Density functional theory study of rutile VO2 surfaces

Multimodal Artificial Synapses for Neuromorphic Application

The rapid development of neuromorphic computing has led to widespread investigation of artificial synapses. These synapses can perform parallel in-memory computing functions while transmitting signals, enabling low-energy and fast artificial intelligence. Robots are the most ideal endpoint for the application of artificial intelligence. In the human nervous system, there are different types of synapses for sensory input, allowing for signal preprocessing at the receiving end. Therefore, the development of anthropomorphic intelligent robots requires not only an artificial intelligence system as the brain but also the combination of multimodal artificial synapses for multisensory sensing, including visual, tactile, olfactory, auditory, and taste. This article reviews the working mechanisms of artificial synapses with different stimulation and response modalities, and presents their use in various neuromorphic tasks. We aim to provide researchers in this frontier field with a comprehensive understanding of multimodal artificial synapses.

Plasmonic switches based on VO2as the phase change material

In this paper, a comprehensive review of the recent advancements in the design and development of plasmonic switches based on vanadium dioxide (VO2) is presented. Plasmonic switches are employed in applications such as integrated photonics, plasmonic logic circuits and computing networks for light routing and switching, and are based on the switching of the plasmonic properties under the effect of an external stimulus. In the last few decades, plasmonic switches have seen a significant growth because of their ultra-fast switching speed, wide spectral tunability, ultra-compact size, and low losses. In this review, first, the mechanism of the semiconductor to metal phase transition in VO2is discussed and the reasons for employing VO2over other phase change materials for plasmonic switching are described. Subsequently, an exhaustive review and comparison of the current state-of-the-art plasmonic switches based on VO2proposed in the last decade is carried out. As the phase transition in VO2can be activated by application of temperature, voltage or optical light pulses, this review paper has been categorized into thermally-activated, electrically-activated, and optically-activated plasmonic switches based on VO2operating in the visible, near-infrared, infrared and terahertz frequency regions.

Effects of oxygen pressure on the morphology and surface energetics of β-PbO2: insight from DFT calculations

For over a century, lead dioxide (PbO₂) has been investigated in lead-acid batteries and extensively utilized in a variety of applications. Identifying the surface properties and equilibrium morphology of β-PbO₂ (rutile phase) particles is mandatory for industrial utilization and surface engineering. Using density-functional calculations within the generalized gradient approximation revised for solids (PBEsol), we investigate a variety of surface properties of β-PbO₂. The surface energies of low-Miller-index stoichiometric surfaces are firstly determined, and the (110) surface is found to be the most thermodynamically stable. The relative energetics of these surfaces are represented by a Wulff construction which shows an acicular shape, mostly dominated by the (110) and (100) surfaces. Besides, we investigate the surface chemistry of β-PbO₂ under reduction and oxidation conditions as a function of oxygen pressure, finding that most surfaces except for (100) and (110) are likely to be oxidized. Under oxygen pressure at 1 atm and oxygen-rich limit, the (101) surface is the most thermodynamically stable, dominating the Wulff construction with pyramidal shapes. Our results indicate that the growth conditions that cause non-stoichiometry of the surface could modify the equilibrium Wulff shape of β-PbO₂. Our predicted Wulff shapes and dominant facets agree with the experimental results in which the pyramidal shape of the β-PbO₂ grains has often been observed with the (101) preferred orientation.

Decoupled ultrafast electronic and structural phase transitions in photoexcited monoclinic VO2

Photoexcitation has emerged as an efficient way to trigger phase transitions in strongly correlated materials. There are great controversies about the atomistic mechanisms of structural phase transitions (SPTs) from monoclinic (M₁-) to rutile (R-) VO₂ and its association with electronic insulator-metal transitions (IMTs). Here, we illustrate the underlying atomistic processes and decoupling nature of photoinduced SPT and IMT in nonequilibrium states. The photoinduced SPT proceeds in the order of dilation of V-V pairs and increase of twisting angles after a small delay of ~40 fs. Dynamic simulations with hybrid functionals confirm the existence of isostructural IMT. The photoinduced SPT and IMT exhibit the same thresholds of electronic excitations, indicating similar fluence thresholds in experiments. The IMT is quasi-instantaneously (<10 fs) generated, while the SPT takes place with time a constant of 100 to 300 fs. These findings clarify some key controversies in the literature and provide insights into nonequilibrium phase transitions in correlated materials.

