Substitution Of Zn Atoms Research Articles

Double Helix of Icosahedra Structure and Spin Glass Magnetism of the δ-Co2.5Zn17.5-xMnx (x = 0.4-3.5) Pseudo-Binary Alloys.

We have synthesized δ-Co2.5Zn17.5-xMnx (x = 0.4-3.5) pseudo-binary alloys of 10 different compositions by a high-temperature solid-state synthetic route, determined their crystal structures and the Mn substitution pattern, and estimated the existence range of the δ-phase. The alloys crystallize in two chiral enantiomorphic space groups P62 and P64, where the basic atomic polyhedron of the chiral structure is an icosahedron and the neighboring icosahedra share vertices to form an infinitely long double helix along the hexagonal axis (like in the δ-Co2.5Zn17.5 parent binary phase). The alloys are pure δ-phase up to the Mn content x ≈ 3.5. The Mn atoms partially substitute Zn atoms at particular crystallographic sites located on the icosahedra. The study of magnetism was performed on the Co2.5Zn17.1Mn0.4 alloy with the lowest Mn content. Contrary to the expectation that structural chirality may induce the formation of a nontrivial magnetic state, a spin glass state with no relation to the structural chirality was found. The magnetic sublattice contains all of the necessary ingredients (randomness and frustration) for the formation of a spin glass state. Typical out-of-equilibrium dynamic phenomena of a spin system with broken ergodicity were detected below the spin freezing temperature Tf ≈ 8 K.

Local electron-rich structures facilitating the efficient synthesis of dimethyl carbonate from propylene carbonate and methanol over a ZnO/ZrO2 solid solution catalyst

ZnO/ZrO2 solid solution was introduced as the catalyst for synthesizing dimethyl carbonate (DMC) from propylene carbonate (PC) and methanol. Homogeneous catalysts with strong alkaline nature are the current main catalysts for this reaction. Therefore, to supplant the former with a more applicable multiphase one, it is necessary to construct stable and efficient alkaline active centers on the surface of solid catalysts. The ZnO/ZrO2 solid solution created defective structures of coordinative unsaturation in the form of oxygen vacancies (OV) by substituting Zn atoms for Zr atoms in ZrO2 crystals. Characterizations showed that electrons were transferred to the surrounding Zn atoms upon the formation of these OV, leading to a local enrichment of electrons and the formation of basic active centers. The number of oxygen defects and the alkalinity were effectively regulated by changing the ratio of the two metal atoms in the solid solution. Experiments demonstrated that the catalyst had the best DMC selectivity (92.2%) and the highest DMC yield (57.2%) at a 1:1 ratio of Zn to Zr, at 160 °C and 4 h. This superior performance not only establishes the feasibility of solid solution catalysts in synthesizing DMC but also provides a new perspective on the essential understanding of the active centers of such catalysts.

Effects of Surface Polarity on the Structure and Magnetic Properties of Co Implanted and Co-Sm Co-Implanted Polar ZnO Films.

We present a comprehensive experimental and theoretical study of the effects of surface polarity on the structure and ferromagnetic properties of Co implanted and Co-Sm co-implanted polar ZnO films deposited on sapphire substrates by molecular beam epitaxy. Substantial intrinsic ferromagnetism (FM) is found for all the implanted polar ZnO films. The magnetization of O-polar ZnO is observed to be higher than that of Zn-polar ZnO under the same implantation conditions, and the magnetization is enhanced for Co-Sm co-implanted ZnO in contrast with unimplanted and Co implanted films. First-principles calculations reveal that the Sm 4f and Co 3d states have strong hybridization with the O 2p state in O-polar ZnO, leading to larger magnetic moments for Co and Co-Sm substituting Zn atoms on the O-polar surface. Meanwhile, X-ray photoelectron spectroscopy results confirm that more oxygen vacancies are introduced into O-polar films by implantation and annealing. We consider that the stronger ferromagnetism in O-polar ZnO is associated with the combined influence of more oxygen vacancies and larger local moments related to Co and Sm doping. These results not only contribute to understanding the origin of FM in diluted magnetic semiconductors but also highlight the feasibility of developing polar spintronic devices for future polar thin film systems.

