Metal-insulator Transition In VO2 Research Articles

Switching dynamics of silicon waveguide optical modulator driven by photothermally induced metal-insulator transition of vanadium dioxide cladding layer.

We investigated the switching dynamics of optical modulators consisting of a Si waveguide with a VO2 cladding layer by utilizing the photothermal effect, which induces a metal-insulator transition in VO2. The devices exhibited stable optical switching with a high extinction ratio exceeding 16 dB. The switching time of the insulator-to-metal transition (heating process) ranged from tens of nanoseconds to microseconds depending on the incident light power, and that of the metal-to-insulator transition (cooling process) was several microseconds regardless of the incident light power. The heat transfer in the devices was numerically simulated to reproduce the switching characteristics and revealed that the temperature change in the first few micrometers of the VO2/Si waveguide governed the switching time. The thermal structural design of the device is thus of key importance to improve the switching speed of the device.

Strain mediated Filling Control nature of the Metal-Insulator Transition of VO2 and electron correlation effects in nanostructured films

The Metal-Insulator transition (MIT) in VO2 is characterized by the complex interplay among lattice, electronic and orbital degrees of freedom. In this contribution we investigated the strain-modulation of the orbital hierarchy and the influence over macroscopic properties of the metallic phase of VO2 such as Fermi Level (FL) population and metallicity, i.e., the material ability to screen an electric field, by means of temperature-dependent X-ray Absorption Near Edge Structure (XANES) and Resonant Photoemission spectroscopy (ResPES). We demonstrate that the MIT in strained VO2 is of the Filling Control type, hence it is generated by electron correlation effects. In addition, we show that the MIT in Nanostructured (NS) disordered VO2, where the structural phase transition is quenched, is driven by electron correlation. Therefore a fine tuning of the correlation could lead to a precise control of the transition features.

Quantitative functional imaging of VO2 metal-insulator transition through intermediate M2 phase

VO2 exhibits metal-insulator transition (MIT) accompanied by structural phase transformation between rutile R phase and monoclinic M1 phase, and an intermediate monoclinic phase M2 may also emerge, stabilized by strain. The evolution of microstructures and properties across phase transition is not only critical for understanding the nature of MIT, but also important for numerous device applications, yet they are quite challenging to characterize. Utilizing advanced atomic force microscopy (AFM) techniques in combination with polarized light microscopy, X-ray diffraction, and Raman spectroscopy, we map the microstructure evolution of VO2 when it is heated from room temperature M1 phase to high temperature R phase through intermediate M2 phase, and acquire functional imaging of VO2 spanning these three phases as well. These result in quantitative mapping of electric conduction and Young's modulus of VO2 in one-to-one correspondence to its domain patterns, especially those with austenite-martensite interface between M2 and R phases. Young's modulus of M1, M2, and R phase of VO2 are determined to be 95 GPa, 65-117 GPa, and 98-100 GPa respectively, and significant anisotropy is observed in M2 phase. Rigorous continuum analysis has also been carried out to analyze the complicated domain pattern, validating our experimental observations that match theoretical expectation well.

Continuous Phase Control of Vanadium Dioxide Films

Abstract Vanadium dioxide (VO2) undergoes a metal-insulator transition (MIT) at approximately 68 °C, with associated sharp changes in its physical (e.g., optical, electrical, and mechanical) properties. This behavior makes VO2 films of interest in many potential applications, including memory devices, switches, sensors, and optical modulators. For ON/OFF like digital applications, an abrupt switching behavior is ideal. However, to continuously change VO2 metal/insulator phase ratio for analog-like operation, the intrinsic hysteresis characteristic of VO2 MIT renders the phase control becoming a formidable challenge. This paper considers the problem of controlling and tracking desired optical transmittance via continuous phase ratio change. The problem becomes worse while considering the differences of individual thin-film samples and the hysteresis associated with the phase change within a narrow temperature range. This paper reports a robust feedback controller using an optical transmittance measurement and based on an uncertainty and disturbance estimator (UDE) architecture. The proposed controller is capable of mitigating the adverse effect of hysteresis, while also compensating for various uncertainties. The effectiveness of the proposed methodology is demonstrated with experimental validation.

