Enhanced hydrophilicity of the Si substrate for deposition of VO2 film by sol–gel method

We have illustrated the role of hydrophilic nature of Si substrate played in the improvement of the contact performance between the vanadium dioxide (VO₂) film and Si substrate. The VO₂ films were fabricated by sol–gel method on single crystal Si substrate, which was pre-treated with hydrophilic solution and obtained a quite improved hydrophilicity. The bonding of Si substrate with precursor V₂O₅ gel was interpreted. The morphology and crystalline structure of the films were investigated by field-emission scanning electron microscopy, atomic force microscopy and X-ray diffraction. It is shown that the surface of the film on Si substrate with enhanced hydrophilicity is quite homogeneous and uniform. The film exhibits the formation of VO₂ phase with (011) preferred orientation. Moreover, the optical pump induced phase transition property of the film was studied by terahertz time-domain spectroscopy, which revealed around 70% reduction of transmission at 0.1–1.5?THz in the VO₂ film across the phase transition.     

Electrically tunable mid-infrared antennas based on VO2

A large change in optical constants of phase-change vanadium dioxide enables active control over the transmission and reflection properties of structures incorporating VO₂. In this paper, we demonstrate electrically tunable mid-infrared strip array antennas based on metal–insulator transition of vanadium dioxide. The antennas consist of an interdigitated metal strip array separated from a metallic ground plane by a film of vanadium dioxide. The interdigitated metal strips serve as both antennas and electrodes. As the insulator-to-metal phase transition of vanadium dioxide is induced with applied voltages, resonance of the strip antenna array redshifts with reduced absorbance before it is eventually switched off. A tuning magnitude of 30% in reflectivity is measured at 25.5 THz. Our measurements of sample temperature reveal that tuning mechanism of the antennas is primarily thermal in nature. The demonstrated electrical tuning of mid-infrared antennas could be used for reconfigurable bolometric sensing, camouflaging and modulation of infrared radiations.     

二氧化钒薄膜智能窗的研究进展

二氧化钒具有良好的半导体-金属相变特性,在常温下,二氧化钒的晶体结构为单斜晶系结构(M相),随着温度的升高达到相变温度,二氧化钒的晶型变成四方晶红石结构(R相),当温度降低到相变温度时,二氧化钒的晶型又变回单斜晶系结构(M相)。这种典型可逆热色特征,使二氧化钒成为当前建筑用智能窗材料的最佳选择。综述了近些年来制备VO_2薄膜的几种常用方法,并针对VO_2薄膜在热色智能窗应用方面存在的主要问题,从掺杂和复合薄膜结构两方面总结了提高VO_2薄膜性能的改进工艺,为推进VO_2薄膜智能窗的进一步研究提供了依据。     

The large modification of phase transition characteristics of VO2 films on SiO2/Si substrates

The vanadium oxide (VO₂) films have been prepared on SiO₂/Si substrates by using a modified Ion Beam Enhanced Deposition (IBED) method. During the film deposition, high doses of Ar⁺ and H⁺ ions have been implanted into the deposited films from the implanted beam. The resistance change of the VO₂ films with temperature has been measured and the phase transition process has been observed by using the X-ray Diffraction technique. The phase transition of the IBED VO₂ films starts at a low temperature of 48 °C and ends at a high temperature of 78 °C. It is found that the phase transition characteristics can be adjusted by changing the annealing temperature or the time and the phase transition characteristics of the IBED VO₂ films depend on the quantity and location of argon atoms in the film matrix.     

Two-dimensional metamaterials for epitaxial heterostructures

We review the use of two-dimensional psuedomorphic materials to accommodate an extraordinary range of misfit and allow novel new phases to be grown epitaxially. These materials assume the structure of the substrate and can thus be regarded as metamaterials. We illustrate these principles through a number of systems, including a detailed structural and spectroscopic study of epitaxial VO₂/NiO heterostructures. In this case the metamaterial is VO_1+x which is structurally and electronically distinct from the bulk of the VO₂ film. In the transition region the crystal structure adopts that of the NiO layer, while the oxidation state of vanadium increases from ∼3+ to ∼4+ with thickness, accompanied by increasing lattice disorder. The formation and evolution of this interfacial phase, VO_1+x, accommodates the change in crystal symmetry across the interface from the rock-salt structure of NiO to the rutile structure of VO₂. The use of two-dimensional metamaterials opens a wealth of new opportunities for the growth of new materials with novel properties.     

