VO2 Phase Research Articles

Machine learning guided hydrothermal synthesis of thermochromic VO2 nanoparticles

Vanadium dioxide (VO2) is a promising material for energy-saving smart windows due to its reversible metal-to-insulator transition near room temperature, concomitantly with a structural phase transition between monoclinic VO2(M) phase and rutile VO2(R) phase. However, the fact that VO2 has a complex crystalline phase makes its reliable synthesis an obstacle to its practical application. Machine learning (ML), a specific subset of artificial intelligence, can be utilized to generate virtual representations of experimental conditions and outcomes for the purpose of predicting experiments. Therefore, in the paper, four machine learning models were trained to perform optimization of the VO2 hydrothermal synthesis. A random forest model achieved a classification accuracy of 87.27%. The synthetic parameter space was explored to filter combinations with a synthetic probability above 90%. Random forest models were used to guide the experimental synthesis, and the obtained products were characterized using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and differential scanning calorimetry. The results showed that phase-pure VO2(B) and VO2(M) were successfully synthesized, demonstrating the effectiveness of machine learning in optimizing material synthesis, alleviating the stochasticity of material synthesis caused by the control of synthesis conditions, and promoting the application research of VO2 materials.

Study on the Surface Morphology of Thermochromic Rf-Sputtered VO2 Films Using Temperature-Dependent Atomic Force Microscopy

Vanadium dioxide (VO2) is a well-known phase-changing material that goes from a semiconducting state to a metallic one at a critical temperature of 68 °C, which is the closest to room temperature (25 °C). The electrical transition is also accompanied by structural and optical changes. The optical transition upon heating-also known as thermochromism-makes VO2 a possible coating for “intelligent” windows. In this work, the relationship between the thermochromic performance of VO2 films and the surface morphology was investigated using Temperature-dependent Atomic Force Microscopy (T-AFM) in conjunction with the X-ray Diffraction technique and Scanning Electron Microscopy. In particular, VO2 films were deposited using the rf sputtering technique on Silicon and glass substrates at a substrate temperature of 300 °C, which is one of the lowest for this technique to grow the thermochromic monoclinic phase of VO2. It was found that upon heating (25–100 °C), there was a decrease in RMS roughness for all films independent from the substrate; the value of RMS roughness, however, varied depending on the substrate. Finally, the thermochromic parameters of the VO2 films were correlated with the surface morphology and appeared to be dependent on the kind of substrate used.

Acidity of Carboxylic Acid Ligands Influences the Formation of VO2(A) and VO2(B) Nanocrystals under Solvothermal Conditions.

Vanadium dioxide (VO2) can adopt many different crystal structures at ambient temperature and pressure, each with different, and often desirable, electronic, optical, and chemical properties. Understanding how to control which crystal phase forms under various reaction conditions is therefore crucial to developing VO2 for various applications. This paper describes the impact of ligand acidity on the formation of VO2 nanocrystals from the solvothermal reaction of vanadyl acetylacetonate (VO(acac)2) with stoichiometric amounts of water. Carboxylic acids examined herein favor the formation of the monoclinic VO2(B) phase over the tetragonal VO2(A) phase as the concentration of water in the reaction increases. However, the threshold concentration of water required to obtain phase-pure VO2(B) nanocrystals increases as the pKa of the carboxylic acid decreases. We also observe that increasing the concentration of VO(acac)2 or the concentration of acid while keeping the concentration of water constant favors the formation of VO2(A). Single-crystal electron diffraction measurements enable the identification of vanadyl carboxylate species formed in reactions that do not contain enough water to promote the formation of VO2. Increasing the length of the carbon chain on aliphatic carboxylic acids did not impact the phase of VO2 nanocrystals obtained but did result in a change from nanorod to nanoplatelet morphology. These results suggest that inhibiting the rate of hydrolysis of the VO(acac)2 precursor either by decreasing the ratio of water to VO(acac)2 or by increasing the fraction of water molecules that are protonated favors the formation of VO2(A) over VO2(B).

