Effective Radiative Cooling by Paint-Format Microsphere-Based

Feb 7, 2018 - We demonstrate that photonic media, when properly randomized to minimize the photon transport mean free path, can be used to coat a blac...
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Letter Cite This: ACS Photonics XXXX, XXX, XXX−XXX

Effective Radiative Cooling by Paint-Format Microsphere-Based Photonic Random Media Sarun Atiganyanun,† John B. Plumley,‡ Seok Jun Han,‡ Kevin Hsu,‡ Jacob Cytrynbaum,‡ Thomas L. Peng,∥ Sang M. Han,†,‡,§ and Sang Eon Han*,†,‡,§ †

Nanoscience and Microsystems Engineering, University of New Mexico, Albuquerque, New Mexico 87131, United States Chemical & Biological Engineering, University of New Mexico, Albuquerque, New Mexico 87131, United States § Center for High Technology Materials, University of New Mexico, Albuquerque, New Mexico 87131, United States ∥ Air Force Research Laboratory, Kirtland AFB, New Mexico 87117, United States ‡

S Supporting Information *

ABSTRACT: We demonstrate that photonic media, when properly randomized to minimize the photon transport mean free path, can be used to coat a black substrate and reduce its temperature by radiative cooling. Even under strong solar radiation, the substrate temperature could reach substantially below that of the ambient air. Our random media that consist of silica microspheres considerably outperform commercially available solar-reflective white paint for daytime cooling. We have achieved the outstanding cooling performance through a systematic study on light scattering, which reveals that the structural parameters of the random media for maximum scattering are significantly different from those of the commercial paint. We have created the random media to maximize optical scattering in the solar spectrum and to enhance thermal emission in the atmospheric transparency window. In contrast to previous studies, our random media do not require expensive processing steps or materials, such as silver, and can be applied to almost any surface in a paint format. The facile and scalable processing steps for our random media point to the possibility that low-cost coatings can be used for efficient radiative cooling. KEYWORDS: light scattering, photon transport mean free path, diffusion approximation, microspheres, random media

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For daytime cooling, the desired property other than radiation is to strongly reflect or scatter the sunlight to minimize solar heating. To reduce the sunlight absorption, sophisticated high-precision nanostructures can be used.3,5 Alternatively, a thin metal film, such as silver, can keep the solar absorptivity below 4% without such structures.5 While other metal films can also reflect the sunlight, most metals absorb more strongly than silver. A mere 1% increase over silver degrades the cooling performance significantly because the increased absorptivity accounts for a decrease in cooling power by ∼10% (Supporting Information). Following this understanding, recent studies have used a silver film in combination with dielectric materials of high solar reflectance for efficient radiative cooling under sunlight.3,5−7 In some of these studies, the cooling materials are rendered into a thin flexible sheet

eat radiated by a terrestrial object in a select mid-infrared (IR) spectral range, known as the atmospheric transparency window, can transmit through the atmosphere into the outer space. When this radiative heat loss is greater than the heat gain from the ambient sources, the object cools below the ambient temperature until it reaches a steady-state temperature where the net heat transfer rate is zero.1−3 In 1963, Trombe demonstrated that this effect can reduce the temperature of an object by ∼35 °C below the ambient temperature at night.1 The study of radiative cooling recently gained new momentum with the influential work by Raman et al. in 2014,3 which demonstrated radiative cooling of an object by 5 °C below the ambient temperature under direct sunlight. Preceding the effort in academia, the paint industry had also started developing products, which can achieve a similar degree of daytime cooling for rooftop applications, and these paint products have been in the market.4 © XXXX American Chemical Society

