Interstitial Zn Research Articles

Enhancing blue luminescence from Ce-doped ZnO nanophosphor by Li doping.

Undoped ZnO, Ce-doped ZnO, and (Li, Ce)-codoped ZnO nanophosphors were prepared by a sol-gel process. The effects of the additional doping with Li ions on the crystal structure, particle morphology, and luminescence properties of Ce-doped ZnO were investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy and photoluminescence spectroscopy. The results indicate that the obtained samples are single phase, and a nanorod shaped morphology is observed for (Li, Ce)-codoping. Under excitation with 325 nm light, Ce-doped ZnO phosphors show an ultraviolet emission, a green emission, and a blue emission caused by Zn interstitials. The spectrum of the sample codoped with a proper Li concentration features two additional emissions that can be attributed to the Ce3+ ions. With the increase of the Li doping concentration, the Ce3+ blue luminescence of (Li, Ce)-codoped ZnO is obviously enhanced, which results not only from the increase of the Ce3+ ion concentration itself but also from the energy transfer from the ZnO host material to the Ce3+ ions. This enhancement reaches a maximum at a Li content of 0.02, and then decreases sharply due to the concentration quench. These nanophosphors may promise for application to the visible-light-emitting devices.PACS78.55.Et; 81.07.Wx; 81.20.Fw

Room temperature diluted magnetism in Li, Na and K co-doped ZnO synthesized by solution combustion method

In this effort the alkali dopants Li, Na and K are co-doped with ZnO by solution combustion technique. This work is the production of room temperature ferromagnetism along with a rare orange emission from ZnO with equally concentrated Li, Na and K impurities. The XRD analysis reveals the hexagonal wurtzite structure and the substitution of the alkali in ZnO. The SEM and EDAX study shows the morphology and impurity free samples of ZnAO. The interstitial Zn atoms and the strain produced in the crystal structure due to alkali dopants is reflected in the FT Raman spectra. The band gap determined from the UV-Vis-NIR shows a variation that follows the variation of the grain size. The PL emission spectra brings to limelight the abundance of interstitial O atoms that involves in orange emission. The oxygen vacancies in the sample produce a weak green emission that decreases and then increases with alkali content. There is also a weak UV emission due to bound acceptor–donor recombination. The magnetometry establishes the fact that the direct bandgap semiconductor ZnO becomes magnetically sensitive due to the presence of alkali thereby making ZnO:Li+Na+K as a member of the family of dilute magnetic semiconductors. To the best of our knowledge this is the first report on the ferromagnetism observed in Li, Na and K co-doped ZnO.

Augmentation of band gap and photoemission in ZnO by Li doping

The monovalent impurity Lithium is chosen to dope with Zinc oxide (ZnO) in four concentrations by auto-combustion route. The influence of Li on the structural and optical properties of ZnO are discussed. The Li incorporation happens both as substitution and interstitial doping with an increase of grain size and the optical band gap of ZnO. The optical phonon modes are identified from Raman spectra that also gives information about the stress in the samples. The UV and visible emission characteristics of the samples are found from the fluorescence spectra. The origin of the visible emission is explained by defect chemistry. When Li lodges Zn site new acceptor levels of Li are created that causes the yellow emission that is absent in undoped ZnO. Li interstitial creates Zn interstitials that are responsible for blue emission. The green emission is explained as the outcome of the transition between Zni and oxygen vacancies.

Near-infrared emission from ZnO nanorods grown by thermal evaporation

We report the growth of ZnO nanorods on Si/SiO2 subtrates by the thermal evaporation method at different distances (substrate temperatures) from vapor source to substrates. SEM images showed that morphologies of nanorods were significantly affected by distance from the substrate to vapor source. Energy dispersive X-ray spectroscopy (EDS) spectra present change of the ratio of zinc to oxygen in ZnO nanostructures as the substrate temperature varied. X-ray diffraction patterns revealed that the prepared ZnO nanorods are preferentially oriented in the c-axis at lower substrate temperature. The shift towards small angle of the XRD pattern peaks is consistent with the presence of the redundant zinc and the lack oxygen in the ZnO lattice. The photoluminescence (PL) spectra of the ZnO nanorods show beside the near band edge UV emission, a very broad emission ranges from green to near-infrared (NIR). The NIR emission is interpreted as due to the transition of carriers between radiative recombination centers related to Zn interstitials and oxygen interstitials.