Metal–insulator transition of vanadium dioxide and the role of grain boundaries

The metal–insulator transition (MIT) in polycrystalline vanadium dioxide (VO2) is less sharp than in single-crystal VO2 due to the presence of grain boundaries (GBs). The MIT causes a resistivity change (Ron/Roff) across the MIT to decline from the single-crystal value of ∼104 and also causes a larger hysteresis across the transition. The properties of GBs in VO2 are studied as a function of GB tilt angle. We find that V–V site pairing, which causes a monoclinic phase VO2 to be semiconducting in bulk, also controls bandgap opening at GBs, thus influencing the overall metal–insulator behavior. The almost linear relationship between the ratio of adhesion energy divided by two times the surface energy and the GB energy indicates that the GB energy of twin GBs is primarily determined by re-binding across the GB rather than by the corresponding surface energy. Moreover, a steeper linear fit, characterized by a slope of 0.62, observed in the monoclinic phase VO2 suggests that the GB energy and stability have a higher tolerance to interfacial distortion at the boundary, compared to the rutile phase VO2 and TiO2.

Sandwich-like MXene bridged heterostructure electrode enables anti-aggregation and superior storage for aqueous zinc-ion batteries

Severe aggregation and sluggish reaction kinetics are the main research challenges that impair the storage performance of cathode materials for aqueous zinc-ion batteries. In this work, we report the concept of sandwich-like Ti3C2Tx MXene-bridged VO2 heterostructure (VO2/Ti3C2Tx) design to simultaneously relieve the self-aggregation and boost the interfacial storage kinetics. When acting as cathode material, such VO2/Ti3C2Tx electrode can deliver a high specific capacity 415 mAh g−1 at 0.1 A g−1 after 110 cycles, as well as 338 mAh g−1 at 1 A g−1 after 300 cycles, much higher than that of the pure VO2 case. Moreover, such electrode also exhibits remarkably rate capability and cyclic stability, which can retain a specific capacity of 170 mAh g−1 at the high current density of 5 A g−1 after 1500 cycles. The superior storage performance should be mainly benefited from the mixed-dimensional heterogeneous engineering. In addition, the evolution of crystal structure and valence state of electrode upon cycling were investigated, thus a storage mechanism can be revealed. That is, single VO2 phase would be transformed into the VO2 and ZnxV2O5·nH2O mixed phase, with a highly reversible H+/Zn2+ insertion/extraction behavior. Such strategy proposed in this work can be also applied for other kinds of cathode materials and promoting the development of aqueous zinc-ion batteries.

Single‐Phase Growth, Stabilization, and Electrical Properties of B Phase VO2 Films Grown on Mica by Reactive Magnetron Sputtering

AbstractThe VO2 metastable (B) phase is of interest for applications in temperature sensing, bolometry, and Li‐ion batteries. However, single‐phase growth of thin films of this metastable phase is a challenge because vanadium oxide exhibits many polymorphs and the VO2 stable (M1) phase is usually present as a secondary phase. Additionally, the phase transition at 350 °C in the (B) phase severely narrows the processing window for achieving phase‐pure films. Here, single‐phase growth of 5‐to 50‐nm thick VO2 (B) films on muscovite, mica, by pulsed direct‐current reactive magnetron sputtering at 400 °C is demonstrated. The films are phase‐pure and exhibit a high density of 4.05 g cm−3 and low resistivity of about 50 mΩcm at 30 °C. Increasing the film thickness to 100 nm results in a V2O5‐capped VO2 (B) film with a resistivity of 8000 mΩcm. These results indicate that the stability of the VO2 (B) phase is sensitive to in situ annealing during deposition. These findings should serve as a basis to design processes to exclusively obtain phase‐pure VO2 (B) films.

