VO2 Thin Films Research Articles

Polarized Raman mapping and phase-transition by CW excitation for fast purely optical characterization of VO2 thin films

Vanadium dioxide has attracted much interest due to the drastic change of the electrical and optical properties it exhibits during the transition from the semiconductor state to the metallic state, which takes place at a critical temperature of about 68 °C. Much study has been especially devoted to developing advanced fabrication methodologies to improve the performance of VO2 thin films for phase-change applications in optical devices. Films structural and morphological characterisation is normally performed with expensive and time consuming equipment, as x-ray diffractometers, electron microscopes and atomic force microscopes. Here we propose a purely optical approach which combines Polarized Raman Mapping and Phase-Transition by Continuous Wave Optical Excitation (PTCWE) to acquire through two simple measurements structural, morphological and thermal behaviour information on polycrystalline VO2 thin films. The combination of the two techniques allows to reconstruct a complete picture of the properties of the films in a fast and effective manner, and also to unveil an interesting stepped appearance of the hysteresis cycles probably induced by the progressive stabilization of rutile metallic domains embedded in the semiconducting monoclinic matrix.

Crystallographic Pathways to Tailoring Metal-Insulator Transition through Oxygen Transport in VO2.

The metal-insulator (MI) transition of vanadium dioxide (VO2) is effectively modulated by oxygen vacancies, which decrease the transition temperature and insulating resistance. Oxygen vacancies in thin films can be driven by oxygen transport using electrochemical potential. This study delves into the role of crystallographic channels in VO2 in facilitating oxygen transport and the subsequent tuning of electrical properties. A model system is designed with two types of VO2 thin films: (100)- and (001)-oriented, where channels align parallel and perpendicular to the surface, respectively. Growing an oxygen-deficient TiO2 layer on these VO2 films prompted oxygen transport from VO2 to TiO2. Notably, in (001)-VO2 film, where oxygen ions move along the open channels, the oxygen migration deepens the depleted region beyond that in (100)-VO2, leading to more pronounced changes in metal-insulator transition behaviors. The findings emphasize the importance of understanding the intrinsic crystal structure, such as channel pathways, in controlling ionic defects and customizing electrical properties for applications.

On the enhanced electrochromic performance of nano-structured V2O5 thin films developed by substrate temperature induced orientated Growth

Electrochromic society is keen to develop devices with multicolor displays with high stability, using eco-friendly inorganic materials. In the present research, Influence of substrate temperature on the electrochromic V2O5 active electrodes deposited using automated nebulizer spray pyrolysis was examined to demonstrate V2O5 nanostructured films with good cyclic stability, high color contrast, and a robust memory effect. The structural characterization of the V2O5 thin films was conducted through R-ray diffraction and thermogravimetric analysis to assess material stability. Nanostructure morphology under different conditions was analysed using SEM and TEM. The electrochromic studies revealed that the higher optical modulation (ΔT) was achieved (with 70.8 % at 550 nm) in a very short switching time. Feature studies on V2O5 under different potentials were conducted to understand the capacitive and diffusion effects. The electrochromic response V2O5 thin films to various potentials was analysed using ex-situ XRD, elucidates phase changes from V2O5 to LiVO5 and V4O9, and a 2000-cycle CV measurement comprehended the material stability. The significantly improved electrochromic performance is primarily attributed to the nanosheet structure of the V2O5 thin films, since the structures enhance the material stability during lithium-ion intercalation/deintercalation processes.

