Niobium Oxide Thin Films Research Articles

Molybdenum-Modified Niobium Oxide: A Pathway to Superior Electrochromic Materials for Smart Windows and Displays

Electrochromic materials enable the precise control of their optical properties, making them essential for energy-saving applications such as smart windows. This study focuses on the synthesis of molybdenum-doped niobium oxide (Mo-Nb2O5) thin films using a one-step hydrothermal method to investigate the effect of Mo doping on the material’s electrochromic performance. Mo incorporation led to distinct morphological changes and a transition from a compact granular structure to an anisotropic rod-like feature. Notably, the MN-3 (0.3% Mo) sample displayed an optimal electrochromic performance, achieving 77% optical modulation at 600 nm, a near-perfect reversibility of 99%, and a high coloration efficiency of 89 cm2/C. Additionally, MN-3 exhibited excellent cycling stability, with only 0.8% degradation over 5000 s. The MN-3 device also displayed impressive control over color switching, underscoring its potential for practical applications. These results highlight the significant impact of Mo doping on improving the structural and electrochromic properties of Nb2O5 thin films, offering improved ion intercalation and charge transport. This study underscores the potential of Mo-Nb2O5 for practical applications in energy-efficient technologies.

Niobium Oxide Thin Films Grown on Flexible ITO-Coated PET Substrates

Niobium oxide thin films were grown on both rigid and flexible substrates using DC magnetron sputtering for electrochromic applications. Three experimental series were conducted, varying the oxygen to argon flow rate ratio and deposition time. In the first series, the oxygen to argon ratio was adjusted from 0 to 0.32 while maintaining a constant growth time of 30 min. For the second and third series, the oxygen to argon ratios were fixed at 0.40 and 0.56, respectively, with deposition times ranging from 15 to 60 min. A structural transition from crystalline to amorphous was observed at an oxygen to argon flow rate ratio of 0.32. This transition coincided with a change in appearance, from non-transparent with metallic-like electrical conductivity to transparent with dielectric behavior. The transparent niobium oxide films exhibited thicknesses between 51 nm and 198 nm, with a compact, dense, and featureless morphology, as evidenced by both top-view and cross-sectional images. Films deposited on flexible indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrates displayed a maximum surface roughness (Sq) of 9 nm and a maximum optical transmission of 83% in the visible range. The electrochromic response of niobium oxide thin films on ITO-coated PET substrates demonstrated a maximum coloration efficiency of 30 cm2 C−1 and a reversibility of 96%. Mechanical performance was assessed through bending tests. The ITO-coated PET substrate exhibited a critical bending radius of 6.5 mm. Upon the addition of the niobium oxide layer, this decreased to 5 mm. Electrical resistance measurements indicated that the niobium oxide film mitigated rapid mechanical degradation of the underlying ITO electrode beyond the critical bending radius.

Memristive effect in niobium oxide thin films obtained by the pulsed laser deposition

NbOx thin films demonstrating the analog resistive switching characteristics in planar geometry were fabricated by the droplet-free pulsed laser deposition from Nb metal targets. The memristive properties of the films were controlled by tuning the oxygen pressure from 0.7 to 8 Pa and the wavelength of the ablative laser (λ = 248 nm or λ = 532 nm). The stoichiometry of the NbOx films was controlled by the X-ray photoelectron spectroscopy, optical and electrical investigations. Variation of the film thickness from 20 to 50 nm affects the resistive switching characteristics. The 20 nm NbO2 films obtained at the oxygen pressure of 1.3 Pa show a volatile memristive switching with a memory window of ON/OFF resistances ∼ 101 after 500 direct current cycles at an operating voltage of 4.5 V. The 30 nm Nb2O5 films fabricated at the oxygen pressure of 4 Pa, showed a gradual increase in device conductivity with successive positive voltage sweeps with an amplitude of +5 V, which is characteristic of non-volatile memristive switching. Thus, the use of NbOx memristive films of different compositions and crystallinity degree will allow creating neuromorphic systems based on the same material by controlling the memristive behavior by the laser synthesis conditions.

