Metal-insulator Phase Transition Temperature Research Articles

Role of the double-glow plasma pre-sputtering in the growth mechanisms and metal–insulator transition of VO2 film

High phase transition temperature and low utilization rate of sunlight of vanadium dioxide (VO2) have been an important obstacle to expand its application in smart semiconductor materials. In this paper, we propose a novel method by the double-glow plasma pre-sputtering to induce to generate a lot of surface vacancy defects/residual stresses, which can promote the high-quality growth of VO2 thin films on quartz substrates. The resultsshowed that the plasma pretreatment and annealing temperature affected the stability of phase structureto cause the formation of surface residual stresses of VO2films. The optical-electrical properties of the treated VO2 film indicated that the incident light at 2.5 μm has a great transmittance contrast of 63.25% and the solar transmittance modulation efficiency (ΔTsol) of 14.9 %before and after the phase transition. It is found that the metal–insulator phase transition temperature of VO2 after plasma treatment is 39.7℃, which is 20.4℃ lower than that of conventional VO2 film, greatly increases its application in intelligent windows and photoelectric fields. Our research provides an effective and low-cost approach to prepared high-quality VO2 thin films with low phase transition temperature and large-phase-transition amplitude on quartz substrate.

Reconfigurable broadband polarisation conversion metasurface based on VO2

Polarisation is an important characteristic parameter of electromagnetic waves. The controllable adjustment of the polarisation of electromagnetic waves is important for applications in fields such as antenna radiation, satellite communication, and radar stealth. In this study, a reconfigurable metasurface polarisation converter with a square ring structure and an opening was developed based on the reversible metal–insulator phase transition (MIT) of vanadium dioxide (VO2). VO2 is in the insulating high-resistance state (M phase) when its temperature is lower than the MIT temperature (68 °C). Finite element simulations showed that the metasurface could convert linearly polarised waves into cross-polarised reflected waves at frequencies of 8–18 GHz, corresponding to the X and Ku bands. The polarisation conversion rate (PCR) was over 90% and the relative bandwidth was up to 79%. VO2 is in the metallic low-resistance state (R phase) when its temperature is higher than the MIT temperature. At frequencies > 9.4 GHz, the PCR of the metasurface was less than 20%. Under these conditions, the polarisation of most of the reflected waves was the same as that of the incident wave. The polarisation converter was designed to work over a wide frequency range, and thus multiple resonances could be produced based on multiple resonance points in the working frequency range. Moreover, the electromagnetic wave component in the v-direction was rotated along 180°; hence, the polarisation of the incident electromagnetic wave rotated along 90°. When the resistance was low, the phases in the u- and v-directions were basically the same and the electromagnetic waves were reflected in the same direction. A smaller VO2 resistance denotes a higher co-polarisation reflection coefficient (Rxx). Rxx > 80% when the resistance of VO2 < 10 Ω, and Rxx > 90% when the resistance was 1 Ω. The maximum VO2 power loss was achieved when the resistance of VO2 was 188 Ω, i.e., the impedance matched with that of air. The experimental and simulation results were generally consistent. The proposed polarisation converter could have widespread applications in the dynamic control of electromagnetic waves, with uses in antenna radiation and radar cross-section reduction.

Optimizing VO2 film properties for use in SAW devices

The aim of this study is to optimize the conditions for producing thin VO2 films for their use in SAW devices. We used the pulsed laser deposition (PLD) method to produce VO2 films. We used a metallic vanadium target. The dependence of the oxygen pressure during PLD on the resistive metal–insulator transition (MIT) on substrates of [Formula: see text]-sapphire and LiNbO3 YX/128[Formula: see text] was studied. The resulting films had sharp metal–insulator temperature transitions on [Formula: see text]-Al2O3. The most high-quality films showed resistance change by four orders of magnitude. At the lower point of the hysteresis, the resistance of these samples was in the range of 50–100 [Formula: see text]. The synthesized VO2 films had a sharp temperature transition and a relatively small width of thermal hysteresis. The SAW damping during its passage through a VO2/ZnO/LiNbO3 YX/128[Formula: see text] film at the metal–insulator phase transition temperature was studied. Attenuation SAW decreases with increasing temperature from 52 dB/cm to 0 dB/cm.

