VO2 Films Research Articles

Preparation of VO2/graphene/SiC film by water vapor oxidation

Abstract Vanadium dioxide (VO2) has attracted extensive attention due to the specific metal-insulator phase transition as well as the wide device applications. The practical performance of VO2-based device strongly depends on the quality of VO2, since the higher quality of VO2 film always shows much more pronounced phase transition behavior. Thus, the preparation of high quality VO2 film is essential and highly desirable. In this work, we have prepared high-quality VO2 film on SiC substrate by water vapor oxidation with graphene (G) buffer layer, which showed excellent phase transformation properties. Compared with the VO2/SiC sample without G buffer layer, the VO2/G/SiC films show the resistance changes up to four-orders of magnitude across the phase transition boundary and superior optoelectronic properties, which indicates the significant role of G layer in the film growth process. The current study not only provides an economical and feasible method for VO2/G/SiC thin film preparation with high quality, but also supply some clues for the application of G-based VO2 devices in the future.

Detailed pathway for a fast low-temperature synthesis of strongly thermochromic W-doped VO2 films with a low transition temperature

We report the discharge characteristics, and population of chosen atomic neutral (V, O and Ar) and ionic (V+, V2+, O+ and Ar+) species in the discharge during low-temperature (350 °C) depositions of W-doped VO x films onto a 170 nm thick Y-stabilized ZrO2 layer on a 1 mm thick conventional soda-lime glass. The depositions were performed using reactive deep oscillation magnetron sputtering with a pulsed O2 flow control and to-substrate O2 injection into a high-density plasma in front of the sputter V-W target. The pre-selected critical values of the oxygen partial pressure were in the range from 82 mPa to 92 mPa. Optical emission spectroscopy proved a relatively high density of O atoms in front of the substrate, which is important for production of the W-doped VO2 films. It was found that setting of the amount of O2 in the gas mixture determines not only the elemental and phase composition of the films and their deposition rate but also the crystallinity of the thermochromic VO2 phase in them. The crystalline W-doped VO2 films with a low transition temperature of 28 °C exhibited a high modulation of the solar energy transmittance of 8.9%. The results are important for further improvement of this new scalable sputter deposition technique for a fast (53 nm min−1 at a target-substrate distance of 100 mm in this work) low-temperature synthesis of high-performance durable thermochromic VO2-based multilayer coatings designed for smart-window applications.

Resistive switching localization by selective focused ion beam irradiation

Materials displaying resistive switching have emerged as promising candidates for implementation as components for neuromorphic computing. Under an applied electric field, certain resistive switching materials undergo an insulator-to-metal transition through the formation of a percolating filament, resulting in large resistance changes. The location and shape of these filaments are strongly influenced by hard-to-control parameters, such as grain boundaries or intrinsic defects, making the switching process susceptible to cycle-to-cycle and device-to-device variation. Using focused Ga+ ion beam irradiation, we selectively engineer defects in VO2 and V2O3 thin films as a case study to control filament formation. Using defect pre-patterning, we can control the position and shape of metallic filaments and reduce the switching power significantly. A greater than three orders of magnitude reduction of switching power was observed in V2O3, and a less than one order of magnitude reduction was observed in VO2. These experiments indicate that selective ion irradiation could be applied to a variety of materials exhibiting resistive switching and could serve as a useful tool for designing scalable, energy efficient circuits for neuromorphic computing.

Effect of ZnO dopant on V2O5:ZnO films electrochromic properties

Due to its unique physical and chemical properties, vanadium pentoxide has become a popular material for electrochromic devices, and using doping with other transition metals can enhance its overall performance. Therefore, the peroxovanadate sol-gel method was used to produce ZnO-doped V2O5 thin films. The results of this work show how ZnO doping affected the electrochromic and structural characteristics of thin films of V2O5. The main finding was that adding ZnO to the V2O5 films improved their cyclic stability and reversibility. Besides that, the incorporation of the dopant resulted in a mixture of crystalline phases, according to the X-ray diffraction. The charge density has increased from 28.79 for undoped to 41.68 mC/cm2 for doped films. The UV–Vis transmittance values between bleached and colored states - ΔT at 633 nm - changed from 14.4 to 55.5% for undoped and doped films, respectively. The roughness (root mean square, RMS) of the film remained the same, i.e., 23.5 ± 4.3 for undoped to 23.8 ± 5.1 nm for doped, as revealed by atomic force microscopy. Finally, the doping of V2O5 with ZnO improved the electrochemical and electrochromic characteristics of these new films, making them promising candidates for electrochromic device applications.

