Pure Metallic Phase Research Articles

Modulation of the optical and transport properties of epitaxial SrNbO3 thin films by defect engineering

The discovery of strontium niobate (SNO) as a potentially new transparent electrode has generated much interest due to its implications in various optoelectronic devices. Pristine SNO exhibits exceptionally low resistivity (∼10−4 Ω cm) at room temperature. However, this low resistivity occurs due to large number of carrier concentration in the system, which significantly affects its optical transparency (∼40%) in the visible range and hinders its practical applications as a transparent electrode. Here, we show that modulating the growth kinetics via oxygen manipulation is a feasible approach to achieve the desired optoelectronic properties. In particular, epitaxial (001) SNO thin films are grown on (001) lanthanum aluminate by pulsed laser deposition at different oxygen partial pressures and are shown to improve the optical transparency from 40% to 72% (λ = 550 nm) at a marginal cost of electrical resistivity from 2.8 to 8.1 × 10−4 Ω cm. These changes are directly linked with the multi-valence Nb-states, as evidenced by x-ray photoelectron spectroscopy. Furthermore, the defect-engineered SNO films exhibit multiple electronic phases that include pure metallic, coexisting metal-semiconducting-like, and pure semiconducting-like phases as evidenced by low-temperature electrical transport measurements. The intriguing metal-semiconducting coexisting phase is thoroughly analyzed using both perpendicular and angle-dependent magnetoresistance measurements, further supported by a density functional theory-based first-principles study and the observed feature is explained by the quantum correction to the conductivity. Overall, this study shows an exciting avenue for altering the optical and transport properties of SNO epitaxial thin films for their practical use as a next-generation transparent electrode.

Investigating on the microstructure and optical properties of Au, Ag and Cu implanted TiN thin films: The effects of surface oxidation and ion-induced defects

In the present paper, the effects of metal ion implantation on the structural and optical properties of TiN thin films have been investigated. TiN films of 170 nm thickness were grown by d.c. reactive sputtering on Si (100) wafers and then irradiated at 5 × 1016 ions/cm2 with either Au, Ag, or Cu ions by using two different energies per each implanted metal. The results showed that as deposited TiN crystallizes in the form of a fcc cubic structure, with the crystallites preferentially oriented along the (111) plane. For all implanted layers, the cubic structure was preserved, but compared to as deposited TiN the crystallites were smaller and the lattice was contracted. These changes were correlated with the depth distribution of Au, Ag and Cu ions and assigned to implantation-induced damage that was larger when higher ion energies were used. High-resolution XPS spectra of the surface of as deposited sample showed the coexistence of TiN, TiO2 and TiOxNy phases and this was related to the surface oxidation of the films due to the exposure to air. After implantation, the results were almost similar for all metals, showing an increase in TiO2 contribution and the formation of pure metallic Au and Ag phases, while copper is in the Cu2+ state, which is attributed to Cu(II)-oxide and Cu(OH)2. The microstructural characteristics including defect formation, changes in crystallite size and lattice contraction, and also growth of different metallic phases during implantations were correlated with the findings of the optical characterization of the implanted films. For the as deposited film we found an energy gap of 2.91 eV, which was lower than the value typical for TiN. After implantation the gap was shifted to higher energies, while at the visible part of the region, the existence of additional energy levels, at photon energies below 2.9 eV was observed. Besides, all implanted films showed degraded photocatalytic activity compared to as deposited TiN, among which Cu-implanted samples exhibited the best photocatalytic performances. The lower photocatalytic activity of Au and Ag implanted films compared to Cu implantations was ascribed to larger structural defects and the formation of less favorable electronic states.

Phase distributions in YB3 alloys (B[dbnd]Cr, Mn, Fe, Co, Ni, Cu) and related hydrogen storage performances

The phase distributions and hydrogen absorption and desorption performances for YB3-type alloys were investigated in the paper. The compositions of the alloys can be generally divided into three categories, and the pure metal phase will disappear gradually with the increase of B-side atomic number in the YB3 alloys. The hydrogen storage capacity and desorption rate increase first, and then decrease with the atomic number. The dehydrogenation temperatures and the molar enthalpies of the hydrides during the desorption process are also discussed.

