Polycrystalline VO2 Research Articles

Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model.

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 °C. To grow VO2 on a wide variety of substrates, with wafer-scale uniformity and angstrom level control of thickness, the method of atomic-layer deposition was chosen. This ALD process enables high-quality, low-temperature (≤150 °C) growth of ultrathin films (100-1000 Å) of VO2. For this demonstration, the VO2 films were grown on sapphire substrates. This low temperature growth technique produces mostly amorphous VO2 films. A subsequent anneal in an ultra-high vacuum chamber with a pressure of 7x10-4 Pa of ultra-high purity (99.999%) oxygen produced oriented, polycrystalline VO2 films. The crystallinity, phase, and strain of the VO2 were determined by Raman spectroscopy and X-ray diffraction, while the stoichiometry and impurity levels were determined by X-ray photoelectron spectroscopy, and finally the morphology was determined by atomic force microscopy. These data demonstrate the high-quality of the films grown by this technique. A model was created to fit to the data for VO2 in its metallic and insulating phases in the near infrared spectral region. The permittivity and refractive index of the ALD VO2 agreed well with the other fabrication methods in its insulating phase, but showed a difference in its metallic state. Finally, the analysis of the films' optical properties enabled the creation of a wavelength- and temperature-dependent model of the complex optical refractive index for developing VO2 as a tunable refractive index material.

Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 °C. To grow VO2 on a wide variety of substrates, with wafer-scale uniformity and angstrom level control of thickness, the method of atomic-layer deposition was chosen. This ALD process enables high-quality, low-temperature (≤150 °C) growth of ultrathin films (100-1000 Å) of VO2. For this demonstration, the VO2 films were grown on sapphire substrates. This low temperature growth technique produces mostly amorphous VO2 films. A subsequent anneal in an ultra-high vacuum chamber with a pressure of 7x10-4 Pa of ultra-high purity (99.999%) oxygen produced oriented, polycrystalline VO2 films. The crystallinity, phase, and strain of the VO2 were determined by Raman spectroscopy and X-ray diffraction, while the stoichiometry and impurity levels were determined by X-ray photoelectron spectroscopy, and finally the morphology was determined by atomic force microscopy. These data demonstrate the high-quality of the films grown by this technique. A model was created to fit to the data for VO2 in its metallic and insulating phases in the near infrared spectral region. The permittivity and refractive index of the ALD VO2 agreed well with the other fabrication methods in its insulating phase, but showed a difference in its metallic state. Finally, the analysis of the films' optical properties enabled the creation of a wavelength- and temperature-dependent model of the complex optical refractive index for developing VO2 as a tunable refractive index material.

Investigation on V2O5 Thin Films Prepared by Spray Pyrolysis Technique

The spray pyrolysis technique was employed to deposit V2O5 thin films on a glass substrate. By varying the precursor solution volume from 10[Formula: see text]mL to 50[Formula: see text]mL in steps of 10[Formula: see text]mL, films of various thicknesses were prepared. Orthorhombic polycrystalline V2O5 films were inferred from the XRD pattern irrespective of precursor solution volume. The micro-Raman studies suggested that annealed V2O5 thin film has good crystallinity. The effect of precursor solution volume on morphological and optical properties were analysed and reported.

Rapid Thermal Oxidation of Sputtering Power Dependent Vanadium Thin Films for VO2 Thin Films Preparation

Rapid thermal oxidation of sputtering power dependent vanadium thin films for VO2 thin films preparation is investigated. Metallic vanadium thin films are deposited by direct current magnetron sputtering at different sputtering power. They are characterized by the atomic force microscope (AFM) and the field emission-scanning electron microscope (FE-SEM). In our experiment, we increase the sputtering power while reducing the sputtering time to maintain the same thickness of the film. In this process, the particle size of metallic vanadium increases. The vanadium thin films are oxidized at the same annealing conditions by rapid thermal annealing. X-ray diffraction (XRD) results indicate there is polycrystalline VO2 in oxidation products of the metallic vanadium with large average particle radius. Surface morphology and metal-to-insulator transition (MIT) properties of VO2 thin films are characterized. When average particle radius of the metallic vanadium is 8.78 nm, the VO2 thin film has the largest particles and maximum phase transition magnitude. These results reveal the relationship of sputtering power and the particle size of vanadium thin films, and illustrate the effect of the particle size of vanadium thin films on VO2 thin films preparation through the oxide growth rate law for the oxidation of spherical particles.

Optimization of metal-to-insulator phase transition properties in polycrystalline VO2 films for terahertz modulation applications by doping

A low TC and small ΔH were successfully achieved without significantly impairing THz MD by doping through an appropriate annealing process.

