NiO Films Research Articles

Formation of NiO films by reactive sputtering and application to BaSi2 heterojunction solar cells as hole-selective interlayer material

Semiconducting BaSi2 has attractive features for thin-film solar cell applications. In this study, we investigated the potential of NiO as a hole transport layer in NiO/BaSi2 heterojunction solar cells both by simulation and by experiment. To find deposition conditions to form NiO layers, a NiO target was sputtered on glass substrates under various O2-to-Ar gas flow ratios. The hole concentration of the NiO films was controlled in the range 1017–1021 cm−3 mainly by the substrate temperature during deposition. After that, NiO/BaSi2 heterojunction solar cells were designed using a one-dimensional simulation software (AFORS-HET v2.5). The conversion efficiency exceeded 16% for 400 nm thick n-BaSi2 absorption layers. We actually formed NiO/BaSi2 heterojunction solar cells on glass substrates by radio-frequency sputtering, and demonstrated that the carriers photogenerated in the BaSi2 films contributed to the internal quantum efficiency spectrum at wavelengths shorter than approximately 900 nm, corresponding to the band gap of BaSi2.

A flexible bilayer p-NiO/n-ZnO films with photodetecting properties in self power mode

We report highly efficient flexible p-NiO/n-ZnO heterojunction UV photodetectors. NiO and ZnO thin films were grown on plastic polyethylene terephthalate (PET) substrates by dc magnetron sputtering for constructing high quality interfaces. The diode has shown an excellent rectification ratio i.e. 106 under dark mode and observed more than 80% transparency in the visible region. Investigated structural, compositional, and optical properties of the film to correlate the UV photodetector transport characteristics. Further, UV illumination devices exhibit an excellent responsivity of 0.24 A W−1 with a detectivity of 1.25 × 1011 jones and the highest external quantum efficiency of 83.14% achieved at −5 V of bias. The device shows the fastest speed of response with rise/fall times of 86.10 ms/106.60 ms, accomplished at −1 V with the lowest optical power density. The photodiode has shown incredible characteristics that are mainly attributed to the built-in potential and the transport mechanism at the interface.

Structural, Morphological, and Optical Analysis of La-Doped NiO Films Fabricated by the Sol-Gel Spin-Coating Technique for Solid-State Electronics

Structural, Morphological, and Optical Analysis of La-Doped NiO Films Fabricated by the Sol-Gel Spin-Coating Technique for Solid-State Electronics

Effect of reactive pressure on direct current-sputtered NiO films with improved p-type conduction ability

The NiO films with improved p-type conduction ability are direct current sputtered on glass substrates at different reactive pressures (Preact). For NiO films, Preact has a threshold value of 1.0 Pa for the continuous glow discharge. The NiO film at 1.0 Pa Preact has more point defects than other NiO films. That is related to the blue glow discharge of oxygen atoms at 1.0 Pa Preact that confirms the occurrence of the energy transfer between Ar+ and O atoms. Although the decrease in point defect concentration of the films with increase of Preact result in the improvement in crystallization and the reduction in p-type conduction ability, the p-type conduction of the films is still greatly improved compared with other reports. The p-type NiO films at different Preact all have a near-violet absorption edge of ca. 3.6 eV.

Disentangling electrical switching of antiferromagnetic NiO using high magnetic fields

Recent demonstrations of the electrical switching of antiferromagnets (AFs) have given an enormous impulse to the field of AF spintronics. Many of these observations are plagued by non-magnetic effects that are very difficult to distinguish from the actual magnetic ones. Here, we study the electrical switching of thin (5 nm) NiO films in Pt/NiO devices using magnetic fields up to 15 T to quantitatively disentangle these magnetic and non-magnetic effects. We demonstrate that these fields suppress the magnetic components of the electrical switching of NiO, but leave the non-magnetic components intact. Using a monodomainization model the contributions are separated, showing how they behave as a function of the current density. These results show that combining electrical methods and strong magnetic fields can be an invaluable tool for AF spintronics, allowing for implementing and studying electrical switching of AFs in more complex systems.