The effect of Cr doped in amorphous carbon films on electrical conductivity: Characterization and mechanism

Metallic bipolar plates (BPPs) are prospective candidates for BPPs in PEMFCs due to their lower cost and higher power density than traditional graphite. Coatings are used to enhance the electrical conductivity and anti-corrsive quality of BPPs, and amorphous carbon (a-C) films have attracted broad attentions from both industry and academia. In this study chromium was incorporated into a-C to further enhance conductivity. A series of Cr-doped a-C films (a-C:Cr) with different doping contents were prepared by CFUBMSIP. Moreover, Microstructure analyses and composition characterizations were performed to explore the mechanism. The results show that Cr decreases the interfacial resistance with satisfying the anti-corrosive behavior. XRD patterns indicate that Cr will form chromium carbide phase and pure metal phase. Moreover, TEM results also show that Cr atoms bond with C atoms. SEM photographs show that Cr atoms refine the grain size and compact the film all the same. C 1s spectrum of XPS shows that C-sp2/C-sp3 increases first and then decreases, reaching the maximum level of 1.708 at Cr0.205, indicating the resistance drops first and then increases. In addition, Cr doping will also bring changes in nano-hardness and surface roughness.

Surfactant-free synthesis of carbon-supported silver (Ag/C) nanobars as an efficient electrocatalyst for alcohol tolerance and oxidation of sodium borohydride in alkaline medium

We have synthesized carbon-supported silver (Ag/C) nanobars by a simple surfactant-free hydrothermal method using glucose as the reducing reagent as well as the source of carbon in Ag/C nanobars. Physicochemical characterization of the materials was performed by X-ray Diffraction (XRD), field emission scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The XRD pattern confirmed the presence of a pure metallic silver phase. No carbon phase was detected, which indicates that the carbon exists mainly in the amorphous form. The electrocatalytic activity of Ag/C in different electrolyte solutions such as 0.5 M NaOH, 0.5 M NaOH + 1 M ethanol (EtOH), 0.5 M NaOH + 1 M ethylene glycol (EG), and 0.5 M NaOH + 0.01 M NaBH4 (sodium borohydride) was studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronoamperometry (CA) study. Alcohol tolerance of the catalysts was also established in the presence of ethanol and ethylene glycol. The forward-to-backward current ratio from cyclic voltammetry (CV) study of Ag/C-20 (20 h) in 0.5 M NaOH + 1 M ethanol solution at 100 mV s−1 scan rate is 4.13 times higher compared to that of Ag/C-5 (5 h). Hence, Ag/C-20 is a better candidate for the tolerance of ethanol. In the presence of ethylene glycol (1 M) in 0.5 M NaOH solution, it is obtained that the forward-to-backward current ratio at the same scan rate for Ag/C-20 is lower than that in the presence of ethanol. The durability of the catalyst was studied by chronoamperometry measurement. We studied the electrochemical kinetics of Ag/C catalysts for borohydride oxidation in an alkaline medium. The basic electrochemical results for borohydride oxidation show that Ag/C has very well strength and activity for direct borohydride oxidation in an alkaline medium. The reaction of borohydride oxidation with the contemporaneous BH4−. hydrolysis was noticed at the oxidized silver surface. Among all the synthesized Ag/C catalysts, Ag/C-20 exhibited the best electrocatalytic performance for borohydride oxidation in an alkaline medium. The activation energy and the number of exchange electrons at Ag/C-20 electrode surface for borohydride electro-oxidation were estimated as 57.2 kJ mol−1 and 2.27, respectively.

Corrosion and transformation of solution combustion synthesized Co, Ni and CoNi nanoparticles in synthetic freshwater with and without natural organic matter

Pure metallic Co, Ni, and their bimetallic compositions of Co3Ni, CoNi, and CoNi3 nanomaterials were prepared by solution combustion synthesis. Microstructure, phase composition, and crystalline structure of these nanoparticles (NPs) were characterized along with studies of their corrosion and dissolution properties in synthetic freshwater with and without natural organic matter (NOM). The nanomaterials consisted of aggregates of fine NPs (3–30 nm) of almost pure metallic and bimetallic crystal phases with a thin surface oxide covered by a thin carbon shell. The nanomaterials were characterized by BET surface areas ranging from ~ 1 to 8 m2/g for the Ni and Co NPs, to 22.93 m2/g, 14.86 m2/g, and 10.53 m2/g for the Co3Ni, CoNi, CoNi3 NPs, respectively. More Co and Ni were released from the bimetallic NPs compared with the pure metals although their corrosion current densities were lower. In contrast to findings for the pure metal NPs, the presence of NOM increased the release of Co and Ni from the bimetallic NPs in freshwater compared to freshwater only even though its presence reduced the corrosion rate (current density). It was shown that the properties of the bimetallic nanomaterials were influenced by multiple factors such as their composition, including carbon shell, type of surface oxides, and the entropy of mixing.

