Search

In this work, we investigate the improvement of the thermochromic properties of W-doped vanadium dioxide (VO2) films induced by annealing temperatures. Firstly, W-doped VO2 films with different W contents were successfully prepared on quartz substrates by direct current (DC) reactive magnetron sputtering. The results show that the W element not only decrease the phase transition temperature of VO2 film but also seriously decrease the infrared modulation performance of the film. Secondly, Undoped and W-doped VO2 films with different annealing temperatures were prepared and characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM) tests. The effect of annealing temperature on the chemical states, crystal structure, and surface morphology of W-doped VO2 films was analyzed. It is found that VO2 film with the 1.4 at.% ratio of W contents has excellent metal–insulator phase transition performance compared to undoped VO2 film at the annealing temperature of 400 °C, i.e., narrower hysteresis width (4.3 °C) and lower phase transition temperature (37.4 °C). More importantly, the infrared amplitude modulation of the film is closer to that of undoped VO2 films. This is attributed to the enhanced (011) diffraction peak intensity and larger particle size and obvious grain boundary in the film. This work shows proper annealing temperatures can maintain the phase-change amplitude modulation performance of W-doped VO2 films while lowering the phase transition temperature and have great potential for infrared modulation applications. [ABSTRACT FROM AUTHOR]

Nanostructured zirconium (Zr)-doped vanadium oxide (VOx) films were prepared at low temperature on glass substrates by reactive direct current magnetron sputtering followed by in-situ annealing process. The effect of Zr content on the chemical composition, structure, morphology and metal–semiconductor transition properties of the deposited films was investigated systematically. It was found that Zr doping significantly reduced the grain size of VOx films due to increased density of nucleation centers, but was found almost not to influence the structure and the + 4 valence vanadium concentration. Interestingly, the hysteresis width was hugely modulated from 30.0 to 5.8 °C while the phase transition temperature upon cooling increases from 42.0 to 56.7 °C as the Zr/V atomic ratio in the deposited film increases from 0 to 13.9%. However, the phase transition temperature upon heating initially decreases with Zr/V atomic ratio, attains a minimum of 53.8 °C at a Zr/V atomic ratio of 8.5%, and then increases with increase in Zr/V atomic ratio. The decrease in hysteresis width with the increase in Zr contents can be attributed to the increased density of nucleation defects introduced by Zr doping. This work shows that Zr doping can effectively regulate the morphology and the phase transition characteristics of VOx films. [ABSTRACT FROM AUTHOR]

Vanadium dioxide (VO 2) is a potential candidate in smart window application field owing to its reversible and automatic first-order insulator-to-metal transition (IMT) at 341K which can induce dramatic changes of transmittance in near infrared band through MIT. However, the low T lum (visible light transmittance) below 40% and ΔT sol (solar modulation efficiency) below 10% of pure VO 2 films have severely constrained the practical application in smart windows. In this paper, by etching the VO 2 films fabricated by magnetron sputtering with sodium hydroxide solution, the VO 2 films with improved optical properties were obtained. Specifically, the T lum increased to 42.6% and the ΔT sol decreased to 15.5% for 150 nm VO 2 films after 9 min etching, the T lum increased to 56.4% and the ΔT sol decreased to 12.2% for 150 nm VO 2 films after 12 min etching, respectively, comparing with the unetched film of 150 nm (T lum :16.9%, ΔT sol :16.3%). Furthermore, we synthetically investigated the influences of etching on the IMT properties and molecular structure of the VO 2 films through characterizations such as Raman spectra, XRD, XPS and SEM. The proposed methods represented a facile and cost-effective approach for enhancing thermochromic properties that could promote the application of VO 2 thin films in smart windows. • Optimizing the etching process of vanadium oxide film. • The main mechanism of sodium hydroxide etching vanadium oxide film is figured out by characterization. • Optimized optical properties (T lum : 42.6%, ΔT sol : 15.5% and T lum : 56.4%, ΔT sol : 12.2%) are obtained. • This alkalinity etching method could be an alternative approach to improve the thermochromic properties of VO 2 thin films. [ABSTRACT FROM AUTHOR]

Abstract: Vanadium oxide (VOx) thin films were deposited on glass substrates at room temperature by pulsed DC reactive magnetron sputtering. The effects of the duty cycle of the pulsed power and oxygen flow on the surface morphology, deposition rate, composition and electrical properties of VOx films were systematically investigated. Experimental results revealed that decreasing the duty cycle or increasing the oxygen flow leaded to the decrease of the root mean square (RMS) roughness of VOx films. Moreover, the deposition rate decreased with the reduction of the duty cycle, and it also became more sensitive to the duty cycle at higher oxygen flow. Electrical measurements indicated that if the oxygen flow was increased from 3.1 sccm to 3.5 sccm, the resistivity and the temperature coefficient of resistivity (TCR) value of VOx films varied from 0.005 Ω cm to 0.409 Ω cm and from −0.209%/K to −1.485%/K, respectively. Particularly, by adjusting the duty cycle, the maximum value of the resistivity and TCR of VOx films can be achieved as 604.2 Ω cm and −2.864%/K, respectively. X-ray photoemission spectroscopy (XPS) analyses revealed that decrease of the duty cycle or increase of the oxygen flow enhanced the oxidation of vanadium. Therefore, with adjustment of the duty cycle in combination with regulation of the oxygen flow, the thermistor properties of VOx films can be well controlled. [Copyright &y& Elsevier]

