Zn-rich Conditions Research Articles

First-principles calculation study on the mechanism of p-type ZnO through Mi-nNO (M= Mg and Cu) complexes

First principles calculation based on density functional theory is applied to study the possible approach to realize p-type ZnO through Mi-nNO (Mi is an interstitial defect; n = 2 or 3) complexes. In codoping method, the substitutional defects usually serve as the donors in donor-acceptor complexes, which are seen as the source of p-type ZnO. However, our research shows that the interstitial defects can replace the substitutional defects to become the donors. In Mg + N and Cu + N codoped ZnO, the binding, formation and ionization energies of possible complexes are investigated. Mgi-2NO and Cui-NO are two neutral complexes which can introduce a fully occupied impurity band, 0.66 and 0.87 eV above the VBM of the pure ZnO, respectively. It is found that Mgi-3NO and Cui-nNO (n = 2,3) can become the stable source of p-type conductivity under Zn-rich condition. In addition, the requirements of “M” in Mi-nNO complex are discussed.

Five-fold symmetry structure inhibiting the growth of an otherwise perfect η2 phase in Al-Zn-Mg-Cu alloys

Using high angle annular dark-field scanning transmission electron microscopy, we report the formation of a five-fold symmetry structure at the (1¯100)η//(110)Al edge interface of otherwise defect-free η2 precipitates in Al-Zn-Mg-Cu alloys. This quasicrystal entity has a demonstrated ability to hinder growth at the interface where it resides. Assisted by calculations based on density functional theory, the five-fold symmetry structure is shown to form easily at Zn-rich conditions due to favorable interfacial energies at all stages of evolution. These findings offer an enhanced perspective of precipitate stabilization in an alloy of widespread industrial interest, presenting a potential alternative path to alloy design.

Air and Vacuum Annealing Effect on the Highly Conducting and Transparent Properties of the Undoped Zinc Oxide Thin Films Prepared by DC Magnetron Sputtering

In this study, we aim to investigate the effect of zinc interstitials (Zni) and oxygen vacancies (VO) on the ZnO electrical conductivity. ZnO films were synthesized via DC magnetron sputtering process using pure Zn target in gases mixture of Ar/O2 = 80/17.5 (sccm). In order to improve the optical and electrical prosperities, the obtained films were subjected to air and vacuum annealing treatment. Several techniques such as field emission scanning electron microscopy (FESEM), Grazing Incidence X-ray Diffraction (GIXRD), Raman spectroscopy, photoluminescence spectroscopy (PL) and UV-visible were used to study the influence of heat treatment on structural and physical properties of ZnO films. Electrical conductivity of ZnO thin films was determined by measuring the sheet resistance and thickness of the films. XRD results confirm the synthesis of annealed ZnO films of the hexagonal structure with a preferential orientation along the (002) plane. The average crystallite size is altered between 22.6 to 28.4 nm dependent on the plan orientation of the ZnO film. Morphology and crystallinity of the ZnO structure could efficiently control the transmittance, electrical resistivity and optical band gap. As deposited ZnO film showed a lower electrical resistivity of 2.72×10-3 Ωcm due to the Zn-rich conditions. Under vacuum annealing, a combination of low resistivity (1.17×10-2 Ωcm) and better optical transmittance (87 %) are obtained. ZnO films developed in this study with high transmittance and low resistivity and good electro-optical quality supports their use in transparent and conductive electrode applications. The plan presentation was visualized using Vesta, with the lattice parameter set as follows: a = b = 3.249 Å; c = 5.207 Å; α = β = 90°; γ = 120°. Based on the construction and optimization of primitive cells, the supercells were constructed and then optimized. Finally, (002) and (103) planes were cut and the planar supercell structure was constructed. In order to make a plane representation for the solid bulk with 10 Å of thickness.

Nickel and zinc micronutrient availability in Phanerozoic oceans.

