Photoluminesence and FTIR study of ZnO nanoparticles: the impurity and defect perspective (original) (raw)
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Correlations among size, defects, and photoluminescence in ZnO nanoparticles
Journal of Applied Physics, 2007
We studied the correlations among size, defects, and photoluminescence emissions in ZnO nanoparticles of sizes ranging from 25 to 73 nm. The impurities and defects were characterized by Fourier-transform infrared spectroscopy and Raman spectroscopy. Particles of larger size revealed fewer surface impurities and enhanced E 2 mode of hexagonal ZnO crystals, while the oxygen vacancy centers did not vary significantly with particle size. A simultaneous increase of excitonic luminescence and defect luminescence intensities with the increase of particle size is shown, indicating both emissions are subjected to nonradiative quenching by near surface defects. The study on the size-dependent green luminescence in our samples suggests that the emission might be a bulk property instead of having a surface origin in nanostructured ZnO. Two different radiative recombination processes are involved in the excitonic emission of ZnO. While the slow decay component ͑370 ps͒ did not depend on particle size, the fast component varied from 56 to 96 ps. We attribute the slow component to free exciton recombination, while the fast component is attributed to near surface exciton recombination.
The influence of vacuum and annealing on the visible luminescence in ZnO nanoparticles
Journal of Luminescence, 2017
The ZnO nanoparticles synthesized by a simple solution based chemical bath deposition method were characterized using different experimental techniques. Photoluminescence (PL) studies were performed in ambient as well as in vacuum conditions. The emission spectra showed two bands corresponding to UV emission at 380 nm and a wide visible luminescence centered at 571 nm due to surface defects in ambient conditions. Under vacuum condition, the spectra showed a reduction in the intensity of the wide visible luminescence and an enhancement in the UV emission. These nanoparticles were annealed at high temperatures in air. The wide visible luminescence remains at the same intensity in both ambient and in vacuum condition for the annealed samples indicating that some of the surface adsorbed defects are removed due to annealing. Fourier Transform Infrared Spectroscopy (FTIR) and Electron Spin Resonance (ESR) results confirm the presence of [OH-] related groups on the surface of the samples. An analysis of the O1s peak in ZnO using X-ray Photoelectron Spectroscopy (XPS) measurement confirms the presence of intrinsic defects such as oxygen related vacancies and adsorbed oxygen species in the sample. Our investigation shows that the green emission observed in ZnO samples is primarily due to oxygen vacancies.
Annealing effects on photoluminescence of ZnO nanoparticles
Materials Letters, 2013
In this study, the effects of annealing temperature on photoluminescence (PL) of ZnO nanoparticles were studied. ZnO was annealed at various temperatures between 600 and 900 1C. The X-ray diffraction (XRD) results demonstrated that grain size increased with increase of annealing temperature. As the annealing temperature increased from 600 to 800 1C, the intensities of both UV peak and that of green luminescence (GL) enhanced monotonously but reduced at 900 1C. The enhancement in the UV peak intensity is attributed to the decrease of grain boundaries and surface states; whereas, the remarkable improvement in the GL is assigned to the out-diffusion of oxygen from the sample up to 800 1C. It supports that GL is induced by the singly ionized oxygen vacancies. These oxygen vacancies are saturated due to the finiteness of the defects at 800 1C. So, it is speculated that the deterioration of GL intensity at 900 1C is due to the evaporation of Zn which is predominant at temperatures higher than 850 1C.
EPR and photoluminescence spectroscopy studies on the defect structure of ZnO nanocrystals
Structural and optical properties of ZnO nanoparticles can be fine tuned by a novel variant of milling performed at cryogenic temperatures. In this study intrinsic defect centers such as oxygen and zinc vacancies are characterized using electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. Three different surface defects with different g factors were identified by EPR for which the spectral intensities change upon decreasing the crystal size. EPR and PL intensities revealed a linear correlation giving detailed information about optical and electronic properties of ZnO. The core-shell model established from optical emission and EPR suggests distinguished electronic states in the band gap belonging to negatively charged Zn vacancies and positively charged oxygen vacancies. This model indicates a correlation between red emission and positively charged oxygen vacancies, which lead to a possible transition from a typical n-type to a p-type ZnO semiconductor.
physica status solidi (a), 2013
We report the controlled variation of luminescence of ZnO nanostructures from intense ultraviolet to bright visible light. Deliberate addition of surfactants in the reaction medium not only leads to growth anisotropy of ZnO, but also alters the luminescence property. ZnO nanoclusters comprising of very fine particles with crystallite sizes $15-22 nm were prepared in a non-aqueous medium, either from a single alcohol or from their mixtures. Introduction of the aqueous solution of the surfactant helps in altering the microstructure of ZnO nanostructure to nanorods, nanodumb-bells as well as the luminescence property. The as-prepared powder material is found to be well crystallized. Defects introduced by the surfactant in aqueous medium play an important role in substantial transition in the optical luminescence. Chromaticity coordinates were found to lie in the yellow region of color space. This gives an impression of white light emission from ZnO nanocrystals, when excited by a blue laser. Oxygen vacancy is described as the major defect responsible for visible light emission as quantified by X-ray photoelectron spectroscopy and Raman analysis.