Abstract: Nanostructured vanadium oxide (nano-VOx) films were deposited at low temperature on glass substrates by reactive direct current (DC) magnetron sputtering followed by in-situ annealing process. The chemical compositions, crystal structures, morphologies and metal-insulator transition (MIT) properties of nano-VOx films were investigated. The experimental results indicate that deposition conditions (reactive O2 gas flow rates, annealing temperatures and annealing times) have important effects on film morphologies (grain shapes and grain sizes) which profoundly affect the MIT properties of VOx films, especially the hysteresis width. The film consisting of spheroidal small particles shows a large hysteresis width due to the small density of nucleating defects and large interfacial energies, whereas the film consisting of the misshapen and/or large particles shows a small hysteresis width due to the large density of nucleating defects. These experimental results suggest that reactive DC magnetron sputtering is an effective process for preparing high-quality thermochromic nano-VOx films with tunable hysteresis widths and surface morphologies. [Copyright &y& Elsevier]

Vanadium oxide thin films have been deposited by reactive DC magnetron sputtering. The results showed that the sputtering voltage has a special change behavior in Ar condition and Ar/O2mixed conditions. With O2gas flow input instantaneously, the sputtering voltage climbs quickly, and then tends to be stable shortly. Also, the sputtering voltage evolution appears a hysteretic loop characteristics with O2gas supplied increasing and decreasing. The sputtering voltage behavior observed is related to the ion induced secondary electron emission (ISEE) coefficient of the target material. The ISEE coefficient of the oxidized vanadium on target surface is lower than the ISEE coefficient of the pure vanadium metal, which is not in agreement with the generally accepted idea that the ISEE coefficient of oxides is much larger than for metals. [ABSTRACT FROM AUTHOR]

Titanium oxide (TiO) thin films were deposited on thermally grown SiN-film-coated Si substrates by dc reactive magnetron sputtering. The crystallographic structure, microscopic morphology, electrical resistivity, temperature coefficient of resistance (TCR) and activation energy of the films were proven as functions of the oxygen partial pressure ( pO) and substrate temperature ( T). The mixed valence TiO thin films deposited with temperature from 30 to 250 °C in different pO present various electrical properties even though all the samples are amorphous. The TCR and activation energy of the films vary with different pO and T. It has been demonstrated that the sputtered TiO films are promising heat sensitive materials for uncooled Infrared detector application. [ABSTRACT FROM AUTHOR]

Abstract: Titanium oxide (TiOx) thin films were deposited on K9 glass substrates using a pure titanium target in a dc sputtering system. In this present work, we have investigated the dependence of electrical properties of deposited TiOx thin films on the different process condition, i.e., as the substrate temperature increases from 50°C to 300°C, the oxygen partial pressure (pO2) ranged from 2.5% to 10%, the sputtering time increases from 20min to 60min, and annealed in different oxygen concentration atmospheres. The film''s thickness and composition has been performed by profilometer and X-ray Photoelectron Spectroscopy (XPS) respectively. The films’ electrical resistance and the temperature coefficient of resistance (TCR) were studied as a function of the sputtering time, growth temperature, oxygen partial pressure (pO2) and annealing process. The TCR value of the films varied from 0.6% (K-1) to 2.2% (K-1) by controlling deposition process. [Copyright &y& Elsevier]

Abstract: Amorphous thin films of titanium oxide (TiOx) have been deposited on K9 glass substrates by dc reactive magnetron sputtering from a pure titanium target at different oxygen partial pressure. The structure and composition of these films have been characterized by x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS), respectively. The effects of the oxygen partial pressure on the composition and optical properties of amorphous TiOx thin films have been mainly investigated. Analyses of transmittance, reflectance, optical band gap of the films demonstrate that the oxygen partial pressure during the sputtering plays a very important role in their optical properties. [Copyright &y& Elsevier]

Abstract: Nanostructured vanadium oxide (nano-VOX) thin films were prepared by direct current magnetron reactive sputtering in mixed O2/Ar discharges and subsequent annealing in oxygen atmosphere. X-ray diffraction and field emission scanning electron microscope were employed to characterize the crystal structures and morphologies, respectively. Fourier transform infrared spectroscopy was applied to analyze the vanadium–oxygen bonds of films. X-ray photoelectron spectroscopy revealed the compositions of the surface and inner portion of nano-VOX thin films. It was shown that the as-deposited films were amorphous, and in-situ annealing of these films in ambient oxygen for 10min can lead to the growth of nano-VOX thin films. Results of electrical studies indicated that the absolute values of temperature coefficient of resistance and activation energy of films increased significantly after oxygen annealing. Optical investigations carried out in the UV-visible range showed that the absorption edges of nano-VOX films exhibited large red shifts compared with as-deposited film, and that dual optical absorptions were observed in nano-VOX films. [Copyright &y& Elsevier]