Boron doped M-phase VO2 nanoparticles with low metal-insulator phase transition temperature for smart windows

Vanadium dioxide (VO2) is considered to be a promising candidate for energy-efficient smart windows because of its special reversible Metal-Insulator Transition (MIT) near the ambient temperature. However, its use is constrained by its high transition temperature (TC) relative to the room temperature. In this paper, VO2 doped by boron, could achieve an outstanding metal-insulator phase transition property with a low TC (28.1 °C) close to the room temperature. This enhancement strongly contributes to the studies of the VO2-based smart windows. A limit doping level of around 9.0 at% is observed for the boron-doped VO2. Moreover, the particle size is getting smaller and more uniform and the particle distribution becomes more equal and compact with the continued increase in the doping content. Such uniform grain size and grain boundary conditions suppress the extension of the hysteresis loop (ΔT decreases from 25 °C to 7 °C). In addition, the TC first declines with the increase in the boron content and it starts to increase after reaching its minima of 28.1 °C at 6.0 at% doping level. This feature is the consequence of the competition between the inhibition on the phase transition caused by the V5+ and the promotion on the phase transition caused by the heterogeneous defect-nucleation sites. VO2 doped with 6.0 at% boron exhibits a favorable thermochromic performance with ΔTsol of 12.5% and Tlum up to 54.3%, which is promising for the smart windows.

Metal–Insulator Phase Transition in Thin Films of a Nickel-Doped Vanadium Dioxide

The electrical conductivity of thin polycrystalline VNixO2 films has been studied in a wide range of temperatures, which covers the regions of both metallic and insulator phase. It is shown that the metal–insulator phase transition temperature decreases as the nickel concentration increases, and the temperature range of coexistence of the phases increases monotonically. The temperature dependence of the conductivity of the insulating VNixO2 phase is explained using the hopping conduction model that takes into account the influence of thermal atomic vibrations on the resonance integral. Parameter e has been calculated as a function of the doping level of VO2.

Фазовый переход металл-диэлектрик в тонких пленках диоксида ванадия, легированного никелем

AbstractThe electrical conductivity of thin polycrystalline VNi_ x O_2 films has been studied in a wide range of temperatures, which covers the regions of both metallic and insulator phase. It is shown that the metal–insulator phase transition temperature decreases as the nickel concentration increases, and the temperature range of coexistence of the phases increases monotonically. The temperature dependence of the conductivity of the insulating VNi_ x O_2 phase is explained using the hopping conduction model that takes into account the influence of thermal atomic vibrations on the resonance integral. Parameter ε has been calculated as a function of the doping level of VO_2.

Localization induced by pressure in pyrochlore Bi2Ir2O7

In this work, the resistivity and magnetization of Bi2Ir2O7 are investigated under hydrostatic pressure. At ambient pressure, the resistivity of Bi2Ir2O7 exhibits a metallic behavior with the decrease of temperature. When the pressure is applied, a metal-insulator phase transition at low temperature is induced under a pressure of ∼ 0.48GPa. The metal-insulator phase transition temperature (TMI) increases linearly with pressure as dTMI/dP=3.4±0.3K/GPa. The temperature dependence of resistivity [ρ(T)] in the pressure-induced insulating phase exhibits a thermal activation behavior (ρ=ρ0eΔE/kBT), where the thermal activation energy (ΔE) increases monotonously with the pressure. Meanwhile, the magnetization is enhanced by the pressure, which indicates an enhancement of magnetic ordering. The results suggest that localization occurs due to the magnetic ordering induced by the pressure, which confirms the magneto-electronic coupling in Bi2Ir2O7

Magnetic Effects on the Dielectric and Polarization Properties in BiAlO3 /La0.67 Sr0.33 MnO3 Heterostructure

BiAlO3/La0.67Sr0.33MnO3 heterostructure was fabricated on LaAlO3 (111) substrate by pulsed laser deposition technology. A remarkable magneto-resistivity effect was detected at H = 1 T near the metal–insulator phase transition temperature (TMI) of La0.67Sr0.33MnO3. The magneto-dielectric constant shows an anomaly near TMI of La0.67Sr0.33MnO3, whereas the ferroelectric polarization increases under magnetic field and its variation shows the similar anomaly near TMI of La0.67Sr0.33MnO3, which could be assigned to the important role of the phase transformation of La0.67Sr0.33MnO3. The magnetic moment of the BiAlO3/La0.67Sr0.33MnO3 heterostructure distinctly arises compared to the single La0.67Sr0.33MnO3 layer, probably on account of the spin–orbit coupling effect at the interface, and the relative magnetization variation also shows an anomaly near TMI of La0.67Sr0.33MnO3.

