NiO Thickness Research Articles

Characteristic magnetic domain size in Fe with exchange-coupled antiferromagnetic NiO underlayer

Characteristic size of magnetic domains in Fe films deposited on NiO layers was measured by spin-polarized low-energy electron microscopy. Our observation results and statistical analysis revealed the increase of magnetic domain sizes with the Fe-thickness increase. Interestingly, the evolution of magnetic domain sizes in Fe showed a strong dependence on the thickness of NiO underneath, or thicker NiO lead the formation of smaller magnetic domains in Fe. This phenomenon can be explained by the increase of the interlayer exchange coupling between Fe and the polycrystalline NiO. It is consistent with the results of recent studies that spin dynamics at a ferromagnet/antiferromagnet interface is affected by the domain structure. Our results suggest a new route to manipulate the micromagnetic structure without the application of external magnetic field.

Outstanding Photocurrent Density and Incident Photon‐to‐Current Conversion Efficiency of Liquid‐State NiO Perovskite‐Sensitized Solar Cells

The efficiency and photocurrent density reported for p‐type‐sensitized solar cells up to now are still lagging behind that of the n‐type counterparts, limiting the successful development of p–n tandem cells. To circumvent this issue, NiO thin film is fabricated by the aerosol‐assisted chemical vapor deposition (AACVD) technique and used in p‐type solar cells. A systematic study is conducted to comprehend the correlation between NiO thickness and the power conversion efficiency (PCE) of liquid‐state NiO‐based sensitized solar cells. By carefully designing the cell components, this type of device demonstrates the highest photocurrent density (Jsc) exceeding 18 mA cm−2 when using iodine/triiodide as the redox shuttle matching the one produced by the TiO2 counterpart. This is accomplished by 1) using the AACVD technique for the one‐step deposition of compact and mesoporous NiO electrodes, 2) optimizing the thickness of the NiO layer through controlling the deposition time, and 3) adopting methylammonium lead iodide (CH3NH3PbI3) as a light harvester prepared via a sequential deposition method.

Flame Assisted Chemical Vapour Deposition of NiO hole transport layers for planar perovskite cells

Thin films of polycrystalline NiO were deposited by Flame Assisted Chemical Vapour Deposition, which is an ideal process for in-line, atmospheric pressure deposition of wide area coatings. This, along with the ability to use aqueous salts rather than organic precursors or solvents makes it well suited for industrial integration. To establish the capability of FACVD deposited NiO for use as a low cost, commercially viable option planar perovskite cells were fabricated under ambient conditions. The resulting cells showed the importance of both the flame composition and NiO thickness. A continuous NiO Hole Transport Layer (HTL) was achieved for ca. 36 nm thick film, which showed a maximum higher efficiency of 12.3% over that of the control (11.8%) which used a spin coated HTL. This was mainly driven by the large improvement in the current density from 16.6 mA/cm2 to 19.0 mA/cm2.

Sol-gel fabrication of NiO and NiO/WO3 based electrochromic device on ITO and flexible substrate

Electrochromic devices (ECDs) with reversible transmittance change represent a promising alternative to smart windows. However, the low−cost facile fabrication of ECDs, particularly flexible devices, remains challenging. In this study, novel NiO is synthesized by a solid state method, and the as−prepared NiO is introduced as an electrochromic anodic layer and fabricated onto a transparent conductive electrode (indium tin oxide, ITO or flexible silver nanowires, AgNW) by a sol–gel spin coating and low temperature annealing (80 °C-150 °C). The solvent, thickness of NiO, and annealing temperature are evaluated to obtain higher ECD performance. NiO/ITO ECDs exhibit very high transmittance variation (ΔT = ~84%) at 700 nm with applied potentials of −3.0 and 0 V. The stability and transmittance variation of NiO/ITO are significantly improved in the presence of a WO3 cathodic electrode at lower applied voltages of 1.5 to 0 V. The low processing temperature of 80 °C demonstrates the potential of the flexible ECDs. The flexible NiO–WO3 device achieves a transmittance variation of ~38% at 700 nm with applied potential of 2.0 and 0 V, and retains the ECD performance. The application of low−cost solution−processed NiO and NiO/WO3−based ECDs in flexible transparent conductive electrodes provides a new pathway for the fabrication of optical devices and printed electronics.