Preparation of Mg-Zr alloys through direct electro-deoxidation of MgO-ZrO2 in CaCl2-NaCl molten salt

The electro-deoxidation of MgO does not proceed efficiently due to the electrical insulation properties of MgO and large molar volume of metallic magnesium. In this experiment, an attempt was done to prepare Mg-Zr alloy from their oxides using the method of electro-deoxidation. The electrochemical mechanism of the deoxidation process of MgO, ZrO2 and MgO-ZrO2 composite was studied through cyclic voltammetry using metallic cavity electrode (MCE). There was an intermediate phase (Zr3O) in the reduction process of ZrO2. The electro-deoxidation of MgO occurred for MgO-ZrO2 composite in CaCl2-NaCl molten salt. Moreover, the electro-deoxidation of MgO was promoted by the addition of polyvinyl butyral (PVB). The formation of metal phase Mg was confirmed by XRD, EDS, Raman spectroscopy, and inductively-coupled plasma spectroscopy. The electro-deoxidation products showed significant differences with the concentration of PVB and the ratio between MgO and ZrO2. The electro-deoxidation of MgO and ZrO2 was favorable when the PVB concentration was 5.0% (wt.%). Pure metal phase of Mg-Zr alloys was formed when the molar ratio of MgO and ZrO2 was 1:2. The atomic fraction of Mg in the electrolytic product was ca. 28.0 mol.%. This work shows that MgO could be directly electro-deoxidized to Mg without the formation of intermediate phase oxide.

Hydrodeoxygenation of m-cresol as a depolymerized lignin probe molecule: Synergistic effect of NiCo supported alloys

Three bimetallic Ni-Co (Ni:Co ratio 1:3, 1:1 and 3:1) and two monometallic (Ni and Co) nanoparticles supported on Al2O3 were synthesized by incipient wet impregnation and characterized by various technics (N2-physisorption, XRD, H2-TPR, CO-chemisorption and elemental analysis). It was demonstrated by XRD that NiCo alloys nanoparticles were present on bimetallic solids. The catalytic properties of all catalysts were determined for the hydrodeoxygenation of m-cresol at 340 °C under 4 MPa of total pressure. It was demonstrated that NiCo alloy developed better deoxygenation catalytic properties than pure Ni metallic phase, these properties being evaluated both by the total reaction rate (kTOT) and the selectivity into deoxygenation products. Indeed, bimetallic NiCo(3:1)/Al2O3 was 1.2 times more active than Ni/Al2O3 and 8.8 times than Co/Al2O3, deoxygenated products being favored on bimetallic catalysts compared to Ni one. In addition, the kTOT values seems to be related to the amount of CO uptakes, indicating that active sites in HDO were of similar nature than those allowing the adsorption of CO, and could be oxygen vacancies which were promoted in bimetallic Ni-Co particles.

The effect of surfactants on the particle size of iron, cobalt and nickel nanopowders

This paper studies the effect of surfactants on the particle size of metal nanopowders (NPs): iron, cobalt and nickel synthesized using chemical-metallurgy method – hydrogen reduction of hydroxide compounds FeOOH, Co(OH) 2 and Ni(OH) 2 at 400, 285, and 280 °С, respectively. These hydroxides were pre-synthesized via chemical deposition from the corresponding nitrate solutions with NaOH alkali solution (10 wt.%) using sodium dodecyl sulfate (SDS) (0.1 %) and ethylenediaminetetraacetic acid disodium salt (EDTA) (0.3 %). The obtained NPs were studied using such methods as X-ray diffractometry (XRD), scanning electron microscopy (SEM), and measurements of the specific surface area by low-temperature nitrogen adsorption. According to XRD shows that all the obtained samples of NPs Fe, Co and Ni contain pure metallic phases. The results of electron microscopic analysis and measurement of the specific surface area of powder samples show that the addition of various surfactants to the initial synthesis medium of hydroxide compounds has a significant effect on the size and morphology of the obtained NPs. It was found that the addition of 0.1 % SDS leads to a decrease in the average size of the obtained particles, and the presence of 0.3 % EDTA contributes to the formation of larger metal particles. It was shown that the use of 0.3 % EDTA in deposition of initial hydroxide precursors makes it possible to obtain metal NPs with the narrowest crystallite size distributions.