Printed and Room Temperature Processed Nanoparticulate VO2 Thin Films Toward Memristive Device Applications

Vanadium dioxide (VO2) has received tremendous research interest in recent years for its versatile electronic application possibilities based on its reversible metal-insulator transition (MIT) behavior, which can be triggered by a large set of stimuli, such as current, electric field, or light. VO2 undergoes a phase transition from a semi-insulating monoclinic to metallic rutile phase at a low critical temperature of 68 °C, which is very close to room temperature, and compatible with any inexpensive flexible substrate. However, owing to the existence of various vanadium oxide polymorphs, the synthesis of the pure monoclinic VO2 (M) phase is a challenge. In fact, the difficulty level increases further when it is to be solution-processed. However, synthesis protocol for phase-pure VO2 (M) following wet chemical routes is essential to obtain high throughput, cost-efficient, and substrate-independent fabrication routine for the large volume of devices. In this regard, in the present study, we report a solution-based synthesis of ultra-small VO2 (M) nanoparticles that are phase-pure and the small nanoparticles may easily be used to formulate inkjet printable VO2 nanoparticulate. The printed VO2 (M) films have been found to be homogeneous, crack-free and the memristor device thus fabricated has demonstrated temperature-dependent phase transition and a resistance change of more than three orders of magnitude around 68 °C; on the other hand, retention of the metallic and insulating state has also been demonstrated for 180 minutes.

Tunable IR perfect absorbers enabled by tungsten doped VO2 thin films

The temperature tunability of complex dielectric constants of vanadium dioxide (VO2) makes it a promising phase-change material for use in active, dynamic, tunable photonics applications. Specifically, the semiconductor-to-metal phase transition in VO2 enables reversible, broadband, and large complex refractive index variation and paves the way for a plethora of applications. Although the critical temperature for phase-transition is 68 °C for VO2 films, its transition temperature can be reduced to room temperature by tungsten-doping of vanadium dioxide. Such a degree of freedom in controlling the critical temperature through tungsten doping provides further tunability of the thermochromic behavior. In this work, we investigate a variety of W-doped VO2 thin films deposited by laser ablation of targets with increasing W doping content and report detailed infrared characterization together with numerical simulations. Our experimental results indicate that the perfect absorption can be achieved at different temperatures, within the VO2 insulator-to-metal phase transition process, as a function of W doping content. Tunable subwavelength layers allow perfect absorption under different temperature conditions around λ = 12 µm. We show that a high dynamic range of reflectivity can be achieved when the temperature is increased above the phase transition temperature. Furthermore, we observe perfect absorption at 11.8 µm at room temperature for a W content of 0.75%. We believe that W-doped VO2 thin films with tunable and controllable perfect absorption will open the way for a class of promising thermo-optical devices including thermos-photovoltaics, infrared filters, radiative cooling devices, and thermal emitters.

Study on nonreciprocal radiation and photothermal conversion of VO2-based tunable inverse opal thermal emitter

In this work, a novel tunable nonreciprocal thermal emitter, consisting of an InAs magneto-optic inverse opal film and a VO2 film, is designed to achieve strong and tunable nonreciprocal radiation in the mid-infrared band of 6.8–7.8 μm. A more appropriate Lorentz-Drude model is used to describe the material properties of InAs, and the nonreciprocal band in the mid-infrared range for doped InAs is obtained through the Lorentz-Drude model. The effect of different middle layers on the structure of films and inverse opals is investigated under a magnetic field at an incident angle of 26° The inverse opal structure is found to have better nonreciprocal properties, while VO2 has the role of replacing noble metals in the tuning of nonreciprocal radiation. In addition, the strengthening mechanism of the inverse opal structure and the physical mechanism of the nonreciprocal radiation are further analysed and revealed through electric field energy distribution plots and impedance matching theory. A comparison of the nonreciprocal radiation and the photothermal effect of three inverse opal structures with different pore distributions and porosities for different parameters shows that the simple cubic structure has the strongest nonreciprocal radiation performance and a relatively high photothermal conversion effect, which can further increase the radiation capacity of the thermal emitter over a range. Therefore, the designed inverse opal composite structure can be used for the fabrication of tunable thermal emitters with nonreciprocal effects, where the amount of light absorbed in one channel of the thermal emitter can be controlled to tune the radiation capacity of the other channel. Providing a degree of assistance in the development and application of nonreciprocal thermal radiation devices in the mid-infrared wavelength band.

