Metal-insulator Transition Properties Research Articles

All-2D CVD-grown semiconductor field-effect transistors with van der Waals graphene contacts

Two-dimensional (2D) semiconductors and van der Waals (vdW) heterostructures with graphene have generated enormous interest for future electronic, optoelectronic, and energy-harvesting applications. The electronic transport properties and correlations of such hybrid devices strongly depend on the quality of the materials via chemical vapor deposition (CVD) process, their interfaces and contact properties. However, detailed electronic transport and correlation properties of the 2D semiconductor field-effect transistor (FET) with vdW graphene contacts for understanding mobility limiting factors and metal-insulator transition properties are not explored. Here, we investigate electronic transport in scalable all-2D CVD-grown molybdenum disulfide (MoS2) FET with graphene contacts. The Fermi level of graphene can be readily tuned by a gate voltage to enable a nearly perfect band alignment and, hence, a reduced and tunable Schottky barrier at the contact with good field-effect channel mobility. Detailed temperature-dependent transport measurements show dominant phonon/impurity scattering as a mobility limiting mechanisms and a gate-and bias-induced metal-insulator transition in different temperature ranges, which is explained in light of the variable-range hopping transport. These studies in such scalable all-2D semiconductor heterostructure FETs will be useful for future electronic and optoelectronic devices for a broad range of applications.

Boosting Polysulfide Redox Kinetics by Temperature-Induced Metal-Insulator Transition Effect of Tungsten-Doped Vanadium Dioxide for High-Temperature Lithium-Sulfur Batteries.

The practical application of Li-S batteries is still severely restricted by poor cyclic performance caused by the intrinsic polysulfides shuttle effect, which is even more severe under the high-temperature condition owing to the inevitable increase of polysulfides' solubility and diffusion rate. Herein, tungsten-doped vanadium dioxide (W-VO2) micro-flowers are employed with first-order metal-insulator phase transition (MIT) property as a robust and multifunctional modification layer to hamper the shuttle effect and simultaneously improve the thermotolerance of the common separator. Tungsten doping significantly reduces the transition temperature from 68 to 35 °C of vanadium dioxide, which renders the W-VO2 easier to turn from the insulating monoclinic phase into the metallic rutile phase. The systematic experiments and theoretical analysis demonstrate that the temperature-induced in-suit MIT property endows the W-VO2 catalyst with strong chemisorption against polysulfides, low energy barrier for liquid-to-solid conversion, and outstanding diffusion kinetics of Li-ion under high temperatures. Benefiting from these advantages, the Li-S batteries with W-VO2 modified separator exhibit significantly improved rate and long-term cyclic performance under 50°C. Remarkably, even at an elevated temperature (80°C), they still exhibit superior electrochemical performance. This work opens a rewarding avenue to use phase-changing materials for high-temperature Li-S batteries.

Magnetic field sensors using VO2/Fe3O4 nanoparticle devices * *This work was supported in part by the USTAR Program at the University of Utah.

We combined the metal-insulator transition (MIT) properties of VO2 and the magnetic properties of Fe3O4 to realize a magnetometer with very large nonlinearity and switching characteristics. VO2, Fe3O4 nanoparticles, and a conductive binder (silver paint) were mixed and drop-casted onto two-terminal gap junction devices. The device’s current–voltage characteristics exhibited current-switching behavior related to MIT in VO2 which changed with the external magnetic field. The magnetoresistance and magnetostriction in Fe3O4 both contributed to the field sensitivity of the sensor. Sensitivities as high as 1 A nT−1 (or 50.8 V T−1 with a current bias) were observed near the MIT voltage. The resulting minimum detectable signal was 20 pT/SQRT(Hz).

Optimization of the electrical transport properties of La0.7Ca0.3MnO3 films by sintering temperature adjustment

With the development of the radiation thermometers, perovskite manganites have been fallen into the focus of highly sensitive infrared bolometers due to its metal-insulator transition properties. In this study, La0.7Ca0.3MnO3 (LCMO) films were prepared on (l00) LaAlO3 substrates using sol-gel spin-coating technology by adjusting different sintering temperatures (Ts, 1000 °C ≤ Ts ≤ 1400 °C). The analysis of the microstructure, surface morphology, ion valence state, and electrical transport properties revealed that the crystalline quality enhanced and the concentration of Mn4+ increased with the increasing of sintering temperature, promoting the DE effect and improving the electrical properties of the LCMO films. At Ts = 1200 °C, the temperature coefficient of resistivity (TCR) value of the LCMO film reached the maximum value (22.95% K−1) and corresponding peak TCR temperature of 246.85 K indicating that the LCMO film has great potential in infrared bolometers.

