Stress-induced domain dynamics and phase transitions in epitaxially grown VO₂ nanowires

Modulation of Switching Characteristics in a Single VO2 Nanobeam with Interfacial Strain via the Interconnection of Multiple Nanoscale Channels

We demonstrate the modulation of electrical switching properties through the interconnection of multiple nanoscale channels (∼600 nm) in a single VO₂ nanobeam with a coexisting metal-insulator (M-I) domain configuration during phase transition. The Raman scattering characteristics of the synthesized VO₂ nanobeams provide evidence that substrate-induced interfacial strain can be inhomogeneously distributed along the length of the nanobeam. Interestingly, the nanoscale VO₂ devices with the same channel length and width exhibit distinct differences in hysteric current-voltage characteristics, which are explained by theoretical calculations of resistance change combined with Joule heating simulations of the nanoscale VO₂ channels. The observed results can be attributed to the difference in the spatial distribution and fraction ratios of M-I domains due to interfacial strain in the nanoscale VO₂ channels during the metal-insulator transition process. Moreover, we demonstrate the electrically activated resistive switching characteristics based on the hysteresis behaviors of the interconnected nanoscale channels, implying the possibility of manipulating multiple resistive states. Our results may offer insights into the nanoscale engineering of correlated phases in VO₂ as the key materials of neuromorphic computing for which nonlinear conductance is essential.

Synchrotron radiation based X-ray techniques for analysis of cathodes in Li rechargeable batteries

Li-ion rechargeable batteries are promising systems for large-scale energy storage solutions. Understanding the electrochemical process in the cathodes of these batteries using suitable techniques is one of the crucial steps for developing them as next-generation energy storage devices. Due to the broad energy range, synchrotron X-ray techniques provide a better option for characterizing the cathodes compared to the conventional laboratory-scale characterization instruments. This work gives an overview of various synchrotron radiation techniques for analyzing cathodes of Li-rechargeable batteries by depicting instrumental details of X-ray diffraction, X-ray absorption spectroscopy, X-ray imaging, and X-ray near-edge fine structure-imaging. Analysis and simulation procedures to get appropriate information of structural order, local electronic/atomic structure, chemical phase mapping and pores in cathodes are discussed by taking examples of various cathode materials. Applications of these synchrotron techniques are also explored to investigate oxidation state, metal-oxygen hybridization, quantitative local atomic structure, Ni oxidation phase and pore distribution in Ni-rich layered oxide cathodes.

Tuning the Doping Ratio and Phase Transition Temperature of VO2 Thin Film by Dual-Target Co-Sputtering

A new simple way for tuning the phase transition temperature (PTT) of VO₂ thin films has been proposed to solve the problem of changing the doping ratio by using the dual-target co-sputtering method. A series of samples with W doping ratios of 0%, 0.5%, 1%, 1.5% and 2% have been fabricated by sputtering V films with the power of pure and 2% W-doped V targets from 500 W: 0 W, 500 W: 250 W, 500 W: 500 W, 250 W: 500 W to 0 W: 500 W respectively and then annealed in an oxygen atmosphere to form VO₂. The XRD results of both pure and W-doped VO₂ samples reveal that VO₂ forms and is the main component after annealing. The PTT can be tuned by controlling the sputtering power ratio of the pure and doped targets. It can be tuned easily from 64.3 °C to 36.5 °C by using the pure and 2% W-doped targets for demonstration, with W doping ratios from 0% to 2%. It is also valid for other doping elements and is a promising approach for the large-scale production of sputtering.

Effect of Zr Doping on the Magnetic and Phase Transition Properties of VO₂ Powder

In this work, V_1-xZr_xO₂ powder (x = 0, 0.01, 0.02, 0.04) was synthesized by two step hydrothermal method. The micro-topography, magnetic and phase transition properties have been investigated using various measurement techniques. All prepared V_1-xZr_xO₂ powder samples exhibit monoclinic structure at room temperature. With the Zr⁴⁺ ions doping concentration increased, the shapes of VO₂ particles change from spherical to rectangular slice. Besides, the saturation magnetic moment of the samples decrease with the increase of doped Zr⁴⁺ ions concentration, while their phase transition temperature increase gradually with Zr ions doping at a rate of around 2 °C/at% on average. We investigated the Zr doping effects on V-V dimers and confirmed the role of V-V dimers in phase transition. We speculate that more V-V dimers form with Zr doping by magnetic measurements, which result in the monoclinic phase of Zr-doped VO₂ sample is more stable than rutile phase. Therefore the phase transition temperature is elevated by Zr doping in our experiment. We further consider that the VO₂ phase transition should be ascribed to Peierls transition caused by the changing of V-V dimers.

