NiO Films Research Articles

High Stability Flexible Deep-UV Detector Based on All-Oxide Heteroepitaxial Junction

Transparent flexible electronics constitute a significant research field. Flexible deep-ultraviolet (UV) detectors have received much attention due to their potential in the applications of healthcare, communications, astronomy, and environment monitoring. Recent studies have investigated a variety of flexible photodetectors but show that the transparent, flexible, chemical, and thermal stability performances of these detectors cannot meet the requirements for practical applications. In this study, we demonstrate transparent flexible deep-UV detectors based on the combination of high-quality epitaxial n-type β-Ga2O3 and p-type NiO films as a photodiode on a flexible muscovite substrate. The electrical current of this heterojunction is increased over a 1000 times for on/off ratio under 265 nm wavelength illumination with a reasonable response (<1 s). Moreover, these photodetectors also exhibit good thermal stability as well as excellent mechanical flexibility. Our results exhibit the superior performance of the oxide-based solar-blind deep-UV detectors for advanced flexible sensing and smart applications.

ZnO/NiO heterostructures with enhanced photocatalytic activity obtained by ultrasonic spraying of a NiO shell onto ZnO nanorods

Degradation of organic pollutants such as methylene blue (MB) from water resources is currently of particular interest. Employment of a heterojunction device with optimized layer properties and proper interface engineering can enhance the photocatalytic performance by taking advantage of efficient charge separation. In this work, we develop an efficient photocatalytic system for the MB degradation based on ZnO nanorod (ZnONR)/NiO core-shell heterostructure with an optimized chemical and electronic structure for achieving record MB degradation efficiency of ∼70 %. ZnONR were grown by hydrothermal technique, whereas homogeneous crystalline NiO thin films were prepared by a robust and easy for up-scaling method of ultrasonic spray pyrolysis (USP). The optimum preparation conditions of photocatalytically efficient ZnONR/NiO heterostructures imply NiO film deposition from two USP cycles at 500 °C followed by air annealing heterostructures at 600 °C. The photocatalytic performance of ZnONR/NiO core-shell structure was investigated in comparison to counterpart layers and ZnO/NiO bilayer system. Chemical composition and band alignment at the ZnONR/NiO interface were investigated by X-ray photoelectron spectroscopy, Kelvin probe and photoelectron yield spectroscopy. Current transport studies indicated the presence of built-in electric field at the n-ZnO/p-NiO heterointerface responsible for the enhanced photocatalytic activity and based on this the degradation mechanism of MB is discussed.

(Digital Presentation) Optimization of MIM Rectifiers for Terahertz Rectennas

Metal-Insulator-Metal (MIM) rectifiers comprising thin films of Al2O3, ZnO, NiO and Nb2O5 and metal configurations of Au/Au, Au/Zn and AuCr/AuCr, have been fabricated using atomic layer deposition and radio-frequency sputtering. The effect of device area scaling from 104 µm2 to 1 µm2 on rectification properties, in particular zero-bias dynamic resistance (R0) and zero-bias responsivity (β0) has been studied and found to be of critical importance in improving diode coupling efficiency. A significant increase of current has been found for Au/3.3 nm ZnO/Au diode when compared to the reference Au/3 nm Al2O3/Au diode, that resulted in obtaining the lowest R0 of 540 W for a device area of 104 µm2. The best performing device is found to be 1 µm2 AuCr/6.77 nm NiO/AuCr featuring (R0, b0) = (461 kW, 0.76 A/W) and a coupling efficiency of 1.5 ´ 10-5 %.