Gas Sensing of NiO‐SCCNT Core–Shell Heterostructures: Optimization by Radial Modulation of the Hole‐Accumulation Layer

AbstractHierarchical core–shell (C–S) heterostructures composed of a NiO shell deposited onto stacked‐cup carbon nanotubes (SCCNTs) are synthesized by atomic layer deposition (ALD). A film of NiO particles (0.80–21.8 nm in thickness) is uniformly deposited onto the inner and outer walls of the SCCNTs. The electrical resistance of the samples is found to increase of many orders of magnitude with the increasing of the NiO thickness. The response of NiO–SCCNT sensors toward low concentrations of acetone and ethanol at 200 °C is studied. The sensing mechanism is based on the modulation of the hole‐accumulation region in the NiO shell layer upon chemisorption of the reducing gas molecules. The electrical conduction mechanism is further studied by the incorporation of an Al2O3 dielectric layer at NiO and SCCNT interfaces. The investigations on NiO–Al2O3–SCCNT, Al2O3–SCCNT, and NiO–SCCNT coaxial heterostructures reveal that the sensing mechanism is strictly related to the NiO shell layer. The remarkable performance of the NiO–SCCNT sensors toward acetone and ethanol benefits from the conformal coating by ALD, large surface area of the SCCNTs, and the optimized p‐NiO shell layer thickness followed by the radial modulation of the space‐charge region.

Spin transport in an insulating ferrimagnetic-antiferromagnetic-ferrimagnetic trilayer as a function of temperature

We present a study of the transport properties of thermally generated spin currents in an insulating ferrimagnetic-antiferromagnetic-ferrimagnetic trilayer over a wide range of temperature. Spin currents generated by the spin Seebeck effect (SSE) in a yttrium iron garnet (YIG) YIG/NiO/YIG trilayer on a gadolinium gallium garnet (GGG) substrate were detected using the inverse spin Hall effect (ISHE) in Pt. By studying samples with different NiO thicknesses, the spin diffusion length of NiO was determined to be ∼3.8 nm at room temperature. Surprisingly, a large increase of the SSE signal was observed below 30 K, and the field dependence of the signal closely follows a Brillouin function for an S=7/2 spin. The increase of the SSE signal at low temperatures could thus be associated with the paramagnetic SSE from the GGG substrate. Besides, a broad peak in the SSE response was observed around 100 K. These observations are important in understanding the generation and transport properties of spin currents through magnetic insulators and the role of a paramagnetic substrate in spin current generation.

A model for oxidation-induced stress analysis of Ni-based anode-supported planar solid oxide fuel cell

A critical hurdle in realizing commercially viable anode-supported solid oxide fuel cell (SOFC) is the re-oxidation of cermet anode during the cell abnormal operation. In this paper, the analysis of energy dispersive X-ray spectroscopy on a partially oxidized half-cell demonstrates a particular inhomogeneous oxidation mechanism that a portion of anode near the air/anode interface is oxidized with a graded NiO content, while the remaining region hold a reduced status. Based on this observation, an analytical oxidation-induced stress model is developed to provide in-depth information about the mechanical behavior of the half-cell suffered from various oxidation. The dependences of the mechanical performance of half-cell on the NiO distribution and thickness of oxidation-graded zone are revealed. The results show that an increase in oxidation-graded zone elevates the stress level of electrolyte, but decreases the curvature under the same global degree of oxidation (DoO). In addition, the influence of thickness of oxidation-graded zone on the electrolyte failure probability is also investigated. According to these results, we conclude that the thickness of oxidation-graded zone should be as thin as possible in order to delay the electrolyte cracking.

Antiferromagnetic NiO thickness dependent sign of the spin Hall magnetoresistance in γ-Fe2O3/NiO/Pt epitaxial stacks

We study the spin Hall magnetoresistance (SMR) in epitaxial γ–Fe2O3/NiO(001)/Pt stacks, as a function of temperature and thickness of the antiferromagnetic insulating NiO layer. Upon increasing the thickness of NiO from 0 nm to 10 nm, we detect a sign change of the SMR in the temperature range between 10 K and 280 K. This temperature dependence of the SMR in our stacks is different compared to that of previously studied yttrium iron garnet/NiO/Pt, as we do not find any peak or sign change as a function of temperature. We explain our data by a combination of spin current reflection from both the NiO/Pt and γ-Fe2O3/NiO interfaces and the thickness-dependent exchange coupling mode between the NiO and γ-Fe2O3 layers, comprising parallel alignment for thin NiO and perpendicular alignment for thick NiO.