Abstract: 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. [ABSTRACT FROM AUTHOR]

Abstract: Nanostructured vanadium oxide (nano-VO x ) films were prepared on indium-tin oxide (ITO) glass substrate at low temperature by means of direct current (DC) reactive magnetron sputtering from pure vanadium target in Ar + O2 atmosphere. Field emission scanning electron microscope (SEM) reveals that the VO x film is composed of spheroidal nanoparticles whose diameters are in the range of 20–40 nm. This nano-VO x film shows a broad hysteresis loop whose width is as large as 41.6 °C. Moreover, the metal-insulator transition (MIT) can also be triggered by Joule heat produced by electrical current through the ITO sublayer. Compared to traditional heating of the sample by heating plate, this Joule heating is more efficient and convenient, which enables potential applications of this nano-VO x film on ITO conductive glass in compact storage devices. [Copyright &y& Elsevier]

Vanadium dioxide (VO 2) thin films were fabricated by means of DC reactive magnetron sputtering at low temperature, and then were aged in air for 5 years. The thermochromic properties of the fresh VO 2 film and aged one were investigated by UV–visible spectrophotometer. The experimental results show that the fresh VO 2 film shows excellent thermochromic properties with a high visible transmittance of 40.8% and a large switching efficiency of 73% at 2500 nm. For the film aged in ambient air for 5 years, the visible transmittance is increased to 54.0% due to a blue-shift of absorption edge and a decrease in luminous absorbance. More interestingly, the aged VO 2 film still keeps a relatively large switching efficiency of 51% at 2500 nm, suggesting the deposited VO 2 film has a good thermochromic stability in ambient air. These characteristics indicate the practical applications of VO 2 film to smart optical devices. • Nanoporous VO 2 film was prepared on ITO conductive glass at low temperature. • The VO 2 film was aged in ambient air for 5 years. • Comparative study of thermochromic properties on the fresh VO 2 and the aged one. • The VO 2 film shows a good long-time thermochromic stability in ambient air. [ABSTRACT FROM AUTHOR]

Abstract: Hydrogenated silicon film was fabricated by using plasma enhanced chemical vapor deposition method. The influence of crystalline volume fraction variation on the thermal conductivity was investigated. The relation between crystalline volume and film thickness was characterized by using spectroscopic ellipsometry with Bruggeman effective medium (BEMA) model. The thermal conductivity of silicon film was measured based on Fourier thermal transmitting law using sputtering platinum as electrode. The results demonstrate that the thermal conductivity of silicon film is proportional to the volume fraction of crystalline silicon, and there is crystalline and thermal conductive gradient between surface and bottom in the microcrystalline film. [Copyright &y& Elsevier]

The VOx films with composition from metal-rich to maximum stoichiometry can be accessed by tuning the pulse frequency, without varying any other parameters, especially, the flow rate of reactive gas. With the increase of pulse frequency, which can also facilitate the formation of a denser microstructure and smoother surface morphology in the films. The temperature dependent electrical properties of the VOx films can also be tailored via pulse frequency in a wide range to satisfy specific applications, such as microbolometer. [Display omitted] • • VOx films were prepared by reactive pulsed DC magnetron sputtering. • • The role of pulse frequency on the deposition process and properties of VOx films were investigated. • • Composition and some important properties of VOx films can be tailored by pulse frequency effectively. • • The corresponding underlying mechanisms were discussed in detail. In this study, vanadium oxide (VOx) thin films were prepared by reactive pulsed DC magnetron sputtering (PDMS). The influence of pulse frequency (F p) on the deposition process and properties of VOx films was investigated. The obtained results indicate that with the increase of F p , which not only can enhance the oxidation of vanadium but also can facilitate the formation of a denser microstructure and smoother surface morphology in the films. More importantly, films with composition from metal-rich to maximum stoichiometry can be accessed by tuning the F p , without varying any other parameters, especially, the flow rate of reactive gas. And temperature dependent electrical properties of the VOx films can also be tailored via the F p to fall in the desirable range for microbolometer applications. The impacts of F p on the reaction between V and O, thin film growth and some dynamic processes occurring during reactive PDMS were analyzed. The corresponding underlying mechanisms were discussed in detail. Our present work may provide a practical and effective way to develop the reactive sputtering towards hysteresis-free, and also paves the way for optimizing the reactive sputtered VOx films with desirable properties via F p. [ABSTRACT FROM AUTHOR]