Pseudocapacitive and dual-functional electrochromic Zn batteries

The similarity in electrochemical mechanisms in electrochromic and energy storage devices has inspired explorations of dual-functional devices. However, complementary dual-functional devices often neglect charge balance and suffer from unsatisfactory capacity. Here, electrochemically deposited NiO was balanced by Zn anode, constructing an electrochromic Zn battery (EZB) in both aqueous mild (ZnCl2) and alkaline (KOH) electrolytes. With well-adjusted electrochemical deposition condition, nanoporous instead of dense NiO film was obtained, manifesting faster switching for electrochromism and higher capacity for charge storage. In addition, the alkaline EZB outperforms the mild counterpart, due to the formation of larger content of Ni3+ species upon oxidation. The alkaline EZB harvests areal capacity of 20 mAh/m2 at 0.02 mA/cm2 and high-rate performance (17.1 mAh/m2 at 0.2 mA/cm2) for energy storage, while achieving optical contrast of 36%, high transmittance at clear state (>90%), fast switching time (tb = 5.3s, tc = 1.9s), and excellent switching stability (no contrast degradation after 2000 cycles) for electrochromism. A 5∗5 cm2 charged alkaline NiO//Zn EZB can power a timer for more than 30 min, accompanied by a gradual bleaching process, demonstrating the outstanding dual functionalities. This work presents the construction of excellent dual-functional EZB.

Structural Defect Impact on Changing Optical Response and Raising Unpredicted Ferromagnetic Behaviour in (111) Preferentially Oriented Nanocrystalline NiO Films

NiO thin films deposed on a glass substrate, “NiO/glass”, are successfully prepared using a spray pyrolysis technique (SPT) at 460 °C and characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray, Atomic force microscopy (AFM), spectroscopic ellipsometry (SE), Photoluminescence (PL) and diverse electric and magnetic studies. The structural investigation shows that the synthesized films crystallized in a cubic structure with (111) preferential orientation. The NiO layers exhibit a uniform grain of regular sizes with aggregates randomly distributed across their surface. The optical properties of the NiO thin films evidenced a normal optical dispersion as well as good transparency of the NiO films. An unpredicted ferromagnetic aspect was raised due to the high oxygen presence in the synthetized material. A high thermal dependency of the conductivity, as well as a semiconductor behavior of the grown NiO material, is also demonstrated.

Nano-faceted stabilization of polar-oxide thin films: The case of MgO(111) and NiO(111) surfaces

Molecular beam epitaxy growth of polar MgO(111) and NiO(111) films demonstrates that surface stabilization of the films is achieved via the formation of neutral nano-faceted surfaces. First-principles modeling of the growth of polar MgO(111) films reveals that the growth does not proceed layer-by-layer. Instead, the Mg or O layers grow up to a critical sub-monolayer coverage, beyond which the growth of the next layer becomes energetically favorable. This non-layer-by-layer growth is accompanied by complex relaxations of atoms both at the surface and in the sub-surface, and leads to the experimentally observed surface nano-faceting of MgO and NiO (111) films through formation of neutral nano-pyramids terminated by {100} facets. These facets are limited in size by an asymptotical surface energy relation to their height; with the reconstruction being much more stable than previously reported surface terminations across a wide range of growth conditions. The termination offers access to lower coordinated atoms at the intersection of the neutral {100} planes whilst also increasing the surface area of the film. The unique electronic structures of these surfaces can be utilized in catalysis, as well for forming unique heterostructures for electronic and spintronic applications.

Synergistic Effect and Electrochromic Mechanism of Nanoflake Li‐doped NiO in LiOH Electrolyte

Inorganic metal oxide electrochromic materials have good application prospects for energy‐saving windows in buildings and smart display applications. Therefore, the development of electrochromic films with good cycling stabilities, fast color‐change response times, and high coloring efficiencies has attracted considerable attention. In this study, nanoflake Li‐doped NiO electrochromic films were prepared using a hydrothermal method, and the films exhibited superior electrochromic performances in the LiOH electrolyte. Li+ ions doping increased the ion transmission rates of the NiO films, and effectively promoted the transportation of ions from the electrolyte into NiO films. Meanwhile, the nanoflake microstructure caused the NiO films to have larger specific surface areas, providing more active sites for electrochemical reactions. It was determined that the NiO‐Li20% film exhibited an ultra‐fast response in the LiOH electrolyte (coloring and bleaching times reached 3 and 1.5 s, respectively). Additionally, the coloration efficiency was 62.1 cm2 C−1, and good cycling stability was maintained beyond 1500 cycles. Finally, the simulation calculation results showed that Li doping weakened the adsorption strengths of the NiO films to OH−, which reduced the generation and decomposition of NiOOH and helped to improve the cycling stabilities of the films. Therefore, the research presented in this article provides a strategy for designing electrochromic materials in the future.