Optical and electrical properties and simulation curves of NiO/Al/NiO transparent conductive film

In this paper, NiO/Al/NiO transparent conductive films were prepared by magnetron sputtering at the room temperature. Effects of the NiO and Al layers thicknesses on the optical and electrical properties of the NiO/Al/NiO laminated films were analyzed. When the light wavelength falls in range 300–900 nm, with the increase of the NiO and Al layers thicknesses, the transmittance of the laminated film first increases significantly and then decreases slightly, finally tends to be stable. The laminated film obtained the best optical and electrical properties when the NiO layer is 40 nm and the Al layer is 12 nm. The maximum transmittance is 83%, the average transmittance is 77.3%, the film resistivity is [Formula: see text] and the carrier concentration is [Formula: see text]. At the same time, the transmittance of laminated film is simulated by FDTD software. But the simulation curve is different from the experimental data. Analysis results show that, with the NiO dielectric is added on both sides of the metal Al film, the light reflection characteristic of laminated film has been completely different from that of the single Al metal film because of the change of interface characteristics between Al film and NiO film, and the actual luminous transmittance greatly increases.

Spin transport in multilayer systems with fully epitaxial NiO thin films

We report on the generation and transport of thermal spin currents in fully epitaxial {\gamma}-Fe$_2$O$_3$/NiO(001)/Pt and Fe$_3$O$_4$/NiO(001)/Pt trilayers. A thermal gradient, perpendicular to the plane of the sample, generates a magnonic spin current in the ferrimagnetic maghemite ({\gamma}-Fe$_2$O$_3$) and magnetite (Fe$_3$O$_4$) thin films by means of the spin Seebeck effect. The spin current propagates across the epitaxial, antiferromagnetic insulating NiO layer, before being detected in the Pt layer by the inverse spin Hall effect. The transport of the spin signal is studied as a function of the NiO thickness, temperature and ferrimagnetic material where the spin current is generated. In epitaxial NiO grown on maghemite, the spin Seebeck signal decays exponentially as a function of the NiO thickness, with a spin-diffusion length for thermally-generated magnons of {\lambda}$_{MSDL}$ = $1.6 \pm 0.2$ nm, largely independent on temperature. We see no enhancement of the spin current signal as previously reported for certain temperatures and thicknesses of the NiO. In epitaxial NiO grown on magnetite, the temperature-averaged spin diffusion length is {\lambda}$_{MSDL}$ = $3.8 \pm 0.3$ nm, and we observe an enhancement of the spin signal when the NiO thickness is 0.8 nm, demonstrating that the growth conditions dramatically affect the spin transport properties of the NiO even for full epitaxial growth. In contrast to theoretical predictions for coherent spin transport, we do not see vastly different spin diffusion lengths between epitaxial and polycrystalline NiO layers.

Intelligent Modeling of Metal Oxide Gas Sensor

Due to the complexity of the gas detection process, traditional modeling techniques cannot provide accurate modeling performance to reproduce the behavior of this difficult process. In this paper, an intelligent modeling technique is utilized to develop an accurate model to represent the complex and nonlinear gas detection process. In particular, in this study nickel Oxide NiO gas sensor, which was specifically fabricated by a simple chemical spray pyrolysis technique. In the process, the nickel chloride hexahydrate salt was used at a concentration of (0.05 M) and a temperature of 350 ºC. Because of this process, the thickness of NiO was 0.1μm. Inspection was done using three different testing techniques; X-ray diffraction, scanning electron microscopy, and the sensitivity test of NiO for Methane gas CH4 in the range of (0-500) ppmv. Inspection results show that the film was crystalline, has a cubic system, and without cracks or open pores. On the other hand, the sensitivity results were disparate and low in value within the considered range. From the real-time experiment described above, training samples were gathered to develop the desired process model. The considered modeling technique was based on exploiting the wavelet network (wavenet) to represent the nonlinear function of the nonlinear autoregressive with exogenous input (NARX) structure. In model development process, the experimental data were utilized as the training samples for the wavenet-based NARX model. As the modeling accuracy, the proposed wavenet-based NARX model attained a value of 1.895 × 10-12 for the root mean square of error (RMSE) criterion.

Effect of interface NiO layer on magnetism in Fe/BaTiO3 thin film

The insertion effect of an interface NiO layer on magnetism in Fe/BaTiO3 is investigated by X-ray absorption spectroscopy (XAS). From X-ray magnetic circular dichroism (XMCD) analysis, the enhancement of remanent magnetization in Fe is observed with increasing NiO thickness. We also find that the NiO layer is partially composed of metallic Ni, and its thickness is estimated to be ∼0.4 nm from the NiO-thickness dependence of the Ni XAS intensity by assuming that Ni is localized at the interface between Fe and NiO. Moreover, extended X-ray absorption fine structure data shows that the Ni–O distance of the interface NiO layer is shorter than that of bulk NiO, which leads to the pseudomorphic growth of NiO on BaTiO3. We thus suppose that the improved Fe growth promoted by the interface NiO layer enhances the magnetic moment in Fe.