Received: December 6, 2017 Published: February 7, 2018 A

DOI: 10.1021/acsphotonics.7b01492 ACS Photonics XXXX, XXX, XXX−XXX

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a white coating in the photograph consists of randomly packed SiO2 microspheres of d = 0.9 μm in diameter. The solar radiation is scattered from the microsphere coating without being absorbed, while the heat escapes from the surface by midIR emission. Figure 1b shows the emissivity (green) from the coating produced in our laboratory against solar (red) and atmospheric (blue) radiation intensity. The emissivity spectrum was measured using a spectrophotometer with an integrating sphere (Vertex 70, Bruker). The emissivity (= absorptivity) of the sample is negligible in most of the solar radiation spectral range (0.3−3 μm), whereas the emissivity is high in the atmospheric transparency window (8−13 μm) highlighted in orange. Absorptivity peaks in the near IR (1.9 μm < 3 μm) are due to intrinsic absorption of SiO2 and correspond to a less than 0.1% increase in the solar absorptivity by absolute value. The average IR emissivity of this coating over the atmospheric transparency window is >0.94, which exceeds that in ref 7 where 8-μm-diameter microspheres were used. The reason for the high emissivity of our random media is that, in comparison to the case of a solid SiO2 film, the porous media provide improved optical impedance matching with the air. According to Maxwell−Garnett effective medium theory,14 we estimate that the average emissivity in the normal direction over the atmospheric transparency window for our random media would be greater than that of a solid SiO2 film by more than 0.13. Here, we assume a fill fraction of 0.55 for our random media. The high emissivity outside the atmospheric transparency window decreases the cooling performance.2 However, this effect is significant only when heat transfer by convection and conduction from the ambient air is negligible.6,15 Because the coating for radiative cooling would be exposed to the ambient air in common outdoor applications, where convective/ conductive heat transfer is substantial, the high emissivity of our coating outside the window would not significantly affect the cooling performance. In fact, the high emissivity outside the window is even desirable for cooling when the coating temperature is higher than the ambient air temperature. While the mid-IR emissivity largely remains intrinsic to the material choice with a modest improvement with optical impedance match, the solar light scattering strongly depends on the structure. We apply the diffusion approximation16 to model the light scattering in our random media. The key parameter that quantitatively describes the scattering strength is the photon transport mean free path (l*), beyond which light propagation is no longer correlated to its original direction. In random media, this l* implicitly quantifies the degree of randomness, where l* decreases as the randomness increases. In practice, the randomness depends on the choice of deposition technique (e.g., colloidal sedimentation and spraycoating) and deposition parameters (e.g., ionic strength and air pressure). Thus, the desired goal is to minimize l*, so that full scattering is achieved within the thinnest possible coating. For l* measurement, we prepare the samples of randomly packed microspheres by colloidal sedimentation and, for comparison, by spray coating. In colloidal sedimentation,17 we add an electrolyte to a stable colloidal suspension of SiO2 microspheres. Specifically, 0.01 M of KCl is added to an aqueous solution of 0.9 μm microspheres and to the solution of 2 μm microspheres. The salt addition induces colloidal instability, and the flocculated microspheres precipitate onto a glass slide as a randomly packed film. For spray coating,18 a 2 vol % aqueous solution of 0.9-μm-diameter microspheres is

format, where silver is the material primarily responsible for solar reflection.6,7 While the previous studies using silver have shown great promise in radiative cooling, it would be desirable for practical applications to eliminate the use of such expensive metal films and their deposition processes, and to have the materials available in a much simpler format such as paint.4,8−11 We note that commercial solar rejection paint products4 provide a cooling performance comparable to recent advances made in academia. In light of these innovations, we have conducted a systematic study of optical scattering in random media whose emissivity in the mid-IR is intrinsically high. The goal was to determine key structural and material properties that provide the cooling performance exceeding pre-existing paint products without the use of precious metals. Popular pigments in commercial white paint include TiO2 particles of 200−250 nm in size12,13 as well as hollow spheres of relatively low refractive index (e.g., ∼1.5) with the size ranging from 50 to 150 μm.4 While these pigments exhibit high mid-IR emissivity, they have their own limitations for daytime cooling applications. For example, TiO2 particles strongly absorb ultraviolet (UV) light, which accounts for 5% of the total solar intensity,13 and the hollow spheres are not as effective in scattering sunlight as TiO2 particles. In general, high-index particles tend to have high UV absorption while low-index particles suffer from inefficient light scattering. Because the intrinsic UV absorption in high-index particles is difficult to eliminate, in this study, we choose to maximize the optical scattering in random media consisting of low-index particles. Specifically, we perform a systematic light scattering study on randomly packed low-index SiO2 microspheres. We demonstrate that, with the right particle size and disorder, these randomly packed particles can surpass the cooling performance of commercial solar rejection paint products. Figure 1a conceptually describes how highly scattering random media can lead to radiative cooling. What appears as

