Insulator Transition Temperature Research Articles

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

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

Terahertz beam reconfigurable phase gradient metasurface of VO2 based on different metal–insulator transition temperatures

Terahertz beam reconfigurable phase gradient metasurface of VO2 based on different metal–insulator transition temperatures

Metastable marvels: Navigating VO2 polymorphs for next-gen electronics and energy solutions

VO2 polymorphs present a unique opportunity to unravel diverse electronic properties possessed by their metastable phases. A highly reproducible, single-phase, and inexpensive synthesis method is challenging for obtaining VO2 polymorphs. Recent years have witnessed some exciting success in the growth and application of a wide range of VO2 polymorphs. This comprehensive review article delves into different polymorphs, including VO2(x) (x = A, B, M, R, C, P, and D), and investigates their distinct physical attributes. The primary focus of this article centers on providing a thorough overview of the recent progress made in stabilizing VO2(A) and VO2(B) polymorphs, emphasizing the significance of the coexistence of nanodomains at the film–substrate interface in stabilizing specific metastable phases. Additionally, the review article delves into advancements in understanding the phase transition mechanism, adjusting the order parameter in resistivity, and modifying the metal–insulator transition (MIT) temperature through doping. It also summarizes the structural, optical, electronic, and interface properties of these polymorphs and highlights their potential applications in next-generation electronic devices, particularly in the fields of sensing and energy storage.

Large temperature coefficient of resistivity (TCR) of La0.67Ca0.33MnO3 polycrystalline ceramics improved by optimizing sintering temperatures

This work investigates fabrication of La0.67Ca0.33MnO3 (LCMO) polycrystalline ceramics using a facile sol–gel method at sintering temperatures (Ts) ranging from 1470 °C to 1540 °C and explores relationship between their electrical transport properties and Ts. X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and X-ray photoemission spectroscopy were employed to characterize crystal structure, morphologies, element distributions, and Mn3+/Mn4+ ratio of the as-obtained LCMO ceramics. The influence of Ts on phase purity, microstructure, morphology, and electrical transport properties was also investigated. LCMO ceramics exhibit increased grain size and higher temperature coefficient of resistivity (TCR) at higher temperatures. An increase in Ts can reduce the strength of electron–lattice interactions and theoretical Curie temperature in LCMO ceramic samples, which can in turn optimize their electrical transport properties. Moreover, an increase in Ts enhances double-exchange (DE) effect to some extent. The DE effect further affects resistivity and metal–insulator transition temperature of material. Additionally, small-polaron hopping model and phenomenological percolation model were used to characterize electrical transport in metal–insulator transition region. The temperature at which TCR is maximum (265.78 K) and the largest TCR value (58.16 %) were achieved at a Ts of 1530 °C. By modulating the sintering temperature, we achieved LCMO ceramics with larger grains and enhanced electrical properties. This optimization renders materials suitable for highly sensitive infrared bolometer applications.

Exploring the Magnetism and Magnetocaloric Effect of W‐Doped VO2 Based on J1–J2 Ising Model

W‐doped VO2 exhibits a significant reduction in the metal–insulator transition temperature. In this study, the authors aim to investigate the magnetic influences of different proportions of W doping in VO2 with a rutile structure, using the J1–J2 model. The J1 and J2 values are calculated as 1.21 and −0.024 eV, respectively, using the generalized gradient approximation (GGA)+U method. Additionally, the J1–J2 model without W doping (x = 0) is simulated using the Ising model, and the obtained phase transition temperature of approximately 361 K closely matches the temperature calculated by Zheng et al. based on the spin dimer model. Furthermore, employing this model, the magnetization curves and magnetic entropy change (ΔS) are simulated for different W doping levels (x = 0–0.2). Through comprehensive analysis, it is found that the system exhibits the most promising research potential at a doping level of x = 0.12. These findings deepen the understanding of the mechanisms underlying the magnetic effects of W‐doped VO2 and provide a theoretical model guidance for future development of vanadium oxide materials.