Metal segregation in planetesimals: Constraints from experimentally determined interfacial energies

High temperature experiments have been performed to constrain interfacial energies in a three-phase system (metal–forsterite–silicate melt) representative of partially differentiated planetesimals accreted early in the solar system history, with the aim of providing new insights into the factors affecting the interconnection threshold of metal-rich phases. Experiments were run under controlled oxygen fugacity (ΔNi-NiO=−3) at 1440°C, typically for 24 h. Quantification of the true dihedral angles requires a resolution of at least 30 nm per pixel in order to reveal small-angle wedges of silicate melt at crystal interfaces. At this level of resolution, dihedral angle distributions of silicate melt and olivine appear asymmetric, an observation interpreted in terms of anisotropy of olivine crystals. Based upon the theoretical relation between dihedral angles and interfacial energies in a three-phase system, the relative magnitudes of interfacial energies have been determined to be: γMelt-Ol<γMelt-Ni<γOl-Ni. This order differs from that obtained with experiments using an iron sulfide liquid close to the Fe–FeS eutectic for which γMelt-Sulfide<γMelt-Ol<γOl-Sulfide, implying a lower interconnection threshold for sulfur-rich melts than for pure metallic phases. This dependence of the interconnection threshold on the sulfur content will affect the drainage of metallic phases during melting of small bodies. Assuming a continuous extraction of silicate melt, evolution of the metal volume fraction has been modeled. Several sulfur-rich melts extraction events are possible over a range of temperatures relevant with thermometric data on primitive achondrites (1200–1400°C and 25% of silicate melt extracted). These successive events provide novel insight into the variability of sulfur content in primitive achondrites, which are either representative of a region that experienced sulfide extraction or from a region that accumulated sulfide melt from overlying parts of the parent body.

Dynamic signature of orbital selective Mott transition in the metallic phase of VO2

An orbital-selective technique, nuclear magnetic resonance (NMR) spectroscopy, which could simultaneously probe the atomic structure and electronic properties, was employed to address the ‘bad metal’ behaviors, i.e., negligible electron thermal conductivity in the metallic phase of VO2. The measured electric field gradients at V sites by 51V NMR were found to evolve significantly in the pure metallic phase within the vicinity of structural phase transition temperatures, unexpected in a simple metal within this temperature region according to structural analysis by diffraction. This abnormal temperature-dependent local symmetry evolution can be well explained by a simple phenomenological model of orbital selective Mott transition in this multiorbital system.

Electrical Investigation of Metal‐Olivine Systems and Application to the Deep Interior of Mercury

AbstractWe report electrical conductivity measurements on metal‐olivine systems at about 5 and 6 GPa and up to 1,675°C in order to investigate the electrical properties of core‐mantle boundary (CMB) systems. Electrical experiments were conducted in the multianvil apparatus using the impedance spectroscopy technique. The samples are composed of one metal layer (Fe, FeS, FeSi2, or Fe‐Ni‐S‐Si) and one polycrystalline olivine layer, with the metal:olivine ratio ranging from 1:0.7 to 1:9.2. For all samples, we observe that the bulk electrical conductivity increases with temperature from 10−2.5 to 101.8 S/m, which is higher than the conductivity of polycrystalline olivine but lower than the conductivity of the pure metal phase at similar conditions. In some experiments, a conductivity jump is observed at the temperature corresponding to the melting temperature of the metallic phase. Both the metal:olivine ratio and the metal phase geometry control the electrical conductivity of the two‐layer samples. By combining electrical results, textural analyses of the samples, and previous studies of the structure and composition of Mercury's interior, we propose an electrical profile of the deep interior of the planet that accounts for a layered CMB‐outer core structure. The electrical model agrees with existing conductivity estimates of Mercury's lower mantle and CMB using magnetic observations and thermodynamic calculations, and thus, supports the hypothesis of a layered CMB‐outermost core structure in the present‐day interior of Mercury. We propose that the layered CMB‐outer core structure is possibly electrically insulating, which may influence the planet's structure and cooling history.

