Metal-insulator Transition Temperature Research Articles

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.

Reversible Entropy-Driven Defect Migration and Insulator-Metal Transition Suppression in VO2 Nanostructures for Phase-Change Electronic Switching.

Oxygen defects are among essential issues and required to be manipulated in correlated electronic oxides with insulator-metal transition (IMT). Besides, surface and interface control are necessary but challenging in field-induced electronic switching towards advanced IMT-triggered transistors and optical modulators. Herein, we demonstrated reversible entropy-driven oxygen defect migrations and reversible IMT suppression in vanadium dioxide (VO2 ) phase-change electronic switching. The initial IMT was suppressed with oxygen defects, which is caused by the entropy change during reversed surface oxygen ionosorption on the VO2 nanostructures. This IMT suppression is reversible and reverts when the adsorbed oxygen extracts electrons from the surface and heals defects again. The reversible IMT suppression observed in the VO2 nanobeam with M2 phase is accompanied by large variations in the IMT temperature. We also achieved irreversible and stable IMT by exploiting an Al2 O3 partition layer prepared by atomic layer deposition (ALD) to disrupt the entropy-driven defect migration. We expected that such reversible modulations would be helpful for understanding the origin of surface-driven IMT in correlated vanadium oxides, and constructing functional phase-change electronic and optical devices.

Oxygen isotope effect in VO2

We investigate the oxygen isotope effect on the ${\mathrm{VO}}_{2}$ metal-insulator transition. Using an alternative method, we synthesize ${\mathrm{V}}^{16}\phantom{\rule{0.16em}{0ex}}{\mathrm{O}}_{2}$ and ${\mathrm{V}}^{18}\phantom{\rule{0.16em}{0ex}}{\mathrm{O}}_{2}$ crystals, finding a 1%--3% phonon softening and a 1.3 K increase in the metal-insulator transition temperature for the latter. A simple calculation, further confirmed by density functional theory, shows that this shift can be attributed to changes in the lattice internal energy. Our results show that lattice dynamics plays a key role in setting the electronic transition temperature, indicating that electronic and structural degrees of freedom remain strongly coupled at the transition.

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.

Vanadyl formate VO(HCOO)2·H2O as a precursor for preparing nanoscale vanadium sesquioxide V2O3

Vanadyl formate monohydrate VO(HCOO)2·H2O has been synthesized by a facile two-stage method. The chemical and structural identity of the synthesized compound has been confirmed by X-ray diffraction, thermogravimetry, vibrational and absorption spectroscopy. It was established that during heating in water and ethylene glycol, VO(HCOO)2·H2O leads to vanadyl hydroxide VO(OH)2 and vanadyl glycolate VO(OCH2CH2O) being formed. When heated in air or in helium at 300 °C and higher, VO(HCOO)2·H2O transforms into vanadium pentoxide or sesquioxide, respectively. VO(HCOO)2·H2O was used as a precursor for the synthesis of nanoscale vanadium sesquioxide (with an average particle size of 50 nm), which is stable under the normal conditions and is characterized by a lower metal-insulator transition temperature than the microsized (bulk) compound. The effect of transition temperature reduction for nanoscale vanadium sesquioxide agrees with the lower value of its unit cell volume (V = 296.74 Å3) as compared with that for bulk V2O3 (297.7 Å3). The main factor affecting the V2O3 particle size is the annealing temperature of precursor in inert atmosphere.

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

Utilization of Ag ions to improve room-temperature TCR of La0.85-Sr0.15Ag MnO3 polycrystalline ceramics