High contrast switchability of VO2 based metamaterial absorbers with ITO ground plane.

A metamaterial consisting of an array of gold micro-disks, separated from a ground plane of indium tin oxide (ITO) by a thin film of vanadium dioxide (VO2), behaves as a perfect absorber at infrared (IR) frequencies at room temperature. This metamaterial, which is transparent to visible light, can be switched to a highly reflecting state for IR light by heating the metamaterial to temperatures larger than the metal-insulator phase transition temperature 68°C of VO2. For a disk diameter of 1.5 μm and VO2 film thickness of 320 nm, two absorption bands are obtained: one, that arises from the metamaterial resonance; and a second peak that arises principally from a Fabry-Pérot resonance. A large change (>78%) occurs in the reflectivity between the low and high temperature phases. IR emittance of the metamaterial was measured with IR cameras and shown to be switchable to result in low emittance at high temperature. Optical readout of the state of VO2 within the metamaterial is demonstrated.

Magnetism variations and susceptibility hysteresis at the metal-insulator phase transition temperature of VO2 in a composite film containing vanadium and tungsten oxides

We report on the magnetic property of 0.67–WO3+0.33–VOx mixture film deposit on the corning glass substrate using the chemical sol–gel and atmospheric pressure chemical vapor deposition (APCVD) methods. The XRD and Raman spectroscopy confirm species of both materials, and the morphological studies with FIB-SEM and TEM reveal segregation of W and V atoms. XPS reveals that V4+ from VO2 forms only 11% of the film; V3+ in the form of V2O3 form 1% of the film, 21% is V5+ from V2O5 and 67% is given to W6+ from WO3. The analysis of the ESR data shows some sharp changes in the magnetism near the metal-to-insulator (MIT), which could be theoretically interpreted as the ordering or alignment of electron spins from net moment nature to parallel alignment of magnetic moment. The derivatives of magnetic susceptibility established the thermally induced magnetic property: two distinct transitions of 339 K for heating data and 338 K for cooling data for 151.2 mT field were obtained. Similar results were also obtained for 308.7 mT field, 336 K for heating data and 335 K for cooling data. VSM results confirm a paramagnetic phase with a small amount of magnetically ordered phase.

Transport properties of a ferroelectric tunnel junction in bilayer ferroelectric/manganite structures

This paper presents the results of an experimental investigation of current flow through a ferroelectric nanolayer in a series of bilayer ferroelectric/manganite structures prepared by metalorganic aerosol deposition on a MgO substrate. It has been shown that a variation in the thickness of the ferroelectric layer in a bilayer ferroelectric/manganite structure leads to a change in the metal-insulator phase transition temperature of the manganite layer and also affects the transport properties of the ferroelectric/manganite structure. Based on the analysis of the experimental results, it is demonstrated that the ferroelectric layer up to 4 nm thick exhibits polarization properties. In the theoretical analysis, it is assumed that electrical conduction of bilayer structures is provided by a dual mechanism combining the electron tunneling through a ferroelectric barrier and the ohmic current flow in a manganite layer. The possibility of retaining the mechanism of charge carrier tunneling in a bilayer ferroelectric/manganite structure through the ferroelectric layer up to 6 nm thick is discussed.

Epitaxial growth and structural transition of VO2/MgF2(001)

We fabricated epitaxial VO2 thin films on MgF2(001) substrates by pulsed laser deposition. Epitaxial films were obtained at deposition temperatures below 520 °C, whereas prominent interdiffusion between VO2 and MgF2 was observed at higher temperatures. We found that epitaxial strain controlled by the deposition temperature results in the shift of the phase transition temperature of the VO2 films. The structural phase transition temperature evaluated by means of X-ray diffraction is coincident with the metal–insulator phase transition temperature.