Nickel and zinc are both bio-essential micronutrients with a nutrient-like distribution in the modern ocean, but show key differences in their biological functions and geochemical behavior. Eukaryotic phytoplankton, and especially diatoms, have high Zn quotas, whereas cyanobacteria generally require relatively more Ni. Secular changes in the relative availability of these micronutrients may, therefore, have affected the evolution and diversification of phytoplankton. In this study, we use a large compilation of Ni and Zn concentration data for Phanerozoic sediments to evaluate long-term changes in Ni and Zn availability and possible links to phytoplankton evolution. Modern data suggest that organic-rich sediments capture the dissolved deep ocean Ni/Zn ratio, regardless of local depositional conditions. We use this observation to constrain Ni/Zn ratios for past oceans, based on data from the sedimentary record. This record highlights long-term changes in the relative availability of these micronutrients that can be linked to the (bio)geochemical conditions on the Earth's surface. Early Palaeozoic oceans were likely relatively Ni rich, with sedimentary Ni/Zn ratios for this interval mostly being around ~1 or higher. A comparison with Phanerozoic strontium-, carbon-, and sulfur-isotopic records suggests that the late Palaeozoic decrease in sulfidic conditions and increase in hydrothermal inputs and organic-carbon burial rates caused a shift towards more Zn-rich conditions. Mesozoic and Cenozoic sediments show relatively Zn-rich oceans for these time intervals, with sedimentary Ni/Zn ratios mostly being around ~1 or lower. These observations imply that the diversification of the dominant groups of modern eukaryotic phytoplankton occurred in relatively Zn-rich oceans and that these organisms still carry this signature in their stoichiometries. However, the Phanerozoic transition to a more Zn-rich ocean pre-dates the origin and diversification of modern eukaryotes and, therefore, this transition was likely not the main direct cause for eukaryotic diversification in the Mesozoic and Cenozoic Eras.

Effects of VZn and Hi interstitials with different valence states and alkaline earth metal doping on carrier lifetime, activity, and oxidation–reduction reaction of ZnO

We applied the generalized gradient approximation plane wave ultrasoft pseudopotential method based on the framework of density functional theory in this work to study the influences of Be/Mg/Ca single doping and the coexistence of Zn vacancies with different valence states and H interstitial on the magnetic and photocatalytic properties of ZnO. Results showed that the formation energy of all doping systems decreases with the increase in the valence state of Zn vacancies, and the doping is easy. The binding energies of all doped systems under O-rich conditions are lower than those under Zn-rich conditions, and the doped systems are more stable under O-rich conditions. The Zn34MO36 (VZn1−, M = Be/Mg/Ca), Zn34MHiO36 (VZn1−, M = Be/Mg/Ca), and Zn34MHiO36 (VZn0, M = Mg/Ca) systems show magnetism. The source of the magnetism of the doped system is generated by the itinerant electrons of unpaired spin polarization of the O atom near VZn. The study found that the Zn34MgHiO36(VZn2−) system has the longest carrier lifetime, relatively large electric dipole moment, significant red shift of the absorption spectrum, and strong redox ability. The Zn34MgHiO36 (VZn2−) system can be used as a photocatalyst for H2 production. The result of this work has theoretical reference value for the design and preparation of new magneto-optical functional materials.

Defects study in zinc blende ZnS utilizing optimized hybrid functional

Due to its wide band gap and widespread availability in single crystals as well as thin film, zinc sulfide (ZnS) has been applied in many areas. Unfortunately, experimental studies alone are not always sufficient to understand the electronic properties of semiconductors, especially their intrinsic point defects. It is thus necessary to perform theoretical calculations along with the characterization experiments to elucidate their defect characteristics. Density functional theory (DFT) with (semi)local exchange correlation is not often appropriate for describing defects in wide band gap semiconductors, while hybrid functional that do not enforce generalized Koopmans theorem (gKT) are not entirely reliable for predicting the properties of point defects in semiconductors. In this study, we have tuned exact-exchange parameters of hybrid functional to not only reproduce the experimental band gap but also satisfy the gKT for zinc blende (ZB) ZnS. With the new optimized hybrid functional, we study the intrinsic defects in ZB ZnS. Using this optimized hybrid functional we study the intrinsic defects of ZB ZnS. The results show that intrinsic defects have deep charge-transition levels. Among these defects, the zinc vacancy (Znvac) is energetically preferable in a n-type undoped sample either in Zn-rich or S-rich conditions. The calculated emission energy by the combination of an electron from the conduction band minimum and a trapped hole at Znvac2- is 3.17 eV. This calculation is the first theoretical result that explains the origin of the deep-blue emission in ZB ZnS.