In this work, silver-doped La0.85-xSr0.15AgxMnO3 (LSAMO, 0.05 ≤ x ≤ 0.20) polycrystalline ceramics were prepared by the sol-gel method. The experimental characterization of ceramics revealed that Ag ions were bounded with the lattice matrix. The resistivity and the corresponding peak temperature coefficient (TCR) of LSAMO ceramics were systematically tuned by changing the Ag dopant content. Adjusting the proportion of Ag enabled one to vary the metal-insulator transition temperature (TMI) for LSAMO specimens in a wide range, and their peak resistivity temperature (Tp) was increased from 299.6 to 335.7 K. At the same time, the peak TCR value achieved its maximum of 15.2% K−1 at the doping molar ratio x of 0.15 and the peak resistivity temperature (Tk) of 294.1 K. In addition, the electrical transport was described in the context of the small polaron hopping (SPH) model and the phenomenological percolation model (PP) over the metal-insulator transition region. Under the combined action of the Jahn-Teller (JT) effect, the double exchange (DE) mechanism and the PP model, LSAMO ceramics possessing high TCR at room temperature were obtained by varying the amount of Ag doping. The observed properties suggest LSAMO material can be used in advanced uncooled infrared bolometers.

Physical Origin of Negative Differential Resistance in V3 O5 and Its Application as a Solid-State Oscillator.

Oxides that exhibit an insulator-metal transition can be used to fabricate energy-efficient relaxation oscillators for use in hardware-based neural networks but there are very few oxides with transition temperatures above room temperature. Here the structural, electrical, and thermal properties of V3 O5 thin films and their application as the functional oxide in metal/oxide/metal relaxation oscillators are reported. The V3 O5 devices show electroforming-free volatile threshold switching and negative differential resistance (NDR) with stable (<3% variation) cycle-to-cycle operation. The physical mechanisms underpinning these characteristics are investigated using a combination of electrical measurements, in situ thermal imaging, and device modeling. This shows that conduction is confined to a narrow filamentary path due to self-confinement of the current distribution and that the NDR response is initiated at temperatures well below the insulator-metal transition temperature where it is dominated by the temperature-dependent conductivity of the insulating phase. Finally, the dynamics of individual and coupled V3 O5 -based relaxation oscillators is reported, showing that capacitively coupled devices exhibit rich non-linear dynamics, including frequency and phase synchronization. These results establish V3 O5 as a new functional material for volatile threshold switching and advance the development of robust solid-state neurons for neuromorphic computing.

The novel preparation method of thermochromic VO2 films with a low phase transition temperature by thermal oxidation of V–Mo cosputtered alloy films

Vanadium dioxide (VO2) has been studied extensively for its unique insulator-metal transition characteristics and potential applications in thermochromic smart windows, switching devices, and infrared detectors. However, how to balance the metal-insulator transition temperature, luminous transmittance (Tlum) and solar modulation ability (ΔTsol) of VO2 thin films remains a challenge. In this work, high-quality thermochromic VO2 thin films were prepared by a two-step method of magnetron sputtering and thermal oxidation annealing. Metallic and alloyed V–Mo layers were first deposited by direct-current reactive magnetron sputtering, and then a thermal oxidation annealing process was used to obtain pure and Mo-doped VO2 thin films. The Mo content in the films was regulated by changing the sputtering power of the vanadium target, and the effect of Mo doping on the crystallinity, microstructure, phase transition temperature and optical properties of VO2 thin films was studied. The shift of the VO2(011) peak to a lower 2θ angle in the XRD patterns showed that Mo was successfully diffused into vanadium dioxide films. The phase transition temperatures were decreased continuously from 57.4 to 32.7 °C by decreasing the sputtering power of vanadium. The thinner Mo-doped VO2 thin films showed higher luminous transmittance and lower transition temperature. Our results were shown to be an innovative preparation method to fabricate thermochromic VO2 films with a low phase transition temperature, balanced luminous transmittance and solar modulation ability by thermal oxidation of V–Mo cosputtered alloy films.

Influence of germanium substitution on the structural and electronic stability of the competing vanadium dioxide phases

We present a density-functional theory (DFT) study of the structural, electronic, and chemical bonding behavior in germanium (Ge)-doped vanadium dioxide $({\mathrm{VO}}_{2})$. Our motivation is to explain the reported increase of the metal-insulator transition temperature under Ge doping and to understand how much of the fundamental physics and chemistry behind it can be captured at the conventional DFT level. We model doping using a supercell approach, with various concentrations and different spatial distributions of Ge atoms in ${\mathrm{VO}}_{2}$. Our results suggest that the addition of Ge atoms strongly perturbs the high-symmetry metallic rutile phase and induces structural distortions that partially resemble the dimerization of the experimental insulating structure. Our work, therefore, hints at a possible explanation of the observed increase in transition temperature under Ge doping, motivating further studies into understanding the interplay of structural and electronic transitions in ${\mathrm{VO}}_{2}$.

