Synthesis and structural characterization of mesoporous V2O5 thin films for electrochromic applications

Mesoporous vanadium pentoxide (V₂O₅) films have been synthesized by hydrolysis of vanadium tri-isopropoxide (VO(OC₃H₇)₃) in the presence of polyethylene glycol (PEG) as a structure-directing agent. The structure, the stoichiometry and the morphology of the films have been studied as a function of the thermal annealing by X-ray diffraction (XRD), micro-Raman spectroscopy, optical microscopy, scanning electron microscopy and atomic force microscopy. XRD patterns and Raman spectra show the presence of two previously unreported crystalline phases. The PEG:V₂O₅ molar ratio affects the temperature of phase formation, the amount and even the order in which the phases appear. The morphological characterization underlines the role of the surfactant to promote porous networks, formed by micrometric clusters of controlled shapes and patterns embedded in a homogeneous host matrix.     

Structural and optical properties of pulsed laser deposited V2O5 thin films

Pulsed laser deposited nanocrystalline V₂O₅ thin films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and optical spectroscopy. The films were deposited on amorphous glass substrates, keeping the O₂ partial pressure at 13.33 Pa and the substrate temperature at 220 °C. The characteristics of the films were changed by varying the laser fluence and repetition rate. XRD revealed that films are nanocrystalline with an orthorhombic structure. XPS shows the sub-stoichiometry of the films, that generally relies on the fact that during the formation process of V₂O₅ films, lower valence oxides are also created. From the HRTEM images, we observed the size evolution and distribution characteristics of the clusters in the function of the laser fluence. From the spectral transmittance we determined the absorption edge using the Tauc plot. Calculation of the Bohr radius for V₂O₅ is also reported.     

Structural and electrical properties of thin Ho2O3 gate dielectrics

This paper describes the structural properties and electrical characteristics of thin Ho₂O₃ gate dielectrics deposited on silicon substrates by means of reactive sputtering. The structural and morphological features of these films after postdeposition annealing were studied by X-ray diffraction, atomic force microscopy, and X-ray photoelectron spectroscopy. It is found that Ho₂O₃ dielectrics annealed at 700 °C exhibit a thinner capacitance equivalent thickness and excellent electrical properties, including the interface trap density and the hysteresis in the capacitance-voltage curves. Under constant current stress, the Weibull slope of the charge-to-breakdown of the 700 °C-annealed films is about 1.7. These results are attributed to the formation of well-crystallized Ho₂O₃ structure and the reduction of the interfacial SiO₂ layer.     

A study of thin films of V2O5 containing molybdenum from an evaporation boat

An attempt was made to produce thin films of vanadium oxide by evaporating V₂O₅ in vacuum using molybdenum boats. Following analysis of the films by X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry, it was found that the films contained a large amount of molybdenum (atomic ratio of Mo:V>1). Films were chemically inhomogeneous along the direction of growth such that the value of the atomic ratio decreased from the substrate side of the film to its interface with the air. However, a study of the optical properties of the films revealed that they were optically homogeneous. The films went through a semiconductor-to-metal phase transition at a temperature of approximately 200 °C. When annealed in vacuum at a temperature of 275 °C, it was found that, (a) the films remained amorphous, (b) there was a loss of oxygen leading to an increase in their electrical conductivity, (c) their thickness decreased leading to a larger refractive index of the films, and (d) their band gap energy shifted to a higher photon energy by approximately 0.1 eV.     

The effect of titanium on the lithium intercalation capacity of V2O5 thin films

The effect of Ti incorporation on lithium intercalation capacity of V₂O₅ thin films prepared by spin coating using metalorganic (MO) and inorganic sol-gel (SG) precursors on indium tin oxide coated glass substrates have been investigated. Earlier studies show that V₂O₅-TiO₂ oxide system has a higher cyclic stability than V₂O₅. However, there is controversy concerning the capacity of these mixed phases. We observe that upon incorporation of 5% Ti in MO films the lithium intercalation capacity decreases from 47 mC/cm² to 27 mC/cm², while for the SG films, the capacity increases from 14 mC/cm² to 27 mC/cm². We attribute this difference in the lithium intercalation capacity of the 5% Ti doped V₂O₅ films prepared by MO and SG precursors to the variation in the nonstoichiometry and the particle size. We find that it is essential to have a critical V:O ratio to achieve high intercalation capacity. Any deviations from this critical V:O ratio leads to a decrease in capacity and films having similar nonstoichiometry have similar values of intercalation capacity and diffusion coefficient.     