Electrochemical Self-Assembly of ZnO Nanosheetlike Structures

ZnO nanosheetlike structures were synthesized on zinc (Zn) foil substrates by electrochemical deposition method in ZnCl2aqueous solutions at a temperature of 90 °C. In addition, the synthetic parameters in this work allow additional structural direction for ZnO nanoscaled structures. The morphology growth from smooth plane structures to nanosheet like structures could be accomplished by modifying the current densities of electrodeposition. In the photoluminescence (PL) spectra of the as-synthesized ZnO samples, typically there are few oxygen vacancies or interstitial Zn centers would be produced when the electrochemical deposition was performed out with a low current density. The UV peak is usually considered as the characteristic emission of ZnO nanosheetlike structures and attributed to the band edge emission or the exciton transition. All XRD diffraction peaks of ZnO nanosheetlike structures are shown in a good agreement with hexagonal structure. The average particle size was calculated using the Debye-Scherrer formula. ZnO nanosheetlike structures processed for various current densities have different size.

Nature of red luminescence band in research-grade ZnO single crystals: A “self-activated” configurational transition

By implanting Zn+ ions into research-grade intentionally undoped ZnO single crystal for facilitating Zn interstitials (Zni) and O vacancies (VO) which is revealed by precise X-Ray diffraction rocking curves, we observe an apparent broad red luminescence band with a nearly perfect Gaussian lineshape. This red luminescence band has the zero phonon line at ∼2.4 eV and shows distinctive lattice temperature dependence which is well interpreted with the configurational coordinate model. It also shows a low “kick out” thermal energy and small thermal quenching energy. A “self-activated” optical transition between a shallow donor and the defect center of Zni-VO complex or VZnVO di-vacancies is proposed to be responsible for the red luminescence band. Accompanied with the optical transition, large lattice relaxation simultaneously occurs around the center, as indicated by the generation of multiphonons.

Structural and Optical Properties of Al Co-Doped ZnCoO Thin Film

Al co-doped ZnCoO thin film has been prepared by a sol-gel method. The structural and optical properties of the sample were investigated. X-ray diffraction and UV absorption spectroscopy analyses indicate that Al3+ and Co2+ substitute for Zn2+ without changing the wurtzite structure. With the Al doping, the visible emission increased and the UV emission decreased, which is attributed to the increase of O vacancies and Zn interstitials.

Observation of negative differential resistance and electrical bi-stability in chemically synthesized ZnO nanorods

Zinc oxide nanorods/p-Si heterostructures have been fabricated by depositing the chemically synthesized ZnO nanorods on p-type silicon substrate. Heterostructure shows electrical bi-stability and negative differential resistance (NDR) only at the beginning of the forward bias region, and these phenomena have been explained with the help of energy band diagram. An explanation is proposed for the origin of electrical bi-stability in light of the electric field induced charge transfer across the junction, and the NDR phenomena could be attributed to interfacial traps and defect level that arises due to oxygen and zinc interstitial vacancies. Room temperature photoluminescence measurement of ZnO nanorods exhibits the emission peaks at about 466 nm and 566 nm which are attributed to oxygen vacancies and Zn interstitials. A correlation between NDR and blue emission phenomena in the ZnO nanorods due to defects states has been established.

Structural and Optical Properties of (Al, Co) Codoped-ZnO Thin Film

Zn0.99-xCo0.01AlxO (x=0, 0.01,) thin films have been prepared by a sol-gel method. The structural and optical properties of the samples were investigated. X-ray diffraction and EDX spectrum indicate that Al3+ and Co2+ substitute for Zn2+ without changing the wurtzite structure. With the Al doping,the intensity ratio of the visible to UV emission increases, which is attributed to the increase of O vacancies and Zn interstitials .

