Controllable Fabrication of Two-Dimensional Patterned VO2

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Controllable Fabrication of Two-Dimensional Patterned VO2 Nanoparticle, Nanodome, and Nanonet Arrays with Tunable Temperature-Dependent Localized Surface Plasmon Resonance Yujie Ke, Xinglin Wen, Dongyuan Zhao, Renchao Che, Qihua Xiong, and Yi Long ACS Nano, Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017

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Controllable Fabrication of Two-Dimensional Patterned VO2 Nanoparticle, Nanodome, and Nanonet Arrays with Tunable TemperatureDependent Localized Surface Plasmon Resonance Yujie Ke,1 Xinglin Wen,2 Dongyuan Zhao,3 Renchao Che,4 Qihua Xiong,2,5 and Yi Long 1,* 1

School of Materials Science and Engineering, Nanyang Technological University, Singapore, 639798, Singapore 2

Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore

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Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Laboratory of Advanced Materials, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Fudan University, Shanghai, 200433, China

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Department of Materials Science, Laboratory of Advanced Materials, iChEM (Collaborative

Innovation Center of Chemistry for Energy Materials), Fudan University, Shanghai, 200433, China

ACS Paragon Plus Environment

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NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore, 639798, Singapore * Corresponding author: Dr. Yi Long, Fax: (65)67911604, Email: [email protected]

KEYWORDS: Vanadium dioxide (VO2); localized surface plasmon resonance (LSPR); patterned nanostructure; near infra-red (NIR) modulation; thermochromics; smart devices; nanosphere lithography

ABSTRACT:

A universal approach to develop various two-dimensional ordered nanostructures, namely nanoparticle, nanonet and nanodome arrays with controllable periodicity, ranging from 100 nm to 1 um, has been developed in centimeter-scale by nanosphere lithography technique. Hexagonally patterned vanadium dioxide (VO2) nanoparticle array with average diameter down to sub-100 nm as well as 160 nm of periodicity is fabricated, exhibiting distinct size-, media-, and temperature-dependent localized surface plasmon resonance (LSPR) switching behaviors, which fits well with the predication of simulations. We specifically explore their decent thermochromic performance in energy saving smart window and develop a proof-of-concept demo which proves the effectiveness of patterned VO2 film to serve as a smart thermal radiation control. This versatile and facile approach to fabricate various ordered nanostructures integrated with attractive phase change characteristic of VO2 may inspire the study of temperaturedependent physical responses and the development of smart devices in extensive areas.


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Ordered arrays of colloidal nanocrystals (NCs) represent an important class of metamaterials with tunable structures and properties.1,2 Two-dimensional (2D) patterned nanostructures, such as 2D ordered nanostructure arrays, surface-patterned nanostructures, and free-standing 2D patterned films, have attracted intensive interests due to the pattern-dependent properties.3 The fabrication approach based on monolayer colloidal crystal (MCC) templates has been recognized as a facile, inexpensive, efficient, and flexible nanolithography technique for preparing functional 2D patterned nanostructures with high reproducibility.4 Previous studies of artificial 2D patterned nanostructures via the templates of hexagonal self-assembly of polystyrene (PS) spheres have been widely used in different fields including sensor,5 solar cells,6 photocatalysis,7 surface enhanced Raman scattering (SERS),8,9

light-emitting diodes (LED),10 antireflection

layer,11 hydrophobia structures,12 and so on.13,14 Due to its intriguing near-room-temperature first-order phase transition, a metal-insulator transition (MIT), accompanying with strong electronic, optical, and mechanical changes,15,16 vanadium dioxide (VO2) has been widely used in field-effect devices,17,18 energy storage,19-21 thermal camouflage,22 smart window,23,24 four-dimensional imaging,25 and so on. However, only a few works on 2D patterned VO2 were reported, which may be mainly due to limited fabrication methods. Periodic VO2 nanonet array was produced via the colloidal lithography approach and was further developed by a template-free dual-phase transformation method.26,27 However, these methods were limited to nanonet structure as well as relatively large periodicity. VO2 nanopillar arrays through nanoimprinting and truss network via controlled directed growth cannot reach sub-100 nm region.28,29 Ordered VO2 nanoparticles have successfully been prepared via a combination of ion beam lithography, pulsed laser deposition followed by thermal oxidation,30


