ZnO Supercell Research Articles

Resistance switching characteristics of Ag/ZnO/graphene resistive random access memory

In this work, an Ag/ZnO/Graphene structure resistive random access memory (RRAM) is prepared through RF magnetron sputtering. Compared to the Ag/ZnO/Au structure device, the endurance of the device using a graphene electrode up to 3 × 103 cycles. It has a self-current compliance function of 2 × 10−4 A and 30 times switching resistance ratio and characteristics without electroforming. The density of states (DOS) of ZnO supercells with different space groups are calculated by using the first-principles, and it is analyzed that the contributions of different supercells to the conductivity and contact type of the RRAM. Combined with the I–V curve of the RRAM, it illustrated that the conduction path is created by the space-charge-limited conduction (SCLC) of electrons captured by oxygen vacancy traps. The biocompatibility of the device is tested by a cell toxicity test. It proves that the Ag/ZnO/Graphene RRAM has high biocompatibility and broad applicability in implantable biomedical devices.

The UV Effect on the Chemiresistive Response of ZnO Nanostructures to Isopropanol and Benzene at PPM Concentrations in Mixture with Dry and Wet Air

Towards the development of low-power miniature gas detectors, there is a high interest in the research of light-activated metal oxide gas sensors capable to operate at room temperature (RT). Herein, we study ZnO nanostructures grown by the electrochemical deposition method over Si/SiO2 substrates equipped by multiple Pt electrodes to serve as on-chip gas monitors and thoroughly estimate its chemiresistive performance upon exposing to two model VOCs, isopropanol and benzene, in a wide operating temperature range, from RT to 350 °C, and LED-powered UV illumination, 380 nm wavelength; the dry air and humid-enriched, 50 rel. %, air are employed as a background. We show that the UV activation allows one to get a distinctive chemiresistive signal of the ZnO sensor to isopropanol at RT regardless of the interfering presence of H2O vapors. On the contrary, the benzene vapors do not react with UV-illuminated ZnO at RT under dry air while the humidity’s appearance gives an opportunity to detect this gas. Still, both VOCs are well detected by the ZnO sensor under heating at a 200–350 °C range independently on additional UV exciting. We employ quantum chemical calculations to explain the differences between these two VOCs’ interactions with ZnO surface by a remarkable distinction of the binding energies characterizing single molecules, which is −0.44 eV in the case of isopropanol and −3.67 eV in the case of benzene. The full covering of a ZnO supercell by H2O molecules taken for the effect’s estimation shifts the binding energies to −0.50 eV and −0.72 eV, respectively. This theory insight supports the experimental observation that benzene could not react with ZnO surface at RT under employed LED UV without humidity’s presence, indifference to isopropanol.

Oxygen vacancy in ZnO-w phase: pseudohybrid Hubbard density functional study

The study of zinc oxide, within the homogeneous electron gas approximation, results in overhybridization of zinc 3d shell with oxygen 2p shell, a problem shown for most transition metal chalcogenides. This problem can be partially overcome by using LDA + U (or, GGA + U) methodology. However, in contrast to the zinc 3d orbital, Hubbard type correction is typically excluded for the oxygen 2p orbital. In this work, we provide results of electronic structure calculations of an oxygen vacancy in ZnO supercell from ab initio perspective, with two Hubbard type corrections, UZn-3d and UO-2p. The results of our numerical simulations clearly reveal that the account of UO-2p has a significant impact on the properties of bulk ZnO, in particular the relaxed lattice constants, effective mass of charge carriers as well as the bandgap. For a set of validated values of UZn-3d and UO-2p we demonstrate the appearance of a localized state associated with the oxygen vacancy positioned in the bandgap of the ZnO supercell. Our numerical findings suggest that the defect state is characterized by the highest overlap with the conduction band states as obtained in the calculations with no Hubbard-type correction included. We argue that the electronic density of the defect state is primarily determined by Zn atoms closest to the vacancy.

