Polycrystalline VO2 Research Articles

Preparation of polycrystalline VO2 films on glass and TiO2/glass substrates by means of excimer laser assisted metal organic deposition

An excimer laser assisted metal organic deposition process for the preparation of vanadium dioxide (VO2) films is described. When a precursor film grown on a silica glass substrate by preheating at 300°C was irradiated with a KrF laser at room temperature in air, the film was crystallized into polycrystalline VO2 phases. Generally, the preparation of VO2 films requires strict atmosphere control, but in this process, the polycrystalline VO2 films were successfully prepared without it. Moreover, it was revealed that crystallization into the VO2 phases was promoted by a TiO2 buffer layer placed on the glass substrate. This means that the photochemical reaction induced by the TiO2 absorbing the laser participates in the crystallization. The electrical resistivity of the film grown on the TiO2/glass substrate was lowered compared to that of the film grown on the glass substrate due to improvement of the film uniformity. The crystallinity of the obtained film was dependent on the irradiation time. With the improved crystallinity of the film, the resistivity drop through the metal-insulator transition became large.

Application of V<sub>2</sub>O<sub>5</sub> in Thin Film Microbatteries Prepared by RF Magnetron Sputtering

The polycrystalline V2O5 films as the anode in V2O5 /LiPON /LiCoO2 lithium microbattary were prepared by RF magnetron sputtering system. The V2O5 films’ crystal structures, surface morphologies and composition were characterized and analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The microbatteries were fabricated by micro electro-mechanical system (MEMS) technology. The battery active unit area is 500μm×500μm, and the thickness of V2O5, LiPON and LiCoO2 films was estimated to be 200, 610, and 220nm, respectively. The discharge volumetric capacity is between 9.36μAhcm-2μm-1 and 9.63μAhcm-2μm-1 after 40 cycles.

Experimental routes to in situ characterization of the electronic structure and chemical composition of cathode materials for lithium ion batteries during lithium intercalation and deintercalation using photoelectron spectroscopy and related techniques

Three different experimental routes to in situ characterization of electronic structure and chemical composition of thin film cathode surfaces used in lithium ion batteries are presented. The focus is laid on changes in electronic structure and chemical composition during lithium intercalation and deintercalation studied by photoelectron spectroscopy and related techniques. At first, results are shown obtained from spontaneous intercalation into amorphous or polycrystalline V2O5 thin films after lithium deposition. Although this technique is simple and clean, it is nonreversible and only applicable to the first lithium intercalation cycle into the cathode only to be applied to host materials stable in the delithiated stage. For other cathode materials, as LiCoO2, a real electrochemical setup has to be used. In our second approach, the experiments are performed in a specially designed electrochemical cell directly connected to the vacuum system. First experimental results of RF magnetron sputtered V2O5 and LiCoO2 thin film cathodes are presented. In the third approach, an all solid-state microbattery cell must be prepared inside the vacuum chamber, which allows electrochemical processing and characterization by photoelectron spectroscopy in real time. We will present our status and experimental difficulties in preparing such cells.

Electrical and optical properties of metal-insulator-transition VO2 thin films

Vanadium dioxide thin films were deposited using in situ pulsed laser deposition technique on a-, c-, and r-plane Al2O3, and MgO(100) substrates. Microstructure of the films was varied between epitaxial VO2, polycrystalline VO2, mixtures of VO2 and V6O13 with VO2 as the main phase, and mixtures of VO2 and V6O13 with V6O13 as the main phase by controlling the deposition oxygen partial pressure. Detailed XRD, SPM, and FESEM measurements were performed in order to analyze the structure of the films. Resistivity as a function of temperature, current–voltage characteristics in electric current induced Joule heating transition process, and the optical transmittance both in insulator and metal states were measured. MIT effect led to 103–105 change in resistivity with varying transition temperature and hysteresis loop widths. The largest and the steepest transitions were found in the films with polycrystalline microstructure deposited at higher oxygen pressures. Epitaxial films had 2.5 times higher insulator state conductivity than polycrystalline films, which lead to a clearly smaller switching powers in MIT effect generated by Joule heating. However, the optical properties in both states were not considerable affected by conductivity or microstructure properties. The relationships between the microstructure, electrical and optical properties, as well as MIT switching effect together its dynamics in the films are discussed.