Green emission in carbon doped ZnO films

The emission behavior of C-doped ZnO films, which were prepared by implantation of carbon into ZnO films, is investigated. Orange/red emission is observed for the films with the thickness of 60–100 nm. However, the film with thickness of 200 nm shows strong green emission. Further investigations by annealing bulk ZnO single crystals under different environments, i.e. Ar, Zn or C vapor, indicated that the complex defects based on Zn interstitials are responsible for the strong green emission. The existence of complex defects was confirmed by electron spin resonance (ESR) and low temperature photoluminescence (PL) measurement.

Annealing temperature dependence of the structures and properties of Co-implanted ZnO films

• To avoid the forming of Co clusters and explore the origin of the magnetism, detailed investigation on the properties of the Co-implanted ZnO films with a rather low dose of 8 × 10 15 cm −2 and high implantation energy of 1 MeV were carried out. • The crystalline structure of the damaged region caused by ion-implantation has been recovered via the thermal annealing treatment at the temperature of 900 °C and above. • The low temperature magnetic hysteresis loops have indicated paramagnetism for the annealed films with weak ferromagnetic characteristics. • The zero-field cooling (ZFC) magnetization curves of the Co-implanted ZnO samples have varied from concave shape to convex one as the annealing temperature increased from 750 °C to 1000 °C. The effects of thermal annealing treatment on the structural, electrical, optical and magnetic properties of Co-implanted ZnO (0 0 0 1) films have been investigated in detail. The crystalline structure of the damaged region caused by ion implantation has been recovered via the thermal annealing at the temperature of 900 °C and above, and no Co clusters or its related oxide phases have been observed. The electrical and optical properties of the annealed films have shown strong dependence on the annealing temperature. The zero field cooling magnetization curves of the annealed films have varied from concave shape to convex one as the annealing temperature increased from 750 °C to 1000 °C, which are possibly tuned by the changes of the ratio of the itinerant carriers over the localized spin density. The low temperature magnetic hysteresis loops have indicated paramagnetic behavior for the annealed films with weak ferromagnetic characteristics. The ferromagnetism is attributed to the substituted Co 2+ ions and vacancy defects, while the paramagnetism could be induced by ionized interstitial Zn defects.

Room temperature ferromagnetism and photoluminescence of nanocrystalline Zn1 − xCoxO thin films prepared by sol–gel MOD

Zn1 − xCoxO (ZC) [x = 0, 1, 3, 5, 7 and 9 mol%] thin films were prepared by sol–gel combined metallo-organic decomposition method. The films were deposited on the Si substrate with spin-coating technique and annealed at 600 °C for 3 h. X-ray diffraction pattern shows the formation hexagonal wurtzite phase and distortion (c/a) decreases with increasing Co concentration in ZnO. The average grain size is measured using Scherer relation. Atomic force microscopy is used to confirm the formation of nanograins resulted by the use of polyethylene glycol as surfactant. The photoluminescence was recorded by using He-Cd laser of excitation wavelength 325 nm in wavelength region of 350–650 nm which exhibits some influence of Co doping on the multiplication of defects such as O vacancies, Zn interstitials and grain boundary defects. All thin films show room temperature ferromagnetism except pure ZnO which is diamagnetic and 9 mol% of Co shows paramagnetism. This behaviour is interpreted as due to fluctuations in the magnetic ordering, depending on grain size and site location in grain boundaries or oxygen vacancies.

The study on crystal defects-involved energy transfer process of Eu3+ doped ZnO lattice

Eu3+ doped ZnO nanoparticles are synthesized by solvothermal reaction and following annealing process. When molar ratio of Eu/Zn is increased from 0.01 to 0.03, a small percentage of Eu3+ is slowly separated from host ZnO lattice accompanied by decrease of average particle size. However, the proportion of individual Eu2O3 particles in doped ZnO is extremely low. After doping, a great deal of crystal defects are generated in host ZnO lattice, in which O vacancy, interstitial Zn and interstitial O are involved in energy transfer from host lattice to Eu3+. Among them interstitial O plays an important bridge role in this process.

