Phase transition of single crystal VO2(R) nanorods in solution revealed by reversible change in surface charge state and structure

The VO₂ single crystal nanorods were synthesized by hydrothermal method and investigated by XRD, SEM, SPM and TEM. Phase transition revealed by a reversible surface charge transfer for VO₂ nanorods during heating-cooling circle in solution was observed by zeta potential measurement. The monoclinic VO₂ nanorods were negatively charged because the exposed crystal surface planes are mostly composed of oxygen atoms as obtained by structure simulation, while the corresponding crystal planes change to a mostly positively charged state in VO₂(R) during phase transition since the exposed planes change into V-atom dominated ones. The study provided a novel method to characterize the phase transition in solution for VO₂ nanocrystals, and the phenomenon may have important potential applications.     

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.     

Chemical Modulation of Metal–Insulator Transition toward Multifunctional Applications in Vanadium Dioxide Nanostructures

The metal–insulator transition (MIT) of vanadium dioxide (VO₂) has been of great interest in materials science for both fundamental understanding of strongly correlated physics and a wide range of applications in optics, thermotics, spintronics, and electronics. Due to the merits of chemical interaction with accessibility, versatility, and tunability, chemical modification provides a new perspective to regulate the MIT of VO₂, endowing VO₂ with exciting properties and improved functionalities. In the past few years, plenty of efforts have been devoted to exploring innovative chemical approaches for the synthesis and MIT modulation of VO₂ nanostructures, greatly contributing to the understanding of electronic correlations and development of MIT-driven functionalities. Here, this comprehensive review summarizes the recent achievements in chemical synthesis of VO₂ and its MIT modulation involving hydrogen incorporation, composition engineering, surface modification, and electrochemical gating. The newly appearing phenomena, mechanism of electronic correlation, and structural instability are discussed. Furthermore, progresses related to MIT-driven applications are presented, such as the smart window, optoelectronic detector, thermal microactuator, thermal radiation coating, spintronic device, memristive, and neuromorphic device. Finally, the challenges and prospects in future research of chemical modulation and functional applications of VO₂ MIT are also provided.     

Electrically tunable sign of capacitance in planar W-doped vanadium dioxide micro-switches

Abstract

Negative capacitance (NC) in a planar W-doped VO₂ micro-switch was observed at room temperature in the low-frequency range 1 kHz–10 MHz. The capacitance changed from positive to negative values as the W-doped VO₂ active layer switched from semiconducting to metallic state under applied voltage. In addition, a capacitance–voltage hysteresis was observed as the applied voltage was cycled from ?35 to 35 V. These observations suggest that NC results from the increase of the electrically induced conductivity in the active layer. This NC phenomenon could be exploited in advanced multifunctional devices including ultrafast switches, field-effect transistors and memcapacitive systems.     

钨掺杂二氧化钒薄膜的THz波段相变性能的研究

通过溶胶–凝胶法制备纯的VO₂和W掺杂的VO₂薄膜, 并且进行了XPS、AFM和XRD的分析与表征, 并观察了其微观形貌和结构. 同时研究了VO₂和W掺杂VO₂在红外光谱(λ=4 μm)和THz(0.3~1.0 THz)区域的金属–绝缘转变性能. 结果表明: 室温下W掺杂的VO₂薄膜在红外和THz区域的初始透过率都比纯的VO₂薄膜低. 在THz波段, W掺杂的VO₂表现出更低的相变温度. 同时在VO₂和W掺杂VO₂相变过程中, 观察到了金属–绝缘转变和结构转变的现象, W掺杂VO₂具有明显的峰位偏移现象.     

Porous nano-structured VO2 films with different surfactants: synthesis mechanisms, characterization, and applications

Porous nano-structured vanadium dioxide (VO₂) thin films have been prepared on mica substrates via sol–gel process using surfactant cetyltrimethyl ammonium bromide, nonionic surfactant polyethylene glycol, and anionic surfactant sodium dodecyl sulfate as nano-structure directing agents. Models concerning the structure forming were proposed to explain the synthesis mechanisms between V₂O₅ colloid and different surfactants. Porous nano-structured VO₂ films with sphere-shaped, island-shaped and strip-shaped nanocrystals are synthesized in the experiments, and the optical properties and thermochromic properties of these films are compared. The porous nano-structured VO₂ films showed excellent infrared transmittance (nearly 70 %), low transition temperature (59.7 °C without doping), wide hysteresis width (37.8 °C), and different optical transmittance difference before and after the phase transition (39–67 %). The results suggest that these porous nano-structured VO₂ films have significant importance in practical application in VO₂-based optical and electronic devices.     

