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Abstract
Smart windows based on VO2 can control the infrared radiation entering the building based on the temperature, however, the visible part of the spectrum is not controlled. Liquid crystal (LC) based privacy windows, on the other hand, control the visibility either with temperature or applied voltage, however, the total transparency remains fixed as the scattering is mainly in the forward direction. To be able to control both the visibility and the temperature in the house, here we combine both layers in which the LC layer is made of a composite of nanoporous organic microparticles called Cochleates at small concentrations embedded in the LC matrix, thus acting as a tunable scattering metamaterial. The VO2-LC interface has less Fresnel reflectivity and therefore higher solar modulation is expected in an optimized window. In addition, being hidden under the LC layer, VO2 will be protected from oxidation. Electro-optic and thermo-optic properties of the device are investigated including the response time measurements. A non-reciprocity effect is observed showing better performance when the VO2 layer is facing the outside world, in which the window becomes more transparent from inside than from outside and also showing higher solar modulation. Response time is 1 ms for the rise and 10 ms fall time at 70 V. This approach opens up a new possibility of thermochromic VO2 and LC-based systems to satisfy the real-life requirements on smart window applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Windows were invented since humans started to build houses for privacy, safety and to protect themselves from cold or hot weathers. Recent advances in modern technology, however are re-inventing the window by making it smarter to be able to self-control its visibility and heat transfer. With the increasing awareness in reducing the emission of harmful green-house gases that adversely affect our environment there is race for reducing energy consumption in buildings. Buildings are responsible for about 40% of the total energy consumption used for heating, cooling, lightening or air conditioning [1]. Buildings continuously exchange energy through their windows by means of conduction, convection and radiation. Managing this heat exchange through windows is a possible approach to effectively control the building energy consumptions. In summer, solar radiation and the visible light entering buildings should be controlled to reduce the air-conditioning energy consumption. On the contrary, in winters, buildings should allow the inflow of solar radiation to retain the heat [2]. Smart windows which adjust the transmittance of solar irradiation are considered to be the promising candidates in reducing energy consumption of the buildings [3].
One of the effective routes to achieve this goal is by employing smart coatings on building windows that control the solar radiation. In this regard, several smart coatings based on electrochromism [4–6], gasochromism [7,8], thermochromism [9,10] and photochromism [11,12] have been widely investigated for the energy-efficient coating applications. Among them, thermochromic windows have gained enormous interest as they can modulate near-infrared radiation (NIR) from transmissive to opaque in response to the rise in environmental temperature, which does not require extra stimuli, in turn reduces the energy consumption. Thermochromic windows can be operated in two states: a transparent state with a higher solar transmittance and an opaque state with a lower solar transmittance [13].
Vanadium oxide (VO2) is one such promising material for energy saving applications due to its reversible metal-insulator (MIT) phase transition that occurs at critical transition temperature (Tc) at 68 °C [14] accompanied by the changes in the optical properties [15]. Below Tc, VO2 is in dielectric monoclinic phase that is transmissive to NIR. Above Tc, VO2 in its metallic tetragonal phase is reflective for NIR. This feature makes VO2 a promising material to be employed in the thermochromic smart coatings [16,17].
VO2 coatings are mostly used as VO2-based multilayer films [18–20] and VO2 nanoparticles based flexible foils [21–23]. However, there are several obstacles limiting the applicability of VO2 smart coatings [24]. (i) Phase-transition temperature (Tc) for pure VO2 (68 °C) is too high to be applied on building windows, while Tc around 40 °C is acceptable; (ii) Luminous transmittance (Tlum) for single layer VO2 with desirable solar modulation (ΔTsol) is usually less than 40% due to the absorption in the short-wavelength range in both the insulating and metallic states of VO2, needs to be larger than 50% for daily applications; (iii) VO2 can easily transform into V2O5 phase when exposed to the environment, this phase is not just stable but does not possess the thermochromic property [25]. For practical applications, VO2 must possess excellent thermochromic performance for several years. Therefore, environmental stability of VO2 is a great challenge for practical applications as smart coatings. Recently, a unique core-shell structure made of Au/VO2 was introduced which could reduce the Tc of VO2 up to 10 °C, explained as a result of internal stress when metal nanoparticles are introduced in the VO2 matrix, and not due to Plasmonic heating as it was believed before [26]. A machine learning approach to design and optimize VO2 based nanostructured smart window performance was introduced to simplify the heavy numerical simulations usually required when complicated photonic structures of VO2 are required to obtain better performance [27]. Improved transmittance in NIR and solar modulation are obtained by incorporating nanostructured VO2 films, which can be achieved by femtosecond pulsed laser patterning [28] or patterned nano-holes [29]. Apart from their applications in smart windows, such structures are also being explored for their tunable photonic bandgap properties [30].
In parallel to this, there is also a great interest in controlling the visible radiation for privacy and human comfort in particular in places where windows cannot be opened for safety reasons such as in trains, hospitals, hotels and high buildings. Hence privacy windows in which the visibility can be controlled are in demand. Most of the existing solutions are based on voltage controlled light scattering using polymer dispersed LCs [31,32], dye-doped LCs [33,34], particle dispersions in LCs [35] and by employing transparent electrodes [36–38]. However, the scattering is both for the visible and infrared light and usually in the forward direction, meaning that the heat cannot be controlled based on these privacy windows. On the other hand, the smart windows that control the heat do not control the visible light. Hence there is a need for a smarter window that can control both the heat and the visibility. This is the main motivation for this work in which we present the first demonstration of such hybrid smart window showing the two functions by combining VO2 layer with a tunable scattering metamaterial layer made of LC-nanoporous microparticles composite which we have shown recently [39,40] to be able to control the transparency by voltage and temperature. The VO2–LC interface has less Fresnel reflectivity and therefore higher solar modulation is expected in an optimized window. In addition, being hidden under the LC layer, VO2 will be protected from oxidation.
