Zinc Vacancy Defects Research Articles

Photocatalytic CO2 Reduction Using Zinc Indium Sulfide Aggregated Nanostructures Fabricated under Four Anionic Conditions.

Zinc indihuhium sulfide (ZIS), among various semiconductor materials, shows considerable potential due to its simplicity, low cost, and environmental compatibility. However, the influence of precursor anions on ZIS properties remains unclear. In this study, we synthesized ZIS via a hydrothermal method using four different anionic precursors (ZnCl2/InCl3, Zn(NO3)2/In(NO3)3, Zn(CH3CO2)2/In(CH3CO2)3, and Zn(CH3CO2)2/In2(SO4)3), resulting in distinct morphologies and crystal structures. Our findings reveal that ZIS produced from Zn(CH3CO2)2/In2(SO4)3 (ZIS-AceSO4) exhibited the highest photocatalytic CO2 reduction efficiency, achieving a CO production yield of 134 μmol g-1h-1. This enhanced performance is attributed to the formation of more zinc and indium vacancy defects, as confirmed by EDS analysis. Additionally, we determined the energy levels of the valence band maximum (VBM) and the conduction band minimum (CBM) via UPS and absorption spectra, providing insights into the band alignment essential for photocatalytic processes. These findings not only deepen our understanding of the anionic precursor's impact on ZIS properties but also offer new avenues for optimizing photocatalytic CO2 reduction, marking a significant advancement over previous studies.

Tailoring p-Type Behavior in ZnO Quantum Dots through Enhanced Sol-Gel Synthesis: Mechanistic Insights into Zinc Vacancies.

The synthesis and control of properties of p-type ZnO is crucial for a variety of optoelectronic and spintronic applications; however, it remains challenging due to the control of intrinsic midgap (defect) states. In this study, we demonstrate a synthetic route to yield colloidal ZnO quantum dots (QD) via an enhanced sol-gel process that effectively eliminates the residual intermediate reaction molecules, which would otherwise weaken the excitonic emission. This process supports the creation of ZnO with p-type properties or compensation of inherited n-type defects, primarily due to zinc vacancies under oxygen-rich conditions. The in-depth analysis of carrier recombination in the midgap across several time scales reveals microsecond carrier lifetimes at room temperature which are expected to occur via zinc vacancy defects, supporting the promoted p-type character of the synthesized ZnO QDs.

Facile Synthesis of Ni-Doped ZnO Nanoparticles Using Cashew Gum: Investigation of the Structural, Optical, and Photocatalytic Properties

This work adopted a green synthesis route using cashew tree gum as a mediating agent to obtain Ni-doped ZnO nanoparticles through the sol–gel method. Structural analysis confirmed the formation of the hexagonal wurtzite phase and distortions in the crystal lattice due to the inclusion of Ni cations, which increased the average crystallite size from 61.9 nm to 81.6 nm. These distortions resulted in the growth of point defects in the structure, which influenced the samples’ optical properties, causing slight reductions in the band gaps and significant increases in the Urbach energy. The fitting of the photoluminescence spectra confirmed an increase in the concentration of zinc vacancy defects (VZn) and monovacancies (Vo) as Zn cations were replaced by Ni cations in the ZnO structure. The percentage of VZn defects for the pure compound was 11%, increasing to 40% and 47% for the samples doped with 1% and 3% of Ni cations, respectively. In contrast, the highest percentage of VO defects is recorded for the material with the lowest Ni ions concentration, comprising about 60%. The influence of dopant concentration was also reflected in the photocatalytic performance. Among the samples tested, the Zn0.99Ni0.01O compound presented the best result in MB degradation, reaching an efficiency of 98.4%. Thus, the recovered material underwent reuse tests, revealing an efficiency of 98.2% in dye degradation, confirming the stability of the photocatalyst. Furthermore, the use of different inhibitors indicated that •OH radicals are the main ones involved in removing the pollutant. This work is valuable because it presents an ecological synthesis using cashew gum, a natural polysaccharide that has been little explored in the literature.