Oxygen-vacancy induced ferroelectricity in nitrogen-doped nickel oxide

This paper reports the onset of ferroelectricity in NiO by breaking the crystallographic symmetry with oxygen vacancies created by N doping. Nitrogen-doped NiO was grown at room temperature by RF sputtering of Ni target in Ar–O2–N2 plasma on silicon and fused silica substrates. The impact of the nitrogen doping of NiO on microstructural, optical, and electrical properties has been investigated. According to x-ray diffraction investigations, by increasing the N doping level in NiO, a transition from (002) to a (111) preferential orientation for the cubic NiO phase was observed, as well as a lattice strain relaxation, that is usually ascribed to structural defect formation in crystal. The x-ray diffraction pole figures the presence of a distorted cubic structure in NiO and supports the Rietveld refinement findings related to the strain, which pointed out that nitrogen doping fosters lattice imperfections formation. These findings were found to be in agreement with our far-infrared measurements that revealed that upon nitrogen doping a structural distortion of the NiO cubic phase appears. X-ray photoemission spectroscopy measurements reveal the presence of oxygen vacancies in the NiO film following nitrogen doping. Evidence of ferroelectricity in nitrogen-doped NiO thin films has been provided by using the well-established Sawyer–Tower method. The results reported here provide the first insights on oxygen-vacancy induced ferroelectricity in nitrogen-doped nickel oxide thin films.

The effect of rare-earth element (Gd, Nd, La) doping of NiO films on UV photodetector

The semiconductor-based UV photodetectors are the most essential devices in the field of space observations, military, DNA sequencing, analysis of protein, medical imaging, checking of atmospheric pollution, optical communications radiation, etc. With such a wide range of possible applications, the nanostructured pristine and rare-earth (RE) doped NiO ((NiO: Gd(1%), NiO:Nd(1%), NiO:La(1%)) thin films were prepared and investigated for their suitability as UV photodetectors. The films were prepared by nebulized spray pyrolysis (NSP) at a substrate temperature of 450 °C. The x-ray diffraction studies confirm the cubic single phase with the polycrystalline nature of the prepared films. The spectroscopic studies such as absorbance and photoluminescence confirm that increase in the optical bandgap and 391 nm PL emission is attributed to the near band edge emission of the NiO. The x-ray photoelectron spectroscopy reveals the presence of nickel and the doped elements with their oxidation states. The UV photodetector performance of the prepared NiO films was carried out under the irradiation of 365 nm light. The NiO:Gd exhibits the best responsivity (0.353 AW−1), external quantum efficiency (120%), detectivity (1.72 × 1010 Jones) and rise time (2.0 s), and fall time (2.2 s). Importantly, strategies such as limited doping (1 at.%) and larger ionic radii of Gd incorporation into the host NiO cause a moderate increase in the lattice distortion and inhibit the recombination rate instead of behaving as a recombination center. In addition, the conduction band (CB) electrons are trapped by a greater number of oxygen vacancies residing at the Gd3+ 4f state and cause a good separation of charge carriers. Overall, these modifications enhance the mean lifetime of electrons, consequently reducing the recombination rate and enhancing the photoresponse.

