Ultrathin VO2 Research Articles

Ultrathin VO2(B) nanosheets assembled into 3D micro/nano structured flower-like microspheres for high-performance cathode materials of Li-ion batteries

Vanadium oxides, as a potential cathode material for lithium-ion batteries (LIBs), have received extensive attention from researchers owing to their significant advantages in terms of theoretical specific capacity, energy density, and power density. However, vanadium oxides lead to irreversible decay of specific capacity during cycling for reasons such as poor structural stability and electrical conductivity. In this paper, the flower-like VO2(B) microsphere assembled from ultrathin nanosheets was synthesized successfully by hydrothermal method, and their effects on the morphology as well as the phase of VO2(B) are investigated by varying the hydrothermal reaction time and the concentration of oxalic acid. The results show that the flower-like VO2(B) microsphere electrode material not only provides a larger active area, but also offers a faster lithium ion diffusion rate and a shorter charge transport path, resulting in a high reversible capacity, outstanding rate performance, and excellent long cycle life. The flower-like VO2(B) microsphere electrode exhibited a first discharge specific capacity of 282.9 mAh/g (at a current density of 0.1 A/g) and a capacity retention of 80.7 % after 50 cycles. In addition, the electrode material shows a first discharge specific capacity of 209.6 mAh/g at a high current density (1 A/g) and a capacity retention rate of 83.1 % after 200 cycles. Consequently, the VO2(B) provides great development potential as a cathode material for next-generation LIBs.

Ultrathin VO2 grown with oxygen plasma molecular beam epitaxy on TiO2 (001) and Al2O3 (0001)

Bulk VO2 undergoes an insulator-to-metal transition (IMT) with up to five orders of magnitude change in the resistance at 340 K. However, when VO2 is deposited as a film on a substrate, the strain from the substrate can alter the IMT temperature, resistivity ratio of IMT, and hysteresis. Here, we present single-phase VO2 ultrathin films (thickness less than 20 nm) grown using oxygen plasma molecular beam epitaxy (MBE) on TiO2 (001) and Al2O3 (0001) substrates. First, we modify existing recipes employing ozone MBE and reproduce the best films from literature on TiO2 (001), maintaining an almost three orders of magnitude transition in a 12 nm thick film with TIMT of 308 K (296 K) upon warming (cooling). We then extend our recipe to Al2O3 (0001) substrates where we stabilize a 12 nm thin single-phase VO2 film and observe two orders of magnitude transition at 337 K (329 K) upon warming (cooling), expanding the possible growth methods for ultrathin VO2 films on Al2O3 (0001).

Enhancement of phase transition properties of ultrathin VO2 thin films on microsphere array by vacuum thermal pre-annealing

Vanadium dioxide (VO2) thin films were fabricated on SiO2 microsphere array surface by vacuum thermal pre-annealing and rapid thermal oxidization annealing of magnetron sputtering vanadium films. The effects of vacuum thermal pre-annealing temperature on phase transition properties of VO2 thin films were investigated. The resistance transition ranges of phase transition are improved from 7.58 to 10.23 as the increasing of vacuum thermal pre-annealing temperature. The phase transition temperature decreases and the width of thermal hysteresis loop keeps constant. We analyzed the effects of vacuum thermal pre-annealing temperature on phase transition properties of VO2 thin film in the light of vanadium thin film shrinking effects. The results are important for fabrication of ultrathin VO2 film on microsphere array and application in optical regulation field.

Epitaxial Grown VO2 with Suppressed Hysteresis and Low Room Temperature Resistivity for High-Performance Thermal Sensor Applications

The ability of VO2 to undergo semiconductor-to-metal phase transition (SMT) upon heating makes it a very attractive material for uncooled bolometers. The SMT of VO2 represents a large temperature coefficient of resistance, which is an important parameter for the development of highly responsive microbolometers. However, other characteristics of the SMT of VO2 such as its high transition temperature (341.2 K), the sharpness of the transition, its hysteresis, and the high room temperature resistivity limit the performance of this material in microbolometers. In this work, we grow a high-quality epitaxial ultrathin film VO2 on c-plane Al2O3 by pulsed laser deposition. The low deposition temperature and tuning the oxygen partial pressure during the growth process enable control over the grain size and oxygen vacancy concentration. This allowed controlling the SMT parameters of the samples. In particular, we show that the high density of grain boundaries associated with nanosized grains suppresses the thermal hysteresis of the SMT. Simultaneous control over the density of oxygen vacancies and the size of grains enables the adjustment of the temperature coefficient of resistance, room temperature resistivity, SMT temperature, sharpness, and thermal hysteresis toward suitable values for the fabrication of efficient VO2-based uncooled bolometers. Compared with other VO2 fabrication methods, this approach can be viewed as a simpler alternative for VO2 fabrication with favorable properties for practical bolometer applications.