Elucidating the process of hydrogen generation from the reaction of sodium hydroxide solution and ferrosilicon

For the first time, the process of hydrogen evolution from ferrosilicon 75 using sodium hydroxide solution has been investigated as a function of temperature using a combination of X-ray photoelectron spectroscopy, X-ray diffraction and physical measurements. Ferrosilicon 75, a mixture of silicon (~50wt.%) and iron disilicide (~50wt.%), has been shown to produce hydrogen by the action of sodium hydroxide solution on the silicon only, with the iron disilicide acting in the role of spectator/protector species for the silicon. Neither iron disilicide alone nor ferrosilicon 45, which does not contain a pure metallic silicon phase, was found to generate hydrogen under similar reaction conditions, further indicating that the presence of a pure metallic silicon phase is essential for hydrogen generation. As the iron disilicide acts as a diluent for the active silicon, it is hypothesized that this would result in a slower release of hydrogen than that which would be obtained from the reaction of silicon alone, which may be useful for applications which require a long-term, sustained release of hydrogen. A hydrogen yield of 462.5mL/g and a maximum hydrogen generation rate of 83mL/min g were obtained within 10min of reaction with 40wt.% NaOH at 348K.

Pure and stable metallic phase molybdenum disulfide nanosheets for hydrogen evolution reaction.

Metallic-phase MoS2 (M-MoS2) is metastable and does not exist in nature. Pure and stable M-MoS2 has not been previously prepared by chemical synthesis, to the best of our knowledge. Here we report a hydrothermal process for synthesizing stable two-dimensional M-MoS2 nanosheets in water. The metal–metal Raman stretching mode at 146 cm−1 in the M-MoS2 structure, as predicted by theoretical calculations, is experimentally observed. The stability of the M-MoS2 is associated with the adsorption of a monolayer of water molecules on both sides of the nanosheets, which reduce restacking and prevent aggregation in water. The obtained M-MoS2 exhibits excellent stability in water and superior activity for the hydrogen evolution reaction, with a current density of 10 mA cm−2 at a low potential of −175 mV and a Tafel slope of 41 mV per decade.

Magneto-resistance, magneto-caloric and bolometric properties in Pr2/3Sr1/3MnO3:PdO composites near room temperature

Temperature coefficient of resistance (TCR), magneto-resistance (MR) and magneto-caloric properties of polycrystalline pristine Pr2/3Sr1/3MnO3 (PSMO) and its composites with PdO (10–30 mole %) (PSMO:Pd) have been studied. Rietveld refinement of their x-ray diffraction (XRD) patterns confirms their single phase crystalline structure with orthorhombic Pbnm space group and Pd appears in pure metallic phase. Of the various composites investigated, PSMO:Pd30 composite exhibits significant TCR of 9.7% K−1 (at 281 K) and low field MR value of 33.5% (H = 1 T). Magnetic entropy change (ΔSM) has been found to be 3.5 J kg−1K−1 (1 T), 6.3 J kg−1K−1 (2 T) and 8.7 J kg−1K−1 (3 T) at 283 K for PSMO:Pd10 sample which show a decreasing trend with increasing Pd content. Present investigations reveal excellent characteristics for bolometric, magnetic sensing and magnetic refrigerating applications near room temperature by PSMO:Pd composites.

L-cysteine protected copper nanoparticles as colorimetric sensor for mercuric ions

This report demonstrates a novel, simple and efficient protocol for the synthesis of copper nanoparticles in aqueous solution using l-cysteine as capping or protecting agent. UV–visible (UV–vis) spectroscopy was employed to monitor the LSPR band of l-cysteine functionalized copper nanoparticles (Cyst-Cu NPs) based on optimizing various reaction parameters. Fourier Transform Infrared (FTIR) spectroscopy provided information about the surface interaction between l-cysteine and Cu NPs. Transmission Electron Microscopy (TEM) confirmed the formation of fine spherical, uniformly distributed Cyst-Cu NPs with average size of 34±2.1nm. X-ray diffractometry (XRD) illustrated the formation of pure metallic phase crystalline Cyst-Cu NPs. As prepared Cyst-Cu NPs were tested as colorimetric sensor for determining mercuric (Hg2+) ions in an aqueous system. Cyst-Cu NPs demonstrated very sensitive and selective colorimetric detection of Hg2+ ions in the range of 0.5×10−6–3.5×10−6molL−1 based on decrease in LSPR intensity as monitored by a UV–vis spectrophotometer. The developed sensor is simple, economic compared to those based on precious metal nanoparticles and sensitive to detect Hg2+ ions with detection limit down to 4.3×10−8molL−1. The sensor developed in this work has a high potential for rapid and on-site detection of Hg2+ ions. The sensor was successfully applied for assessment of Hg2+ ions in real water samples collected from various locations of the Sindh River.