Chemical states and ferromagnetism in heavily Mn-substituted zinc oxide thin films

A concentration of Manganese as high as 8% was successfully diluted into Zinc Oxide epitaxial films deposited by pulsed laser deposition. The films showed strong ferromagnetism with a large coercivity. Low temperature X-ray absorption spectroscopy measurements indicated that all the Manganese ions substitute for Zinc sites of the wurtzite lattice in the valency of +2. Photoluminescence measurements excluded the presence of Zinc vacancies, as well as Zn interstitials. All the magnetic moments measured were to ascribe to the formation of bound magnetic polarons, with no other contribution due to Manganese-secondary phases or Zinc vacancy-mediated double exchange interaction.

Defect induced ferromagnetism in undoped ZnO nanoparticles

Undoped ZnO nanoparticles (NPs) with size ∼12 nm were produced using forced hydrolysis methods using diethylene glycol (DEG) [called ZnO-I] or denatured ethanol [called ZnO-II] as the reaction solvent; both using Zn acetate dehydrate as precursor. Both samples showed weak ferromagnetic behavior at 300 K with saturation magnetization Ms = 0.077 ± 0.002 memu/g and 0.088 ± 0.013 memu/g for ZnO-I and ZnO-II samples, respectively. Fourier transform infrared (FTIR) spectra showed that ZnO-I nanocrystals had DEG fragments linked to their surface. Photoluminescence (PL) data showed a broad emission near 500 nm for ZnO-II which is absent in the ZnO-I samples, presumably due to the blocking of surface traps by the capping molecules. Intentional oxygen vacancies created in the ZnO-I NPs by annealing at 450 °C in flowing Ar gas gradually increased Ms up to 90 min and x-ray photoelectron spectra (XPS) suggested that oxygen vacancies may have a key role in the observed changes in Ms. Finally, PL spectra of ZnO showed the appearance of a blue/violet emission, attributed to Zn interstitials, whose intensity changes with annealing time, similar to the trend seen for Ms. The observed variation in the magnetization of ZnO NP with increasing Ar annealing time seems to depend on the changes in the number of Zn interstitials and oxygen vacancies.

Tuning of energy gap, microstructure, optical and structural properties of Cr doped Zn0.96Cu0.04O nanoparticles

Zn0.96−xCu0.04CrxO nanoparticles with different Cr concentrations from 0 to 4% were successfully synthesized by co-precipitation method. The X-ray diffraction pattern showed the crystalline nature with hexagonal wurtzite structure. The diminution in activation energy and the enhanced strain by Cr-doping from 0 to 3% reduced the average crystal size from 27 to 23.8nm while the formation of secondary phase such as spinel ZnCr2O4 enhanced it to 25.6nm after Cr=3% via increasing the activation energy. The solubility limit of Cr in the present system is fixed as 3%. The energy dispersive X-ray spectra confirmed the presence of Cu and Cr in Zn-O. The optical absorption spectra showed the three distinct bands corresponding to UV region (305–324nm) due to electronic transition between the bands; blue region (382–386nm) mainly due to the Zn/Cr interstitials and bluish green (481–482nm) region due oxygen vacancies and interstitials. The noticed poor transmittance throughout the region and the broader absorption bands in Cr=1% doped Zn0.96Cu0.04O confirmed that it had more defects such as oxygen vacancies, Zn interstitials and other defects. The blue shift of energy gap from 3.68eV (Cr=0%) to 3.71eV (Cr=1%) was due to the creations of more defects. The red shift of Eg from 3.71eV (Cr=1%) to 3.6eV (Cr=3%) was explained by sp–d exchange interactions between the band electrons and the localized d-electrons of the Cr3+ ions. Presence of chemical bonding was confirmed by FTIR spectra.

Role of zinc interstitials and oxygen vacancies of ZnO in photocatalysis: a bottom-up approach to control defect density.