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but its sophisticated procedure, difficulty in scalability, and high dependence on equipment limit it to fundamental science. Nanocrystals with tunable localized surface plasmon resonance (LSPR) attracted great interest as their significance in medicine, sensing, electrocatalysts, microelectronic devices.31-33 This particularly important property is increasingly playing critical roles in nanophotonics and nanoelectronics. VO2 was studied as the host material to introduce tunability for Au nanoparticles.34,35 SiO2-coated VO2 nanorods were calculated with tunable LSPR position by varying the fill factor and the ratio aspect.36 Nevertheless, to our knowledge, systematical experiment focusing on the tunable LSPR of pure VO2 nanocrystal is rare. This may due to the relatively harsh requirement for the VO2 preparation compared to other widely studied nanocrystals such as gold- and cadmium-based materials. In this work, we report 2D patterned VO2 nanocrystals with different nanostructures, namely nanoparticle, nanonet, and nanodome arrays as well as controlled periodicity. The periodicity of structure achieves down to 200 nm with sub-100 nm of nanoparticle size, while the area of samples can reach up to centimeter-scale. Their size-, media-, and temperature-dependent LSPR tunabilities in near infra-red (NIR) range are observed. LSPR red-shifts are observed with increase of the particle size and the media reflective index, respectively, while the relative LSPR intensity can be dynamically adjusted by varying temperature. Moreover, the hexagonally patterned VO2 nanoparticle array with the periodicity of 160 nm has been demonstrated with good performance in thermochromic smart window application as well as being a smart thermal radiation control. RESULTS AND DISCUSSION


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Monolayer hexagonal-close-packed (HCP) VO2 nanoparticle, nanodome and nanonet arrays are prepared by varying the viscosity of vanadium precursor and plasma etching (PE) duration as well as controlling the separation distance between adjacent PS spheres in MCC template (Scheme 1). Low separation distance results in the infiltrated precursor forming as islands in interspace among PS spheres under low precursor viscosity condition; in contrast, covering MCCs as contiguous domes under high precursor viscosity circumstance. Meanwhile, high separation distance in MCCs allows large quantity of the low viscosity precursor to infiltrate and adhere to the substrate, which develops into the nanonet structure to cover the lower part of the spheres. The final morphology of VO2 films is highly dependent on the patterning of infiltrated vanadium precursor in related to PS spheres before annealing, and can be largely preserved after the removal of the PS spheres and the VO2 crystallization. The template with PS spheres diameter down to 160 nm can be assembled in hexagonally close-packed way (Figure 1a). The height of template is measured to be 160 nm which is consistent with the diameter of PS sphere, suggesting it is monolayer assembled (Figure 1b). The inter distance between two adjacent PS spheres is controlled via Ar/O2 PE. For example, the size of PS spheres with diameter of 640 nm (inset of Figure 1c) is diminished after Ar/O2 PE treatment for 300 s (Figure 1c), while the sphere morphology maintains, which demonstrates the etching process happens isotropically. The HCP structure remains and adheres well on quartz substrate after the treatment. The diameter of spheres decreases linearly with etching duration with a speed of 1.55±0.06 nm/s (Figure 1d) measured by AFM (Figure S1). These characters combining with the high reproducibility demonstrate that the PE method is a facile way to precisely control the size of PS sphere in MCC templates.


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As shown in Scheme 1, the high-quality 2D VO2 nanoparticle (Figure 2a-c), nanonet (Figure 2d-f), and nanodome arrays (Figure 2g-i) are prepared with variable periodicities, which are 160, 490, and 830 nm showing from left to right columns in Figure 2. The HCP structure and periodicity of final VO2 crystals are well maintained from the PS spheres MCC template for all morphologies. Except for the monoclinic VO2, no other phase is detectable after annealing (Figure S2). The VO2 nanoparticle shows quasi-sphere shape (Figure 2a-c), which is thermodynamically stable since such shape lowers the surface energy under high annealing temperature. The periodicities of hexagonal arrays, across 160 to 830 nm, are the same as used nanosphere masks (denoting as the yellow hexagons in Figures 2a-c). Average diameters of VO2 particles are measured to be as 67, 125, and 287 nm, respectively (Figure S3a-c), increasing with size of PS nanospheres (Figure S3d). The interspace among PS spheres performs as templates to generate size-controlled island particles. This MCC templated-assisted growth is believed to be broadly suitable for nanoparticles with various compositions, because of its low requirement for the precursor properties, while is more time-saving, cost-efficient, and facile than other methods, such as dip-pen nanolithography and ion beam lithography.30,37 Ordered VO2 nanonet arrays with various periodicities are prepared by increasing the interspheres distance (Figure 2d-f) and the net width was further tuned by controlling PE duration (Figure S4). Short duration, such as 50 s, gives “egg carton” nanonet structure, where VO2 crystals on nodes are higher than on lines (Figure S4a), as the MCC offers more space for precursor accumulation in the intersections formed among three adjacent PS spheres during dipcoating process. Long duration turns the final nanonet structure flattened (Figure S4b).


