Dual emission in Ho³⁺ doped ZnO showing red and green emission600 dpi in TIF format)??> under UV (DC) and IR (UC) excitation.

Luminescence of colloidal ZnO nanoparticles synthesized in alcohols and biological application of ZnO passivated by MgO

This report presents the results of spectroscopic measurements of colloidal ZnO nanoparticles synthesized in various alcohols. Luminescence of colloidal ZnO was monitored under different reaction conditions to elucidate the mechanism of the visible emission. We performed the process in different alcohols, temperatures and reaction times for two different reactants: water and NaOH. Based on the presented and previously published results it is apparent that the luminescence of the nanoparticles is influenced by several competing phenomena: the formation of new nucleation centers, the growth of the nanoparticles and surface passivation. Superimposed on the above effects is a size dependent luminescence alteration resulting from the quantum confinement. The study contributes to our understanding of the origin of ZnO nanoparticles' green emission which is important in a rational design of fluorescent probes for nontoxic biological applications. The ZnO nanoparticles were coated with a magnesium oxide layer and introduced into a HeLa cancer cell.