Strain-Induced Modulation of Resistive Switching Temperature in Epitaxial VO2 Thin Films on Flexible Synthetic Mica.

The resistive switching temperature associated with the metal-insulator transition (MIT) of epitaxial VO2 thin films grown on flexible synthetic mica was modulated by bending stress. The resistive switching temperature of polycrystalline VO2 and V2O5 thin films, initially grown on synthetic mica without a buffer layer, was observed not to shift with bending stress. By inserting a SnO2 buffer layer, epitaxial growth of the VO2 (010) thin film was achieved, and the MIT temperature was found to vary with the bending stress. Thus, it was revealed that the bending response of the VO2 thin film depends on the presence or absence of the SnO2 buffer layer. The bending stress applied a maximum in-plane tensile strain of 0.077%, resulting in a high-temperature shift of 2.3 °C during heating and 1.8 °C during cooling. After 104 bending cycles at a radius of curvature R = 10 mm, it was demonstrated that the epitaxial VO2 thin film exhibits resistive switching temperature associated with MIT.

Improvement of electrical and magnetic properties in La0.67Ca0.33Mn0.97Co0.03O3 ceramic by Ag doping

For the perovskite manganite La1-xCaxMnO3, achieving high temperature coefficient of resistance (TCR) and magnetoresistance (MR) is the key to realize its potential applications. In this study, high-quality La0.67Ca0.33Mn0.97Co0.03O3:Agx polycrystalline ceramics were prepared by the sol-gel method. The results show that Ag doping has important impact on metal-insulator and ferromagnetic-paramagnetic transitions. A Ag doping amount increases, the grain size of the samples increases at x = 0.05 and then decreases. The doping of Ag can improve the crystalline quality of the samples and enhance the connectivity between grains, thereby improving the metallicity of the system. Additionally, with Ag doping amount increases, the resistivity of the samples gradually decreased, while the Curie temperature and the metal-insulator transition temperature gradually increased. Especially after Ag doping, both the TCR peak (TCRpeak) and the MR peak (MRpeak) values are significantly improved. The TCRpeak reaches 65.2%·K−1 at x = 0.1, while the MRpeak is as high as 82.6% at x = 0.05 under 1 T magnetic field. Doping perovskite manganite ceramics with Co and Ag can greatly optimize their TCR and MR, favoring the potential applications of these materials.

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.

Evidence of lattice deformation induced metal-insulator transition in Ti2O3

We synthesized ${\mathrm{Ti}}_{2}{\mathrm{O}}_{3}$ epitaxial films with continuously varying ratios of the c-axis to a-axis lattice constants ($c/a$ ratios) on 4H-SiC (0001) substrates and investigated their structural and electronic properties. ${\mathrm{Ti}}_{2}{\mathrm{O}}_{3}$ films with a wide range of $c/a$ ratios were fabricated in a controllable fashion by changing the growth temperature. As the $c/a$ ratio at room temperature increased, the metal-insulator transition (MIT) temperature systematically decreased and eventually the MIT disappeared. Detailed analyses revealed that the MIT occurred at a critical $c/a$ ratio of 2.68. The critical $c/a$ ratio for the occurrence of the MIT was also reproduced by density functional theory calculations. These results provide evidence for the origin of the MIT in ${\mathrm{Ti}}_{2}{\mathrm{O}}_{3}$. The MIT is not a Mott transition induced by temperature, but a gradual semimetal-to-semiconductor transition induced by lattice deformation.