Growth and phase transition characteristics of pure M-phase VO2 epitaxial film prepared by oxide molecular beam epitaxy

VO2 epitaxial film with large size has been prepared by oxide-molecular beam epitaxy method on Al2O3 (0001) substrate. The VO2 film shows a perfect crystal orientation, uniformity, and distinct metal-insulator phase transition (MIT) characteristics. It is observed that the MIT character is closely associated with the crystal defects such as oxygen vacancies. By controlling the growth condition, the MIT temperature can be tuned through modifying the content of oxygen vacancies. The role of the oxygen vacancies on the phase transition behavior of this VO2 film is discussed in the framework of the hybridization theory and the valence state of vanadium.

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.

Effects of lattice deformation on magnetic properties of electron-doped La0.8Hf0.2MnO3 thin films

The lattice deformation effects on electric and magnetic properties of electron-doped La0.8Hf0.2MnO3 (LHMO) thin films have been systematically investigated. LHMO films with various thicknesses (15 nm, 40 nm, and 80 nm) were grown on (001) SrTiO3 and (001) LaAlO3 substrates, which induces in-plane tensile and compressive biaxial stress, respectively. The metal-insulator phase transition temperature (TP) and magnetoresistance (MR) effect show a strong dependence on film thickness. TP increases with a decrease in thickness and is enhanced as the lattice strain rises, regardless of whether it is tensile or compressive. The maximum MR ratio is suppressed by reduction of the film thickness. These anomalous phenomena may be attributed to the competition between the strain induced modification of the Mn-O bond length and the eg orbital stability.

Structural and magnetic inhomogeneity, phase transitions, magnetoresonance and magnetoresistive properties of La0.6 − x Pr x Sr0.3Mn1.1O3 (x = 0–0.6)

Ceramic samples of manganite perovskites La0.6 − xPrxSr0.3Mn1.1O3 (x = 0−0.6) have been studied using the X-ray diffraction, resistive, magnetic (χac, 55Mn NMR), microscopic, and magnetoresistive methods. It has been found that an increase in the praseodymium concentration x leads to a transition from the rhombohedral R\(\bar 3\)c (x = 0–0.3) to orthorhombic Pbnm (x = 0.4–0.6) perovskite structure. It has been shown that the real perovskite structure contains anion and cation vacancies, whose concentrations increase with an increase in the praseodymium concentration x. A decrease in the metal-insulator phase transition temperature Tmi and the ferromagnetic-paramagnetic phase transition temperature Tc with increasing x correlates with an increase in the concentration of vacancies weakening the high-frequency electronic exchange Mn3+ ↔ Mn4+. For compositions with x = 0 and 0.1, when the lattice contains not only vacancies but also nanostructured clusters with Mn2+ in the A-positions, there is an anomalous hysteresis. An analysis of the asymmetrically broadened 55Mn NMR spectra of the compounds has revealed a high-frequency electronic exchange of the ions Mn3+ Mn4+ in the B-positions and a local heterogeneity of their surrounding by other ions (La2+, Pr3+, Sr2+) and vacancies. The phase diagram has demonstrated that there is a strong correlation between the composition, imperfection of the perovskite structure, phase transition temperatures Tmi and Tc, and magnetoresistive properties.

Metal insulator transition of bulk and nanocrystalline La1−xCaxMnO3 perovskite manganite materials through in-situ ultrasonic measurements

Abstract Bulk and nanocrystalline La 1 − x Ca x MnO 3 ( x = 0.75, 0.80 and 0.85) perovskite manganite samples were prepared by using the solid-state reaction and sonochemical reactor methods, respectively. The X-ray diffraction studies confirmed that the prepared bulk and nanoperovskite manganite samples were crystalline in nature with an orthorhombic crystal structure. The microscopic studies revealed that the particle sizes of the bulk and nanoperovskite samples were 0.75–0.90 μm and 120–160 nm, respectively. In addition, ultrasonic velocities and attenuations of the bulk and nanosamples were measured by using the in-situ ultrasonic through transmission method. The temperature-dependent ultrasonic parameters showed an interesting anomaly in both bulk and nanoperovskite samples. The observed marked softening and hardening in sound velocities or attenuation were related to metal–insulator phase transition temperature. In nanosamples, the metal insulator transition took place at a lower temperature compared with the corresponding bulk samples. The observed results confirmed that nanoperovskite samples have a broad transition compared with the corresponding bulk samples. The observed broad peak at transition temperature in nanoperovskite samples was used to confirm the nanocrystalline nature of perovskites.