2. Materials and methods
2.1 VO2 film deposition on ITO substrate
VO2 thin films were prepared through physical vapor deposition (PVD). The vanadium source is a 390 cm2 99.9% purity vanadium target provided by Cemecon. During the deposition, the deposition chamber was evacuated to under 0.5 mPa and maintained at 420 °C throughout the experiment. Argon was pumped into the chamber with a flow rate of 500 sccm to serve as the protective gas, and the substrates were subjected to a DC bias of 120 V. High power impulse magnetron sputtering (HiPIMS) power was then supplied with a pulse frequency of 1500 Hz and a pulse on-time of 200 μs, which resulted in a duty cycle of 30%. After forming a monolayer of vanadium metal, oxygen gas was then introduced into the chamber with a flow rate of 53 sccm. The pressure was kept at 500 mPa during the deposition process [28].
2.2 Characterization
The surface morphology and the topography of VO2 film on ITO were studied by Scanning Electron Microscopy (SEM, Quanta 200, FEI) and Atomic Force Microscopy (AFM, MFP-3D-Bio, Asylum Research, Oxford Instruments). SEM images were captured at an accelerating voltage of 3 kV, and AFM was operated in the tapping mode. The XPS data were collected using an X-ray photoelectron spectrometer ESCALAB 250 (Termo Scientific, England) ultrahigh vacuum (1*10−9 bar) apparatus with an Al Kα X-ray source and a monochromator. The X-ray beam size was 500 μm and survey spectra were recorded with pass energy (PE) 150 eV and high energy resolution spectra were recorded with pass energy (PE) 20 eV. To correct for charging effects, all spectra were calibrated relative to a carbon C 1s peak positioned at 284.8 eV. Processing of the XPS results was carried out using AVANTGE program.
2.3 Sample preparation of LC cell assembly
Cochleates are lipid-based supramolecular tubular structures with a morphology accompanied by tightly packed bilayers [41]. A composite mixture of cochleates with nematic LC, BL036 (a mixture of alkylcyanobiphenyls obtained from Canaan Chem, China) is used for the device fabrication. Measurements are performed on the device made of 12 wt% cochleate in the BL036 [42]. The weighed amount of cochleate and LC is ultrasonicated for 15 mins in the presence of ethanol. The composite was subjected to magnetic stirring for 3 hrs in the presence of heat. The process helps in evaporating the ethanol, aiding to the uniform dispersion of the material.
Schematic illustration of the VO2 based LC-cochleate device is shown in Fig. 1. Out of the two substrates employed in the LC cell assembly, one was Indium-Tin oxide (ITO) coated SF11 glass (used to enable the light transmission in the range 420–2300 nm) and the other was VO2 coated ITO glass substrate. These two substrates were spin coated with a photoalignment layer ROP-108, obtained from ROLIC Technologies Ltd.-Switzerland. The substrates were baked at 180 °C for 15 mins. Thereafter, the substrates were treated with a linearly polarized ultraviolet (UV) light of power density of 0.5 mW/cm2 for 30 mins. Coating of the substrates with the photoalignment layer and further treatment with UV light helps in the uniform alignment of the LC molecules in the device. These two glass substrates were assembled together with a cell gap of 10 µm. The prepared LC-cochleate mixture was then filled into the device in the isotropic phase of LC using capillary action.
Electro-optic measurements on the device were performed with a tungsten-halogen light source (190–2500 nm) and the output response is recorded from the spectrometer for the entire Vis-IR range of the spectrum (StellarNet UV-Vis-NIR spectrometers (Blackcomet-SR) for the range 200–1080 nm and (NIR) for the range 900–2500 nm). Haze-transmittance measurements on the LC device were performed using an integrating sphere (Thorlabs). A bipolar sine wave input AC signal of specific amplitude at 1 kHz frequency from the function generator (GW Instek, AFG-2005) amplified by high-voltage amplifier (Tabor Electronics-9100A) is fed to the device. Instec hot-stage with mk 2000 controller is employed for the temperature dependent measurements.
3. Results and discussion
3.1 Device
The LC device can be operated between scattering and transparent states with the application of external voltage. The device switching for a sine wave input AC signal applied at 1 kHz frequency at different RMS voltages is shown in Fig. 2. As the applied voltage increases, gradual increase in the transparency from opaque state (voltage OFF) to transparent state (voltage ON) is achieved.
Fig. 2. Images taken through the LC device for a sine wave input AC signal applied at 1 kHz frequency in the scattering state obtained with RMS voltage of 0 V, translucent state at 30 V and the transparent state at 70 V in the absence of polarizers.