Investigation on electrically-driven semiconductor-metal transition of polycrystalline VO2 thin films on two kinds of substrates

Electrical driving is one of frequently-used stimuli for the semiconductor-metal transition (SMT) of VO2. But the driving mechanism is still under debate. We investigated the DC electrically-driven SMT features of polycrystalline VO2 thin films deposited on two kinds of substrates (quartz and silicon) with obviously-different thermal conductivity and the influence of cooling by a thermo electric cooler (TEC) on the SMT of VO2. Interestingly, the SMT doesn’t happen at a high voltage at very start, but at a relatively low one. Moreover, the SMT of VO2 thin films on silicon substrate is completely restrained by cooling through a TEC although the electric field strength across VO2 reaches 1.1×107 V/m. Our findings reveal that the Joule-heating effect plays an important role in the DC electrically-driven SMT of VO2.

Crystallization and inter-diffusional behaviors in the formation of VO2(B) thin film with layered W-doping

To achieve a better material for uncooled infrared (IR) detector, polycrystalline VO2(B) thin films with layered W-doping were fabricated on Si substrates by magnetron sputtering, and the best temperature coefficient of resistance (TCR) value reached −4.1%/K. The film synthesis was in a two-step route, first deposition at room temperature and then post-deposition annealing at 450 °C, to better control the crystallization behavior. Various transmission electron microscopy (TEM) methods were employed to investigate three sets of multi-layered films with different deposition time, 10, 20, and 30 min, with especial emphasis on the effect of layered W-doping scheme on the formation of multiple VO2(B) layers. Spatial-resolved energy dispersive X-ray spectroscopy (EDS) revealed the alternative patterns of W-rich layers and W-poor layers, while the thinner films exhibited better crystallinity and texturing. By comparison with an as-deposited film, it was found that the inter-diffusion between the two types of layers was completed in the deposition step while both remained in amorphous structure. A stable W solution of about 8 cat% in VO2(B) layers measured from all these films indicated that the layered doping can tailor the multi-layered microstructure to optimize the performance of VO2(B) films.

Influence of Discharge Current on Phase Transition Properties of High Quality Polycrystalline VO₂ Thin Film Fabricated by HiPIMS.

To fabricate high-quality polycrystalline VO2 thin film with a metal–insulator transition (MIT) temperature less than 50 °C, high-power impulse magnetron sputtering with different discharge currents was employed in this study. The as-deposited VO2 films were characterized by a four-point probe resistivity measurement system, visible-near infrared (IR) transmittance spectra, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. The resistivity results revealed that all the as-deposited films had a high resistance change in the phase transition process, and the MIT temperature decreased with the increased discharge current, where little deterioration in the phase transition properties, such as the resistance and transmittance changes, could be found. Additionally, XRD patterns at various temperatures exhibited that some reverse deformations that existed in the MIT process of the VO2 film, with a large amount of preferred crystalline orientations. The decrease of the MIT temperature with little deterioration on phase transition properties could be attributed to the reduction of the preferred grain orientations.

(Invited) Aspects of Vibrational Spectroscopy in Investigation of Low-Temperature Pigmented V-Oxide Films