STRUCTURE, OPTICAL PROPERTIES AND PHOTOCATALYTIC ACTIVIYY OF UNDOPED, La2O3-DOPED ZnO NANOCOMPOSITES

La-doped ZnO nanocomposites with di­ffe­rent content of La2O3 (1–5%) were obtained by the Pechini method from their nitrate solutions. The solutions of Zn2+ and La3+ nitrates were preliminary obtained by dissolving of zinc and lanthanum oxides with a content of the main component of 99.99% in nitric acid. The influence of lanthanum doping the on the microstructure, morphology, optical pro­perties and photocatalytic activity of the ZnO nanopowders were examined. The properties of the nanopowders were studied by using X-ray phase analysis, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy. The samples were subjected to X-ray powder diffraction using a DRON-3 diffractometer (Cu-K radiation) at room temperature. The scan angle was 0.05–0.1 ° in the range 2 = 15–90 °. X-ray phase ana­lysis confirms the formation of single phase of La2O3-doped ZnO powders on diffractograms. Raman light scattering and photoluminescence spectra were recorded using a Horiba Jobin‑Yvon T64000 spectrometer equipped with a CCD detector at room temperature in the inverse scattering geometry. According to SEM results, the powders characterized a conglomerate structure. The undoped ZnO has an average particle size of 43 nm, while the average particle size of La3+-doped ZnO ranges from 64 to 80 nm. It was established that the morphology of powder particles primarily depends on the content of La3+ in the material. An increase in the amount of La3+ in zinc oxide leads to an increase in the specific surface area (from 3.8 to 11.8 m2/g). In the photoluminescence spectra of ZnO powders, with increasing La2O3 concentration, bands at 400 nm are observed due to the appearance of impurities that cause of interstitial zinc and zinc vacancy defects and their broade­ning with a shift to the long-wave region. Photocatalytic properties of ZnO pow­ders doped with lanthanum oxide were in­vestigated using Methyl Orange as a model dye under Osram Ultra-Vitalux lamp (300 W) irradiation. A present result indicates that the obtained powders are potential candidate for the practical application in photocatalysis.

Photocatalysis activity of ZnO nanorods arrays prepared via hydrothermal

In the present work, ZnO nanorods have been synthetized via hydrothermal method. The solution temperature was varied in order to control the nanorods alignment. X diffraction rays (XRD) analysis and scanning electron microscopy (SEM) observations results confirm the nanorods morphology change caused by the solution temperature variation. The variation of the solution temperature engenders the evolution of the nanorods morphology from well aligned nanorods at low temperature towards entangled nanorods nanostructure. The photocatalysis activity of the realized nanostructures was tested for the degradation of methylene blue dye. The results indicate that the ZnO nanorods prepared at higher temperature exhibits a superior photocatalysis activity due its entangled structure that offers larger reactive sites with the surrounding aqueous solution in one hand and on the other hand, to the low recombination of the photo generated electrons, due to the absence of zinc vacancy defects (Vzn) deduced from photoluminescence measurements.

Investigation on structural, photoluminescence and magnetic behavior of Mn–ZnO nanoparticles via solvothermal route

Mn doped ZnO (Zn: Mn) nanopowder is successfully synthesized using the solvothermal route with concentration (X = 2%–8%) and annealing at 450 °C. The resulting material is investigated using various techniques. XRD studies confirm the structure of hexagonal wurtzite and good crystallinity in nature. From FESEM analysis confirms the formation of nanoparticles and allows for evaluation of the particle size of the sample. UV–Vis studies are used to determine absorbance wavelength and energy bandgap values. PL and EPR spectra confirm of the existence of zinc vacancy defects and g values are calculated. Interestingly, magnetic measurements revealed that the low concentration exhibited superparamagnetic behavior, while the high concentration showed a soft ferromagnetic structure using VSM.