PHASE TRANSFORMATION AND R–T CHARACTERISTICS OF VACUUM ANNEALED VANADIUM OXIDE THIN FILMS

Polycrystalline vanadium oxide ( VO x) thin films with mixed phases of V 6 O 13 + VO 2 (M) are deposited onto p-doped Si (100) substrates at 430°C using magnetron sputtering. By vacuum annealing on as-deposited VO x thin films at 450°C, at different annealing times we obtained nanoscale polycrystalline VO x thin films with two or more mixed phases. With the annealing time increasing, the phases of the films underwent the following transformation: V 6 O 13 + VO 2 ( M ) → VO 2 ( M ) + VO 2 ( B ) → VO 2 ( B ) + VO 2 ( M ) + V 5 O 9 → V 4 O 7 + V 5 O 9. The surface of VO x thin films prepared in different conditions is of high quality with mean grain size ranging from 80 to 300 nm and the R a being about 20nm. The lowest temperature coefficient of resistance at ambient temperature (20°C) of -2% K-1 is found in the VO x thin film annealed at 450°C for 8 h with mixed phases of VO 2 ( M ) + VO 2 ( B ), which can meet the needs of microbolometer materials.

V O 2 films with strong semiconductor to metal phase transition prepared by the precursor oxidation process

We describe a relatively simple, reliable, and reproducible preparation technique, the precursor oxidation process, for making VO2 films with strong semiconductor-to-metal phase transition. Sputter-deposited metal precursor V films were oxidized in situ in the deposition chamber for 2.5–7h at 370–415°C in 0.2Torr O2 to form 22–220nm VO2. The strength [resistivity ratio, RR=ρS∕ρM] and sharpness (hysteresis width ΔTC) of T-dependent semiconductor-to-metal hysteretic phase transition in VO2 were our most immediate and relevant quality indicators. In 200-nm-range films, the process was optimized to yield RR=(1–2)×103, ΔTC∼11°C and absolute resistivity in a semiconducting phase ρS=0.4±0.2Ωm, close to resistivity in bulk single crystals of VO2. Films were characterized by scanning electron microscopy, atomic force microscopy, grazing-incidence x-ray diffraction, and Raman spectroscopy, and found to be polycrystalline single-phase VO2. We also measured optical reflectivity RT(λ) from 200to1100nm, and Rλ(T) from 20to100°C. RT(λ) measured in thin-film interference structures allowed us to calculate the index of refraction in the two phases, which agrees well with the published data and, together with structural measurements, confirms that our films are essentially pure VO2. The limited study of these films in terms of stability, aging, lithographic processing, and thermal cycling shows that they can be used in applications.

Morphological transformations of vanadium oxide films during low-temperature reduction in hydrogen electron cyclotron resonance plasma

Morphological transformations of amorphous vanadium oxide films obtained by the sol-gel method and polycrystalline V2O5 films are studied during their low-temperature (295–623 K) reduction in a hydrogen electron cyclotron resonance plasma. The morphology of films is analyzed using atomic force microscopy and high-resolution electron microscopy. It is found that a homogeneous amorphous film during the reduction process transforms to an island film and then bulk amorphous islands of a regular shape appear. These islands resemble microcrystals, and their concentration depends on the temperature and the reduction time. The low-temperature reduction of polycrystalline V2O5 films leads to their amorphization; however, the microcrystals in the polycrystalline film do not change their shape in this process. A mechanism of the reduction process is proposed. This mechanism explains the regularities of morphological transformations in amorphous sol-gel films of vanadium oxides based on the suggestion of a competition between the ion-stimulated nucleation and growth of nuclei of the crystalline phase and the amorphization of the growing nuclei.

Synthesis and Characterization of Polymeric V<SUB>2</SUB>O<SUB>5</SUB>/AlO(OH) with Nanopores on Alumina Support

Polymeric vanadium pentoxide gel was formed via the reaction of V2O5 powder with hydrogen peroxide. The polymeric vanadium pentoxide gel was then dispersed in alumina gel. Different vanadium loading composites were coated on alumina support and calcined at 500 degrees C for 1 hr. These composite layers were characterized using TGA, FT-IR, XRD, SEM, and Autosorb. It was found that the lamellar structure of polymerized vanadium pentoxide was retained in the inorganic matrix. Crystalline alumina in gamma phase was formed after calcinations. However, the vanadium-alumina mixed oxides are lack of the well defined PXRD peaks for polycrystalline V2O5. This is possibly because the vanadia species are highly dispersed in the alumina matrix or the vanadia species are dispersed as crystalline which is smaller than 4 nm. In addition, the imbedded polymeric vanadium oxide improved the specific area and average pore diameter of the composite layer.

The effects of vacuum annealing on the structure of VO 2 thin films

Noncrystalline VO x thin films were deposited onto p-doped Si (100) substrates at 400 °C using magnetron sputtering. By vacuum annealing, we obtained polycrystalline VO 2 thin films with two different structures under a variety of annealing conditions. With the annealing temperature increasing and the annealing time developing, structures of the films underwent the following transformation: amorphous structure→metastable VO 2 (B)→VO 2 (B) + VO 2 (M). Vacuum annealing is useful of acquiring VO 2 thin films with high surface quality, but too high annealing temperature (500 °C) and too long time (15 h) are harmful, which make the surface degenerate.