First-principles study on the effect of biaxial strain on the carrier lifetime and absorption spectrum redshift of (S, Se, Te) double-doped ZnO

Considering the mismatch of lattice constants between the substrate and the (S, Se, Te) double-doped ZnO system and of the thermal expansion coefficients between the substrate and the doped system, the doped system was subject to external strain. Previous studies have neglected this issue. In the current research, we first applied biaxial strain on the model and then performed density functional theory calculation. The effects of applying different strains on the stability of the doped system, the red shift of the absorption spectrum, the trap effect, and the carrier lifetime were determined. The research results showed that Zn36SSeO34, Zn36STeO34, and Zn36SeTeO34 systems had smaller formation energies and better stability under the Zn-rich condition than under the O-rich condition. All doped systems had negative cohesive energies and good stability. With the synergistic effect of strain and doping, when the biaxial compressive strain of the Zn36STeO34 system was − 5%, the hole–electron separation was good, the trap effect was substantial, the hole life increased, the absorption spectrum had a better red shift, and the oxidation reaction improved. These findings had a certain theoretical guiding effect on photocatalysts for the decomposition of water for oxygen production.

First-principle study of new insights on the magnetic mechanism of ZnO through rare-earth Nd doping and Zn vacancies

Theoretical and experimental studies on the magnetic mechanism of Nd doping or Nd doping and Zn vacancies on ZnO have been reported. However, the magnetic source and mechanism of the doping system are controversial. Thus far, no reasonable theoretical explanation exists. First principles can solve such problems. In this study, the first principles are used to investigate the influence of Nd doping or Nd doping and Zn vacancies on the magnetic source and mechanism of ZnO based on the generalized gradient approximation plane wave supersoft pseudopotential method under the framework of spin density functional theory. The calculation results show that the formation energy of the Zn15NdO16 system or the Zn14NdO16 system is smaller and the doping is easier under the O-rich condition than those under the Zn-rich condition. Both doping systems have high binding energy and high stability. The study found that the magnetism of the Zn15NdO16 system is mainly produced by the spin polarization of the s, p, d, and f states of a single Nd3+ ion. This phenomenon is consistent with the mechanism by which a single ion in a compound produces magnetism. The magnetism of the Zn14NdO16 system is produced not only by the double exchange of the p, d, and f state itinerant electrons of a single Nd3+ ion but also by the d and f state exchange mediated of the O1−-2p state itinerant electrons. This finding is consistent with the explanation of the carrier-mediated double exchange mechanism. The new insights on the magnetic origin and mechanism of doped Nd or doped Nd and Zn vacancy coexisting ZnO mediated by itinerant electron have a leading role in studying similar dilute magnetic semiconductors.

The origin of opto-functional enhancement in ZnO/CuO nanoforest structure fabricated by submerged photosynthesis

Semiconductor hetero-nanostructures are of great interest for practical industry use. In this report, we demonstrated ZnO/CuO nanoforest (NFRs) fabricated by galvanic submerged photo-synthesis (G-SPSC) method, which utilizes light illumination in pure water without additives. For the first time, we elucidated the origin of its enhanced opto-functional properties. At the hetero-epitaxial interface, ZnO(001)/CuO(001) planes linkage were established, even though with 13.62 – 28.15% local lattice discrepancies along the c axis. Formed under Zn rich condition and photo-induced {110} ZnO surface, the NFRs exhibited photoluminescence emissions at 450 nm and 650 nm, due to zinc interstitial (IZn) and oxygen vacancies (VO). Ternary oxide of ZnCuO2 was discovered. The interface exhibited significant emission at 650 – 700 nm and absorbance at 450 – 500 nm. On the basis of STEM-VEELS spectra and ab initio calculations, electrons occupancy at Cu 3p was responsible for 2.0 eV peak of the interface absorption coefficient. In particular, an interface dipole under interface-induced gap states (IFIGS) was clarified, caused by quasi defects zinc antisite (ZnO). This led to an incommensurate charge density (ICCD) for a coherent ZnO(001)/CuO(001) interface. This is the origin for the opto-functional enhancement of the ZnO/CuO NFRs, where a maximum 12% IPCE at 550 nm, a 20% increase from similar NFRs morphology was demonstrated. Our results indicated a promising method and strategy for novel hetero-epitaxial nanostructures fabrication in the field of optoelectrical devices.