Enhancing thermochromic properties of VO2 amorphous films on glass substrates by Sn-W co-doping

Co-doping is an effective method for VO2 films to meet the requirements of smart window applications. In this research, we prepared pure VO2, Sn-doped, and Sn-W co-doped amorphous films on glass substrates. Compared with pure and Sn-doped VO2 films, Sn-W co-doped samples showed better thermochromic properties. For the 4.8 % Sn-2.1 % W co-doped sample, the luminous transmittance (Tlum) reached 76 %, the metal-insulator transition (MIT) temperature (Tc) decreased to 46.7 °C, the solar transmittance modulation capacity (ΔTsol) was maintained at around 5.2 %, and the near-infrared modulation capacity (ΔTIR) was up to 13.5 %. These results show that co-doping Sn and W into VO2 is feasible for smart window applications.

Highly Tunable MOCVD Process of Vanadium Dioxide Thin Films: Relationship between Structural/Morphological Features and Electrodynamic Properties.

The monoclinic structures of vanadium dioxide are widely studied as appealing systems due to a plethora of functional properties in several technological fields. In particular, the possibility to obtain the VO2 material in the form of thin film with a high control of structure and morphology represents a key issue for their use in THz devices and sensors. Herein, a fine control of the crystal habit has been addressed through an in-depth study of the metal organic chemical vapor deposition (MOCVD) synthetic approach. The focus is devoted to the key operative parameters such as deposition temperature inside the reactor in order to stabilize the P21/c or the C2/m monoclinic VO2 structures. Furthermore, the compositional purity, the morphology and the thickness of the VO2 films have been assessed through energy dispersive X-ray (EDX) analyses and field-emission scanning electron microscopy (FE-SEM), respectively. THz time domain spectroscopy is used to validate at very high frequency the functional properties of the as-prepared VO2 films.

Lattice stress engineering induced by rare-earth ion in regulating phase transition and thermochromic properties of VO2 films

Reduction of metal-insulator transition temperature (θMIT) of VO2 has been a key topic in turning its potential into practical applications. Electron injection as induced by high-valence ions’ doping of W, Mo and Nb has been studied extensively due to its high efficiency in declining θMIT. However, little attention has been given to the impact of lattice stress/strain caused by larger radius ions on the phase transition and thermochromic properties of VO2. Herein, we investigate the dependence of θMIT on lattice stress by varying the radius of rare-earth (RE) ions in RE-Mo co-doped VO2 films. As the radius of the RE ion increases, the lattice stress gradually intensifies, resulting in a lower θMIT and enhanced luminance transmittance (Tlum) at the same doping concentration. The TEM results reveal that the apical V–O bond length of the VO6 octahedron is significantly influenced by the radius of the RE ion, thereby altering the energy gap between the d// and π∗ bands and adjusting θMIT. These findings demonstrate an additional degree of freedom to manipulate the phase transition of VO2 through lattice stress engineering.

Tailoring the thermochromic properties of sputter-deposited VO2 films by O2+ ion irradiation

Vanadium dioxide (VO2) thin film is a well-known phase transition material showing extensive modification in their physical properties, such as electronic, magnetic, and optical characteristics when the metal–insulator transition (MIT) occurs. Many approaches (including doping, micro-structure, external stress, and defect engineering) can enhance the MIT characteristics of vanadium dioxide thin films. More recently, low-energy ion irradiation has been well-established to modify the electrical, optical, and physical properties of metal oxide thin films. In this paper, the effect of low-energy O2+ ion irradiation on the thermochromic properties of VO2 thin films was investigated. From the temperature-dependent near infrared transmittance measurements, both MIT temperature and thermal hysteresis width of VO2 thin films showed an opposite trend and decreased as the ion fluence increased. The values of solar modulation and luminous transmittance of irradiated VO2 films were significantly altered by introducing low-energy ion irradiation, compared to the pristine VO2 thin films. According to the XPS analyses, the vanadium ion are in mixed of two valence states (V4+ and V5+). Moreover, the concentration of oxygen vacancies in VO2 films increases as the fluence of low-energy O2+ ion increases. Our study concludes that low-energy O2+ ion irradiation is an effective means to optimize the MIT characteristics of VO2 thin films to a certain extent.