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When using the vanadium precursor with high viscosity, uniform VO2 nanodomes are produced with HCP monolayer (Figure 2h), with the morphology preserved from the structure before annealing (Figure S5). In the VO2 nanodome arrays, each dome is constructed by numbers of nanoparticles with clear boundaries (inset of Figure 2h). The bubbles form below the domes, because the crystallized vanadium precursor develops enough mechanical strength before the removal of PS spheres during heating up. Different from previous report,38 it is demonstrated the deformation of MCC templates is caused by melting of PS spheres, starting from 120 °C (Figure S6), rather than by being dissolved by vanadium precursor. The nanodome structures are widely investigated in photovoltaic (PV) devices,39 chemical and biological sensors,40 plasmonic sensing relays on surface enhanced Raman scattering (SERS) or surface plasmon polaritons (SPP) modes,41,42 and so on, which cover materials from semiconductor, metallic to composite. None of those has the smart functionality as VO2 nanodome arrays exhibiting temperature-dependent MIT (Figure S7). Periodicity of the patterned films is adjusted from 160 to 830 nm by tuning the PS sphere size of MCC templates. Morphologies are consistent for the nanoparticle (Figure 2a-c) and nanonet arrays (Figure 2d-f) with the change of periodicity. For the nanodome arrays, the distinct dome structure maintains well in 490- (Figure 2h) and 830-nm (Figure 2i) periodicity samples, but becomes obscure when 160-nm PS spheres are used (Figure 2g). This is because 160-nm PS templates provide limited interspace among spheres, which prevents the high viscosity precursor from penetrating into the lower part of spheres. This demonstrates periodicity is another critical parameter to affect morphology of final products via adjusting volume ratio of the PS sphere to the infiltrated vanadium precursor. Maintaining nanostructure with periodicity less than 200 nm


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is always challenging,11,23 the current method achieves the VO2 nanoparticle, nanonet, and nanodome arrays with periodicity as low as 160 nm and can be easily applied to other systems. The samples with 160-nm periodicity are presented to illustrate the effect of PE duration. By prolonging PE duration, the patterned nanostructure changes from isolated small VO2 particles (Figure S8a) to large particles (Figure S8b), then to partially linked particles (Figure S8c). Finally, nanonet structure forms after PE treatment for 60 s and sufficient precursor infiltration (Figure S8d). This evolution is experimentally verified as shown in SEM images (Figure S8e-h). The PE technique can maintain the periodicity, while change the interspace between neighboring spheres. Besides precursor viscosity and periodicity of the template as explained above, the PE technique is also demonstrated to be an efficient method for patterned morphology control, introducing more flexibility for the template-based nanostructure fabrication. LSPR phenomenon on VO2 is interesting since its temperature-dependent behavior, which is not observable on pure noble metals and heavily-doped semiconductors.31,43 The transmittance spectra measured in air of three samples with particles size of 67 (Figure 2a), 125 (Figure 2b), and 287 nm (Figure 2c), are normalized and presented in Figure 3a. The LSPR in NIR range can be observed at 95 oC instead of at 20 oC (Figure S9), which is consistent with previous report,36 due to the metal phase formation above the critical temperature as well as controllable size of the particles down to sub-100 nm region. At temperature of 95 oC, a red-shift of LSPR from 1120 to 1210 nm (inset of Figure 3a) is observed with increasing diameter of VO2 nanoparticles (Figure 3b) which are fabricated with increasing nanosphere template (Figure S3). This size-dependent LSPR red-shift is further confirmed by simulation. The simulated transmission as shown in the dash line in Figure 3a matches the experimental results very well. Both the resonance wavelength of simulation and experiment are extracted and compared in Figure 3b, the minor