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.     

Ramp‐Reversal Memory and Phase‐Boundary Scarring in Transition Metal Oxides

Transition metal oxides are complex electronic systems that exhibit a multitude of collective phenomena. Two archetypal examples are VO₂ and NdNiO₃, which undergo a metal–insulator phase transition (MIT), the origin of which is still under debate. Here this study reports the discovery of a memory effect in both systems, manifested through an increase of resistance at a specific temperature, which is set by reversing the temperature ramp from heating to cooling during the MIT. The characteristics of this ramp‐reversal memory effect do not coincide with any previously reported history or memory effects in manganites, electron‐glass or magnetic systems. From a broad range of experimental features, supported by theoretical modelling, it is found that the main ingredients for the effect to arise are the spatial phase separation of metallic and insulating regions during the MIT and the coupling of lattice strain to the local transition temperature of the phase transition. We conclude that the emergent memory effect originates from phase boundaries at the reversal temperature leaving “scars” in the underlying lattice structure, giving rise to a local increase in the transition temperature. The universality and robustness of the effect shed new light on the MIT in complex oxides.     

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.     

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.     

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.     

Oxidation potential control of VO2 thin films by metal oxide co-sputtering

For metal-to-insulator transition (MIT) in vanadium oxide thin film, a thermodynamically stable vanadium dioxide (VO₂) phase is essential. In VO₂ films sputter-deposited on a quartz substrate from a V₂O₅ target, a radio-frequency (RF) magnetron sputter system at working pressure of 7 mTorr is used. Due to the lower sputtering yield of oxygen compared to vanadium leading to oxygen-ion deficiency, the reduction of V ions is resulted to compensate charge with the oxygen ions. Under lower working pressures, the deposition rate increases, but a simultaneous oxygen-ion deficiency causes the destabilization of VO₂. To prevent this, titanium oxide co-deposition is suggested to enrich the oxygen source. When TiO₂ is used, it is found that the Ti ion has a stable +4 charge state so that the use of extra oxygen in sputtering prevents the destabilization of VO₂. However, this is not the case for TiO. For the latter, Ti ions are oxidized from the +2 state to the +3 and +4 states, and V ions with less oxidation potential are reduced to +3 or so. Pure VO₂ thin film exhibits MIT at 66 °C and a large resistivity ratio of four orders of magnitude from 30 to 90 °C. The (V₂O₅ + TiO₂) system under working pressure as low as 5 mTorr yields fairly good films comparable to pure VO₂ deposited at 7 mTorr, whereas the use of TiO yields films with MIT absent or considerably weakened.     

A 1D Vanadium Dioxide Nanochannel Constructed via Electric‐Field‐Induced Ion Transport and its Superior Metal–Insulator Transition

Nanoscale manipulation of materials' physicochemical properties offers distinguished possibility to the development of novel electronic devices with ultrasmall dimension, fast operation speed, and low energy consumption characteristics. This is especially important as the present semiconductor manufacturing technique is approaching the end of miniaturization campaign in the near future. Here, a superior metal–insulator transition (MIT) of a 1D VO₂ nanochannel constructed through an electric‐field‐induced oxygen ion migration process in V₂O₅ thin film is reported for the first time. A sharp and reliable MIT transition with a steep turn‐on voltage slope of <0.5 mV dec^?1, fast switching speed of 17 ns, low energy consumption of 8 pJ, and low variability of <4.3% is demonstrated in the VO₂ nanochannel device. High‐resolution transmission electron microscopy observation and theoretical computation verify that the superior electrical properties of the present device can be ascribed to the electroformation of nanoscale VO₂ nanochannel in V₂O₅ thin films. More importantly, the incorporation of the present device into a Pt/HfO₂/Pt/VO₂/Pt 1S1R unit can ensure the correct reading of the HfO₂ memory continuously for 10⁷ cycles, therefore demonstrating its great possibility as a reliable selector in high‐density crossbar memory arrays.     