Investigating the Intrinsic Anisotropy of VO2(101) Thin Films Using Linearly Polarized Resonant Photoemission Spectroscopy

VO2 is one of the most studied vanadium oxides because it undergoes a reversible metal-insulator transition (MIT) upon heating with a critical temperature of around 340 K. One of the most overlooked aspects of VO2 is the band’s anisotropy in the metallic phase when the Fermi level is crossed by two bands: π* and d||. They are oriented perpendicularly in one respect to the other, hence generating anisotropy. One of the parameters tuning MIT properties is the unbalance of the electron population of π* and d|| bands that arise from their different energy position with respect to the Fermi level. In systems with reduced dimensionality, the electron population disproportion is different with respect to the bulk leading to a different anisotropy. Investigating such a system with a band-selective spectroscopic tool is mandatory. In this manuscript, we show the results of the investigation of a single crystalline 8 nm VO2/TiO2(101) film. We report on the effectiveness of linearly polarized resonant photoemission (ResPES) as a band-selective technique probing the intrinsic anisotropy of VO2.

Atomic scale interfacial magnetism and origin of metal-insulator transition in (LaNiO_3)_n/(CaMnO_3)_m superlattices: a first principles study

Interfacial magnetism and metal-insulator transition at LaNiO_3-based oxide interfaces have triggered intense research efforts, because of the possible implications in future heterostructure device design and engineering. Experimental observation lack in some points a support from an atomistic view. In an effort to fill such gap, we hereby investigate the structural, electronic, and magnetic properties of (LaNiO_3)_n/(CaMnO_3)_m superlattices with varying LaNiO_3 thickness (n) using density functional theory including a Hubbard-type effective on-site Coulomb term. We successfully capture and explain the metal-insulator transition and interfacial magnetic properties, such as magnetic alignments and induced Ni magnetic moments which were recently observed experimentally in nickelate-based heterostructures. In the superlattices modeled in our study, an insulating state is found for n=1 and a metallic character for n=2, 4, with major contribution from Ni and Mn 3d states. The insulating character originates from the disorder effect induced by sudden environment change for the octahedra at the interface, and associated to localized electronic states; on the other hand, for larger n, less localized interfacial states and increased polarity of the LaNiO_3 layers contribute to metallicity. We discuss how the interplay between double and super-exchange interaction via complex structural and charge redistributions results in interfacial magnetism. While (LaNiO_3)_n/(CaMnO_3)_m superlattices are chosen as prototype and for their experimental feasibility, our approach is generally applicable to understand the intricate roles of interfacial states and exchange mechanism between magnetic ions towards the overall response of a magnetic interface or superlattice.

Charge-orbital Ordering, Magnetic State, and Exchange Couplings in Quasi-One-Dimensional Vanadate V6O13

Charge and orbital ordering, magnetic state, and exchange couplings in quasi-one-dimensional vanadate V6O13, a potential cathode material for Li-ion batteries, are investigated using the density functional theory with Coulomb interaction correction method (DFT + U). While the difference between {{t}_{{2g}}} orbital occupancies of V4+ (with a nominal 3{{d}^{1}} electronic configuration) and V5+ ions is large and gives direct evidence for charge and orbital ordering, the screening is so effective that the total 3d charge disproportionation is rather small. Our results show that the occupied {{t}_{{2g}}} states of V4+ ions in the single V–V layer form a spin-singlet molecular orbital, while the rest half of V4+ ions in the structurally distinct double V–V layers order antiferromagnetically in the low-temperature insulating phase of V6O13. We conclude that the metal-insulator transition and low-temperature magnetic properties of V6O13 involve the spin-Peierls transition assisted by orbital ordering and concomitant distortions of the crystal structure.