It should be mentioned that the voltage drop can be calculated based on the equivalent circuit model where each layer is represented by capacitance and resistor in parallel and the two layers are connected together in series [43]. Based on this the voltage drop is mostly on the LC layer because resistivity of VO2 is much smaller (kΩ to few hundred kΩ at room temperature and few Ω above the MIT temperature) than that of the LC and the capacitance of the VO2 layer is much larger than that of the LC (dielectric constant of VO2 around 36 at room temperature and around 6000 above the MIT temperature) [44].
3.2 Microscopy and XPS analysis of VO2 film
The surface morphology of the VO2 thin-film is studied from SEM and AFM images shown in Fig. S1 of Supplement 1. Morphology confirms the uniform and dense coating of the film with the film thickness of ∼ 150 nm. In order to verify the contents of the film, XPS surface measurement was performed on the VO2 film deposited on ITO substrate. The XPS spectra can be resolved into two main peaks in the V 2p3/2 and V 2p1/2 regions in Fig. S2. The parameters obtained after profile fitting such as peak position, full width at half-maximum (FWHM) and area are tabulated. The photoelectron peaks for the V 2p3/2 line in the range from 514–518 eV and for the V 2p1/2 line from 522–526 eV are the characteristic binding energies of V4+ (VO2 phase) [45,46]. Traces of V2O5 are found in the peak fitting of XPS spectra. The higher binding energies for the V 2p3/2 line in the range from 516–520 eV and for the V 2p1/2 at 525 eV indicate the presence of V5+ (V2O5 phase), from the surface oxidation of the film [46,47]. Therefore, the XPS peak fitting confirms the presence of vanadium multiphases such as VO2 and V2O5. This can be a result of the prolonged exposure of the films to air as they were prepared in NTU-Singapore (SHARE, CREATE labs), tested and shipped to BGU-Israel for device assembly.
3.3 Optical microscopy
Polarizing optical microscopic images of the VO2 based LC-Cochleate device obtained without and in the presence of external AC voltages are shown in Fig. 3. The images are collected with device positioned at 0° (optic axis parallel to one of the polarizers), rotated by 45° with respect to the crossed polarizers, and in the absence of one of the polarizers. The uniform dispersion of the particles in the LC is observed both with and without applied voltage. Some of the LC molecules can get trapped within the hollow cylindrical anisotropic cochleate particles. With the application of voltage, LC molecules transfer their torque to the cochleate particles which in turn orient along the field direction. The system reverts back to the stable state upon withdrawal of the applied voltage, the state governed by the elastic forces. For more detailed description of the tunable scattering metamaterial LC-Chochleate device the reader is referred to our recent work [42].
Fig. 3. Polarizing optical microscopy images of the VO2 based LC-Cochleate device at different voltages. The images are obtained in the transmission mode with the LC cell positioned with optic axis at 0°, and 45° with respect to the polarizer axis between the crossed polarizers.
3.4 Electro-optic measurements
The transmission spectrum of the VO2 based LC-Cochleate device was measured by a UV−VIS−NIR spectrometer (Stellarnet). The experimental set-up employed for the measurements is shown in Fig. 4. The light beam from the source is collimated using the lens and directed to the device in the hot-stage from the pinhole. The scattered beam from the device gets collected by the collection lens. The lens focuses the beam to the fiber which in turn is fed to the spectrometer and PC.
The transmittance spectra of the VO2 based LC-cochleate device obtained at three different temperatures of 25 °C, 50 °C and 80 °C for various voltages are shown in Fig. 5(a-c). The transmittance increases with the increase in the applied voltage. As the VO2 is more transmissive to the light in IR region in its insulating state, at 25 °C and 50 °C the transmittance continues to increase throughout the IR wavelength range (900–2500 nm). The transmittance increases from 5% to 23% at the highest applied voltage of 70 V. At high temperature 80 °C, VO2 is in its metallic state reflecting most of the IR radiation, hence the profiles show a drop in the overall transmittance of light from the device.
Fig. 5. Transmission spectra of VO2 based LC-cochleate device plotted as a function of wavelength at three different temperatures a) 25°C b) 50°C and c) 80°C for various voltages.
Figure 6(a-c) show the transmittance profiles at fixed AC RMS voltages (0 V, 50 V and 70 V) for three different temperatures, 25 °C, 50 °C and 80 °C. In the absence of external applied field, at room temperature the device is in its scattering state owing to the refractive index (RI) mismatch between cochleate and LC molecules, and hence the transmittance is low in the visible region. As the VO2 is in its insulating state at low temperatures, this allows the transmission of NIR through the device. As the applied voltage increases, the index matching between cochleate and LC molecules increases the transmittance from the device. At 70 V, the device becomes almost transparent, as the cochleates and LC molecules orient along the field direction reducing the scattering in the device. This further increases the transmittance. The profiles at 80 °C, show a reduction in transmittance in NIR region due to the fact that VO2 is in its metallic state reflecting most of the IR radiation. Transmission spectra of pure VO2 in the cold (20 °C) and hot (90 °C) states are shown in Fig. 6(d). Pure VO2 allows up to 40% of transmission of visible light, but the device with LC molecules transmits about 23% of the incoming light. This can be optimized further by choosing more transparent glass than SF11 as well as more transparent ITO layers.
Fig. 6. Transmission spectra of LC device at a) 0 V b) 30 V c) 70 V for three different temperatures and d) Pure VO2 sample without the LC at 20 °C and 90 °C. The lines through the data points for VO2 are simulated using the fit parameters obtained from Drude-Lorentz classical oscillator model described below.
