Optimized Atmospheric-Pressure Chemical Vapor Deposition Thermochromic VO2 Thin Films for Intelligent Window Applications.
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ABSTRACT:
SUBMITTER: Malarde D
PROVIDER: S-EPMC6640985 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Over the years, vanadium dioxide, (VO2(M1)), has been extensively utilised to fabricate thermochromic thin films with the focus on using external stimuli, such as heat, to modulate the visible through near-infrared transmittance for energy efficiency of buildings and indoor comfort. It is thus valuable to extend the study of thermochromic materials into the mid-infrared (MIR) wavelengths for applications such as smart radiative devices. On top of this, there are numerous challenges with synthesising pure VO2 (M1) thin films, as most fabrication techniques require the post-annealing of a deposited thin film to convert amorphous VO2 into a crystalline phase. Here, we present a direct method to fabricate thicker VO2(M1) thin films onto hot silica substrates (at substrate temperatures of 400 °C and 700 °C) from vanadium pentoxide (V2O5) precursor material. A high repetition rate (10 kHz) femtosecond laser is used to deposit the V2O5 leading to the formation of VO2 (M1) without any post-annealing steps. Surface morphology, structural properties, and UV-visible optical properties, including optical band gap and complex refractive index, as a function of the substrate temperature, were studied and reported below. The transmission electron microscopic (TEM) and X-ray diffraction studies confirm that VO2 (M1) thin films deposited at 700 °C are dominated by a highly texturized polycrystalline monoclinic crystalline structure. The thermochromic characteristics in the mid-infrared (MIR) at a wavelength range of 2.5-5.0 μm are presented using temperature-dependent transmittance measurements. The first-order phase transition from metal-to-semiconductor and the hysteresis bandwidth of the transition were confirmed to be 64.4 °C and 12.6 °C respectively, for a sample fabricated at 700 °C. Thermo-optical emissivity properties indicate that these VO2 (M1) thin films fabricated with femtosecond laser deposition have strong potential for both radiative thermal management or control via active energy-saving windows for buildings, and satellites and spacecraft.
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Project description:The ability to achieve energy saving in architectures and optimal solar energy utilisation affects the sustainable development of the human race. Traditional smart windows and solar cells cannot be combined into one device for energy saving and electricity generation. A VO2 film can respond to the environmental temperature to intelligently regulate infrared transmittance while maintaining visible transparency, and can be applied as a thermochromic smart window. Herein, we report for the first time a novel VO2-based smart window that partially utilises light scattering to solar cells around the glass panel for electricity generation. This smart window combines energy-saving and generation in one device, and offers potential to intelligently regulate and utilise solar radiation in an efficient manner.
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Project description:Fenestration elements that enable spectrally selective dynamic modulation of the near-infrared region of the electromagnetic spectrum are of great interest as a means of decreasing the energy consumption of buildings by adjusting solar heat gain in response to external temperature. The binary vanadium oxide VO2 exhibits a near-room-temperature insulator-metal electronic transition accompanied by a dramatic modulation of the near-infrared transmittance. The low-temperature insulating phase is infrared transparent but blocks infrared transmission upon metallization. There is considerable interest in harnessing the thermochromic modulation afforded by VO2 in nanocomposite thin films. However, to prepare a viable thermochromic film, the visible-light transmittance must be maintained as high as possible while maximizing thermochromic modulation in the near-infrared region of the electromagnetic spectrum, which necessitates the development of high-crystalline-quality VO2 nanocrystals of the optimal particle size embedded within the appropriate host matrix and refractive index matched to the host medium. Here, we demonstrate the preparation of acrylate-based nanocomposite thin films with varying sizes of embedded VO2 nanoparticles. The observed strong size dependence of visible-light transmittance and near-infrared modulation is explicable on the basis of optical simulations. In this article, we elucidate multiple scattering and absorption mechanisms, including Mie scattering, temperature-/phase-variant refractive-index mismatch between VO2 nanocrystals and the encapsulating matrix, and the appearance of a surface plasmon resonance using temperature-variant absorptance and diffuse transmittance spectroscopy measurements performed as a function of particle loading for the different sizes of VO2 nanocrystals. Nanocrystals with dimensions of 44 ± 30 nm show up to >32% near-infrared energy modulation across the near-infrared region of the electromagnetic spectrum while maintaining high visible-light transmission. The results presented here, providing mechanistic elucidation of the size dependence of the different scattering mechanisms, underscore the importance of nanocrystallite dimensions, refractive-index matching, and individualized dispersion of particles within the host matrix for the preparation of viable thermochromic thin films mitigating Mie scattering and differential refractive-index scattering.