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.     

Fabrication of thermochromic composite using monodispersed VO2 coated SiO2 nanoparticles prepared by modified chemical solution deposition

Monodispersed thermochromic VO₂ particles were fabricated by VO₂ coating onto monodispersed SiO₂ nanoparticles with the modified chemical solution deposition technique using vanadium isopropoxide solution and monodispersed SiO₂ particle suspension solution. The average size of the resultant VO₂–SiO₂ particle was 57 nm and the coating thickness of the VO₂ layer was 6 nm. A thermochromic composite was fabricated using the VO₂–SiO₂ particles and a poly lactose acid polymer as a transparent matrix, and the transmittance of the composite at a high temperature was 10% less than that at a low temperature.     

Controlled synthesis and electrochemical properties of vanadium oxides with different nanostructures

Vanadium oxides (V₃O₇·H₂O and VO₂) with different morphologies have been selectively synthesized by a facile hydrothermal approach using glucose as the reducing and structure-directing reagent. The as-obtained V₃O₇·H₂O nanobelts have a length up to several tens of micrometers, width of about 60?C150?nm and thickness of about 5?C10?nm, while the as-prepared VO ₂ (B) nanobelts have a length of about 1·0?C2·7???m, width, 80?C140?nm and thickness, 2?C8?nm. It was found that the quantity of glucose, the reaction temperature and the reaction time had significant influence on the compositions and morphologies of final products. Vanadium oxides with different morphologies were easily synthesized by controlling the concentration of glucose. The formation mechanism was also briefly discussed, indicating that glucose played different roles in synthesizing various vanadium oxides. The phase transition from VO₂(B) to VO₂(M) were investigated and the phase transition temperature of the VO₂(M) appeared at around 68 °C. Furthermore, the electrochemical properties of V₃O₇·H₂O nanobelts, VO₂(B) nanobelts and VO₂(B) nanosheets were investigated and they exhibited a high initial discharge capacity of 296, 247 and 227 mAh/g, respectively.     

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。     

Fabrication, structural and electrical characterization of VO2 nanowires

The structural and electrical properties of VO₂ nanowires synthesized on Si₃N₄/Si substrates or molybdenum grids by a catalyst-free vapour transport method were investigated. The grown VO₂ nanowires are single crystalline and rectangular-shaped with a preferential axial growth direction of [1 0 0], as examined with various structural analyses such as transmission electron microscopy, electron diffraction, X-ray diffraction, and X-ray photoelectron spectroscopy. In particular, it was found that growing VO₂ nanowires directly on Si₃N₄ deposited molybdenum transmission electron microscopy grids is advantageous for direct transmission electron microscopy and electron diffraction characterizations, because it does not involve a nanowire-detachment step from the substrates that may cause chemical residue contamination. In addition to structural analyses, VO₂ nanowires were also fabricated into field effect transistor devices to characterize their electrical properties. The transistor characteristics and metal-insulator transition effects of VO₂ nanowires were investigated.     

Molybdenum-doped vanadium dioxide coatings on glass produced by the aqueous sol-gel method

Thin films of vanadium dioxide (VO₂) on glass substrates were produced by the aqueous sol-gel method. Various levels of doping were achieved by adding small quantities of a water-soluble molybdenum compound to the sol. After dip coating, the substrates were reduced by heat treatment in a low-pressure carbon monoxide/carbon dioxide (CO/CO₂) atmosphere. The change in electrical conductivity with temperature, and optical reflectance in the semiconductor and metallic phases were measured and compared to undoped VO₂ films. Doping the VO₂ films with molybdenum lowered the transition temperature of the semiconductor-to-metal phase change; at a doping level of 7 at.% the transition temperature was measured at 24 °C, as indicated by the electrical conductivity. All the films showed a substantial change in reflectance upon heating through the transition. The optical reflectance in the semiconductor state increased slightly with additional dopant, while the reflectance in the metallic state remained constant.     

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.