High temperature coefficients of resistance of VO2 films grown by excimer-laser-assisted metal organic deposition process for bolometer application

With bolometer application in mind, we prepared VO₂ films on TiO₂ (001) substrates by an excimer-laser-assisted metal organic deposition process at 300 °C or less. A metal-to-insulator transition of VO₂ is expected to induce high temperature coefficient of electrical resistance (TCR) useful for high-performance infrared sensors, but the practical use of crystalline VO₂ films has been prevented due to the accompanied wide hysteresis. In this study, by forming the epitaxial phase only near the substrate interface, the transition of the film was successfully broadened and the hysteresis disappeared. The maximum TCR of the film was more than -10%/°C near room temperature, and the temperature range in which TCR was higher than -4%/°C was very wide (280-320 K).     

New intelligent multifunctional SiO2/VO2 composite films with enhanced infrared light regulation performance,solar modulation capability,and superhydrophobicity

Abstract

Highly transparent, energy-saving, and superhydrophobic nanostructured SiO₂/VO₂ composite films have been fabricated using a sol–gel method. These composite films are composed of an underlying infrared (IR)-regulating VO₂ layer and a top protective layer that consists of SiO₂ nanoparticles. Experimental results showed that the composite structure could enhance the IR light regulation performance, solar modulation capability, and hydrophobicity of the pristine VO₂ layer. The transmittance of the composite films in visible region (T_lum) was higher than 60%, which was sufficient to meet the requirements of glass lighting. Compared with pristine VO₂ films and tungsten-doped VO₂ film, the near IR control capability of the composite films was enhanced by 13.9% and 22.1%, respectively, whereas their solar modulation capability was enhanced by 10.9% and 22.9%, respectively. The water contact angles of the SiO₂/VO₂ composite films were over 150°, indicating superhydrophobicity. The transparent superhydrophobic surface exhibited a high stability toward illumination as all the films retained their initial superhydrophobicity even after exposure to 365 nm light with an intensity of 160 mW^.cm^?2 for 10 h. In addition, the films possessed anti-oxidation and anti-acid properties. These characteristics are highly advantageous for intelligent windows or solar cell applications, given that they can provide surfaces with anti-fogging, rainproofing, and self-cleaning effects. Our technique offers a simple and low-cost solution to the development of stable and visible light transparent superhydrophobic surfaces for industrial applications.     

Growth of VO2 films with metal-insulator transition on silicon substrates in inductively coupled plasma-assisted sputtering

Single-phase monoclinic vanadium dioxide (VO₂) films were grown on a Si(100) substrate using inductively coupled plasma (ICP)-assisted sputtering with an internal coil. The VO₂ film exhibited metal-insulator (M-I) transition at around 65 °C with three orders of change in resistivity, with a minimum hysteresis width of 2.2 °C. X-ray diffraction showed structural phase transition (SPT) from monoclinic to tetragonal rutile VO₂. For conventional reactive magnetron sputtering, vanadium oxides with excess oxygen (V₂O₅ and V₃O₇) could not be eliminated from stoichiometric VO₂. Single-phase monoclinic VO₂ growths that are densely filled with smaller crystal grains are important for achieving M-I transition with abrupt resistivity change.     