Vanadium(V) oxide (V2O5) has attracted attention mainly due to its use in lithium secondary batteries. Consequently, various studies of its lithiated phases were made, leading to their identification as α, ε, δ, γ [1]. When prepared in the form of thin films V2O5 can be applied in a battery-type electrochromic (EC) devices as counter electrode, recognized for its multicolor electrochromism, electrochemical cycling stability in a specified potential range, good ion-storage capacity and relatively high transparency in the bleached state [2]. However, as for other transition metal oxides a temperature of above 300 °C is demanded for V2O5 formation, which rules out the possibility of its deposition on plastic substrates. With the aim to overwhelm this circumstance, a two-step procedure according to paint formulation procedures was developed in our laboratory. This preparation procedure has already been proven to be effective for Ni1-xO, TiO2, WO3 pigmented films [3,4]. During the first step, an oxide crystalline powder is prepared via sol-gel route and thermal treatment at elevated temperatures. As a second step, the milling process is performed in an appropriate solvent and with the use of dispersants to prevent the agglomeration of so-formed nanoparticles. The final dispersion can be used for spin-coating deposition of films on polymeric substrates, followed by thermal treatment at 150 °C. The first aim of this work is to investigate the possibility of the application of the described two-step procedure for the preparation of V-oxide pigmented films. The sol for the preparation of crystalline powder in the first step is made of vanadium(V) oxoisopropoxide diluted in 2-propanol. Other routes as hydrothermal synthesis will be investigated as well. In the second step, the size distribution of nanoparticles in dispersion is a factor that crucially determines the deposition possibilities of films. Therefore, the relation between crystalline particles and dispersant is studied via transmission electron microscopy (TEM). The structural and surface characterization of pigmented films is further made using infrared (IR) and Raman spectroscopies, scanning electron (SEM) microscopy, contact angles and surface energy. In situ UV-visible absorbance in a custom-made trielectrode cell is used to detect optical changes during intercalation/deintercalation processes. The possibilities of formation of EC devices with appropriate pigmented films will be considered and the performance compared to already existing EC systems. The second aim of this work is the study of possibilities of combined analytical tools for the investigation of intercalation/deintercalation in EC films and the developed V-oxide pigmented films will be used as an example. Especially spectroelectrochemistry, with a wide range of spectroscopic techniques can be advantageous. The above mentioned in situ UV-visible absorbance spectroelectrochemistry is widely used in investigation of EC films and devices for detection of optical modulation. More deep insight into the structural changes that occur in films can be performed using vibrational spectroscopies. Different techniques are characterized by certain advantages and disadvantages. For example, IR reflection-absorption spectroscopy can be coupled with electrochemical technique ex situ or in situ. While ex situ approach lack due to uncertainty during the transfer from the electrochemical cell to the spectrometer, the disadvantage of in situ cell is absorption of the electrolyte. Consequently, a cell with a very thin layer of electrolyte between the silicon window and EC film on reflective substrate should be designed. While ex situ technique offers usual IR RA spectra with regard to the uncovered background, only changes that occur in the structure during intercalation (or deintercalation) are detected in in situ spectra, which makes the identification much more difficult.IR absorbance spectroelectrochemistry can be performed ex situ after deposition of films on partly IR transparent silicon wafers. However, despite galvanostatic charging large overpotentials of silicon wafers may blur the direct comparison with electrochemical measurements of films on conductive substrates. With regard to IR spectroscopy, Raman spectroscopy is more complementary than competitive. As reported, the structural changes of polycrystalline V2O5 films can be significantly different than that of bulk materials [1]. From this reason, the ranking of the Raman spectra of novel V-oxide pigmented films is welcomed. With this aim there is constructed a new custom-made three-electrode in situ Raman cell and the V-oxide pigmented films studied for structural changes during coloration/bleaching processes. [1] Baddour-Hadjean et al., Chem. Rev. 2010, 110, 1278–1319.[2] Surca et al., J. Electrochem. Soc. 1999, 146, 232-242.[3] Mihelčič et al., Sol. Energy Mater. Sol. Cells 2012, 107, 175-187.[4] Mihelčič et al., Sol. Energy Mater. Solar Cells 2014, 120, 116-130.

The influence of post annealing on the structure and optical properties of vanadium oxide thin films

Vanadium and vanadium oxide films were deposited onto glass and silicon substrates at room temperature by DC magnetron sputtering and reactive DC magnetron sputtering, respectively. The effect of post annealing in air atmosphere were explored. The as-deposited vanadium and vanadium oxide films were annealed under air atmosphere at different temperatures (100-500°C) and annealing times (6, 12 hours). The structural properties of the films were studied by X-ray diffraction and scanning electron microscopy. The optical properties of the films were studied by UV-Vis spectrophotometer. The results showed that the annealed vanadium films have a formation of polycrystalline V2O5 structure while the optical transmittance is low due to the imperfect thermal oxidation. For the annealed vanadium oxide films, the vanadium oxide with the phase of V2O5 is also presented while the transmittance decreased when the annealing temperature is increased. This is due to the light loss by the scattering at the rough surface. A comparison of the results with the annealed vanadium and annealed vanadium oxide films will be discussed. Moreover, the effect of the post annealing temperature and the annealing time on the structural and optical properties will also be discussed.

Oxygen vacancy induced band gap narrowing of the low-temperature vanadium dioxide phase