Structure and Photocatalytic Activity of Copper and Carbon-Doped Metallic Zn Phase-Rich ZnO Oxide Films

ZnO is one of the most important industrial metal oxide semiconductors. However, in order to fully realise its potential, the electronic structure of ZnO has to be modified to better fit the needs of specific fields. Recent studies demonstrated that reactive magnetron sputtering under Zn-rich conditions promotes the formation of intrinsic ZnO defects and allows the deposition of metallic Zn phase-rich ZnO films. In photocatalytic efficiency tests these films were superior to traditional ZnO oxide, therefore, the purposeful formation of intrinsic ZnO defects, namely Zn interstitials and oxygen vacancies, can be considered as advantageous self-doping. Considering that such self-doped ZnO remains a semiconductor, the natural question is if it is possible to further improve its properties by adding extrinsic dopants. Accordingly, in the current study, the metallic Zn phase-rich ZnO oxide film formation process (reactive magnetron sputtering) was supplemented by simultaneous sputtering of copper or carbon. Effects of the selected dopants on the structure of self-doped ZnO were investigated by X-ray diffractometer, scanning electron microscope, X-ray photoelectron spectroscope and photoluminescence techniques. Meanwhile, its effect on photocatalytic activity was estimated by visible light activated bleaching of Methylene Blue. It was observed that both dopants modify the microstructure of the films, but only carbon has a positive effect on photocatalytic efficiency.

Impact of growth conditions on the structural and magnetic properties of (Zn,Fe)Te thin films grown by molecular beam epitaxy (MBE)

We have investigated the effect of growth conditions on the structural and magnetic properties of (Zn,Fe)Te. Thin films of Zn1-xFexTe are grown by molecular beam epitaxy (MBE) with an almost same Fe composition, x = 1.4% but under two different growth conditions: (a) Te-rich (Te/Zn flux ratio of 2.3) and (b) Zn-rich (Te/Zn flux ratio of 0.7). Structural analysis by using x-ray absorption fine structure (XAFS) reveals that both films are mainly composed of pure diluted phase with substitutional Fe atoms on the Zn-site, but the valence state of Fe atoms in the film grown under Zn-rich condition has deviated more from isoelectronic Fe2+ as compared to the one grown in Te-rich condition. Accordingly, the magnetization measurement using SQUID confirms a significant difference in magnetic properties among these films; a linear dependence of magnetization on magnetic field (M−H), typical of van-Vleck paramagnetism under Te-rich growth condition turns into ferromagnetic behaviors with hysteretic M−H curve in the Zn-rich condition. The ferromagnetism exhibited by the Zn-rich growth film may reflect the deviation of substitutional Fe valence state from Fe2+ to Fe2+/3+ mixed states.

Effects of oxygen flow ratio and thermal annealing on defect evolution of aluminum doped zinc oxide thin films by reactive DC magnetron sputtering

Al doped ZnO (AZO) is a promising transparent conducting oxide to replace the expensive Sn doped In2O3 (ITO). Understanding the formation and evolution of defects in AZO is essential for its further improvement. Here, we synthesize transparent conducting AZO thin films by reactive DC magnetron sputtering. The effects of oxygen flow ratio as well as the rapid thermal annealing (RTA) in different conditions on their structural and optoelectrical properties were investigated by a variety of analytical techniques. We find that AZO thin films grown in O-rich conditions exhibit inferior optoelectrical performance as compared with those grown in Zn-rich conditions, possibly due to the formation of excessive native acceptor defects and/or secondary phases (e.g. Al2O3). Temperature-dependent Hall measurements indicate that mobilities of these highly degenerate AZO films with N > 1020 cm−3 are primarily limited by ionized and neutral impurities, while films with relatively low N ∼ 1019 cm−3 exhibit a temperature-activated mobility owing to the grain-barrier scattering. As N increases, the optical band gap of AZO thin film increases as a result of Burstein–Moss shift and band gap narrowing. RTA treatments under appropriate conditions (i.e. at 500 °C for 60 s in Ar) can further improve the electrical properties of AZO thin film, with low resistivity of ∼6.2 × 10−4 Ω cm achieved, while RTA at high temperature with longer time can lead to the formation of substantial sub-gap defect states and thus lowers the electron mobility. X-ray photoelectron spectroscopy provides further evidence on the variation of Al (Zn) content at the surface of AZO thin films with different processing conditions.