Modulating Photoluminescence of Monolayer Molybdenum Disulfide by Metal–Insulator Phase Transition in Active Substrates

The atomic thickness and flatness allow properties of 2D semiconductors to be modulated with influence from the substrate. Reversible modulation of these properties requires an “active,” reconfigurable substrate, i.e., a substrate with switchable functionalities that interacts strongly with the 2D overlayer. In this work, the photoluminescence (PL) of monolayer molybdenum disulfide (MoS₂) is modulated by interfacing it with a phase transition material, vanadium dioxide (VO₂). The MoS₂ PL intensity is enhanced by a factor of up to three when the underlying VO₂ undergoes the thermally driven phase transition from the insulating to metallic phase. A nonvolatile, reversible way to rewrite the PL pattern is also demonstrated. The enhancement effect is attributed to constructive optical interference when the VO₂ turns metallic. This modulation method requires no chemical or mechanical processes, potentially finding applications in new switches and sensors.     

Characteristics of CeOx–VO2 composite thin films synthesized by sol–gel process

Pure vanadium dioxide (VO₂) and CeOx–VO₂ (1.5 < x < 2) composite thin films were grown on muscovite substrate by inorganic sol–gel process using vanadium pentaoxide and cerium(III) nitrate hexahydrate powder as precursor. The crystalline structure, morphology and phase transition properties of the thin films were systematically investigated by X-ray diffraction, Raman, X-ray photoelectron spectroscopy, FE-SEM and optical transmission measurements. High quality of the VO₂ and CeOx–VO₂ composite films were obtained, in which the relative fractions of +4 valence state vanadium were above 70 % though the concentrations of cerium reached 9.77 at %. However, much of cerium compounds were formed at the edge of grains and the addition of cerium resulted in more clearly defined grain boundaries as shown in SEM images. Meanwhile, the composite films exhibited excellent phase transition properties and the infrared transmittance decreased from about 70 to 10 % at λ = 4 μm bellow and above the metal–insulator phase transition temperature. The metal–insulator phase transition temperatures were quite similar with about 66 °C of the pure VO₂ and CeOx–VO₂ composite thin films. But hysteresis widths increased with more addition of cerium, due to the limiting effect of grain boundaries on the propagation of the phase transition. Particularly, the CeOx–VO₂ composite film with an addition of 7.82 at % Ce showed a largest hysteresis width with about 20.6 °C. In addition, the thermochromic performance of visible transmittance did not change obviously with more addition of cerium.     

Minimization of phase distortions of transmitted radiation upon optical switching in vanadium dioxide film

We have numerically modeled transmission of radiation with a wavelength of λ=10.6 or 3.4 μm through a VO₂ film and a change in phase of the transmitted radiation upon a transition of the film material from semiconductor to metallic state. It is established that there are optimum values of the film thickness for which the phase changes tend to zero. Conditions favoring minimization of the phase distortions are determined for a single VO₂ film and a multilayer interferometer with such a film.     

On the triggering mechanism for the metal–insulator transition in thin film VO<Subscript>2</Subscript> devices: electric field versus thermal effects

Vanadium dioxide (VO₂) has been shown to undergo an abrupt electronic phase transition near 70 °C from a semiconductor to a metal, with an increase in dc conductivity of over three orders of magnitude, making it an interesting candidate for advanced electronics as well as fundamental research in understanding correlated electron systems. Recent experiments suggest that this transition can be manifested independent of a structural phase transition in the system, and that it can be triggered by the application of an electric field across the VO₂ thin film. Several experiments that have studied this behavior, however, also involve a heating of the VO₂ channel by leakage currents, raising doubts about the underlying mechanism behind the transition. To address the important question of thermal effects due to the applied field, we report the results of electro-thermal simulations on a number of experimentally realized device geometries, showing the extent of heating caused by the leakage current in the “off” state of the VO₂ device. The simulations suggest that in a majority of the cases considered, Joule heating is insufficient to trigger the transition by itself, resulting in a typical temperature rise of less than 10 K. However, the heating following a field-induced transition often also induces the structural transition. Nevertheless, for certain devices, we identify the possibility of maintaining the field-induced high conductivity phase without causing the structural phase transition: an important requirement for the prospect of making high-speed switching devices based on VO₂ thin film structures. Such electronically driven transitions may also lead to novel device functionalities including ultra-fast sensors or gated switches incorporating ferroelectrics.     