Current-induced metal–insulator transition in VO x thin film prepared by rapid-thermal-annealing

The phenomenon of metal–insulator transition (MIT) in polycrystalline VO x thin films and their preparations have been studied. The films were prepared by sputtering of vanadium thin films succeeded by Rapid Thermal Annealing (RTA) in oxygen ambient at 500 °C. Crystalline, compositional, and morphological characterizations reveal a continuous change of phase from vanadium metal to the highest oxide phase, V 2O 5, with the time of annealing. Electrical MIT switching has been observed in these films. Sweeping mode, electrode area, and temperature dependent MIT has been studied in Pt/VO x /Pt vertical structure. The important parameters for MIT in VO x have been found to be the current density and the electric field, which depend on carrier density in the films.

Fabrication of 128×128 element optical switch array by micromachining technology

Polycrystalline VO2 thin films were obtained on Si substrates by ion beam sputtering deposition and annealing in flowing Ar gas. SEM images indicate that VO2 thin films were grown into compact surfaces. Four-probe measurements indicated that the VO2 thin films own good electrical homogeneity. After the films' production, micromachining technology including lithography, reaction ion etching and metallization connection processes was used to produce the optical switch array. As a result, the 128×128 element optical switch array was achieved.

Fabrication of 128×128 element optical switch array by micromachining technology

Polycrystalline VO 2 thin films were obtained on Si substrates by ion beam sputtering deposition and annealing in flowing Ar gas. SEM images indicate that VO 2 thin films were grown into compact surfaces. Four-probe measurements indicated that the VO 2 thin films own good electrical homogeneity. After the films’ production, micromachining technology including lithography, reaction ion etching and metallization connection processes was used to produce the optical switch array. As a result, the 128×128 element optical switch array was achieved.

Preparation of 128 element of IR detector array based on vanadium oxide thin films obtained by ion beam sputtering

Polycrystalline VO x thin films were deposited on Si substrates by ion beam sputtering deposition in oxidation atmosphere. SEM images show that VO x thin films were grown into compact and smooth surfaces. Four-point probe measurements indicated that the VO x thin films own good homogeneity and high temperature coefficient of resistance (TCR ∼ −0.02 K −1). After the films’ deposition, micromachining processes including lithography and metallization connection was used to fabricate the detector array. As a result, the 128 element of IR detector array with a responsivity of 2 × 10 8 cm Hz 1/2/W and a detectivity of 5 kV/W at current bias of 100 μA were achieved.

Preparation of homogeneous VO X thin films by ion beam sputtering and annealing process

Polycrystalline VO x thin films that were prepared for thermal-sensitive material of far infrared sensor had been deposited on Si substrates by ion beam sputtering deposition. Scanning electron microscopy images indicated that VO x thin films (oxygen pressure of 1.5×10 −3 Pa) were grown into compact and ultra-fine grains (⩽50 nm), the film surfaces seemed smooth and uniform. Four-point probe measurements showed that the homogeneity of the films was better than 98% in a size of 30×30 mm 2. The four-point probe measurement on hot plate presented the sheet resistance and the temperature coefficient of resistance of the VO x thin film that were 50 kΩ/square and −0.021 K −1 at 28°C, respectively. In addition, some samples annealed in Ar atmosphere had their resistance decreased. Thus, vanadium oxide films containing more amount VO 2 were obtained.

VO x thin films obtained by ion beam sputtering and oxidation process

Polycrystalline VO x thin films were grown on Si substrates by ion beam sputtering deposition and annealing in oxygen atmosphere, which were prepared for thermal-sensitive material of far infrared (IR) sensors. As the deposition temperature (∼150 °C) is lower compared to that (∼400 °C) used to deposit single phase of VO 2, scanning electron microscopy (SEM) morphology indicates that the as-deposited vanadium oxide (VO x ) thin films were very smooth in surface and their grains are not able to be resolved. After annealing at higher temperature, the thin films were grown finally into ball-like and skeleton-like grains. The four-probe test presents their sheet resistance and the temperature coefficient of resistance (TCR) of the VO x thin film, which are 50 kΩ/□ and −0.021 K −1 at 30 °C, respectively. In addition, IR absorption in the range of 2–20 μm has been characterized.

