Monoclinic VO2 Research Articles

Ultrahigh Infrared Photoresponse from Core–Shell Single‐Domain‐VO2/V2O5 Heterostructure in Nanobeam

Infrared (IR) harvesting and detection in red and near‐IR (NIR) part of the solar spectrum have always been a long‐term research area of intense interest. However, limited choices of current photoactive materials have significantly hampered the realization of ultrahigh IR sensitivity under room temperature conditions. The trigger for this requires the exploration of new photo­active materials and the ability to fabricate new photoactive structural design. Herein, a new oxide‐catalogue photoconductive NIR detector with ultrahigh performance built by core/shell nanobeam heterostructures (CSNHs) with the inner single‐domain monoclinic VO2 (M) core and outer V2O5 shell, which is the first example of photoconductive IR detector made from transition metal oxides (TMOs), is presented. Benefited from the well‐defined TMO hetero­junction interface, the ultrahigh responsivity (Rλ) of 2873.7 A W‐1 and specific detectivity (D*) of 9.23 × 1012 Jones are achieved at room temperature (at 990 nm; 0.2 mW cm‐2), recording the best performance compared with those reported IR detectors based on heavy‐metal‐free materials, and even comparable/superior to those traditional ones made from materials including heavy metals. These findings pave a new way to design oxide heterostructures for intriguing applications in optoelectronic and energy harvesting nanodevices.

One-step hydrothermal conversion of VO2(B) into W-doped VO2(M) and its phase transition and optical switching properties

W-doped monoclinic vanadium dioxide VO2(M) nanobelts have been transformed from VO2(B) nanobelts in the presence of tungstic acid by a facile one-step hydrothermal route for the first time. The as-prepared samples were characterized by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and variable-temperature infrared spectroscopy (IR). The results show that W atoms are doped into the crystal lattice of VO2(M) matrix, and the morphology and size of the as-obtained VO2(M) nanobelts are dependent on that of the precursor VO2(B). The phase transition temperature (Tc) of W-doped VO2(M) can be tuned by W doping. The variable-temperature IR reveals that the as-synthesized W-doped VO2(M) nanobelts have outstanding thermochromic characters and optical switching properties.

Direct synthesis of thermochromic VO2 through hydrothermal reaction

Thermochromic VO2 was directly synthesized using hydrothermal techniques. The effects of formation conditions on the structure and morphology of the final product were studied through X-ray diffraction (XRD), and scanning electron microscopy (SEM). Unique hollow sphere morphology was observed for the synthesized VO2 powders. Ex-situ XRD studies after heat treatment confirmed the thermal stability of the VO2 structure. Thermochromic properties, as a consequence of the reversible structural transformation between monoclinic VO2 and tetragonal phases, were observed by Fourier transform infrared spectroscopy (FTIR).

Growth of oriented vanadium pentaoxide nanostructures on transparent conducting substrates and their applications in photocatalysis

A novel, hydrothermal and hard-template-free method was developed for the first time to grow oriented, single-crystalline monoclinic VO2 (B) flower-like nanorod films on transparent conductive fluorine-doped tin oxide (FTO) substrates. The length and morphology of the nanorods can be tuned by changing the growth parameters, such as growth time and initial precursor concentration. The flower-like V2O5 films were obtained after post-calcination treatment of VO2 (B) films. The photocatalytic activity of V2O5 films was investigated by the degradation of methylene blue (MB) under UV and visible light. The prepared V2O5 film exhibited good photocatalytic performance (74.6% and 63% under UV and visible light for 210min, respectively) and more practical application in industry.

Correlation between structure and semiconductor-to-metal transition characteristics of VO2/TiO2/sapphire thin film heterostructures

This study focuses on the role of strain and thin film epitaxy on the semiconductor-to-metal transition (SMT) characteristics of single crystalline VO2 thin films. The VO2/TiO2 heterostructures of controlled orientations were epitaxially grown on m-cut, r-cut and c-cut sapphire substrates. Detailed structural investigations were performed using high-resolution X-ray diffraction (2θ–θ and φ scans) and high-resolution transmission electron microscopy techniques to correlate SMT properties with microstructural characteristics. Monoclinic (M1) VO2 thin films with (100), (001) and (2¯01) out-of-plane orientations were grown on TiO2(101)/r-sapphire, TiO2(100)/c-sapphire and TiO2(001)/m-sapphire platforms, respectively. The in-plane alignments across the interfaces were established to be [010](100)VO2||[010](101)TiO2, [100](001)VO2||[001](100)TiO2 and [010](2¯01)VO2‖[010](001)TiO2 for r-sapphire, c-sapphire and m-sapphire substrates, respectively. We were able to tune the SMT temperature of VO2 epilayers from ∼313K to 354K (bulk Tc≈340K). The SMT characteristics were interpreted based upon the residual strain in the VO2 lattice, particularly along the c-axis of tetragonal VO2. This research introduces the VO2-based single crystalline heterostructures as a potential candidate for a wide range of applications where different transition temperatures are required.