Effect of Rotational Speed on the Structural, Morphological, and Optical Properties of Biosynthesized Nickel Oxide Thin Films for Selective Solar Absorber Nanocoatings

This article presents a simple and low-cost green synthesized single-layer NiO selective solar absorber nanocoating prepared by spin coating on a Cu substrate at different rotational speeds (RS). The effects of substrate RS on the structural, morphological, chemical, and optical properties of the NiO nanocoatings were thoroughly investigated. The XRD results reveal the formation of pure diffraction peaks indexed to face-centered cubic NiO nanocoatings. SEM confirmed the uniform distribution of the NiO thin films with a nanosphere-like structure and the influence of RS variation on the formation of NiO nanostructures. EDS and XPS confirmed the presence of Ni and O in the green synthesized NiO coatings. AFM showed homogeneous nanopillar-like NiO thin films with the average surface roughness decreasing from 13.6 to 9.06 nm as the RS increased from 700 to 1300 RPM. Raman spectroscopy of the nanocoatings showed normal modes related to longitudinal optical and transverse optical phonons, and a combination of both, which implies the presence of a defect-rich or anti-ferromagnetically ordered NiO film responsible for the occurrence of more scattering peaks. UV–Vis–NIR and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the optical properties of the nanocoatings. The green synthesized NiO nanocoatings deposited at 700 RPM exhibited excellent solar absorptance (α) of 0.92 and low thermal emittance (ɛ) of 0.11. The optical properties of the selective materials obtained in the present work were correlated with the non-stoichiometric nature of the spin-coated sample, multiple reflections, and interference-induced light absorption on the green synthesized surface. These results suggest that the NiO thin films prepared through this simple and environmentally benign green synthesis method can be promising candidates for efficient solar selective absorbers.

Influence of heat treatment on H2S gas sensing features of NiO thin films deposited via thermal evaporation technique

In this study, NiO thin films are prepared on Ni foil using thermal evaporation method and the effect of deposition conditions on structural, morphological and H2S gas sensing properties of NiO thin films is investigated. Structural analysis of the NiO films are conducted by means of X-ray diffraction (XRD), and the films are found to have the FCC phase with preferred (111) and (200) Bragg reflections. Based on XRD and scanning electron microscopy results, it is shown that the crystallite size and the size of homogeneous nanoclusters on the surface of the films increase after increasing oxidation temperature. Also, the EDS patterns of NiO films demonstrate a rise in the intensity of O peak upon increasing the oxidation temperature due to the enhancement of oxygen in the NiO films. The Raman spectrum of NiO thin films several bands correspond to Ni–O vibrations. The obtained gas sensing results demonstrate the sensing response to 20 ppm of hydrogen sulfide at 700 °C. Upon increasing the amount of hydrogen sulfide gas to more than 20 ppm, the fabricated sensors show no significant change in their response, indicating that no additional active sites were exist to interplay with hydrogen sulfide molecules for the higher concentration of hydrogen sulfide. The response of the sensors, recorded 10 s after H2S gas exposure, depended on the deposition temperature. The current study shows that NiO thin films-based sensors prepared by a simple thermal evaporation method are potentially useful for the detection of H2S gas.

The Microstructures and Characteristics of NiO Films: Effects of Substrate Temperature.

The influence of the substrate temperature on the structural, surface morphological, optical and nanomechanical properties of NiO films deposited on glass substrates using radio-frequency magnetron sputtering was examined by X-ray diffraction (XRD), atomic force microscopy (AFM), UV-Visible spectroscopy and nanoindentation, respectively. The results indicate that the substrate temperature exhibits significant influences on both the grain texturing orientation and surface morphology of the films. Namely, the dominant crystallographic orientation of the films switches from (111) to (200) accompanied by progressively roughening of the surface when the substrate temperature is increased from 300 °C to 500 °C. The average transmittance of the NiO films was also found to vary in the range of 60-85% in the visible wavelength region, depending on the substrate temperature and wavelength. In addition, the optical band gap calculated from the Tauc plot showed an increasing trend from 3.18 eV to 3.56 eV with increasing substrate temperature. Both the hardness and Young's modulus of NiO films were obtained by means of the nanoindentation continuous contact stiffness measurements mode. Moreover, the contact angle between the water droplet and film surface also indicated an intimate correlation between the surface energy, hence the wettability, of the film and substrate temperature.