Influence of vacancy defects on the electronic structure and magnetic properties of Cu-doped ZnO monolayers: A first-principles study

The development of available electronic structures and magnetism has received considerable interest for applications in nanoelectronic and spintronic devices. In this paper, we investigated the electronic and magnetic properties of Cu-doped defective ZnO monolayers by first-principles calculations. The results demonstrate that the formation energies of the VO + nCuZn complex under Zn-rich conditions are lower than those under O-rich conditions. However, the formation energies of the VZn + nCuZn complex under O-rich conditions are lower than those under Zn-rich conditions. Furthermore, the Cu dopant and vacancies both play an important role in the contribution of the magnetic moment. The total magnetic moments of the Cu2Zn14O15 monolayers remain at 1.822–1.868 µB, and the average local magnetic moment is approximately 0.439–0.443 µB per Cu atom. The magnetic moments of the O atoms close to the Cu atom are 0.124–0.163 µB per O atom. The magnetic moments of the VO + nCuZn complexes satisfy the odd-even doping rule, and the VO + nCuZn complexes undergo a transition from NM to FM with increasing Cu concentration. For the VZn + nCuZn complex, the total magnetic moments increase monotonically with an increase in the number of Cu atoms. The absolute value of total magnetic moment of the CuZn14O16 monolayer is 0.225 µB, which is contributed by the Cu dopant (0.433 µB), as well as the nearest O atoms around the Cu dopant. Furthermore, the combined effect of defect vacancy and Cu dopant could tune the ZnO monolayer from a direct semiconductor to an indirect semiconductor and even regulate the band gap. These findings are potentially useful for nanoelectronic and spintronic applications.

Structural Properties of Thin ZnO Films Deposited by ALD under O-Rich and Zn-Rich Growth Conditions and Their Relationship with Electrical Parameters.

The structural, optical, and electrical properties of ZnO are intimately intertwined. In the present work, the structural and transport properties of 100 nm thick polycrystalline ZnO films obtained by atomic layer deposition (ALD) at a growth temperature (Tg) of 100–300 °C were investigated. The electrical properties of the films showed a dependence on the substrate (a-Al2O3 or Si (100)) and a high sensitivity to Tg, related to the deviation of the film stoichiometry as demonstrated by the RT-Hall effect. The average crystallite size increased from 20–30 nm for as grown samples to 80–100 nm after rapid thermal annealing, which affects carrier scattering. The ZnO layers deposited on silicon showed lower strain and dislocation density than on sapphire at the same Tg. The calculated half crystallite size (D/2) was higher than the Debye length (LD) for all as grown and annealed ZnO films, except for annealed ZnO/Si films grown within the ALD window (100–200 °C), indicating different homogeneity of charge carrier distribution for annealed ZnO/Si and ZnO/a-Al2O3 layers. For as grown films the hydrogen impurity concentration detected via secondary ion mass spectrometry (SIMS) was 1021 cm−3 and was decreased by two orders of magnitude after annealing, accompanied by a decrease in Urbach energy in the ZnO/a-Al2O3 layers.

Bipolar self-doping in ultra-wide bandgap spinel ZnGa2O4

The spinel group is a growing family of materials with general formulation AB2X4 (the X anion typically being a chalcogen like O and S) with many advanced applications for energy. At the time being, the spinel zinc gallate (ZnGa2O4) arguably is the ternary ultra-wide bandgap bipolar oxide semiconductor with the largest bandgap (∼5eV), making this material very promising for implementations in deep UV optoelectronics and ultra-high power electronics. In this work, we further demonstrate that, exploiting the rich cation coordination possibilities of the spinel chemistry, the ZnGa2O4 intrinsic conductivity (and its polarity) can be controlled well over 10 orders of magnitude. p-type and n-type ZnGa2O4 epilayers can be grown by tuning the pressure, oxygen flow and cation precursors ratio during metal-organic chemical vapor deposition. A relatively deep acceptor level can be achieved by promoting antisites (ZnGa) defects, while up to a (n > 1019 cm−3) donor concentration is obtained due to the hybridization of the Zn–O orbitals in the samples grown in Zn-rich conditions. Electrical transport, atomic and optical spectroscopy reveal a free hole conduction (at high temperature) for p-ZnGa2O4 while for n-ZnGa2O4 a (Mott) variable range hopping (VRH) and negative magnetoresistance phenomena take place, originated from “self-impurity” band located at Ev+ ∼3.4 eV. Among arising ultra-wide bandgap semiconductors, spinel ZnGa2O4 exhibit unique self-doping capability thus extending its application at the very frontier of current energy optoelectronics.