Effect of Li doping and point vacancy on photocatalytic properties and mechanism of ZnO

Controlling the point defect in Li-doped ZnO is difficult. In addition, the absorption spectrum distribution in Li-doped ZnO with O vacancy (VO) has two reports of red- and blue-shift experimental predictions. The photocatalytic properties of Li doping and point defect on ZnO was studied using first-principles method based on generalized gradient approximation (GGA) plane-wave super-soft pseudopotential + U (GGA + U) to solve this problem. Compared with undoped ZnO, ZnO supercell with Li replacing Zn (LiZn) and VO, LiZn and Zn vacancy (VZn), interstitial Li (Lii) and VZn, and ZnO supercell with LiZn, Lii, and VZn have a narrower band gap and a red-shifted absorption spectrum distribution. By contrast, ZnO supercell with Lii and VZn have a wider band gap and a blue-shifted absorption spectrum distribution. The effective mass calculations show that the ZnO supercell with LiZn and VO has the smallest effective mass of carriers and can thus accelerate the separation of electron–hole pairs, thereby improving photocatalytic activity. The analysis of the electric dipole moment showed that ZnO supercell with LiZn and VO has the strongest activity of carries. This condition is advantageous for designing and preparing novel photocatalysts.

Optoelectronic properties of AZO/ZnO bilayer

The series of ZnO thin films (such as pure ZnO, Al doped ZnO, and AZO/ZnO bilayer) were deposited on p-Si(100) substrates by RF magnetron sputtering technique respectively, which were investigated by numerical simulation and experiments from two aspects to study its interfacial and photoelectric properties. It’s found that the interface properties of Si-based films are affected remarkably by annealing process, and the AZO/ZnO bilayer has poor optical properties. According to the prepared films, 2 × 2 × 2 ZnO supercell, 2 × 2 × 2Al0·0625Zn0·9375O, Al0·0625Zn0·9375O/ZnO interface, and Al0·125Zn.0.875O/ZnO bilayer were constructed and analyzed based on first principles. Compared with other cases, the TDOS reveal that valence band of AZO/ZnO bilayer from −20eV to −17eV becomes wide, but the conduction band narrows considerably. The dielectric function of AZO/ZnO bilayer slightly moves to the lower energy directions (red shift), and its conductivity is much lower in the range of 7eV–35eV. Moreover, there is little difference between AZO/ZnO and AZO/ZnO bilayer in electrical and optical properties. It can be inferred that AZO/ZnO bilayer has adverse impact on the solar cells and other optoelectronic devices, which may supply some theoretical value for studying ZnO based optoelectronics.

Theoretical study of the effects of Mn: N ratios on electronic structure and magnetic properties of (Mn, N)-codoped ZnO

(Mn, N)-codope is an effective method to achieve both magnetism and p-type stability of ZnO system. In this paper, the magnetism and the p-type stability of (Mn, N)-codoped ZnO with the ratios of 1:1, 1:2 and 2:1 were analyzed through establishing a (2 × 2 × 2) ZnO supercell, in which Zn site was replaced by Mn atom and O site was replaced by N atom. The results of electronic structure analysis show that (Mn, N)-codope is easier to form stable p-type ZnO in comparison to N-mono dope. The p-d coupling between Mn 3d and N 2p states weakens the repulsive force between ions under (Mn, N)-codope condition and results in stable p-type. Especially, the (Mn, N)-codoped ZnO with 2:1 ratio has lower formation energy and higher p-type stability. Furthermore, the results of magnetic analysis also show that the (Mn, N)-codoped ZnO system with 2:1 ratio has higher total magnetic moment than 1:1 and 1:2. A significant ferromagnetic coupling raised by strong p-d interaction between Mn 3d and N 2p in the Mn-N-Mn complex is responsible for these results. However, high N doping ratio is not beneficial to the enhancement of ferromagnetism and p-type stability.