The transmission spectra of VO2 are calculated from the optical constants obtained from the fit parameters of classical Drude-Lorentz classical oscillator model [48]. The model assumes the complex dielectric constant is given by the dispersion relation:
The first term on the right ${\epsilon _\infty }$ represents a contribution to the real part of the dielectric constant from high-frequency electronic transitions. The second term gives the free-electron contribution to the dielectric constant. Here ωn = (4πnce2/m*)1/2 is the carrier density parameter and is related to the plasma frequency, and ωc = e/µoptm* is the collision frequency, where nc is the number of conduction electrons and m* the so-called optical mass of the electrons. The third term, which is a sum over Lorentz classical oscillators, takes into account the contributions to the dielectric constant from lattice vibrations. Herein, Si, ωi and Γi represent strength, frequency and linewidth respectively, of the ith oscillator. Parameters of optical constants obtained from the fitting in the insulating phase (at 20 °C) and metallic phase (at 90 °C) of VO2 are tabulated in Table S1.
The integrated values of luminous transmittance (Tlum, 380−780 nm), infrared transmittance (TNIR, 780−2500 nm), and solar transmittance (Tsol, 380−2500 nm), which are used to quantify the amounts of visible, NIR, and solar energy entering a smart window, can be determined from the following equations [29].
The modulation levels of transmitted luminous, NIR, and solar radiation at cold and hot weather conditions can be represented as ΔTlum, ΔTNIR and ΔTsol, respectively, and it can be calculated from the following equations:
T(λ) represents the measured spectral transmittance. The photopic luminous efficiency of the human eye (φlum) [49] and the air mass (AM) 1.5 solar irradiance spectrum distributions (corresponding to the sun standing 37° above the horizon) [50] are used as weighting functions for the wavelength-dependent transmittance coefficients. Cold and hot states stand for the insulating state of VO2 at 20 °C and the metallic state of VO2 at 90 °C, respectively.
Thermochromic parameters of bulk VO2 show Tlum (Avg) = 39.7%, ΔTNIR = 8% and ΔTsol = 10.2%, whereas the device with LC-cochleate exhibits slightly lower performance with Tlum(Avg) = 5.3%, ΔTNIR = 1.9% and ΔTsol = 3.6% at 0 V, the values improve at 70 V (Tlum(Avg) = 17.3%, ΔTNIR = 10.1% and ΔTsol = 4.16%). The low values of Tlum are attributed to the strong reflection and absorption in the visible region both due to non-optimized glass substrate and ITO layers. Employing antireflection coatings (ARCs) have proved to be one of the effective strategies for enhancing the low Tlum without degrading the thermochromic properties of the VO2 films. Another problem for the low modulation values at 0 V is the fact that the device is scattering in this state and therefore the collected light to the optical fiber is not the whole light transmitted through the device.
The simulated transmittance profiles from the device at 0 V and 70 V for two different temperatures 25 °C and 80 °C are given in Fig. 7. The simulation was performed using 4 × 4 matrix formalism [51]. The measured transmittance values are lower in compared to simulated results for both the temperatures at 0 V. This is due to the fact that the device is in its scattering state without the application of voltage, while the simulation does not take into account the scattering. With VO2 being in its insulating state at 25 °C it transmits most of the IR radiation causing the rise in transmittance in the infrared region.
Fig. 7. Measured (red and green curves) and simulated transmittance profiles (black curves) from the device at (a) 0 V and (b) 70 V for two different temperatures 25°C and 80°C.
The transmittance at 80 °C is mainly contributed by two factors: Firstly, the increase in the transmittance observed from the device as the LC-Cochleate composite gets more diluted upon approaching closer to the isotropic phase, which reduces the scattering. Secondly, at this temperature VO2 being in its metallic state reflects most of the IR radiation, reducing the transmission in the IR region. At 70 V, most of the LC-Cochleate molecules are oriented along the applied field direction leading to increase in the transmittance from the device. The measured transmittance change between the low and high temperature profiles in the visible region were 66% and 26% respectively at 0 V and 70 V. Similarly the change in the IR region is about 71% at 0 V and 89% at 70 V. The simulated transmittance change between 25 °C and 80 °C in the visible region was about 30% for both 0 V and 70 V. The change is about 89% and 92% at 0 V and 70 V respectively in the IR region.
Temperature dependence of transmittance in the wavelength range (900–2500 nm) at different voltages (0 V, 30 V and 70 V) is shown in Fig. 8. Evidently, the transmittance increases with the increase in wavelength for all the voltages below 70 °C. This originates from the fact that VO2 is in its insulating state below 70 °C, allowing the IR radiation to pass through the device. Above 70 °C, the VO2 is in its metallic state blocking the IR, lowering the total transmittance from the device. An interesting result observed in Fig. 8 is the fact that there is almost no sensitivity to the wavelength in the IR at the MIT transition, particularly at the scattering state (0 V).
Fig. 8. Transmittance of VO2 based LC-Cochleate device plotted as a function of temperature at three different RMS voltages a) 0 V b) 30 V and c) 70 V in the wavelength range (900–2500 nm). Transmittance increases with the increase in wavelength.
