VOx Films Research Articles

Performance improvement of nitrogen dioxide gas sensors based on novel p-n heterojunction gold black/VOx bi-sensing membranes

In this work, novel NO2 gas sensors having p-n heterojunction bi-sensing membranes obtained using gold black/VOx films were developed. VOx and gold black films were sequentially deposited on quartz substrates through radio frequency magnetron sputtering system and low-temperature vapor cooling condensation system, respectively. Scanning electron microscopy and atomic force microscopy images revealed that a rougher surface was obtained for gold black/VOx bi-sensing membranes than for VOx sensing membranes. After the detection of NO2 gas by using an NO2 gas sensor with a gold black sensing membrane and another with a VOx sensing membrane, results confirmed that the p-n heterojunction could be formed using gold black/VOx bi-sensing membranes. Because of a large surface area-to-volume ratio and the formation of the p-n junction of gold black/VOx bi-sensing membranes, the gas response of prepared NO2 gas sensors was improved from 78.78% to 188.15% compared with NO2 gas sensors with VOx membranes operated under 100 ppm NO2 concentration at its respectively optimal operating temperature. Furthermore, in NO2 gas sensors with gold black/VOx bi-sensing membranes, a minimum NO2 concentration of 1 ppm could be detected.

Evolution of structures and optical properties of vanadium oxides film with temperature deposited by magnetron sputtering

In this study, mid infrared optical modulation property of VOx films with temperature was researched. VOx film which contain both VO2 and V2O5 was deposited on Al2O3 substrate by radio frequency magnetron sputtering. The mid infrared transmittance undergoes two reductions when increasing the temperature to 590 °C, the first reversible reduction around 68 °C as the phase transition of VO2 and the second irreversible gradually reduction starting around 350 °C because the oxidation state changed from V2O5 to VO2. The transmittance increased from 5.49% to 70.29% when cooling the temperature around 54℃. The film has the reversible 64% optical modulation around 60 °C during the second thermal cycling and keep stable even increased the temperature higher than 550 °C. The difference caused by high temperature thermal cycling in surface morphology structures, and chemical composition, were characterized by AFM, XRD, Raman, and XPS. The process of phase transition and the oxidation state changed during the thermal cycles were monitored with temperature dependent Raman spectra. This study provides new information about the transition between different structures and oxidation states of VOx film and a good candidate for infrared thermochromic window.

Effect of Addition of Mo or V on the Structure and Cutting Performance of AlCrN-Based Coatings

This study focuses on a comparative analysis of AlCrXN (X = Mo or V) coatings with the reference AlCrN coating via arc ion plating technique (AIP). The XRD and XPS results showed that the AlCrXN coatings were mainly composed of fcc-(Cr,Al)N solid solution phases. Both the AlCrMoN and AlCrVN coatings exhibited much higher hardness and adhesive strength than the AlCrN coating. The addition of Mo or V decreased the coefficient of friction (COF) and wear rate, which was due to the formation of lubricant oxides containing Mo or V on the coating surfaces. The cutting results showed that abrasive wear, adhesive wear, and oxidation wear were the main wear mechanisms for the coated tools at the cutting speeds of 60 m/min and 94 m/min. The addition of Mo or V dramatically improved the cutting performance of AlCrXN-coated tools by increasing the anti-wear ability due to the high hardness and the formed lubricant VOx or MoOx films.

Performance improvement of Y-doped VOx microbolometers with nanomesh antireflection layer.

In the study, the yttrium (Y)-doped vanadium oxide (VOx:Y) films used as the sensitive layers of microbolometers were deposited using a radio frequency magnetron co-sputtering system. The temperature coefficient of resistance (TCR) of the VOx:Y films was enhanced from -1.88%/°C to -2.85%/°C in comparison with that of the VOx films. To further improve the performance of microbolometers, the nanomesh antireflection layer was placed on the top surface of the microbolometers to reduce the infrared reflection. The responsivity, thermal time constant, thermal conductivity, absorptance, and detectivity of the VOx:Y microbolometers with nanomesh antireflection layer were 931.89 ± 48 kV/W, 4.48 ms, 6.19×10-8 W/K, 74.41% and 2.20×108 cmHz0.5W-1, respectively.

