Defects In ZnO Research Articles

First-Principles Investigation of the Structure and Properties of Au Nanoparticles Supported on ZnO

We present a first-principles investigation of the structure, stability, and reactivity of Au nanoparticles (NPs) supported on ZnO. The morphologies of supported Au NPs are predicted using the formation energy of Au surfaces and the adhesion energy between Au and the dominant ZnO surfaces exposed on ZnO tetrapods. We show how Zn interstitials (a stable intrinsic defect in ZnO) are attracted toward the Au/ZnO interface and in the presence of oxygen can lead to the encapsulation of Au by ZnO, an effect that is observed experimentally. We find that O2 molecules absorb preferentially at the perimeter of the NP in contact with the ZnO support. These results provide atomistic insight into the structure of ZnO-supported Au NPs with relevance to CO oxidation.

Er3+ induced point defects in ZnO and impact of Li+/Na+/K+ on the vacancy defects in ZnO:Er studied by positron annihilation spectroscopy

The defects species in as-prepared ZnO, Er-doped, and co-doped (with Li/Na/K) nanocrystallites synthesized by combustion method were examined by Positron Annihilation Lifetime (PAL) and Doppler Broadening (DB) spectroscopy. The S and W parameters were derived from positron trapping of valence and core electrons extracted from the central and wing regions of DB spectra to identify the vacancy-type defects. For as-prepared Er-doped ZnO samples, the observed high value of S indicated that the vacancies and vacancy cluster defects were located at the grain surface and two-grain junction. With co-doping of Li/Na/K, the S value was decreased due to the dopant occupying the vacancy clusters. On annealing, the major recovery of such defects was observed with enormous decreasing S for Li co-doped sample. Positron lifetime variations signified the presence of defects compared with bulk ZnO. Er doping in ZnO evidenced a small increase of defects with a gradual decrease when co-doped with Li/Na/K indicating the co-dopant impurity occupied the vacancies in the grain boundary. The first lifetime component of 193 ps owing to the Zn-type vacancies in as-prepared ZnO were increased on Er doping, evidencing the replacement of the Zn2+ by Er3+ ion and thereby created point defects due to charge compensation. On annealing to 800 °C, Li co-doped ZnO:Er resulted to enormous reduction of second lifetime τ2, mean lifetime τave, and S value signifying the Li ion localised in Zn vacancy site. Annealed ZnO:Er + Na/K resulted in considerable intensification in defect concentrations than ZnO:Er + Li due to the vacancy migration to form as clusters. The defect type, defect modification, the influence of Er together with co-doped Li/Na/K in ZnO, and their effect on annealing are elucidated in detail from the positron annihilation characteristics.

Performance of graphene oxide-modified electrodeposited ZnO/Cu2O heterojunction solar cells

We report the fabrication of ZnO/Cu2O heterojunction solar cells by means of the electrodeposition technique. The effect of electrolyte medium for the ZnO deposition, annealing treatment and interface modification with graphene oxide (GO) layer on the photoelectrical properties was analyzed. The electrochemical results indicated a markedly dependent Cu2O film electrodeposition on the GO-modified ZnO films. The modification of ZnO/Cu2O interface with GO nanosheets and annealing treatment results in improved interface properties, varying morphology and defects in ZnO lattice that further lead to enhanced performance of the proposed heterojunction solar cells. While the obtained results indicate that the properties of GO coating need to be tailored for improved performance, a synergetic effect of the GO addition and annealing treatment on the photoelectric properties of the electrodeposited heterojunction is achieved.

Oxygen partial pressure-dependent growth mechanism of low-dimensional zinc oxide on indium tin oxide glass

The influence of O2 partial pressure on the growth mechanism of quasi-one-dimensional thermal-evaporated ZnO structure was explained by the preferential interface nucleation. At the relatively low temperature of 550 °C, the growth was governed by different growth mechanisms as the O2 partial pressure increased. At a very low O2 partial pressure of 0.3 Pa, the growth of ZnO nanowire on indium-tin oxide glass was Au-catalyzed vapor-liquid-solid (VLS) mechanism instead of vapor-solid (VS) mechanism. As increasing O2 partial pressure to 3 and 6 Pa, the ZnO needle-like structure was grown by self-catalyzed VLS mechanism. While, the VS mechanism was suggested for ZnO micropillar and two-dimensional ZnO sheet grown with the O2 partial pressure of 9 Pa and above. The near band edge (NBE) at 3.26 eV from the photoluminescence (PL) spectra strongly depended on the as-grown structure. In addition, PL at 2.32 eV corresponded to the ZnO defects due to the oxygen vacancies within 14 nm beneath the one-dimensional ZnO surface.