VOx Phase Mixture of Reduced Single Crystalline V2O5: VO2 Resistive Switching

The strongly correlated electron material, vanadium dioxide (VO2), has seen considerable attention and research application in metal-oxide electronics due to its metal-to-insulator transition close to room temperature. Vacuum annealing a V2O5(010) single crystal results in Wadsley phases (VnO2n+1, n > 1) and VO2. The resistance changes by a factor of 20 at 342 K, corresponding to the metal-to-insulator phase transition of VO2. Macroscopic voltage-current measurements with a probe separation on the millimetre scale result in Joule heating-induced resistive switching at extremely low voltages of under a volt. This can reduce the hysteresis and facilitate low temperature operation of VO2 devices, of potential benefit for switching speed and device stability. This is correlated to the low resistance of the system at temperatures below the transition. High-resolution transmission electron microscopy measurements reveal a complex structural relationship between V2O5, VO2 and V6O13 crystallites. Percolation paths incorporating both VO2 and metallic V6O13 are revealed, which can reduce the resistance below the transition and result in exceptionally low voltage resistive switching.

Embedded metallic nanoparticles facilitate metastability of switchable metallic domains in Mott threshold switches

Mott threshold switching, which is observed in quantum materials featuring an electrically fired insulator-to-metal transition, calls for delicate control of the percolative dynamics of electrically switchable domains on a nanoscale. Here, we demonstrate that embedded metallic nanoparticles (NP) dramatically promote metastability of switchable metallic domains in single-crystal-like VO₂ Mott switches. Using a model system of Pt-NP-VO₂ single-crystal-like films, interestingly, the embedded Pt NPs provide 33.3 times longer ‘memory’ of previous threshold metallic conduction by serving as pre-formed ‘stepping-stones’ in the switchable VO₂ matrix by consecutive electical pulse measurement; persistent memory of previous firing during the application of sub-threshold pulses was achieved on a six orders of magnitude longer timescale than the single-pulse recovery time of the insulating resistance in Pt-NP-VO₂ Mott switches. This discovery offers a fundamental strategy to exploit the geometric evolution of switchable domains in electrically fired transition and potential applications for non-Boolean computing using quantum materials.

Control of percolative dynamics of metal and insulator domains during electrically triggered insulator-metal transition underlies applications in energy-efficient switches. Jo et al. show that embedded metallic nanoparticles enhance the metastability and memory effects of metallic domains in VO₂ switches.

Unravelling the Catalytic Activity of MnO2, TiO2, and VO2 (110) Surfaces by Oxygen Coadsorption on Sodium-Adsorbed MO2 {M = Mn, Ti, V}

Metal-air batteries have attracted extensive research interest owing to their high theoretical energy density. However, most of the previous studies have been limited by applying pure oxygen in the cathode, without taking into consideration the effect of the catalyst, which plays a significant role in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Adsorption of oxygen on (110) Na-MO₂ is investigated, using density functional theory (DFT) calculations, which is important in the discharging and charging of Na-air batteries. Adsorption of oxygen on Na/MO₂ was investigated, and it was observed that the catalysts encourage the formation of the discharge product reported in the literature, i.e., NaO₂. The surface NaO₂ appears to have bond lengths comparable to those reported for monomer NaO₂.

Photo-induced non-volatile VO2 phase transition for neuromorphic ultraviolet sensors

In the quest for emerging in-sensor computing, materials that respond to optical stimuli in conjunction with non-volatile phase transition are highly desired for realizing bioinspired neuromorphic vision components. Here, we report a non-volatile multi-level control of VO₂ films by oxygen stoichiometry engineering under ultraviolet irradiation. Based on the reversible regulation of VO₂ films using ultraviolet irradiation and electrolyte gating, we demonstrate a proof-of-principle neuromorphic ultraviolet sensor with integrated sensing, memory, and processing functions at room temperature, and also prove its silicon compatible potential through the wafer-scale integration of a neuromorphic sensor array. The device displays linear weight update with optical writing because its metallic phase proportion increases almost linearly with the light dosage. Moreover, the artificial neural network consisting of this neuromorphic sensor can extract ultraviolet information from the surrounding environment, and significantly improve the recognition accuracy from 24% to 93%. This work provides a path to design neuromorphic sensors and will facilitate the potential applications in artificial vision systems.