Interplay among work function, electronic structure and stoichiometry in nanostructured VOx films.

The work function is the parameter of greatest interest in many technological applications involving charge exchange mechanisms at the surface. The possibility to produce samples with a controlled work function is then particularly interesting, albeit challenging. We synthetized nanostructured vanadium oxide films by a room temperature supersonic cluster beam deposition method, obtaining samples with tunable stoichiometry and work function (3.7-7 eV). We present an investigation of the electronic structure of several vanadium oxide films as a function of the oxygen content via in situ Auger, valence-band photoemission spectroscopy and work function measurements. The experiments probed the partial 3d density of states, highlighting the presence of strong V 3d-O 2p and V 3d-V 4s hybridizations which influence 3d occupation. We show how controlling the stoichiometry of the sample implies control over work function, and that the access to nanoscale quantum confinement can be exploited to increase the work function of the sample relative to the bulk analogue. In general, the knowledge of the interplay among work function, electronic structure, and stoichiometry is strategic to match nanostructured oxides to their target applications.

Enhanced Nucleation of Atomic Layer Deposited Contacts Improves Operational Stability of Perovskite Solar Cells in Air

AbstractMetal‐halide perovskites show promise as highly efficient solar cells, light‐emitting diodes, and other optoelectronic devices. Ensuring long‐term stability is now a major priority. In this study, an ultrathin (2 nm) layer of polyethylenimine ethoxylated (PEIE) is used to functionalize the surface of C60 for the subsequent deposition of atomic layer deposition (ALD) SnO2, a commonly used electron contact bilayer for p–i–n devices. The enhanced nucleation results in a more continuous initial ALD SnO2 layer that exhibits superior barrier properties, protecting Cs0.25FA0.75Pb(Br0.20I0.80)3 films upon direct exposure to high temperatures (200 °C) and water. This surface modification with PEIE translates to more stable solar cells under aggressive testing conditions in air at 60 °C under illumination. This type of “built‐in” barrier layer mitigates degradation pathways not addressed by external encapsulation, such as internal halide or metal diffusion, while maintaining high device efficiency up to 18.5%. This nucleation strategy is also extended to ALD VOx films, demonstrating its potential to be broadly applied to other metal oxide contacts and device architectures.

High temperature coefficient of resistance and low noise tungsten oxide doped amorphous vanadium oxide thin films for microbolometer applications

In this study, we report tungsten oxide (WO3) doped vanadium target in order to sputter amorphous vanadium tungsten oxide (a-VWOx) thin films by reactive direct-current magnetron sputtering technique. No post annealing and post oxidation steps were applied to improve bolometric properties. Thin films with high temperature coefficient of resistance, up to 3.02%/K, were prepared with resistivity range of 1–10 Ω cm thereby controlling the oxygen flow rate and sputtering conditions. Thin film composition was determined using X-ray photoelectron spectroscopy analysis which showed that the film has a mixed phase of V2O5, VO2 and V2O3. The noise level of the deposited film at 20 Hz was calculated as 1.9 × 10−15 V2/Hz from the noise spectrum density analysis. This noise level was found out to be comparable to the literature for VOx films deposited by ion beam deposition technique.