Effects of Sputtering Pressure on Electrochromic Properties of NiO films Prepared by DC Magnetron Sputtering

In this paper, nickel oxide films were deposited on ITO-coated glass substrates by DC magnetron sputtering at different pressures(1.2 Pa ∼ 3.0 Pa). The effects of sputtering pressure on microstructure and electrochromic properties of nickel oxide films were investigated. The film thickness was measured by a surface profilometer. The crystal structure and surface morphology of the films were observed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrochromic properties of the films were studied by combining UV-visible spectrophotometer with electrochemical workstation. The results showed that the nickel oxide film obtained the best surface morphology (uniform grain size and the fewer surface cracks) and outstanding electrochromic performances, including large transmittance modulation (ΔT = 57.19%), high coloration efficiency (CE = 33.59 cm2·C−1) and fast switching speed (tc = 4.63 s, tb = 4.87 s) at a wavelength of 550 nm when the sputtering pressure was 2.4 Pa. And after 500 electrochemical cycles, the transmittance modulation could continue to increase to 61.49% and the coloration efficiency can still be maintained at about 28.21 cm2·C−1, which showed excellent cycling durability.

Asymmetric encryption by optical Kerr nonlinearities exhibited by electrochromic NiO thin films.

Herein is analyzed how an electric field can induce a band gap shift in NiO films to generate an enhancement in their third-order optical nonlinearities. An electrochromic effect seems to be responsible for changes in absorbance and modification in off-resonance nonlinear refractive index. The optical Kerr effect was determined as the dominant physical mechanism emerging from the third-order optical susceptibility processes present in a nanosecond two-wave mixing configuration at 532 nm wavelength. Absence of any important multi-photonic absorption was validated by the constant trace of high-irradiance optical transmittance in single-beam mode. The inspection of nonlinear optical signals allowed us to propose an exclusive disjunctive logic gate assisted by an electrochromic effect in an optical Kerr gate. Asymmetric encryption by our XOR system with the influence of a switchable probe beam transmittance and electrical signals in the sample was studied. Immediate applications for developing multifunctional quantum systems driven by dynamic parameters in electrochromic and nonlinear optical materials were highlighted.

Electrical and Optical Properties of P-Type NiO Films Grown Via Nickelocene Precursor in a Hollow-Cathode Plasma-ALD Reactor

Electrical and Optical Properties of P-type NiO Films Grown via Nickelocene Precursor in a Hollow-Cathode Plasma-ALD Reactor In contrast to the relative abundance of as-grown unintentionally doped n-type semiconductor materials, there are only a few alternative alloys showing p-type conduction without needing extra high-temperature doping processes. Among them, CuO, Bi2O3, SnO, and NiO represent the most widely studied metal oxides which exhibit native p-type properties. NiO is of particular interest mainly due to its stability and promising performance as hole-transport layers in emerging solar cell device structures. However, to broaden the NiO application domain towards potential back-end-of-the-line (BEOL) transistor devices as potential p-type channel layers, the transport properties of NiO needs to be improved significantly. While low-temperature thermal, plasma, and ozone-assisted ALD efforts have resulted in p-type NiO films with atomic-level precision and large-area uniformity, the hole mobility is far from being sufficient, typically lower than 1 cm2/Vs.In an attempt to enhance the hole transport properties in low-temperature as-grown NiO layers, in this study we have carried out a systematic study on plasma-enhanced ALD (PEALD) of NiO films on Si and glass substrates using nickelocene (NiCp2) and O2/Ar plasma mixture in two separate reactors: (i) 8” plasma-ALD reactor with a quartz-based inductively-coupled plasma (ICP) source; (ii) 4” plasma-ALD reactor with a stainless steel-based hollow-cathode plasma (HCP) source, equipped with an in-situ ellipsometer. 300 cycle ICP-ALD growth experiments with 300 W O2/Ar plasma at 150 °C substrate temperature were carried out to identify the optimal NiCp2 precursor temperature (50 - 110 °C) and observe the influence of precursor pulse time (0.1 - 1 s). Detailed saturation studies using in-situ ellipsometer monitoring were carried out in the HCP-ALD reactor, scanning for NiCp2 pulse time, O2 plasma exposure time, purge time, and plasma power. Optimal growth conditions were identified as 90 ms NiCp2 pulse / 5 s purge / 10 s O2-plasma at 100 W plasma exposure / 5 s purge. To check the growth linearity and obtain thicker films for materials characterization, 800 cycle runs were conducted to evaluate the substrate temperature impact (100 - 250 °C) on growth-per-cycle (GPC) and film properties. The last growth parameter studied was the plasma gas composition, where several O2/Ar mixtures (20/0, 20/20, 50/0 sccm) were tested at the optimal substrate temperature of 200 °C.The resulting NiO films are characterized for their structural, optical, and electrical properties. Films grown at optimal conditions (200 °C) exhibit refractive index values reaching 2.3, which is in good agreement with reported values for the best polycrystalline NiO films in the literature. NiO films deposited on both Si and glass substrates exhibit polycrystalline single-phase cubic structure (c-NiO). Hall measurements of the as-grown NiO layers on glass substrates, revealed film resistivity values about 40 and 150 ohm-cm for HCP and ICP samples, respectively. Post-deposition annealing within 200 – 400 °C temperature range resulted in a significant reduction of resistivity for more than two orders of magnitude. However, the relatively volatile and unstable Hall voltage values led to unreliable carrier concentration and Hall mobility values, therefore we were not able to confirm the type of transport via Hall measurements.In order to determine the carrier type and extract field-effect mobility values, we have fabricated vertical NiO/n-Si p-n junction diodes as well as NiO/Al2O3/Si thin-film-transistor (TFT) device prototypes to characterize diode I-V behavior and confirm p-type layer structure for NiO, and extract the field-effect carrier mobility, respectively. Our I-V measurement results confirmed p-n junction diode characteristics with decent ON/OFF ratios for forward and reverse diode current values. TFT characterization results and findings will be presented as well.