Excess noise and thermoelectric effect in magnetron-sputtered VO2 thin films

This work presents the excess noise and thermoelectric (Seebeck) measurements on polycrystalline vanadium dioxide (VO2) thin films. Noise spectral power density (SPD) of current fluctuations in the semiconducting (SC) phase had a typical flicker noise (f−γ) characteristic with an average slope parameter γ of 1.13. Normalized SPD (Sn) values obtained in the SC-phase indicate that the noise originates in the bulk of the film. On the contrary, in the metallic (M)-phase, γ values were greater than unity, and the observed Sn values indicated that the origin of the noise is most likely from the contacts or surface rather than the bulk. A general decrease was observed in Sn by a factor of 4–5 from the SC- to M-phase. Moreover, Sn in the SC-phase showed no temperature dependence. An interpretation based on the number of charge carrier fluctuations in Hooge's model led to an unrealistically high Hooge parameter and had to be ruled out. We propose that the fluctuations are related to the mobility fluctuations of carriers arising primarily from grain-boundary scattering which explains the observed characteristics well. The Seebeck coefficients (S) obtained under both heating and cooling schedules showed the n-type nature of magnetron-sputtered VO2 films in the SC-phase. Differently, in the M-phase, the S value was positive. The S values obtained from the cooling schedule signified the low percolation threshold of the metal-to-insulator transition already demonstrated for VO2 thin films grown on r-cut sapphire using the Efros–Shklovskii percolation model.

Epitaxy of Monoclinic VO2 on Large-Misfit 3m Template Enabled by a Metastable Interfacial Layer.

We report the epitaxial growth of a monoclinic VO2 thin film on the CoFe2O4(111) template, assisted by an interfacial layer of the metastable orthorhombic phase. The interface between orthorhombic VO2 and CoFe2O4 is atomically sharp without noticeable interfacial diffusion. The (010)-faceted orthorhombic VO2 layer is lattice-matched to both the CoFe2O4(111) template and the monoclinic phase, although they have different surface symmetries. The occurrence of an orthorhombic VO2 thin layer significantly lowers the in-plane misfit strains of the monoclinic VO2 epilayer, along both the [100] and [001] axes. Our first-principles calculations confirm that the low-misfit orthorhombic VO2 is preferred on CoFe2O4(111) over the large-misfit monoclinic phase, at the initial growth stage. Additionally, upon increasing the film thickness to ∼8 nm, the orthorhombic phase is no longer favored, and the bulk stable monoclinic VO2 appears to minimize the free energy of the system. Moreover, we show that the metal-to-insulator transition of our VO2 epilayer can be efficiently triggered by both the temperature and Joule self-heating effect.

Microstructure of VO2 Thin Films Synthesized by Pulsed Laser Deposition

Microstructure of VO2 Thin Films Synthesized by Pulsed Laser Deposition

The synergistic effect of Ta-doping and antireflective TaOx layer on the thermochromic VO2 thin films for smart windows

Current efforts to promote the use of VO2 materials as a promising candidate for thermochromic smart windows continue to be hindered by the low luminous transmittance (Tlum), the limited solar modulation ability (ΔTsol), the high phase transition temperature (Tt, ∼68 °C) and the wide thermal hysteresis (ΔT). The present work addresses these issues by fabricating an antireflective TaOx layer on the Ta-doped VO2 (i.e., VO2(Ta)) thin films. The VO2(4.35 at% Ta) thin films with low Tt of 29.87 °C and narrowed ΔT of almost zero were grown on glass substrates by magnetron sputtering at room temperature followed by rapid thermal annealing. However, the VO2(Ta) thin films show slightly enhanced Tlum of 38.31 % compared with undoped VO2 films (35.46 %) and decreased the ΔTsol by 1.83 %. The thermochromic properties are further enhanced by depositing the antireflective TaOx layer onto the VO2(Ta) thin film by reactive magnetron sputtering, which significantly increases the Tlum from 38.31 % to 48.58 %, marginally decreases the ΔTsol from 4.44 % to 4.12 %, and maintain the Tt and ΔT constant. These results verify the possibility of developing VO2 films with simultaneously increased optical transmittance, decreased Tt to suitable values, and extremely narrow ΔT by using the synergistic effect of element doping and constructing an antireflective layer.

Room-temperature tuning of mid-infrared optical phonons and plasmons in W-doped VO2 thin films

The development of mid-infrared nanophotonics relies on the availability of new materials combining metal- and insulator-like optical features. We systematically studied the phase transition of thermochromic vanadium dioxide (VO2) using substitutional tungsten doping at room temperature. Our results reveal that vanadium dioxide thin films, doped with tungsten and grown via pulsed laser deposition techniques on sapphire substrates, can be precisely engineered to exhibit tailored infrared phonon and plasmon polaritonic responses. By controlling the extent of tungsten concentration, starting from VO2-WO3 cold-pressed powder targets, we demonstrate the ability to continuously adjust both the amplitude and frequency of optical phonon resonances. Furthermore, we observe tunable free-electron response due to varying tungsten concentrations. The adopted fabrication technique makes it possible to create multilayer structures by alternating layers with different concentrations of tungsten. Our results pave the way for the development of tunable mid-infrared metamaterial devices operating at room-temperature.