One-pot Hydrothermal Synthesis of Vanadium Dioxide Nanoparticles

Vanadium dioxide (VO2) shows excellent thermochromic properties suitable for potential application in optical devices and smart windows coating. VO2 is interesting since actual coating on glass materials is possible along with large-scale production. This paper, VO2 nanoparticles were synthesized successfully using the sol-gel-assisted hydrothermal method. The as-prepared VO2 nanoparticles were annealed under vacuum, and the crystal phase formation, composition, morphology, and metal-insulator phase transition properties of the nanoparticles were studied. X-ray Diffraction (XRD) spectra revealed that a purely monoclinic phase of VO2 nanoparticles with an average crystallite size of 33.27 nm was synthesized. Scanning Electron Microscopy (SEM) micrographs also confirmed the formation of crystalline VO2 nanoparticles. Differential Scanning Calorimetry (DSC) curve shows that the change in temperature (∆T_c) between the exothermic and endothermic peaks is 13ºC. Moreover, Fourier Transform Infrared (FTIR) spectroscopy peaks ascertain the presence of VO2 functional groups in the synthesized nanoparticles.

The Local Structure and Metal-Insulator Transition in a Ba3Nb5-xTixO15 System.

The local structure of the filled tetragonal tungsten bronze (TTB) niobate BaNbTiO (x = 0, 0.1, 0.7, 1.0), showing a metal-insulator transition with Ti substitution, has been studied by Nb K-edge extended X-ray absorption fine structure (EXAFS) measurements as a function of temperature. The Ti substitution has been found to have a substantial effect on the local structure, that remains largely temperature independent in the studied temperature range of 80–400 K. The Nb-O bonds distribution shows an increased octahedral distortion induced by Ti substitution, while Nb-Ba distances are marginally affected. The Nb-O bonds are stiffer in the Ti substituted samples, which is revealed by the temperature dependent mean square relative displacements (MSRDs). Furthermore, there is an overall increase in the configurational disorder while the system with Nb 4d electrons turns insulating. The results underline a clear relationship between the local structure and the electronic transport properties suggesting that the metal-insulator transition and possible thermoelectric properties of TTB structured niobates can be tuned by disorder.

Statistical metal–insulator transition properties of electric domains in NdNiO3 nanowires

Neodymium nickelate, NdNiO3 (NNO), with a perovskite structure shows resistance change of 1–2 orders owing to insulator–metal-transition (IMT) and metal–insulator-transition (MIT) at around 200 K and its IMT/MIT properties are affected by strain effects (Ni–O–Ni angular distribution). Since the resistance changes in the NNO system are considered to be dominated by competing nanoscale electronic phases, the reduction in sample size down to the individual domain scale could realize the direct investigation of single electric domains. In this study, 100 nm wide NNO nanowire structures were produced on NdGaO3(110) and LSAT(100) substrates, and the statistical IMT/MIT properties of electric domains under different strained structures were investigated. The nanowires showed prominent step resistance changes reflecting intrinsic first-order transition properties with different transition temperature distributions. A statistical transition model unveils the quantitative relationship between the IMT properties of the NNO nano-electronic phase and the strain effect due to the Ni–O–Ni angular distribution in NNO.

Tuning the Metal-Insulator Transition Properties of VO2 Thin Films with the Synergetic Combination of Oxygen Vacancies, Strain Engineering, and Tungsten Doping.

Vanadium oxide (VO2) is considered a Peierls–Mott insulator with a metal–insulator transition (MIT) at Tc = 68° C. The tuning of MIT parameters is a crucial point to use VO2 within thermoelectric, electrochromic, or thermochromic applications. In this study, the effect of oxygen deficiencies, strain engineering, and metal tungsten doping are combined to tune the MIT with a low phase transition of 20 °C in the air without capsulation. Narrow hysteresis phase transition devices based on multilayer VO2, WO3, Mo0.2W0.8O3, and/or MoO3 oxide thin films deposited through a high vacuum sputtering are investigated. The deposited films are structurally, chemically, electrically, and optically characterized. Different conductivity behaviour was observed, with the highest value towards VO1.75/WO2.94 and the lowest VO1.75 on FTO glass. VO1.75/WO2.94 showed a narrow hysteresis curve with a single-phase transition. Thanks to the role of oxygen vacancies, the MIT temperature decreased to 35 °C, while the lowest value (Tc = 20 °C) was reached with Mo0.2W0.8O3/VO2/MoO3 structure. In this former sample, Mo0.2W0.8O3 was used for the first time as an anti-reflective and anti-oxidative layer. The results showed that the MoO3 bottom layer is more suitable than WO3 to enhance the electrical properties of VO2 thin films. This work is applied to fast phase transition devices.