Manipulation of magnetic properties of ferromagnetic Ni thin films grown on Cu(001) by antiferromagnetic NiO and effects of voltage application

The magnetic properties of ferromagnetic Ni thin films grown on a Cu(001) single crystal are modified by the growth of a NiO overlayer, as well as by the voltage application through the NiO/Ni interface. A spin reorientation transition from in-plane to perpendicular magnetization is induced with increasing NiO thickness, and the coercive field significantly increases by further growth of the NiO overlayer. The remanent magnetization of the films is found to be modulated by the voltage application. Moreover, a small exchange-bias effect arising from the ferromagnetic–antiferromagnetic interaction at the interface is observed, and the amplitude of the effect is modified by the applied voltage.

Bottle-brush-shaped heterostructures of NiO–ZnO nanowires: growth study and sensing properties

We present here heterostructured ZnO–NiO nanowires (NWs), constituted by a core of single crystalline ZnO NWs, covered by poly-crystalline NiO nanorods (NRs). The bottle-brush shape was investigated by scanning electron microscopy and transmission electron microscope, confirming that a columnar growth of NiO occurred over the ZnO core, with a preferred orientation of NiO over ZnO NWs. The heterostructured devices are proposed for gas sensing application. Bare ZnO NWs and heterostructured sensors with two different thicknesses of NiO poly-crystalline NRs were analysed for acetone, ethanol, NO2 and H2 detection. All sensors maintained n-type sensing mechanism, with improved sensing performance for lower thickness of NiO, due to high catalytic activity of NiO. The sensing dynamic is also strongly modified by the presence of heterojunction of NiO/ZnO, with a reduction of response and recovery times towards ethanol and acetone at 400 °C.

Negative tunneling magnetoresistance of Fe/MgO/NiO/Fe magnetic tunnel junction: Role of spin mixing and interface state

Motivated by a recent tunneling magnetoresistance (TMR) measurement in which the negative TMR is observed in MgO/NiO-based magnetic tunnel junctions (MTJs), we have performed systematic calculations of transmission, current, and TMR of Fe/MgO/NiO/Fe MTJ with different thicknesses of NiO and MgO layers based on noncollinear density functional theory and non-equilibrium Green's function theory. The calculations show that, as the thickness of NiO and MgO layers is small, the negative TMR can be obtained which is attributed to the spin mixing effect and interface state. However, in the thick MTJ, the spin-flipping scattering becomes weaker, and thus, the MTJs recover positive TMR. Based on our theoretical results, we believe that the interface state at Fe/NiO interface and the spin mixing effect induced by noncollinear interfacial magnetization will play important role in determining transmission and current of Fe/MgO/NiO/Fe MTJ. The results reported here will be important in understanding the electron tunneling in MTJ with the barrier made by transition metal oxide.

Influence of cycling temperature on the electrochromic properties of WO3//NiO devices built with various thicknesses

Electrochromic smart windows, which are able to vary their optical transmittance by the application of a voltage, improve the indoor comfort in buildings by controlling light and temperature. Among electrochromic devices, ECDs, WO3 and NiO are often associated leading to neutral color. This work focuses on the influence of the thickness of WO3 and NiO single layers on the ECD electrochemical and optical properties. Initial characterizations in lithium-based liquid electrolyte, show a non-linear evolution of the electrochemical capacity for each oxides vs. thickness while for similar thickness, WO3 thin films exhibit larger EC properties than NiO ones. The combinations of various WO3 and NiO thicknesses lead to ECDs of which capacity appears limited by the one of NiO, while the switch to a neutral color always remain. Besides, the influence of each oxide on the ECD cyclic voltammogram shape is discussed. Finally, the study of the cycling temperature, in the range −40°C to 80°C on the electrochromic properties of ITO/650nm-WO3/LiClO4-PC +PMMA/360nm-NiO/ITO shows an increase in performance with temperature nevertheless associated with signs of degradation above 60°C. At low temperature, the decrease in capacity is interestingly associated with a remaining significant optical contrast and the capacity recovery if the ECD is further cycled at RT.

Tunable Sign Change of Spin Hall Magnetoresistance in Pt/NiO/YIG Structures.

