We report a novel effect that the visible photoluminescence (in the blue-green band) from ZnO anoparticles can be controlled by changing the ionic or polar nature of the medium in which the nanoparticles are dispersed. We find that the presence of sufficient amount of electrolytes can even quench the emission. We propose an explanation based on surface charge of the ZnO nanoparticles which control the band bending in the depletion layer at the surface of the anoparticles. The band bending in turn, decides the predominant nature of the visible emission. The explanation is validated by establishing a direct correlation between the visible emission and the zeta potential.
We present a theoretical study of broadening of defect luminescence bands due to vibronic coupling. Numerical proof is provided for the commonly used assumption that a multi-dimensional vibrational problem can be mapped onto an effective one-dimensional configuration coordinate diagram. Our approach is implemented based on density functional theory with a hybrid functional, resulting in luminescence lineshapes for important defects in GaN and ZnO that show unprecedented agreement with experiment. We find clear trends concerning effective parameters that characterize luminescence bands of donor…
We analyse the morphological, structural and luminescence properties of self-assembled ZnO nanowires grown by chemical vapour transport on Si(001). The examination of nanowire ensembles by scanning electron microscopy reveals that a non-negligible fraction of nanowires merge together forming coalesced aggregates during growth. We show that the coalescence degree can be unambiguously quantified by a statistical analysis of the cross-sectional shape of the nanowires. The examination of the structural properties by X-ray diffraction evidences that the nanowires crystallize in the wurtzite phase, …
In this work, a reversible control over the visible luminescence of phosphor ZnO is achieved by the application of a few volts (<5V) to nanostructured ZnO film sandwiched between ITO and LiClO4/PEO solid electrolyte. Since ZnO is a good absorbing material but ITO-Glass substrate is transparent in the range of 320-375 nm, the ZnO-ITO interface has been illuminated by a 345 nm light passing through the ITO glass substrate and the emitted light near 545 nm has been collected in reflection mode. When the solid polymer electrolyte is negatively biased, up to 107 percent enhancement in the visible l…
Uniaxial hot pressing has been used to obtain ceramics based on zinc oxide, and their optical, x-ray-structure, luminescence, and scintillation characteristics have been studied. It is shown that, by changing the concentration of the dopant (Ga) and the codopant (N), it is possible to change the intensities of the edge band (397.5 nm) and the intraband luminescence (510 nm) of the ZnO luminescence, as well as their ratio. Undoped ZnO ceramic has good transparency in the visible region and fairly high luminous yield: 9050 photons per MeV. Ceramic ZnO:Ga possesses intense edge luminescence with …
Er-doped phosphate glasses were fabricated by melt-quenching technique. The changes in their thermal, structural and luminescence properties with the addition of Al2O3, TiO2 or ZnO were studied. Physical and thermal properties were investigated through density measurement and differential thermal analysis. Structural characterization was performed using the Raman and Infrared spectroscopy. In order to study the influence of the composition on the luminescence properties of the glasses, the refractive index, the luminescence spectra and the lifetime values were measured. The results show that w…
There are two characteristic optical transitions associated with Cu ions in ZnO, the 0.72 eV infrared and the 2.86 eV structured green luminescence (SGL). While the former is unambiguously related with the d(Cu2+) intra-shell transition, there is no generally accepted mechanism of the SGL. Besides the original model of Dingle [Phys. Rev. Lett. 23, 579 (1969)], two other mechanisms were recently proposed. We report an analysis of the optical properties of Cu in ZnO by ab initio calculations. The GGA+U approach is used, with the +U corrections applied to d(Zn), p(O) and d(Cu) orbitals. The resul…
Thin films of laser molecular-beam epitaxy grown \textit{n}-type Ga-doped ZnO were investigated with respect to their optical properties. Intense room-temperature photoluminescence (PL) in the near-band edge (NBE) region was observed. Moreover, its broadening of PL band was significantly larger than predicted by theoretical results modeled in terms of potential fluctuations caused by the random distribution of donor impurities. In addition, the lineshape was rather asymmetrical. To explain these features of the NBE bands, a vibronic model was developed accounting for contributions from a serie…
The spectral--kinetic characteristics of a ZnO:Ga single crystal upon excitation in the vacuum UV region have been studied. At a temperature of 8 K, the exciton luminescence line peaking at 3.356 eV has an extremely small half-width (7.2 meV) and a short decay time (360 ps). In the visible range, a wide luminescence band peaking at ~2.1 eV with a long luminescence time at 8 K and a decay time in the nanosecond range at 300 K is observed. The luminescence excitation spectra of ZnO:Ga have been measured in the range from 4 to 12.5 eV
We investigated the optical properties of epitaxial \textit{n}-type ZnO films grown on lattice-matched ScAlMgO$_4$ substrates. As the Ga doping concentration increased up to $6 \times 10^{20}$ cm$^{-3}$, the absorption edge showed a systematic blueshift, consistent with the Burstein-Moss effect. A bright near-bandedge photoluminescence (PL) could be observed even at room temperature, the intensity of which increased monotonically as the doping concentration was increased except for the highest doping level. It indicates that nonradiative transitions dominate at a low doping density. Both a Sto…
The spectral characteristics of ZnO:Ga and ZnO:Ga,N ceramics prepared by uniaxial hot pressing have been investigated. At room temperature, the edge (exciton) band at 3.12 eV dominates in the luminescence spectra of ZnO:Ga, while a wide luminescence band at 2.37 eV, which is likely to be due to zinc vacancies, is observed in the spectra of ZnO:Ga,N. Upon heating, the edge band maximum shifts to lower energies and the bandwidth increases. The extrapolated position of the edge-band maximum at zero temperature, Em(0) = 3.367 +/- 0.005 eV, is in agreement with the data for thin zinc oxide films. T…