PVDF-based (V2O5)x. (Mn0.4Fe2.6O4)(2−x), x = [0.2, 0.4, 0.6, 0.8, and 1] nanocomposites for tailoring the optical and nonlinear optical properties of PVDF

Nanocomposites comprising (V2O5)x. (Mn0.4Fe2.6O4)(2−x), where x = [0.2, 0.4, 0.6, 0.8, and 1], were successfully synthesized by milling each single phase of V2O5 and Mn0.4Fe2.6O4 nanostructured samples. We studied the structure and morphology of the prepared samples through X-ray diffraction and scanning electron microscopy. UV–Vis. spectrum analysis was used to study the optical characteristics. As an attempt to improve the optical and nonlinear optical properties of the PVDF blend, the prepared nanocomposites were used as nanofillers for the poly (vinylidene fluoride) (PVDF) polymer. Optical and nonlinear optical properties of the PVDF polymer were tuned using (V2O5)x. (Mn0.4Fe2.6O4)(2−x), where x = [0.2, 0.4, 0.6, 0.8, and 1] nanocomposite. The obtained results showed a tunable spectrum, absorption, refractive index, and dielectric constant at wavelengths ranging from 200 to 800 nm. However, the results showed that the (V2O5)0.6. (Mn0.4Fe2.6O4)1.4, sample was the best for obtaining the lowest transmittance percentage and bandgap energy. The nonlinear optical properties and Kerr effect (quadratic electro-optic effect) showed a significant response, which can be tuned in the range from 200 to 1200 nm of wavelength using the investigated samples. Finally, incorporating (V2O5)x. (Mn0.4Fe2.6O4)(2−x), where x = [0.2, 0.4, 0.6, 0.8, and 1], nanoparticles in the PVDF polymer improved the hydrophilic behavior of the PVDF films. This study suggests a promising nanocomposite material for tunable optoelectronics and useful polymer membranes.

The Feasibility of Tungsten-Doped VO2 Films on Soda-Lime Glass with Low Thermal Budget by High Density Plasma Source for Smart Windows

Thermochromic W-doped VO2 thin films on soda-lime glass are successfully fabricated by co-sputtering technique using a high density plasma source, which is equipped by high power impulse magnetron sputtering (HIPIMS). A post-rapid thermal annealing of 500 °C is performed for the purpose of low thermal budget. The effect of doping amount of tungsten on thermochromic properties such as transmittance and transition temperature are addressed. The transition temperature can be lower down to 30 °C with a small amount of 4.5% tungsten added in VO2 nanocrystals. A solar regulation efficiency, ΔTsol = 10% is achieved at thicker TiO2 thickness. The competition of secondary phase of V2WO7.5 and oxygen-rich phase of V2O5 under different O2/Ar ratio is studied. The crystalline behavior of monoclinic phase is examined by X-ray diffraction (XRD) pattern and high resolution transmission electron microscope (HR-TEM). The good endurance property ensures the feasible use for the energy-saving applications.

Positive and Negative Pressure Regimes in Anisotropically Strained V2O3 Films

AbstractThe metal‐insulator phase transitions in V2O3 are considered archetypal manifestations of Mott physics. Despite decades of research, the effects of doping, pressure, and anisotropic strains on the transitions are still debated. To understand how these parameters control the transitions, anisotropically strained pure V2O3 films are explored with nearly the same contraction along the c‐axis, but different degrees of ab‐plane expansion. With small ab‐plane expansion, the films behave similar to bulk V2O3 under hydrostatic pressure. However, with large ab‐plane expansion, the films are driven into the “negative pressure” regime, similar to that of Cr‐doped V2O3, exhibiting clear coexistence of paramagnetic insulator and paramagnetic metal phases between 180–500 K. This shows that c‐axis contraction alone, or an increase in c/a ratio is insufficient for inducing “negative pressure” effects. Actually, c‐axis contraction alone destabilizes the two insulating phases of V2O3, whereas a‐axis expansion tends to stabilize them. The effects of strain are modeled using density functional theory providing good agreement with experimental results. The findings show that chemical pressure alone cannot account for the phase diagram of (V1−xCrx)2O3. This work enables to manipulate a Mott transition above room temperature, thereby expanding the opportunities for applications of V2O3 in novel electronics.