Variation of the total transmittance (TT) is shown in Fig. 9, obtained using an integration sphere in the wavelength range of 420–2500 nm at 25 °C and 80 °C for different voltages (0 V, 50 V and 70 V) at two different configurations, depending on which side the output signal is collected. The Fig. 9 (a, b), show the TT collected from the VO2 side of the device showing that the transmittance increases as the applied voltage increases. At 25 °C the increase in transmittance is about 15% in the visible region and 17% in the IR region for the highest applied voltage 70 V. At 80 °C the change in transmittance is about 61% in the visible region and 8% in the IR region. The reflectance measured with integration sphere for 0 V, 50 V and 70 V at 25 °C and 80 °C for the output collected from LC side and VO2 side of the device is shown in Fig. S3. The 3 mm diameter beam was collimated with a lens from an optical fiber, then hit the sample on the opposite side of the sphere. The incidence angle is supposed to be zero, however small angles are possible due to misalignment and some divergence of the beam.
Fig. 9. (a, b) Variation in the total transmittance TT for different voltages 0 V, 50 V and 70 V when the output collected from the VO2 side at 25 °C and 80 °C respectively. (c, d) TT for the output collected from the LC side at 25 °C and 80 °C respectively.
Thermotropic parameters calculated from the data obtained using the integration sphere gives more accurate values, with Tlum (at 25 °C) = 5.06%, Tlum (at 80 °C) = 15.1%, ΔTNIR = 1.6% and ΔTsol = 5.1% at 0 V, the values being Tlum (at 25 °C) = 6.7%, Tlum (at 80 °C) = 30.3%, ΔTNIR = 0.8% and ΔTsol = 10.7% at 70 V. The solar modulation gets doubled at 70 V. For the output collected from the LC side of the device (see Fig. 9(c, d)) the transmittance at 80 °C almost reduces to half the value of the signal from the VO2 side. Parameters calculated in this configuration have Tlum (at 25 °C) = 5.4%, Tlum (at 80 °C) = 12.1%, ΔTNIR = 16% and ΔTsol = 4.8%, at 0 V, while at 70 V, the values being Tlum (at 25 °C) = 9.2%, Tlum (at 80 °C) = 15.5%, ΔTNIR = 12.4% and ΔTsol = 2.8%. The reason for the nonreciprocal effect is due to two effects: (i) the asymmetric lossy structure, and (ii) the scattering. The losses in the ITO layer and the VO2 can cause nonreciprocal behavior however this is a small effect as can be seen in Fig. S4 showing simulated reflectance/transmittance at normal incidence for unpolarized light from both sides of the device and different temperatures assuming uniform non-scattering device. The non-reciprocity due to losses and asymmetric structure showing around 5% maximum differences between the two sides. On the other hand in the true measurement (Fig. S3) the differences between the two sides can be as large as 30% depending on the temperature, voltage and wavelength. The scattering has larger effect because of the reflectivity dependence of VO2 on the incidence angle, as when the light incident first on the VO2 side, the reflectivity is mainly that of small angles, while the light is incident from the LC side, due to the forward scattering, many large angles hit the VO2 and therefore changing the reflected and thus transmitted signals. As a privacy window it can be recommended to have the LC side facing the inside of the room so that the window is more transparent for the people inside and less when looking from outside. This orientation also gives the higher ΔTsol, which is better in terms of energy consumption.
Figure 10 shows the schematic of experimental set-up employed in the measurements using integration sphere. The scattered beam from the device enters the integration sphere from the entrace port. Upon undergoing reflections within the sphere, the signal collected by the detector port is collected by the fiber to the spectrometer.
One of the important parameters in determining the performance of switchable devices is the response time. The schematic of experimental set-up employed for the response time measurements is shown in Fig. S5. A low frequency square wave input signal for the amplitude modulation given from the function generator to the amplifier is fed to the LC device for the response time measurements. Rise time and fall times at different voltages for temperatures 25 °C, 50 °C and 80 °C is shown in Fig. 11(a, b). The rise time is measured between 10% and 90% values during the voltage ON state, while the fall time is measured from 90% and 10% in the voltage OFF state. A decreasing trend is observed in the rise time with the increase in applied voltage. The measured rise and fall times are about 1 ms and 10 ms at 70 V. It is worth mentioning that the fall time at voltages above 10–15 V of 10 ms is much less than the visco-elastic relaxation time. The anchoring of the LC molecules to the walls of Cochleate tubes and the existence of domains prevents the backflow effect and thus responsible for such shorter relaxation times. The LC molecules in the device start to orient above 1 V and then continue to gradually increase without indication of cochleates re-orientation. The only indication of the reorientation of the cochleates is the abrupt reduction in the fall time around 10-15 V which is believed to be result of the formation of cylindrical walls created by cochleates separated by an average distance of around 5 μm (due to the 10% concentration of cochleates with diameter of 500 nm). Since the device thickness is 10 μm, the switching time is expected to reduce by at least factor 4. Additional reductions can be coming from variation in the anchoring conditions. Also worth mentioning that the LC molecules inside the cochleates will be switching at much fast speed because of the small diameter of 500 nm.
Fig. 11. Device response time to the applied voltage, with a) the rise time and b) fall time measured during the voltage ON and OFF states for different temperatures 25 °C, 50 °C and 80 °C.