Bioinspired neuromorphic vision components are highly desired for the emerging in-sensor computing technology. Here, Ge et al. develop an array of optoelectronic synapses capable of memorizing and processing ultraviolet images facilitated by photo-induced non-volatile phase transition in VO₂ films.

First principles studies of the electronic and structural properties of the rutile VO2(110) surface and its oxygen-rich terminations

We present a density functional theory (DFT) study of the structural and electronic properties of the bare metallic rutile VO₂(110) surface and its oxygen-rich terminations. Due to the polyvalent nature of vanadium and abundance of oxide phases, the modelling of this material on the DFT level remains a challenging task. We discuss the performance of various DFT functionals, including PBE, PBE +U(U= 2 eV), SCAN and SCAN + rVV functionals with non-magnetic and ferromagnetic spin ordering, and show that the calculated phase stabilities depend on the chosen functional. We predict the presence of a ring-like termination that is electronically and structurally related to an insulating V₂O₅(001) monolayer and shows a higher stability than pure oxygen adsorption phases. Our results show that employing the spin-polarized SCAN functional offers a good compromise, as it offers both a reasonable description of the structural and electronic properties of the rutile VO₂bulk phase and the enthalpy of formation for oxygen rich vanadium phases present at the surface.

Surfaces of VO2 -Polymorphs: Structure, Stability and the Effect of Doping

Vanadium dioxide is an interesting and frequently applied material due to its metal‐insulator phase transition. However, there are only few studies of the catalytic activity and surface properties of different VO₂ polymorphs. Therefore, we investigated the properties of the surfaces of the most stable VO₂ phases theoretically at density‐functional theory level using a self‐consistent hybrid functional which has demonstrated its accuracy for the prediction of structural, electronic and energetic properties in a previous study. We found that the surfaces of the rutile R phase of VO₂ are not stable and show a spontaneous phase transition to the monoclinic M₁ phase. Doping with Mo stabilizes the surfaces with rutile structure even for small dopant concentrations (6.25 %). Both M₁ and R surfaces strongly relax, with and without doping. In particular the metal‐metal distances in the uppermost layers change by up to 0.4 Å. Mo segregates in the topmost layer of both R and M₁ phases. The electronic structure is only slightly changed upon doping.

The formation of TiO2/VO2 multilayer structure via directional cationic diffusion

The alternative VO2/TiO2 nanostructure is a potential candidate for application in optical or electrical devices. A promising and straightforward route to form tunable alternative VO2/TiO2 nanostructure is in high demand. Herein, we demonstrate that the VO2/TiO2 nanostructure could be self-assembled from the VO2 film/TiO2 substrate via directional cationic migration, characterizing Ti-rich nano-lamellas with nanoscale spacing along the c-axis. Through aberration-corrected high-resolution transmission electron microscopy, it has been shown that the realization of directional cationic migration is assisted by the interstitial position inside the VO2 lattice. Non-equilibrium cationic diffusion could even retain these interstitial atoms in the form of incoherent strain lines, which affect the local electronic structure as validated by theoretical calculation. Due to Ti-rich nano-lamellas and incoherent strain lines, the phase transition temperature decreased (∼10 °C). The idea of tailoring the elemental distribution by directional cationic diffusion significantly broadens the functional application of VO2 films.

The morphology of VO2/TiO2(001): terraces, facets, and cracks

Vanadium dioxide (VO₂) features a pronounced, thermally-driven metal-to-insulator transition at 340 K. Employing epitaxial stress on rutile \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text{TiO}_{2}(001)$$\end{document}TiO2(001) substrates, the transition can be tuned to occur close to room temperature. Striving for applications in oxide-electronic devices, the lateral homogeneity of such samples must be considered as an important prerequisite for efforts towards miniaturization. Moreover, the preparation of smooth surfaces is crucial for vertically stacked devices and, hence, the design of functional interfaces. Here, the surface morphology of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text{VO}_2/\text{TiO}_2(001)$$\end{document}VO2/TiO2(001) films was analyzed by low-energy electron microscopy and diffraction as well as scanning probe microscopy. The formation of large terraces could be achieved under temperature-induced annealing, but also the occurrence of facets was observed and characterized. Further, we report on quasi-periodic arrangements of crack defects which evolve due to thermal stress under cooling. While these might impair some applicational endeavours, they may also present crystallographically well-oriented nano-templates of bulk-like properties for advanced approaches.