A dual-functional flexible sensor based on defects-free Co-doped ZnO nanorods decorated with CoO clusters towards pH and glucose monitoring of fruit juices and human fluids

Herein, ZnO nanorods were doped with Co and decorated with CoO clusters through an in situ technique to create a CoO/Co-doped ZnO (CO/CZO) heterostructure at low temperatures (150 °C) on a flexible PET substrate. In the CO/CZO heterostructure, the Co dopant has a low energy barrier to substitute Zn atoms and adsorb over oxygen atoms and their vacancies. Therefore, it decreased the charge density (ND = 2.64 × 1019 cm−3) on non-active sites of ZnO and lowered the charge transfer resistance (317 Ω) at Co-doped-ZnO/electrolyte interface by suppressing the native defects and reducing the Schottky barrier height (− 0.35 eV), respectively. Furthermore, CoO clusters induced a p-n heterostructure with Co-doped ZnO, prevented corrosion, increased the active sites for analyte absorption, and increased the ultimate tensile strength (4.85 N m−2). These characteristics enabled the CO/CZO heterostructure to work as a highly sensitive, chemically stable, and flexible pH and glucose oxidation electrode. Therefore, CO/CZO heterostructure was explored for pH monitoring in human fluids and fruit juices, demonstrating a near-Nernst-limit pH sensitivity (52 mV/pH) and fast response time (19 s) in each human fluid and fruit juice. Also, it demonstrated high sensitivity (4656 µM mM−1 cm−2), low limit of detection (0.15 µM), a broad linear range (0.04 mM to 8.85 mM) and good anti-interference capacity towards glucose-sensing. Moreover, it demonstrated excellent flexibility performances, retained 53% and 69% sensitivity of the initial value for pH and glucose sensors, respectively, after 500 bending, stretching, and warping cycles.Graphical

Three-zone model for Ti, Al co-doped ZnO films deposited by magnetron sputtering

Al-doped ZnO is well known as an alternative Transparent Conductive Oxide (TCO) to Indium Tin Oxide (ITO). Recently, co-doped ZnO with Ti and Al (TAZO) have been explored to improve the conductivity while maintaining moisture durability and opto-electronic properties. In this context, the structural, morphological, and optical properties of TAZO films deposited by reactive magnetron are investigated by experiments and Density Functional Theory (DFT) simulations. The results show that TiO2 units substitute Zn atoms in the ZnO wurtzite structure and, whatever the concentration of Ti, the formation of Al2O3 is taking place. In addition, a three-regime model as a function of Ti content is proposed to explain the evolution of the properties. First, for Ti content 〈 2.2 at%, TiO2 is incorporated by substitution of Zn by TiO2 units in the ZnO network leading to a more oxidized Zn state and a hybridization between Zn4s and Ti3d orbitals. This opens the bandgap and increases the compressive stress of TAZO films. Between 2.2 at% and 7.0 at% of Ti content, cauliflower shape grains appear due to the compressive stress induced by small clusters of TiO2. For Ti content 〉 7.0 at%, amorphous TAZO films are observed and are composed of TiO2, Al2O3 and ZnO.