Effect of the calcination temperature on temperature coefficient of resistance and magnetoresistance of La0.67Ca0.33MnO3 polycrystalline ceramics

La0.67Ca0.33MnO3 (LCMO) has attracted considerable attention due to its superior colossal magnetoresistance properties, as well as the metal-insulator transition and enhanced temperature coefficient of resistance (TCR) characteristics. Here, the co-precipitation method is utilized in order to prepare the LCMO-based ceramics, whose magnetotransport properties as a function of calcination temperature (Tcal) were thoroughly investigated. The TCR of LCMO exhibits initially an increasing pattern and then decreases by elevating the Tcal, whereas the metal-insulator transition temperature (TMI) is shifted towards a lower temperature value. In addition, the magnetoresistance (MR) was enhanced by enforcing a bigger Tcal and reached the value of 82.4% at Tcal = 800 °C. The underlying mechanism for the manifestation of such improved properties is thoroughly examined by employing electrical measurements in various temperature ranges. An optimal TCR of 32.3%·K−1 in LCMO-based ceramic was attained with Tcal = 500 °C, indicating that the co-precipitation method could be employed in order to facilitate the potential use of LCMO for infrared detecting and magnetoresistive switching applications.

Structural, electrical and magnetic transport properties of Pr1−xSrxMnO3 (0.30 ≤ x ≤ 0.35) ceramics prepared by sol-gel method

In order to promote practical application of Pr1−xSrxMnO3 (PSMO, 0.30 ≤ x ≤ 0.35) ceramics in uncooled infrared bolometer, the effect of Sr content on the microstructure, surface morphologies, ionic valence, electrical and magnetic transport properties was investigated in detail. The crystal structure parameters such as, cell volume, MnO bond length increased as x rose, while the MnOMn bond angle decreased. In terms of electrical transport properties, more Sr content was beneficial to increase in Tp (metal-insulator (M-I) transition temperature) and Mn4+ ion concentration, the percentage of Mn4+ was increased from 18.71% to 23.33%. The Tp increased gradually as amount of Sr doping rose, which also led to the increase of peak temperature corresponding to temperature coefficient of resistivity (TCR). The reduction in resistivity was beneficial to improve TCR values. These phenomena could be explained by the double exchange (DE) interaction mechanism and the Jahn-Teller (JT) distortion effect. The increase in Tp is a result of enhanced DE interactions. The TCR peak temperature (Tk) of Pr0.65Sr0.35MnO3 ceramic was reached room temperature (5.77% K−1; 298.63 K), which is suitable for use in Pr-based uncooled infrared bolometers and is worthy of further investigation.

Nb-doped VO2 single crystal microtube arrays

Nb-doped VO2 microtube arrays were fabricated by a thermal oxidation process of V–Nb alloy foils in air. The Nb-doped VO2 microtubes are featured with the rod-like, hollow, and rectangular cross-sectional single crystal, where arrays aligned nearly vertically on the surface of a Nb-doped V2O5 substrate. Phase transition examinations demonstrate that Nb doping can decrease the metal-insulator transition temperatures of VO2 by approximately 15 °C/at. % Nb, which reaches the highest decrease efficiency for metal-insulator transition temperatures of Nb-doped VO2. The phase transition route and mechanism of Nb-doped VO2 microtube arrays are revealed.

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).

Thermochromic SmNiO3-δ thin films deposited by magnetron sputtering and crystallized by soft-annealing in air

Samarium nickelate (SmNiO3) exhibits a metal-insulator transition (MIT) around 120 °C and therefore appears to be an excellent candidate for a new generation of thermochromic solar absorbers. Nevertheless, this kind of perovskite is classically obtained under extreme conditions to stabilize the metastable Ni3+ oxidation state. Here, we show that SmNiO3-δ thin films can be synthesized under soft-annealing as the habitual orthorhombic distorted perovskite structure. The layers were deposited by reactive magnetron co-sputtering and subsequently annealed for crystallization for 2 h at 500 °C in air. Structural characterization was carried out using X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques, while optical and electrical properties were determined by Fourier transform infrared spectroscopy (FTIR) and the four-point probe method, respectively. The Ni3+ ion was confirmed by Electron energy loss spectroscopy (EELS). Finally, reversible thermochromic behavior was observed with an MIT temperature estimated at 138 °C and an IR contrast of 19%.