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deviation from experiment may be due to the ideal and uniform size and shape used in simulation is slightly different with the real size distribution and shape variations. Figure 3c-e display the electric field distribution of the nanoparticle with the size of 67, 125, and 287 nm at their resonance wavelength, respectively. The highly confined electromagnetic field at two ends of nanoparticle indicates the electric moment is in resonance with external light at the resonance wavelength. In addition, the electric-field distribution suggests that the coupling between two adjacent particles is negligible and the optical response comes from the individual particles. In addition, by changing the dielectric medium that encapsulates the VO2 nanoparticles, the LSPR exhibits adjustability with 67-nm VO2 nanoparticle array (Figure 3f). The LSPR is measured with distinct difference of VO2 nanoparticles in air (1120 nm as shown in Figure 3a) and in polymer matrixes, which is 1220 nm in polyethtlene glycol (PEG), 1270 nm in polyvinyl alcohol (PVA), and 1350 nm in polyvinylpyrrolidone (PVP) (inset of Figure 3f). A red-shift of LSPR peaks can be observed with a higher reflective index of overcoat (Figure 3g), this effect is consistent with the observation in heavily doped semiconductor system and is extensively applied to chemical as well as biological sensors.44 The dipole moment inside the nanoparticle can be expressed as: p = 4πε 0ε m a3

ε (λ ) − ε m E ε (λ ) + 2ε m 0

(1)

where ε (λ ) and ε m is the permittivity of metal and surrounding dielectric respectively. At the resonant condition, the permittivity of the VO2, ε (λ ) = -2 ε m , so that the resonance wavelength will change once the surrounding dielectric changed. Interestingly, LSPR intensity in VO2 nanoparticles can be dynamically tuned by temperature (Movie S1). The corresponding spectra are recorded in Figure 3h showing that LSPR appears at high temperature, and gradually quenches at low temperature of 20°C. The enhanced LSPR


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intensity with higher temperature is due to the emergence of intermediate states during the metalinsulator gradual transition and the structural phase transition.45 These intermediate states display the temperature-dependence of the metallic fraction with enhanced carrier density by increasing temperature, bringing about gradual enhancement of the relative LSPR intensity. Moreover, the relative LSPR intensity can be reversibly tuned during cooling and heating processes (Figure 3i). In addition, it is interesting that a gradual red-shift of the resonance peak is observed when temperature is above 75oC during heating and the blue-shift to its original position when the sample is under 70 oC during cooling down process (Figure 3j). Both the intensity and peak position are hysteretic in the heating-cooling circle, which is consistent with the hysteresis nature of the VO2 phase transition. The dynamically and reversibly tunable LSPR characters in VO2 system make it great potential in programmable devices. The VO2 nanoparticle arrays exhibit size-, media-, and temperature- dependent LSPR in NIR range, which is of great significant in various applications. We specifically discuss the LSPR effects on the thermochromic properties as LSPR is favorable in enhancing the solar modulation. Because the LSPR intensity increases at higher temperature which results in higher absorption and lower transmittance, and moreover the LSPR peak position tunes to the region with higher solar energy density in the solar spectrum range.46 Thermochromic smart window is considered as a promising way to reduce the architectural energy consumption by blocking the NIR transmittance at high temperature.47 2D patterned VO2 nanocrystal array is a promising candidate for thermochromic modulation due to its LSPR on rutile phase at high temperature,48 self-generated antireflection behavior,49 and porosity-induced visible transmittance enhancement.50