Design of Wavelength Selectively Metamaterial Antenna for Thermal Camouflage

Phase transition materials offer exciting opportunities to dynamically control the optical properties of photonic devices. As a special phase change material (PCM), vanadium dioxide (VO2) is an excellent alternative for thermal camouflage technology research. Due to the unique property of VO2, coupled with the metallic structure can enable the design of thermally reconfigurable absorbers that allow resonance tuning. We propose a very simple, multi-layered with metamaterial excellent thermal emitter nanoantenna. The size of our design is only 1365 nm for the infrared wavelength range from 0.8 to 12 µm. Concept silver (Ag), VO2, dielectric separator and silver (Ag) configuration are used for finite difference time domain (FDTD) analysis. The absorption band can be further expanded with more deposition layers with various metals. In this paper, a thermal camouflage mid-infrared adaptive metamaterial antenna based on a PCM metamaterial is designed to fit perfectly into atmospheric windows. The spectral properties of the structure were calculated using FDTD method. The distinctive feature of the proposed structure is that adaptations can be made by placing an ultra-thin VO2 interlayer between the metallic structures. We believe that the proposed design is very promising in terms of simple fabrication and modifiable aspects for wide-area broadband unity absorption.

Insulator-to-metal transition in ultrathin rutile VO2/TiO2(001)

An insulator-to-metal transition (IMT) is an emergent characteristic of quantum materials. When the IMT occurs in materials with interacting electronic and lattice degrees of freedom, it is often difficult to determine if the energy gap in the insulating state is formed by Mott electron–electron correlation or by Peierls charge-density wave (CDW) ordering. To solve this problem, we investigate a representative material, vanadium dioxide (VO2), which exhibits both strong electron–electron interaction and CDW ordering. For this research, VO2 films of different thicknesses on rutile (001) TiO2 substrates have been fabricated. X-ray diffraction (XRD) data show that ultrathin VO2 films with thickness below 7.5 nm undergo the IMT between rutile insulator below Tc and rutile metal above Tc, while an ultrathin VO2 film with a thickness of 8 nm experiences the structural phase transition from the monoclinic structure below Tc to the rutile structure above Tc. Infrared and optical measurements on a film of 7.2 nm thickness, below Tc, reveal the energy gap of 0.6 eV in the rutile insulator phase and the absence of the 2.5 eV bonding-antibonding CDW structure. Above Tc, a Drude feature in the optical conductivity reveals the IMT to a metallic phase. These results suggest that for VO2 films below a critical thickness of about 7.5 nm, the IMT occurs in the rutile structure of VO2 without the Peierls lattice distortion.

Actively Tunable Metasurfaces via Plasmonic Nanogap Cavities with Sub-10-nm VO2 Films.

Actively tunable optical materials integrated with engineered subwavelength structures could enable novel optoelectronic devices, including reconfigurable light sources and tunable on-chip spectral filters. The phase-change material vanadium dioxide (VO2) provides a promising solid-state solution for dynamic tuning; however, previous demonstrations have been limited to thicker and often rough VO2 films or require a lattice-matched substrate for growth. Here, sub-10-nm-thick VO2 films are realized by atomic layer deposition (ALD) and integrated with plasmonic nanogap cavities to demonstrate tunable, spectrally selective absorption across 1200 nm in the near-infrared (NIR). Upon inducing the phase transition via heating, the absorption resonance is blue-shifted by as much as 60 nm. This process is reversible upon cooling and repeatable over more than ten temperature cycles. Dynamic, ultrathin VO2 films deposited by ALD, as demonstrated here, open up new potential architectures and applications where VO2 can be utilized to provide reconfigurability including three-dimensional, flexible and large-area structures.

Highly transparent ultrathin vanadium dioxide films with temperature-dependent infrared reflectance for smart windows

Vanadium dioxide (VO2) is a suitable material for energy-efficient smart windows that can modulate infrared (IR) light. Previous studies on smart windows using thick VO2 films achieved high modulation ratios in the IR region, but the transmittance in the visible region was low. To address this problem, we fabricated and investigated the characteristics of ultrathin VO2 films with pronounced metal–insulator transition. We prepared highly transparent smart windows by depositing a 10 nm-thick VO2 film on F-doped tin oxide film-coated glass. The IR transmittance of the smart window dramatically decreased with increasing temperature without a decrease in visible light. The IR transmittance change at a wavelength of 2500 nm was 50.1%, and the average transmittance in the visible region of 450–750 nm was 76.7%. These results are superior than those reported in earlier works discussing the IR modulation of ultrathin VO2 and the visible-light transmittance of thick VO2 films. We examined the switching behavior of the smart window. This work demonstrates, for the first time, the high IR modulation and rapid switching behaviors of highly transparent smart windows with ultrathin VO2 films, which could be very useful for energy savings in buildings.