Effect of Oxide Buffer Layer on the Thermochromic Properties of VO2 Thin Films

VO2 thin films were deposited on soda lime glass substrates with ZnO, TiO2, SnO2, and CeO2 thin films applied as buffer layers between the VO2 films and the substrates in order to investigate the effect of buffer layer on the formation and the thermochromic properties of VO2 film. Buffer layers with thicknesses over 50 nm were found to affect the formation of VO2 film, which was confirmed by XRD spectra. By using ZnO, TiO2, and SnO2 buffer layers, monoclinic VO2 (VO2(M)) film was successfully fabricated on soda lime glass at 370 °C. On the contrary, films of VO2(B), which is known to have no phase transition near room temperature, were formed rather than VO2(M) when the film was deposited on CeO2 buffer layer at the same film deposition temperature. The excellent thermochromic properties of the films deposited on ZnO, TiO2, and SnO2 buffer layers were confirmed from the temperature dependence of electrical resistivity from room temperature to 80 °C. Especially, due to the tendency of ZnO thin film to grow with a high degree of preferred orientation on soda lime glass at low temperature, the VO2 film deposited on ZnO buffer layer exhibits the best thermochromic properties compared to those on other buffer layer materials used in this study. These results suggest that deposition of VO2 films on soda lime glass at low temperature with excellent thermochromic properties can be achieved by considering the buffer layer material having structural similarity with VO2. Moreover, the degree of crystallization of buffer layer is also related with that of VO2 film, and thus ZnO can be one of the most effective buffer layer materials.

Transport behavior and electronic structure of phase pure VO2 thin films grown on c-plane sapphire under different O2 partial pressure

We grew highly textured phase pure VO2 thin films on c-plane Al2O3 substrates with different oxygen partial pressure. X-ray absorption and photoemission spectroscopy confirm the identical valence state of vanadium ions despite the different oxygen pressure during the deposition. As the O2 flow rate increases, the [010] lattice parameter for monoclinic VO2 was reduced and coincidently distinctive changes in the metal-semiconductor transition (MST) and transport behaviors were observed despite the identical valence state of vanadium in these samples. We discuss the effect of the oxygen partial pressure on the monoclinic structure and electronic structure of VO2, and consequently the MST.

Ultrathin Nanosheets of Half‐Metallic Monoclinic Vanadium Dioxide with a Thermally Induced Phase Transition

Only the half of it: A modified intercalation-deintercalation strategy that takes advantage of the characteristic thermally induced phase transition of monoclinic VO2 has been developed for the preparation of ultrathin VO2 nanosheets. The resultant nanosheets exhibit half-metallic character and a temperature-dependent phase transition. The half-metallicity could lead to applications in spintronic nanodevices.

Elastic properties of VO2 from first-principles calculation

We used first-principles methods to calculate the elastic properties of rutile (R) structure and monoclinic (M1: space group P21/c, M2: space group C2/m) structure VO2, including single-crystal elastic constants cij's, polycrystalline bulk modulus, shear modulus, Young's modulus and elastic anisotropy ratio. We found that the energy difference among the R, M1 and M2 phases is small, indicating that it is easy to transit among them under a perturbation. Furthermore, from the pressure dependence of cij's, we also found that the structural instability (or phase transition) will occur when the volumes of the three phases are slightly smaller than their equilibrium volumes. Additionally, the R and M2 phases are predicted to be harder than the M1 phase, indicated by their larger bulk moduli and shear moduli. The elastic anisotropy of the M2 phase is larger than the M1 and R phases. The presently predicted elastic properties of VO2 provide helpful guidance for the strain energy estimation and stress analysis in nano-electronic devices.