Bi-functional electrochromic supercapacitor based on hydrothermal-grown 3D Nb2O5 nanospheres

The design and development of novel cathodic materials with suitable crystal structure and nanostructure is an essential basis for both electrochromism and electrochemical energy storage devices. In this study, Niobium oxide (Nb2O5) thin films as high-performance cathodic materials were synthesized using a facile hydrothermal method. The influence of the precursor concentration on the structural, morphological, electrochromic, and energy storage properties was investigated successfully. The X-ray diffraction analysis confirmed the highly crystalline orthorhombic nature of Nb2O5 thin films. Nanostructured Nb2O5 nanospheres effectively improved the charge transportation and mobility of charge carriers. Nb01 thin film most effectively accommodated lithium ions and facilitated the desired bifunctional performance. An electrochromic supercapacitor (ESC) of Nb01 composition showed the highest optical modulation of (76.92%), excellent reversibility of (98%), high coloration efficiency (CE) (98.75 cm2 C−1), and high-energy storage capacity (93.33 C cm−2). The fully colored ESC devices (4 × 5 cm2) have lightened a red and yellow light-emitting diode (LED) which confirms the energy storage level with the color changing ability of the 3D Nb2O5 nanospheres as a potential application in ESC.

Morphology of lithium niobium oxide thin film ultrasonic transducers deposited by RF magnetron sputtering

Lithium niobium oxide (LNO) thin film high temperature ultrasonic transducers have been deposited using radio frequency magnetron sputtering. Effects of the sample position in the sample plane on the thin film morphology are studied. In the center of the sputtering area, where the radial distance is 0 mm, the LNO thin film transducers are a mixture of LiNbO3/LiNb3O8 phases with well developed columnar structure and good ultrasonic response. At the radial distance of 10–40 mm, complex “sunflower-like” morphological features are formed, which may develop due to preferable crystallization of the sputtered material on particle seeds, significant difference in atomic mass of Li and Nb, and space confinement. The formation of wedge-like “sunflowers” reduces the performance of the ultrasonic transducers. In addition, the impacts of annealing temperature and time on morphology, crystal structure, element distribution, and ultrasonic response are also observed. The long-term annealing at 700 °C (for 100 h) upgrades the ultrasonic response, and for prolonged annealing up to 312 h, the ultrasonic response gradually degrades.

Optical and physical properties of annealed amorphous niobium oxide thin films.

The physical and optical properties of coatings deposited using physical vapor deposition techniques exhibit changes during post deposition annealing. Optical properties including the index of refraction and spectral transmission vary when coatings are annealed. Physical and mechanical properties such as thickness, density, and stress are also impacted by annealing. In this paper, we study the source of these changes by examining the impact of 150-500°C annealing on N b 2 O 5 films deposited using thermal evaporation and reactive magnetron sputtering. Models based on the Lorentz-Lorenz equation and potential energy considerations explain the data, and rationalize conflicts between previously reported results.

Effect of niobium oxide thin film on the long-term immersion corrosion of the 2198-T851 aluminium alloy

This work aims to study the effect of niobium oxide coating on the long-term immersion corrosion of the 2198-T851 aluminium alloy. The niobium oxide thin film was successful obtained on freshly plates of 2198-T851 alloy by reactive sputtering technique. Scanning vibrating electrode tests, electrochemical impedance spectroscopy and immersion tests measurements were used to reveal the insights of global and localised corrosion behaviour of the functionalised material in different concentrations of sodium chloride solution. The electrochemical impedance spectroscopy spectra showed that the coating significantly influenced on the corrosion resistance of the 2198-T851 alloy, since the functionalised alloy immersed in 0.6 mol L−1 NaCl solution during 720 h (30 days) displays the same result as the bare material for 3 h of immersion. The scanning vibrating electrode tests demonstrated the coated material was free of local corrosive attack by the electrolyte in the first 10 h of immersion, whilst the bare alloy exhibited active anodic and cathodic sites over the surface with increasing current density values for longer immersion times. The deposited coating was able to increase the corrosion resistance of the 2198-T851 aluminium alloy used as fuselage skin material by the aeronautical industry as verified by the immersion tests. Finally, experimental results confirm the extraordinary properties of niobium oxide as-deposited coating.