Atomic structures and electronic properties of Cr-doped ZnO() surfaces

An integrated approach combining scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) is used to investigate the atomic structures and electronic properties of Cr-doped ZnO(▪) surfaces. When deposited at 300 K, Cr at low surface coverage (< 0.1 ML) appeared either as isolated atoms on the surface terrace of ZnO(▪) or substituting Zn atoms in the ZnO lattice. Their structural models could be identified from atomic-resolution STM images and their oxidation states were found as Cr3+ based on XPS measurements. Rectangular islands nucleated at step edges along the [0001] direction could also be observed during the initial growth of Cr at 300 K and were assigned as Cr islands. The density of Cr islands as well as their average size increased with the increasing of Cr surface loading. Thermal treatments at above 600 K could facilitate the decomposition of Cr islands and the re-dispersion of Cr atoms into the ZnO lattice, indicating a strong interaction between Cr and ZnO. The adsorption of CO at 78 K showed no preferential adsorption at Cr3+ sites embedded in the surface lattice of ZnO. However, the re-dispersion of Cr atoms into the ZnO bulk at above 600 K could induce a significant upward band bending, causing a negative shift of core level XPS peaks of Zn 2p and O 1s by ~0.5–0.7 eV. Our study has thus constructed a model catalyst for Cr-doped ZnO and provided atomic insight for understanding ZnO-based catalysts.

Synergy between Cu doping and catalytic platform in 2D Ni-MOFs/Cu-Zn0.5Cd0.5S for efficient water-to-hydrogen conversion

Doping strategy has been widely studied and applied to design visible light driven photocatalysts, which can introduce defect level and modulate the band structure of photocatalysts to improve the photocatalytic activity. In addition, it is of great significance to introduce a catalytic platform in the photocatalytic H2 evolution (PHE) system, which can also greatly improve the photogenerated carriers transport capacity. As is known to all, 2D materials have been proven to be crucial platform for PHE reactions. In view of this, we have successfully prepared the 2D Ni-MOFs/Cu-doped Zn0.5Cd0.5S-x (abbreviated as NMCZ-x) heterojunction with the preferential orientation of crystals, which endows the catalyst with extremely excellent PHE activity and chemical stability. DFT calculations indicated that Cu atoms served as the center of electron capture after substituting Zn atoms, and Cu doping will introduce defect level. The NMCZ-10 complex achieves the best PHE activity and superior structural stability. This work highlights the importance of the synergistic effect between element doping and catalytic platform for the preparation of advanced photocatalysts.

Electronic and optical properties of ZnO nanosheet doped and codoped with Be and/or Mg for ultraviolet optoelectronic technologies: density functional calculations

Theoretically, the density functional calculations have been effectuated for investigating electronic and optical properties of zinc oxide nanosheet doped and codoped with Be and/or Mg utilizing the generalized gradient approximation modified Becke–Johnson (GGA-mBJ) approach. The computed results show that the ZnBeO, ZnMgO and ZnBeMgO in nanosheet structure, referring to their low formation energy values, are more stable than those in bulk one. Furthermore, the bandgap of ZnO monolayer can be effectively modulated through substitution of Zn atoms by Be and/or Mg. In addition to that, by incorporating Be and/or Mg, the absorption peaks of ZnO nanosheet shift into the shorter UV-wavelength side as well as its reflectivity becomes lower. These results indicate that doping and codoping process of ZnO monolayer with Be and/or Mg are two efficient ways to modulate electronic and optical properties for ultraviolet optoelectronic technologies.

Stability and band offsets between c-plane ZnO semiconductor and LaAlO3 gate dielectric

Wurtzite-perovskite heterostructures composed of a high dielectric constant oxide and a wide bandgap semiconductor envision promising applications in field-effect transistors. In the present paper, the structural and electronic properties of LaAlO3/ZnO heterojunctions are investigated by first-principles calculations. We study the initial adsorption of La, Al, and oxygen atoms on ZnO (0001) and (0001¯) surfaces and find that La atoms may occupy interstitial sites during the growth of stoichiometric ZnO (0001). The band gap of the stoichiometric ZnO (0001) surface is smaller than that of the stoichiometric ZnO (0001¯) surface. The surface formation energy indicates that La or Al atoms may substitute Zn atoms at the nonstoichiometric ZnO (0001) surface. The atomic charges, electronic density of states, and band offsets are analyzed for the optimized LaAlO3/ZnO heterojunctions. There is a band gap for the LaAlO3/ZnO (0001¯) heterostructures, and the largest variation in charge occurs at the surface or interface. Our results suggest that the Al-terminated LaAlO3/ZnO (0001¯) interfaces are suitable for the design of metal oxide semiconductor devices because the valence and conduction band offsets are both larger than 1 eV and the interface does not produce any in-gap states.