Enhanced room temperature ferromagnetism and green photoluminescence in Cu doped ZnO thin film synthesised by neutral beam sputtering

The Cu (3 to 15 at%) is incorporated into ZnO thin film by atomic beam co-sputtering has been investigated for enhancement in room temperature ferromagnetism and green photo-luminance. These Cu-ZnO thin films examined with Raman spectroscopy, X-Ray Diffraction (XRD), UV-Visible spectroscopy, Hall measurement, magnetic force microscopy (MFM) and magnetic hysteresis. Raman spectroscopy, XRD confirms wurtzite structure and improvement in the crystallinity of ZnO upto 7% Cu. Further increase in Cu concentration results in growth in Cu nanoparticles. On increasing Cu concentration, there is decrement in transparency and increase in band gap with increase in n-type carrier concentration as confirmed from UV-Visible and Hall measurement studies. Magnetic measurement exhibited unique feature of room temperature ferromagnetic ordering in undoped and doped sample upto 3% Cu. The enhancement in magnetic moment as well as green emission in photoluminescence response with increase in Cu doping indicates that generation of large defects in ZnO by Cu doping, which can be attributed to combined effect of the presence of oxygen vacancies and/or structural inhomogeneity as well as formation of bound magnetic polarons. Importantly, synthesised Cu doped ZnO thin films can be used as spin LEDs and switchable spin-laser diodes.

Decontamination and disinfection of wastewater by photocatalysis under UV/visible light using nano-catalysts based on Ca-doped ZnO

Nano-catalysts based on ZnO-Ca x% (with x = 0, 0.1, 0.3, 0.5, 0.7, and 1.0 mol % Ca2+) were synthesized with a bio-friendly adaptation of the sol-gel method using gelatin as template. These materials were characterized by Fourier Transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Micro-Raman, transmission electron microscopy (TEM), scanning electron microscopy (SEM), N2 physisorption, photoacoustic absorption spectroscopy (PAS), and photoluminescence spectroscopy (PL). The Raman results indicated that the signal, attributed to an E1(LO) mode at 580 cm−1, was characteristic of oxygen vacancies that decreased with the increased Ca2+ content in doped oxides. This agreed with the PL results, which showed that the green emission centered at 510 nm and attributable to structural defects in ZnO decreased for Ca-doped ZnO. Our oxides are constituted by nanoparticles with rod-like and spherical morphologies. All the nano-catalysts exhibited the band gap characteristics of semiconductor materials around 3.0 eV. ZnO-Ca 1.0% exhibited the best photocatalytic performance for degradation of Methyl Orange (MO) model dye, degrading about 82% after 240 min of UV–Vis irradiation at pH 7.0. The reaction mechanism was influenced by hydroxyl (OH) and superoxide (O2-) radicals and mainly by active holes (h+). This doped oxide also demonstrated efficiency in wastewater disinfection against heterotrophic bacteria and total coliforms, exhibiting a potential use as an antimicrobial agent for the treatment of hospital wastewater. Furthermore, our nanoparticles did not show significant cytotoxic effects on L929 fibroblast cells.

Noble gas as a functional dopant in ZnO

Owing to fully occupied orbitals, noble gases are considered to be chemically inert and to have limited effect on materials properties under standard conditions. However, using first-principles calculations, we demonstrate herein that the insertion of noble gas (i.e. He, Ne, or Ar) in ZnO results in local destabilization of electron density of the material driven by minimization of an unfavorable overlap of atomic orbitals of the noble gas and its surrounding atoms. Specifically, the noble gas defect (interstitial or substitutional) in ZnO pushes the electron density of its surrounding atoms away from the defect. Simultaneously, the host material confines the electron density of the noble gas. As a consequence, the interaction of He, Ne, or Ar with O vacancies of ZnO in different charge states q (ZnO:VOq) affects the vacancy stability and their electronic structures. Remarkably, we find that the noble gas is a functional dopant that can delocalize the deep in-gap VOq states and lift electrons associated with the vacancy to the conduction band.