Size-induced metal–insulator transition temperature decrease of VO2 nanoparticles obtained by laser irradiation in water

Vanadium dioxide (VO2) is a highly attractive and multifunctional material for technological applications due to its metal–insulator transition, which is accompanied by several orders of magnitude changes in electrical resistivity and optical transmissivity. In this work, we report on the metal–insulator transition of VO2 nanoparticles which are produced by a simple, efficient, and low-cost two-step preparation method based on infrared laser irradiation of V2O5 powder dispersed in water and subsequent stoichiometric stabilization by vacuum annealing at 773 K. VO2 nanoparticles have an average size of 36 nm with facet-like contours and undergo a structural phase transition from a monoclinic to a tetragonal phase, accompanied by a metal–insulator transition occurring around 323 K, which is about 17 K lower than bulk VO2. No evidence of the formation of intermediary metastable phases is observed. Our findings reveal that the size of VO2 nanoparticles is a key parameter to moving the transition closer to room temperature, which is important for many applications.

Ion irradiation induced modification in the electrical properties and the electronic structures of vanadium dioxide thin films

Ion irradiation modifies the physical properties of oxide materials. The present study focuses on the effect of 150 keV Fe and Ni ion irradiations in VO2 thin films. These films were deposited by pulsed laser on an r-cut sapphire substrate (1 1‾ 02). The range of ion fluences was from 1 × 1014 to 1 × 1016 ions/cm2 and these samples were characterized by using grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, Temperature-dependent resistivity measurements (R-T), and Hall effect measurement. While the presence of monoclinic phases is confirmed by GIXRD and Raman measurements, the ion irradiation on VO2 modifies the metal to insulator transition temperature (MIT). The R-T measurement with supporting analysis of carrier concentration authenticates a decrease in transition temperature and an increase in conductivity with an increment of ion-fluences. Synchrotron-based X-ray photoemission spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) revealed that Vanadium ions are in mixed oxidation states of 4+ and 5+ after ion irradiation. Further, the Photo-Electron Spectroscopy (PES) measurement indicates the metallization of VO2 after ion irradiation consistent with R-T measurements.

Metal-insulator transition tuned by valence variation of Nb dopants in Nb-doped VO2 films

High-valence cation doping was proved to be an effective method in reducing the high metal–insulator transition (MIT) temperature (θc). However, the mechanism of cation-dopants reducing θc is not quite clear and the reported data about the θc reduction induced by Nb dopants are dispersive. In this work, we prepared Nb-doped VO2 films by hydrothermal process and investigated the effects of Nb dopants on the MIT behavior. Valence variation of Nb dopants was observed in the Nb-doped VO2. The Nb5+ dopants elevate the valence band top to higher level than the Nb4+-containing sample. Correspondingly, the Nb5+ doping reduces θc in a higher rate of ∼21 ℃/at%Nb than the Nb4+-containing sample which has the decline rate of ∼18 ℃/at%Nb. The results imply Nb5+ dopants contributing excess electrons modify the band structure and consequently promoting the MIT transition. In addition, the Nb doping realizes the θc reduction almost without detriment to the optical properties of VO2, suggesting the promising applications in smart window and other optical switching devices.

Effects of Li substitution on structural, magnetic properties and electroresistance behaviour in La0.8Na0.2-xLixMnO3 monovalent-doped manganite

The effects of Li substitution on structural, magnetic properties, and electroresistance (ER) behaviour in La0.8Na0.2-xLixMnO3 (x = 0 – 0.15) manganites prepared via the conventional solid-state method were reported. All samples exhibited a metal–insulator transition behaviour with a decrease in the metal–insulator transition temperature, TMI from 270 K (x = 0) to 255 K (x = 0.15) and reduction in Curie temperature, TC from 272 K (x = 0.10) to 235 K (x = 0.15). All samples exhibited a reduction in resistivity under higher applied current of 5 mA than that of 1 mA, which led to an ER effect. Large ER value of −59 % is achieved for sample × = 0.15 at 300 K, thereby indicating its potential application for spintronic-based devices. The competition between the occurrence of lattice distortion and the presence of magnetic inhomogeneity induced by A-site disorder (σ2) is suggested responsible for the observed ER effect.