Figure 1. (a) Conceptual description of passive radiative cooling. (b) Experimentally determined emissivity spectrum of randomly packed silica microspheres (green) against solar (red) and atmospheric (blue) radiation intensity. The main atmospheric transparency window is highlighted in orange. B

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ACS Photonics ejected through an air-brush nozzle (Badger 100G, Badger AirBrush) at 69 kPa. The spray coating condition is identical for a solution of 2-μm-diameter microspheres. The nozzle is located 22.5 cm above the substrate. The substrate is subjected to a linear periodic motion to ensure coating uniformity and is heated at >100 °C to evaporate water quickly before microspheres are significantly rearranged. We obtain l* by measuring optical transmittance of randomly packed SiO2 microsphere films of varying thickness (20 to 100 μm). For these films, transmittance (T) is related to l* by19,20 T=

l*(1 + s) L + 2l*s

(1)

In eq 1, L is the thickness of the film, and extrapolation length ratio s = 2(1 + R)/[3(1 − R)] where R is the internal reflectance. To estimate s, we apply Maxwell−Garnett effective medium approximation for the randomly packed SiO2 microspheres. Using the refractive index of SiO2 (n = 1.46) and the fill fraction range (f ≈ 0.55 to 0.64), one can estimate s to be approximately 1.4. Note that T is a function of wavelength. We plot 1/T vs L at each wavelength, and apply linear regression to estimate the slope: 1/[l*(1 + s)]. The slope yields l* as a function of wavelength [see Figure 2a]. To theoretically investigate how l* is related to the microsphere size and fill fraction, we apply mean field theory to the scattering system.21,22 The system is modeled by two scattering units surrounded by an effective medium.22 One unit is an entirely empty microsphere, and the other unit is a solid SiO2 microsphere concentrically surrounded by an empty sphere. Light propagation in the effective medium is characterized by a complex wave vector k + i/(2l), where l is the so-called scattering mean free path. The l is found by requiring that the forward scattering amplitude is zero on average when the scattering units are within the effective medium. Using a relation between l and l* in the low concentration limit,21 we calculate l* from the knowledge of l. Figure 2a shows both experimentally measured and theoretically calculated l* as a function of wavelength. The samples prepared by colloidal sedimentation and spray coating exhibit a similar l* spectrum. For these samples, the calculated result for the fill fraction ( f) of 0.64, which corresponds to random close packing,23 shows an excellent agreement with the experimental result. Our model also accurately captures the resonant response in l* [see Figure 2a], which shows up as peaks and valleys. In comparison, the results from other theoretical models in previous studies24,25 show a significant difference from the experimentally measured l*. Notably, these models show an overall trend where l* decreases as the wavelength increases, which is opposite to the experimental results.24,25 While the calculation assuming random close packing ( f = 0.64) accurately captures the experimental results, the agreement does not necessarily prove that f = 0.64. We note that our calculation does not fully account for short-range ordering22 and that any level of ordering in the realistic structures would naturally increase l* due to less efficient scattering. What this entails is that l* for a fill fraction lower than 0.64 [e.g., calculated l* for f = 0.55 for random loose packing shown in Figure 2a] would start to rise from where it is shown in Figure 2a with increasing level of ordering and may match our experimental outcome. In a previous study, the fill fraction resulting from colloidal sedimentation was found to be as low as f ≈ 0.55,24 which corresponds to the lower limit of random

Figure 2. (a) Comparison of experimentally measured and theoretically calculated l* for close ( f = 0.64) and loose ( f = 0.55) random packing of silica microspheres of d = 0.9 μm. (b) Dependence of the sunblock power, 1/⟨l*⟩, on the sphere diameter and fill fraction of randomly arranged silica microspheres. (c) Absorption of a substrate with a microsphere-based coating for d = 2 μm and f = 0.6 as a function of coating thickness for two extreme values of internal reflectance Rs at the coating/substrate interface.

loose packing.23 As the fill fraction in randomly packed microspheres is difficult to determine accurately,26 we only estimate the fill fraction of our samples in the present study. C