Three‐Dimensional Investigations of Inclusions in Ferroalloys

AbstractAs the requirements on material properties increase, there has been a demand on an additional knowledge on the effect of impurities in the ferroalloys on the properties. Thus, the number, morphology, size, and composition of inclusions in four different ferroalloys (FeTi, FeNb, FeSi, and SiMn) were investigated. This was done in three dimensions (3D) by using scanning electron microscopy in combination with energy dispersive spectroscopy after electrolytic extraction of the ferroalloy samples. The non‐metallic and metallic inclusions were successfully analyzed on the surface of film filter. Thereafter, the particle size distribution was plotted for most of the non‐metallic inclusions. The non‐metallic inclusions were found to be REM oxides in FeTi, FeSi, and SiMn, Al2O3, Ti–Nb–S–O oxides in FeNb and silicon oxides in SiMn. Moreover, the intermetallic inclusions were found to be a Ti–Fe phase in FeTi, Ca–Si, and Fe–Si–Ti phases in FeSi and a Mn–Si phase in SiMn. In addition, the almost pure single metallic phases were found to be Ti in FeTi, Nb in FeNb, and Si in FeSi.

Microstructures of Al–Cu–Ag Ternary Eutectics Under Near-Isothermal Conditions

The three-phase eutectic microstructures in Al–Cu–Ag ternary alloys are complex and only directional solidification (DS) structures have been systematically studied in the literature. In order to better understand the large variation of eutectic structures in this system, three off-eutectic alloys in the vicinity of the nonvariant eutectic composition were solidified in a differential scanning calorimeter (DSC). The DSC was operated between 450 and 550 °C and at a heating and cooling rate of 1 °C/min to produce near-isothermal conditions. The intermetallic phase ζ-Ag2Al is found to behave like pure metallic phase and forms primary dendrites. The ζ-Ag2Al phase has a regular morphology in eutectics, while the θ-Al2Cu exhibits big anisotropy of intermetallic nature and shows elongated morphology in the unconstraint grown eutectics. Three basic eutectic structures are categorized based on the relative phase arrangement: (1) semi-regular eutectic with elongated θ-Al2Cu; (2) regular eutectic with rod-like ζ-Ag2Al embedded in θ-Al2Cu and (3) irregular eutectic with θ-Al2Cu in ζ-Ag2Al. They have nearly the same phase fraction and composition but distinct phase size distribution. The DSC eutectic structures are compared with reported DS eutectic structures.

Synthesis and characterization of C/Cu core–shell particles by hydrogen-free spray pyrolysis assisted with citric acid and sucrose

C/Cu core–shell particles were synthesized by hydrogen-free spray pyrolysis. In order to obtain pure metallic Cu particles having a thin carbon layer, the idea of this study was to use organic additives (citric acid and sucrose) which can act as a reducing agent and carbon source. The crystal phase and the microstructure of C/Cu particles were investigated by changing the content of the organic additives and the pyrolysis temperature. When no organic additive was used, the formation of CuO was inevitable in the hydrogen-free spray pyrolysis. Conversely, a pure Cu metal phase was obtained by adding citric acid or sucrose to the precursor solution. It was found that using both citric acid and sucrose makes it possible to synthesize a C/Cu powder having a complete core–shell structure. Finally, the optimal molar ratio of citric acid and sucrose with respect to the Cu metal was found to be 0.1 and 0.2, respectively. The thickness of the carbon layer could be controlled simply by varying the synthesis temperature from 800°C to 1100°C.

Electrically controlled metal–insulator transition process in VO2 thin films

We report the evolution process of VO2 thin films from the insulating phase to the metallic phase under current injection for the two-electrode-based thin-film devices. Based on electrical characterization and Raman microscopic detection, it was found that there exist two critical current densities, based on which the insulator-to-metal transition process can be divided into three stages. In stage I with low current injection, the VO2 film in the insulating (semiconducting) phase acts as a resistor until the first critical current density, above which the insulator–metal transition is a percolation process with metallic rutile and insulating monoclinic phases coexisting (stage II); while beyond a second critical current density, a filamentary current path with pure metallic phase is formed with the remaining part outside of the current path receding back to the pure insulating phase (stage III). We confirm that a critical current density is required for the onset of electrically induced insulator-to-metal transition in VO2 thin films.