Spin Hall magnetoresistance (SMR) has been investigated in Pt/NiO/YIG structures in a wide range of temperature and NiO thickness. The SMR shows a negative sign below a temperature that increases with the NiO thickness. This is contrary to a conventional SMR theory picture applied to the Pt/YIG bilayer, which always predicts a positive SMR. The negative SMR is found to persist even when NiO blocks the spin transmission between Pt and YIG, indicating it is governed by the spin current response of the NiO layer. We explain the negative SMR by the NiO "spin flop" coupled with YIG, which can be overridden at higher temperature by positive SMR contribution from YIG. This highlights the role of magnetic structure in antiferromagnets for transport of pure spin current in multilayers.

Electrical Detection of Spin Backflow from an Antiferromagnetic Insulator/ Y3Fe5O12 Interface

Spin Hall magnetoresistance (SMR) has been observed in Pt/NiO/Y_{3}Fe_{5}O_{12} (YIG) heterostructures with characteristics very different from those in Pt/YIG. This phenomenon indicates that a spin current generated by the spin Hall effect in Pt transmits through the insulating NiO and is reflected from the NiO/YIG interface. The SMR in Pt/NiO/YIG shows a strong temperature dependence dominated by effective spin conductance, due to antiferromagnetic magnons and spin fluctuation. Inverted SMR has been observed below a temperature which increases with the NiO thickness, suggesting a spin-flip reflection from the antiferromagnetic NiO exchange coupled with the YIG.

Nickel oxide as back surface field buffer layer in CdTe thin film solar cell

In this work, we report that NiO thin film can be used as a back contact buffer layer in CdTe thin film solar cells. The NiO layer is prepared by electron beam evaporation. To optimize the thickness of the NiO thin film, we fabricate some CdTe solar cells with different NiO thickness values. A NiO/Au back contact CdTe solar cell with an efficiency of 12.17% and an open-circuit voltage Voc of 789 mV is obtained, which are comparable to those of a standard Cu/Au back contact solar cell. The X-ray photoelectron spectroscopy (XPS) is used to quantitatively characterize the band alignment at the CdTe/NiO interface. It can be seen from the band alignment that the valence band offset (EVBO) is 0.52 eV and the conduction band offset (ECBO) is 2.68 eV. The EVBO presents no energy barrier for hole to transport from CdTe to NiO. The value of ECBO indicates that NiO can act as a back surface field layer (BSF) to dramatically reduce carrier recombination in the contact region of a CdTe cell, leading to an improved Voc. The band alignment obtained from XPS measurement shows that the band alignments of NiO and CdTe are perfectly matched. However, the conductivity of NiO film is poor. The insertion of a NiO buffer layer in the back contact increases the series resistance and reduces the fill factor (FF). We propose to use Cu/NiO composite structure as a bi-layer contact to improve the conductivity of the NiO buffer layer, which at the same time can be used to dope the CdTe film surface by Cu to obtain a low resistive contact. We fabricate a cell with a contact structure of 3-nm-Cu/20-nm-NiO/Au and the cell has a Voc of 796 mV, a Jsc (short-circuit currrent) of 24.2 mA/cm2, an FF of 70.2% and an efficiency of 13.5%. In order to study the stability of the solar cell with a Cu/NiO/Au back contact, a thermal stressing test is carried out at a temperature of 80 ℃ in the air atmosphere. For the Cu/NiO/Au back contact structure solar cell, the efficiency decreases from 13.1% to 12.9% after the cell is stressed for 80 h, showing that the stability of the Cu/NiO/Au back contact cell is significantly improved compared with that of the standard Cu/Au contact cell. In summary, the experimental results obtained in this study demonstrate that NiO thin film is a promising buffer layer for manufacturing stable and high efficiency CdTe thin film solar cells.

Spin transport in antiferromagnetic NiO and magnetoresistance in Y3Fe5O12/NiO/Pt structures

We have studied spin transport and magnetoresistance in yttrium iron garnet (YIG)/NiO/Pt trilayers with varied NiO thickness. To characterize the spin transport through NiO we excite ferromagnetic resonance in YIG with a microwave frequency magnetic field and detect the voltage associated with the inverse spin-Hall effect (ISHE) in the Pt layer. The ISHE signal is found to decay exponentially with the NiO thickness with a characteristic decay length of 3.9 nm. This is contrasted with the magnetoresistance in these same structures. The symmetry of the magnetoresistive response is consistent with spin-Hall magnetoresistance (SMR). However, in contrast to the ISHE response, as the NiO thickness increases the SMR signal goes towards zero abruptly at a NiO thickness of ≃ 4 nm, highlighting the different length scales associated with the spin-transport in NiO and SMR in such trilayers.