Temperature-responded tunable metalenses based on phase transition materials

Once the metalenses are fabricated, the functions of most metalenses are invariable. The tunability and reconfigurability are useful and cost-saving for metalenses in realistic applications. We demonstrate this tunability here via a novel hybrid metalens with the strategic placement of an ultra-thin VO2 layer. The hybrid metalens is capable of dynamically modulating the focusing intensity of transmitted light at a wavelength of 1550 nm, and demonstrate a 42.28% focusing efficiency of the incident light and 70.01% modulation efficiency. The hybrid metalens’ optothermal simulations show an optothermal conversion process of dynamic focusing, and a maximum laser density of 1.76×103 W/cm2 can be handled at an ambient temperature lower than 330 K. The hybrid metalens proposed in this work, a light-dose sensitive tunable smart metalens that can protect other instruments/systems or materials from being damaged, has its specific applications such as in anti-satellite blinding, bio-imaging, etc.

Vanadium Dioxide Nanoparticle Doped Polyimide Hybrid Alignment Layers for Flexible Liquid Crystal Displays

In this paper, a flexible liquid crystal display (LCD) is developed by applying hybrid alignment layers made of vanadium dioxide (VO2) nanoparticles and homeotropic-polyimide (h-PI) onto colorless plastic substrates, and the proposed hybrid thin layers with randomly embedded agglomerated VO2 nanoparticles exhibit a transparency over 85% at 550 nm including the substrate and a strong surface irregularity. After rubbing, liquid crystals (LCs) are vertically aligned between the hybrid thin layers with a tilt angle of ∼90°, and due to the significant surface plasmon polariton (SPP) of VO2 nanoparticles, the switch of LCs under the external electrical field has been remarkably accelerated. The operating voltage to switch LCs in a 5 μm cell assembled using a pair of 0.5% VO2 nanoparticle doped h-PI hybrid alignment layers has been reduced to 3.536 V, and simultaneously, the decaying time is also shortened more than 18%. Notably, the vertical alignment of LCs was also stabilized under bending thanks to the use of undulating hybrid alignment layers. The fabricated flexible LC cell using 0.5% VO2 nanoparticle doped h-PI alignment layers yielded good flexibility at a bending radius of 1500 mm without aggravated light leakages and deduced electro-optical performance, and the VO2 agglomerates hindering the slide of LCs by providing adequate obstructions are believed as the main contributors in firmly stabilizing the vertical alignment of LCs during bending. Compared to micropatterned substrates, ultrathin VO2 nanoparticle doped h-PI alignment layers employed in flexible LC cells demonstrating their priorities in easy investigation and displaying quality improvement are emerging, highly appreciated, and of great interest for the potential applications in flexible LCDs.

Heterogeneous integration of single-crystalline rutile nanomembranes with steep phase transition on silicon substrates

Unrestricted integration of single-crystal oxide films on arbitrary substrates has been of great interest to exploit emerging phenomena from transition metal oxides for practical applications. Here, we demonstrate the release and transfer of a freestanding single-crystalline rutile oxide nanomembranes to serve as an epitaxial template for heterogeneous integration of correlated oxides on dissimilar substrates. By selective oxidation and dissolution of sacrificial VO2 buffer layers from TiO2/VO2/TiO2 by H2O2, millimeter-size TiO2 single-crystalline layers are integrated on silicon without any deterioration. After subsequent VO2 epitaxial growth on the transferred TiO2 nanomembranes, we create artificial single-crystalline oxide/Si heterostructures with excellent sharpness of metal-insulator transition (triangle rho /rho > 103) even in ultrathin (<10 nm) VO2 films that are not achievable via direct growth on Si. This discovery offers a synthetic strategy to release the new single-crystalline oxide nanomembranes and an integration scheme to exploit emergent functionality from epitaxial oxide heterostructures in mature silicon devices.