Preparation and characterization of VO2/Cu-Zr NPs/VO2 composite films with enhanced thermochromic properties

The contradiction between luminous transmittance (Tlum) and solar modulation ability (ΔTsol) of vanadium dioxide (VO2) smart windows can be reconciled through the regulation of surface morphology. In this study, Cu-Zr bimetallic nanoparticles (Cu-Zr NPs) were embedded into VO2/VO2 bilayer films by pulsed laser deposition to tailor the VO2 surface morphology. XRD characterization revealed that the introduction of Cu-Zr NPs can significantly improve the crystallinity of VO2 films. FESEM characterization illustrated that the grain size and morphology of top-VO2 layer were sensitive to the density of Cu-Zr NPs. From the transmission spectrum, evident localized surface plasmon resonance (LSPR) phenomena could be observed in the range of 1300–1500 nm. When the pulse number for depositing Cu-Zr NPs is 35, the composite film exhibits both high Tlum of 62.4% and excellent ΔTsol of 10.6% simultaneously. In addition, the finite difference time domain (FDTD) simulation suggested that the improvement of thermochromic properties could be attributed to the LSPR effect induced by the unevenly distributed VO2 grains on the surface. This work confirmed the existence of non-noble Cu-Zr NPs could both improve the crystallinity and regulate the surface morphology of VO2 film, which simplified the fabricating process of VO2 thermochromic smart windows and was cost-effective.

Enhanced Thermochromic Performance of VO2 Nanoparticles by Quenching Process.

Vanadium dioxide (VO2) has been a promising energy-saving material due to its reversible metal-insulator transition (MIT) performance. However, the application of VO2 films has been seriously restricted due to the intrinsic low solar-energy modulation ability (ΔTsol) and low luminous transmittance (Tlum) of VO2. In order to solve the problems, the surface structure of VO2 particles was regulated by the quenching process and the VO2 dispersed films were fabricated by spin coating. Characterizations showed that the VO2 particles quenched in deionized water or ethanolreserved VO2(M) phase structure and they were accompanied by surface lattice distortion compared to the pristine VO2. Such distortion structure contributed to less aggregation and highly individual dispersion of the quenched particles in nanocomposite films. The corresponding film of VO2 quenched in water exhibited much higher ΔTsol with an increment of 42.5% from 8.8% of the original VO2 film, because of the significant localized surface plasmon resonance (LSPR) effect. The film fabricated from the VO2 quenched in ethanol presented enhanced thermochromic properties with 15.2% of ΔTsol and 62.5% of Tlum. It was found that the excellent Tlum resulted from the highly uniform dispersion state of the quenched VO2 nanoparticles. In summary, the study provided a facile way to fabricate well-dispersed VO2 nanocomposite films and to facilitate the industrialization development of VO2 thermochromic films in the smart window field.

Evaluation of low-voltage-driven multi-colored electrochromic device based on dry-deposited V2O5