Interplay of dopant and polarons in trifunctional semimagnetic semiconductor for supercapacitor applications: Local structure and electronic structure investigations

Diluted magnetic semiconductors (DMS), fractionally doped by magnetic ions, are peculiar, and a single semiconductor effectively contributes to charge and spin properties for various applications, including data and energy storage. Here, a low temperature assisted co-precipitation method produced a series of Co doped ZnO nanoparticles. After structural characterizations, zinc vacancy defects were realized via the hyperfine signal with g ∼ 1.96 (spin resonance) and the emissions (photoluminescence) at 410 and 465 nm. The synergistic effect among the Co dopant, native, and polaron defects makes the system exhibit enhanced polarization and electrochemical reactions. Magnetization hysteresis curves demonstrate the intrinsic magnetic nature of the materials that are essential for realizing useful polaronic states. As an anode material, 20 at. % Co doped electrode has a higher specific capacitance and superior cycling stability with charge retention above 98 %, which is much higher than the other ZnO-based electrodes and is associated with the polaron-assisted rapid redox reactions. These polarons also stimulate a ferroelectric polarization of 0.28 μC cm−2 via local lattice distortion. All these findings confirm that the Co doped semi-magnetic ZnO system with multifunctionality is adequate for employment in device fabrication as a low-cost, highly stable material.

Structure and defects-related optical properties of highly (002)-oriented zinc oxide thin films

We report on the highly (002)-oriented ZnO thin films deposited on glass substrates at different substrate temperatures using the recently developed pulsed laser deposition (PLD) facility at our institution. Effects of the substrate temperature on the structural, compositional, morphological, and optical properties of the ZnO thin films are studied. The XRD studies revealed the deposition of high-quality ZnO thin films with preferred hexagonal wurtzite structure along the (002) plane above 200 °C substrate temperatures. An increase in the crystallite size estimated using the Scherrer formula from 9.58 nm to 13.25 nm of the deposited thin films reflects the enhancement of the crystallinity with the increasing substrate temperature. Scanning electron microscope (SEM) micrographs revealed a smooth surface morphology with small grains for the thin film deposited at room temperature. The thin films deposited at high substrate temperatures showed agglomeration of nanoparticles resulting in nano rod-like structures. The optical band gap energy was found to be increased with an increase in the crystallinity of the thin films. In the photoluminescence (PL) spectra; near band edge (NBE) emissions were observed in all of the deposited thin films. The deep-level emissions of ZnO thin films were found to be highly dependent on the substrate temperature during film deposition. The thin films grown at 500 °C showed the visible emission in blue, green, and yellow-orange regions, which corresponded to zinc vacancy (VZn), oxygen vacancy (VO), and oxygen interstitial (Oi) defect levels, respectively. These crystal defects in the deposited ZnO thin films are responsible for the deep level emission linked to the visible emission.

Investigation of Electronic and Optical Properties of Al/Ag and Al/N Co-Implanted ZnO Thin Films

Either metal/metal or metal/non-metal co-doping is one of the most effective methods to modulate the visible emission of ZnO. In this paper, ZnO, aluminum-doped ZnO (Al-ZnO), aluminum and silver co-doped ZnO (Al/Ag-ZnO), and aluminum and nitrogen co-doped ZnO (Al/N-ZnO) are deposited. Combining the substitution of zinc ions using Al and/or Ag doping and the substitution of oxygen ions using N doping is expected to introduce more interstitial zinc and oxygen vacancy defects related to visible light emission in ZnO films. The results indicate that the PL spectrum of ZnO shows a violet emission peak at 406 nm and other weak visible emission peaks. After Al doping, we observe a strong blue emission at 421 nm, and its intensity is further enhanced and attains the maximum for Al/N-ZnO. However, for Al/Ag-ZnO, the blue emission shifts toward a longer wavelength, and the intensity of the blue emission conversely decreases. Then, the band structures, the density of states (DOS), the partial density of states (PDOS), and the optical constant of doped ZnO are calculated using density functional theory (DFT). Based on the experimental and theoretical results, the enhancement mechanism of visible light is discussed.