How Annealing and Charge Scavengers Affect Visible Emission from ZnO Nanocrystals
The Journal of Physical Chemistry C, 2016
Simultaneous transmission infrared (IR) absorption and photoluminescence (PL) spectroscopies are used to reveal the correlation of free electron and defect densities during the stepwise annealing of ZnO nanocrystals in high vacuum. For increasing annealing temperatures between 700 and 1000 K, the free electron density increases with a negligible increase in PL yield. With increased annealing temperature above 1000 K, the free electron density decreases and the PL yield increases in inverse proportion. Accompanying the free electron loss are indications of increased bound charges and changes in the multiphonon bands in the infrared spectrum, which collectively suggest that structural change and defect formation accompanies the loss of free electrons and the increase in PL. Exposure of the previously annealed sample to electron (O 2) and hole (MeOH) scavengers shows that the buildup of holes quenches visible emission, while additional electrons have a marginal effect on the PL yield. Given that certain neutral donors bind excitons and facilitate energy transfer to visible emitting sites, the buildup of free holes appears to quench PL intensity by ionizing those neutral donors.
Journal of Applied Physics, 2010
The present article describes the size induced changes in the structural arrangement of intrinsic defects present in chemically synthesized ZnO nanoparticles of various sizes. Routine X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) have been performed to determine the shapes and sizes of the nanocrystalline ZnO samples. Detailed studies using positron annihilation spectroscopy reveals the presence of zinc vacancy. Whereas analysis of photoluminescence results predict the signature of charged oxygen vacancies. The size induced changes in positron parameters as well as the photoluminescence properties, has shown contrasting or non-monotonous trends as size varies from 4 nm to 85 nm. Small spherical particles below a critical size (~ 23 nm) receive more positive surface charge due to the higher occupancy of the doubly charge oxygen vacancy as compared to the bigger nanostructures where singly charged oxygen vacancy predominates. This electronic alteration has been seen to trigger yet another interesting phenomenon, described as positron confinement inside nanoparticles. Finally, based on all the results, a model of the structural arrangement of the intrinsic defects in the present samples has been reconciled.
Influence of reactant concentration on optical properties of ZnO nanoparticles
Materials Technology: Advanced Performance Materials, 2000
Zinc oxide (ZnO) nanoparticles have been prepared by wet chemical method from zinc acetate. Particle size was controlled by adjusting the reactant concentration. The size of nanoparticles was investigated using ultraviolet-visible absorption spectra and photoluminescence spectra. The present nanoparticles exhibit non-linear optical behaviour with blue shift of the wavelengths as the particle size decreases. Furthermore, yellow emission is observed in ambient air while it disappears in the presence of nitrogen gas and gets substituted by blue violet emissions. While the blue violet emissions are familiar and likely to be attributed to electronic transitions from localised states (e.g. shallow donor states on Zn interstitials 'Zn i ') or the conduction band edge to the valence band, the yellow emission in the absence of nitrogen remains unclear. Our results of the present investigation suggest that the bubbling with nitrogen should fill the oxygen vacancies, substitute the oxygen interstitials, passivate the dangling bonds and introduce shallow acceptor states, which allow electronic transitions with shorter wavelengths (i.e. blue violet emissions). In the absence of nitrogen, surface defects such as oxygen interstitials and Zn(OH) 2 and possibly other point defects become again active and induce deep acceptor states of ,1 eV above the valence band edge, which allow electronic transitions of longer wavelength (i.e. yellow emission). Our results are compared to several available experimental data and first principle calculations in order to support our claims and conclusions.
ZnO nanoparticles (ZnO–NPs) were synthesized using sol–gel method. The structural and optical properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL). In this study, the effects of annealing temperature on photoluminescence (PL) of ZnO-NPs were studied. ZnO was annealed at various temperatures between 500 and 800 °C. The X-ray diffraction (XRD) results demonstrated that grain size increased with increase of annealing temperature. The average size of the nanoparticles was determined by SEM as well as XRD data and found to be 50 nm after annealing at 800 °C. As the annealing temperature increased from 500 to 800 °C, the intensities of both UV peak and that of green luminescence (GL) enhanced monotonously. The enhancement in the UV peak intensity is attributed to the decrease of grain boundaries and surface states; whereas, the remarkable improvement in the GL is assigned to the out-diffusion of oxygen from the sample up to 800 °C. It supports that GL is induced by the singly ionized oxygen vacancies. These oxygen vacancies are saturated due to the finiteness of the defects at 800 °C.
Science of Advanced Materials, 2015
Size dependent elemental binding energies in solution phase synthesized zinc oxide (ZnO) nanoparticles have been investigated using X-ray Photoelectron Spectroscopy. It is shown that the reduction in particle size is accompanied with an increase in the density of oxygen vacancies and a red shift in the Zn2p 3/2 photoelectron binding energy. The concentration of oxygen vacancies have been shown to be further enhanced after an oxygen scavenging treatment of ZnO using hydrazine. Size induced compressive strain in ZnO nanoparticles investigated using X-ray diffraction technique have been shown to contribute to the increase in oxygen vacancy population. The nanoparticles have also been characterized using transmission electron microscopy, atomic force microscopy, optical absorption and luminescence spectroscopy. The oxygen vacancy related defect states have been shown to give rise to green luminescence band in ZnO.