VO2-based colorful smart windows with self-cleaning function

Smart windows hold promise for mitigating energy demand for indoor heating or cooling. However, VO2-based thermochromic smart windows face challenges such as high phase transition temperature, limited window color options, and a lack of functional diversity. Herein, we investigated the colorful smart windows utilizing the tungsten-doped VO2 thin films with the phase transition temperature of approximately 23.5 °C. The surface color of these smart windows can be dynamically adjusted from brown to purple, cyan, yellow, and red by tuning the thickness of the wave-impedance matching layer of HfO2 film, resulted from the interference effect of the HfO2 layer and the underlying WxV1-xO2 layer. Moreover, the HfO2-coated WxV1-xO2 thin film with the HfO2 thickness of 132 nm demonstrates superior optical performance with a solar modulation ability of 9.35 %, the low-temperature luminous transmission of 36.81 %, and the high-temperature luminous transmission of 38.03 %. In addition, the incorporation of SiO2 nanoparticles into the HfO2-coated WxV1-xO2 thin films results in the superhydrophobic property with a water contact angle of 162.1° due to the formed rough surface, which is favor to the self-cleaning of the windows surface.

VO2 thin films: various microstructures for hysteresis manipulations

Vanadium dioxide reveals a phase change transition at 68 °C associated to typical hysteresis curves with strong variations in physico-chemical properties. The study presented here aims to investigate the influence of the structure and the microstructure of VO2 condensed matter on optical transmittance and electrical resistivity hysteresis curves during the metal-insulator transition. The characteristics of VO2 thin films synthesized through conventional laser ablation and of VO2 nanoparticle stacks generated via a laser vaporization cluster source are investigated. This study reveals the capability to modulate the transition temperature by as much as 20 °C, the transition width within the range of 4–30 °C, and the resistivity dynamics across the semiconducting-to-metallic phase transition by more than two orders of magnitude. These effects appear to arise because of the change of microstructure, the physical percolation of nanoparticles and associated collective effects.More complex hysteresis behaviors, taking benefit of the fabrication processes presented in this paper, and coupling the two previous distinct VO2 architectures (thin film and nanoparticles stack) lead to optical transmittance curves with three transition levels. This approach offers flexibility beyond binary responses, facilitating the design of innovative optoelectronic devices with n-stages modulation capabilities. Near room temperature applications of VO2 metal-insulator transition are underscored, highlighting opportunities for tailored architectures to exploit its properties effectively.

Ag decorated V2O5 electrodes as a promising option for electrochromic, photovoltaic, and energy-saving applications

This study reports the fabrication of electrochromic Ag decorated V2O5 thin film by thermal evaporation method at 10−5 torr vacuum medium and the effects of the Ag doping to the V2O5 matrix. Besides, the electrochromic capabilities and energy-saving characteristics of the films are also in the scope of this study. Structural and morphological properties are investigated by XRD, FESEM, and AFM. The films have a polycrystalline structure and a porous surface. FTIR and Raman spectra verify the molecular formation of the films. The performance of the electrochromic films is characterized by a variety of techniques such as cyclic voltammetry, chronoamperometry, chronocolumetry, and electrochemical impedance spectroscopy. The color of the films changes from yellow to green and then from green to blue in the range of −1.2 and + 1.0 V vs Ag/AgCl with a reversibility of 92.9 %. With the response times of tb = 13.6 s and tc = 9.4 s, an optical modulation of 51.7 % (from yellow to blue), and a coloration efficiency of 93.7 cm2 C−1, Ag:V2O5 becomes an impressive electrochromic material produced by physical methods. Linear sweep voltammetry studies show that the photocurrent of both V2O5/ITO and Ag:V2O5/ITO electrodes increases under solar illumination. The supercapacitive performance of the film is investigated by cyclic voltammetry, and Ag:V2O5 is an encouraging electrode material with a specific capacitance of 720.9 F g−1 at 5 mV s−1 scan rate. An equivalent circuit is fitted from the film Nyquist data and the results also confirm the positive contribution of Ag doping on the response times, energy saving, and photoresponse of V2O5 thin film. In this regard, the produced electrodes are highly promising for potential applications in electrochromic, energy storage, and photovoltaic devices.