Room temperature photoluminescence and spectroscopic ellipsometry of reactive co-sputtered Cu-doped ZnO thin films

In this present work, reactive co-sputtering (DC) has been used to deposit the pure ZnO and Cu-doped ZnO (Cu-ZnO) thin films on a glass substrate. The copper atoms are subsumed into ZnO thin film by co-sputtering(RF) of a pure copper target. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Raman spectroscopy, Photoluminescence spectroscopy (PL), UV-Visible spectroscopy, and ellipsometry were used to examine pure ZnO and Cu-doped ZnO thin films. XRD reveals the wurtzite crystal structure for both pure and Cu-doped ZnO thin films. FESEM images show the dense and uniform surface morphology of all thin films. Energy-dispersive X-ray analysis (EDS) confirms the uniform distribution of copper dopant and variation in doping percentage in the Cu-doped ZnO films. The PL spectra of Cu-doped ZnO film have strong violet emission which shows a variation in zinc vacancy (VZn) defect density levels. UV-Vis-NIR spectroscopy has confirmed a slight increase in optical band gap by copper doping. The refractive index, roughness, and film thickness have been calculated using spectroscopic ellipsometry and found to increase with Cu doping concentration.

Effect of vacancy defects on the electronic transport properties of an\xa0Ag\u2013ZnO\u2013Pt sandwich structure

The effect of zinc and oxygen vacancy defects on the electronic transport properties of Ag(100)–ZnO(100)–Pt(100) sandwich structures is studied using density functional theory in combination with the nonequilibrium Green’s functional formalism. Defect-free systems show clear current rectification due to voltage dependent charge localization in the system as revealed in our transmission eigenstates analysis. Regardless of the location, oxygen vacancies result in enhanced current in the system, whereas Zn vacancy defects reduce the charge transport across the junction. The current rectification becomes less pronounced in the presence of both types of vacancy defects. Our findings can be of practical importance for developing metal-insulator-metal diodes.

Complex features of the photoluminescence from ZnO nanorods grown by vapor-phase transport method

We present photoluminescence (PL) studies of ZnO nanorods synthesized by high-temperature vapor-phase transport method under pulse laser excitation. The PL spectrum contains a strong near band edge (NBE) emission located at around 387 nm and no emission bands in the visible range were observed. Our studies show that an increase in power density of pulse UV laser results in appearance of a so-called P-band centered at 389 nm and it overlaps the NBE emission. The variations in excitation intensity and detection angles revealed that the band at 389 nm can be deconvoluted into Lorentzian components at ~382 and ~395 nm. Excitation intensity dependence of the band at 389 nm exhibits a clear threshold behavior while the intensity of the band at ~395 nm grows almost linearly with the increase in the excitation intensity. It is possible that the emission band at 395 nm is associated with emission centers caused by zinc vacancy defects or their complexes in ZnO nanorods.

Anti‐Thermal‐Quenching Bi3+ Luminescence in a Cyan‐Emitting Ba2ZnGe2O7:Bi Phosphor Based on Zinc Vacancy

AbstractThermal quenching (TQ) of phosphor is one of the biggest challenges to develop high‐quality white light‐emitting diodes (w‐LEDs). Herein, an anti‐thermal‐quenching (anti‐TQ) property in cyan‐emitting Ba2ZnGe2O7:Bi3+ phosphor is reported. At 150 °C, its emission intensity increases to 114% of the original intensity at 25 °C. Especially, the integrated emission intensity reaches 138%, 148%, and 134% at 150, 200, and 250 °C, respectively, by artificially creating zinc and oxygen vacancy defect. The anti‐TQ phenomenon is mainly attributed to high structure rigidity and strong ability to compensate emission loss during thermal generation process. Thermal‐induced emission compensation mainly stems from self‐oxidization behavior of Bi2+ in zinc vacancy and the presence of oxygen vacancy defect. Oxygen vacancy is induced by native zinc vacancy and charge imbalance between Bi3+ and Ba2+ ions. The strategy to create oxygen vacancy defect and design self‐oxidization process of Bi opens a new insight to exploit anti‐TQ phosphors for high‐quality w‐LEDs applications.