Magneto transport behavior of La0.67Pb0.33MnO3manganite system with various additives

The nanostructured manganites show low field magnetoresistance, where grain boundary effects influence the phenomenon. The current investigation aims to comprehend the impact of the grain boundaries on the magnetic and magneto transport characteristics of La0.67Pb0.33MnO3 manganite with 1% of additives ZrO2, Bi2O3, and Li2CO3. Two step synthesis processes was adopted to prepare the pure and additive samples. The samples were characterized with XRD, XPS and FESEM to establish the structure, phase purity, oxidation states, elemental analysis and morphology respectively. The prepared samples were formed in single-phase rhombohedral structure with the enhanced crystallite and grain size with additives when compared to the pure sample. The observed magnetic transition temperature, are well above room temperature (TC = 353 K) and a soft ferromagnetic nature with 70% of ferromagnetic fraction is noticed at T = 300 K for all the studied samples. The metal insulator transition temperature (TP) remains invariant for ZrO2, Bi2O3 additives, while it reduces for Li2CO3 additive. It is observed that 1% of additives are acting as conduction barriers at the grain boundaries as a consequence there is an increase in resistivity. The contribution to magnetoresistance is seen as intrinsic and extrinsic in pure and additive materials exhibiting high percentage both at low and room temperature. All the sample shows an appreciable temperature coefficient of resistance percentage at their respective transition temperatures which can be made useful for bolometric applications. Conduction mechanisms in the ferromagnetic and paramagnetic phases were examined with the electron-magnon scattering and small polaron hopping model respectively.

Wide temperature range magnetoresistance enhancement of La0.67Ca0.33MnO3: NiO nanocomposites

This paper reports the addition of NiO nanoparticles into the LCMO nanocomposites through a simplified methodology in the heat treatment process compared to the previous work. Structural analysis showed that LCMO and NiO co-existed in the samples without the formation of other phases and confirmed their composite structures. The grain size distribution of samples was observed to become narrower and left-skewed as the concentration of NiO nanoparticles increased. This indicates that the NiO nanoparticles were distributed near the grain boundaries or on the grain surfaces. The presence of La, Ca, Mn, O, and Ni was confirmed by EDX analysis, and no additional impurities were noticeable, which revealed that the composites here are chemically pure in the composition. Temperature dependence magnetisation measurement (100–300 K) revealed the occurrence of ferromagnetic to paramagnetic transition. The Curie temperature, TC recorded at 260 K for all samples. However, the metal–insulator transition temperature, TMI was found to decrease from 246 to 120 K as the NiO content increased. Magnetoresistance of composites was observed to enhance over a wide temperature range at 10 kOe. The findings of LCMO: NiO composites in this work can set up a new perspective and unlock its potential to be utilised in magnetic field sensor element functioning at a wide range of temperatures.

Analysis of the temperature dependent optical properties of V1−xWxO2 thin films

Vanadium dioxide is one of the most promising materials that can be used to control and tune in real time the optical properties of nanoscale devices due to its ability to perform a Metal-Insulator Transition (MIT). In this paper, we report the optical and structural properties of undoped and W-doped V1-xWxO2 vanadium dioxide thin films deposited on oxidized silicon substrates using magnetron sputtering. The structural properties determined by X-ray diffraction showed not only the transition from a monoclinic to a rutile phase when crossing the metal–insulator transition temperature (TMIT) but also a coexistence of phases for the tungsten-doped VO2 even at room temperature. It is also shown that TMIT decreases from 69.6°C to 41.6°C as x increases from 0 to 3%. A point-by-point fitting method is proposed to reliably extract the refractive index and extinction coefficient from spectroscopic ellipsometry measurements as a function of temperature and tungsten concentration. This approach avoids all the correlations between the thickness and the refractive index well known in absorbent films calling into question the uniqueness of the results. The dielectric function is then calculated from the point by point refractive index. A model based on Tauc-Lorentz oscillators in the insulating phase to which a Drude component is added in the metallic phase provides information on the levels of the energy bands and the plasma frequency of the films. The calculation of reflectance and transmittance of V1-xWxO2 thin films highlights the effect of W doping on thermochromic performance.