Oxygen pressure dependent VO2 crystal film preparation and the interfacial epitaxial growth study

High quality VO₂ crystal films have been prepared on sapphire substrates by pulsed laser deposition method and the effects of oxygen pressure on the crystal phase structure are investigated. Results indicate that the phases and microstructures of VO₂ films are strongly sensitive to oxygen pressure. High oxygen pressure tends to form coarse B-VO₂ nanocrystals while low pressure favors a flat M1-VO₂ film epitaxial growth. X-ray diffraction φ-scan patterns confirm the [020] epitaxial growth orientation of the M1-VO₂ film and the in-plane lattice epitaxial relationship at the interface is also examined. Raman spectra indicate that M1-VO₂ phase has much stronger Raman scattering modes than B-VO₂, and the clear phonon modes further confirm the idea stoichiometry of VO₂ crystal film. Infrared transmittance spectra as the function of temperature are recorded and the results show that M1-VO₂ crystal films undergo a distinct infrared transmittance variation across metal insulator transition boundary, while B-VO₂ exhibits negligible thermochromic switching properties in the temperature range concerned. The pronounced phase transition behavior of the M1-VO₂ crystal film makes it a promising candidate for optical filter/switch and smart window applications in the future.     

A novel wet process for the preparation of vanadium dioxide thin film

Thin films of vanadium oxide have been prepared from an aqueous solution system of (V₂O₅–HF aq.) with the addition of aluminium metal by a novel wet-preparation process which is called liquid-phase deposition (LPD). From X-ray diffraction measurements, the as-deposited film was found to be amorphous and it was then crystallized to V₂O₅ by calcination at 400 °C under an air flow. In contrast, the monoclinic VO₂ phase was obtained when the deposited film was calcined under a nitrogen atmosphere. The deposited film showed excellent adherence to the substrate and was characterized by a homogeneous flat surface. The deposited VO₂ film exhibited a reversible semiconductor–metal phase transition around 70 °C and its transition behaviour depended on the way in which the film was prepared. This revised version was published online in November 2006 with corrections to the Cover Date.     

Metal-insulator transition in thin films of vanadium dioxide: The problem of dimensional effects

Vanadium dioxide has various potential applications in electronics due to the metal-insulator transition (MIT). It is known that oxide structures with nanometric dimensions exhibit properties different from bulk oxide materials because of the spatial confinement and the proximity of the substrate. However, in order to integrate VO₂ into the thriving nano-scale electronics, it is necessary to explore the MIT in this material in thin film nano-structures. In this work, it is shown that there is a fundamental dimensional restriction for the transition to occur even for pure epitaxial VO₂ nano-films and nano-wires. This is associated with the fact that any phase transition turns out to be impossible when the system size is decreased below a certain characteristic length d_c. This dimension is estimated to be d_c ∼ ξ (where ξ is the correlation length, ∼ 2 nm for VO₂), and, on the basis of the available experimental data, it is shown that the transition temperature falls as the characteristic size (film thickness or nano-wire radius) diminishes, though the predicted theoretical limit of 2 nm is not still experimentally achieved by now.Experimental results concerning the dependence of the threshold voltage on the film thickness at MIT-induced switching in VO₂ based sandwich structures are presented. Finally, the comparison of the authors' experimental data with the literature data, as well as with the analogous features of superconducting phase transitions, is carried out.     

Recent progresses on physics and applications of vanadium dioxide

As a strongly correlated electron material, vanadium dioxide (VO₂) has been a focus of research since its discovery in 1959, owing to its well-known metal–insulator transition coupled with a structural phase transition. Recent years have witnessed both exciting discoveries in our understanding of the physics of VO₂ and developments in new applications of VO₂-related materials. In this article, we review some of these recent progresses on the phase transition mechanism and dynamics, phase diagrams, and imperfection effects, as well as growth and applications of VO₂. Our review not only offers a summary of the properties and applications of VO₂, but also provides insights into future research of this material by highlighting some of the challenges and opportunities.     