Single-source chemical vapor deposition of clean oriented Al2O3 thin films

Clean oriented Al₂O₃ thin film with a dominant Al₂O₃ <1 1 3> plane was deposited on Si <1 0 0> substrate at 550 °C, by single-source chemical vapor deposition (CVD) using aluminium(III) diisopropylcarbamate, Al₂(O₂CNⁱPr₂)₆. This process represents a substantial reduction in typical CVD film growth temperatures which are typically > 1000 °C. Through the studies of thermal stability of this precursor, we propose a specific β-elimination decomposition pathway to account for the low temperature of the precursor decomposition at the substrate, and for the lack of carbon impurity byproducts in the resulting alumina films that are characterized using X-ray photoelectron spectroscopy and depth profiling.     

A review of the growth of V2O5 films from 1885 to 2010

This year is the 125th anniversary of the first synthesis of V₂O₅ gels. The fascinating properties and wide application range of V₂O₅ thin films have attracted significant attention over the past decades. Its wide optical band gap, layered structure, good chemical and thermal stability and excellent thermoelectric and electrochromic properties have made V₂O₅ a promising material for industrial applications such as gas sensors, electrochromic devices, optical switching devices, and reversible cathode materials for Li batteries. Gels were the first form in which V₂O₅ was synthesized at the end of the 19th century. Interest started to grow in the 1980s due to the discovery of their semiconducting properties and their use in antistatic coatings in the photographic industry. The rapid development of the sol-gel process brought new interest in V₂O₅ gels. Following a short discussion of vanadium oxides and V₂O₅, I summarize all thin film preparation techniques known up to now and use reported optical band gaps to characterize different growth methods. An estimation of the Bohr radius for V₂O₅ is also presented. This article provides an up-to-date review of more than a century (1885-2010) of research on the growth of vanadium oxide thin films. Nonetheless, due to the huge number of publications in the field, only those are selected and described which, according to the author, contribute the most to the field's further development.     

IR spectra of VO2 and V2O3

IR spectra of the tetragonal modification of VO₂ and of the trigonal form of V₂O₃ are recorded at room temperature and compared with that of V₂O₅. The investigated samples of the two lower-valent vanadium oxides, obtained on temperature-programmed reduction treatment, were also characterized with diffuse reflectance and electron-paramagnetic resonance spectra. The effect of atmospheric oxygen on these materials was revealed with XPS measurements and also studied with IR spectra.     

Low temperature chemical vapor deposition of nanocrystalline V2O5 thin films

Spatially uniform, carbon-free thin films of V₂O₅ were deposited on silicon by chemical vapor deposition using vanadium oxide triisopropoxide and water as gaseous precursors, in the temperature range of 100-300 °C. Films with substantial crystallinity were obtained for deposition temperatures as low as 180 °C. The “neat” chemistry that nominally leaves no fragments of ligand or water in the solid promotes film purity and reduces the deposition temperature needed for crystallization. Such deposition temperatures also open up additional possibilities for using crystalline vanadia on fragile substrates such as polymers for electronics and optical applications.     

Electronic structure of amorphous InGaO3(ZnO)0.5 thin films

The electronic structure of amorphous semiconductor InGaO₃(ZnO)_0.5 thin films, which were deposited by radio-frequency magnetron sputtering process, was investigated using X-ray photoelectron spectroscopy and O K-edge X-ray absorption spectroscopy. The overall features of the valence and conduction bands were analyzed by comparing with the spectra of Ga₂O₃, In₂O₃, and ZnO films. The valence and conduction band edges are mainly composed of O 2p and In 5sp states, respectively. The bandgap of the films determined by spectroscopic ellipsometry was approximately 3.2 eV. Further, it is found that the introduction of oxygen gas during the sputter-deposition does not induce significant variations in the chemical states and band structure.     