Switching of vanadium dioxide (VO2) from low-temperature insulating phase to high-temperature rutile phase can be induced by photons with a certain energy. Photoinduced insulator-metal transition is found experimentally in VO2 polycrystalline film by photos with energy even below 0.67 eV. However, insulator-metal transition in single crystal can only be induced when photo energyis above 0.67 eV. In order to understand these experimental phenomena, we make a first-principle study on low-temperature non-magnetic M1 phase of VO2 with oxygen vacancy by density functional theory calculations based on the Heyd-Scuseria-Ernzerhof screened hybrid functional. According to symmetry, M1 phase has two kinds of different oxygen vacancies, O1 and O2 vacancies. Calculations are made on structures and electronic properties of nonmagnetic M1 phases with O1 and O2 vacancies, respectively. The present theoretical results show that neither the short vanadium-vanadium (VV) bond length near O1 or O2 vacancy nor the lattice parameters almost change but the long VV bond length near O1 or O2 vacancy decreases due to the oxygen vacancy. The long VV bond lengths near O1 and O2 vacancies are about 2.80 and 2.95 , respectively, but the long VV bond length is 3.17 in pure M1. The insulating band gap is opened between V 3d bands, and hybridization happens between V 3d and O 2p orbitals. Furthermore, the present theoretical results demonstrate that the band gap of pure nonmagnetic M1 is 0.68 eV while M1 with O1 vacancy, O2 vacancy, and two oxygen vacancies including O1 and O2, have band gaps of 0.23 eV, 0.20 eV, and 0.15 eV, respectively. The band gap decreases probably because oxygen vacancy results in the decease of the long VV bond length near it. The present results can explain the experimental results well.

Current induced polycrystalline-to-crystalline transformation in vanadium dioxide nanowires.

Vanadium dioxide (VO2) exhibits a reversible insulator-metal phase transition that is of significant interest in energy-efficient nanoelectronic and nanophotonic devices. In these applications, crystalline materials are usually preferred for their superior electrical transport characteristics as well as spatial homogeneity and low surface roughness over the device area for reduced scattering. Here, we show applied electrical currents can induce a permanent reconfiguration of polycrystalline VO2 nanowires into crystalline nanowires, resulting in a dramatically reduced hysteresis across the phase transition and reduced resistivity. Low currents below 3 mA were sufficient to cause the local temperature in the VO2 to reach about 1780 K to activate the irreversible polycrystalline-to-crystalline transformation. The crystallinity was confirmed by electron microscopy and diffraction analyses. This simple yet localized post-processing of insulator-metal phase transition materials may enable new methods of studying and fabricating nanoscale structures and devices formed from these materials.

Electrical Switching in Semiconductor-Metal Self-Assembled VO2 Disordered Metamaterial Coatings.

As a strongly correlated metal oxide, VO2 inspires several highly technological applications. The challenging reliable wafer-scale synthesis of high quality polycrystalline VO2 coatings is demonstrated on 4” Si taking advantage of the oxidative sintering of chemically vapor deposited VO2 films. This approach results in films with a semiconductor-metal transition (SMT) quality approaching that of the epitaxial counterpart. SMT occurs with an abrupt electrical resistivity change exceeding three orders of magnitude with a narrow hysteresis width. Spatially resolved infrared and Raman analyses evidence the self-assembly of VO2 disordered metamaterial, compresing monoclinic (M1 and M2) and rutile (R) domains, at the transition temperature region. The M2 mediation of the M1-R transition is spatially confined and related to the localized strain-stabilization of the M2 phase. The presence of the M2 phase is supposed to play a role as a minor semiconducting phase far above the SMT temperature. In terms of application, we show that the VO2 disordered self-assembly of M and R phases is highly stable and can be thermally triggered with high precision using short heating or cooling pulses with adjusted strengths. Such a control enables an accurate and tunable thermal control of the electrical switching.

Polycrystalline VO2 thin films via femtosecond laser processing of amorphous VO x

Femtosecond laser processing of pulsed laser-deposited amorphous vanadium oxide thin films was investigated. Polycrystalline VO2 thin films were achieved by femtosecond laser processing in air at room temperature. The electrical transport properties, crystal structure, surface morphology, and optical properties were characterized. The laser-processed films exhibited a metal–insulator phase transition characteristic of VO2, thus presenting a pathway for the growth of crystalline vanadium dioxide films on low-temperature substrates.

Influence of lattice distortion on phase transition properties of polycrystalline VO2 thin film

In this work, high power impulse magnetron sputtering was used to control the lattice distortion in polycrystalline VO2 thin film. SEM images revealed that all the VO2 thin films had crystallite sizes of below 20nm, and similar configurations. UV–vis-near IR transmittance spectra measured at different temperatures showed that most of the as-deposited films had a typical metal–insulator transition. Four-point probe resistivity results showed that the transition temperature of the films varied from 54.5 to 32°C. The X-ray diffraction (XRD) patterns of the as-deposited films revealed that most were polycrystalline monoclinic VO2. The XRD results also confirmed that the lattice distortions in the as-deposited films were different, and the transition temperature decreased with the difference between the interplanar spacing of the as-deposited thin film and standard rutile VO2. Furthermore, a room temperature rutile VO2 thin film was successfully synthesized when this difference was small enough. Additionally, XRD patterns measured at varied temperatures revealed that the phase transition process of the polycrystalline VO2 thin film was a coordinative deformation between grains with different orientations. The main structural change during the phase transition was a gradual shift in interplanar spacing with temperature.