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The optimized thermochromic performance is observed on sample with structure shown in Figure S8f. Samples were prepared on quartz glasses with centimeter-scale shown as the inserted photograph in Figure 4a. The performance is further promoted by simply overlaying the foils for 1, 2, and 3 layers (Figure 4a). All spectra display conspicuous damps at high temperature in NIR transmittance (Figure 4b), which are consistent with the nature of thermal-induced monoclinic to rutile phase transition of VO2. When increasing the number of layers, transmittance crossing visible to NIR decreases, while the thermal-induced NIR contrast increases. The optimized thermochromic properties is observed on double-layered sample, which exhibits an average high transmittance (Tlum) of 46%, accompanying with a transmittance contrast at 2000 nm (∆TNIR = 44%) and a solar energy modulation (∆Tsol) up to 13.2% (inserted table of Figure 4a), and no transmittance change observed at 2500 nm after 100 cycles, which indicates the high stability of the thermochromic performance (Figure 4c). This performance largely surpasses those reported on periodic VO2 nanostructures in NIR modulation ability (Figure 4d), and is comparable to the best thermochromic performance reported among anti-reflective multilayer,51 controlled porosity,52 biomimetic moth-eyed structure,49 VO2 nanoparticles/transparent media composite,53 and doping (Figure 4e).54 In addition, this inexpensive and facile method is easily incorporated with other technologies, such as doping and composite, to further enhance the thermochromic performance. The result demonstrates the great potential of the periodic VO2 nanoparticle array in smart window application. The periodic patterned VO2 film is further demonstrated to be a smart thermal radiation control by a proof-of-concept demo, in which the incident light passes through the testing sample, then traverses holes of the heating plates, aluminum foil, and support layer in sequence, finally projects to the thermo-responsive foils (Figure 5a). These thermo-responsive foils, containing


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poly N-isopropylacrylamine hydrogel,55 with the characteristic of change from transparent to blurry during heating up, can serve as observable verifications of thermal radiation. The heat can be observed with accumulative effect during the test, where the appearance changed gradually with prolonging exposure duration (Figure S10). Four sets of samples (namely pure glass, aluminum foil, and VO2 coated glasses heated at 20 and 95 oC) have been exposed under spotlight exposure for 1 hour. As shown in Figure 5b, the thermo-responsive foil under the pure glass turns into blurry and in contrast it remains clear under the aluminum foil (Figure 5c), which demonstrates the negligible conductive and convective heat transfers and efficient thermalradiation-blocking ability of aluminum foil in this demo. The thermo-responsive foil under VO2 (M) at room temperature become blurry (Figure 5d) similar to what it is observed in pure glass (Figure 5b), while it changes to slightly obscure when VO2 (R) glass heated at 95 oC is inserted between the light source and the thermal-responsive foil (Figure 5e). The appearance contrast is induced by the difference of optical-energy transmittance in VO2 (M) and VO2 (R), where VO2 (R) is able to largely hinder radiation heat transfer by blocking more transmitted light comparing with its monoclinic phase. This demo proves that such nanocrystals can transmit light smartly which can be applied to smart thermal radiation control and temperature-dependent smart optical devices. CONCLUSION In this paper, periodic two-dimensional VO2 nanonet, nanodome, and nanoparticle arrays are reported with controllable periodicity as well as well-defined morphologies in centimeter-scale via nanosphere lithography. Patterned VO2 nanodome and nanoparticle arrays with average diameter of sub-100 nm are produced. More importantly, such nano patterning gives rise to LSPR exhibiting distinct temperature-response switching behavior as well as size- and media-


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dependent characteristic position tunabilities. In addition, the pattern arrays are demonstrated with high NIR modulation for thermochromic smart window application, and are further demonstrated to be a smart thermal radiation control. The inexpensive, and universal fabrication process, combined with the intriguing phase change characteristic of VO2 nanocrystals may inspire various development in processable and functional devices. METHODS Chemicals. Vanadium pentoxide (V2O5, 99.6%, Alfa Aesar), oxalic acid (99.99%, Alfa Aesar), hydrogen peroxide (H2O2, 30%, Sigma-Aldrich), sodium dodecyl sulfate (SDS, 99%, Sigma-Aldrich), polyethtlene glycol (PEG, Mw 600, Sigma-Aldrich), polyvinyl alcohol (PVA, Mw 85000-124000, 99%, Sigma-Aldrich), polyvinylpyrrolidone (PVP, Mw 40000, SigmaAldrich), and polystyrene spheres with nominal average diameters of 200, 500, 600, and 800 nm (PS spheres, 10%, Sigma-Aldrich), which were experimentally measured to be 160, 490, 640, and 830 nm, respectively (Figure S11), were used as received. Preparation of vanadium precursors. The preparation methods of precursors were reported previously.23,56 Generally, low viscosity vanadium precursor was prepared by mixing 182 mg of V2O5 powder with 15 ml of H2O2 in 90°C for 1 h, then, 400 mg of oxalic acid was added to reduce the vanadium and turn the solution to blue color. High viscosity precursor was synthesized via mixing 182 mg of V2O5 powder with 15 ml of H2O2 at room temperature until a homogeneous clear yellow solution formed. The high viscosity precursor was ready for use after aging for several days. The reaction of V2O5 powder with H2O2 releases heat violently, which should be done with fully careful. Monolayer PS sphere template fabrication. Close-packed monolayer PS spheres were prepared via an interface method.11,23 In brief, 0.2 wt% PS sphere dispersion in a 1 : 1 by volume