Novel approach for improving electrochromic and mechanical properties of NiO film: Experiment and molecular dynamics simulation

In this paper, we systematically studied the effect of substrate surface roughness on the electrochromic and mechanical properties of NiO films deposited on FTO conductive glass by magnetron sputtering. Results showed that increasing substrate surface roughness led to an increase in optical contrast of the film and decreased response time. The NiO film deposited on FTO (Ra 20 nm) had the largest optical contrast (54.7 % at 600 nm) and fastest response time (7.0 s and 1.7 s). Moreover, SEM, XPS, and EDS analyses revealed that the resultant increase in nanoporosity with increasing surface roughness was the main reason for this occurrence. The growth process of the NiO film was simulated by molecular dynamics, and the mechanism of the nanoporous structure was explained theoretically. The mechanical test showed that the composite hardness of FRa20 was 4.2 GPa, while the elastic modulus of the NiO film deposited on it was 74.9 GPa, which was better than all the other films. Thus, this study provides a novel and convenient approach for improving the electrochromic and mechanical properties of NiO films.

Ginkgo Leaf Inspired Fabrication of Micro/Nanostructures and Demonstration of Flexible Enzyme-Free Glucose Sensors.

Flexible enzyme-free glucose sensors have attracted widespread attention due to their importance and potential applications in clinical diagnosis, flexible wearable devices, and implanted devices in vivo. At present, there are still major problems in fabricating flexible enzyme-free glucose sensors with low detection limits, high stability, and high sensitivity at low cost, hindering their practical application. Here, we report a facile strategy for the fabrication of flexible non-enzymatic glucose sensors using ginkgo leaf as a template. NiO film and PEDOT:PSS composite film were deposited on the surface of the ginkgo leaf induced micro-nano hierarchical structure as a sensitive layer and a conductive layer, respectively. The as-prepared, flexible, enzyme-free glucose sensor exhibited excellent electrochemical performance toward glucose oxidation with a sensitivity of 0.7413 mA·mM−1/cm−2, an operating voltage of 0.55 V, a detection limit of 0.329 μM, and good anti-interference. Due to the simple fabrication process and performance reliability, the novel flexible enzyme-free glucose sensor is an attractive candidate for next generation wearable and implantable non-enzymatic glucose diagnostic devices.