Self-Selective Resistive Device With Hybrid Switching Mode for Passive Crossbar Memory Application

In this letter, we experimentally demonstrated a novel resistive device with a hybrid switching mode that can be alternated between volatile threshold switching and non-volatile resistive switching. The device consists of dual-functional layers VO2 /HfO2 sandwiched by symmetrical TiN electrodes. A >20 unified ratio for selectivity and memory window is obtained. Owing to the stable resistive behavior of HfO2 and insulator-metal transition of VO2, the device shows excellent uniform switching parameters in both switching modes with a high on-state current density (1E4 A/cm2) and fast switching/recovery speed (< 30 ns). This self-selective resistive memory is of great potential in the high-density crossbar array, particularly for the future 3D-Vertical resistive random access memory (RRAM) integration.

(Invited) A Reconfigurable Remotely Epitaxial VO2 Electrical Heterostructure

VO2 has been regarded as an archetypal phase transition building block with superior metal-insulator transition characteristics. However, reconfigurable VO2-based heterostructure and the associated devices are rare due to the fundamental challenge in integrating high quality VO2 in technologically important substrates. In this report, for the first time, we show the remote epitaxy of VO2 and the demonstration of a vertical diode device in graphene/epitaxial VO2/single crystalline BN/graphite structure with VO2 as a reconfigurable phase change material and hexagonal boron nitride (h-BN) as an insulating layer. By diffraction and electrical transport studies, we show that the remote epitaxial VO2 films exhibits higher structural and electrical quality than direct epitaxial one. By high resolution transmission electron microscopy and Cs-corrected scanning transmission electron microscopy we show that graphene buffered substrate leads to less strained VO2 film than bare substrate. In the reconfigurable diode, we find that the Fermi level change and spectral weight shift along with metal-insulator transition of VO2 could modify the transport characteristics. The work suggests the feasibility of developing single crystalline VO2-based reconfigurable heterostructure with arbitrary substrates and sheds light on designing novel adaptive photonics and electrical devices and circuits.

Sliding twin-domains in self-heated needle-like VO2 single crystals

The prototypical metal-insulator transition in VO2 at 340 K is from a high-temperature rutile phase to a low-temperature monoclinic phase. The lower symmetry of the monoclinic structure removes the degeneracy of the two equivalent directions of the tetragonal structure, giving rise to twin domains. Since formation of domain walls require energy most needle-like monoclinic single crystal are single-domain. The mixed metal-insulator state in self-heated needle-like single crystals exhibits various domain patterns, the most remarkable being static insulating triangular domains embedded in the metal and narrow insulating domains sliding along the metallic background in the direction of the electric current. Reported here are results obtained for some rare needle-like twinned VO2 single crystals. Such sample revealed a unique feature: joint static triangular twins emit sliding twin domains, first overlapping and later disjoining. Dark and bright twins and dim metallic background were seen for optimal orientation under a microscope, due to polarization by reflection.

Modulation of VO2 metal-insulator transition by co-doping of hydrogen and oxygen vacancy

As a well-known thermochromic material, Vanadium dioxide (VO2) has drawn widespread interest due to its particular phase transition feature. Many endeavors have been implemented to decrease critical temperature in order to realize VO2-based smart window, while there is still much room to improve. Herein, we provide a facile way to control the critical temperature of VO2 by co-introducing of hydrogen and oxygen vacancy while without deteriorating its original metal-insulator transition behavior. SIMS was carried out to detect the depth profile of hydrogen element, and XPS was conducted to investigate the chemical state. Based on the first-principle calculation, it was found that the hydrogen played a crucial role on the structural evolution of VO2. For seldom hydrogen doping, rutile-like VO2 with metallic state was formed, in agreement with the XRD observation. Strikingly, it transformed into a more monoclinic structure as further introducing hydrogen atoms. In addition, the energy-efficient demonstration experiment exhibited a promising application for VO2 material in energy-saving field. The current findings offer a novel strategy to modulate the critical temperature of VO2, which will promote the development of VO2 in smart window and bring insight into comprehensive understanding the role of H in VO2 phase transition process.

Mixed-Dimensional Van der Waals Heterostructure Photodetector.