Bond model of second-harmonic generation in wurtzite ZnO(0002) structures with twin boundaries

We describe second-harmonic generation (SHG) in ZnO(0002) wurtzite structures with [11¯00]/(1102) twin boundaries using the simplified bond hyperpolarizability model (SBHM). We show explicitly how the reflective second-harmonic generation (RSHG) intensity profile for the s-incoming fundamental and s-outgoing SHG polarized light arise via superposition of the SHG dipole fields. The nonlinear fields originate from anharmonic motions of electric charges along each bond. The total dipole fields from within the ZnO bulk sum up to zero but produce a constructive SHG radiation from the unreduced surface and twin boundary. In addition, we compare the third-order susceptibility tensor obtained from group theory and the SBHM and calculate the values for the nonzero components. We found that the off-resonance RSHG intensity data in diatomic wurtzite structures even with the presence of twin boundaries can be modeled using only three independent fitting parameters, namely, the effective bulk, reduced surface, and twin boundary SHG hyperpolarizability. The results show that the SBHM can be used to investigate impurities and surface defects in ZnO as well as their contribution to nonlinear radiation with potential application to frequency conversion in nanoscale optical circuitry.

Bistability of a hydrogen defect with a vibrational mode at 3326 cm−1 in ZnO

Results from illumination-sensitive infrared absorption and photocurrent spectroscopy measurements on the hydrogen defect in ZnO characterized by a local vibrational mode with a frequency of 3326 ${\mathrm{cm}}^{\ensuremath{-}1}$ are reported. It is shown that photoexcitation with $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\ge}2.7$ eV results in two new axial oriented $\mathrm{O}\text{\ensuremath{-}}\mathrm{H}$ stretch vibrational modes at 3170 and 3589 ${\mathrm{cm}}^{\ensuremath{-}1}$, which appear at the expense of the previously reported azimuthal oriented 3326- and 3358-${\mathrm{cm}}^{\ensuremath{-}1}$ modes. We propose a tentative configurational-coordinate energy diagram of the defect and discuss its microscopic origin.

Determination of Diffusion Coefficient of Copper in ZnO (001) Single Crystals at 1000 °C

Copper from a solid source was diffused into undoped n-type bulk ZnO (001) single crystals at 1000 °C under a nitrogen atmosphere at different diffusion times. The Cu diffusion profiles were obtained by Secondary ion mass spectroscopy (SIMS), and the fitting reveals a diffusion case from a constant concentration source. A value for the diffusion coefficient of 2.42(±0.2) × 10−12 cm2∙s−1 was obtained. Electrical measurements present an increment of carrier concentration with diffusion time, but remains n-type which indicates an increase in the donor levels produced by structural defects in ZnO. Photoluminescence (PL) spectra showed an increment of green emission intensity associated with Cu incorporation.

Vacancy cluster in ZnO films grown by pulsed laser deposition

Undoped and Ga-doped ZnO films were grown on c-sapphire using pulsed laser deposition (PLD) at the substrate temperature of 600 °C. Positron annihilation spectroscopy study (PAS) shows that the dominant VZn-related defect in the as-grown undoped ZnO grown with relative low oxygen pressure P(O2) is a vacancy cluster (most likely a VZn-nVO complex with n = 2, 3) rather than the isolated VZn which has a lower formation energy. Annealing these samples at 900 °C induces out-diffusion of Zn from the ZnO film into the sapphire creating the VZn at the film/sapphire interface, which favors the formation of vacancy cluster containing relatively more VZn. Increasing the P(O2) during growth also lead to the formation of the vacancy cluster with relatively more VZn. For Ga-doped ZnO films, the oxygen pressure during growth has significant influence on the electron concentration and the microstructure of the VZn-related defect. Green luminescence (GL) and yellow luminescence (YL) were identified in the cathodoluminescence study (CL) study, and both emission bands were quenched after hydrogen plasma treatment. The origin of the GL is discussed.

A first-principles study on magnetic properties of the intrinsic defects in wurtzite ZnO.

Since the origin of magnetism in ZnO-based diluted magnetic semiconductors (DMSs) is still controversial, in this work, we presented a detailed study on the magnetic, structural, and electronic properties of wurtzite ZnO-based DMS systems with point and complex intrinsic defects. Two outer electrons from neutral oxygen vacancy (VO) occupy the a1 orbital, making the inducted magnetic moment to be zero, while a cluster including three VOs leads to a magnetic moment of ∼1 μB. The magnetic moment of the system with a Zn vacancy (VZn) is 1.65 μB. When two neutral VZns in different relative distances were created in respective supercells, the systems showed different magnetic moments induced by the unequal level between the highest electron occupied orbital of the defect state introduced by different VZn sites and the valence band maximum. The system of a neutral O occupying an octahedral site gives rise to a magnetic moment of 2 μB, while zinc interstitial and antisite defects do not cause spin polarization. The system with a complex defect of VO and VZn is magnetic when those vacancies are adjacent but still do not cause the compensation effect. The oxygen interstitial defect is unstable, and VZn easily turns into the complex defect. We suggest that VO clusters and VZn complex defects could likely be the origin of ferromagnetism in undoped ZnO.