Experimental and theoretical study on the structural, electrical and optical properties of tantalum-doped ZnO nanoparticles prepared via sol-gel acetate route

Pure and Ta-doped ZnO were prepared by the sol-gel method in acetic medium using different annealing temperature treatments. The effects of low Ta doping on the crystallinity and electrical properties of ZnO nanoparticles were analyzed by means of X-ray diffraction and micro-Raman, UV–visible, and impedance spectroscopies.We also performed first principles calculations in order to study the predicted changes in the structural, vibrational and electronic properties induced by the inclusion of the impurities, and to complement the experimental measurements.We showed that above thermal treatments at 600°C the precursor samples take the hexagonal wurtzite ZnO phase with high crystallinity. For the doped samples, we found that the synthesizing method has good Ta doping efficiency of the ZnO host structure. Also, Ta doping substantially decreases the resistivity compared to pure ZnO. These results confirm that Ta impurities can substitute Zn atoms and act as donor impurities in the host semiconductor.

Structure, phase transition and impedance of Zn1−x Li x O (0.10 ≤ x ≤ 0.70) ceramic

Structural transformations of Li-doped Zn1−xLixO (0.10 ≤ x ≤ 0.70) which was synthesised by solid-state reaction were investigated. XRD carried out on powder specimens of the samples show that they are polycrystalline in nature with a hexagonal wurtzite structure having minor impurities. The result indicates the maximum limit of substitution of Zn atoms by Li is at x = 0.4. The lattice parameter a reduced from 3.01 to 2.99 A, while c reduced from 5.21 to 5.19 A. However, the Zn–O bond length reduced from 1.88 to 1.87 A for the undoped, to x = 0.60 for the doped, respectively. The c/a ratio is 1.73 and is almost constant for all samples. The grain size of the (100) peak of the undoped ZnO is 41.73 nm and that of x = 0.10 is 41.76 nm. For x = 0.2–0.70, the grain size is 41.72 nm indicating that the grain size is almost independent of doping. The SEM results indicate a variation of grain size from 2.18 to 5.15 µm for the undoped ZnO to x = 0.50, which shows increase in grain size and reduction in grain boundaries as doping increases. The results show that x = 0.50 has the highest grain size and the one with the highest transition temperature is x = 0.6. DTA results indicate the structural phase transition temperature of the doped ZnO ranged from ~371 to ~409 K and increased as the amount of Li increases. A single arc is observed in all the impedance plots of the ZnO together with the presence of a relaxation process which is non-Debye. The impedance data show reduced resistance with increase in lithium content. A general increase in dielectric constant with increase in lithium content is observed.

Influence of the Ba-Doping to Improvement of Ferroelectric, Optical and Electronic Properties of Wurtzite-ZnO Material: A DFT Study

The effects of Ba doping on structural, electronic and optical properties of crystalline wurtzite-ZnO were analyzed by DFT/B3LYP calculations. The substitution of Zn atoms causes an increase of lattice parameters and small variation of unit cell angles. Likewise, the doping changes the optical properties, reducing the band-gap by 0.2 eV and modifying the associated wavelength to the visible spectrum range. In electronic properties, the number of charge carriers for pure and doped materials were also evaluated and it was verified for both materials that the number of electrons are higher than holes; therefore, the doping does not change the ZnO electronic nature, an n-type semiconductor. After the doping process, the ferroelectric properties monitored through the ionic bond character, dielectric constants and polarizability show a good improvement. Keywords: Computer modelling and simulation, electronic structure, oxides, semiconductors, ferroelectricity.