Fabrication of V2O5/NiO double-layer film with hydrophilic property and evaluation of the effects of V2O5 coating on electro-optical parameters of NiO film

Fabrication of V2O5/NiO double-layer film with hydrophilic property and evaluation of the effects of V2O5 coating on electro-optical parameters of NiO film

Preparation and characteristics study of self-powered and fast response p-NiO/n-Si heterojunction photodetector

The growth of crack-free nanostructured NiO films with good crystalline quality is of high importance for photodetectors to avoid performance failure. In this work, physical properties of spin coated NiO films were controlled by changing diethanolamine (DEA) to nickel acetate (NiAc) molar ratio (0:1–1:1) and post annealing temperature (300–650 °C). NiO film coated at DEA:NiAc molar ratio of 0:1 suffered from severe cracks and poor crystallinity, and by increasing the molar ratio to 1:1 a crack-free NiO with enhanced grain growth was obtained. With the increase of annealing temperature from 300 C to 600 °C, the crystallite size increased from 12.79 to 37.31 nm, and the bandgap decreased from 3.81 to 3.42 eV, indicating an enhancement in NiO film quality. A self-powered photodetector based on p-NiO/n-Si heterojunction showed broadband (UV-NIR) photodetection owing to synergistic photoelectric effect from both NiO film and Si substrate. The responsivity, detectivity, and external quantum efficiency were measured as 13.08, 46.02, 44.49, mA/W, 1.03 × 1011, 3.65 × 1011, 3.53 × 1011, Jones, and 4.43%, 8.62%, 6.47% upon illumination with UV (365 nm), red (660 nm), and NIR (850 nm) lights, respectively. The photodetector showed high on/off current ratio of 1.210 × 103 and fast response (less than 85 ms). These findings introduce p-NiO/n-Si heterojunction as a promising candidate for next generation optoelectronics.

Enhanced electrochromic performance on novel W@NiO doped composite electrode via pre-annealing

To improve the electrochromic properties of traditional sputtering NiO films, we prepare a novel W@NiO composite films depending on dual impact of doping and pre-annealing. The small quantity of doping tungsten is guaranteed by unique stacked targets sputtering mode. On this basis, different substrate temperature is set to obtain a series of samples. The obtained 200 °C W@NiO films show obvious improvement compared with room-temperature pure NiO films and literature results. Specially, the charge density can get ∼60 mC/cm2, the optical modulation can realize ∼83% at 550 nm, the coloration efficiency can up to 65.27 cm2/C at 550 nm, while the specific capacitance can achieve ∼30 mF/cm2. Additionally, the W@NiO-200 shows potential in prominent cyclic reversibility, rapid response time, prolonging cycle lifespan. This study provides a new strategy for preparation of high-quality electrochromic NiO films by sputtering technology.

Improving electrochromic performance of porous nickel oxide electrode via Cu doping

Porous Cu-doped NiO films were fabricated by a cost-effective chemical bath deposition method. The structural characterizations show that Cu doping significantly increase the porosity of the electrodes and the ratio of Ni3+/Ni2+, which are conducive to the electrolyte penetration and chemical reactivity. But the electrochromic and electrochemical behaviors show a nonmonotonic change with Cu doping content. The transmittance modulation increases from 54.56% to 67.72% with the doped Cu/Ni molar ratio increasing from 0 to 0.5%, and the corresponding coloration/bleaching time reduces from 10.4/7.9 s to 6.0/4.1 s, then gets worse with the further Cu doping because of the ionized impurity scattering effects. The optimal electrode with Cu doping of 0.5% show a good cyclic stability, which maintains 84.4% of ΔT after 200 discharge/charge cycles, the excellent cycle performance can be attributed to the inhibition of the irreversible transition of Ni2+ to Ni3+ by the doping of Cu during cycling.