Lithium-Ion-Based Conjugated Polyelectrolyte as an Interface Material for Efficient and Stable Non-Fullerene Organic Solar Cells.

An eco-friendly n-type water/alcohol-soluble conjugated polyelectrolyte PFEO SO3 Li was synthesized and applied as a cathode interfacial layer in organic solar cells. The π-delocalized polyfluorene backbone has an intimate connection with the hydrophobic active layer, and the side chain with lithium ion may move toward the ZnO layer through the self-assembly property of conjugated polyelectrolytes. UV photoelectron spectroscopy indicated that modification with PFEO SO3 Li dramatically lowers the work function of indium-doped tin oxide (ITO)/ZnO and may form strong interfacial dipoles between ZnO and the active layer. Meanwhile, introduction of lithium ions as spectator cations may contribute to reduction of the intrinsic surface defects of ZnO. The green emission in the photoluminescence spectrum of ZnO disappeared after modification with PFEO SO3 Li. In addition, the roughness of ZnO barely changed after coating with PFEO SO3 Li, and the surface became more hydrophobic, which demonstrates that the thin conjugated polyelectrolyte layer exhibits good adhesion with both ZnO and the active layer. These phenomena indicate that the introduction of PFEO SO3 Li makes ITO/ZnO an efficient cathode. As a result, inverted organic solar cell devices with ZnO/PFEO SO3 Li double-interlayers exhibit high efficiencies of 11.7 and 10.6 % for PBDB-T:IT-M and PBDB-T:ITIC blend systems, respectively.

Defect annealing kinetics in ZnO implanted with Zn substituting elements: Zn interstitials and Li redistribution

It is known that the behavior of residual Li in ion implanted ZnO depends on the preferential localization of the implants, in particular, forming characteristic Li depleted or Li pile-up regions for Zn or O sublattice occupation of the implants due to the corresponding excess generation of Zn and O interstitials in accordance with the so-called “+1 model.” However, the present study reveals that conditions for the radiation damage annealing introduce additional complexity into the interpretation of the Li redistribution trends. Specifically, four implants residing predominantly in the Zn-sublattice, but exhibiting different lattice recovery routes, were considered. Analyzing Li redistribution trends in these samples, it is clearly shown that Li behavior depends on the defect annealing kinetics which is a strong function of the implanted fluence and ion species. Thus, Li depleted and Li pile-up regions (or even combinations of the two) were observed and correlated with the defect evolution in the samples. It is discussed how the observed Li redistribution trends can be used for better understanding a thermal evolution of point defects in ZnO and, in particular, energetics and migration properties of Zn interstitials.

ZnO Synthesized Using Bipolar Electrochemistry: Structure and Activity.

The photoactive materials broadly applied in catalysis and energy conversion are generally composed of metal oxides. Among these oxides, ZnO showed a promising photocatalytic activity; however, traditional synthetic routes generated by-products and large amounts of secondary waste. Herein, we report the use of bipolar electrochemistry to generate ZnO nanoparticles using deionized water and a zinc metal to conform to green chemistry practices. TEM imaging demonstrated that the sizes of the bipolar-made ZnO particles were smaller than the commercial sample. The presence of structural defects in ZnO was correlated with the chemical shifts analyzed by X-ray photoelectron spectroscopy (XPS) and by different concentrations of O2− ions in stoichiometric and defected lattice. Further, the diffuse reflectance UV–Vis studies demonstrated a blue-shift in the reflectance spectrum for the bipolar-made oxide. This was also an indication of defects in the ZnO lattice, which related to the formation of shallow levels in the bandgap of the material. The structural and morphological differences influenced the photocatalytic characteristics, revealing a higher photocurrent for the bipolar-made ZnO when compared to the reference sample. This was further manifested in lower total resistivity for all anodes made from the non-stoichiometric ZnO, and also in their shorter diffusion length for charge exchange and electron lifetimes.

The fight against multidrug-resistant organisms: The role of ZnO crystalline defects

This article reports the excellent antimicrobial response of nanoparticulate ZnO against multidrug-resistant organisms (MDROs). We demonstrate that the enhanced antimicrobial activity against MDROs depends on the crystalline defects of ZnO. Hence, this work provides insights on the ZnO-microorganism interactions, and we pose combined physico-chemical action mechanisms against resistant bacteria.