In Situ Conversion of Metal-Organic Frameworks into VO2 -V3 S4 Heterocatalyst Embedded Layered Porous Carbon as an "All-in-One" Host for Lithium-Sulfur Batteries

Although lithium-sulfur batteries exhibit a fivefold higher energy density than commercial lithium-ion batteries, their volume expansion and insulating nature, and intrinsic polysulfide shuttle have hindered their practical application. An alternative sulfur host is necessary to realize porous, conductive, and polar functions; however, there is a tradeoff among these three critical factors in material design. Here, the authors report a layered porous carbon (LPC) with VO₂ /V₃ S₄ heterostructures using one-step carbonization-sulfidation of metal-organic framework templates as a sulfur host that meets all the criteria. In situ conversion of V-O ions into V₃ S₄ nuclei in the confined 2D space generated by dynamic formation of the LPC matrix creates {200}-facet-exposed V₃ S₄ nanosheets decorated with tiny VO₂ nanoparticles. The VO₂ /V₃ S₄  @ LPC composite facilitates high sulfur loading (70 wt%), superior energy density (1022 mA h g^-1 at 0.2 C, 100 cycles), and long-term cyclability (665 mA h g^-1 at 1 C, 1000 cycles). The enhanced Li-S chemistry is attributed to the synergistic heterocatalytic behavior of polar VO₂ and conductive V₃ S₄ in the soft porous LPC scaffold, which accelerates polysulfide adsorption, conversion, and charge-transfer ability simultaneously.

Direct Probing of Oxygen Loss from the Surface Lattice of Correlated Oxides during Hydrogen Spillover

Hydrogen spillover is a catalytic process that occurs by surface reaction and subsequent diffusion to reversibly provide a massive amount of hydrogen dopants in correlated oxides, but the mechanism at the surface of correlated oxides with metal catalyst are not well understood. Here we show that a significant amount of oxygen is released from the surface of correlated VO₂ films during hydrogen spillover, contrary to the well-established observation of the formation of hydrogen interstitials in the bulk part of VO₂ films. By using ambient-pressure X-ray photoelectron spectroscopy, we prove that the formation of surface oxygen vacancies is a consequence of a favorable reaction for the generation of weakly adsorbed H₂O from surface O atoms that have low coordination and weak binding strength. Our results reveal the importance of in situ characterization to prove the dynamic change during redox reaction and present an opportunity to control intrinsic defects at the surface.

Growth of vanadium dioxide thin films on hexagonal boron nitride flakes as transferrable substrates

Vanadium dioxide (VO2) is an archetypal metal-insulator transition (MIT) material, which has been known for decades to show an orders-of-magnitude change in resistivity across the critical temperature of approximately 340 K. In recent years, VO2 has attracted increasing interest for electronic and photonic applications, along with advancement in thin film growth techniques. Previously, thin films of VO2 were commonly grown on rigid substrates such as crystalline oxides and bulk semiconductors, but the use of transferrable materials as the growth substrates can provide versatility in applications, including transparent and flexible devices. Here, we employ single-crystalline hexagonal boron nitride (hBN), which is an insulating layered material, as a substrate for VO2 thin film growth. VO2 thin films in the polycrystalline form are grown onto hBN thin flakes exfoliated onto silicon (Si) with a thermal oxide, with grains reaching up-to a micrometer in size. The VO2 grains on hBN are orientated preferentially with the (110) surface of the rutile structure, which is the most energetically favorable. The VO2 film on hBN shows a MIT at approximately 340 K, across which the resistivity changes by nearly three orders of magnitude, comparable to VO2 films grown on common substrates such as sapphire and titanium dioxide. The VO2/hBN stack can be picked up from the supporting Si and transferred onto arbitrary substrates, onto which VO2 thin films cannot be grown directly. Our results pave the way for new possibilities for practical and versatile applications of VO2 thin films in electronics and photonics.