Characterization of polycrystalline VO2 thin film with low phase transition temperature fabricated by high power impulse magnetron sputtering

VO2 is a unique material that undergoes a reversible phase transformation around 68[Formula: see text]C. Currently, applications of VO2 on smart windows are limited by its high transition temperature. In order to reduce the temperature, VO2 thin film was fabricated on quartz glass substrate by high power impulse magnetron sputtering with a modulated pulsed power. The phase transition temperature has been reduced to as low as 32[Formula: see text]C. In addition, the VO2 film possesses a typical metal–insulator transition. X-ray diffraction and selected area electron diffraction patterns reveal that an obvious lattice distortion has been formed in the as-deposited polycrystalline VO2 thin film. X-ray photoelectron spectroscopy proves that oxygen vacancies have been formed in the as-deposited thin film, which will induce a lattice distortion in the VO2 thin film.

Formation of VO2 by rapid thermal annealing and cooling of sputtered vanadium thin films

Sputtered vanadium-rich films were subjected to rapid thermal annealing-cooling (RTAC) in air to produce vanadium dioxide (VO2) thin films with thermochromic switching behavior. High heating and cooling rates in the thermal oxidation process provided an increased ability to control the film's microstructure. X-ray diffraction patterns of the films revealed less intense VO2 peaks compared to traditional polycrystalline samples fabricated with a standard (slower) cooling time. Such films also exhibit a high optical switching reflectance contrast, unlike the traditional polycrystalline VO2 thin films, which show a more pronounced transmittance switching. The authors find that the RTAC process stabilizes the VO2 (M2) metastable phase, enabling a rutile-semiconductor phase transition (R-M2), followed by a semiconductor–semiconductor phase transition (M2-M1).

Manipulating Surface Potentials of Metal Oxides Using Semiconductor Heterojunctions

Controlling the free-surface electrostatic potential of semiconducting metal oxides offers possibilities for improving the performance of sensors and catalysts and photocatalysts. However, methods to exert such control have typically proven to be inexact and unreliable. The present work demonstrates an approach based on semiconductor heterojunctions, wherein an oxide substrate with controlled carrier concentration supports a much thinner layer. The layer is too thin to absorb all the charge that would normally transfer, so some of the excess charge propagates to the free surface and changes the surface potential. A combination of standard heterojunction analysis via Poisson’s equation and surface potential measurements verifies the workability of this concept for thin polycrystalline V2O5 grown on polycrystalline anatase TiO2.

Thermochromic VO2 films by thermal oxidation of vanadium in SO2

Thermochromic films of VO2 were prepared by a two-step procedure: Sputtering was first used to deposit metallic vanadium, and such layers were subsequently oxidized in SO2 at a temperature in the 600–650°C range. X-ray diffraction, Raman spectroscopy, measurements of temperature-dependent electrical resistance, and spectrophotometric transmittance data at different temperatures were employed to demonstrate that the films consisted of polycrystalline VO2 with good thermochromism, especially when oxidized at the highest temperature. Oxidation in SO2 is able to produce VO2 without the stringent process control that can be an obstacle for making VO2 by oxidation in O2.

Effect of yttrium-doping on the microstructures and semiconductor-metal phase transition characteristics of polycrystalline VO2 thin films

We investigate the effect of yttrium-doping on the microstructures and semiconductor-metal phase transition characteristics of polycrystalline VO2 thin films prepared by reactively co-sputtering process. XPS analyses indicate the existence of Y3+ in the Y-doped VO2 films, but Y-doping hardly influences the chemical states of V and O elements. X-ray diffraction patterns and Raman spectra reveal that both undoped and Y-doped VO2 thin films have a polycrystalline structure of monoclinic VO2. The introduction of Y greatly reduces the grain size of VO2 thin films as evidenced by scanning electron microscopy analyses. The relationship between the hysteresis width and doping level is not monotonic although the grain size is monotonically reduced with increasing the doping level. Y-doped VO2 films with optimal doping level (1.82at%) have a notably narrower hysteresis width (4.6°C) than undoped VO2 films (10.7°C). This is ascribed to increased heterogeneous nucleation centers due to Y in the VO2 lattice. With the further increase of doping level, the size effect gradually plays a prominent role in SMPT, and the hysteresis width of Y-doped VO2 films increases instead. The SMPT temperature of Y-doped VO2 films obviously decreases compared with undoped VO2 films due to reduced grain size and deformation of local structure around Y atom.