Influence of Sr-doping on the metal–insulator transition temperature and magnetoelectric properties of La0.67Ca0.33Mn0.98V0.02O3 polycrystalline ceramics

Influence of Sr-doping on the metal–insulator transition temperature and magnetoelectric properties of La0.67Ca0.33Mn0.98V0.02O3 polycrystalline ceramics

Electrical transport behavior and thermo electric power studies of Nd0.67Sr0.33MnO3/ZrO2 CMR composites

In this work, we reported structural, morphological, electrical, magnetic transport and thermo electric power properties of (1-x) Nd0.67Sr0.33MnO3 - x ZrO2 (abbreviated as NSMO, where × = 0%, 5%, 10%, 15%, and 20% mol.%) composites and were synthesized by a double stage sol–gel method. The X-ray diffraction (XRD) revealed a pure perovskite phase of NSMO. The Scanning electron microscopy (SEM) studies displayed uniform microstructure and the average grain size of NSMO was estimated to be 30 nm and was increased to 80 nm with the substitution of ZrO2. With addition of ZrO2 the metal to insulator transition temperature (Tp) shifted towards lower temperature and the resistivity order of the samples are enhanced from 5 to 10 times. The magnetic transition temperature (Tc) is shifted to higher temperature with increase the ZrO2. The temperature dependent resistivity data is well fitted with polynomial equation, which indicates that the high temperature resistivity behavior has been explored with adiabatic small polaron and variable range hopping (VRH) models.

Low-temperature electrical transport and tunable optical properties of Mo-doped V2O3 thin films

This work reports the structural and low-temperature electrical transport properties of Mo doped V2O3 single-layer thin films (i.e., with compositions between Mo and V2O3 [V2−xMoxO3−δ (x = 0, 0.05–0.1)]) deposited on alumina (Al2O3) substrates by sol–gel dip-coating technique. The crystallinity of the V2O3 films is modified with the high concentration of Mo doping and exhibits a relatively defective structure compared to that of pure V2O3 films. The low temperature (i.e., over a temperature range of 253.15 to 273.15 K) electrical hysteresis curves reveal that the triggering of metal–insulator transition at low temperatures. The metal–insulator transition (MIT) temperature (Tc) values are found to be 258.49 and 261.11 K for 8 and 10 mol% Mo doped V2O3 films, respectively, which are higher than that of undoped V2O3 (Tc = 253.09 K). The experimental results contribute a promising method to develop a novel material for high-performance low-temperature engineering applications.

Structural, Magnetic, and Electrical Properties and Magnetoresistance of Monovalent K-Substituted La0.7Ba0.3−xKxMnO3 (x = 0 and 0.04) Manganite

The effects of K+ substitution at the Ba-site on the structural, magnetic, and electrical properties and magnetoresistance (MR) of La0.7Ba0.3−xKxMnO3 (x = 0 and 0.04) manganites prepared via the solid-state method were investigated. Rietveld refinement of X-ray diffraction data confirmed that both samples were crystallized in the rhombohedral structure with the R3c¯ space group. In addition, the unit cell volume, V, and the average grain size also increased with K+ ions. Magnetization versus applied field (M–H) measurement was carried out, and the saturation magnetization (Ms) was found to increase from 1.81 μB/f.u. (x = 0) to 4.11 μB /f.u. (x = 0.04), implying that K+ ions strengthened the ferromagnetic (FM) interaction. Furthermore, the metal–insulator transition temperature, TMI, increased from 257 K (x = 0) to 271 K (x = 0.04). The observed behaviour may be related to the enhancement of double-exchange (DE) interaction due to the increase in Mn-O-Mn bond angle and electronic bandwidth (W), favouring the increasing rate of the eg electron hopping process. The fitting of the electrical resistivity data in the metallic region describes the significance of residual resistivity, electron–electron and electron–magnon scattering processes to elucidate the electronic transport properties. Within the insulating region, variable range hopping (VRH) and small polaron hopping (SPH) models are proposed to describe the conduction mechanism.