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nonzero sphere size. In practice, the short-range correlations may be stronger than the theoretical approximation and will depend on the microsphere deposition techniques and parameters. With strong correlations, the coating may exhibit l* deviating from the mean field theory predictions. To investigate the impact of short-range correlations on l*, we compared coatings of randomly packed 2-μm-diameter microspheres, where different surfactant concentrations were used in the spray coating solution. Figure 3a−c shows scanning electron micrographs (SEM) of the top surface of microsphere coatings prepared with different

Using the experimentally validated theoretical model [Figure 2a], we calculate sunlight scattering power, 1/⟨l*⟩, as a function of microsphere diameter and fill fraction [see Figure 2b]. Here, ⟨ ⟩ represents the average value with the solar intensity spectrum as a weighting factor. Figure 2b shows that the maximum scattering power (Region A) is obtained when d = 1.5 μm and f = 0.375, where 1/⟨l*⟩ is 0.20 μm −1, and therefore ⟨l*⟩ is 5.0 μm. In contrast, the common sizes of TiO2 particles (200−250 nm) and hollow spheres (50−150 μm) in commercial solar rejection paint products are very far from the optimum d determined for SiO2 microspheres. While small size particles are generally desired to reduce the coating thickness, TiO2 particles absorb in UV and near-IR, which compromises their cooling performance compared to our SiO2 microsphere coating. For large diameter hollow spheres, Kubelka−Munk theory27 gives an empirical estimate of l* as l* ≅ d/(3f) (see Supporting Information). For d = 50−150 μm and f = 0.6, this estimate gives l* = 28−83 μm. This photon transport mean free path is much greater than the minimum value of ⟨l*⟩ = 6.4 μm (Region B) that can be achieved for f = 0.6 with 2 μm microspheres [see Figure 2b]. What this comparison implies is that the commercial paint containing large hollow spheres must be 4 to 13 times thicker than our SiO2 microsphere films to achieve similar performance. As for the fill fraction, f = 0.55 is typically considered to be the lower limit in loose random packing of spheres.23 However, Figure 2b indicates that fill fractions much lower than 0.55 can maximize the scattering power (Region A), and the remaining question is whether one can experimentally achieve such low fill fractions. Giera et al. recently discovered that the fill fraction can be as low as 0.4 when the microspheres undergo highvoltage-driven sedimentation.28 At this low fill fraction, we predict that light scattering with properly sized microspheres can be stronger than at f ≥ 0.55. For f < 0.55, we surmise that the size of vacancies can be larger than individual microspheres and that these vacancies are inhomogeneously distributed in the coating. Depending on the scale of intervacancy distance, the current theory may need to be modified to accurately describe the scattering system. To estimate the required coating thickness for effective sunlight rejection, we calculate absorptivity as a function of film thickness when the microspheres are coated on an absorbing substrate. Figure 2c shows an example of absorptivity vs coating thickness for d = 2 μm and f = 0.6. These experimental parameters are not optimum for maximum scattering power [Region A in Figure 2b], but the fill fraction (f = 0.6) can be easily realized in experiment. For f = 0.6, the minimum ⟨l*⟩ occurs at the microsphere diameter (d = 2 μm). The two curves in Figure 2c correspond to two extreme values of the internal reflectance, Rs, at the coating/substrate interface. The two cases Rs = 0 and Rs = 0.96 represent an ideal blackbody and a highly reflective metal as a substrate, respectively. The difference in absorptivity between the two limiting cases becomes smaller as the coating thickness increases. For 2-μm-diameter SiO2 microspheres packed randomly at f = 0.6, the absorptivity reaches less than 3% as the coating thickness approaches ∼500 μm. At such low absorptivity, previous studies3,5,6 achieved substrate cooling 5−10 °C below the ambient temperature, and we will show that our SiO2 coating achieves a similar level of cooling. The mean field theory that we use for theoretical predictions does not take into account the correlations of microsphere positions except for short-range correlations induced by the

Figure 3. (a−c) SEM images of silica microspheres of d = 2 μm deposited by spray coating when the surfactant concentration is (a) 0%, (b) 2 × 10−4%, and (c) 2 × 10−3%. The scale bar represents 20 μm. (d) l* spectrum for the three cases and the sample prepared by colloidal deposition. Theoretical predictions for close ( f = 0.64) and loose (f = 0.55) random packing of silica microspheres are also displayed for comparison in (d).