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Project description:Window coatings with dynamic solar transmittance represent an excellent opportunity to reduce building heating and cooling loads, which account for >40% of energy consumed by the built environment. In particular, inorganic vanadium dioxide-based thermochromic coatings offer long lifetimes (>30 years) and can be passively integrated into a window system without additional electronics or power requirements. However, their limited solar modulation depth and wide phase-change hysteresis have traditionally restricted their ability to adapt to changing weather conditions. Here, we derive an optical performance limit for thin film vanadium dioxide coatings, which we find to be far beyond the current literature. Furthermore, we experimentally demonstrate a solution-processed multilayer thin film coating that uses temperature-dependent optical impedance matching to approach the optical performance limit. The thin film coating demonstrated has a record solar transmittance modulation of 21.8% while maintaining a high level of visible transparency (?50%) and minimal hysteresis (?10 °C). This work represents a step-change in thin film thermochromic window coatings and, as a result, establishes planar thin film vanadium dioxide as the most viable morphology for high-performance thermochromic windows.
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Project description:Laminated metal dichalcogenides are candidates for different potential applications ranging from catalysis to nanoelectronics. However, efforts are still needed to optimize synthesis methods aiming to control the number of layers, morphology, and crystallinity, parameters that govern the properties of the synthesized materials. Another important parameter is the thickness and the length of the samples with the possibility of large-scale growth of target homogeneous materials. Here, we report a chemical vapor deposition method at atmospheric pressure to produce vertically aligned tin dichalcogenide based-materials. Tin disulfide (SnS2) and tin diselenide (SnSe2) vertically aligned nanosheets have been synthesized and characterized by different methods showing their crystallinity and purity. Homogenous crystalline 2H-phase SnS2 nanosheets with high purity were synthesized with vertical orientation on substrates; sulfur vacancies were observed at the edges of the sheets. Similarly, in the crystalline 2H phase SnSe2 nanosheets selenium vacancies were observed at the edges. Moreover, these nanosheets are larger than the SnS2 nanosheets, show lower nanosheet homogeneity on substrates and contamination with selenium atoms from the synthesis was observed. The synthesized nanomaterials are interesting in various applications where the edge accessibility and/or directionality of the nanosheets play a major role as for example in gas sensing or field emission.
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Project description:In this work, a simple, fast and dry method for the fabrication of a thermochromic product with a high load of VO2 (M1) consisting of the controlled heat treatment of pure vanadium nanoparticles in air is presented. After a complete design of experiments, it is concluded that the most direct way to attain the maximum transformation of V into VO2 (M1) consists of one cycle with a fast heating ramp of 42 °C s-1 , followed by keeping 700 °C for 530-600 seconds, and a subsequent cooling at 0.05 °C s-1 . Careful examination of these results lead to a second optimum, even more suitable for industrial production (quicker and less energy-intensive because of its lower temperatures and shorter times), consisting of subjecting V to two consecutive cycles of temperatures and times (625 °C for 5 minutes) with similar preheating (42 °C s-1 ) but a much faster postcooling (∼ 8 °C s-1 ). These green reactions only use the power for heating a tube open to atmosphere and a vanadium precursor; without assistance of reactive gases or catalysts, and no special vacuum or pressure requirements. The best products present similar thermochromic properties but higher thermal stability than commercial VO2 particles. These methods can be combined with VO2 doping.
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Project description:Chemical vapor deposition (CVD) is a powerful method employed for high-quality monolayer crystal growth of 2D transition metal dichalcogenides with much effort invested toward improving the growth process. Here, we report a novel method for CVD-based growth of monolayer molybdenum disulfide (MoS2) by using thermally evaporated thin films of molybdenum trioxide (MoO3) as the molybdenum (Mo) source for coevaporation. Uniform evaporation rate of MoO3 thin films provides uniform Mo vapors which promote highly reproducible single-crystal growth of MoS2 throughout the substrate. These high-quality crystals are as large as 95 μm and are characterized by scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy. The bottom-gated field-effect transistors fabricated using the as-grown single crystals show n-type transistor behavior with a good on/off ratio of 106 under ambient conditions. Our results presented here address the precursor vapor control during the CVD process and is a major step forward toward reproducible growth of MoS2 for future semiconductor device applications.
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Project description:Thermochromic vanadium dioxide (VO2)-based smart radiator devices (SRDs) display emittance variation with changes in temperature, making them very promising for energy-efficient thermal control of spacecrafts in general, and nanosatellites in particular. However, the high solar absorptance of the VO2-based SRDs remains too high for their intended application. Based on an approach combining optical simulation and experimental work, I demonstrate that an additional top stack layer alternating between high and low refractive indices made of a-Si(25 nm)/SiO2(67 nm) reduces the solar absorptance of a VO2-based SRD by 35% (from 0.43 to 0.28) while keeping the emittance performance of the SRD within the requirements for the intended application (low-temperature emittance εL = 0.35, high-temperature emittance εH = 0.81 and emittance tuneability with temperature Δε = 0.46). I also discuss factors to consider while designing additional top stack layers alternating between high and low refractive indices to further decrease the SRD's solar absorptance without affecting its emittance performance.
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