Defect-engineered epitaxial VO2±δ in strain engineering of heterogeneous soft crystals

The success of strain engineering has made a step further for the enhancement of material properties and the introduction of new physics, especially with the discovery of the critical roles of strain in the heterogeneous interface between two dissimilar materials (for example, FeSe/SrTiO₃). On the other hand, the strain manipulation has been limited to chemical epitaxy and nanocomposites that, to a large extent, limit the possible material systems that can be explored. By defect engineering, we obtained, for the first time, dense three-dimensional strongly correlated VO_2±δ epitaxial nanoforest arrays that can be used as a novel "substrate" for dynamic strain engineering, due to its metal-insulator transition. The highly dense nanoforest is promising for the possible realization of bulk strain similar to the effect of nanocomposites. By growing single-crystalline halide perovskite CsPbBr₃, a mechanically soft and emerging semiconducting material, onto the VO_2±δ, a heterogeneous interface is created that can entail a ~1% strain transfer upon the metal-insulator transition of VO_2±δ. This strain is large enough to trigger a structural phase transition featured by PbX₆ octahedral tilting along with a modification of the photoluminescence energy landscape in halide perovskite. Our findings suggest a promising strategy of dynamic strain engineering in a heterogeneous interface carrying soft and strain-sensitive semiconductors that can happen at a larger volumetric value surpassing the conventional critical thickness limit.

Role of annealing temperature on the sol–gel synthesis of VO2 nanowires with in situ characterization of their metal–insulator transition

Among the techniques to create VO₂ nanostructures, the sol–gel method is the most facile and benefits from simple, manipulable synthetic parameters. Here, by utilizing various TEM techniques, we report the sequential morphological evolution of VO₂ nanostructures in a sol–gel film spin-coated on a customized TEM grid, which underwent oxygen reduction as the annealing temperature increased. In situ TEM dark-field imaging and Raman spectroscopy allowed us to confirm the sharp phase transition behavior of an individual nanowire by illustrating the effect of electrode-clamping-induced tensile stress on the nucleation of the R phase from the M1 phase. The electrical transport properties of a single-nanowire device fabricated on a customized TEM grid showed excellent control of the stoichiometry and crystallinity of the wire. These results offer critical information for preparing tailored VO₂ nanostructures with advanced transition properties by the sol–gel method to enable the fabrication of scalable flexible devices.

The single-VO₂ nanowire device synthesized via sequential morphological evolutions with oxygen reduction during annealing features a sharp metal-insulator transition.

Metastable state-induced consecutive step-like negative differential resistance behaviors in single crystalline VO2 nanobeams

We demonstrate the current-dependent consecutive appearance of two different negative differential resistance (NDR) transitions in a single crystalline VO₂ nanobeam epitaxially grown on a c-cut sapphire substrate. It is revealed that the first NDR occurs at an approximately constant current level as a result of the carrier injection-induced transition, independent of a thermally induced phase transition. In contrast, it is observed that the second NDR exhibits a temperature-dependent behavior and current values triggering the metal-insulator transition (MIT) are strongly mediated by Joule heating effects in a phase coexisting temperature range. Moreover, we find that the electrically and thermally triggered MIT behavior can be closely related with the alternate occurrence of current-induced multiple insulating and metallic phase coexistence in the nanobeam. These findings indicate that the current density passing through VO₂ plays a critical role in both the electrical and structural phase transitions.

Ramp-Reversal Memory and Phase-Boundary Scarring in Transition Metal Oxides

Transition metal oxides are complex electronic systems that exhibit a multitude of collective phenomena. Two archetypal examples are VO₂ and NdNiO₃ , which undergo a metal-insulator phase transition (MIT), the origin of which is still under debate. Here this study reports the discovery of a memory effect in both systems, manifested through an increase of resistance at a specific temperature, which is set by reversing the temperature ramp from heating to cooling during the MIT. The characteristics of this ramp-reversal memory effect do not coincide with any previously reported history or memory effects in manganites, electron-glass or magnetic systems. From a broad range of experimental features, supported by theoretical modelling, it is found that the main ingredients for the effect to arise are the spatial phase separation of metallic and insulating regions during the MIT and the coupling of lattice strain to the local transition temperature of the phase transition. We conclude that the emergent memory effect originates from phase boundaries at the reversal temperature leaving "scars" in the underlying lattice structure, giving rise to a local increase in the transition temperature. The universality and robustness of the effect shed new light on the MIT in complex oxides.