CdSe quantum dots (QDs) with different organic linker molecules are attached to ZnO nanowires (NWs) to study the luminescence dynamics and the electron tunneling from the QDs to the nanowires in time-resolved photoluminescence (PL) and photoconductivity measurements. The PL transients of the QD luminescence indicate two different recombination channels: the direct recombination inside the QD core and the recombination via QD surface defect states. After linking the QDs to the ZnO NW surface, photo-induced electron tunneling from an excited state of the QD into the conduction band of the nanowi…
This work explores the Zn vacancy in ZnO using hybrid density functional theory calculations. The Zn vacancy is predicted to be an exceedingly deep polaronic acceptor that can bind a localized hole on each of the four nearest-neighbor O ions. The hole localization is accompanied by a distinct outward relaxation of the O ions, which leads to lower symmetry and reduced formation energy. Notably, we find that initial symmetry-breaking is required to capture this effect, which might explain the absence of polaronic hole localization in some previous hybrid density functional studies. We present a …
ZnO films have been grown on gold (111) by electrodeposition using two different OH- sources, nitrate and peroxide, in order to obtain a comparative study between these films. The morphology, structural and optical characterization of the films were investigated depending on the solution used (nitrate and peroxide) and the applied potential. Scanning Electron Microscopy pictures show different morphologies in each case. X-Ray Diffraction confirms that the films are pure ZnO oriented along the (0002) direction. ZnO films have been studied by photoluminescence to identify the emission of defects…
We generalize and systematize basic experimental data on optical and luminescence properties of ZnO single crystals, thin films, powders, ceramics, and nanocrystals. We consider and study mechanisms by which two main emission bands occur, a short-wavelength band near the fundamental absorption edge and a broad long-wavelength band, the maximum of which usually lies in the green spectral range. We determine a relationship between the two luminescence bands and study in detail the possibility of controlling the characteristics of ZnO by varying the maximum position of the short-wavelength band. …
In this paper we show that the ionic conduction through surface chemisorbed ambient moisture leads to the remarkably high room temperature selective response towards hydrogen gas. The surface adsorbed moisture acts as surface states and shows ionic conduction, as a result of smaller size of ZnO nanoparticles of 20 +/- 5 nm. This response is enhanced remarkably i.e. from 10% to 190% for 1200 ppm H2 gas when p-type NiO quasi-nanowires (width ~50 nm) are mixed with these n-type ZnO nanoparticles to form a homogenous NiO/ZnO nano-bulk p-n heterostructure. The maximum response is obtained for about…
Optical properties of ZnO nanoparticles prepared from a simple chemical method using sodium zincate bath show strong white light emission. X-ray absorption fine structure studies reveal a completely different local environment around Zn in these ZnO nanoparticles. The observed luminescence properties and local structural changes have been explained on the basis of a linkage between Zn and OH- ions in the surface layers of ZnO nanoparticles.
The effect of thermal annealing on wurtzite ZnO, terminated by two surfaces, (0 0 0 $\bar 1$) (which is oxygen-terminated) and (0 0 0 1) (which is Zn-terminated), is investigated via molecular dynamics simulation using reactive force field (ReaxFF). As a result of annealing at a threshold temperature range of 700~K $ < T_{\mbox{\small t}} \leq 800$~K, surface oxygen atoms begin to sublimate from the (0 0 0 $\bar 1$) surface, while no atom leaves the (0 0 0 1) surface. The ratio of oxygen leaving the surface increases with temperature $T$ (for $T \geq T_{\mbox{\small t}}$). The relative lumines…
Ionic layer epitaxy (ILE) has recently been developed as an effective strategy to synthesize nanometer-thick 2D materials with a non-layered crystal structure, such as ZnO. The packing density of the amphiphilic monolayer is believed to be a key parameter that controls the nanosheet nucleation and growth. In this work, we systematically investigated the growth behavior of single-crystalline ZnO nanosheets templated at the water-air interface by an anionic oleylsulfate monolayer with different packing densities. The thicknesses of ZnO nanosheets were tuned from one unit cell to four unit cells,…
Previous luminescence and absorption experiments in Co-doped ZnO revealed two ionization and one intrashell transition of $d(\textrm{Co}^{2+})$ electrons. Those optical properties are analyzed within the generalized gradient approximation to the density functional theory. The two ionization channels involve electron excitations from the two $\textrm{Co}^{2+}$ gap states, the $t_{2\uparrow}$ triplet and the $e_{2\downarrow}$ doublet, to the conduction band. The third possible ionization channel, in which an electron is excited from the valence band to the $\textrm{Co}^{2+}$ level, requires ener…
Samarium doped Na$_2$O-ZnO-TeO$_2$ (NZT) glasses were prepared by the melt quenching method. The glass-forming ability and glass stability of prepared glass was estimated by Hruby parameter using Differential Thermal Analysis (DTA) and Thermo-gravimetric Analysis (TGA). The study of FTIR spectra and X-ray diffraction described the ionic nature and the amorphous pattern of glass respectively. The absorption peaks were observed for the transitions 6H5/2-4P3/2 at 402 nm,6H5/2-4M19/2 at 418 nm, 6H5/2-4I15/2 at 462 nm and 6H5/2-4I11/2 at 478 nm in the absorption spectra. The optical band gap energy…
We present a theoretical study on the impact of an active optical layer on the emission properties of an ultrathin luminescent film. While the study can be generalized to any material, we focus here on a simple layered medium composed of a conjugated polymers (CPs) thin film, a zinc oxide layer (ZnO) and a sapphire substrate. The study spreads throughout variable aspects including the effect of the structure parameters on the CP luminescence and radiation pattern and more specifically the influence of the absorption and emission properties of the active layer. Comparing between the passive and…