Growth and characterization of electron beam evaporated NiO thin films for room temperature formaldehyde sensing

Herein, we report the formaldehyde gas sensing performance of NiO thin films deposited by the electron beam evaporation technique. NiO thin films were grown over glass substrates by the variation of deposition temperature from room temperature (RT) to 500⁰C. The structure, chemical composition and morphological studies were conducted by XRD, FESEM, EDS, AFM, XPS, RBS and HRTEM techniques. The XRD characterization revealed the formation of NiO in the polycrystalline phase with the face-centered cubic structure without impurity phases. The surface and morphological characterization confirmed the growth of the NiO thin layer and the homogeneous distribution of Ni and O elements on the surface of the films. The optical transmittance increased with the rise in deposition temperature. The optical band gap was estimated using Tauc relation and it was shown a decreasing trend with the rise in deposition temperature. The electrical characteristics of thin films were carried out using Hall measurements and presented. The developed gas sensors tested at room temperature have displayed good selectivity towards formaldehyde with a low detection limit of 10 ppm. The NiO thin film sensor prepared at a deposition temperature of 400⁰C has exhibited a short response and recovery time of 25 s and 9 s respectively towards 50 ppm concentration of formaldehyde. The detailed formaldehyde sensing mechanism for p-type NiO thin films was discussed. • E-beam evaporated NiO thin films were deposited by varying the substrate temperature. • The developed NiO films were tested to detect various VOCs at room temperature. • NiO sensor (400⁰C) has exhibited enhanced formaldehyde sensing performance. • The response and recovery times were determined to be 25 s and 9 s towards 50 ppm. • The sensing mechanism involved in redox reactions was projected.

Atomic Layer Deposition of SnO2 as an Electron Transport Material for Solid-State P-type Dye-Sensitized Solar Cells

Tin oxide (SnO2) as an electron transport material was prepared by atomic layer deposition in dye-sensitized NiO films to fabricate solid-state p-type dye-sensitized solar cells using two organic dyes PB6 and TIP as photosensitizers. Due to the excellent electron mobility and satisfactory penetration of SnO2 material into the NiO film, a record photocurrent density over 1 mA cm–2 was achieved with a power conversion efficiency of 0.14%. The effect of an inserted Al2O3 layer between the dye-sensitized NiO and SnO2 layer on photovoltaic performance of the devices was also investigated. The results suggest that the charge recombination between NiO and SnO2 can be significantly suppressed, showing prolonged charge lifetime and enhanced photovoltage.

Routes to increase performance for antimony selenide solar cells using inorganic hole transport layers

Simple compound antimony selenide (Sb2Se3) is a promising emergent light absorber for photovoltaic applications benefiting from its outstanding photoelectric properties. Antimony selenide thin film solar cells however, are limited by low open circuit voltage due to carrier recombination at the metallic back contact interface. In this work, solar cell capacitance simulator (SCAPS) is used to interpret the effect of hole transport layers (HTL), i.e., transition metal oxides NiO and MoO x thin films on Sb2Se3 device characteristics. This reveals the critical role of NiO and MoO x in altering the energy band alignment and increasing device performance by the introduction of a high energy barrier to electrons at the rear absorber/metal interface. Close-space sublimation (CSS) and thermal evaporation (TE) techniques are applied to deposit Sb2Se3 layers in both substrate and superstrate thin film solar cells with NiO and MoO x HTLs incorporated into the device structure. The effect of the HTLs on Sb2Se3 crystallinity and solar cell performance is comprehensively studied. In superstrate device configuration, CSS-based Sb2Se3 solar cells with NiO HTL showed average improvements in open circuit voltage, short circuit current density and power conversion efficiency of 12%, 41%, and 42%, respectively, over the standard devices. Similarly, using a NiO HTL in TE-based Sb2Se3 devices improved open circuit voltage, short circuit current density and power conversion efficiency by 39%, 68%, and 92%, respectively.