Crystal orientation dependence of metal–insulator transition for VO2 microwires fabricated on TiO2(110) substrates with step and terrace structures

Vanadium dioxide (VO2) thin films exhibit a metal–insulator transition (MIT) with sensitivity to the lattice strain. Substrates with step and terrace structures are an attractive platform for growing high-quality thin films. Thus, a prominent lattice strain effect could be derived using VO2 thin films on these substrates. In this study, VO2/TiO2 microwires were fabricated following the thin film growth to investigate the microwire-direction dependence of the MIT property. The in-plane crystal orientation dependence of the MIT property was enhanced for VO2/TiO2 microwires with step and terrace structures, which is promising for strain engineering in device applications.

Step-like resistance changes in VO2 thin films grown on hexagonal boron nitride with in situ optically observable metallic domains

Vanadium dioxide (VO2) thin films grown on hexagonal boron nitride (hBN) flakes show three orders of magnitude resistance change due to metal–insulator transition (MIT). The MIT property of VO2 thin films is strongly dependent on the metallic domain size, which should be identified to derive the resistance change owing to the single metallic domain. In this study, we investigated the relationship between the metallic domain size and the device-size-dependent MIT property of VO2 thin films grown on hBN. We observed by temperature-dependent Raman spectroscopy and optical microscopy the emergence of the metallic domains and determined the metallic domain size in VO2 thin films grown on hBN. The metallic domain size of the VO2 thin films grown on hBN was determined to be ∼500 nm on average in length and up to sub-micrometer scale. Electric transport measurements revealed that VO2/hBN microwires exhibit multi-level step-like resistivity changes that change by one to two orders when the length and width are ∼2 μm owing to the confined metallic domains in the micrometer scale. Our results open a way for VO2 devices, showing a steep and large resistance change even in the micrometer scale.

Critical conductance of two-dimensional electron gas in silicon-on-insulator metal-oxide-semiconductor field-effect transistor

Conductance of thin (18 nm) silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistor is investigated from the viewpoint of metal insulator transition (MIT). Conductance characteristics taken in the parameter space defined by the front- and back-gate voltages in a temperature range on the order of 10 K reveal that the critical conductance G C, separating the metallic and insulating states, varies with the gate voltages. In particular, it is found that G C of the double (front and back) channel mode is not a simple sum of G C for the two single (front or back) channel modes, but becomes lower than the sum. This is a unique feature of the MIT in thin SOIs, and strongly suggests that modulation of the electron wavefunction due to the vertical confinement affects the MIT properties in thin SOIs.

Cytotoxicity and genotoxicity of low-dose vanadium dioxide nanoparticles to lung cells following long-term exposure

Vanadium dioxide nanoparticles (VO2 NPs) have been massively produced and widely applied due to their excellent metal-insulator transition property, making it extremely urgent to evaluate their safety, especially for low-dose long-term respiratory occupational exposure. Here, we report a comprehensive cytotoxicity and genotoxicity study on VO2 NPs to lung cell lines A549 and BEAS-2B following a long-term exposure. A commercial VO2 NP, S-VO2, was used to treat BEAS-2B (0.15–0.6 μg/mL) and A549 (0.3–1.2 μg/mL) cells for four exposure cycles, and each exposure cycle lasted for 4 consecutive days; then various bioassays were performed after each cycle. Significant proliferation inhibition was observed in both cell lines after long-term exposure of S-VO2 at low doses that did not cause apparent acute cytotoxicity; however, the genotoxicity of S-VO2, characterized by DNA damage and micronuclei, was only observed in A549 cells. These adverse effects of S-VO2 were exposure time-, dose- and cell-dependent, and closely related to the solubility of S-VO2. The oxidative stress in cells, i.e., enhanced reactive oxygen species (ROS) generation and suppressed reduced glutathione, was the main toxicity mechanism of S-VO2. The ROS-associated mitochondrial damage and DNA damage led to the genotoxicity, and cell proliferation retard, resulting in the cellular viability loss. Our results highlight the importance and urgent necessity of the investigation on the long-term toxicity of VO2 NPs.

Microstructure scaling of metal–insulator transition properties of VO2 films

The metallic to semiconducting resistivity ratios and the transition temperature sharpness of annealed atomic layer deposited VO2 films on amorphous silicon dioxide are found to depend on the grain size in a closely similar way to films from previous sputtered or pulse laser deposited work. This occurs because the dominance of grain growth and compaction processes leads to a common scaling of properties with the grain size. Density functional simulations find that V–V dimerizations allow grain boundaries to remain semiconducting but with a reduced bandgap, while others create metallic grain boundaries, reducing the resistivity ratio for smaller grains in both cases.