4. Conclusions
A true smart window that controls both the infrared light and the visibility is demonstrated based on a hybrid VO2-LC tunable scattering metamaterial structure. While VO2 controls the IR transmission as the temperature varies it cannot control the visible light transparency. On the otherhand the LC based layer controls the transparency in the whole spectral range by voltage and temperature however it is based on forward scattering. The combination then is essential in order for the window to act both as privacy window in which the visibility is controlled and to control the heat as an energy saving window. Temperature and voltage dependent transmittance measurements were performed to determine the electro-optic and thermo-optic properties of the device. A non-reciprocity effect is reported in which the electro-optic and thermo-optic properties depend on which side the window is facing the outside world. The reason for the nonreciprocal effect is partially due to the asymmetric lossy structure, however the large effect originates due to the reflectivity dependence of VO2 on the incidence angle, as when the light incident on the VO2 side, the reflectivity is mainly that of small angles, while the light is incident from the LC side, due to the forward scattering, many large angles hit the VO2 and therefore changing the reflected and thus transmitted signals. As a privacy window it can be then recommended to have the LC side facing the inside of the room so the window is more transparent for people inside and less when looking from outside. This orientation also gives the higher ΔTsol, which is better in terms of energy consumption. Response time is faster in this system. Further optimization is possible through choosing more transparent glass substrates and ITO coatings, less absorbing LC in the IR, and addition of anti-reflection coatings. Combination of other VO2 structures that gave better performance can now be combined with LC privacy window to give even better performance. Using fresh VO2 prepared samples and the device construction immediately before exposing the VO2 to air will help maintain its thermochromic properties at optimum. The LC layer in this window helps in protecting the VO2 layer from further oxidation once the device is assembled. In addition the LC-VO2 interface exhibits less Fresnel reflectivity and this fact is expected to improve the window performance. This device opens up a new possibility of thermochromic VO2 and LC based systems to satisfy the real-life requirements on smart window applications.
Funding
Ministry of National Infrastructure, Energy and Water Resources; Ministry of Science, Technology and Space; National Research Foundation Singapore.
Acknowledgements
The research is supported partially by The Israel Ministry of National Infrastructure, Energy and Water Resources, and the binational collaboration project supported jointly by the Ministries of Science and Technology of Israel and Taiwan. This research is supported by grants from the National Research Foundation, Prime Minister’s Office, Singapore under its Campus of Research Excellence and Technological Enterprise (CREATE) program.
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
References
1. A. M. Omer, “Energy, environment and sustainable development,” Renewable and Sustainable Energy Reviews 12(9), 2265–2300 (2008). [CrossRef]
2. C. G. Granqvist, “Electrochromics and thermochromics: towards a new paradigm for energy efficient buildings,” Materials Today: Proceedings 3, S2–S11 (2016). [CrossRef]
3. H. Kim and S. Yang, “Responsive smart windows from nanoparticle–polymer composites,” Adv. Funct. Mater. 30(2), 1902597 (2020). [CrossRef]
4. J. Wang, L. Zhang, L. Yu, Z. Jiao, H. Xie, X. W. Lou, and X. Wei Sun, “A bi-functional device for self-powered electrochromic window and self-rechargeable transparent battery applications,” Nat. Commun. 5(1), 4921 (2014). [CrossRef]
5. C. G. Granqvist, “Oxide electrochromics: An introduction to devices and materials,” Sol. Energy Mater. Sol. Cells 99, 1–13 (2012). [CrossRef]
6. S. K. Deb, “Opportunities and challenges in science and technology of WO3 for electrochromic and related applications,” Sol. Energy Mater. Sol. Cells 92(2), 245–258 (2008). [CrossRef]
7. V. Wittwer, M. Datz, J. Ell, A. Georg, W. Graf, and G. Walze, “Gasochromic windows,” Sol. Energy Mater. Sol. Cells 84(1-4), 305–314 (2004). [CrossRef]
8. W.-L. Jang, Y.-M. Lu, C.-L. Chen, Y.-R. Lu, C.-L. Dong, P.-H. Hsieh, W.-S. Hwang, J.-L. Chen, J.-M. Chen, T.-S. Chan, J.-F. Lee, and W.-C. Chou, “Local geometric and electronic structures of gasochromic VOx films,” Phys. Chem. Chem. Phys. 16(10), 4699 (2014). [CrossRef]
9. J. Zheng, S. Bao, and P. Jin, “TiO2(R)/VO2(M)/TiO2(A) multilayer film as smart window: Combination of energy-saving, antifogging and self-cleaning functions,” Nano Energy 11, 136–145 (2015). [CrossRef]
10. T. Chang, X. Cao, N. Li, S. Long, X. Gao, L. R. Dedon, G. Sun, H. Luo, and P. Jin, “Facile and low-temperature fabrication of thermochromic Cr2O3/VO2 smart coatings: enhanced solar modulation ability, high luminous transmittance and uv-shielding function,” ACS Appl. Mater. Interfaces 9(31), 26029–26037 (2017). [CrossRef]
11. N. Li, Y. Li, G. Sun, Y. Zhou, S. Ji, H. Yao, X. Cao, S. Bao, and P. Jin, “Enhanced photochromic modulation efficiency: a novel plasmonic molybdenum oxide hybrid,” Nanoscale 9(24), 8298–8304 (2017). [CrossRef]