Pulsed laser-deposited VO2 thin films on Pt layers

VO2 films were deposited on Pt (111)/TiO2/SiO2/Si (001) substrates by means of a pulsed laser deposition technique. An x-ray diffraction peak at 2θ = 39.9° was deconvoluted into two pseudo-Voigt profiles of Pt (111) and VOx-originated components. The VOx diffraction peak was more obvious in a VOx/Pt (111)/Al2O3 (0001) sample, having a narrower width compared with a VO2/Al2O3 (0001) sample. Temperature-controlled Raman spectroscopy for the VOx/Pt/TiO2/SiO2/Si sample has revealed the monoclinic VO2 phase at low temperature and the structural phase transition at about 72 °C in a heating process. The electronic conductive nature at the high temperature phase was confirmed by near normal incidence infrared reflectivity measurements. Out-of-plane current-voltage characteristics showed an electric field-induced resistance switching at a voltage as low as 0.2 V for a 50 nm-thick film. A survey of present and previous results suggests an experimental law that the transition voltage of VO2 is proportional to the square root of the electrodes distance.

Theoretical study on the tungsten-induced reduction of transition temperature and the degradation of optical properties for VO2

We have studied the effect of the doped tungsten on the phase transition temperature and the optical properties between the monoclinic phase and the rutile phase of VO2 by performing first-principles calculations. It is found that the introduction of tungsten (W) significantly decreases the transition temperature between the rutile VO2 and the monoclinic VO2, with reducing the energy difference between the two phases. Meanwhile, our calculations indicate that the doped W lowers the infrared light transmittance of the monoclinic VO2, being consistent with the experimental observations. The nature about the reduction of the transition temperature and the change of the optical properties arising from the W dopants is revealed. Our results are valuable for application of VO2 as an optical energy material in future.

Facile Growth of Thermochromic VO2Nanostructures with Greatly Varied Phases and Morphologies

We have successfully and reproducibly synthesized a variety of novel vanadium dioxide (VO2) nanostructures, including metastable monoclinic VO2(B) nanoneedles and nanocorals, orthorhombic VO2(O) nanoparticles, and monoclinic VO2(M) nanofacets by manipulating the aging time of a facile sol-gel approach and the subsequent postannealing conditions. We envision that this previously unreported highly controlled synthesis and the resulting distinct morphologies of VO2 will not only provide a promising route for reliably selecting the phase, size, and morphology of these nanostructures, as well as achieving a fundamental understanding of their unusual temperature-dependent optical transmittance, but also facilitate the synthesis of functional VO2 nanostructures for a number of novel applications.

The Phase Transition of W-Doped VO<SUB>2</SUB> Nanoparticles Synthesized by an Improved Thermolysis Method

High-quality thermochromic monoclinic VO2(M) and series of W-doped V(1-x)W(x)O2(M) nanoparticles were successfully synthesized by an improved thermolysis method. The products were investigated using X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) technologies. The measurement of DSC revealed that the metal-insulator phase transition (MIT) of 2.0% W-doped VO2 sample occurred at 25.6 degrees C, which was much lower than the MIT of host VO2(M) nanoparticles at 67.6 degrees C. The results showed that the proportion of the transmittance of tetragonal phase reached only about 29% of that of monoclinic phase for 0.5% W-doped VO2 at the wavenumber 900 cm(-1), which indicated W-doped VO2(M) was an intelligent window and optical switch materials.

A low-temperature synthesis of monoclinic VO2 in an atmosphere of air

Monoclinic vanadium dioxide (VO2) particles are prepared via the thermolysis of a vanadyl ethylene glycolate precursor in an atmosphere of air. Monoclinic VO2 particles can be obtained at temperatures above 170 °C in an oven. The synthetic VO2 product exhibits high crystallinity and features a pure monoclinic phase and composition. The expected metal–insulator transition at around 68 °C is revealed by differential scanning calorimetry, variable-temperature X-ray diffraction and temperature-dependent resistance curves. The exothermic decomposition of the precursor elevates the temperature of the sample and results in a final sample consisting of monoclinic VO2 and having a high crystallinity at room temperature.

Growth and physical properties of vanadium oxide thin films with controllable phases

ABSTRACTVanadium oxides thin films with variable oxidation states have attracted great attention due to their unique electrical, optical properties and many important applications in microelectronics, infrared optical devices, and energy harvest systems. However, to fabricate vanadium oxide thin films with controllable phases and desired transport properties is still a challenge by using a chemical solution deposition (CSD) technique. In this paper, we report that vanadium oxide thin films with well controlled phases such as rhombohedral V2O3 and monoclinic VO2 could be synthesized on Al2O3 (0001) substrates using a CSD technique ---- polymer assisted deposition (PAD). Both V2O3 and VO2 thin films can be well controlled with good epitaxial quality by optimizing the fabrication parameters. The electrical resistivity changes 3∼4 orders of magnitude at metal insulator transition for both epitaxial V2O3 and VO2 thin films. The correlation between the physical properties and the microstructures of the films will be discussed.