surfactant concentrations. The microsphere arrangement appears to be quite random in all three images. We have confirmed the randomness by taking autocorrelation functions from the images, following the method described by Garcı ́a et al.17 The 2D autocorrelation functions (not shown here) are circularly symmetric, indicating that the random arrangement is isotropic. The value of autocorrelation functions shows the maximum at the center and decays precipitously when traced along the line radially extending from the center. This behavior is a clear signature of random microsphere arrangement. The functions also exhibit small bumps approximately at integer D

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ACS Photonics multiples of d. These bumps show short-range correlations that are present because of the nonzero size of the spheres.17 Because contrast and brightness in the SEM images vary from sample to sample, it is difficult to quantitatively compare the autocorrelation functions to evaluate the degree of randomness in the three samples. Instead of using autocorrelation functions, we optically measure the l* of the samples for comparison. As shown in Figure 3d, our measurements reveal that l* for spray-coated samples increases as the surfactant concentration increases. In the visible wavelengths, with respect to the microsphere film formed by spray coating without surfactant (black scattered dots), l* increases by ∼1.7 μm when 2 × 10−3 vol % of surfactant is added. We also note that the spray-coated sample without the surfactant exhibits l* that is larger than that of the sample prepared by colloidal sedimentation (pink scattered dots). While spray-coating is a much more convenient technique than sedimentation, the shorter l* for sedimented sample suggests that the sedimented microspheres are more randomly distributed or their fill fraction is lower than the spray-coated microspheres. In all cases, however, the measured l* exhibits deviations from theoretical predictions for both f = 0.55 and f = 0.64. The comparison with the theory suggests that while the sphere positions shown in all three images of Figure 3a−c appear random, the surface images alone cannot quantitatively represent the optical scattering strength in the bulk. Further study is necessary to elucidate how the degree of randomness can be more accurately characterized, how it is related to the scattering strength, and how it changes from the coating surface to the bulk. To evaluate the cooling performance of our coatings under the sunlight, we deposited 2-μm-diameter microspheres on 2.5 cm × 2.5 cm glass slides, using colloidal sedimentation. The bottom of the glass slides is painted black (Specialty Black High Heat Ultra, Rust-Oleum). The colloidal method is used rather than the spray coating because the sedimentation consistently gave a lower l* spectrum as seen in Figure 3d. We selected the coating thickness to be ∼700 μm where solar absorptivity is 0.02 based on Figure 2c. In principle, we could determine the required coating thickness by directly measuring the absorptivity. However, the accuracy of our measurement based on reflectometry turns out to be unsatisfactory because of the very strong scattering property of the samples (Supporting Information).29 For comparison, we prepare a sample where our microsphere coating is replaced by a commercial solar reflective white paint (Chromaflo Technologies, Spartacryl PM 60312) with the same thickness. We have experimentally determined that this paint achieves the lowest temperature under the sunlight among several different brands. The paint was applied without cracks and air pockets. To characterize the cooling properties of the samples under sunlight, we set up fixtures shown in Figure 4a,b. The experimental setup is designed to accurately measure the temperature of the air surrounding the samples, so that the sample temperatures can be properly compared to the ambient air temperature. Typically, the ambient temperature is measured in a Stevenson screen, which consists of a whitepainted box with slits on its side walls to allow air flow. The white-painted surfaces minimize the solar heating of the box. The inside of the box is dark enough to prevent radiative heating of a thermometer in the box. The thermometer is located about 1−2 m above the ground. Our fixtures resemble a Stevenson screen, but significant modifications are made to

Figure 4. (a) Optical image and (b) schematic diagram of our outdoor experimental setup for temperature measurement. (c) Measured temperature variation over 3.5 days for our microsphere sample, commercial solar-rejection white paint, and ambient air.