Calculation Evidence of Staged Mott and Peierls Transitions in VO2 Revealed by Mapping Reduced-Dimension Potential Energy Surface

Unraveling the metal-insulator transition (MIT) mechanism of VO2 becomes tremendously important for understanding strongly correlated character and developing switching applications of VO2. First-principles calculations were employed in this work to map the reduced-dimension potential energy surface of the MIT of VO2. In the beginning stage of MIT, a significant orbital switching between σ-type d(z(2)) and π-type d(x(2)-y(2))/d(yz) accompanied by a large V-V dimerization and a slight twisting angle change opens a band gap of ∼0.2 eV, which can be attributed to the electron-correlation-driven Mott transition. After that, the twisting angle of one chain quickly increases, which is accompanied by the appearance of a larger change in band gap from 0.2 to 0.8 eV, even though orbital occupancy is maintained. This finding can be ascribed to the structure-driven Peierls transition. The present study reveals that a staged electron-correlation-driven Mott transition and structure-driven Peierls transition are involved in MIT of VO2.

Substrate-mediated strain effect on the role of thermal heating and electric field on metal-insulator transition in vanadium dioxide nanobeams

Single-crystalline vanadium dioxide (VO₂) nanostructures have recently attracted great attention because of their single domain metal-insulator transition (MIT) nature that differs from a bulk sample. The VO₂ nanostructures can also provide new opportunities to explore, understand, and ultimately engineer MIT properties for applications of novel functional devices. Importantly, the MIT properties of the VO₂ nanostructures are significantly affected by stoichiometry, doping, size effect, defects, and in particular, strain. Here, we report the effect of substrate-mediated strain on the correlative role of thermal heating and electric field on the MIT in the VO₂ nanobeams by altering the strength of the substrate attachment. Our study may provide helpful information on controlling the properties of VO₂ nanobeam for the device applications by changing temperature and voltage with a properly engineered strain.

Size effects on metal-insulator phase transition in individual vanadium dioxide nanowires

We report the size effects on the metal-insulator phase transition of vanadium dioxide (VO2) nanowires prepared by chemical vapor deposition. The phase transition temperature can be tuned from 67 °C in the bulk VO2 to as low as 29°C by reducing the diameter of VO2 nanowires to nanoscale. Temperature-dependent Raman spectra display a clear dynamic picture on the metal-insulator phase transition process of the VO2 nanowires. Whilst, Raman study shows no remarkable strain effect on the phase transition behaviors of our samples. The increasing surface defect density with reducing nanowire size facilitates the decreasing phase transition temperature. In addition, the polarized-photocurrent effect was observed, resulting from the anisotropy of the photoresponse and also caused by the reduced dimensionality.Our results indicate that size of VO2 nanostructures can dominate their thermoelectric and photoelectrical properties.

In situ probing of doping- and stress-mediated phase transitions in a single-crystalline VO₂ nanobeam by spatially resolved Raman spectroscopy

We demonstrate an experimental in situ observation of the temperature-dependent evolution of doping- and stress-mediated structural phase transitions in an individual single-crystalline VO₂ nanobeam on a Au-coated substrate under exposure to hydrogen gas using spatially resolved Raman spectroscopy. The nucleation temperature of the rutile R structural phase in the VO₂ nanobeam upon heating under hydrogen gas was lower than that under air. The spatial structural phase evolution behavior along the length of the VO₂ nanobeam under hydrogen gas upon heating was much more inhomogeneous than that along the length of the same nanobeam under air. The triclinic T phase of the VO₂ nanobeam upon heating under hydrogen gas transformed to the R phase and this R phase was stabilized even at room temperature in air after sample cooling. In particular, after the VO₂ nanobeam with the R phase was annealed at approximately 250 °C in air, it exhibited the monoclinic M1 phase (not the T phase) at room temperature during heating and cooling cycles. These results were attributed to the interplay between hydrogen doping and stress associated with nanobeam-substrate interactions. Our study has important implications for engineering metal-insulator transition properties and developing functional devices based on VO₂ nanostructures through doping and stress.

Hydrogen-induced morphotropic phase transformation of single-crystalline vanadium dioxide nanobeams

We report a morphotropic phase transformation in vanadium dioxide (VO2) nanobeams annealed in a high-pressure hydrogen gas, which leads to the stabilization of metallic phases. Structural analyses show that the annealed VO2 nanobeams are hexagonal-close-packed structures with roughened surfaces at room temperature, unlike as-grown VO2 nanobeams with the monoclinic structure and with clean surfaces. Quantitative chemical examination reveals that the hydrogen significantly reduces oxygen in the nanobeams with characteristic nonlinear reduction kinetics which depend on the annealing time. Surprisingly, the work function and the electrical resistance of the reduced nanobeams follow a similar trend to the compositional variation due mainly to the oxygen-deficiency-related defects formed at the roughened surfaces. The electronic transport characteristics indicate that the reduced nanobeams are metallic over a large range of temperatures (room temperature to 383 K). Our results demonstrate the interplay between oxygen deficiency and structural/electronic phase transitions, with implications for engineering electronic properties in vanadium oxide systems.