Nickel compound quantum dots as inorganic hole transporting layer in perovskite solar cells

Nickel oxide has grabbed the attention as one the most efficient materials for inorganic hole transporting layer in perovskite solar cells. However, it is quite a challenge to control the growth conditions of NiO film. In this work, the nickel compound quantum dots prepared via the electrochemical process are investigated as HTL for the PSCs. From absorbance spectra, the mixed phase of NiCl2 and NiOOH are observed. The average size of nickel compound quantum dots is about 2–4 nm from transmission electron microscopy. The solutions of nickel compound quantum dots are used to fabricate the PSCs with a p-i-n structure consisting of Fluorine doped Tin Oxide glass/nickel compound quantum dot/perovskite/PCBM/Ag. It is found that the NiO films prepared from nickel compound quantum dots show the full covered film, resulting in higher power conversion efficiency of 12.47% as compared to that of the reference NiCl2. The improvement in efficiency can be explained by increasing short circuit current density and fill factor. In addition, the stability of PSC based on nickel compound quantum dots demonstrates higher stability than the other condition. Thus, PSCs based on nickel compound quantum dots can be used to improve the interface of HTL and perovskite layer in PSC and result in the enhancement of the PSC performance.

Oxygen vacancy-enriched ALD NiO sub-50 nm thin films for enhanced triethylamine detection

p-type metal oxide semiconductors have received significant interest in the field of gas sensors; however, it is quite challenging to achieve high sensor response because of inferior surface and electronic properties. Herein, we report a high-performance gas sensor fabricated by plasma-etching an NiO thin film deposited by atomic layer deposition. Ar plasma treatment is found to introduce a large number of oxygen vacancies, which effectively adjusts the electronic and chemical characteristics of the p-type NiO films to afford improved response to toxic triethylamine. The effects of the thickness of the sensing layer on sensor properties are also studied, which reveals that the NiO film with a thickness of 40 nm has the greatest gas sensing performance. After Ar plasma treatment, the response of the NiO thin films is significantly enhanced to enable an excellent limit of detection of 27.4 ppb, which is much lower than the threshold limit of 1 ppm proposed by American Conference of Governmental Industrial Hygienists. The demonstrated strategy and excellent sensor properties suggest a pathway to high performance gas sensors.

Synthesis and Investigation of Pure and Cu-Doped NiO Nanofilms for Future Applications in Wastewater Treatment Rejected by Textile Industry

Pure and Cu-doped NiO films were synthesized via a soft chemical process. They were deposited on glass substrates heated to 400 °C. Different atomic percentage ratios (2, 4, 6, 8, and 10%) of Cu-doping were used. The prepared samples were characterized by several techniques such as X-ray diffraction for crystallographic study, SEM and AFM for microstructural and morphological properties, and UV-Visible spectroscopy for optical and photocatalytical analysis. XRD results of pure and Cu-doped NiO films indicated the formation of NiO polycrystalline phases under a cubic structure with a favored orientation along the (200) plane noticed in all sprayed films. SEM images revealed the formation of NiO nanoparticles of spherical forms whose sizes increase and agglomerate with increasing Cu-doping. At 10% Cu-doping, NiO agglomeration was extended to the whole surface. AFM images showed a textured and rough surface composed of NiO nanoparticles of average size varying from 16 to 10 nm depending on Cu-doping concentration. UV-visible spectroscopy confirmed the transparency of NiO films and their semiconducting character with a band gap ranging from 3.4450 eV to 2.8648 eV. The photocatalytical properties of pure and Cu-NiO films were enhanced by Cu-doping particles as revealed by the degradation of methylene blue (MB) solution subjected to irradiation.

Stochastic Local Breakdown of Oxide Film on Ni from Identical-Location Imaging: One Single Site at a Time.

The electrochemical breakdown of a metal oxide film can directly affect the performance of functional electrochemical devices. However, revealing the structural insight into the breakdown sites is challenging because of heterogeneity: different breakdown sites are spatially distributed over the surface. Herein, we combine scanning electrochemical cell microscopy with identical-location microscopies to reveal the heterogeneity in the breakdown of NiO film on Ni in a site-by-site manner. Local critical breakdown potential varies by ∼500 mV, corresponding to an excess energy of 0.02-0.12 J/m2. Correlative composition imaging using time-of-flight secondary ion mass spectrometry shows Ni crystal grains with thinner NiO films are more resistant to breakdown. This high resistance is explained using classical nucleation theory, where the electrical energy is affected by the film thickness through the local interfacial capacitance. The correlative imaging approach overcomes the issue of heterogeneity, providing conclusive insight into the stability of the electrochemical interfaces.