K-doped V2O5 derived from V-MOF precursor as high-performance cathode for aqueous zinc-ion batteries

The safety and low toxicity of aqueous zinc-ion batteries (ZIBs) are piqueing intense scientific interest. Slow reaction kinetics and substantial structural degradation of the cathode materials, however, prevent the creation of ZIBs. As a better cathode for ZIBs, tunnel-structured V2O5 hierarchical with a high preintercalated K cation content (K-V2O5) are proposed here. The K-V2O5 are specifically made using a self-sacrificial template approach that also involves hydrothermal intercalation in a neutral solvent and phase change during annealing. A subsequent H+ and Zn2+ intercalation procedure at various voltage platforms is clarified when tested as cathodes for ZIBs. Prior to the subsequent insertion of Zn2+ into V2O5, the water-solvated H+ first enters tunnel cavities, which partially transforms the V2O5 phase from a tunnel-type structure to a layered-type structure. The pre-intercalated K cations with a high content that act as pillars in the layered-type matrix stabilize the layered structures and widen the migration channels for Zn2+, which can aid in the diffusion of Zn2+ in the V2O5 cathodes.

Optically tunable guide-mode resonance grating based on VO2 phase transition material

In this paper, highly efficient tunable guide-mode resonance (GMR) grating patterned on top of a film of the phase transition material, vanadium dioxide (VO2), is demonstrated. The optimal optical transmission and tuning can be achieved by optimizing the device structure. Based on the large insulator-to-metal transition (IMT) characteristic tuned by thermally driven of VO2 in the near-infrared range, we show that the full width at half maximum (FWHM) of about 100 nm of the transmittance resonance spectra of the dielectric GMR in VO2 grating offers remarkable extinction modulation depths (68%) at 1535 nm for tunability. The tunability is accompanied by a hysteresis-like behavior that can be exploited for versatile memory effects. In addition, we experimentally demonstrate tuning of the reflection phase over 20°. This work further improves the research topic of GMR and expands the design of novel reconfigurable devices for photonics.

Coexistent VO2 (M) and VO2 (B) Polymorphous Thin Films with Multiphase-Driven Insulator-Metal Transition.

Reversible insulator-metal transition (IMT) and structure phase change in vanadium dioxide (VO2) remain vital and challenging with complex polymorphs. It is always essential to understand the polymorphs that coexist in desired VO2 materials and their IMT behaviors. Different electrical properties and lattice alignments in VO2 (M) and VO2 (B) phases have enabled the creation of versatile functional devices. Here, we present polymorphous VO2 thin films with coexistent VO2 (M) and VO2 (B) phases and phase-dependent IMT behaviors. The presence of VO2 (B) phases may induce lattice distortions in VO2 (M). The plane spacing of (011)M in the VO2 (M) phase becomes widened, and the V-V and V-O vibrations shift when more VO2 (B) phase exists in the VO2 (M) matrix. Significantly, the coexisting VO2 (B) phases promote the IMT temperature of the polymorphous VO2 thin films. We expect that such coexistent polymorphs and IMT variations would help us to understand the microstructures and IMT in the desired VO2 materials and contribute to advanced electronic transistors and optoelectronic devices.

Modelling a near perfect temperature tunable multiband VO2 based photonic-plasmonic absorber within visible and near infrared spectra

This work demonstrates thermal and optical stability with multiple near perfect absorption in a polarizing sensitive one-dimensional vanadium dioxide (VO2) graphene-based nanocomposite plasmonic structure within visible to infrared region (IR). Proposed structure in each phase of VO2 has multiple perfect optical absorption peaks within visible range with a Q-factor near ∼45 which leads the structure to work like an optical switch. An increase in absorption is achieved through plasmonic polaritons of a nearby Ag thin layer directly or through photonic crystal. The temperature turnability of VO2 leads the proposed structure a tunable optical absorber with a hysteresis loop from low to nearly perfect absorbance spectrum. The octonacci quasi periodic proposed structure has highest multiple perfect absorber spectrum comparing other photonic sequences. The peaks of absorption have blue shifted with an increase in volume fraction in both phases. The optical absorption of the structure in metal phase mode alters from wide band to multiple narrow bands with the increment of permittivity of the host composite material within the 1000–1600 nm range. The absorption maximizes in TE mode comparing TM mode due to polarization sensitivity. The structure with optimal thickness and volume mixing ratio in composite has temperature dependency with a thin layer of VO2 has ∼90% absorption. The absorption of the structure can be tuned in both heat and cool mode function of the phase change material (PCM) at an optimum frequency within visible to IR spectrum. These thermal bistable activities may enhance the optical features of the devices.

