VO2 Films Research Articles

Hysteresis loop variations induced by nitrogen flow rate in solution-based VO2 films

Hysteresis loop variations induced by nitrogen flow rate in solution-based VO2 films

Facile preparation of vanadium oxide films under varied calcination temperature and their multicolor electrochromic behaviors

A vanadium oxide (VO) film was prepared on indium tin oxide (ITO) glass using a simple electrophoretic method and the VO film and its device demonstrated multicolor electrochromic properties of yellow, green, orange-red, and their intermediate colors. The X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS) were employed to characterize the composition of the as-prepared films. The ratio of V5+/V4+ in the XPS is significantly different in the yellow, green, and orange-red states and this ratio can be adjusted by applied voltages. The detailed effect of calcination temperature as a post-treatment method on the electrochromic performance of VO film was explored. With the increase of calcination temperature, the optical modulation ability of the films was enhanced, and the color change contrast of the films was most obvious at 200 °C. Meanwhile, the cyclic stability of the VO films was also enhanced. The cyclic voltammetry (CV) curves show that the prepared V2O5 films have high cyclic stability at a calcination temperature of 300 °C. The color efficiency (CE) values of the VO devices were 3.4 cm2C-1 (from initial yellow to green) and 21.6 cm2C-1 (from green to orange-red) at 700 nm, and the coloring time (Tc) and bleaching time (Tb) of the VO devices were 6 s and 10 s, respectively. This work develops a simple and environmentally friendly method to prepare VO films and their devices with multicolor electrochromic properties.

In situ synthesis and structural morphology analysis of 3D porous hierarchical V2O5 films for transmissive-to-black all-solid-state electrochromic devices

3D, porous, low-dimensional, and nanostructured V2O5 films have significant potential for application in ECDs, but their development is still in its nascency. This paper reports a facile strategy of using PEG as a structure-directing agent and short-time heat treatments to obtain 3D, porous, and hierarchical V2O5 films comprising interpenetrating stacked ultra-long nanowire surfaces and nanorod/nanosheet mixed interiors deposited on ITO-glass substrates. The mechanism of morphological transformation was studied in detail, and the analysis shows that the formation of such special structured films is a synergetic result of the guided intercalation of PEG, atomic rearrangement, Ostwald ripening, anisotropy of crystal growth, and orientation attachment mechanism. Among them, the preferred PEG addition amount of 25 % for the 350-3-V2O5 film was applied in two types of all-solid-state ECDs: one using a gel quasi-solid electrolyte and the other using a solid-film electrolyte. The 350-3-V2O5 film served as an ion storage coating and WO3 was the electrochromic layer. These devices achieved ideal reversible switching of transmissive-to-black electrochromic characteristics. Notably, the ‘Integrated-Type’ ECD, composed of inorganic all-solid-state films, has a large optical modulation range ΔT = 72.0 %, rapid switching responses (colouration 25.4 s and bleaching 20.8 s), and sustained electrochromic performance.

Low Hysteresis Vanadium Dioxide Integrated on Silicon Using Complementary Metal‐Oxide Semiconductor Compatible Oxide Buffer Layer

VO2 undergoes a metal‐insulator transition (MIT) at ≈70 °C, which induces large variations in its electrical and wavelength‐dependent optical properties. These features make VO2 a highly sought‐after compound for optical, thermal, and neuromorphic applications. To foster the development of VO2‐based devices for the microelectronic industry, it is also imperative to integrate VO2 on silicon. However, high lattice mismatch and the formation of silicates at the interface between VO2 and Si degrade the quality and functionality of VO2 films. Moreover, VO2's polymorphic nature and stable VO phases pose integration issues. To address these challenges, the MIT of VO2 thin films integrated on Si with a complementary metal‐oxide semiconductor‐compatible HfxZr1−xO2 (HZO) buffer layer is investigated. Using in situ high‐resolution X‐ray diffraction and synchrotron far‐infrared spectroscopy, combined with multiscale atomic and electronic structure characterizations, it is demonstrated that VO2 on the HZO buffer layer exhibits an unusually low thermal hysteresis of ≈4 °C. In these results, the influence of strain on M2 phase nucleation, which controls the hysteresis, is unraveled. Notably, the rate of phase transition is symmetric and does not change for the heating and cooling cycles, implying no incorporation of defects during cycling, and highlighting the potential of an HZO buffer layer for reliable operation of VO2‐based devices.