Tunable broadband absorption and broadband linear polarization converter based on vanadium dioxide

A composite dielectric metamaterial based on vanadium dioxide (VO2) is proposed to achieve flexible switching between two functions, broadband absorption, and polarization conversion, by adjusting the VO2 conductivity. The designed metamaterial functions as a broadband absorber when VO2 is in the metal phase. The absorber consists of a VO2 top structure, a silicon dioxide (SiO2) dielectric layer, and a VO2 thin film. Numerical simulation ns show that the absorber can absorb up to more than 90% in the frequency range of 3.22 ∼ 8.51 THz, and due to the symmetry of the structure, the absorber is characterized by polarization-insensitive properties and good absorption over a wider incidence angle. When VO2 is in the insulator phase, the designed metamaterial has a cross-polarization conversion function. The linear polarization converter primarily comprises an I-beam metal, a SiO2 dielectric layer, and a gold substrate layer. Numerical simulations demonstrate that the linear polarization converter accomplishes a line polarization conversion rate (PCR) greater than 90% within the 1.40 ∼ 4.11 THz frequency range, attains a close to 100% cross-polarization conversion rate (PCR) at 1.46, 1.95, 3.0, and 3.97 THz. To confirm the wave absorption mechanism of the absorber, we utilize the impedance matching theory to analyze it. The proposed switchable bifunctional metamaterials present significant potential for broader applications in future terahertz communication, imaging, stealth technology, and related fields.

Flexible composite film utilizing VO2 self-adaptive photothermal and infrared radiative cooling for continuous energy harvesting.

Self-adaptive photothermal (PT) and radiative cooling (RC) based on insulation-metal phase transition vanadium dioxide (VO2) are among the most promising continuous energy harvesting technologies recently. However, previous work relies on rigid substrates that cannot fit complex or non-planar surfaces. Here, we propose a flexible composite film by bonding a VO2 thin film and a polyimide (PI) substrate with polymethyl methacrylate (PMMA), which achieves efficient spectrally self-adaptive broadband absorption/emission and can convert between the daytime PT mode and nighttime RC mode. Because of the inherent absorption of VO2 and the intricate interplay within multi-layer structure, the solar absorptance of the film could to up to 0.886 in the PT mode with the incorporation of an Al2O3 anti-reflection layer. On the other hand, due to the phase change properties of VO2, this film exhibits a broadband infrared emissivity modulation from 0.32 to 0.82 and reaches a maximum RC power of approximately 244.59 W/m2 in the RC mode at night. Moreover, the film maintains the infrared spectrum switching capability and high emissivity in RC mode even after 104 bending cycles. Our work shows potential to broaden the applications of VO2 smart coatings, including tunable selective emitters, thermal management of spacecraft and smart skins.

Raman scattering of TixV1‐xO2 thin films on (110) rutile TiO2 in the low and high temperature phase adjacent to the metal–insulator transition