Investigation of humidity sensor based on Au modified ZnO nanosheets via hydrothermal method and first principle

In this paper, Au modified ZnO (Au/ZnO) nanosheets with high humidity performance were prepared by hydrothermal method. The position and the energy band of Au atoms in ZnO single crystals were simulated by first-principles based on density-functional theory (DFT), and the mechanism of Au/ZnO humidity sensors was studied further. The calculations show that Au atoms are adsorbed on the surface of ZnO supercell to provied more oxygen vacancies. Therefore, more water molecules can be adsorbed on the Au/ZnO humidity sensor to react with more electrons and hence to enhance the performance of the humidity sensor. Experiments show that the performance of humidity sensor based on Au/ZnO can be improved by changing the concentration of the modifier Au nanoparticles. Au/ZnO (0.5 mM HAuCl4) humidity sensor exhibits approximately 5 orders of magnitude variation in impedance realization with RH in a range from 11% to 95%, and it exhibits good linearity and narrow hysteresis loop (˜3.0%), as well as short response (16 s) time and recovery time (28 s). The performance of Au/ZnO humidity can be improved by modifier Au nanoparticles, because the Au/ZnO have more surface oxygen defects and special electronic storage capabilities. These results indicate that Au/ZnO nanosheets have a potential in the manufacture of humidity sensors and provide a new idea for high performance of ZnO humidity sensors.

Effects of stress and point defect on the physical properties of ZnO:Nd

The magnetism sources and mechanism of Nd doping and point defect are frequently controversial. To solve these controversies, ZnO supercell models with different doping modes of Nd were constructed, and the geometric structure optimization and energy calculation of the established models were calculated based on the generalized gradient approximation method of density functional theory. Calculation results indicated that the formation energy of Nd doped ZnO systems completely followed the sequence Zn15NdO16 > Zn15NdO15 > Zn14NdO16. For systems in which Nd doping and Zn vacancy coexist, a strong hybrid coupling electron exchange effect was present among the unpaired O-2p that was nearest to the Zn vacancy, the Zn-4 s orbit that was second nearest to the Zn vacancy, and the Nd-3d orbit. Therefore, the magnetism originated from the RKKY double-exchange interaction, which was based on the hole carriers in the complexus as the medium. In contrast with the Nd doping system, systems with coexisting Nd doping and O vacancy did not exhibit increased magnetism. Thus, the systems are worthless in designing and preparing dilute magnetic semiconductors. The magnetic moment of the systems with coexisting Nd doping and Zn vacancy was the largest. Thus, the systems are valuable in such design and preparation. The absorption spectra of Zn16NdiO16 and Zn15NdO16 were red shifted under the condition without stress, whereas the absorption spectrum of Zn15NdO16 with stress was blue shifted and the magnetic moment increased. The Curie temperature of Nd double-doped ZnO system was higher than room temperature when the Nd-Nd distance was the shortest.

First-principles research on the optical and electrical properties and mechanisms of In-doped ZnO

The absorption spectra and conductivity of In-doped ZnO still exhibit differences. To resolve this contradiction, the ZnO supercell models with different In doping amounts and the Zn0.9375In0.0625(Zni)0.0625O supercell model were both constructed. When the geometrical structure of all the models was optimized, the GGA + U and GGA used to calculate the energy. In the range of In doping used in this study, the formation energy of In-doped ZnO under Zn-rich conditions is lower than that under O-rich conditions, thereby implying a more stability of In-doped ZnO under Zn-rich than that under O-rich. With the increased In doping content, the volume and the formation energy of the doped system increase, the doped systems become unstable, and doping becomes difficult. Furthermore, the band gaps are narrowed, and the red shift of absorption spectrum is enhanced. In the In doping amount ranging within 0.01389–0.05556, the electron effective mass decreases first and subsequently increases, and the electron concentration increases. The mobility and conductivity also increase first and subsequently decrease. These results are in accordance with the experimental results. The volume of Zn0.9375In0.0625(Zni)0.0625O with the coexistence of In replacing Zn and interstitial Zn is large. The band gap is widened and the absorption spectrum is blue-shifted in the UV region.