A V2O5 film fabricated by a nanoparticle deposition system (NPDS) was used to demonstrate a highly durable electrochromic device (ECD). The V2O5 film deposited on both the electrodes functioned as a complementary layer during the redox reaction to balance the charge inside the device. Thereby, it mitigated the load applied to the ECD by enabling its electrochromic reaction within the range of ±1 V. The fabricated ECD showing different colors at opposite voltages were applied to both the electrodes. Meanwhile, a TiO2-based opaque electrolyte was used to show different colors of V2O5 thin films independently. The fabricated ECD can display three colors: yellow, green, and blue. Furthermore, each color variation is completed within 10 s. According to the International Commission on Illumination (CIE) data, the color differences (ΔE) between yellow and green, green and blue, and yellow and blue were measured to be approximately 27, 17, and 44, respectively. Meanwhile, the fabricated ECD showed almost a similar level of electrochromic performance even after 500 repeated runs. Moreover, the charge capacity of the ECD after a 10,000-cycle cyclic voltammetry (CV) analysis was 49.01 mC/cm2 (the initial charge capacity was measured to be 47.31 mC/cm2). This verified the high durability of the device. Thus, an ECD capable of coloration under a low voltage range of ±1 V was fabricated using a dry deposition method with a TiO2-based opaque electrolyte, to vividly display different colors at both sides of the device simultaneously. The unique device can independently display different colors on both the electrodes. The dry deposition method was applied to deposit an identical V2O5 electrochromic film on the electrodes and an opaque electrolyte. The device has potential applications in functional electrochromic displays.

Editors’ Choice—Equilibrium Boundaries of VO2 Phase Stability: Determination by Solid Electrolyte EMF Measurements and Effect on Optical Changes during Metal-Insulator Transition

Vanadium dioxide is widely known for its metal-insulator transition (MIT), in which drastic changes in resistivity and IR-transparency occur. This makes VO2 thin films promising materials for high-frequency optoelectronic devices. To get the most MIT sharpness, thin films should not contain impurities of hyper-oxygen or hypo-oxygen phases arising during VO2 synthesis. To ascertain the conditions of single-phase VO2 existence, the equilibrium boundaries of VO2 with neighboring phases were determined using the electromotive force method (EMF) with a solid electrolyte ZrO2(Y2O3). Our data for the high-oxygen boundary of VO2 existence in equilibrium with the V6O13 phase agree with the only data known in the literature. We established that VO2 is, in equilibrium with the V9O17 phase at the low-oxygen boundary, which forms V8O15 under further reduction. The temperature of the peritectoid decomposition of V9O17 is established, and the corresponding corrections to the phase diagram of the vanadium-oxygen system are introduced. The Gibbs energies for V9O17, V8O15, and V6O13 formation reactions are calculated. It is also shown that the IR reflectance of VO2 films brought to equilibrium at the high-oxygen boundary is much greater than that of films equilibrated at the low-oxygen boundary.

Correlation of metal-to-insulator transition and strain state of VO2 thin films on TiO2 (110) substrates

We explore the possibility of tuning the metal-to-insulator transition (MIT) of crystalline VO2 thin films by strain engineering. We deposit high-quality VO2 epitaxial films of different thicknesses on TiO2 (110) substrates by pulsed laser deposition. The strain state of the deposited film varies with its thickness. This allows us to correlate the MIT characteristics with the strain state of the VO2 film by a careful characterization of the structural and electrical properties. Thin VO2 films on TiO2 (110) substrates are almost fully strained up to thicknesses of about 20 nm and exhibit tensile strain along the c axis of the (high-temperature) metallic rutile phase leading to an increase in the MIT temperature by as much as 30 °C in comparison to the almost fully relaxed 300 nm-thick VO2 film. The strain gradient within the thicker samples leads to a continuous serial switching of layered regions of the VO2 film from the insulating to the metallic state with increasing temperature.

Manipulation of Spatial Infrared Emission Based on W‐Doped Vanadium Oxide Films toward Thermal Coding

AbstractActive thermal radiation control of an object requires adaptive and flexible strategies to cope with emissivity engineering and its temperature dependence. On the other hand, the concept of digital coding allows a high degree of freedom in manipulating the interacting physical quantities based on local discretized features. Learning from this idea, this study put forward the manipulation of spatial infrared emission based on tungsten (W)‐doped VO2 films toward thermal coding. Here, 1‐bit coding worksheets created by low‐emissivity, that is, code ‘0′, and high‐emissivity, that is, code ‘1′, shift with materials metal‐insulator phase transition to encode emissive information in the temperature domain. It is shown experimentally that this concept has marked implications on thermal radiation control at Mid‐infrared, among which are as follows: digital anti‐counterfeiting by in‐plane spatial coding of a group of VO2 thin films with various W‐doping density and infrared camouflage by out‐of‐plane spatial coding of them. With a W‐doping gradient of 0%, 0.8%, and 1.7%, the three‐phase transition points in the temperature domain, that is, 72, 63, and 45 °C, give a remarkable modulation range of 27 °C. These phenomena pave the way for new classes of emissivity engineering for more accurate, flexible, and adaptive thermal radiation control.