12. N. Li, Y. Li, W. Li, S. Ji, and P. Jin, “One-step hydrothermal synthesis of TiO2 @MoO3 core–shell nanomaterial: microstructure, growth mechanism, and improved photochromic property,” J. Phys. Chem. C 120(6), 3341–3349 (2016). [CrossRef]
13. S. Hoffmann, E. S. Lee, and C. Clavero, “Examination of the technical potential of near-infrared switching thermochromic windows for commercial building applications,” Sol. Energy Mater. Sol. Cells 123, 65–80 (2014). [CrossRef]
14. F. J. Morin, “Oxides which show a metal-to-insulator transition at the Neel temperature,” Phys. Rev. Lett. 3(1), 34–36 (1959). [CrossRef]
15. M. Taha, S. Walia, T. Ahmed, D. Headland, W. Withayachumnankul, S. Sriram, and M. Bhaskaran, “Insulator–metal transition in substrate-independent VO2 thin film for phase-change devices,” Sci. Rep. 7(1), 17899 (2017). [CrossRef]
16. Y. Cui, Y. Ke, C. Liu, Z. Chen, N. Wang, L. Zhang, Y. Zhou, S. Wang, Y. Gao, and Y. Long, “Thermochromic VO2 for energy-efficient smart windows,” Joule 2(9), 1707–1746 (2018). [CrossRef]
17. X. P. Zhao, S. A. Mofid, T. Gao, G. Tan, B. P. Jelle, X. B. Yin, and R. G. Yang, “Durability-enhanced vanadium dioxide thermochromic film for smart windows,” Materials Today Physics 13, 100205 (2020). [CrossRef]
18. H. Liu, D. Wan, A. Ishaq, L. Chen, B. Guo, S. Shi, H. Luo, and Y. Gao, “Sputtering deposition of sandwich-structured V2O5 /Metal (V, W)/ V2O5 multilayers for the preparation of high-performance thermally sensitive VO2 thin films with selectivity of VO2 (B) and VO2 (M) polymorph,” ACS Appl. Mater. Interfaces 8(12), 7884–7890 (2016). [CrossRef]
19. N. R. Mlyuka, G. A. Niklasson, and C. G. Granqvist, “Thermochromic VO2 -based multilayer films with enhanced luminous transmittance and solar modulation,” phys. stat. sol. (a) 206(9), 2155–2160 (2009). [CrossRef]
20. T. Chang, X. Cao, L. R. Dedon, S. Long, A. Huang, Z. Shao, N. Li, H. Luo, and P. Jin, “Optical design and stability study for ultrahigh-performance and long-lived vanadium dioxide-based thermochromic coatings,” Nano Energy 44, 256–264 (2018). [CrossRef]
21. Y. Chen, X. Zeng, J. Zhu, R. Li, H. Yao, X. Cao, S. Ji, and P. Jin, “High performance and enhanced durability of thermochromic films using VO2 @ZnO core–shell nanoparticles,” ACS Appl. Mater. Interfaces 9(33), 27784–27791 (2017). [CrossRef]
22. Y. Ke, X. Wen, D. Zhao, R. Che, Q. Xiong, and Y. Long, “Controllable fabrication of two-dimensional patterned VO2 nanoparticle, nanodome, and nanonet arrays with tunable temperature-dependent localized surface plasmon resonance,” ACS Nano 11(7), 7542–7551 (2017). [CrossRef]
23. M. Li, H. Wu, L. Zhong, H. Wang, Y. Luo, and G. Li, “Active and dynamic infrared switching of VO2 (M) nanoparticle film on ITO glass,” J. Mater. Chem. C 4(8), 1579–1583 (2016). [CrossRef]
24. S.-Y. Li, G. A. Niklasson, and C. G. Granqvist, “Thermochromic fenestration with VO2-based materials: Three challenges and how they can be met,” Thin Solid Films 520(10), 3823–3828 (2012). [CrossRef]
25. Y.-X. Ji, S.-Y. Li, G. A. Niklasson, and C. G. Granqvist, “Durability of thermochromic VO2 thin films under heating and humidity: Effect of Al oxide top coatings,” Thin Solid Films 562, 568–573 (2014). [CrossRef]
26. I. Balin, S. Wang, P. Wang, Y. Long, and I. Abdulhalim, “Enhanced transition-temperature reduction in a half-sphere au/ vo2 core-shell structure: local plasmonics versus induced stress and percolation effects Igal,” Phys. Rev. Appl. 11(3), 034064 (2019). [CrossRef]
27. I. Balin, V. Garmider, Y. Long, and I. Abdulhalim, “Training artificial neural network for optimization of nanostructured VO2 -based smart window performance,” Opt. Express 27(16), A1030 (2019). [CrossRef]
28. S. Bhupathi, S. Wang, M. Abutoama, I. Balin, L. Wang, P. G. Kazansky, Y. Long, and I. Abdulhalim, “Femtosecond laser-induced vanadium oxide metamaterial nanostructures and the study of optical response by experiments and numerical simulations,” ACS Appl. Mater. Interfaces 12(37), 41905–41918 (2020). [CrossRef]
29. C. Liu, I. Balin, S. Magdassi, I. Abdulhalim, and Y. Long, “Vanadium dioxide nanogrid films for high transparency smart architectural window applications,” Opt. Express 23(3), A124 (2015). [CrossRef]
30. Y. Ke, I. Balin, N. Wang, Q. Lu, A. I. Y. Tok, T. J. White, S. Magdassi, I. Abdulhalim, and Y. Long, “Two-dimensional SiO2/VO2 photonic crystals with statically visible and dynamically infrared modulated for smart window deployment,” ACS Appl. Mater. Interfaces 8(48), 33112–33120 (2016). [CrossRef]
31. E. P. Pozhidaev, A. V. Kaznacheev, S. I. Torgova, V. V. Kesaev, and V. A. Barbashov, “Polymer dispersed liquid crystals with electrically controlled light scattering in the visible and near-infrared ranges,” Opt. Mater. Express 10(12), 3030 (2020). [CrossRef]
32. N. Nasir, H. Hong, M. A. Rehman, S. Kumar, and Y. Seo, “Polymer-dispersed liquid-crystal-based switchable glazing fabricated via vacuum glass coupling,” RSC Adv. 10(53), 32225–32231 (2020). [CrossRef]
33. J. R. Talukder, Y.-H. Lee, and S.-T. Wu, “Photo-responsive dye-doped liquid crystals for smart windows,” Opt. Express 27(4), 4480 (2019). [CrossRef]
34. H. Sun, Z. Xie, C. Ju, X. Hu, D. Yuan, W. Zhao, L. Shui, and G. Zhou, “Dye-doped electrically smart windows based on polymer-stabilized liquid crystal,” Polymers 11(4), 694 (2019). [CrossRef]
35. E. Castellón and D. Levy, “Smart windows based on liquid crystal dispersions,” in Transparent Conductive Materials (Wiley-VCH Verlag GmbH & Co. KGaA, 2018), pp. 337–365.