Identification by deuterium diffusion of a nitrogen-related deep donor preventing the p-type doping of ZnO

Deuterium diffusion is investigated in nitrogen-doped homoepitaxial ZnO layers. The samples were grown under slightly Zn-rich growth conditions by plasma-assisted molecular beam epitaxy on m-plane ZnO substrates and have a nitrogen content [N] varied up to 5 × 1018 at cm−3 as measured by secondary ion mass spectrometry (SIMS). All were exposed to a radio frequency deuterium plasma during 1 h at room temperature. Deuterium diffusion is observed in all epilayers, while its penetration depth decreases as the nitrogen concentration increases. This is strong evidence of a diffusion mechanism limited by the trapping of deuterium on a nitrogen-related trap. The SIMS profiles are analyzed using a two-trap model including a shallow trap, associated with a fast diffusion, and a deep trap, related to nitrogen. The capture radius of the nitrogen-related trap is determined to be 20 times smaller than the value expected for nitrogen–deuterium pairs formed by coulombic attraction between D+ and nitrogen-related acceptors. The (N2)O deep donor is proposed as the deep trapping site for deuterium and accounts well for the small capture radius and the observed photoluminescence quenching and recovery after deuteration of the ZnO:N epilayers. It is also found that this defect is by far the N-related defect with the highest concentration in the studied samples.

Correlations of thermal properties with grain structure, morphology, and defect balance in nanoscale polycrystalline ZnO films

Two sets of ZnO thin films grown by atomic layer deposition method were investigated to analyse the correlation between morphology, chemical defects and thermal properties. This analysis became possible by measuring series of similarly thick wurtzite ZnO films synthesized using identical number of deposition cycles at two temperatures – 100 °C and 200 °C. By comparing these sample series, exhibiting the thickness variations from 12 to 118 nm, we observed several prominent correlations for the thermal properties as measured by high-resolution scanning thermal microscopy at room temperature. Specifically, comparing thinnest samples in the series, a lower thermal conductivity was revealed in samples grown at 100 °C and these variations were interpreted in terms of changing populations of native donors, affecting the free carrier concentration and, as such, the thermal conductivity. Further, the increase in thickness for the samples grown at 200 °C was accompanied with a continuous shift to Zn-rich conditions and the corresponding increase in the thermal conductivity, specifically from 0.28 Wm−1 K−1 to 2.81 Wm−1 K−1. As such, we have decoupled the thickness effect on the thermal conductivity from the rest of the phenomena, explaining the thermal transport in terms of the bulk thermal contributions within the grains combined with additional thermal resistance introduced by the grain boundaries.

Tuning electronic structure of ZnO nanowires via 3d transition metal dopants for improved photo-electrochemical water splitting: An ab initio study

ZnO nanowires have been proposed as potential photo-anode materials for photo-electrochemical water splitting due to their low toxicity, simple synthesis and easy modification routes. However, ZnO suffers from low PEC activity and photo-corrosion eff ;ects, and therefore, application of ZnO nanowires in PEC water splitting still awaits development of effective design and synthesis strategies to improve its PEC efficiencies to commercially viable levels. Here, we present ab initio Density Functional Theory calculations considering 3d transition metal doping as a potential route towards attainment of ZnO nanowires with superior PEC activity. Our results show that the stability of 3d transition metal dopants in ZnO NWs is dependent on the d character of the transition metal dopant as well as their concentration and doping site, with most transition metal atoms being energetically most favorable at the Zn substitutional site both in O-rich and Zn-rich conditions considered. Specifically, we find all 3d transition metal dopants in ZnO NW under O-rich conditions as well as Sc, Ti and V under Zn-rich conditions have negative formation energies at the considered dopant concentrations of 1−6 atm. %, indicating that these dopants can readily be incorporated into ZnO NWs at thermodynamic equilibrium conditions. The electronic properties of Ti and V at 2% and 4% dopant concentration, respectively, yield a staggered band-structure configuration, while Sc, Cr, Mn, Co, Ni, and Cu dopants in ZnO NWs induce band-edge states. In addition, 3d TM dopants induces significant red-shift of the absorption edge of ZnO NW due to reduction in band gap, and are projected to improve visual light harvesting capabilities. Finally, the band alignment relative to the redox potential of water revealed that the valence band maximum of Sc, V, Ni and Cu doped ZnO NWs remains strongly positive above the oxidation potential of O2/H2O, while their reduction potential remain negative below the reduction potential of H+/H2, favouring PEC applications.