Thermal evolution of point defects in indium doped ZnO transparent conducting films

High visible light transparency and conductivity of ZnO enable the use of its material as transparent conducting films, in which point defects are vitally important for determining its optoelectronic properties. Herein, Indium doped ZnO thin films (ZnO:In) were prepared by radio-frequency magnetron sputtering technique. Thermal evolution of point defects and their effects on the microstructure and photoelectric properties of ZnO:In transparent conducting films were systematically investigated. The results show that thermal annealing induces the dissociation of extensive zinc interstitial clusters, causing their concentration gradually decreasing with increasing annealing temperature. Furthermore, when annealing at low temperature (≤ 500°C), the concentration of oxygen vacancy and zinc vacancy defects also decreases with increasing temperature. However, after annealing at higher temperature (600 and 700°C), additional vacancy defects are generated due to the evaporation of zinc and oxygen atoms from the film surface. It is demonstrated that the evolution of point defects caused by thermal annealing directly affects the microstructure and photoelectric properties of ZnO:In transparent conducting films.

Optical band gap engineering of ZnO nanophosphors via Cu incorporation for ultraviolet–violet LED

The tunability of optical band gap and augmentation of luminescence intensity of Cu-doped ZnO nanophosphors for ultraviolet and violet light-emitting diodes grown by microwave-assisted aqueous solution technique were studied at room temperature for various doping concentrations. The UV–Vis spectral analysis demonstrated that band gap engineering of ZnO nanophosphors can be performed through incorporation of Cu ions into the ZnO matrix by controlling the dopant content. The photoluminescence investigations revealed that a strong UV emission centred around 346–348 nm and an intense violet band comprising of two peaks centred about 403 nm and 424 nm occur in the absence of any other band. A thorough study of the defect-related PL spectra elucidated that these violet peaks originated from energetically more probable deep-acceptor zinc vacancy and shallow-donor zinc interstitial defect transitions. The luminescence intensity of nanophosphors can be optimized through modification of intrinsic defect density by adjusting Cu concentration in the ZnO host. The results are significant for ZnO-based photoluminescence cutting-edge technology.

Room Temperature Ferromagnetism in D-D Neutron Irradiated ZnO Single Crystals

Room temperature ferromagnetism (RTFM) was observed in unirradiated ZnO single crystal owing to zinc vacancy (VZn) defects. D-D neutron irradiation was used to regulate the defect concentration and types avoiding interference of impurity elements. D-D neutron mainly collides elastically with ZnO single crystals, producing VZn, oxygen vacancy (VO), and other point defects, the density and kind of defect associated with neutron irradiation fluence. It can be found the saturation magnetization (Ms) first increases and then decreases with the irradiation fluence increases, meaning there is an optimum value for VZn concentration. When the neutron fluence is greater than 5× 109 n/cm2, more unstable VZn combined with VO to form zinc-oxygen divacancy (VZnO), indicating neutron irradiation changes the defect types. VZnO induces a smaller magnetic moment; thus, the Ms decreased. To further verify the origin of RTFM, the CASTEP module was employed to calculate the magnetic and structural properties of native vacancy defects in ZnO. VZn induces 2.12 μB magnetic moment, and only VZnO formed by the nearest VZn and VO has 2.09 μB magnetic moment, while 3VO cluster introduces a magnetic moment of $\sim $1 μB. Combined experiment and CASTEP calculation results, VZn is the most likely origin of RTFM.