AbstractVanadium dioxide (VO2) undergoes a reversible first‐order metal‐to‐insulator transition (MIT) from a high‐temperature metallic phase to a low‐temperature insulating phase at a critical temperature Tc of 68°C. The MIT is accompanied by a structural phase transition. In addition to the metallic high‐temperature rutile phase, several insulating phases may be involved depending on doping, interfacial stress, or external stimuli. Unambiguously identifying the crystal phases involved in the phase transition is of key interest from the point of view of application as well as fundamental science. We study the impact of Ti doping of VO2 thin films on (110) rutile TiO2 substrates. We conduct a careful analysis of structural properties by combining results of x‐ray diffraction, Raman spectroscopy, and transmission electron microscopy. The transition temperature Tc of the deposited thin films decreases with increasing Ti‐content. All our thin film samples undergo a structural phase transition from the monoclinic M1‐phase to the rutile R‐phase with increasing temperature without passing the intermediate monoclinic M2‐phase. A careful analysis of polarization and angle‐dependent Raman data reveals that, above Tc, the unit cell of the high‐temperature rutile TixV1‐xO2 phase is aligned with that of the rutile TiO2 substrate whereas, below Tc, 180°‐domains of the M1‐phase of TixV1‐xO2 are observed. The structural relationship between TiO2 substrate and the high respective low‐temperature phase of the TixV1‐xO2 determined by Raman spectroscopy is in excellent agreement with TEM results on these samples. Raman spectroscopy is a powerful tool for studying structural changes of VO2‐based samples in the vicinity of MIT.

Temperature-Dependent Residual Stresses and Thermal Expansion Coefficient of VO2 Thin Films

This study aims to investigate the thermomechanical properties of vanadium dioxide (VO2) thin films. A VO2 thin film was simultaneously deposited on B270 and H-K9L glass substrates by electron-beam evaporation with ion-assisted deposition. Based on optical interferometric methods, the thermal–mechanical behavior of and thermal stresses in VO2 films can be determined. An improved Twyman–Green interferometer was used to measure the temperature-dependent residual stress variations of VO2 thin films at different temperatures. This study found that the substrate has a great impact on thermal stress, which is mainly caused by the mismatch in the coefficient of thermal expansion (CTE) of the film and the substrate. By using the dual-substrate method, thermal stresses in VO2 thin films from room temperature to 120 °C can be evaluated. The thermal expansion coefficient is 3.21 × 10−5 °C−1, and the biaxial modulus is 517 GPa.

Optical, Electrical, Structural, and Thermo-Mechanical Properties of Undoped and Tungsten-Doped Vanadium Dioxide Thin Films.

The undoped and tungsten (W)-doped vanadium dioxide (VO2) thin films were prepared by electron beam evaporation associated with ion-beam-assisted deposition (IAD). The influence of different W-doped contents (3-5%) on the electrical, optical, structural, and thermo-mechanical properties of VO2 thin films was investigated experimentally. Spectral transmittance results showed that with the increase in W-doped contents, the transmittance in the visible light range (400-750 nm) decreases from 60.2% to 53.9%, and the transmittance in the infrared wavelength range (2.5 μm to 5.5 μm) drops from 55.8% to 15.4%. As the W-doped content increases, the residual stress in the VO2 thin film decreases from -0.276 GPa to -0.238 GPa, but the surface roughness increases. For temperature-dependent spectroscopic measurements, heating the VO2 thin films from 30 °C to 100 °C showed the most significant change in transmittance for the 5% W-doped VO2 thin film. When the heating temperature exceeds 55 °C, the optical transmittance drops significantly, and the visible light transmittance drops by about 11%. Finally, X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to evaluate the microstructure characteristics of VO2 thin films.

Atomically Resolved Phase Coexistence in VO2 Thin Films.

Concurrent structural and electronic transformations in VO2 thin films are of 2-fold importance: enabling fine-tuning of the emergent electrical properties in functional devices, yet creating an intricate interfacial domain structure of transitional phases. Despite the importance of understanding the structure of VO2 thin films, a detailed real-space atomic structure analysis in which the oxygen atomic columns are also resolved is lacking. Moreover, intermediate atomic structures have remained elusive due to the lack of robust atomically resolved quantitative analysis. Here, we directly resolve both V and O atomic columns and discover the presence of intermediate monoclinic (M2) phase nanolayers (less than 2 nm thick) in epitaxially grown VO2 films on a TiO2 (001) substrate, where the dominant part of VO2 undergoes a transition from the tetragonal (rutile) phase to the monoclinic M1 phase. Strain analysis suggests that the presence of the M2 phase is related to local strain gradients near the TiO2/VO2 interface. We unfold the crucial role of imaging the spatial configurations of the oxygen anions (in addition to V cations) by utilizing atomic-resolution electron microscopy. Our approach can be used to unravel the structural transitions in a wide range of correlated oxides, offering substantial implications for, e.g., optoelectronics and ferroelectrics.