Effects of different valence states of Mo and point vacancies on magneto-optical properties of ZnO

Experimental studies have been conducted to characterize the physical properties of Mo-doped ZnO. However, they have not considered the effects of different valence states and point vacancies on the magneto-optical performance of the system. The present study adopted the plane-wave ultrasoft pseudopotential + U method within the framework of spin density functional theory. The formation energy, electronic structure, and optical properties of ZnO systems with coexisting different valence states of Mo and point vacancies were calculated. Results showed that the formation energy of all doping systems was negative under Zn-rich and O-rich conditions, but the formation energy was relatively lower under O-rich conditions than under Zn-rich conditions. Compared with Mo6+ and Mo5+ when the concentration of O vacancies or Zn vacancies was the same, Mo6+-doped ZnO had the lowest formation energy, was the easiest to dope, and had the most stable system structure. Zn34Mo5+O36 and Zn34Mo6+O36 systems exhibited ferromagnetism, and their Curie temperature was above room temperature. The Zn34Mo6+O36 system had the largest magnetic moment, which is beneficial for designing and preparing new types of diluted magnetic semiconductors. The band gap widths of the Zn34Mo5+O36 (3.04 eV) and Zn34Mo6+O36 (2.92 eV) systems were both narrower than that of the pure ZnO system. By comparison, the band gap widths of the Zn35Mo5+O35 and Zn35Mo6+O35 systems were 4.07 and 3.94 eV, respectively. Although the band gap width widened, under light excitation conditions, the valence band electrons could absorb the photons of lower energy to transition to the impurity energy level, and then transition further from the impurity energy level to the conduction band. During the hierarchical transition, visible light and infrared light effects still occurred. The range of photon energy response of all doping systems expanded, and the absorption spectrum red-shifted in the light-absorption edge within the low-energy (0–3.2 eV) range. In particular, the Zn35Mo6+O35 system exhibited the strongest polarization ability, photoelectric field strength, and charge-binding ability in the low-energy region. Moreover, the dielectric and absorption peaks of the imaginary part of the dielectric function were relatively large, and the red-shift of the absorption spectrum was relatively substantial. Therefore, the Zn35Mo6+O35 system is useful in designing and preparing novel ZnO-based photocatalysts.

Intermittent sulfurization—a method promoting Macro-Porous Cu-Poor Zn-Rich—kesterite CZTS as HTM for inverted perovskite solar cell application

Porous semiconductors have emerged as a lucrative apparatus for energy-harvesting applications with more exceptional ability as compared to their bulk counterpart. Exposure of a new porous formation strategy for the abundant semiconductor CZTS could initiate an extensive exploration for solar cell applications. Available porous formation strategies for the CZTS have neglected a significant factor of its compositional balance, which needs to be assisted carefully. In this work, intermittent sulfurization technique is used to promote macro-porous CZTS towards perovskite solar cell applications. This technique not only promotes the macro-porosity of CZTS but also successfully controls its compositional balance to fulfill the favorable Cu-poor Zn-rich condition. Every tuned step of this technique causes a different morphology and composition, which has been investigated by FESEM and EDX mapping. Meanwhile, characterization of phase, crystallinity, and optical bandgap have also been investigated by XRD, Raman spectroscopy, and UV–Vis spectroscopy, respectively, for the resulted in macro-porous Kesterite CZTS. Inverted (p-i-n)-structured perovskite solar cell with p-type macro-porous CZTS as hole transporter material delivered 3.33% power conversion efficiency. Synthesis of perovskite material and electron transporting material along with the fabrication of (p-i-n)-structured perovskite solar cells have also been detailed here.