Preparation and black-white electrochromic performance of the non-contact ZnO@NiO core-shell rod array

In this paper, the non-contact ZnO@NiO core-shell rod array films with black-white electrochromic performance were designed and constructed by electrochemical deposition method. The blank porous NiO films were also prepared for comparison. The morphology, component and microstructure of the films were characterized using X-ray diffractometer (XRD), scanning electron microscope (SEM) and high resolution transmission electron microscope (HRTEM). And their electrochromic (EC) performance was examined. The results show that the ZnO transition layer presents a hexagonal rod array nature, which is approximately perpendicular to the surface of substrate, thus, offered a three-dimensional frame for the growth of porous NiO. The ZnO@NiO film is composed of a large number of non-contact core-shell rods. The introduction of ZnO rod array transition layer not only led to the improvement of film/substrate connection but also resulted into the high EC performance including sharp black-white contrast, rapid switch speed and good stability. The heterojunction and P-N junction formed at the SnO2/ZnO/NiO interfaces might also play a role in the EC performance.

Effect of Gd doping on spray pyrolyzed NiO thin films for optoelectronic applications

In the present work, we systematically investigated the effect of Gd (1, 3, and 5%) doping on the structural, morphological, electrical, and optical properties of NiO thin films deposited on glass substrate using the spray pyrolysis method. X-ray diffraction study revealed that the deposited Gd:NiO(0–5%) films exhibit cubic structure with preferred orientation along (111) direction. The average crystallite size decreases from 15 nm to 12 nm with increasing Gd doping concentration. The atomic force microscope (AFM) topography study revealed that there is a decrease in grain size and variation of roughness with increasing Gd content into NiO lattice . The energy-dispersive X-ray spectroscopy (EDX)/ scanning electron microscope (SEM) mapping studies show the presence and uniform distribution of Gd in the Gd:NiO films. The average optical transparency (70%) and the optical band gap energy values decrease in Gd doped NiO films. The refractive index values are found to be in the range of 1.5–2.6. The higher values of linear and nonlinear optical parameters like χ1, χ3 and n2 are found to be 0.98 esu, 3.31 × 10−10 esu and 2.92 × 10−9 esu, respectively in Gd doped NiO films. The current study indicates that the Gd doping concentration has a strong influence on linear and non-linear optical properties of NiO films and makes them useful in several optoelectronic device applications.

A general and facile solution approach for deposition of high-quality metal oxide charge transport layers

Metal oxide charge transport layers (CTLs) are significant components for fabricating high-performance thin film solar cells. Generally, sol-gel methods have been widely used to deposit high-quality metal oxide CTLs, but they usually suffer from the choices of solvents, ligands and metal precursors. Here we report a general and facile ionic liquid (n-butylammonium butyrate) approach to prepare V2O5, MoO3, WO3, NiO, ZnO and SnO2 thin films, which exhibits strong dissolving ability and good film-forming property. The thermal volatilization of ionic liquid is mild, which provides a slow and stable decomposition process of metal compounds and favors the formation of smooth and compact CTLs. All thin films are highly transparent in the visual range from the optical transmittance spectra and suitable to be CTLs in photovoltaic devices. Encouragingly, we have received the efficiencies of 15.06% and 13.38% based on the area of 0.09 and 1.21 cm2 (NiO as the hole transport layer), and 19.13% and 16.71% based on the area of 0.09 and 1.21 cm2 (SnO2 as the electron transport layer), which are obviously higher than those of devices prepared by traditional sol-gel methods.

Two-Dimensional Porous Structure of V-Doped NiO with Enhanced Electrochromic Properties.

In this work, a two-dimensional porous structure of a V-doped NiO film with excellent electrochromic properties on an ITO substrate was synthesized by a hydrothermal method. The influence of V5+ ions on the NiO film was explored by adjusting the amount of V doping, including refining the crystal grains, increasing the specific surface area of the film, and accelerating the diffusion rate of OH– in the film. Compared with the undoped NiO film, a 3 atom % V-doped NiO film comes out with superior electrochromic properties with large optical transmittance modulation (81.9% at 600 nm), fast response times (1.2 and 0.9 s), and excellent cycle stability (90.6%). This work creates innovation direction in the field of intelligent energy-saving window materials with high electrochromic properties.