Photo-induced high modulation depth terahertz modulator based on VOx–Si–VOx hybrid structure

We demonstrate a photo-induced high modulation depth terahertz (THz) modulator based on a VOx/Si/VOx hybrid structure. The thickness of the embedded high resistance silicon substrate is 510 µm and two layers of VOx films are 100 nm, respectively. Compared with other THz modulators based on the phase transition of vanadium oxide which were fabricated by complex chemical vapor deposition and sputtering processes, the VOx film in this device is fabricated using a simple spin coating process of VOx solution. The sample is characterized using a THz time-domain-spectroscopy system. In the test experiment, the insulator–metal-transition of the VOx is induced by two oblique incident pump lasers with a central wavelength of 650 nm from two sides. The experimental results show that it can be optically modulated in a wide frequency range from 0.2 to 2.0 THz, and the maximum modulation depth of 93% is realized when the optical power is 120 mW. Film theory and Drude theory are introduced to interpret the mechanism of modulation. This modulator has broadband, a simple fabrication process and very low optical power, which are important for broadband THz communication and all-optical THz systems.

Atomic-layer-deposited ultra-thin VOx film as a hole transport layer for perovskite solar cells

In this study, we report the use of ultra-thin VOx film deposited by low temperature (50 °C) atomic layer deposition (ALD) as a hole transport layer (HTL) for perovskite solar cells (PSCs). High efficient PSCs with a power conversion efficiency of 11.53% are achieved with a ∼1 nm VOx layer. It is found that compared to the pristine ALD-VOx films, UV post-treatment significantly enhances the hole transportation ability of the VOx films. To understand the hole transporting mechanism in the VOx films, the ALD-VOx films grown on fluorine-doped tin oxide are investigated by photoelectron spectroscopy. Our investigation confirms that the defect states below the Fermi level of the VOx facilitate hole extractions, and that a greater V5+ oxidation state ratio is found in the UV-treated VOx films. This work shows the potential of using low temperature inorganic VOx films as the HTLs for the application of flexible and large-area PSCs.

Low temperature processed PEDOT:PSS/VOx bilayer for hysteresis-free and stable perovskite solar cells

Planar perovskite solar cells (PSCs) have attracted tremendous attention because of their excellent photovoltaic efficiencies and low temperature fabrication. However, most of the PSCs showed low open circuit voltage due to imperfect band alignment between poly(3,4-ethylenedioxythiophene): poly (styrene suffocate) (PEDOT:PSS) and perovskite, which lead to limited performance. In this letter, low temperature solution processed VOx film is used as a hole transport layer (HTL) to modified PEDOT:PSS for inverted PSCs, which could modify the Fermi energy of pristine PEDOT:PSS HTL, resulting in an improved band alignment and increasing open circuit voltage The optimized device shows nearly free hysteresis with maximum power conversion efficiency (PCE) up to 14.22%, which shows better stability than the performance of the device based on PEDOT:PSS.

Reactive Phase Separation during Methanol Oxidation on a V-Oxide-Promoted Rh(110) Surface

The distribution of ultrathin layers of vanadium oxide on Rh(110) (θV ≤ 1 MLE, one monolayer equivalent corresponds to the number of Rh atoms in the topmost Rh(110) surface layer) after exposure to catalytic methanol oxidation in the 10–4 mbar range has been investigated with x-ray photoelectron spectroscopy and spectroscopic low-energy electron microscopy (SPELEEM). The reaction is shown to cause a macroscopic phase separation of the VOx film into VOx-rich and into V-poor phases. For θV = 0.8 MLE compact VOx islands develop whose substructure exhibits several ordered phases. At θV = 0.4 MLE the VOx-rich phase consists of many small VOx islands (0.1–1 μm). Laterally resolved x-ray photoelectron spectroscopy of V 2p3/2 shows an oxidic component at 515.5 eV binding energy (BE) and a component at 513.0 eV BE attributed to metallic or strongly reduced V. On the V-poor phase only the reduced/metallic component is present. The results are compared with the distribution of ultrathin layers of vanadium oxide on R...

Hole patterns in ultrathin vanadium oxide layers on a Rh(111) surface during catalytic oxidation reactions with NO.