Temperature tunable broadband filter based on hybridized vanadium dioxide (VO2) metasurface

Abstract In this paper, a novel temperature tunable terahertz (THz) broadband filter based on hybridized vanadium dioxide (VO2) metasurface (MS) was proposed. The designed MS is composed of subwavelength metallic square-grid structure situated between two layers of metallic square-patch structure integrated with VO2 film pad spaced with two layers of dielectric substrate. Utilizing the insulator-metal phase transition characterizations of VO2 in the THz regime, the operation mode of the proposed MS filter accomplishes the broadband transmission-to-reflection transition. Simulation results show that when VO2 is in the insulating state with a lower external temperature, the proposed MS behaves like a broadband filter with transparent window and a transmittance of above 80% over the frequency range of 0.66–1.12 THz. However, when VO2 undergoes the metallic state with a higher external temperature, the MS becomes a broadband filter with low-transmission shielding and exhibiting a transmittance of below 10% across the frequency spectrum from 0.56 THz to 1.4 THz. The physical mechanism of the proposed MS based tunable broadband filter is illustrated by introducing impedance matching theory, equivalent circuit model and field analysis. In addition, the proposed MS structure offers exceptional angular stability and polarization insensitivity, opening up new opportunities for the utilization of energy selective surface in THz applications.

Active infrared tuning of metal–insulator-metal resonances by VO2 thin film

VO2 is a promising phase change material offering a large contrast of electric, thermal, and optical properties when transitioning from semiconductor to metallic phase. Here we show that a hybrid metamaterial obtained by proper combination of a VO2 layer and a nanodisk gold array provides a tunable plasmonic gap resonance in the infrared range. Specifically, we have designed and fabricated a metal–insulator-metal gap resonance by inserting sub-wavelength VO2 film between a flat gold layer and a gold nanodisk resonator array. The resonance of the hybrid metamaterial is centered in the useful 3–5 μm range when VO2 is in its semiconductor state. The experimental study highlights a monotonical spectral tuning of the resonance when increasing temperature up to 50 °C above the room temperature, providing a continuous resonance shift of almost 1 μm in the mid-infrared range. Wavelength range and intensity tunability can be further optimized by modifying the thicknesses of the layers and metamaterial parameters.

Application Of Vanadyl Alkoxoacetylacetonate In Formation Of v2O5 Electrochromic Films

Crystal structure, morphology and electrochromic properties of V2O5 film, prepared using vanadyl alkoxoacetylacetonate as precursor, were studied. We have shown that the obtained vanadium pentoxide contains significant amount of V4+ cations, which is indicated by low electron work function among other things. This results in material possessing anodic electrochromism – coloring upon oxidation – with rapid bleaching process (1 s upon necessary potential application). Anodic coloration is observed in the whole visible light spectrum, as well as in near IR region up to 1100 nm. Obtained data show high prospects for approach to formation of V2O5-based films using vanadyl acetylacetonate as precursor and application of such films as components of smart windows and displays, optical properties of which could be controlled by electrical current application.

Retraction Note: The influence of rare earth (La, Er and Yb) doped V2O5 films on the structural, linear and nonlinear optical properties and optical limiting for optoelectronic applications

Retraction Note: The influence of rare earth (La, Er and Yb) doped V2O5 films on the structural, linear and nonlinear optical properties and optical limiting for optoelectronic applications

Smart window based on integration of nanoporous microparticles in liquid crystal composite with metamaterial nanostructured VO2 film

Smart window based on integration of nanoporous microparticles in liquid crystal composite with metamaterial nanostructured VO2 film

The influence of structural defects on the electrical and infrared emission properties of VO2 thin films