Van der Waals (vdW) heterostructures, integrated two-dimensional (2D) materials with various functional materials, provide a distinctive platform for next-generation optoelectronics with unique flexibility and high performance. However, exploring the vdW heterostructures combined with strongly correlated electronic materials is hitherto rare. Herein, a novel temperature-sensitive photodetector based on the GaSe/VO2 mixed-dimensional vdW heterostructure is discovered. Compared with previous devices, our photodetector exhibits excellent enhanced performance, with an external quantum efficiency of up to 109.6% and the highest responsivity (358.1 mA·W-1) under a 405 nm laser. Interestingly, we show that the heterostructure overcomes the limitation of a single material under the interaction between VO2 and GaSe, where the photoresponse is highly sensitive to temperature and can be further vanished at the critical value. The metal-insulator transition of VO2, which controls the peculiar band-structure evolution across the heterointerface, is demonstrated to manipulate the photoresponse variation. This study enables us to elucidate the method of manipulating 2D materials by strongly correlated electronic materials, paving the way for developing high-performance and special optoelectronic applications.

Modulation of VO2 Metal–Insulator Transition by Ferroelectric HfO2 Gate Insulator

AbstractFerroelectric gating of functional materials has often suffered because ferroelectric materials are poor insulators. In this study, the recently reported ferroelectric HfO2, a large band‐gap oxide with excellent insulating properties, is exploited for electrostatically gating the archetypical metal–insulator transition material, VO2. By protecting the meta‐stable ferroelectric phase from deterioration, the ferroelectric gating is successfully demonstrated with an ultra‐thin VO2 channel in the back‐gate geometry. The observed modulation of the VO2 channel conductivity is originated from the ferroelectric polarization reversal in the HfO2 gate insulator combined with the VO2 metal–insulator transition. These results demonstrate the significant potential of ferroelectric HfO2 for electrostatically controlling the state of matter for both fundamental research and device application.

Tight-binding based modeling to demonstrate metal-insulator transition in VO2 within Mott-Hubbard picture

The mechanism of metal-insulator transition (MIT) in vanadium dioxide (VO2) is still under debate whether it is due to the structural phase transition or electron correlations. A previous study, indicating that MIT appears far below the critical temperature of Tc ≈ 340 K when a small amount of oxygen is taken out from the system, suggested that oxygen screening and electronic correlations play a dominant role in MIT of VO2. To demonstrate how the Mott-Hubbard picture works in this mechanism, we construct a Hamiltonian with a kinetic part constructed within tight-binding scheme and an interaction part treated within the mean-field approximation for the system mimicking the situation of VO2 without and with oxygen deficiencies. Keeping the Hubbard repulsion parameter, U, unchanged, we propose that the MIT is dominantly controlled by the changes of hopping parameters and the on-site energies due to possible orbital reorientations induced by correlation effects. We calculate the projected density of states (PDOS) using Green’s function technique and demonstrate the opening/closing of the insulating gap within the Mott-Hubbard picture. Our results qualitatively reproduce the band renormalization picture inferred from the experimental data suggested by literature. In addition, we find that ferromagnetic electron spin configuration is necessary for the overall MIT scenario to work.

A Reconfigurable Remotely Epitaxial VO2 Electrical Heterostructure.

The reconfigurability of the electrical heterostructure featured with external variables, such as temperature, voltage, and strain, enabled electronic/optical phase transition in functional layers has great potential for future photonics, computing, and adaptive circuits. VO2 has been regarded as an archetypal phase transition building block with superior metal-insulator transition characteristics. However, the reconfigurable VO2-based heterostructure and the associated devices are rare due to the fundamental challenge in integrating high-quality VO2 in technologically important substrates. In this report, for the first time, we show the remote epitaxy of VO2 and the demonstration of a vertical diode device in a graphene/epitaxial VO2/single-crystalline BN/graphite structure with VO2 as a reconfigurable phase-change material and hexagonal boron nitride (h-BN) as an insulating layer. By diffraction and electrical transport studies, we show that the remote epitaxial VO2 films exhibit higher structural and electrical quality than direct epitaxial ones. By high-resolution transmission electron microscopy and Cs-corrected scanning transmission electron microscopy, we show that a graphene buffered substrate leads to a less strained VO2 film than the bare substrate. In the reconfigurable diode, we find that the Fermi level change and spectral weight shift along with the metal-insulator transition of VO2 could modify the transport characteristics. The work suggests the feasibility of developing a single-crystalline VO2-based reconfigurable heterostructure with arbitrary substrates and sheds light on designing novel adaptive photonics and electrical devices and circuits.