Laser synthesis of volatile memristors based on niobium oxide thin films

Amorphous NbOx (x ≤ 2.5) thin films with the presence of nanocrystallites, which are promising for use as an active region of volatile memristors were fabricated on c-Al2O3 substrates by the laser synthesis. The films were obtained from targets of niobium metallic during ablation of laser with λ = 248 nm or λ = 532 nm in oxygen pressure range from 5 to 50 mTorr. The effect of the oxygen partial pressure and the wavelength of ablating laser on the chemical, structural, electrical, and optical properties of the films was studied. The optimal conditions for the formation of Nb2O5 and NbO2 stable phases in the films were determined. It was found that the amorphous NbOx phase, which was a source of defects associated with oxygen deficiency, required for the creation of memristors, appeared only at low oxygen pressures in the chamber (∼ 5 ⋅ 10−3 − 10−1 Ohm ⋅ cm) during the film synthesis by the laser with both λ = 248 nm and λ = 532 nm. Volatile memristors of Nb/NbОх/Nb2O5/Pt/с-Аl2O3 (x ≤ 2.5) in cross-bar geometry were created demonstrating long-term stability of operation for 60 DC cycles with ΔRHRS/RHRS < 8% at an operating switching voltage of ∼ 0.5 V.

Corrosion behaviour of reactive sputtering deposition niobium oxide based coating on the 2198-T851 aluminium alloy

This work aims to improve the corrosion properties of the 2198-T851 aluminium alloy by coating with a set of niobium oxide thin films, using the reactive sputtering technique. The structural and morphological properties of the niobium oxide thin films were characterized by using SEM/EDX, AFM, FTIR and Raman spectroscopy. Global electrochemical tests (OCP, PPc, CV, and EIS) were performed in 0.6 mol L−1 NaCl solution. The results demonstrated that the reactive sputtering technique was advantageous for producing thin films on the 2198-T851 aluminium surface. Raman spectroscopy results revealed a band near 650 cm−1 related to NbO single bonds, whilst the band centred at 868 cm−1 may be attributed to NbO double bonds. The PPc results indicated that the niobium oxide acts as a protective barrier, since a difference of about 210 mV was observed between Epitting and Ecorr. The coated specimens displayed a superior breakdown potential when compared to the base material (−0.387 vs − 0.505 V/SCE). The impedance modulus has increased more than one order of magnitude and the phase angle is closer to - 90°, demonstrating a capacitive thin film was deposited on the 2198-T851 aluminium alloy surface.

Enhancing the corrosion protection of Ti-6Al-4V alloy through reactive sputtering niobium oxide thin films

Here, a set of polycrystalline niobium oxide thin films were produced by using reactive sputtering technique on the Ti-6Al-4V alloy surfaces and characterized by OM, SEM/EDX, AFM, DRX/Rietveld refinement, and XPS techniques. CPDP and SVET tests were performed on the coated and uncoated material considering an aggressive medium (NaF and Hank's solution). The results demonstrated the technique used in the present survey was advantageous to produce thin films on the Ti-6Al-4V surfaces. XPS analysis demonstrated that the niobium oxide thin films were composed by Nb2O5/NbO2. Measured and Rietveld refined X-ray diffraction patterns for the Nb-oxide thin film demonstrated the presence of the Nb2O5, NbO2, and NbO crystalline phases. The thin film acts as a barrier layer, suggesting a better corrosion performance when compared to the bare alloy (Ecorr = −111 mVSCE vs −290 mVSCE). The CPDP results demonstrated the current density of the coated Ti-6Al-4V specimen, at the same potential, was lower when compared to the bare alloy. SVET results revealed the coated material displayed low values of current density (~100 μA/cm2 for 15 h of immersion time), whilst the bare material (in the range 700–2200 μA/cm2 for 1 h of immersion time), suggesting the niobium oxide thin films act as an effective protective barrier against corrosion process evolution over time.