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mixture of deionized (DI) water : ethanol was dropt slowly on the water/air interface. Then several drops of SDS were added to push the floating PS spheres to form an iridescent film, followed by being picked up with quartz glass substrates. Subsequently, the close-packed monolayer PS spheres were rendered to hydrophilic, or etched to be non-close-packed by a BSETEQ NT1 Plasma Cleaner/Etch System performed using a mixture gas of argon and oxygen with 1 : 5 in pressure, and a RF power of 300 W. The PE duration was 10 s for nanoparticles and nanodomes, while 60 s for the nanonet structures. Preparation of 2D patterned VO2 films. The templates were immersed in certain vanadium oxide precursor (low viscosity precursor for patterned nanoparticles and nanonets, and high viscosity one for nanodome arrays) and dip-coating at a specific withdrawal speed, which was 120 mm/min for nanoparticle arrays, 600 mm/min for nanonet arrays, and 14 mm/min for nanodome arrays. Finally, vanadium oxide was crystalized to VO2, and PS sphere templates were removed via annealing at 550°C for 2h under the protection of argon gas with a ramping speed of 1°C/min. Enclosing VO2 in dielectric media. The 2D patterned VO2 films were dip-coated in dielectric medium (PEG, PVA, or PVP) solutions, followed by drying in oven at 100 oC for 1.5 h to remove DI water. The dried films were ready for measurement. Simulation. The transmission spectra and surface electric field distribution of VO2 array were simulated with three dimensional (3D) finite-different-time-domain (FDTD) method. Perfectly matched layer (PML) was applied to serve as the absorbing boundaries and the structure was set as a periodic disk array with the measured size. The refractive index of air and quartz substrate are 1 and 1.46 respectively. The refractive index n (λ,τ) and extinction coefficient k (λ,τ) of the VO2 under low and high temperature is adopted from previous report.57


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Demo set-up for thermal radiation filter. The customized heating plate with the disk and hole diameters of 6.6 and 1 cm, respectively was integrated with a digital temperature control system (PIKE Technologies, USA). The commercial aluminum foils (Diamond, USA) were used. The hollow support layer was constructed by glass slides (Sail Brand, China). Vertical distances from thermos-responsive foils to the heating plate and the spotlight (Spot R80, 100 W, General Electric) were 5 and 20 cm, respectively. This whole system worked in fume hood under a room temperature around 24 oC. Tests of pure glass, aluminum foil, and VO2(R) were conducted at heating plate temperature of 95 oC, while at 20 oC for the VO2(M) sample. The detailed preparation process of thermal-responsive foils is indicated in supporting Experiment S1. Characterization. The morphology and surface topology were characterized using fieldemission scanning electron microscope (SEM, JEOL JSM-6340F) at an accelerating voltage of 5 kV, and atomic force microscopy (AFM, Bruker, DI-3100) at room temperature. The crystal phases were identified by thin film glancing angle X-ray diffraction (XRD, Shimadzu XRD6000) with Cu K-α X-rays. The transmittance spectra were measured from 250 to 2500 nm by UV-vis-NIR spectrophotometry (Cary 5000, Agilent Ltd) as a function of temperature, which was controlled by a heating and cooling stage (Linkam PE120). Calculation methods for integrated luminous transmittance (Tlum, 380 - 780 nm) and solar energy modulation (∆Tsol, 2502500 nm) are indicated in supporting Experiment S2.58,59 ∆TNIR at 2000 nm is the transmittance contrast between high and low temperature at wavelength of 2000 nm.