Various oxidation reactions with NO as oxidant have been investigated on a partially VOx covered Rh(111) surface (θV = 0.3 MLE) in the 10-4 mbar range, using photoelectron emission microscopy (PEEM) as spatially resolving method. The PEEM studies are complemented by rate measurements and by low-energy electron diffraction. In catalytic methanol oxidation with NO and in the NH3 + NO reaction, we observe that starting from a homogeneous surface with increasing temperature first a stripe pattern develops, followed by a pattern in which macroscopic holes of nearly bare metal surface are surrounded by a VOx film. These hole patterns represent just the inverse of the VOx distribution patterns seen if O2 instead of NO is used as oxidant.

Chemical Wave Patterns and Oxide Redistribution during Methanol Oxidation on a V-Oxide Promoted Rh(110) Surface

Chemical wave patterns and the formation of macroscopic vanadium oxide islands have been investigated in the 10–4 mbar range during catalytic methanol oxidation on ultrathin VOx films (θV ≤ 1 monolayer equivalent) supported on Rh(110). At temperatures around 800 K, wave fragments traveling along the [110] direction and oxidation/reduction fronts exhibiting different front geometries are observed with photoemission electron microscopy. At ≈1000 K, a redistribution of VOx leads to the growth of macroscopic oxide islands under reaction conditions. On these macroscopic V-oxide islands chemical waves including traveling wave fragments propagate. Under conditions close to equistability of oxidized and reduced phase, a dendritic growth of the V-oxide islands is observed. In contrast to Rh(111)/VOx, almost no catalytic activity in formaldehyde production is found on Rh(110)/VOx.

Electric field-triggered metal-insulator transition resistive switching of bilayered multiphasic VOx

Electric field-triggered Mott transition of VO2 for next-generation memory devices with sharp and fast resistance-switching response is considered to be ideal but the formation of single-phase VO2 by common deposition techniques is very challenging. Here, VOx films with a VO2-dominant phase for a Mott transition-based metal-insulator transition (MIT) switching device were successfully fabricated by the combined process of RF magnetron sputtering of V metal and subsequent O2 annealing to form. By performing various material characterizations, including scanning transmission electron microscopy-electron energy loss spectroscopy, the film is determined to have a bilayer structure consisting of a VO2-rich bottom layer acting as the Mott transition switching layer and a V2O5/V2O3 mixed top layer acting as a control layer that suppresses any stray leakage current and improves cyclic performance. This bilayer structure enables excellent electric field-triggered Mott transition-based resistive switching of Pt-VOx-Pt metal-insulator-metal devices with a set/reset current ratio reaching ~200, set/reset voltage of less than 2.5 V, and very stable DC cyclic switching upto ~120 cycles with a great set/reset current and voltage distribution less than 5% of standard deviation at room temperature, which are specifications applicable for neuromorphic or memory device applications.

Tunable thermochromic properties of V2O5 coatings

Thermochromic Di vanadium pentaoxide (V2O5) coatings displaying a variety of colours were synthesised. Tuning of thermochromic behaviour was achieved via a controlled oxidative annealing under ambient air of the as-grown VOx films. Adjusting the oxygen deficiency in V2O5, allows tuning the colour of the films and as a consequence its thermochromic behaviour. Non oxygen deficient V2O5 did not feature any measurable thermochromism

Electronic bipolar resistive switching behavior in Ni/VOx/Al device

In this paper, the Ni/VOx/Al resistive random access memory (RRAM) device is constructed and it shows bipolar resistive switching behavior, low resistive state (LRS) nonlinearity, and good retention. The set and reset processes are likely induced by the electron trapping and detrapping of trapping centers in the VOx films, respectively. The conduction mechanism in negative/positive region are controlled by space charge limited current mechanism (SCLC)/Schottky emission. The temperature dependence of I–V curves for HRS is measured to confirm the defects trapping and detrapping electrons model. activation energy was calculated to analyze the endurance performance of the device. The detailed analysis of the switching behavior with SCLC mechanism and Schottky emission mechanism could provide useful information for electronic bipolar resistive switching (eBRS) characteristics.