Electronic structures and ferroelectric properties of Ba-doped ZnO

Wurtzite ZnO has long been considered to be a promising candidate material for photovoltaic application due to its high power conversion efficiency. More interestingly and very recently, some research results suggested that the ferroelectric property of the photovoltaic material introduced by chemical elements doping can promote its power conversion efficiency significantly. Therefore, in order to understand the effect of Ba doping on the electronic structure and the ferroelectric properties of ZnO and to reveal the potentially optoelectronic properties of Zn1-xBaxO, the energy band structure, the density of states, and the polarizability and the relative dielectric constant of the bulk Ba-doped ZnO supercell system, in which the Zn atoms are partly and uniformly substituted by the Ba atoms, are investigated by using the first-principles method based on the density functional theory and other physical theory. The norm-conserving pseudopotentials and the plane-wave basis set with a cut-off energy of 600 eV are used in the calculation. The generalized gradient approximation refined by Perdew and Zunger (GGA-PBE), the local density approximation (LDA) and the local density approximation added Hubbard energy (LDA+U) are employed for determining the exchange-correlation energy respectively. Brillouin zone is set to be within 4×4×5K point mesh generated by the Monkhorst-Pack scheme. The self-consistent convergence of total energy is at 2.0×10-6 eV/atom. Additionally, in order to obtain a stable and accurate calculation result, the cell structure is optimized prior to calculation. The calculated results suggest that the bulk Ba-doped ZnO semiconductor system is still a semiconductor with a direct wide band gap. The band gap of Zn1-xBaxO increases gradually with Ba atom doping percentage increasing from 12.5% to 87.5%. Consequently, the ferroelectric polarization properties and the dielectric properties of the bulk Ba-doped wurtzite ZnO materials are tailored by doping Ba atoms. It indicates that the polarizability of Zn1-xBaxO system increases with Ba doping atomic percentage increasing, especially, the polarizability reaches to a maximum when the atomic percentage of doping is 75%. Meanwhile, the relative dielectric constant inversely decreases with Ba atomic percentage increasing. This is attributed to the effective contribution of Ba atoms to the density of state at the bottom of the valence band. The diagonalized components of polarizability imply that there are possible micro-domains in the supercell while applying externally electric field to it. And the supercell presents a nearly isotropic polarizability macroscopically due to the strong interaction among the electric dipole moments existing in the different domains.

Efficient diagnostics of the electronic and optical properties of defective ZnO nanoparticles synthesized using the sol–gel method: experimental and theoretical studies

ZnO nanoparticles were synthesized using the sol–gel method with a variation on the sol aging time and refined structural data were taken to simulate the synthesized sample in a first-principles calculation. The ZnO nanoparticles aged at 36 h and showed profound properties with high crystallinity and good visual fit in a Rietveld analysis. Optical analysis revealed only small variation in energy band gap for all samples, ranging between 3.08–3.12 eV, with the presence of oxygen vacancy defects. In first-principles calculation, several exchange-correlation functionals, including LDA, GGA-PBE, GGA-PBESol, LDA + U, GGA-PBE + U and GGA-PBESol + U, were tested to converge with the lowest optimized lattice deviation. GGA-PBE + U (Ud,Zn = 10 eV and Up,O = 6.1 eV) showed the lowest lattice deviation and reproduced the experimental band gap. The ZnO supercell structure with an oxygen vacancy showed that a defect state appears in the valence band and acts as a deep donor.