Electric field driven abnormal increase in conductivity of tungsten-doped VO2 nanofilms

Ion doping is an effective way in tuning the metal-insulator transition (MIT) and physical properties of vanadium dioxide (VO2). However, the mechanism of ion doping effect is not quite clear up to now. The first principle calculations in our last paper indicated that W dopants injected localized d-electrons in rutile VO2 (VO2(R)). Herein, we prepared a group of V1-xWxO2 (0 ≤ x ≤ 2.7%) films, and applied external electric field to drive the MIT transition of W-doped VO2. The variation of electrical conductivity of W-doped VO2 was investigated with different W concentrations and applied voltages. Without external electric field, the conductivity of W-doped VO2(R) is lower than that of undoped VO2(R), and decreases with increasing the W concentration. Upon loading electric field, the conductivity of W-doped VO2(R) is increased by applied voltages and reaches a saturation value which is higher than that of undoped VO2(R) and increases with the W concentration, while the conductivity of undoped VO2(R) is independent of applied voltage. The experimental results suggest external electric field drives the excitation of the localized d electrons injected by W dopants. This work presents useful information for understanding the ion doping effect in VO2, and demonstrates the existence of conduction multiplets in W-doped VO2, proposing potential application of W-doped VO2 in multi-field effect devices such as for high-density storage.

Realization of the three-dimensionally architected nanostructures for the strongly electron correlated metal oxides and investigation of their nanoscale metal-insulator transition properties

Realization of the three-dimensionally architected nanostructures for the strongly electron correlated metal oxides and investigation of their nanoscale metal-insulator transition properties

Investigation of Statistical Metal-Insulator Transition Properties of Electronic Domains in Spatially Confined VO2 Nanostructure

Functional oxides with strongly correlated electron systems, such as vanadium dioxide, manganite, and so on, show a metal-insulator transition and an insulator-metal transition (MIT and IMT) with a change in conductivity of several orders of magnitude. Since the discovery of phase separation during transition processes, many researchers have been trying to capture a nanoscale electronic domain and investigate its exotic properties. To understand the exotic properties of the nanoscale electronic domain, we studied the MIT and IMT properties for the VO2 electronic domains confined into a 20 nm length scale. The confined domains in VO2 exhibited an intrinsic first-order MIT and IMT with an unusually steep single-step change in the temperature dependent resistivity (R-T) curve. The investigation of the temperature-sweep-rate dependent MIT and IMT properties revealed the statistical transition behavior among the domains. These results are the first demonstration approaching the transition dynamics: the competition between the phase-transition kinetics and experimental temperature-sweep-rate in a nano scale. We proposed a statistical transition model to describe the correlation between the domain behavior and the observable R-T curve, which connect the progression of the MIT and IMT from the macroscopic to microscopic viewpoints.

Stable and Tunable Current-Induced Phase Transition in Epitaxial Thin Films of Ca2RuO4.

Owing to the recent discovery of the current-induced metal-insulator transition and unprecedented electronic properties of the concomitant phases of calcium ruthenate Ca2RuO4, it is emerging as an important material. To further explore the properties, the growth of epitaxial thin films of Ca2RuO4 is receiving more attention, as high current densities can be applied to thin-film samples and the amount can be precisely controlled in an experimental environment. However, it is difficult to grow high-quality thin films of Ca2RuO4 due to the easy formation of the crystal defects originating from the sublimation of RuO4; therefore, the metal-insulator transition of Ca2RuO4 is typically not observed in the thin films. Herein, a stable current-induced metal-insulator transition is achieved in the high-quality thin films of Ca2RuO4 grown by solid-phase epitaxy under high growth temperatures and pressures. In the Ca2RuO4 thin films grown by ex situ annealing at >1200 °C and 1.0 atm, continuous changes in the resistance of over 2 orders of magnitude are induced by currents with a precise dependence of the resistance on the current amplitude. A hysteretic, abrupt resistive transition is also observed in the thin films from the resistance-temperature measurements conducted under constant-voltage (variable-current) conditions with controllability of the transition temperature. A clear resistive switching by the current-induced transition is demonstrated in the current-electric-field characteristics, and the switching currents and fields are shown to be very stable. These results represent a significant step toward understanding the high-current-density properties of Ca2RuO4 and the future development of Mott-electronic devices based on electricity-driven transitions.