36. S.-H. Chung and H. Y. Noh, “Polymer-dispersed liquid crystal devices with graphene electrodes,” Opt. Express 23(25), 32149 (2015). [CrossRef]
37. I. C. Kim, T.-H. Kim, S. H. Lee, and B.-S. Kim, “Extremely foldable and highly transparent nanofiber-based electrodes for liquid crystal smart devices,” Sci. Rep. 8(1), 11517 (2018). [CrossRef]
38. H. Hosseinzadeh Khaligh, K. Liew, Y. Han, N. M. Abukhdeir, and I. A. Goldthorpe, “Silver nanowire transparent electrodes for liquid crystal-based smart windows,” Sol. Energy Mater. Sol. Cells 132, 337–341 (2015). [CrossRef]
39. I. Abdulhalim, P. L. Madhuri, M. Diab, and T. Mokari, “Novel easy to fabricate liquid crystal composite with potential for electrically or thermally controlled transparency windows,” Opt. Express 27(12), 17387 (2019). [CrossRef]
40. L. M. Pappu, R. J. Martin-Palma, B. Martín-Adrados, and I. Abdulhalim, “Voltage controlled scattering from porous silicon Mie-particles in liquid crystals,” J. Mol. Liq. 281, 108–116 (2019). [CrossRef]
41. . Shuddhodana, P. W. K. Wong and Z. Judeh, “Continuous, high-throughput production of artemisinin-loaded supramolecular cochleates using simple off-the-shelf flow focusing device,” Materials Science and Engineering: C 108, 110410 (2020). [CrossRef]
42. P. L. Madhuri, Shuddhodana, Z. M. A. Judeh, and I. Abdulhalim, “Cochleate-doped liquid crystal as switchable metamaterial window mediated by molecular orientation modified aggregation,” Part. Part. Syst. Charact. 37(5), 2000067 (2020). [CrossRef]
43. K. Sayyah, “One-dimensional equivalent circuit simulation of a photodiode-based spatial light modulator,” IEEE Trans. Electron Devices 43(12), 2101–2108 (1996). [CrossRef]
44. Z. Yang, C. Ko, V. Balakrishnan, G. Gopalakrishnan, and S. Ramanathan, “Dielectric and carrier transport properties of vanadium dioxide thin films across the phase transition utilizing gated capacitor devices,” Phys. Rev. B 82(20), 205101 (2010). [CrossRef]
45. X. Liu, G. Xie, C. Huang, Q. Xu, Y. Zhang, and Y. Luo, “A facile method for preparing VO2 nanobelts,” Mater. Lett. 62(12-13), 1878–1880 (2008). [CrossRef]
46. O. Monfort, T. Roch, L. Satrapinskyy, M. Gregor, T. Plecenik, A. Plecenik, and G. Plesch, “Reduction of V2O5 thin films deposited by aqueous sol–gel method to VO2(B) and investigation of its photocatalytic activity,” Appl. Surf. Sci. 322, 21–27 (2014). [CrossRef]
47. Y. Li, J.-L. Kuang, Y. Lu, and W.-B. Cao, “Facile synthesis, characterization of flower-like vanadium pentoxide powders and their photocatalytic behavior,” Acta Metall. Sin. (Engl. Lett.) 30(10), 1017–1026 (2017). [CrossRef]
48. H. W. Verleur, A. S. Barker, and C. N. Berglund, “Optical properties of VO2 between 0.25 and 5 eV,” Phys. Rev. 172(3), 788–798 (1968). [CrossRef]
49. T. Smith and J. Guild, “The C.I.E. colorimetric standards and their use,” Trans. Opt. Soc. 33(3), 73–134 (1932). [CrossRef]
50. “American Society for Testing and Materials, “ASTM G173-03 reference spectra,”“ https://www.nrel.gov/grid/solar-resource/spectra-am1.5.html.
51. I. Abdulhalim, “Analytic propagation matrix method for anisotropic magneto-optic layered media,” J. Opt. A: Pure Appl. Opt. 2(6), 557–564 (2000). [CrossRef]