Structural, chemical and optical evaluation of Cu-doped ZnO nanoparticles synthesized by an aqueous solution method

In this work a study of ZnO and Cu-doped ZnO nanoparticles obtained by chemical synthesis in aqueous media was carried out. Structural analysis gave the dominant presence of wurtzite ZnO phase forming a solid solution Zn1−xCuxO. For high Cu doping CuO phase is also present. For low Cu concentration the lattice shrinks due to Cu atoms substitute Zn atoms. For high Cu concentration the lattice enlarges due to predominance of interstitial Cu. From elemental analysis we determined and analyzed the incorporation efficiency of Cu atoms in Zn1−xCuxO as a function of the Cu concentration in the precursor dissolution. Combining structural and chemical results we described the Cu/Zn precursor concentrations rw in which the solid solution of Cu in ZnO is predominant. In the region located at rw≈0.2–0.3 it is no longer valid. For Cu/Zn precursor concentration rw>0.3 interstitial Cu dominates, and some amount of copper oxide appears. As the Cu concentration increases, the effective size of nanoparticles decreases. Photoluminescence (PL) measurements of the Cu-doped ZnO nanoparticles were carried out and analyzed.

Doping-induced electron density modification at lattice sites of ZnO:Ga nanostructures: effects on vibrational and optical properties

Effects of Ga doping on the morphology, microstructure, electron density distribution, and optical properties of hydrothermally grown ZnO nanostructures have been studied by means of scanning electron microscopy, diffuse reflectance spectroscopy, X-ray diffraction, and the maximum entropy methods. It has been shown that while Ga incorporation in ZnO lattice does not result in a large distortion of its wurtzite structure, it affects substantially the electronic charge distribution along the Zn–O bonds. Anisotropic redistribution of the electron charge density around the cation sites consolidates the assumption that the Ga atoms in doped nanostructures incorporate by substituting Zn atoms. The formation of a high density of point defects modifies the lattice dynamics of ZnO; in addition, it introduces a pronounced band-tail in the forbidden band gap.

Anomalous diffusion of Ga and As from semi-insulating GaAs substrate into MOCVD grown ZnO films as a function of annealing temperature and its effect on charge compensation

The diffusion behavior of arsenic (As) and gallium (Ga) atoms from semi-insulating GaAs (SI-GaAs) into ZnO films upon post-growth annealing vis-à-vis the resulting charge compensation was investigated with the help of x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy. The films, annealed at 600 ºC and 700 ºC showed p-type conductivity with a hole concentration of 1.1 × 1018 cm−3 and 2.8 × 1019 cm−3 respectively, whereas those annealed at 800 ºC showed n-type conductivity with a carrier concentration of 6.5 × 1016 cm−3. It is observed that at lower temperatures, large fraction of As atoms diffused from the SI-GaAs substrates into ZnO and formed acceptor related complex, (AsZn–2VZn), by substituting Zn atoms (AsZn) and thereby creating two zinc vacancies (VZn). Thus as-grown ZnO which was supposed to be n-type due to nonstoichiometric nature showed p-type behavior. On further increasing the annealing temperature to 800 ºC, Ga atoms diffused more than As atoms and substitute Zn atoms thereby forming shallow donor complex, GaZn. Electrons from donor levels then compensate the p-type carriers and the material reverts back to n-type. Thus the conversion of carrier type took place due to charge compensation between the donors and acceptors in ZnO and this compensation is the possible origin of anomalous conduction in wide band gap materials.

Theoretic insights into the Ag doping in monolayer and bilayer ZnO armchair nanoribbons: edge effect and position-dependent properties

One-dimensional nanoribbons, thickness of which is smaller than width, exhibit special confinement and edge effects far from uniform in the cross-section. By means of density functional theory, we investigated the influence of the edges and doping positions on the electronic structure of Ag-doped ZnO armchair nanoribbons (ZnOANRs). Ag doping in monolayer and bilayer ZnOANRs (m- and b-ZnOANRs) have all been examined. The results indicated that there is no significant difference on the stability of Ag doping in different positions of both m- and b-ZnOANRs, but exhibits very different electronic properties directly related to the different doping positions. The depth of the acceptor states has essential relationship with the interaction between host O 2p and Ag 4d states in the acceptor. Ag substituting Zn atoms at the inner region of m-ZnOANRs could create shallow acceptors with small hole effective masses, benefit for p-type conduction. Ag doping in the inner region of b-ZnOANRs would create shallow acceptors but larger hole effective masses. The difficulty of hole mobility could be improved by increasing the Ag doping concentration in b-ZnOANRs. The discussion about the mechanism and the suggestion about the achievement in the experiment are interesting and timely.