Elucidating the Influence of Local Structure Perturbations on the Metal–Insulator Transitions of V1–xMoxO2 Nanowires: Mechanistic Insights from an X-ray Absorption Spectroscopy Study

The substitutional doping of Mo within VO2 substantially alters the electronic and structural phase diagrams of the host lattice, most notably by bringing the technologically relevant metal–insulator phase transition temperature in closer proximity to room temperature. Here, we have used X-ray absorption fine structure (XAFS) spectroscopy at V and Mo K-edges to examine the local electronic and geometric structure of both the dopant atoms and the host lattice. A nominal Mo oxidation state of +5 has been determined, which implies electron doping of the VO2 band structure. In addition, XAFS studies suggest that doping with Mo creates locally symmetric domains that are more akin to the high-temperature rutile phase of VO2, thereby lowering activation energy barriers for structural transformation to the metallic phase. Substantial rectification of octahedral canting is also observed upon Mo doping, which has the effect of decreasing V 3d–O 2p hybridization and likely assists in closing the characteristic band gap of the low-temperature monoclinic phase. A correlated set of cationic interactions is seen to emerge with increasing Mo doping, which can be ascribed to local dimerization along the rutile c axis as has been proposed to be a characteristic structural feature of a correlated metallic phase with intermediate mass.

Thermochromic VO2 Thin Films: Solution-Based Processing, Improved Optical Properties, and Lowered Phase Transformation Temperature

This paper describes a solution-phase synthesis of high-quality vanadium dioxide thermochromic thin films. The films obtained showed excellent visible transparency and a large change in transmittance at near-infrared (NIR) wavelengths before and after the metal-insulator phase transition (MIPT). For a 59 nm thick single-layer VO(2) thin film, the integral values of visible transmittance (T(int)) for metallic (M) and semiconductive (S) states were 54.1% and 49.1%, respectively, while the NIR switching efficiencies (DeltaT) were as high as 50% at 2000 nm. Thinner films can provide much higher transmittance of visible light, but they suffer from an attenuation of the switching efficiency in the near-infrared region. By varying the film thickness, ultrahigh T(int) values of 75.2% and 75.7% for the M and S states, respectively, were obtained, while the DeltaT at 2000 nm remained high. These results represent the best data for VO(2) to date. Thicker films in an optimized range can give enhanced NIR switching efficiencies and excellent NIR blocking abilities; in a particularly impressive experiment, one film provided near-zero NIR transmittance in the switched state. The thickness-dependent performance suggests that VO(2) will be of great use in the objective-specific applications. The reflectance and emissivity at the wavelength range of 2.5-25 microm before and after the MIPT were dependent on the film thickness; large contrasts were observed for relatively thick films. This work also showed that the MIPT temperature can be reduced simply by selecting the annealing temperature that induces local nonstoichiometry; a MIPT temperature as low as 42.7 degrees C was obtained by annealing the film at 440 degrees C. These properties (the high visible transmittance, the large change in infrared transmittance, and the near room-temperature MIPT) suggest that the current method is a landmark in the development of this interesting material toward applications in energy-saving smart windows.

Resistance and magnetization of La0.7Ca0.3MnO3 bulk at different resistance states induced by electric pulses

Electric-pulse-induced resistance (EPIR) switching behaviour was observed in bulk La0.7Ca0.3MnO3 polycrystalline. Temperature dependences of resistance and magnetic moment at different resistance states were studied. The metal–insulator phase transition temperature changed with the resistance states, which suggested that the EPIR phenomenon is related to the modulating bulk properties of perovskite manganites. The Curie temperature derived from the magnetic moment measurement did not change with the resistance states, which implied that there was no ferromagnetic–metal and antiferromagnetic–insulator transition occurring in perovskite manganite global materials. These results provided a new evidence for a filamentary mechanism in the EPIR effect.