ASSOCIATED CONTENT Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website at DOI: Supplementary calculation, additional preparation methods, PE-duration-dependent morphology evolution, and additional characterizations (PDF) Temperature-dependent transmittance modulation (AVI)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by Singapore Minster of Education (MOE) Academic Research Fund Tier one, RG124/16, and Singapore Research Foundation under the Campus for Research and Technological Enterprise (CREATE) programme and NRF2015NRF-POC002-019 The characterizations of XRD and SEM were performed at the Facility for Analysis Characterization Testing & Simulation (FACTS). Special thanks to Wang Shanchen for the drawing and Wang Ning for experiment. REFERENCES


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(24) Zhou, Y.; Cai, Y.; Hu, X.; Long, Y. VO2/Hydrogel Hybrid Nanothermochromic Material with Ultra-High Solar Modulation and Luminous Transmission. J. Mater. Chem. A 2015, 3, 1121-1126. (25) Lin, H.; Kwon, O.; Tang, J.; Zewail, A. H. 4D Imaging and Diffraction Dynamics of Single-Particle Phase Transition in Heterogeneous Ensembles. Nano Lett. 2014, 14, 946-954. (26) Zhou, M.; Bao, J.; Tao, M.; Zhu, R.; Lin, Y.; Zhang, X.; Xie, Y. Periodic Porous Thermochromic VO2(M) Films with Enhanced Visible Transmittance. Chem. Commun. 2013, 49, 6021. (27) Liu, M.; Su, B.; Kaneti, V. Y.; Chen, Z.; Tang, Y.; Yuan, Y.; Gao, Y.; Jiang, L.; Jiang, X.; Yu, A. Dual-Phase Transformation: Spontaneous Self-Template Surface-Patterning Strategy for Ultra-Transparent VO2 Solar Modulating Coatings. ACS Nano 2017, 11, 407-415. (28) Paik, T.; Hong, S.-H.; Gaulding, E. A.; Caglayan, H.; Gordon, T. R.; Engheta, N.; Kagan, C. R.; Murray, C. B. Solution-Processed Phase-Change VO2 Metamaterials from Colloidal Vanadium Oxide (VOx) Nanocrystals. ACS Nano 2014, 8, 797-806. (29) Zhang, J.; Jin, H.; Chen, Z.; Cao, M.; Chen, P.; Dou, Y.; Zhao, Y.; Li, J. Self-Assembling VO2 Nanonets with High Switching Performance at Wafer-Scale. Chem. Mater. 2015, 27, 74197424. (30) Lopez, R.; Feldman, L. C.; Haglund, R. F. Size-Dependent Optical Properties of VO2 Nanoparticle Arrays. Phys. Rev. Lett. 2004, 93, 177403.


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(53) Chen, Z.; Gao, Y.; Kang, L.; Cao, C.; Chen, S.; Luo, H. Fine Crystalline VO2 Nanoparticles: Synthesis, Abnormal Phase Transition Temperatures and Excellent Optical Properties of a Derived VO2 Nanocomposite Foil. J. Mater. Chem. A 2014, 2, 2718-2727. (54) Wang, N.; Shun, N. T. C.; Duchamp, M.; Dunin-Borkowski, R. E.; Li, Z.; Long, Y. Effect of Lanthanum Doping on Modulating the Thermochromic Properties of VO2 Thin Films. RSC Adv. 2016, 6, 48455-48461. (55) Zhou, Y.; Cai, Y.; Hu, X.; Long, Y. Temperature-Responsive Hydrogel with Ultra-Large Solar Modulation and High Luminous Transmission for “Smart Window” Applications. J. Mater. Chem. A 2014, 2, 13500-13555. (56) Wang, N.; Liu, S.; Zeng, X. T.; Magdassi, S.; Long, Y. Mg/W-Codoped Vanadium Dioxide Thin Films with Enhanced Visible Transmittance and Low Phase Transition Temperature. J. Mater. Chem. C 2015, 3, 6771-6777. (57) Mlyuka, N. R.; Niklasson, G. A.; Granqvist, C. G. Thermochromic VO2-Based Multilayer Films with Enhanced Luminous Transmittance and Solar Modulation. Phys. Status Solidi A 2009, 206, 2155-2160. (58) Lu, Q.; Liu, C.; Wang, N.; Magdassi, S.; Mandler, D.; Long, Y. Periodic Micro-Patterned VO2 Thermochromic Films by Mesh Printing. J. Mater. Chem. C 2016, 4, 8385-8391. (59) Zhou, Y.; Layani, M.; Boey, F.; Sokolov, I.; Magdassi, S.; Long, Y. ElectroThermochromic Devices Composed of Self-Assembled Transparent Electrodes and Hydrogels. Adv. Mater. Technol. 2016, 1, 1600069.