Scanning nitrogen-vacancy center magnetometry in large in-plane magnetic fields

Scanning magnetometry with nitrogen-vacancy (NV) centers in diamond has emerged as a powerful microscopy for studying weak stray field patterns with nanometer resolution. Due to the internal crystal anisotropy of the spin defect, however, external bias fields—critical for the study of magnetic materials—must be applied along specific spatial directions. In particular, the most common diamond probes made from {100}-cut diamond only support fields at an angle of θ=55° from the surface normal. In this paper, we report fabrication of scanning diamond probes from {110}-cut diamond where the spin anisotropy axis lies in the scan plane (θ=90°). We show that these probes retain their sensitivity in large in-plane fields and demonstrate scanning magnetometry of the domain pattern of Co–NiO films in applied fields up to 40 mT. Our work extends scanning NV magnetometry to the important class of materials that require large in-plane fields.

P-Type Metal Oxide Semiconductor Thin Films: Synthesis and Chemical Sensor Applications.

This review focuses on the synthesis of p-type metal-oxide (p-type MOX) semiconductor thin films, such as CuO, NiO, Co3O4, and Cr2O3, used for chemical-sensing applications. P-type MOX thin films exhibit several advantages over n-type MOX, including a higher catalytic effect, low humidity dependence, and improved recovery speed. However, the sensing performance of CuO, NiO, Co3O4, and Cr2O3 thin films is strongly related to the intrinsic physicochemical properties of the material and the thickness of these MOX thin films. The latter is heavily dependent on synthesis techniques. Many techniques used for growing p-MOX thin films are reviewed herein. Physical vapor-deposition techniques (PVD), such as magnetron sputtering, thermal evaporation, thermal oxidation, and molecular-beam epitaxial (MBE) growth were investigated, along with chemical vapor deposition (CVD). Liquid-phase routes, including sol–gel-assisted dip-and-spin coating, spray pyrolysis, and electrodeposition, are also discussed. A review of each technique, as well as factors that affect the physicochemical properties of p-type MOX thin films, such as morphology, crystallinity, defects, and grain size, is presented. The sensing mechanism describing the surface reaction of gases with MOX is also discussed. The sensing characteristics of CuO, NiO, Co3O4, and Cr2O3 thin films, including their response, sensor kinetics, stability, selectivity, and repeatability are reviewed. Different chemical compounds, including reducing gases (such as volatile organic compounds (VOCs), H2, and NH3) and oxidizing gases, such as CO2, NO2, and O3, were analyzed. Bulk doping, surface decoration, and heterostructures are some of the strategies for improving the sensing capabilities of the suggested pristine p-type MOX thin films. Future trends to overcome the challenges of p-type MOX thin-film chemical sensors are also presented.

Multi – wavelength photodetectors based on porous spin-coated and compact RF-sputtered NiO films grown over Si substrate: Effect of surface morphology

Spin-coated and RF-sputtered NiO films were grown over Si substrate. FESEM analysis showed porous morphology for spin-coated NiO film and compact surface morphology for RF-sputtered NiO film. The photodetection properties were explored in metal-semiconductor-metal (MSM) configuration and were found to be greatly affected by the surface morphology of NiO. The responsivity of spin-coated NiO/Si PD was measured as 6.90, 3.68 and 4.39 A/W, whereas it was measured as 1.73, 1.81 and 2.07 A/W for RF-sputtered NiO/Si photodetector (PD), under exposure to 365, 625 and 850 nm lights, respectively. The photo-to-dark current ratio (PDCR) was calculated as 303, 172, 206 and 31, 35, 41 for spin-coated and RF-sputtered NiO/Si PDs, respectively. The higher photoresponse of spin-coated NiO/Si PD over RF-sputtered NiO/Si PD, especially at 365 nm wavelength, resulted from the larger surface area of porous structured spin-coated NiO film. Both PDs exhibited stable and repeatable photoresponse at different switching frequencies of the illumination source. The rise/fall time were estimated as 1.96/9.76 and 3.49/7.68 ms for spin-coated and RF-sputtered NiO/Si PDs, respectively.