First-principles study of (Ga, Al)-codoped ZnO for optoelectronic device application

The relative stability, electronic structure, and optical properties of (Ga–Al)-codoped ZnO were investigated by first-principles calculations based on density functional theory (DFT). To study the doping effects, ZnO supercells with 32 atoms were built. The results were obtained by using Material Studios 8.0 provided by Accelrys. Ab initio spin-polarized all-electron DFT computations have been performed for substitution. The results indicate that the energy band shifts towards the higher-energy region for Al- and/or Ga-doped ZnO, which endorsed the doping of Al and/or Ga. It has been observed that the preparation of (Ga–Al)-codoped ZnO is difficult compared to Al/Ga-doped ZnO due to the considerably larger formation energy required. The imaginary part of the dielectric function ε2(ω), reflectivity R(ω), absorption coefficient α(ω), and refractive index n(ω) were calculated. The contribution of different density of states in the formation of the conduction and valence band has been analyzed for different configurations of ZnO.

Speeding up GW Calculations to Meet the Challenge of Large Scale Quasiparticle Predictions.

Although the GW approximation is recognized as one of the most accurate theories for predicting materials excited states properties, scaling up conventional GW calculations for large systems remains a major challenge. We present a powerful and simple-to-implement method that can drastically accelerate fully converged GW calculations for large systems, enabling fast and accurate quasiparticle calculations for complex materials systems. We demonstrate the performance of this new method by presenting the results for ZnO and MgO supercells. A speed-up factor of nearly two orders of magnitude is achieved for a system containing 256 atoms (1024 valence electrons) with a negligibly small numerical error of ±0.03 eV. Finally, we discuss the application of our method to the GW calculations for 2D materials.

Electronic Structure and Optical Property Analysis of Al/Ga-Codoped ZnO through First-Principles Calculations.

Using density functional theory and the Hubbard U method, we investigated the geometric structure, electronic structure, and optical property of Al/Ga-codoped ZnO. A 3 × 3 × 3 ZnO supercell was used to construct Al- and Ga-monodoped ZnO structures and Al/Ga-codoped ZnO (AGZO) structures. All three structures showed n-type conduction, and the optical band gaps were larger than that of pure ZnO. For a given impurity concentration, Ga impurities contribute more free carriers than Al impurities in AGZO. However, the presence of Al impurities improves the transmittance. These results can theoretically explain the factors that influence the electrical and optical properties.

Electronic Structure and Optical Properties of ZnO with Antisite Defects

We investigate the electronic structures and optical properties of ZnO with antisite defects OZn using the density function pseudopotential method. Our results show that the Fermi level shifts into the conduction band after introducing one or two OZn defects into ZnO supercell, indicating that the system displays a metallic-like characteristic. Moreover, the antisite defects lead to a redshift of the optical absorption edge and obvious optical absorption in the visible light region. Especially, the optical properties are influenced by the configurations of two OZn defects in our considered ZnO supercell. The strongest optical absorption occurs when the two defects are connected by-Zn-O-Zn-bond in the ab plane. These findings are possibly applicable for designing new optoelectronic and photoelectrochemical devices with improved low energy light absorption.

First-principles study of the electronic structure of Al and Sn co-doping ZnO system

Abstracts The electronic structures of Zn1−x−yAlxSnyO (heavily doping: x=0, 0.06, y=0, 0.06; and lightly doping: x=0, 0.03, y=0, 0.03.) are investigated by using an ab initio pseudopotential method based on density functional calculations. To study the effects of doping concentration on the electronic structure, we have built two ZnO supercells with 32 atoms and 64 atoms. Structural calculations show that, due to the Al and Sn doping, ZnO lattice strains and lattice parameters change. Electronic structural calculations indicated that the energy band of impure ZnO moves toward the low-energy region attributed to the doping of Al and Sn. For different dopants and doping concentrations, the extent of the shift of energy band is different. For Al single-doped ZnO, with Al concentration increase from 3 at% to 6 at%, the shift decreases from 2.0 to 1.06 eV. For Sn single-doped ZnO, the shift increases from 1.83 to 3.57 eV, with Al concentration increase from 3 at% to 6 at%. However, the change of the Al and Sn co-doped samples is not obviously owing to the interaction between Al and Sn. On the other side, Al and Sn doping can induce the narrowing of band-gap. Compared with that of Al doping, Sn doping makes the band-gap narrowing more apparent. With the increase of Sn concentration, the band-gap reduces apparently (reduce is 3 at%:0.68, and 6 at%:0.80). However, the band-gap reduces slightly with Al concentration (reduce is 3 at%: 0.06, and 6 at%:0.29). For Al/Sn co-doping, the gap is in the middle of Al and Sn single doped ZnO. Therefore, the place of energy band and band-gap can be adjusted by Al and Sn co-doping.