Giant persistent photoconductivity of VO2 device by single-pulse femtosecond laser

The manifestation of giant persistent photoconductivity (GPPC) is demonstrated with a fs (femtosecond) Ti:sapphire laser pulse that has a duration of 40 fs and a central wavelength of 400 nm. The femtosecond laser pulse was irradiated on a two-terminal VO2 device fabricated on a corning glass substrate. Under the applied voltages of 9–12 V, the GPPC takes place within 8.6–15 μs after the laser irradiation. The photocurrent from the GPPC in the VO2 device remains stable with the current decreasing slope of ∼0.003%/minute. With one-dimensional thermal model, the temperature (TIR) of the irradiated area is estimated as a function of time, indicating that TIR is above the insulation-to-metal transition temperature of VO2 thin film prior to the onset of GPPC. The ultrafast onset of GPPC of VO2 device can be utilized for ultrafast optoelectronic switch and memory device.

Boosting the UV photo-detecting properties of V2O5 thin films by doping In3+ doping

In recent years, optoelectronic technology has demanded the development of efficient photodetectors by layered semiconductor materials at affordable prices. Herein, we put the efforts to generate V2O5 thin films on the surface of glass slides with diverse insertion of In content (0, 1, 2, 3, 4, and 5 wt%) via cost effective nebulizer spray pyrolysis technique and examined their application in U.V. photosensing. Standard characterization techniques are used for these films to identify the cause of In dopant on its morphological, structural, and opto-electrical properties. X-ray diffraction (XRD) plots confirm the development of single-phase orthorhombic structure grown along [010] direction, without the appearance of foreign phases. Further, the dopant ingredient (In) hikes the crystallinity and crystallite sizes (36–51 nm) up to a certain threshold level (2 wt%) and decreases it for further growing In doping. The Field emission scanning electron microscope (FESEM) images display homogeneous nanorods morphology formation at all doping levels. Doping improved its photon absorption in the U.V. region, and the bandgap obtained from the absorbance spectra tuned to a lower value of 1.98 eV up to 2 wt%. All the created thin films emit intense emission bands at 475 nm and 640 nm. Characterization involving I-V measurements under 365 nm illumination at 5 V bias exhibiting commendable performance in terms of photodetector metrics (Responsivity (R) = 7.96 × 10−1 AW−1, Detectivity (D*)=3.63 × 1011 Jones and External quantum efficiency (EQE) = 186 %) for 2 % In doped V2O5. The same sample exhibits a fast-rising /falling time up to 0.6s /0.7 s, respectively. Our results may widen the field of applications in high-performance U.V. photodetector.

Nitrogen Doping in VO2 Thin Films on Synthetic Mica Substrates Through Mist Chemical Vapor Deposition: Lowering the Metal–Insulator Transition Temperature Toward Smart Windows

AbstractIn this study, nitrogen (N) is doped into VO2 thin films through mist chemical vapor deposition (CVD), and the effect of the doping on metal–insulator transition (MIT) temperatures is investigated. The N‐doped VO2 thin films are grown on an SnO2 buffer layer. The N‐doped VO2 lattice spacing tends to expand as the growth temperature decreased, which indicates that the incorporation of N into the lattice is derived from the Ethylenediamine. Secondary ion mass spectrometry (SIMS) is conducted to investigate the relationship between the decrease in the transition temperature and N concentration. The results reveal that the sample grown at 425 °C contains approximately 2 × 1020 cm−3 of N. Thus, efficient nitrogen doping can be achieved through mist CVD. The temperature‐resistance characteristics of VO2 thin films are measured to investigate their electrical properties and MIT temperatures. The results reveal that for undoped samples, the transition temperature slightly decreases with the decrease in the growth temperature. Furthermore, the sample grown at 425 °C exhibits a considerable change in resistance because of MIT at approximately 29.5 °C. These results prove the potential of using mist CVD N‐doped thin films for smart window applications to address future energy problems.