First‐Principles Studies of Electronic Structures and Optical Properties of Cobalt‐Doped VO2

First‐principles calculations based on density functional theory (DFT) are used to investigate the phase transition characteristics, electronic structures, and optical properties of pure and Co‐doped VO2 (M1 and R phase). Studies show that the metal‐to‐insulator phase transition temperature of VO2 is significantly reduced after Co doping, which is correlated to the decrease of bandgap value. Besides, the decrease of the energy required for electron transition of M1‐phase Co‐doped VO2 corresponds to the imaginary part of the dielectric peak moving to the low‐energy region. For both the M1‐ and R‐phase VO2, the visible light transmissivity of the Co‐doped VO2 is increased than that of pure VO2, which is beneficial to the application of VO2 film as visible windows. In addition, the absorptivity and reflectivity of Co‐doped R‐phase VO2 in the infrared light range are larger than those of M1‐phase VO2, indicating that the Co‐doped VO2 can block more infrared light at higher temperature to fulfill the purpose of lowering temperature. Overall, these results give new insights for the application of Co‐doped VO2 as a photoenergy material to regulate the room temperature.

Tunable heterostructural prism for planar polaritonic switch

The study of phonon polaritons in van der Waals materials at the nanoscale has gained significant attention in recent years due to its potential applications in nanophotonics. The unique properties of these materials, such as their ability to support sub-diffraction imaging, sensing, and hyperlenses, have made them a promising avenue for the development of new techniques in the field. Despite these advancements, there still exists a challenge in achieving dynamically reversible manipulation of phonon polaritons in these materials due to their insulating properties. In this study, we present experimental results on the reversible manipulation of anisotropic phonon polaritons in α-MoO3 on top of a VO2 film, a phase-change material known for its dramatic changes in dielectric properties between its insulating and metallic states. Our findings demonstrate that the engineered VO2 film enables a switch in the propagation of polaritons in the mid-infrared region by modifying the dielectric properties of the film through temperature changes. Our results represent a promising approach to effectively control the flow of light energy at the nanoscale and offer the potential for the design and fabrication of integrated, flat sub-diffraction polaritonic devices. This study adds to the growing body of work in the field of nanophotonics and highlights the importance of considering phase-change materials for the development of new techniques in this field.

Low temperature synthesis of VO2 and hysteresis free VOx thin films with high temperature coefficient of resistance for bolometer applications

Vanadium dioxide (VO2) is an insulator to metal phase transition material, which finds many electronic and optical applications due to its reversible switching while VOx-based bolometers are widely used in commercial uncooled infrared detectors due to their high-temperature coefficient of resistance and low resistivity. Atmospheric pressure thermal oxidation (APTO) of Vanadium (V) is a simple process used to synthesize VO2 films without the need for precise control of oxygen partial pressure unlike other synthesis methods. In this work, we report the effect of reducing the V oxidation temperature to 250–450 °C during APTO and synthesize films with different oxidation durations. For an optimized oxidation duration at each oxidation temperature, VO2 films are obtained with more than two orders of resistance switching. This optimum oxidation duration is found to decrease exponentially with increase in oxidation temperature with an activation energy of 1.05 eV. The phase transition temperature of these VO2 films is found to decrease monotonically with decrease in oxidation temperature. We also observe that partially oxidized (oxidation time less than optimized oxidation duration to form VO2) VOx films show broad resistance switching from room temperature up to 70 °C instead of steep switching at the phase transition temperature. The partially oxidized film shows a temperature coefficient of resistance (TCR) as high as ∼10%/K, which is 3–4 times greater than TCR of commercial VOx thin films used in bolometers while showing similar resistivity. The resistivity of these films also exhibits almost zero hysteresis with temperature, an important requirement for bolometer applications. This study will widen the usefulness of the APTO process for fabricating VO2-based devices and high TCR VOx films for bolometer applications. The demonstrated low temperature process will especially be suitable to integrate VO2/VOx on substrates which can sustain only low temperatures such as some of the flexible substrates.