Reply to ‘Comment on “Zero-thermal-quenching and photoluminescence tuning with the assistance of carriers from defect cluster traps”’

In this response to the comments on our article, a series of Sr8Zn1−xSc(PO4)7:12%Tb3+ (SZ1−xSPO:12%Tb3+) phosphors were synthesized, and their photoluminescence behavior confirmed that zinc vacancy defects play a vital role in the SZ1−xSPO:12%Tb3+ phosphor.

Effect of hydrothermal synthesis on physical property modulation and biological activity of ZnO nanorods

Influence of hydrothermal synthesis route on various defect states of ZnO nanomaterial and this specific synthesis route induced physical property modulation and the biological activity of ZnO nanomaterial has been considered here for the first time ever. X-ray diffraction (XRD) and Field emission scanning electron microscopy (FESEM) reveal the presence of single phase wurtzite structure of ZnO nanomaterial with hexagonal rod like morphology. Photoluminescence (PL) study confirms the presence of structural defects in form of oxygen vacancy defects, zinc vacancy defects, zinc interstitial defects etc in the ZnO nanomaterial. The detail analysis of magnetic response at different temperatures reveals the presence of vacancy mediated room temperature ferromagnetism (RTFM) in rod like ZnO nanomaterial. Dielectric response as a function of frequency has also been measured for ZnO nanorods. The result of this dielectric study shows that the nanomaterial of ZnO exhibits superior dielectric constant due to the grain boundary defect. Hydrothermal synthesis route influences the structure of ZnO nanomaterial mostly towards rod like in nature and the grain boundary defect of rod like structure creates large space charge polarization. In addition to the significant modulation of all these physical properties based on the hydrothermal synthesis route, various biological activities such as antibacterial and antifungal activities of these rod like ZnO nanomaterial has also been observed on the Gram negative clinical isolates like, Escherichia coli, Klebsiella pneumoniae, and plant pathogen Alternaria alternate. Hydrothermal synthesis route creates such an impact on the growth of ZnO nanomaterial that the material evolves with significant modulations of physical properties and biological activities.

A High-Resistivity ZnO Film-Based Photoconductive X-Ray Detector

High-resistivity ZnO film-based photoconductive detectors have been fabricated on glass substrate by the magnetron sputtering growth method, and tested as X-ray detectors for the first time in this letter. The devices exhibited a low dark current $\sim ~100$ pA at 40 V bias and a fast transient reproducible response to X-ray illumination (generated by bremsstrahlung with a tungsten target with tube voltage 30 kV) $\sim ~200$ ms, which was mainly attributed to the high resistivity $\sim ~10^{9} \Omega $ cm of the ZnO film deposited in the oxygen atmosphere. The high resistivity was due to the zinc vacancy defects’ compensation with the n-type defects in ZnO film as revealed by photoluminescence spectra. Furthermore, the detector achieved an X-ray pulse detection, and the rise time and full width at half maximum of the outputs signal were $139~\mu \text{s}$ and 0.9 ms at 200 V bias, respectively. Associated with material characterizations, it was demonstrated that the high-resistivity ZnO film detector had the potential to be promising for fast X-ray detection application.

Vacancies inducing electronic and optical properties in 2D ZnO:Be/Mg

The structural, electronic and optical properties of the 2D ZnO:Mg/Be with oxygen or zinc vacancies were studied by First-principles calculation. Results show that the formation of vacancy defect in DL system is easier than in SL system. The band gap become narrow with introducing oxygen and zinc vacancy defects. It was also found that oxygen vacancies can induce an impurity level near VBM to improve the conductivity of system and Zn vacancies can not. Optical absorption spectra show that oxygen vacancy defect will cause blue-shift and Zn vacancy defect will cause red-shift. So, two kinds of vacancies give birth to the opposite optical properties. Therefore, vacancy defects in 2D ZnO:Mg/Be are very important defect to better the conductivity and potential application in photoelectron and photocatalysis.