Scandium Molybdate Microstructures with Tunable Phase and Morphology: Microwave Synthesis, Theoretical Calculations, and Photoluminescence Properties

In this paper, scandium molybdate microstructures have been prepared from solution via a microwave heating method. By controlling the experimental parameters such as molar ratio of reagent and reaction time, scandium molybdates with tunable phase and diverse morphologies including snowflakes, microflowers, microsheets, and branched spindles were obtained. The density of states and surface energies of Sc₂Mo₃O₁₂ were primarily studied from first-principles calculations. An indirect band gap of 3.56 eV was observed for crystalline Sc₂Mo₃O₁₂, and the surface energies of various facets were determined to be 0.27-0.91 J/m². The influence of n(Sc³⁺): n(Mo₇O₂₄^6-) (short for Sc/Mo) molar ratio was systematically investigated and well-characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and UV-vis absorption spectroscopy (UV-vis). Results indicate that the Sc/Mo molar ratio has a great effect on the phase and morphology. Diffuse reflection spectra (DRS) revealed the E_gap can be readily tuned from 3.69 to 4.16 eV, which is in accordance with the theoretical result. The photoluminescence (PL) properties of Eu³⁺-doped Sc₂Mo₃O₁₂ were discussed. This facile synthesis strategy could be extended to the synthesis of other molybdates.

Computational Approach for Epitaxial Polymorph Stabilization through Substrate Selection

With the ultimate goal of finding new polymorphs through targeted synthesis conditions and techniques, we outline a computational framework to select optimal substrates for epitaxial growth using first principle calculations of formation energies, elastic strain energy, and topological information. To demonstrate the approach, we study the stabilization of metastable VO2 compounds which provides a rich chemical and structural polymorph space. We find that common polymorph statistics, lattice matching, and energy above hull considerations recommends homostructural growth on TiO2 substrates, where the VO2 brookite phase would be preferentially grown on the a-c TiO2 brookite plane while the columbite and anatase structures favor the a-b plane on the respective TiO2 phases. Overall, we find that a model which incorporates a geometric unit cell area matching between the substrate and the target film as well as the resulting strain energy density of the film provide qualitative agreement with experimental observations for the heterostructural growth of known VO2 polymorphs: rutile, A and B phases. The minimal interfacial geometry matching and estimated strain energy criteria provide several suggestions for substrates and substrate-film orientations for the heterostructural growth of the hitherto hypothetical anatase, brookite, and columbite polymorphs. These criteria serve as a preliminary guidance for the experimental efforts stabilizing new materials and/or polymorphs through epitaxy. The current screening algorithm is being integrated within the Materials Project online framework and data and hence publicly available.

Tuning the phase transition temperature, electrical and optical properties of VO2 by oxygen nonstoichiometry: insights from first-principles calculations

The O-vacancy in bulk VO₂ gives rise to an increase in electron concentration, which induces a decrease in T_c. While, O-vacancy and O-adsorption on VO₂(R) (1 1 0) and VO₂(M) (0 1 1) surfaces could alter the work functions and in turn regulate T_c.

Dual electrical behavior of multivalent metal cation-based oxide and its application to thin-film transistors with high mobility and excellent photobias stability

The effect of multivalent metal cations, including vanadium(V) and tin (Sn), on the electrical properties of vanadium-doped zinc tin oxide (VZTO) was investigated in the context of the fabrication of thin-film transistors (TFTs) using a single VZTO film and VZTO/ZTO bilayer as channel layers. The single VZTO TFT did not show any response to the gate voltage (insulator-like behavior). On the other hand, the VZTO/ZTO bilayer TFT exhibited a typical TFT transfer characteristic (semiconducting behavior). X-ray photoelectron spectroscopy revealed that, in contrast to what is commonly true in many oxides, oxygen vacancies (V(O)) in VZTO did not provide a dominant contribution to the total carrier concentration, because the V(O) peak area in the single VZTO film was 5.4% and reduced to 4.5% in VZTO/ZTO bilayer. Instead, Sn 3d5/2 and V 2p3/2 spectra suggest that the significant reduction in Sn and V ions is strongly related to the insulator-like behavior of the VZTO film. In negative-bias illumination tests and illumination tests with various photon energies, the VZTO/ZTO bilayer TFT had much better stability than the ZTO TFT. This result is attributed to the reduction of donor-like states ([Formula: see text]O) that can be positively ionized by blue and green illumination.

A DFT study of the structures, stabilities and redox behaviour of the major surfaces of magnetite Fe3O4

Redox behaviour of magnetite Fe₃O₄ nanoparticles in thermodynamic equilibrium conditions enclosed by non-dipolar reconstructed surfaces.