Fabrication and Characterization of Nb&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; Dopant Al Thin Films Prepared by DC Reactive Plasma Sputtering Technique

The development of niobium oxide (Nb2O5) thin films is an important work as a result of wide applications of this oxide in the field of material science and thin-film applications. In this study, thin-film microstructures of aluminum (Al)-doped Nb2O5 were prepared by DC plasma sputtering on glasses substrate. The ratio of doping was (0.5, 1, and 1.5) wt. % Al. The obtained samples were thermally treated at 450 °C. Characterized and analyzed the physical properties by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), atomic force microscopy (AFM), and UV-Visible spectroscopy for optical properties investigation. Results showed that the average crystalline size of Nb2O5:0.5%Al film was found at 26.47 nm and the structure was a monoclinic phase for all samples. The distribution of grain size was found lower than 36.3 nm and uninformed particles on the surface. The analyzed optical properties showed the absorption decreased from 0.46 to 0.05 with increasing the wavelength and Low energy gap values decreased from 3.10 eV for Nb2O5 samples to 2.84 eV for 1.5%Al samples. In general, the doping by aluminum improved the physical properties of Nb2O5 films.

Engineering the Threshold Switching Response of Nb2O5-Based Memristors by Ti Doping

Two terminal metal-oxide-metal devices based on niobium oxide thin films exhibit a wide range of non-linear electrical characteristics that have applications in hardware-based neuromorphic computing. In this study, we compare the threshold-switching and current-controlled negative differential resistance (NDR) characteristics of cross-point devices fabricated from undoped Nb2O5 and Ti-doped Nb2O5 and show that doping offers an effective means of engineering the device response for particular applications. In particular, doping is shown to improve the device reliability and to provide a means of tuning the threshold and hold voltages, the hysteresis window, and the magnitude of the negative differential resistance. Based on temperature-dependent current-voltage characteristics and lumped-element modelling, these effects are attributed to doping-induced reductions in the device resistance and its rate of change with temperature (i.e., the effective thermal activation energy for conduction). Significantly, these studies also show that a critical activation energy is required for devices to exhibit NDR, with doping providing an effective means of engineering the current-voltage characteristics. These results afford an improved understanding of the physical mechanisms responsible for threshold switching and provide new insights for designing devices for specific applications.

Polyoxoniobates as molecular building blocks in thin films.

Niobium oxide thin films have been prepared by spin-coating aqueous solutions of tetramethylammonium salts of the isostructural polyoxometalate clusters [Nb10O28]6-, [TiNb9O28]7- and [Ti2Nb8O28]8- onto silicon wafers, and annealing them. The [Nb10O28]6- cluster yields films of Nb2O5 in the orthorhombic and monoclinic crystal phases when annealed at 800 °C and 1000 °C, respectively, whereas the [TiNb9O28]7- and [Ti2Nb8O28]8- clusters yield the monoclinic crystal phases of Ti2Nb12O29 and TiNb2O7 (titanium-niobium oxides) in different ratios. We also demonstrate a protocol for depositing successive layers of metal oxide films. Finally, we explore factors affecting the roughness of the films.

Voltage-triggered insulator-to-metal transition of ALD NbOx thin films for a two-terminal threshold switch

We report two-terminal threshold devices that use niobium oxide (NbOx) thin films that were synthesized using atomic layer deposition (ALD) then pulsed-laser annealing.

Effects of preparation conditions on the optical properties of niobium oxide thin films

Major deposition parameters, including oxygen partial pressure and substrate temperature maintained during film deposition, affect the optical properties of electron beam evaporated niobium oxide films. All the films deposited were found to be amorphous as revealed by the X-ray diffraction study. A method requiring measurements at normal incidence of transmission from two films of different thicknesses prepared under identical conditions was used to determine the optical constants. Substantial changes in the optical constants and optical bandgap energy were observed following changes in the preparation conditions. These variations are related to the corresponding compositional changes, as observed by Rutherford Backscattering Spectroscopy, in the atomic ratio of oxygen to niobium in the films prepared under different conditions.

Electron Beam Effects on Oxide Thin Films-Structure and Electrical Property Correlations.