Simulation and calculation of conducting property of Zn1-xTMxO (TM=Al, Ga, In)

Based on the density functional theory (DFT), using first-principles plane-wave ultrasoft pseudopotential method, the models for the unit cell of pure ZnO and Zn1-xTMxO (TM=Al, Ga, In) supercells at the same doping concentration were constructed, and the geometry optimization, total density of states, band structures for all models were carried out. The calculation results show that, In-doped ZnO has the best conductivity at the same doping concentration of 3.125 at% of (Al, Ga, In) high doped in ZnO, the calculation results agree with the experimental results.

Mechanism enhancing gas sensing and first-principle calculations of Al-doped ZnO nanostructures

Al-doped flower-like ZnO nanostructures have been synthesized by a facile hydrothermal method at 95 °C for 7 h. The structure and morphology of the product were characterized by XRD, FTIR and SEM analysis. The sensing tests reveal that the response is significantly enhanced by Al doping, and the 0.3 wt% Al-doped sample exhibits the highest response of 464 to 10 ppm CO at an operating temperature of 155 °C. A change of the structural defects in Al-doped ZnO is responsible for the enhancement of the sensing properties, which has been confirmed by the room temperature photoluminescence (PL) spectra and X-ray photoelectron spectroscopy (XPS). The response time is reduced disproportionately with the increase in CO concentration by modeling the transient responses of the sensor using the Langmuir–Hinshelwood reaction mechanism. The band structures and density of states for pure ZnO and Al-doped supercells have been calculated using first principles based on density functional theory (DFT). The calculated results show that the band gap is narrowed and the conductance is increased by Al doping, which coincides with the experimental results of gas sensing.

The band gap broadening and absorption spectrum of wurtzite Zn1−xCoxO from first-principles calculations

The band structures, density of states and absorption spectra of un-doped and Co-doped ZnO supercells of Zn1−xCoxO (x=0.0417, 0.0625 and 0.1250) have been investigated using first-principles plate-wave uitrasoft pseudopotential method based on the density functional theory. The calculated results showed that the band gaps are broadened by Co doping in ZnO, which is because of the valence band undergoes a greater shift toward the low-energy region than the conduction band lead to. Moreover, the optical absorption edge exhibits a blue-shift, and the heavier the doping concentration is, the more significant the blue-shift will be, which is in agreement with the experimental results.

Possible Origin of Ferromagnetism in an Undoped ZnO d0 Semiconductor

First-principles calculations were employed to investigate the effect of native defects and a hydrogen-related defect complex on ferromagnetism in an undoped ZnO semiconductor. The results show that the zinc vacancy (VZn) could lead to a moment of 1.73 μB in the undoped ZnO supercell, while the oxygen vacancy could not, but the formation energy of the zinc vacancy is much higher than that of the oxygen vacancy. When the hydrogen atom is doped in imperfect ZnO, the formation energy of VZn+HI sharply decreases, compared with that of VZn. Meanwhile, the VZn+HI defect complex can induce a 0.99(0.65) μB moment in the Zn15HIO16 supercell. Furthermore, the total energy of the ZnO supercell with two defect complexes for the ferromagnetic phase is lower than that for the antiferromagnetic phase, and the calculated results show that a strong magnetic coupling exists in the ferromagnetic phase. As an unintentionally doped element, H usually appears in ZnO prepared by many methods. So the ferromagnetism in the ZnO d0 semiconductor most probably arises from the defect complex of the zinc vacancy and H.