Oxygen vacancies (V(O)s) in ZnO are well-known to enhance photocatalytic activity (PCA) despite various other intrinsic crystal defects. In this study, we aim to elucidate the effect of zinc interstitials (Zn(i)) and V(O)s on PCA, which has applied as well as fundamental interest. To achieve this, the major hurdle of fabricating ZnO with controlled defect density requires to be overcome, where it is acknowledged that defect level control in ZnO is significantly difficult. In the present context, we fabricated nanostructures and thoroughly characterized their morphological (SEM, TEM), structural (XRD, TEM), chemical (XPS) and optical (photoluminescence, PL) properties. To fabricate the nanostructures, we adopted atomic layer deposition (ALD), which is a powerful bottom-up approach. However, to control defects, we chose polysulfone electrospun nanofibers as a substrate on which the non-uniform adsorption of ALD precursors is inevitable because of the differences in the hydrophilic nature of the functional groups. For the first 100 cycles, Zn(i)s were predominant in ZnO quantum dots (QDs), while the presence of V(O)s was negligible. As the ALD cycle number increased, V(O)s were introduced, whereas the density of Zn(i) remained unchanged. We employed PL spectra to identify and quantify the density of each defect for all the samples. PCA was performed on all the samples, and the percent change in the decay constant for each sample was juxtaposed with the relative densities of Zn(i)s and V(O)s. A logical comparison of the relative defect densities of Zn(i)s and V(O)s suggested that the former are less efficient than the latter because of the differences in the intrinsic nature and the physical accessibility of the defects. Other reasons for the efficiency differences were elaborated.

Interstitial zinc clusters in zinc oxide

Doped zinc oxide (ZnO) exhibits anomalous Raman modes in the range of 270 to 870 cm${}^{\ensuremath{-}1}$. Commonly, the resonance at 275 cm${}^{\ensuremath{-}1}$ is attributed to the local vibration of Zn atoms in the vicinity of extrinsic dopants. We revisit this assignment by investigating the influence of isotopically purified zinc oxide thin films on the frequency of the vibrational mode around 275 cm${}^{\ensuremath{-}1}$. For this purpose, undoped and nitrogen-doped ZnO thin-films with Zn isotope compositions of natural Zn, ${}^{64}$Zn, ${}^{68}$Zn, and a 1:1 mixture of ${}^{64}$Zn and ${}^{68}$Zn were grown by pulsed laser deposition. The isotopic shift and the line shape of the Raman resonance around 275 cm${}^{\ensuremath{-}1}$ are analyzed in terms of three different microscopic models, which involve the vibration of (i) interstitial zinc atoms bound to extrinsic defects, (ii) interstitial diatomic Zn molecules, and (iii) interstitial zinc clusters. The energy diagram of interstitial Zn-Zn bonds in a ZnO matrix is derived from density functional theory calculations. The interstitial Zn-Zn bond is stabilized by transferring electrons from the antibonding orbital into the ZnO conduction band. This mechanism facilitates the formation of interstitial Zn clusters and fosters the common $n$-type doping asymmetry of ZnO.

Influence of reactant concentration on optical properties of ZnO nanoparticles

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 ‘Zni’) 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.

Effect of Substrate Temperature on Structural and Optical Properties of ZnO:Co Thin Films Fabricated by Laser Molecular Beam Epitaxy

We synthesized ZnO:Co thin films on sapphire (0001) substrates by laser molecular beam epitaxy (LMBE) method. X-ray diffraction (XRD) spectra indicated that all samples possessed wurtzite structure with the preferential c-axis-orientation and the value of the c-axis lattice decreased with increasing substrate temperature. UVvis transmittance spectra and X-ray photoelectron spectroscopy (XPS) implied that Co2+ions incorporated into ZnO lattice with substitution for Zn2+ions and the non-bivalent Zn existed in as-prepared thin films. Two emission bands located at 418 nm (2.97 eV) and 490 nm (2.53 eV) were observed from the photoluminescence (PL) spectra of all samples. The two emission bands were in relation to Zn interstitials and the complex of VOand Zni(VOZni). The quantity of the Zn interstitials remained mostly invariable as substrate temperature increased.