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Scheme 1. Effect of synthesis conditions on the morphology evolution. Route 1: nanoparticle arrays are prepared via short PE duration and low viscosity precursor; Route 2: nanodome arrays are produced, using high viscosity precursor that can stick on the tops of PS spheres; Route 3, nanonet arrays are fabricated by controlling the interval space between adjacent spheres via prolonging PE duration. 152x79mm (300 x 300 DPI)


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Figure 1. (a, b) AFM measurements of high quality 160-nm PS sphere MCC template. Inset of (b) is the AFM analysis along the MCC edge denoting as the red line. (c) SEM images of 640-nm PS sphere MCC template after PE treatment of 300 s, and its original morphology as the inset. (d) Linear relationship of PS sphere diameter and PE duration. 82x83mm (300 x 300 DPI)


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Figure 2. SEM images of patterned VO2 films. (a-c) nanoparticle, (d-f) nanonet, and (g-i) nanodome arrays prepared using 160-, 490-, and 830-nm PS spheres from left to right, respectively. (a-f) are the top-views, while (g-i) are the tilted-views. The inset of (h) is high magnification tilted-view image of 490-nm periodic nanodome arrays on edge. Periodicities of the nanoparticle arrays are illustrated as yellow hexagons, and measured to be 160, 490, and 830 nm for (a-c), respectively. 127x119mm (300 x 300 DPI)


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Figure 3. (a) Calculated (dashed lines) and measured (solid lines) transmittance spectra of nanoparticle arrays with diameters of 67, 125, and 287 nm, respectively. All spectra are normalized for LSPR peak position comparison. Inset of (a) is the magnified spectra for the measured peaks, which indicated by the corresponding triangles. (b) Experimental and simulated correlations of the LSPR peak positions (nm) and particle diameters (nm). (c-e) In-plane (x-y) electric-fields at VO2-air interfaces of the VO2 nanoparticle arrays with diameters of (c) 67, (d) 125, and (e) 287 nm at resonances of 1075, 1085, 1230 nm, respectively. (f) Transmittance spectra of 67-nm VO2 arrays in PEG, PVA, and PVP. Inset of (f) is the magnified spectra for the LSPR peaks pointed out by the triangles. (g) Effect of the medium reflective index to LSPR peak position. (h) Temperature-dependent LSPR tunability of the 67-nm VO2 nanoparticle array in temperature range from 20 to 100 °C. Extinction (A) is calculated via A = -log10 (transmittance). (i) and (j) are the hysteresis loops of the relative LSPR intensity and the LSPR peak position. The relative LSPR intensity relies on normalized integrated area under curves.


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Figure 4. (a) Illustration of the way to overlay VO2 nanoparticle array foils, inserted photograph on left is the 1 layer sample. 2D nanostructured VO2 nanocrystals are prepared with 1.2 mm in length. The inserted table of (a) is calculated thermochromic performance based on (b) the transmittance spectra at 20 and 95°C. (c) Durability test for the double-layered sample. (d-e) The thermochromic performance comparisons of the double-layered sample in this work with (d) reported periodic VO2 nanostructures, and (e) the best through other methods. Data in (d) and (e) include honeycomb,27 2D phonotic crystals,23 periodic nanonets,26 truss network,29 biomimetic moth-eyed structure,49 anti-reflective multilayer,51 controlled porosity,52 VO2 nanoparticles/transparent media composite,53 and doping.54 165x172mm (300 x 300 DPI)


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ACS Nano

Figure 5. (a) Schematic illustration of the proof-of-concept demo. The incident light, heating plates, aluminum foil, support layer, and thermos-responsive foils are indicated as the black arrows. The four testing samples, pure glass, aluminum foil, VO2 (M) at 20 °C, and VO2 (R) at 95 °C are denoted as the red arrows. (b-e) The photographs of four thermos-responsive foils namely 1, 2, 3, and 4 in (a) after 1-h exposure are presented as (b), (c), (d), and (e), respectively. 152x74mm (300 x 300 DPI)


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Controllable Fabrication of Two-Dimensional Patterned VO2

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