In situ transmission electron microscope (TEM) characterization techniques provide valuable information on structure-property correlations to understand the behavior of materials at the nanoscale. However, understanding nanoscale structures and their interaction with the electron beam is pivotal for the reliable interpretation of in situ/ex situ TEM studies. Here, we report that oxides commonly used in nanoelectronic applications, such as transistor gate oxides or memristive devices, are prone to electron beam induced damage that causes small structural changes even under very low dose conditions, eventually changing their electrical properties as examined via in situ measurements. In this work, silicon, titanium, and niobium oxide thin films are used for in situ TEM electrical characterization studies. The electron beam induced reduction of the oxides turns these insulators into conductors. The conductivity change is reversible by exposure to air, supporting the idea of electron beam reduction of oxides as primary damage mechanism. Through these measurements we propose a limit for the critical dose to be considered for in situ scanning electron microscopy and TEM characterization studies.

Sol-gel processed niobium oxide thin-film for a scaffold layer in perovskite solar cells

Sol-gel processed niobium oxide (Nb2O5) thin-film was synthesized for a scaffold layer (SL) of methylammonium lead iodide-based perovskite film. Complementary characterizations, including scanning electron microscopy, X-ray diffraction, optical absorption spectroscopy have revealed that the Nb2O5 SL facilitates efficient crystallization of the perovskite film compared to the conventional mesoporous titanium oxide (m-TiO2) SL. Also, photoluminescence spectroscopy revealed the higher electron-extraction capability at the perovskite/Nb2O5 interface than the perovskite/m-TiO2 interface. The present sol-gel processed Nb2O5 SL and the superior optoelectronic properties of the overlying perovskite film suggest a promising avenue for high-performance and cost-effective perovskite solar cells.

Low-Temperature Thermally Annealed Niobium Oxide Thin Films as a Minimally Color Changing Ion Storage Layer in Solution-Processed Polymer Electrochromic Devices

The limited availability of solution-processable ion storage materials, both inorganic and organic, hinders the adoption of roll-to-roll manufacturing for polymer electrochromic devices (ECDs). The n-type transition metal oxides are known for their ion storage properties. However, the fabrication methods of their amorphous metal oxide thin films typically involve sputtering, thermal deposition, electrical deposition, or sol-gel deposition followed by high-temperature thermal annealing (>300 °C), thus making them incompatible for low-cost roll-to-roll manufacturing on flexible substrates. In this study, we report the synthesis of amorphous niobium oxide(a-Nb2O5) thin films from sol-gel precursors through the combination of photoactivation and low-temperature thermal annealing (150 °C). Coupled with p-type electrochromic polymers (ECPs), solution-processed a-Nb2O5 thin films were evaluated as a minimally color changing counter electrode (MCC-CE) material for electrochromic devices. We found that ultraviolet ozone (UVO) treated and 150 °Cthermally annealed (UVO-150 °C)a-Nb2O5 thin filmsshow excellent electrochemical properties and cycling stability. Notably, a-Nb2O5/ECP-magenta ECD has a high optical contrast of ∼70% and a fast switching time (bleaching and coloring time of 1.6 and 0.5 s for reaching 95% of optical contrast). In addition, the ECD demonstrates a high coloration efficiency of ∼849.5 mC cm-2 and a long cycling stability without a noticeable decay up to 3000 cycles.

Color Mixture of Interference Color by Niobium Oxide Thin Film using Photolithography

Color Mixture of Interference Color by Niobium Oxide Thin Film using Photolithography

Effects of bias pulse frequencies on reactively sputter deposited NbOx films

Within the current work, niobium oxide thin films have been deposited by reactive d.c. magnetron sputtering from cold gas sprayed niobium targets, applying a systematic variation of oxygen partial pressure, bias pulse frequency and substrate temperature. The films grown without additional substrate heating are amorphous. X-ray photoelectron spectroscopy shows the Nb5+ oxidation state, characteristic for the Nb2O5 phase, for films grown at the highest oxygen partial pressure applied, while the Nb4+ state of NbO2 is additionally present at lower oxygen partial pressures. With increasing oxygen partial pressure, an optical shift from non-transparent to nearly fully transparent was observed, whereby the higher bias pulse frequency leads to a transition already at lower oxygen partial pressures. The transparent films are characterized by a maximum transparency of ~80% and a refractive index of ~2.5. Increasing the substrate temperature up to 600 °C results in the formation of well crystalline coatings consisting of the orthorhombic Nb2O5 phase, which are whitish and semi-transparent.