enable the temperature measurement of samples that are exposed to the sunlight. Our box has a window (30 cm × 30 cm) at the top to expose the sample to the sunlight. The bottom of the box is sealed by an optically transparent, lowdensity polyethylene (LDPE) film. If the bottom is sealed with an opaque wall, the sunlight coming through the top window heats the box. Moreover, the side walls are without slits to reduce the fluctuations in the inside temperature due to convection, and the walls are made of Styrofoam covered with aluminum sheets. Two LDPE enclosures of equal volume (2360 cm3) are located inside the box, one beneath the top window and the other beneath the top cover. Thermocouples for the sample and the ambient air are located in the two enclosures, respectively. The heat transfer between the two thermocouples is minimal due to this compartmentalization. The enclosure to measure the ambient air temperature is located far from sunlight illumination even when the sun’s altitude is low. Both thermocouples are located 2 m above the ground. When the elevation of the thermocouples is not sufficiently high above the ground, the temperature is substantially affected by the heated ground. In our design, the ambient temperature represents the temperature of the air surrounding the sample while minimally affected by the sample temperature, so that the cooling performance of the sample can be properly characterized by comparison to the ambient temperature. Using our setup, we measure the temperature of the samples and the ambient air under intense solar radiation in Albuquerque, New Mexico over 3.5 days in May when the E

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sky is relatively clear, as shown in Figure 4c. The results show that our microsphere-based coating on a black substrate, without the use of expensive silver coatings, can reduce the substrate temperature below that of the ambient air by as much as 12 °C under the sunlight. Our coating also outperforms the commercial solar-reflective white paint. At its peak performance, our coating achieves a substrate temperature 7 °C below that of the commercial paint. The average temperature of the substrate under our coating is 4.7 °C below that of the substrate coated with the commercial paint during the time of intense solar radiation (between 11 am and 4 pm). At night (9 pm to 7 am), the microsphere coating and the commercial paint both keep the substrate temperature below the ambient temperature by ∼4 °C. This result suggests that both films have similar radiative properties in the mid-IR, but our microsphere coating has superior scattering properties for the solar spectrum to the commercial paint. We observe that the substrate temperatures of microsphere coating and commercial paint both rise faster than that of the ambient air from 7 am to 12 pm. The reason is that the solar absorptivity of the solid coatings is higher than that of the ambient air, even though the absorptivity in the substrate is approximately 0.02. From 7 pm, when the sun’s altitude is below 10°, until 7 am, the substrate temperature starts to decrease before the ambient temperature does with a time lag of 1−2 h. During this period, the solar heating is negligible, and the substrate is cooled by radiation in the mid-IR. The radiative cooling of solid substrates is more efficient than that of the ambient air; therefore, the substrate temperature decreases below the ambient temperature. In summary, we have demonstrated that the random media based on microspheres can achieve strong radiative cooling under sunlight. The reported results are from microsphere coatings where microsphere size and fill fraction are not yet fully optimized. With strong potential to further improve its performance, our coating cools a black substrate below the ambient temperature by as much as 12 °C under sunlight. On average, our coating also reduces the substrate temperature by 4.7 °C below that of the commercial solar reflective white paint during the period of strong solar radiation (11 am to 4 pm).30 The microsphere coatings can be deposited by facile and scalable methods at a very low cost. These advantages are rather difficult to achieve with ordered structures or other radiative technologies that rely on expensive materials (e.g., silver). We also note that if the coating performance needs to be compromised for esthetical appearance, the paint format is much more amenable to pigment incorporation than other competing technologies. Our study has shown that one can maximize the optical scattering power with judicious choice on microsphere material, size, and fill fraction. In view of the fact that the optimum parameters of random structures deviate substantially from those in current commercial paint products, our results offer a path to low-cost coatings with unprecedented radiative cooling efficiency. Further, the ability of random media to separately control optical properties at different spectral bands (e.g., scattering in visible and emission in mid-IR spectra in our study) has broad technological implications. For practical applications that require durability and antisoiling, we are currently investigating elastomer-based material systems, and we will discuss the results in a future publication.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b01492. Detailed description of cooling power, light scattering by large particles, and light absorption measurement (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sang Eon Han: 0000-0003-0899-6685 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.E.H. and S.M.H. acknowledge the generous financial support from the National Science Foundation (NSF) CAREER Award (DMR-1555290), NSF SEPTET (CHE-1231046), and Air Force Research Laboratory Innovative Research Program (CP0040209).

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REFERENCES

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DOI: 10.1021/acsphotonics.7b01492 ACS Photonics XXXX, XXX, XXX−XXX