The effect of interfaces on electronic switching in VO2 thin films

IR radiance observations and bolometric profile measurements on VO₂ coplanar switching device structures have revealed an anomalous thermal filament that is inconsistent with that expected for the conductivity transition observed in bulk VO₂. This filament nucleates in the interface layer at the VO₂-substrate interface and is due to the exponential dependence of conductivity with temperature at the interface. Once nucleated, the filament thermally drives the balance of the VO₂ film through the conductivity transition and initially dominates the radiance and bolometric measurements. With increased current, a second filament with the expected characteristics emerges and begins to dominate. Devices structures that do not exhibit the interface thermal filament fail catastrophically upon switching.The VO₂ switching results and the electronic properties of the interface layer indicate that stable switching requires an interface layer in which the electronic states are delocalized. This type of interface invariably results when VO₂ films are deposited onto quartz substrates. Unstable switching invariably occurs when the metallic VO₂ phase is nucleated in films deposited onto sapphire substrates and is associated with a sudden electronic delocalization of the interfaces of these systems.     

Optical properties of thin films of amorphous vanadium oxides

The results of an experimental investigation of the optical properties of anodic vanadium oxide films are presented. It is shown that films of different phase composition (VO₂, V₂O₅, or a mixture of two phases) can be obtained, depending on the oxidation regime, and that the absorption and transmission spectra are modified significantly in accordance. The optical properties of the oxides, whose composition is close to stoichiometric vanadium dioxide, demonstrate the occurrence of a metal-semiconductor phase transition in the amorphous films. The results presented are important both from the standpoint of technical applications of thin film systems based on anodic vanadium oxides and for more detailed understanding of the physical mechanism of the metal-semiconductor phase transition and the influence of structural disorder on the transition. Pis’ma Zh. Tekh. Fiz. 25, 81–87 (April 26, 1999)     

Study on optical and electrical switching properties and phase transition mechanism of Mo6+-doped vanadium dioxide thin films

In the present work using V₂O₅ and MoO₃ powders as precursors, a novel method, the inorganic sol-gel method, was developed to synthesize Mo⁶⁺ doped vanadium dioxide (VO₂) thin films. The structure, valence state, phase transition temperature, magnitude of resistivity change and change in optical transmittance below and above the phase transition of these films are determined by XRD, XPS, four-point probe equipment and spectrophotometer. The results showed that the main chemical composition of the films was VO₂, the structure of MoO₃ in the films didn't change, and the phase transition temperature of the VO₂ was obviously lowered with increasing MoO₃ doped concentration. The magnitude of resistivity change and change in optical transmittance below and above phase transition were also decreased, of which the magnitude of resistivity change was more distinct. However, when the MoO₃ concentration was 5 wt%, the magnitude of resistivity change of doped thin films still reached more than 2 orders, and the change in optical transmittance below and above phase transition was maintained. Analysis showed that the VO₂ doped films formed local energy level, and then reduced the forbidden band gap of VO₂ as the donor defect changing its optical and electrical properties and lowering the phase transition temperature.     

Vanadium dioxide thin film with low phase transition temperature deposited on borosilicate glass substrate

A nanostructured vanadium dioxide (VO₂) thin film showing a low metal-insulator transition temperature of 30 °C has been fabricated through reactive ion beam sputtering followed by thermal annealing. The thin film was grown on borosilicate glass substrate at the temperature of 280 °C with a Si₃N₄ buffer layer. Both scanning electron microscopy and atomic force microscopy images have been taken to investigate the configuration of VO₂ thin film. The average height of the crystallite is 20 nm and the grain size ranges from 40 nm to 100 nm. The transmittance measured from low to high temperatures also reveals that the film possesses excellent switching property in infrared light at critical transition temperature, with switching efficiency of 52% at 2600 nm. This experiment paves the way of VO₂ thin film's application in smart windows.