Defect engineering is widely used in the modification of transition metal oxides. In this study, defects are introduced into the polycrystalline VO2 film by the energetic Ar ion irradiation, and the influence of structural defects on the electrical and infrared emission properties of VO2 thin films have been investigated. The decrease in electrical switching ratio with the increase of defect ratio (displacement per atom (dpa)) can be described as R10°C/R90°C=2.032×(dpa+0.005)−1.8069, while the infrared modulation performance is Δε=−0.119×ln(dpa+0.061). The dpa thresholds of the electrical switching ratio and the infrared modulation performance are ηdpaRR = 0.005/atom and ηdpaΔε = 0.0426/atom, respectively, indicating that the electrical switching ratio of VO2 films is more responsive to defects than the infrared modulation performance. The relationship between transition temperature and defect ratio has be determined. Concerning electrical properties, the heating phase transition temperature can be described as TMITHeating=25.88−10.93×ln(dpa+0.01); while for infrared emission modulation, the heating phase transition temperature is given by Tε−MITHeating=31.65−8.18×ln(dpa+0.01). While reducing the phase transition temperature, the VO2 thin films still exhibit an electrical switching ratio of more than two orders of magnitude and obvious infrared modulation performance. It not only offers a viable solution for broadening the application scope of VO2 but also contributes to understanding the role of defects in VO2 thin films for further research.

In-situ XRD study on phase transition kinetics of vanadium dioxide thin film

Vanadium dioxide (VO2) has great potentials to be used in energy-related fields due to its unique reversible thermochromic metal-insulating properties. In this paper, we used in-situ XRD technique to study the phase transition kinetics of VO2 thin films, which can change phase from monoclinic VO2 (M) to tetragonal rutile VO2 (R). The VO2 thin films were prepared by a simple solution-based sol-gel method followed by spin coating to achieve the nanoscale thickness on a quartz substrate. During heating, the critical phase transition of VO2 (M) occurred at 70 °C, which dropped to 45 °C during cooling. A martensite-like crystal phase transformation kinetic model was established based on the Koistinen and Marburger (KM) technique. The relative content of the two phases was quantitatively analyzed using the Rietveld method, and the transformation rate coefficient α was obtained as 0.20345 and 0.12585, for the heating process from VO2 (M) to VO2 (R) and the cooling process from VO2 (R) to VO2 (M), respectively. This research offers an effective tool for understanding the structural characteristics of thin film VO2, paving the way for its future design and applications towards energy-related fields such as in smart windows.

High-quality VO2 thin films sputter-deposited on borosilicate glass substrates for modulating solar transmission and thermal emission

M1-phase VO2 exhibits a reversible phase transition between the insulating and metallic states at T = ∼341 K, and VO2-based smart windows have been extensively studied. The smart window application requires the growth of high-quality VO2 films with a large resistivity change and small hysteresis width (ΔT) on glass substrates using a scalable deposition method such as sputtering. However, the polymorphic nature of VO2 makes it challenging to obtain high-quality VO2 films on ordinary window glasses (borosilicate and soda-lime glasses). Thus far, VO2 films sputtered on glass substrates have exhibited resistivity changes of 1.5–2 orders of magnitude and ΔT = 3–19 °C. In this study, we show that high-quality VO2 films with a resistivity change of 2.5−3.0 orders of magnitude and ΔT = 2 °C can be grown on borosilicate glass substrates via radio-frequency sputtering followed by thermal annealing. The simultaneous achievement of such a large resistivity change and small hysteresis width is highly encouraging not only for smart windows but also for other applications. With respect to the modulation of solar transmission and thermal emission with temperature, the performance levels of the produced VO2 films were 70−75 % of those expected from pure M1-phase VO2 films.

Excellent Electrochromic Properties of Ti4+-Induced Nanowires V2O5 Films

Ti4+-doped V2O5 films with nanowires on top and a dense, long nanorod layer on the bottom were successfully fabricated using the spin-coating route. During the electrochromic cycling, charge transfer resistance (Rct) decreases while ion-diffusion ability (KΩ) rapidly drops in the first ten cycles and then levels off. Low Rct and morphology of nanowires collaboratively improved the electrochromic behavior of Ti4+-doped V2O5 films by enhancing the charge transfer speed and minimizing polarization and dissolution. The obtained Ti4+-doped V2O5 film shows better electrochromic properties than the undoped V2O5 film, with a coloration efficiency (CE) of 34.15 cm2/C, coloration time of 9.00 s, and cyclic retention of 82.6% at cycle 100. In contrast, the corresponding values for the undoped V2O5 film were 23.57 cm2/C, 13.16 s, and 43.6%.