Approaching ultrathin VO2 films on sapphire (001) substrates by biased reactive sputtering: Characteristic morphology and its effect on the infrared-light switching

Ultrathin VO2 films with insulator-metal transition (IMT) were successfully fabricated on sapphire (001) substrates by utilizing radio frequency-biased reactive sputtering. We realized a 6 nm-thick VO2 film that shows resistance change over 2 orders of magnitude. Microscopic observations combined with energy dispersive x-ray analyses revealed characteristic networking morphology in VO2 films with thickness up to around 10 nm. It was found through micro-Raman analyses that a 30 nm-thick film possessed flat surface and ordered lattice with strong in-plane tensile stress. We evaluated the thickness dependence of optical switching performance for infrared-light. The results suggest that the thickness of the VO2 films should be carefully selected for realizing required performances of optical switching, which depends on not only IMT but also characteristic morphological aspects.

Mid-infrared adaptive thermal camouflage using a phase-change material coupled dielectric nanoantenna

Recently, camouflage technology has attracted researchers’ attention in a large variety of thermal applications. As a special phase change material (PCM), vanadium dioxide (VO2) is an excellent candidate for the studies conducted on thermal camouflage technology. VO2 has a transition from the insulator phase to the metal phase with the increase of the temperature. With regards to this unique feature, VO2 can contribute dynamic properties to the camouflage design. In this paper, a PCM–dielectric based metamaterial mid-infrared adaptive thermal camouflage nanoantenna is designed to perfectly mimic the atmospheric windows. The adaptive property of the proposed structure is obtained by using an ultrathin VO2 interlayer embedded within the grating. The spectral responses of the structure are computed using the finite difference time domain method, and the invisibility of the structure is proved using power calculations in the different mid-infrared regions.

Reversible Switching of Interlayer Exchange Coupling through Atomically Thin VO2 via Electronic State Modulation

Changing the interlayer exchange coupling between magnetic layers in-situ is a key issue of spintronics, as it allows for the optimization of properties that are desirable for applications, including magnetic sensing and memory. In this paper, we utilize the phase change material VO2 as a spacer layer to regulate the interlayer exchange coupling between ferromagnetic layers with perpendicular magnetic anisotropy. The successful growth of ultra-thin (several nanometres) VO2 films is realized by sputtering at room temperature, which further enables the fabrication of [Pt/Co]2/VO2/[Co/Pt]2 multilayers with distinct interfaces. Such a magnetic multilayer exhibits an evolution from antiferromagnetic coupling to ferromagnetic coupling as the VO2 undergoes a phase change. The underlying mechanism originates from the change in the electronic structure of the spacer layer from an insulating to a metallic state. As a demonstration of phase change spintronics, this work may reveal the great potential of material innovations for next-generation spintronics.

Tunable broadband, wide angle and lithography-free absorber in the near-infrared using an ultrathin VO2 film

We demonstrate a tunable broadband near-infrared absorber composed of VO2 /Al2O3/ITO layers with a facile lithography-free method. Simulated and experimental spectra show that absorption bands with above 90% absorptance can be tuned from 1.62 ∼ 2.42 μm to 1.2 ∼ 1.5 μm by thermally induced VO2 phase transition. Furthermore, the absorber with VO2 in the intermediate state can yield a broadband perfect absorption over the wavelength range of 1.2 ∼ 2.5 μm. Besides high absorption performance can still be remained at the incident up to 70° for both polarizations due to the presence of ultrathin layer of VO2 (λ/52) in the absorber.

Simple method preparation for ultrathin VO2 thin film and control: nanoparticle morphology and optical transmittance

Nanoscale morphology of vanadium dioxide (VO2) thin films can be controlled to realize smooth ultrathin crystalline films (<10 nm) with a simple way of DC sputtering. Here, crystallization annealing conditions determine whether a continuous ultrathin film or nanoparticle morphology is obtained. The experiments show that ultrathin VO2 thin films possess both a highly crystal orientation (020) and an obvious metal-insulator transition (MIT). Meanwhile, optical transmittance between the visible and near-infrared regions can be modulated by thin film thickness, and the strain in the VO2 lattice is also found to be dependent on the thin film thickness. The 8 nm thickness thin film shows that the change of resistance and visible transmittance is 2.8 orders of magnitude and 77.5%, respectively.