Hydrothermal synthesis and controlled growth of vanadium oxide nanocrystals

0-, 1- and 2-dimensional nanocrystals of different vanadium oxides were synthesized using hydrothermal reaction of commercial bulk monoclinic VO2 and V2O3 precursor, which, depending on reaction conditions, produced nanoparticles, nanoribbons and nanosheets of different vanadium oxides. Addition of acetone was used to direct growth and produce two-dimensional nanosheets, whose aspect ratio depends on acetone-to-precursor concentration, while addition of different alcohols as ligands successfully controlled the nanoribbon size. Variation of pH also changes the product dimensionality, allowing production of nanosheets at pH < 3, nanoribbons at 3 < pH < 7 and, through inhibition of reaction, nanoparticles at high pH. Products of different morphology exhibit systematic increase in unit cell volume with decrease in thickness of nanocrystals. The reaction occurs with VO2+ ion as the primary reactant, requiring only a source of VO2+ ions, and not necessarily crystalline VO2(M), allowing use of other precursors, like VOSO4. Calculations indicate that nanoribbon formation is primarily caused by differences in relative stability of individual crystal planes of VO2. Addition of different amounts of hydrogen peroxide changes the oxidation state of vanadium to produce V3O7 and V2O5, without affecting the product morphology, while use of bulk V2O3 as a precursor leads to the formation of V2O3 nanoribbons, nanoleaves and nanoparticles with similar mechanisms of size and shape control.

Facile solution combustion synthesis of monoclinic VO2: a unique and versatile approach

The present synthesis of single crystalline VO2 (M) via solution combustion (SC) within 5 min at low temperature significantly reduces the complexity of handling the reaction, such as the avoidance of an inert atmosphere and multiple steps. The rod-like morphology of VO2 obtained through SC is the first of its kind.

ChemInform Abstract: Preparation of W‐ and Mo‐Doped VO2(M) by Ethanol Reduction of Peroxovanadium Complexes and Their Phase Transition and Optical Switching Properties.

AbstractThermochromic W‐ and Mo‐doped monoclinic vanadium dioxide VO2(M) with various amounts of dopants is hydrothermally synthesized from aqueous mixtures of V2O5, H2O2, EtOH, and ammonium tungstate or ammonium molybdate (autoclave, 180 °C, 48 h) followed by calcination at 700 °C for 2 h.

Epitaxial variants of VO2 thin films on complex oxide single crystal substrates with 3m surface symmetry

Rutile-based monoclinic VO2 films were grown on (111) LaAlO3, (0001) Al2O3, (111) MgAl2O4, (111) MgO, and Zn-faced (0001) and O-faced (0001̄) ZnO substrates. The out-of-plane alignment of the two-fold screw [010] axis of monoclinic VO2 with three-fold rotational axes of the substrates is made possible by the formation of epitaxial domains. We find that the in-plane [100] axis of VO2 is always oriented with one of the three in-plane eutactic axes of the all of the substrates, despite significant differences in lattice mismatch from substrate to substrate. From diffractometry analysis and upon considering the in-plane symmetry of the substrates with respect to the (010) VO2 film, we conclude the presence of 120° rotational variants and 60° twins. The electrical transport characteristics of all of the films except for those on the ZnO substrates are comparable, showing metal–insulator transitions with bulk-like transition temperatures. The growth of different stoichiometric phases and film orientations on ZnO is attributed to large lattice mismatch, and also leads to the apparent in-plane conductance transition being smeared out in temperature. We find that it is more challenging to stabilize (010) VO2 on O-faced ZnO compared to Zn-faced ZnO and discuss the differences in tetrahedra stacking between the two polarities.

Vanadium Oxide Thin Films Synthesized by Reactive Ion Beam Sputter Deposition: Influence of Processing Parameters

Vanadium oxide thin films were deposited by reactive ion beam sputtering deposition onto glass substrates. The films were prepared by sputtering from a metallic vanadium target with an argon+oxygen ion beam in vacuum. Different processing conditions were evaluated with focus in obtaining monoclinic VO2(M) phase, which is known to exhibit a semiconducting-metal phase transition near room temperature. X-ray diffractometry (XRD) analyses revealed amorphous films for temperatures below 500°C. In crystalline films, the co-existence of VO2(M) with other phases was suppressed by pre-depositing a very thin metallic vanadium seeding layer which showed to promote the formation of single phase VO2(M) films. The VO2(M) films showed clearly the distinctive optical modulation behavior at the near-infrared range when going through the phase transition. The temperature dependence of sheet resistance supports the optical analyses revealing an evident semiconducting-metal behavior change up to over 2 orders of magnitude.