Mg doping in wurtzite ZnO coupled with native point defects: A mechanism for enhanced n-type conductivity and photoluminescence

From first-principles calculations, we show that Mg energetically prefers to substitute Zn atoms rather than occupying interstitial octahedral or tetrahedral sites in ZnO structure. The Mg substitutions significantly reduce the formation energies of Zn/O vacancy and Zn interstitial defects, which consequently promotes the n-type conductivity and photoluminescence in ZnO. The defects form complexes with moderate binding energies, as supported by the shorter Mg-O bonds relative to Zn-O bonds. In agreement with available experimental data, Mg substitutions result in contraction along c-axis and extension along a-axis in wurtzite ZnO.

Influence of interstitial beryllium on properties of ZnO: A first-principle research

First-principles calculation reveals that doping of Be atoms at interstitial positions of ZnO is energetically favorable with negative heats of formation and would be probably co-existed with substitution of Zn by Be in real situations, which was neglected by other theoretical studies in the literature. Calculation also shows that substitution of Zn atoms by Be could enhance the band gap of ZnBeO phases, while interstitial Be atoms would decrease the band gap, and that such changes of band gap are fundamentally due to the increase of covalent and metallic bonding as a result of Be doping, respectively. The calculated results agree well with experimental observations and could clarify the controversies of the ZnBeO phases in the literature.

Crystal structure and thermoelectric properties of KxBa8−xZnyGe46−y clathrates

Polycrystalline samples of degenerate n-type KxBa8−xZnyGe46−y (y∼8-x/2) with the type-I clathrate structure (No. 223, Pm3¯n) were prepared by powder metallurgy to obtain a high-efficiency Ge-based clathrate. Their Zn atoms preferred to exist at the 6c site in the framework, and consequently, the samples with x around 4, such as K4Ba4Zn6Ge40, possessed highly ordered Zn/Ge atom frameworks whose 6c, 16i, and 24k sites were occupied almost solely by Zn, Ge, and Ge atoms, respectively. In spite of such ordered structures and small numbers of substituting Zn atoms, these samples exhibited carrier mobilities lower than those of Ba8Zn8Ge38 and Ba8Ga16Ge30. Band structure calculations implied that the combination of the rattler K and Ba atoms in the cages considerably modified the conduction band edge of the corresponding clathrates; such a modification is considered to strengthen alloy disorder scattering, which reduces carrier mobility. The maximum dimensionless figure-of-merit ZT was 0.51 at 1000 K for the K2Ba6Zn7Ge39 sample, which is similar to that of 0.50 at 900 K for the Ba8Zn8Ge38 sample.

Room temperature magnetic properties of Fe and C implanted ZnO films

ZnO films prepared by radio frequency magnetron sputtering were singly or sequentially implanted with 120 keV Fe ions at a fluence of 5 × 10 16 ions/cm 2 and 20 keV C ions at a fluence of 3 × 10 15 ions/cm 2. Magnetic and optical properties as well as structures of the films have been investigated using various techniques. Magnetic measurements show that the as-deposited ZnO film presents room temperature ferromagnetism. Single Fe or C ion implantation has no contribution to enhancement in the film magnetism, while magnetic moment increases distinctly in the Fe and C ions sequentially implanted film. Results from structural measurements reveal that Fe nanoparticles are formed in the Fe singly implanted ZnO film. The post C implantation induces dissolution of Fe nanoparticles and promotes Fe atoms to substitute Zn atoms in the lattice. Based on the structural results, the effect of magnetic enhancement has been tentatively interpreted.