Frequency Tuning of In-Plane Comb Drive MEMS Resonators Due to VO2 Phase Transition

This letter reports the thermal-mechanical tuning capability of an in-plane comb drive resonator using VO2 phase transition material. By inducing the insulator-to-metal transition (IMT) using a heat conduction method, a shift of approximately 2% was observed. The frequency tuning was attributed to the induced stresses during the IMT. [2022-0200]

Boosting the efficiency of Cu2ZnSnS4 solar cells with VO2 phase transition photonic crystal

Photonic crystal (PhC) has been studied for their potential to improve the efficiency of Cu2ZnSnS4 solar cells by increasing the generated photocurrent by integrating it as a back reflector with almost zero transmission through the absorption active zone of the solar cell. It was found that the thickness of PhC layers greatly affects the width of the photonic bandgap and that increasing the thickness of VO2 causes it to shift to a higher wavelength range. The PhC layers were added at the back side of the solar cell in two different configurations: (Monoclinic (M) VO2/TiO2) and (Tetragonal (T) VO2/TiO2) via SCAPS model. The study found that the (M VO2/TiO2) configuration led to an enhancement of the device's efficiency from 11.02 to 12.79%, while the (T VO2/TiO2) reaches 16.88%. The study concluded that the PhC layers enhance the light-matter coupling and photonic coupling and improvement in the device's performance.

Liquid Precursor‐Guided Phase Engineering of Single‐Crystal VO2 Beams

AbstractThe study of VO2 flourishes due to its rich competing phases induced by slight stoichiometry variations. However, the vague mechanism of stoichiometry manipulation makes the precise phase engineering of VO2 still challenging. Here, stoichiometry manipulation of single‐crystal VO2 beams in liquid‐assisted growth is systematically studied. Contrary to previous experience, oxygen‐rich VO2 phases are abnormally synthesized under a reduced oxygen concentration, revealing the important function of liquid V2O5 precursor: It submerges VO2 crystals and stabilizes their stoichiometric phase (M1) by isolating them from the reactive atmosphere, while the uncovered crystals are oxidized by the growth atmosphere. By varying the thickness of liquid V2O5 precursor and thus the exposure time of VO2 to the atmosphere, various VO2 phases (M1, T, and M2) can be selectively stabilized. Furthermore, this liquid precursor‐guided growth can be used to spatially manages multiphase structures in single VO2 beams, enriching their deformation modes for actuation applications.

Liquid Precursor-Guided Phase Engineering of Single-Crystal VO2 Beams.

The study of VO2 flourishes due to its rich competing phases induced by slight stoichiometry variations. However, the vague mechanism of stoichiometry manipulation makes the precise phase engineering of VO2 still challenging. Here, stoichiometry manipulation of single-crystal VO2 beams in liquid-assisted growth is systematically studied. Contrary to previous experience, oxygen-rich VO2 phases are abnormally synthesized under a reduced oxygen concentration, revealing the important function of liquid V2 O5 precursor: It submerges VO2 crystals and stabilizes their stoichiometric phase (M1) by isolating them from the reactive atmosphere, while the uncovered crystals are oxidized by the growth atmosphere. By varying the thickness of liquid V2 O5 precursor and thus the exposure time of VO2 to the atmosphere, various VO2 phases (M1, T, and M2) can be selectively stabilized. Furthermore, this liquid precursor-guided growth can be used to spatially manages multiphase structures in single VO2 beams, enriching their deformation modes for actuation applications.