Ultrathin VO2(B) Nanosheets as Cathode Material for High-Performance Hybrid Magnesium-Lithium Ion Batteries

With the worldwide demand for electrochemical energy storage continuing to rise, rechargeable magnesium-based batteries have received considerable attention, because of their low cost, high volumetric capacity, and dendrite-free nature. However, limited cathode options and parasitic side reactions hinder the development of magnesium-based batteries. To overcome these issues, a self-designed hybrid magnesium-lithium ion batteries using ultrathin vanadium dioxide nanosheets as the cathode, magnesium as the anode, molybdenum as the current collector and an all-phenyl complex with lithium chloride as the electrolyte was assembled in this work. The battery delivers outstanding electrochemical performance with a high reversible capacity of 275 mAh g−1, standout rate performance of 145 mAh g−1 at a current density of 4 A g−1, long cycle life and high energy density of 484 Wh kg−1. This superior electrochemical performance demonstrates the great potential of ultrathin vanadium dioxide nanosheets for application in hybrid magnesium-lithium ion batteries.

Vacancy-Driven Robust Metallicity of Structurally Pinned Monoclinic Epitaxial VO2 Thin Films.

Vanadium dioxide (VO2) is a strongly correlated material with 3d electrons, which exhibits temperature-driven insulator-to-metal transition with a concurrent change in the crystal symmetry. Interestingly, even modest changes in stoichiometry-induced orbital occupancy dramatically affect the electrical conductivity of the system. Here, we report a successful transformation of epitaxial monoclinic VO2 thin films from a conventionally insulating to permanently metallic behavior by manipulating the electron correlations. These ultrathin (∼10 nm) epitaxial VO2 films were grown on NiO(111)/Al2O3(0001) pseudomorphically, where the large misfit between NiO and Al2O3 were fully relaxed by domain-matching epitaxy. Complete conversion from an insulator to permanent metallic phase is achieved through injecting oxygen vacancies ( x ∼ 0.20 ± 0.02) into the VO2- x system via annealing under high vacuum (∼5 × 10-7 Torr) and increased temperature (450 °C). Systematic introduction of oxygen vacancies partially converts V4+ to V3+ and generates unpaired electron charges which result in the emergence of donor states near the Fermi level. Through the detailed study of the vibrational modes by Raman spectroscopy, hardening of the V-V vibrational modes and stabilization of V-V dimers are observed in vacuum-annealed VO2 films, providing conclusive evidence for stabilization of a monoclinic phase. This ultimately leads to convenient free-electron transport through the oxygen-deficient VO2- x thin films, resulting in metallic characteristics at room temperature. With these results, we propose a defect engineering pathway through the control of oxygen vacancies to tune electrical and optical properties in epitaxial monoclinic VO2.

The ultrathin VO2(M) film with ultrahigh visible transmittance synthesized on the quartz glass substrate by HiPIMS

This paper described a high power impulse magnetron sputtering preparation of ultrathin polycrystalline VO2(M) thin film (<10 nm) on the quartz glass substrate. Combined with our experimental results, we find out that the integral visible transmittance changes with the thickness, and the ultrahigh transmittance values of 75.8% and 77.1% are obtained for the insulator and metal states, respectively. These results represent the best data for VO2 films to date. At lower substrate temperature, the room temperature structures for thicker films and thinner films are VO2(M) and VO2(B), respectively. The thinner VO2(M) films can only be successfully prepared by elevating substrate temperature to 485 °C. The phase transition temperature of the VO2(M) films shows little change with thickness at the same deposition temperature. In addition, the integral visible transmittance exceeds 54.8% for the thinner films deposited at 485 °C. Therefore, they are perfect smart window materials that need high visible transmittance.

Naturally formed ultrathin V2O5 heteroepitaxial layer on VO2/sapphire(001) film

Vanadium dioxide (VO2) and vanadium pentoxide (V2O5) thin films change their properties in response to external stimuli such as photons, temperature, electric field and magnetic field and have applications in electronics, optical devices, and sensors. Due to the multiple valence states of V and non-stoichiometry in thin films, it is challenging to grow epitaxial, single-phase V-oxide on a substrate, or a heterostructure of two epitaxial V-oxides. We report the formation of a heterostructure consisting of a few nm thick ultrathin V2O5 epitaxial layer on pulsed laser deposited tens of nm thick epitaxial VO2 thin films grown on single crystal Al2O3(001) substrates without post annealing of the VO2 film. The simultaneous observation of the ultrathin epitaxial V2O5 layer and VO2 epitaxial film is only possible by our unique reflection high energy electron diffraction pole figure analysis. The out-of-plane and in-plane epitaxial relationships are V2O5[100]||VO2[010]||Al2O3[001] and V2O5[032¯]||VO2[100]||Al2O3[11¯0], respectively. The existence of the V2O5 layer on the surface of the VO2 film is also supported by X-ray photoelectron spectroscopy and Raman spectroscopy.