Local order and the dependence of magnetization on Co content in V2O5 layered films

Local order, electronic structure, and magnetic properties of Co-doped VOx films electrochemically grown onto Si are investigated. The films are studied by means of X-ray absorption spectroscopy (XAS) and magnetic measurements. Freshly made films have V2O5·n(H2O) structure with vanadium valence close to +5. XAS data show that insertion of Co in the films does not affect the close environment around V, when compared to the undoped sample, even varying the concentration of Co by ten times. The site symmetry of Co dopant atoms in the films is consistent with an octahedral coordination where Co is surrounded by six oxygen atoms, as supported by X-ray absorption near-edge structure simulations. Furthermore, there is no evidence of the presence of metallic cobalt (Co0) in the films. The incorporation of small amounts of Co turns ferromagnetic undoped samples into paramagnetic ones. The net magnetic moment per unit volume initially decreases with the incorporation of Co and enhances for higher concentrations. Such behavior is consistent with an O vacancy reduction process driven by the insertion of Co in the films.

Production of VO2 thin films through post-deposition annealing of V2O3 and VOx films

VO2 thin films were produced on sapphire and silicon substrates through post-deposition ex-situ thermal treatment of V2O3 and VOx films. Thin epitaxial films of V2O3 on sapphire and amorphous VOx films on silicon substrates were grown using oxygen assisted molecular beam epitaxy. The post-deposition annealing was performed at different temperatures using an Ar flow. Structural, optical and electrical characterizations were performed to confirm the transformation of the films. The transformed films present a change in resistance across the metal to insulator transition of four orders of magnitude for annealed V2O3 on sapphire and around one order of magnitude in the case of annealed VOx on silicon.

Predicting Thin Film Stoichiometry In V-O2 Reactive Sputtering

The electrical, optical and mechanical properties of compound oxides film depend strongly on the composition of the film, especially for vanadium oxide thin films, since the vanadium-oxygen phase diagram includes mixed valence oxides with two or more oxidation states. Therefore, it is interesting to predict the film stoichiometry in VOx films. However, it has not, to any great extent yet, been possible to predict the composition of vanadium oxides films. In this article, we present a model that enables us to predict film composition of vanadium oxides prepared by reactive sputtering. Based on this model, the fraction ratio of V, V2O3, VO2 and V2O5 in the substrate surface as a function of oxygen flow is investigated. It is concluded that the presented theoretical model for the involved reactions and composition of VOx thin film during reactive sputtering process is in qualitative agreement with the XPS results from experiment and can be used to find optimum processing conditions for deposition of films of a desired composition.DOI: http://dx.doi.org/10.5755/j01.ms.21.2.6910

Thermally grown vanadium oxide films and their electrical properties

A metal–insulator transition (MIT) occurring in vanadium oxide films prepared in different ways has been widely studied in many laboratories. It consists of a resistive change of various orders of magnitude taking place while traversing a temperature close to 67°C. In this work the properties of VOx films synthesized by thermal treatment of vanadium films which were vacuum-evaporated on an oxidized silicon substrate are shown. Such thermal oxidizing treatment was performed under atmospheric air at different temperatures during distinct times. Ellipsometry measurements allowed determining the thickness and optical constants of the layers after the oxidation process. From XRD, Raman and FTIR measurements, several phases with distinct oxygen content, V2O3, V3O5, VO2 and V2O5, were found in the films, depending on the oxidation time and temperature. Current–temperature measurements across the films were carried out by using sandwich-type metal–insulator–metal structures. Unlike former studies on similar structures, no MIT was observed from these measurements. On the other hand, from room-temperature current–voltage measurements a well defined memristive behavior was found as a regular result in most of our structures. This memristive behavior is ascribed to the complex defect structure in the films, including the variable amount of oxygen vacancies in the lattice, rather than to the above-mentioned metal–insulator transition in vanadium oxide.