Optical and Structural Properties of V2O5 Electrochromic Thin Films

The increase in global temperature has led to a significant surge in energy consumption within the air conditioning industry, resulting in environmental deterioration. Electrochromic (EC) windows have emerged as a promising solution to address these challenges. Vanadium pentoxide (V2O5) stands out among all metal oxide materials due to its remarkable EC properties, including substantial Li+ ion insertion capacity and multicolor capabilities. Despite the potential of V2O5, there remains a lack of comprehensive research on the structural and optical properties of V2O5 films with varying thicknesses. Therefore, this study aims to investigate the structural and optical properties of V2O5 thin films with thicknesses ranging from 46 to 344 nm. By employing the sol-gel spin coating method, V2O5 thin films were fabricated and analyzed using X-ray diffraction (XRD) spectroscopy and ultraviolet-visible (UV-Vis) spectrophotometry. The fabricated V2O5 thin films with thicknesses of 46-274 nm demonstrated an average film transparency of 83 %. XRD analysis further revealed that the V2O5 thin films reached their peak crystallinity at a thickness of 344 nm. Moreover, CV analysis revealed that the V2O5 device, with a thickness of 274 nm, exhibited a cathodic peak current of -1.63 mA, indicating its excellent ability to facilitate Li+ ion diffusion. Additionally, CA measurements displayed a high optical modulation of 37.78 %. Ultimately, this research contributes to the development of energy-efficient solutions for sustainable environmental practices.

Robust Dual-Color Electrochromism of Vanadium Oxide Nanorods Embedded on Reduced Graphene Oxide: Unraveling the Mechanism

Vanadium pentoxide (V2O5), associated with both cathodic and anodic coloration, is considered as one of the best electrochromic (EC) materials for energy-saving smart electronics. Here we present the fabrication and detailed mechanism analysis for improving the electrochromic properties of V2O5 incorporated in a reduced graphene oxide (rGO) matrix using a facile wet chemical method. The microstructural study disclosed the formation of prominent V2O5 nanorods embedded in the rGO matrix. The optimized electrochromic film resulted in coloration (tc) and bleaching time (tb) of ∼6.2 and ∼4.8 s, respectively, much faster than the color switching kinetics of the pristine V2O5 sample (tc ∼ 19.4 s, tb ∼ 15.3 s). The more dispersed structure also ensured an approximate 400% enhancement in the optical modulation of EC film and reflected a noticeable improvement in the coloration efficiency (∼347 cm2/C) of V2O5 film. Modification with rGO resulted in an outstanding improvement in the electrochemical redox stability of V2O5 up to 5000 CV cycles with minimum deterioration in the curve area. The formation of nanorod structure was the prime factor for better ion diffusion and thereby facilitating enhanced performance.

Unveiling Intercalation Chemistry via Interference-Free Characterization Toward Advanced Aqueous Zinc/Vanadium Pentoxide Batteries.

Aqueous Zn/V2O5 batteries are featured for high safety, low cost, and environmental compatibility. However, complex electrode components in real batteries impede the fundamental understanding of phase transition processes and intercalation chemistry. Here, model batteries based on V2O5 film electrodes which show similar electrochemical behaviors as the real ones are built. Advanced surface science characterizations of the film electrodes allow to identify intercalation trajectories of Zn2+, H2O, and H+ during V2O5 phase transition processes. Protons serve as the vanguard of intercalated species, facilitating the subsequent intercalation of Zn2+ and H2O. The increase of capacity in the activation process is mainly due to the transition from V2O5 to more active V2O5·nH2O structure caused by the partial irreversible deintercalation of H2O rather than the increase of active sites induced by the grain refinement of electrode materials. Eventually, accumulation of Zn species within the oxide electrode results in the formation of inactive (Zn3(OH)2V2O7·2H2O) structure. The established intercalation chemistry helps to design high-performance electrode materials.