Pulsed laser deposition of (MoO3)1 − x(V2O5)x thin films: Preparation, characterization and gasochromic studies

In this study we demonstrate a new composite oxide thin films of (MoO₃)_1 − x(V₂O₅)_x, x = 0, 0.01, 0.03, and 0.05, fabricated by pulsed laser deposition (PLD). The performance of platinum (Pt) catalyst activated hydrogen gas sensor with modified (MoO₃)_1 − x(V₂O₅)_x thin films were investigated. The thickness of the (MoO₃)_1 − x(V₂O₅)_x thin film is about 600-650 nm and its surface has a uniform morphology. Our results show that the gasochromic sensors prepared by (MoO₃)_0.99(V₂O₅)_0.01 thin film exhibited excellent hydrogen sensibility. The response and recovery time are in the range of 9-15 min for coloration and bleaching at room temperature under H₂ atmosphere. The results also show that (MoO₃)_1 − x(V₂O₅)_x/Pt (x = 0.01, 0.03, 0.05) thin films perform better gasochromic capability than the pristine MoO₃/Pt sample.     

Structural and optical studies of nanocrystalline V2O5 thin films

We report the structural and optical properties of nanocrystalline thin films of vanadium oxide prepared via evaporation technique on amorphous glass substrates. The crystallinity of the films was studied using X-ray diffraction and surface morphology of the films was studied using scanning electron microscopy and atomic force microscopy. Deposition temperature was found to have a great impact on the optical and structural properties of these films. The films deposited at room temperature show homogeneous, uniform and smooth texture but were amorphous in nature. These films remain amorphous even after postannealing at 300 °C. On the other hand the films deposited at substrate temperature T_S > 200 °C were well textured and c-axis oriented with good crystalline properties. Moreover colour of the films changes from pale yellow to light brown to black corresponding to deposition at room temperature, 300 °C and 500 °C respectively. The investigation revealed that nanocrystalline V₂O₅ films with preferred 001 orientation and with crystalline size of 17.67 nm can be grown with a layered structure onto amorphous glass substrates at temperature as low as 300 °C. The photograph of V₂O₅ films deposited at room temperature taken by scanning electron microscopy shows regular dot like features of nm size.     

Impact of substrate temperature on the microstructure, electrical and optical properties of sputtered nanoparticle V2O5 thin films

Applying reactive direct current (DC) magnetron sputtering method, nanoparticle vanadium pentoxide thin films were deposited onto glass slides and KBr substrates at different substrate temperatures. The films were characterized by X-ray photoelectron spectroscopy and atomic force microscope. Infrared spectra were recorded with a Fourier transform infrared spectrophotometer. It was found that, excepting the compositions, the film growth and vanadium oxygen bonds were strongly affected by the substrate temperature. Electrical measurements indicated that the square resistances of films showed an exponential decrease from 46 MΩ/□ to 33 kΩ/□ with substrate temperature increasing from 433 K to 593 K, and that the square resistance-temperature curves of films exhibited typical semiconducting behavior. Optical investigations were carried out in the near infrared and ultraviolet-visible range. Transmittance varied from about 95 to 55% in near-infrared range when the substrate temperature was elevated. In ultraviolet-visible range, optical band gaps and refractive indexes of films were deduced according to the transmission and reflection spectra.     

Formation of a highly oriented FeO thin film by phase transition of Fe3O4 and Fe nanocrystallines

A highly oriented FeO thin film was formed from a Fe₃O₄ thin film containing Fe nanocrystallines by post-annealing at 600°C. Fe₃O₄ thin films were grown on Si(100) substrates by ion beam sputter deposition under oxygen ambient. The stoichiometry of the iron oxide thin film could be precisely controlled by in situ X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) pattern of the Fe₃O₄ thin film grown at substrate temperature of 300°C showed a mixed phase of Fe₃O₄ and Fe nanocrystallines with a preferred orientation (110). However, the mixed phase was converted to a highly oriented FeO(200) phase by post-annealing at 600°C. This could be inverted as a result of Ostwald ripening of the Fe₃O₄ and Fe nanocrystallines.     