Morphological and structural evolution of gas-phase synthesized vanadium nanoparticle films induced by thermal treatment

Temperature-driven genesis of morphology and structure of vanadium nanoparticle films fabricated by magnetron-based gas aggregation sources has been investigated. Three temperature regions were identified. In the first one which covers the range from room temperature to approximately 200 °C, the increasing temperature results in the gradual oxidation of nanoparticle films and a slight increase of the individual nanoparticles that form the coating. Despite this, the nanoparticle films in this temperature range preserve their highly porous architecture. Above 200 °C, the vanadium nanoparticles undergo rapid oxidation, which is accompanied by an abrupt change in their mass, crystallinity, morphology, and optical properties (absorbance, photoluminescence). According to XRD, an orthorhombic V2O5 phase becomes the only detectable crystalline phase in the films in this temperature range. Furthermore, the individual nanoparticles start to coalesce, rapidly forming rod-like structures. The size of such formed structures, as well as the size of crystallites, rapidly increases with the temperature. This lowers the specific surface area of the coatings and causes a shift in the optical band gap from 2.68 eV to 2.48 eV. Finally, the subsequent heating of the nanoparticle films above 650 °C causes the complete collapse of the coatings due to their melting.

Microstructure and gyromagnetic properties of low-sintered LiZnTi ferrite doped with nano-LiZn and V2O5 additives

In this study, nanoparticles of LiZn ferrite (nano-LiZn, 0.0–10.0 wt. %) fabricated via the sol-gel method and V2O5 particles were added into a Li0.43Zn0.27Ti0.13Fe2.17O4 (LiZnTi) ferrite to realize low temperature sintering. The effects of the nano-LiZn ferrite addition on the crystalline phase formation, microstructure, and magnetic properties of LiZnTi ferrite were systematically investigated. X-ray diffraction results reveal that the LiZnTi ferrite doped with nano-LiZn and V2O5 additives present a pure spinel structure, which can be explained by liquid phase of V2O5 and spinel structure of nano-LiZn, as revealed by high-resolution transmission electron microscopy and selected area electron diffraction data. Scanning electron microscopy images show that the addition of nano-LiZn leads to a change in the microstructure, with the grains becoming uniform and dense. Additionally, the variation in the saturation magnetization (Ms) depends on the LiZn content. In particular, Ms increases with increasing LiZn content from 2.0 to 10.0 wt. %. Finally, the LiZnTi ferrite doped 4.0 wt. % nano-LiZn and 0.60 wt. % V2O5 possesses a narrow ferromagnetic resonance linewidth of 197 Oe, a reduced coercivity of 192.6 A/m, and an improved bulk density of 4.73 g/cm3.

Pulsed laser deposited V2O3 thin-films on graphene/aluminum foil for micro-battery applications

The development of new thin-film cathodes triggered a recent research interest in energy storage applications. Over the past years, vanadium oxides have been extensively explored as promising electrodes for batteries owing to their rich valence states and remarkable electrochemical properties. Herein, we report on the synthesis of undoped and Sn doped V2O3 thin-films on graphene (G)/Al foil by pulsed laser deposition followed by rapid thermal annealing in N2 at low temperature (∼430 °C). The obtaining V2O3 phase on graphene/Al foil (G/Al) has been confirmed by X-ray diffraction and Raman and X-ray photoelectron spectroscopy analyses. The synthesized vanadium oxide films were tested as cathodes in coin cells. The electrochemical properties have been systematically investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge discharge (GCD) measurements. A superior electrochemical performance was observed for the V2O3 on G/Al structures, with an initial capacity of around 300 mAh g−1, with respect to the bare G/Al electrode. The use of the Sn-doped (5 mol%) V2O3 thin-films improved slightly the initial capacity up to a value of ca. 311 mAh g−1. Both V2O3/G/Al and Sn-doped V2O3/G/Al exhibited excellent cycling performances after 40 cycles with a capacity maintenance at a C-rate C/20 of 317 mAh g−1. Long-term cycling test (up to 200 cycles) showed that the Sn doping could be an excellent strategy to improve the stability of the electrodes, which yielded a capacity loss of only 0.128% per cycle. Possible mechanisms are presented and discussed. This work could serve as point of reference for future developments in the field of batteries employing vanadium oxide-based thin-films deposited by physical vapor deposition techniques.