Temperature coefficient of resistance of VO2 polycrystalline film formed by ion beam enhanced deposition

The polycrystalline VO2 film was directly prepared from V2O5 powder using the modified ion beam enhanced deposition (IBED) method and a suitable annealing. Testing results show that the IBED polycrystalline VO2 film has a single orientation, an obvious phase transition, a compact structure, and favorable processing properties. The temperature coefficient of resistance (TCR) of the film was up to more 4% and adhered hard to the substrate. The mechanism of the IBED film with higher TCR was discussed in detail. The reason why the film has a high TCR could be as follows: the oxygen vacancy and crystalline boundary in the film was decreased due to the IBED technology; with single oriented and dense texture the activation energy of electrical conduction of the IBED polycrystalline VO2 film was closer to the activation energy of VO2 crystalline in semiconductor phase which is higher than that of the VOx films prepared by other methods

Vanadium‐51 MAS and Static NMR Studies of the Binary V2O5—WO3 System.

Solid-state NMR spectra showing single-quantum excitation of the vanadium-51 (I = 7/2) central and six satellite transitions were obtained for polycrystalline V2O5 and x%V2O5−(100 − x%)WO3 (10 mol % ≤ x ≤ 40 mol %) solids by magic angle spinning (MAS) and static NMR spectroscopies. All MAS spectra show full spinning sideband manifolds resulting from incomplete averaging of the chemical shift and quadrupolar interaction anisotropies. Chemical shift parameters (δiso, δaniso, ηcs) and quadrupolar parameters (CQ, ηQ) were obtained by simulation of both MAS and static spectra. A single vanadium site was observed in the nominally 40 mol % V2O5−60 mol % WO3 and 30 mol % V2O5−70 mol % WO3 compositions. Interpretation of NMR spectral parameters suggests that the vanadium in this single site resides in a square pyramid-like oxygen coordination environment. Three sites were observed for the nominally 20 mol % V2O5−80 mol % WO3 and 10 mol % V2O5−90 mol % WO3 compositions. The spectral parameters for the three sites a...

Study on Thermochromic VO2 Films Grown on ZnO-Coated Glass Substrates for “Smart Windows”

Vanadium dioxide (VO2) is one of the most attractive thermochromic materials, which show large changes in optical and electrical properties at the transition temperature (Tt) close to the atmospheric temperature (approximately 340 K). We already reported for VO2 deposition by rf magnetron sputtering using V2O3 or V2O5 targets that VO2 films thicker than 400 nm showed high thermochromic performance, whereas the VO2 films thinner than 200 nm did not show such performance because of their poor crystallinity and off-stoichiometry. In this study, very thin thermochromic VO2 films with thicknesses of about 50 nm were successfully deposited using highly <001>-preferred oriented ZnO polycrystalline films as a buffer layer between the VO2 film and glass substrate (VO2/ZnO/glass) because of the heteroepitaxial growth of VO2 polycrystalline films. W-doped VO2 films were also deposited on the ZnO-coated glass substrates (ZnO/glass) by cosputtering. It was confirmed that W doping for thin VO2 films deposited on the ZnO/glass can decrease Tt systematically. Such very thin VO2 films should have high potential for application in “smart windows”.

“Doughnut-shaped” Coloration of V2O5 upon Laser Irradiation: Another Evidence of Visible Light Photochromism of V2O5

This short note provides proof of the visible light photochromism of polycrystalline V2O5 using much simpler method than that in a previous report [S. Nishio and M. Kakihana: Chem. Mater. 14 (2002) 3730]. It was found that the “doughnut-shaped” coloration in polycrystalline V2O5 samples is formed around a direct laser spot upon visible laser irradiation; that is, the depth of coloration attains its maximum with increasing laser power. This effect can be a consequence of light and heat having opposite actions (coloration and bleaching, respectively) on polycrystalline V2O5. The result is clear evidence of photochromism based on the definition of the term.

Vanadium-51 MAS and Static NMR Studies of the Binary V2O5−WO3 System

Solid-state NMR spectra showing single-quantum excitation of the vanadium-51 (I = 7/2) central and six satellite transitions were obtained for polycrystalline V2O5 and x%V2O5−(100 − x%)WO3 (10 mol % ≤ x ≤ 40 mol %) solids by magic angle spinning (MAS) and static NMR spectroscopies. All MAS spectra show full spinning sideband manifolds resulting from incomplete averaging of the chemical shift and quadrupolar interaction anisotropies. Chemical shift parameters (δiso, δaniso, ηcs) and quadrupolar parameters (CQ, ηQ) were obtained by simulation of both MAS and static spectra. A single vanadium site was observed in the nominally 40 mol % V2O5−60 mol % WO3 and 30 mol % V2O5−70 mol % WO3 compositions. Interpretation of NMR spectral parameters suggests that the vanadium in this single site resides in a square pyramid-like oxygen coordination environment. Three sites were observed for the nominally 20 mol % V2O5−80 mol % WO3 and 10 mol % V2O5−90 mol % WO3 compositions. The spectral parameters for the three sites a...