Electronic structural analysis of transparent In2O3-ZnO films by hard X-ray photoelectron spectroscopy

The electronic structural analysis of the conductive transparent films was carried out using bulk sensitive hard X-ray photoelectron spectroscopy (HAXPES). The In₂O₃-ZnO film has amorphous structure before and after annealed, and the conduction band spectrum around Fermi level showed the similar spectra with that of as-deposited amorphous In₂O₃ film. In these amorphous films, the conduction band minimum locates at the deeper level than the crystalline In₂O₃ film. The electronic state which comes from randomness of amorphous structure possibly exists around this level or below. These electrons are expected to act as scattering center. We concluded that the electron mobility depends on the density of this electronic state.     

Growth and characterization of nitrogen-doped TiO2 thin films prepared by reactive pulsed laser deposition

Nitrogen-doped titanium dioxide (TiO₂) thin films were grown on (001) SiO₂ substrates by reactive pulsed laser deposition. A KrF* excimer laser source (λ = 248 nm, τ_FWHM ≅ 10 ns, ν = 10 Hz) was used for the irradiations of pressed powder targets composed by both anatase and rutile phase TiO₂. The experiments were performed in a controlled reactive atmosphere consisting of oxygen or mixtures of oxygen and nitrogen gases. The obtained thin film crystal structure was investigated by X-ray diffraction, while their chemical composition as well as chemical bonding states between the elements were studied by X-ray photoelectron spectroscopy. An interrelation was found between nitrogen concentration, crystalline structure, bonding states between the elements, and the formation of titanium oxinitride compounds. Moreover, as a result of the nitrogen incorporation in the films a continuous red-shift of the optical absorption edge accompanied by absorption in the visible spectral range between 400 and 500 nm wavelength was observed.     

A comparison between V2O5 and WO3 thin films as sensitive elements for NO detection

Vanadium oxide (V₂O₅) and tungsten oxide (WO₃) thin films were investigated with the aim to obtain information about their physical and gas sensing properties. The analysis in the presence of different NO concentrations have shown that both materials are able to detect nitrogen oxide, but their responses exhibit different characteristics. In particular, tungsten oxide was found to be more suitable to be used in the field of application for detecting low concentrations. In addition, a mechanism of detection has been considered.     

Structure and surface termination of ZnO films grown on (0001)- and (112¯0)-oriented Al2O3

We have studied the surface termination of ZnO(0001¯) films grown on Al₂O₃ substrates with high epitaxial quality. The structural properties of the ZnO films were investigated by X-ray scattering, revealing a predominant (0001¯)ZnO out-of-plane texture with the [112¯0]_ZnO[0001]_Al2O3 and [112¯0]_ZnO[101¯0]_Al2O3 azimuthal orientations for (112¯0)Al₂O₃ and(0001)Al₂O₃ substrates, respectively. The surface termination was determined by X-ray photoemission spectroscopy (XPS) via pyridine (C₅H₅N) adsorption at the ZnO surface. XPS data recorded at different temperatures after exposure to pyridine revealed that for both orientations of the Al₂O₃ substrates the deposited ZnO films were terminated by oxygen atoms, i.e. corresponding to a ZnO (0001¯) surface.     

Nitridation of nanocrystalline TiO2 thin films by treatment with ammonia

Nanocrystalline anatase (TiO₂) thin films prepared by a physical vapour deposition method were nitrided by annealing in flowing NH₃ at temperatures ranging between 650 °C and 700 °C. It was established that there was a narrow window of temperatures which allowed both incorporation of interstitial nitrogen into the films with retention of the anatase phase without chemical reduction and preservation of the characteristic nanocrystalline morphology. These optimally modified films responded to visible light in photowetting tests and showed the ability to degrade an organic dye under visible light irradiation.     

Growth and structure of electron beam evaporated V2O5 thin films

The growth, structure and room temperature electrical conductivity of electron beam evaporated V₂O₅ thin films were studied in detail as a function of deposition temperature. The films deposited at T_s≈553 K and subsequently annealed in oxygen atmosphere at 693 K exhibited orthorhombic layered structure.