Tailoring Vanadium Dioxide Film Orientation Using Nanosheets: a Combined Microscopy, Diffraction, Transport, and Soft X‐Ray in Transmission Study

AbstractVanadium dioxide (VO2) is a much‐discussed material for oxide electronics and neuromorphic computing applications. Here, heteroepitaxy of VO2 is realized on top of oxide nanosheets that cover either the amorphous silicon dioxide surfaces of Si substrates or X‐ray transparent silicon nitride membranes. The out‐of‐plane orientation of the VO2 thin films is controlled at will between (011)M1/(110)R and (−402)M1/(002)R by coating the bulk substrates with Ti0.87O2 and NbWO6 nanosheets, respectively, prior to VO2 growth. Temperature‐dependent X‐ray diffraction and automated crystal orientation mapping in microprobe transmission electron microscope mode (ACOM‐TEM) characterize the high phase purity, the crystallographic and orientational properties of the VO2 films. Transport measurements and soft X‐ray absorption in transmission are used to probe the VO2 metal–insulator transition, showing results of a quality equal to those from epitaxial films on bulk single‐crystal substrates. Successful local manipulation of two different VO2 orientations on a single substrate is demonstrated using VO2 grown on lithographically patterned lines of Ti0.87O2 and NbWO6 nanosheets investigated by electron backscatter diffraction. Finally, the excellent suitability of these nanosheet‐templated VO2 films for advanced lensless imaging of the metal–insulator transition using coherent soft X‐rays is discussed.

Electrical Pulse Driven Insulator-Metal Transition of VO2 Devices As Leaky-Integrate-and-Fire Artificial Neurons

Both artificial neurons and synapses are needed to efficiently implement artificial neural networks directly in hardware. Memristors have been actively investigated to implement synaptic functions. In contrast, the implementation of artificial neurons are still limited to Si-based circuit components, while single device based artificial neuron has not been well explored. In this study, we show that VO2 two-terminal devices, driven under electrical pulses, can be used to implement the leaky, integrate and fire function of spiking neurons. Since the thermally driven metal-insulator transition of VO2 is volatile and electrical pulses can produce Joule heating, while the heating and cooling have delay times, VO2 devices exhibit characteristic firing time and relaxation time, corresponding to timing difference of the exciting pulse and the resistance switching. In addition, the integrating effect of several small pulses can also trigger the firing from high resistance to low resistance transition. These characteristics suggest the potential using the volatile phase transition of VO2 for artificial spiking neuron. Experimental measurements are further elaborated by a 2-D heterogeneous resistor network based modelling. The insulator-metal transition in VO2 under electrical bias was simulated to confirm the dynamic formation of current filament between two electrodes, and the formation of this path is a flashing process during a single pulse in the pulse train when the integrate effect of these pulses arrives the threshold.

Localized Triggering of the Insulator-Metal Transition in VO2 Using a Single Carbon Nanotube.

Vanadium dioxide (VO2) has been widely studied for its rich physics and potential applications, undergoing a prominent insulator-metal transition (IMT) near room temperature. The transition mechanism remains highly debated, and little is known about the IMT at nanoscale dimensions. To shed light on this problem, here we use ∼1 nm-wide carbon nanotube (CNT) heaters to trigger the IMT in VO2. Single metallic CNTs switch the adjacent VO2 at less than half the voltage and power required by control devices without a CNT, with switching power as low as ∼85 μW at 300 nm device lengths. We also obtain potential and temperature maps of devices during operation using Kelvin probe microscopy and scanning thermal microscopy. Comparing these with three-dimensional electrothermal simulations, we find that the local heating of the VO2 by the CNT plays a key role in the IMT. These results demonstrate the ability to trigger IMT in VO2 using nanoscale heaters and highlight the significance of thermal engineering to improve device behavior.

Search for power-efficient wide-range reversible resistance modulation of VO2 single crystals

The abrupt metal insulator transition in VO2 is attracting considerable interest from both fundamental and applicative angles. We report on DC I–V characteristics measured on VO2 single crystals in the two-probe configuration at several ambient temperatures below the insulator–metal (I–M) transition. The insulator-mixed-metal-insulator transition is induced by Joule heating above ambient temperature in the range of negative differential resistivity (NDR). In this range the stability of I(V) is governed by the load resistance RL. Steady state I(V) is obtained for RL > |dV/dI|max in the NDR regime. For RL < |dV/dI|max there is switching between initial and final steady states associated with peaks in the Joule power, that are higher the lower RL is. The peaks caused by steep switching are superfluous and damaging the samples. On the other hand, the large RL needed for steady state is the main power consumer in the circuit at high currents. The present work is motivated by the need to avoid damaging switching in the NDR regime while reducing the power consumption in the circuit. Large resistance modulation can be obtained under steady state conditions with reduced power consumption by increasing the ambient temperature of the device above room temperature. Under steady state conditions, the transition to the mixed metal-insulator state is smooth and is followed closely by appearance of sliding domains.