Zn-doped V2O5 film electrodes as cathode materials for high-performance thin-film zinc-ion batteries

Zn-doped V2O5 film electrodes were prepared by in-situ growth on indium‑tin oxide (ITO) conductive glass by a low-temperature liquid-phase deposition method and calcined by calcination treatment, and assembled into thin-film zinc-ion batteries (ZIBs). After galvanostatic charge/discharge (GCD) tests with 90 and 200 charge/discharge cycles, the ZIBs system provided specific capacities of 95.7 mAh m−2 and 63.9 mAh m−2 with capacity retention rates of 97.88% and 78.72%, respectively. The electrochemical reaction process of the Zn-doped V2O5 film electrode was analyzed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) to understand the insertion/extraction mechanism of Zn2+. The doping of appropriate amount of Zn2+ in the preparation plays the role of “pillar”, which helps to stabilize the structure of V2O5 and improve the cycling stability and lifetime. Therefore, the research may provide a new idea for the assembly and preparation of thin-film ZIBs with improved performance.

Reversible modulations of insulator–metal transition in an epitaxial VO2 film through thermal crystallization and femtosecond laser-induced-amorphization of capping Ge2Sb2Te5 layer

We demonstrate reversible modulation of an insulator–metal transition (IMT) of a VO2 film grown on an Al2O3 (001) substrate through crystallization and re-amorphization of a chalcogenide germanium–antimony–telluride (Ge2Sb2Te5: GST) capping layer. After succeeding in the negative shift of IMT temperature (Tr) of the VO2 film through the crystallization of the GST layer accompanied by volume reduction, we performed re-amorphization of the crystalline GST by femtosecond laser irradiation. Under the optimized conditions of laser irradiation considering the penetration depth, re-amorphization of the GST layer was fully achieved, resulting in the shift-back of Tr toward a high-temperature side. Such a reversal of IMT through the crystallization and re-amorphization of the capping GST layer was demonstrated over two cycles. It was suggested that capping GST effectively induces interfacial strain modifications in the VO2 film underneath. Although the shifts in the IMT are still small, reversible modulation of IMT shown here will be beneficial for applications of VO2 films with controllable IMT.

Plasmonic Resonance Shifts in Gold Nanoparticles‐Thermochromic VO2 Thin Film Hybrid Platforms: A Joint Experimental and Numerical Study

AbstractThe combination of the phase transition in thermochromic vanadium dioxide (VO2) with plasmonic nanoparticles paves the way for applications in various fields, including optical sensing, advanced coatings, and dynamic optical devices. This study presents a simple fabrication method to control both the size and surface coverage of NPs combined with VO2. First, a thermochromic VO2 coating with a phase transition at 68 °C is synthesized using reactive magnetron sputtering. Then, monodisperse 30 nm diameter gold NPs are bonded to the VO2 surface using (3‐aminopropyl)trimethoxysilane (APTMS) linkers, examining the effect of immersion duration on surface coverage. Two platforms are developed: a VO2 thin film with a monolayer of NPs and a configuration with NPs between two VO2 films. The temperature‐dependent plasmonic response of these platforms is measured by extinction spectroscopy, showing a significant wavelength resonance shift of approximately 10 nm for the first platform and 20 nm for the second. Optical simulations analyze this shift over various geometries, from isolated NPs to fully covered NPs, achieving a 60 nm shift for NPs embedded in a thin VO2 film. This study demonstrates an effective approach to synthesizing thermochromic VO2 coatings with gold NPs, offering insights into the plasmonic properties of hybrid platforms.

Enhancing phase and morphology control of vanadium oxide films for resistive memory applications: A water-assisted chemical approach

This study explores the influence of precursor solution composition and solvent selection on the synthesis of vanadium oxide (VO2) thin films tailored for resistive memory applications. The vanadium pentoxide and oxalic acid molar ratio and the dispersing medium (ethanol, water, and their mixture) were varied. Water content significantly influenced the VO2 phase, morphology, and film thickness. Ethanol yielded mixed vanadium oxide phases VO2 − Tetragonal, V2O5 − Orthorhombic, and Monoclinic) and a film with minor imperfections. Conversely, the active layer prepared using an aqueous medium showed a predominant VO2 − Monoclinic phase, albeit with a non-uniform film. An ethanol–water mixture produced the most uniform morphology and a mixture of vanadium oxide phases (VO2 − Tetragonal, V2O5- Orthorhombic). All devices displayed resistive switching behavior. VO2-containing devices exhibited a more pronounced transition from an ohmic conduction state to a space-charge-limited conduction mechanism. ON-state currents remained consistent, while OFF-state currents varied notably.