Electrical and dielectric properties of polycrystalline VO2 discriminating between bulk and grain boundary conduction

Electrical and dielectric properties were measured on polycrystalline VO2 samples in the neighborhood of the metal → insulator transition (MIT) temperature of ∼340 K, prepared with the ceramic sintering technique. It is shown that in certain temperature ranges in the monoclinic semiconducting M1-phase below the MIT, bulk and grain boundary (GB) effects can be distinguished applying the method of impedance spectroscopy. An enormous literature has accumulated, dealing with electrical properties of VO2 small films and nanowires, with the aim to prepare various kinds of micro-devices for technology. Evidently, investigations to separate bulk from GB effects are largely missing, although many small films and nanowires are polycrystalline. The structure of their GBs doubtless differs from that in macroscopic samples, nevertheless valuable information might be supplied concerning electrical conduction across and along GBs, compared with conduction in the bulk. GB processes in our VO2 samples could be measured between room temperature and the MIT, bulk properties along with GB conduction could only be studied at low temperatures.

Thickness-dependent magnetic and transport properties of La0.7Sr0.3MnO3 thin films grown on Nd0.5Ca0.5MnO3/SrTiO3 epitaxial thin film

We have investigated the proximity effect between the ferromagnetic and the charge-ordered antiferromagnetic bilayer thin film samples. The different thicknesses of the ferromagnetic layer of La0.7Sr0.3MnO3 (LSMO) were deposited on Nd0.5Ca0.5MnO3 (NCMO) layer with a fixed thickness of 90 nm, which is well grown on SrTiO3 (001) substrates. Magnetic and transport measurements show that the ferromagnetic transition and metal to insulator transition temperature decrease with decreasing LSMO thickness, in contrast to the magnetic moments at 70 kOe, and coercivity increases gradually. The transport measurement shows the LSMO thickness down to 5 nm for which LSMO remains insulating at low temperatures. This indicates that the dead layer thickness is 2 nm when LSMO is deposited on NCMO. The 2 nm thick LSMO bilayer thin film exhibits insulating behaviour up to 50 kOe, and the film transforms into a metallic phase and shows a clear metal to insulator transition when a magnetic field ≥70 kOe is applied.

Optimizing phase transition temperature and visible transmittance of VO2 films driven by synergistic effect of La-Mo co-doping

Optimizing the metal–insulator transition temperature (θMIT) and optical switching behavior of vanadium dioxide (VO2) is desirable for boosting thermochromic applications. Ions co-doping engineering has been proved a powerful method in improving the comprehensive performance of VO2. Here, the La-Mo co-doped VO2 films are fabricated on c-cut sapphire substrate by a hydrothermal method. In the case of constant Mo concentration (1.5 at.%), it is found that the increased La dopants contribute to synchronously achieve the reduction of θMIT from 60 °C @ 0 at.% La to 38 °C @ 1 at.% La and the improvement of visible light transmittance. The reduced θMIT along with enhanced luminous transmittance is attributed to the La-Mo co-doping synergetic effect, which effectively reduces the band gap between the V 3d|| and π* band and broadens the band gap between the O 2p and π* band.

Impact of NiO nano-particles on colossal magneto-resistance of La0.70Ca0.30MnO3 composite

Mixed-valent perovskite manganite has gained considerable research interest due to the increasing demand for energy storage materials. Herein, we show that colossal magneto-resistance properties of La0.7Ca0.3MnO3 (LCMO) can be changed via nickel oxide (NiO) doping. A two-step conventional solid-state reaction method is used to synthesize LCMO-10% NiO composite. The phase purity, structural, electrical (resistivity), and magneto-transport properties of as-synthesized LCMO-10% NiO composite are detailed. NiO doping shifts metal–insulator transition temperature (TMI) from 263 K to 195 K and increases resistivity. The magneto-resistance (MR) measurements showed 10% NiO doping resulted in a maximum MR of ∼ -70.32% at 200 K (near TMI) and 10 T applied magnetic field. However, -0.0849% MR is seen at 290 K (RT) and 0.3 T. The increased ∼ -63.85% MR is found at 100 K and at 10 T, which is comparatively much higher than the pristine LCMO sample. This is due to the addition of NiO in LCMO that improves physical properties (i.e., grain size).