Decoherence and collective effects of quantum emitters near a medium at criticality

We investigate the influence of critical phenomena on two distinct physical processes: the center-of-mass decoherence of a single emitter, and the collective radiation of two emitters. We address these different physical mechanisms with an unified formalism relying on standard perturbation theory. We decompose the decoherence and the collective emission rates as a sum of two contributions, accounting for the spontaneous emission and for interference effects respectively. The former is enhanced by the Purcell effect when the material is in the vicinity of the emitter(s). The latter, associated to quantum interferences, experiences a "sudden death" near the critical point of the phase transition. Our findings unveil the interplay between the Purcell and collective effects and its dependence on metal-insulator transitions. We discuss two specific examples of phase transitions: the percolation transition in a metal-dielectric composite, and the metal-insulator transition in VO2. In the latter, decoherence and collective emission rates exhibit a characteristic hysteresis that strongly depends on the material temperature. These results, based on experimental data, suggest that VO2 could be explored as a versatile material platform where decoherence and collective emission can be tuned by varying the temperature.

Broadband switching of mid-infrared atmospheric windows by VO2-based thermal emitter.

Atmospheric windows play an important role in the field of infrared detection and radiative cooling. In this paper, the development of VO2-based metamaterial emitter brings broadband thermal-switching light to mid-infrared atmospheric windows. At room temperature, the emitter radiates light in both 3-5μm and 8-14μm atmospheric windows. At high temperature, the radiation peaks move out of the atmospheric windows and result a strong radiation at 5-8μm. The underlying mechanism relies on the relationship between VO2 metal-insulator transition (MIT) and resonant absorption modes coupling. Corresponding thermal imaging experiment exhibits two distinct phenomena. One is the observation of unchanged thermal radiation around MIT temperature. The other phenomenon regards the concealment of the emitter from Al background at specific temperatures. These two phenomena show potential application in infrared anti-detection.

A dual‐band and wideband omnidirectional circularly polarized antenna based on VO2

In this article, a dual-band and wideband omnidirectional circularly polarized (CP) antenna based on the vanadium dioxide (VO2) is investigated. The operating bandwidth of such an antenna can be regulated by altering the outside temperature (T), which is attained by the insulator-metal transition of VO2. The omnidirectional CP antenna is based on a loop antenna-dipole model, which is composed of four tilted metal and VO2 resonant units that are loaded around a cuboid and a feeding network for broadening bandwidth. The simulated results show that when T = 50°C (State I), the 10-dB impedance bandwidth is 45.7% (1.67-2.66 GHz), and the 3-dB axial ratio (AR) bandwidth is 40% (1.9-2.85 GHz). When T = 80°C (State II), the 10-dB impedance bandwidth is 13.8% (1.62-1.86 GHz), and the 3-dB AR bandwidth is 21.8% (1.68-2.09 GHz). In order to further characterize the concept of the proposed antenna, the related parameters of such an antenna are studied using simulation software HFSS.

Gate-Tunable Thermal Metal-Insulator Transition in VO2 Monolithically Integrated into a WSe2 Field-Effect Transistor.

Vanadium dioxide (VO2) shows promise as a building block of switching and sensing devices because it undergoes an abrupt metal-insulator transition (MIT) near room temperature, where the electrical resistivity changes by orders of magnitude. A challenge for versatile applications of VO2 is to control the MIT by gating in the field-effect device geometry. Here, we demonstrate a gate-tunable abrupt switching device based on a VO2 microwire that is monolithically integrated with a two-dimensional (2D) tungsten diselenide (WSe2) semiconductor by van der Waals stacking. We fabricated the WSe2 transistor using the VO2 wire as the drain contact, titanium as the source contact, and hexagonal boron nitride as the gate dielectric. The WSe2 transistor was observed to show ambipolar transport, with higher conductivity in the electron branch. The electron current increases continuously with gate voltage below the critical temperature of the MIT of VO2. Near the critical temperature, the current shows an abrupt and discontinuous jump at a given gate voltage, indicating that the MIT in the contacting VO2 is thermally induced by gate-mediated self-heating. Our results have paved the way for the development of VO2-based gate-tunable devices by the van der Waals stacking of 2D semiconductors, with great potential for electronic and photonic applications.