Passivating ZnO with a naphthalimide-Schiff base as electron transport layer for inverted polymer solar cells

Abstract Optimizing electron transport layer (ETL) is a key issue to guarantee the performance of polymer solar cells (PSCs). In this work, by passivating ZnO with a naphthalimide-Schiff base (NS) as ETL, the inverted PSCs with polythieno[3,4-b]- thiophene-co-benzodithiophene (PTB7): [6, 6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the light harvesting layer (LHL) exhibited enhanced device performance. Studies demonstrated that the NS introduction passivates the surface defects of ZnO thus suppressing exciton quenching, resulting in enhanced exciton dissociation, efficient charge carrier collection and reduced loss of charge recombination, and thereby giving high power conversion efficiency (PCE). While the PCE of the inverted PSCs with pure ZnO ETL is 7.34%, that of the devices with NS passivated ZnO (NS-ZnO) ETL shows an enhanced PCE of 8.20% (∼11.7% improvement). More importantly, the NS-ZnO can be facilely obtained by adding NS into ZnO precursor, suggesting a simple and efficient way to enhance the PSCs performance.

Impact of Background Oxygen Pressure on the Pulsed-Laser Deposition of ZnO Nanolayers and on Their Corresponding Performance as Electron Acceptors in PbS Quantum-Dot Solar Cells

ZnO films are commonly employed as n-type semiconductors in heterojunctions with PbS colloidal quantum-dot (CQDs) films because of their outstanding optical transparency and electron acceptor properties, yet studies of the impact of ZnO film microstructure, composition, and defect qualities on the solar-cell performance are quite limited. Here we report on the fabrication of ZnO films via pulsed-laser deposition and use these films to investigate how different morphologies affect the PbS CQD solar-cell performance. By modification of the background gas O2 pressures during the ZnO deposition process, the device performance approaching a 7.8% energy conversion efficiency is achieved with an O2 pressure of 100 mTorr. Higher or lower O2 pressures led to significantly lower device efficiency. We employ various materials and device characterizations to highlight the differences in the physical properties introduced by the fabrication oxygen pressure. In particular, we have found that the differences in the type...

Долгоживущие центры фотокатализа, создаваемые в ZnO резонансным возбуждением экситона

Along with TiO_2, ZnO is the main photocatalyst for a wide class of redox reactions used to convert light energy into chemical and for environmental cleanup. It has been shown that the creation in ZnO of surface intrinsic defects in ZnO i.e. vacancies in the anionic and cationic sublattices (F-type and V-type centers) - makes it possible to create long-lived (up to 10^3 s) photocatalysis centers and thus fundamentally (tens of times) to increase the quantum yield of reactions. Slow surface states — photocatalysis centers — are created by the diffusion of electrons and holes generated during interband transitions in the volume of the photoactivated sample. However, the transfer efficiency is sharply reduced due to carrier recombination and losses when overcoming the Schottky surface barrier. In this work, In order to reduce these losses during energy transfer to the surface, we used in this work neutral energy carriers — excitons. The high (60 meV) exciton binding energy in ZnO allows it to move at room temperature without decay. The radiation loss of the exciton energy in our experiments is effectively reduced by the formation of a surface 2D structure. The results obtained confirm the high efficiency of the exciton channel for the formation of surface long-lived F and V centers of photocatalysis in the processes of oxygen photoadsorption and photodesorption, imitating the full cycle of the redox photocatalytic reaction.

Improvement of the performance in Cr-doped ZnO memory devices via control of oxygen defects.

The defect-enhanced resistive switching behavior of Cr-doped ZnO films was investigated in this study, and evidence that the switching effect can be attributed to defects was found. X-ray photoelectron spectroscopy demonstrated the existence of oxygen vacancies in the ZnO-based films, and the concentration of oxygen vacancies in the Cr-doped ZnO film was larger than that in the undoped ZnO film, which can be attributed to Cr doping. We concluded that the defects in Cr-doped ZnO were due to the Cr dopant, leading to excellent performance of Cr-doped ZnO films. In particular, depth-profiling analysis of the X-ray photoelectron spectra demonstrated that the resistive switching effects corresponded to variations in the concentration of the defects. The results confirmed that oxygen vacancies are crucial for the entire class of resistive switching effects in Cr-doped ZnO films. In particular, the Cr-doped ZnO films not only show bipolar resistive switching behavior but also excellent reliability and stability, which should be beneficial for next-generation memory device applications.