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Spectral-Selective Plasmonic Polymer Across the Visible and Near- Infrared † † ‡ † § ∥ Assad U. Khan, Yichen Guo, Xi Chen, and Guoliang Liu*, , , † ‡ § ∥ Department of , Industrial and Systems Engineering, Innovation Institute, and Division of Nanoscience, Academy of Integrated Science, Virginia Tech, Blacksburg, Virginia 24061, United States

*S Supporting Information

ABSTRACT: State-of-the-art commercial -reflecting is coated with a metalized film to decrease the transmittance of electromagnetic waves. In addition to the cost of the metalized film, one major limitation of such light-reflecting glass is the lack of spectral selectivity over the entire visible and near-infrared (NIR) spectrum. To address this challenge, we herein effectively harness the transmittance, reflectance, and filtration of any wavelength across the visible and NIR, by judiciously controlling the planar orientation of two-dimensional plasmonic silver nanoplates (AgNPs) in polymer nanocomposites. In contrast to conventional bulk polymer nanocomposites where plasmonic are randomly mixed within a polymer matrix, our thin-film polymer nanocomposites comprise a single layer, or any desired number of multiple layers, of planarly oriented AgNPs separated by tunable spacings. This design employs a minimal amount of metal and yet efficiently manages light across the visible and NIR. The thin-film plasmonic polymer nanocomposites are expected to have a significant impact on spectral-selective light modulation, sensing, optics, optoelectronics, and photonics. KEYWORDS: plasmonic , polymer , spectral selectivity, visible, near-infrared

urrent tinted glass uses metalized films composed of and there is a need for spectral-selective glass that can control Au, Ag, Cu, Co, Ti, Ce, and Se of various ratios.1 The the transmittance, reflectance, and filtration of any wavelength − metal films are sandwiched between,2 4 or stacked across the visible and NIR. C2,3 with, two layers such as TiO2, ZnO2, SnO2,WO3, In a disparate approach, we investigate polymer nano- and ZnS. Such tinted glass modulates light transmission but composites as alternatives for modern tinted glass. Polymer has a high absorptance, and thus it captures a large amount of fi

Downloaded via VIRGINIA POLYTECH INST STATE UNIV on April 1, 2019 at 14:39:03 (UTC). nanocomposites comprise polymers mixed with llers such as See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. that re-radiates. Instead of full layers of metals, plasmonic fi 5 6,7 black, fumed silica, and clay, which signi cantly metamaterials actively or statically modulate light. The enhance the mechanical strength, flame retardance, and − preparation of metamaterials, however, requires a large amount durability of the polymers.10 12 Conventional polymer nano- of metal and metal . Furthermore, it involves complex composites, however, possess limited optical and aesthetic and expensive top-down fabrication techniques which are time- properties, restricting their use in tinted glass. To prepare consuming, limited to small areas, and inapplicable to − spectral-selective tinted glass from plasmonic nanocomposites substrates of arbitrary shapes.2 4 Alternatively, researchers at industrial scales, there are three primary challenges: have designed electrochromic8 and thermochromic9 glass using selection of appropriate fillers, control over filler dispersion -doped indium (ITO), Nb2O5,andVO2.The electrochromic and thermochromic glass manages light in a and orientation, and suitability for scalable fabrication. certain range of the NIR, but it requires an external energy are emerging that 8 ° 9 interact with light of certain wavelengths depending on their supply or a large temperature gradient from 25 to 100 C, 13 which are impractical under ambient conditions. Recently, size, shape, and composition. Colloidal nanoparticles can colloidal particle suspensions and have been used in tinted glass, but they often result in opaque films.1 Received: December 11, 2018 While all the above glass mitigates the transmittance of light at Accepted: March 25, 2019 some wavelengths, the spectral selectivity is severely limited, Published: March 25, 2019

© XXXX American Chemical Society A DOI: 10.1021/acsnano.8b09386 ACS Nano XXXX, XXX, XXX−XXX ACS Nano Article

Figure 1. Polymer nanocomposites with randomly oriented AgNPs versus planarly oriented AgNPs. (A) Schematic illustration of a polymer composite with randomly oriented AgNPs, which diffusely reflects incident light. (B) Optical photograph of a polymer composite film with randomly oriented AgNPs on a glass slide. (C) TEM image of a thin slice of the . (Inset) Zoomed-in view of a tilted AgNP. Scale bar, 100 nm. (D) Schematic illustration of a polymer composite with planarly oriented AgNPs, which specularly reflects incident light. (E) Optical photograph of a thin-film polymer nanocomposite that contains one layer of planarly oriented AgNPs on a glass slide. PAH instead of PMMA is used to assist the layer-by-layer assembly of AgNPs. (F) Top-down SEM image of planarly oriented AgNPs. (G, H) 3D contour and 2D projected various-angle and various-wavelength transmittance (T), reflectance (R), and absorptance (A) spectra of unpolarized light by the two types of polymer composites that contain (G) randomly embedded AgNPs and (H) planarly oriented AgNPs. fi λ potentially serve as llers for constructing spectral-selective surface resonance (LSPR) wavelengths ( LSPR) and λ polymer nanocomposites via low-cost bottom-up assembly. thus limited spectral selectivity; have tunable LSPR Compared to the particles prepared via nanofabrication (e.g., but are susceptible to percolation. In contrast, two-dimensional 14 λ chemical or physical vapor deposition followed by etching), (2D) Ag nanoplates (AgNPs) have tunable LSPR in the visible the colloidal plasmonic nanoparticles prepared by wet- and NIR26,27 and a high percolation threshold;28 thus they chemistry synthesis have superior crystallinities and thus serve as the most promising candidate for addressing the high-quality optical properties. The use of plasmonic particles challenge of spectral selectivity. To fully utilize the in-plane − in composites dates back to Roman times15 17 and has LSPR but minimize the out-of-plane LSPR, one must control recently flourished with the incorporation of nano- the planar orientation and uniform dispersion of the 2D spheres,12,18,19 nanorods,20,21 nanoplates,22,23 nanocubes,24 AgNPs in the polymer nanocomposites. To this end, layer-by- and nanostars.25 Among these nanoparticles, nanospheres, layer (LbL) assembly is suitable because it has shown excellent nanocubes, and nanostars have limited ranges of localized capability of depositing , graphene, and nano-

B DOI: 10.1021/acsnano.8b09386 ACS Nano XXXX, XXX, XXX−XXX ACS Nano Article

Figure 2. Spectral selectivity of plasmonic polymer nanocomposites. (A) Optical photograph and (B) the corresponding extinction spectra of colloidal suspensions of AgNPs in water. The size of the AgNPs increases from (a) to (h). (C) Representative TEM images of the AgNPs. (D) Optical photographs of the AgNP-polymer composites on glass. As the AgNP size increases from (a) to (h), the polymer composites show characteristic . (E) Representative SEM images of the monolayer AgNPs. (F) Optical properties (T, R, and A) of the monolayer AgNP-polymer composites. (G) A photograph of selected polymer composite thin-films that contain of AgNPs on glass slides. The photograph was taken outdoors against the landmark bridge on the campus of Virginia Tech. The polymer composites selectively reflect light at 480, 550, 600, 750, and 1020 nm. The last polymer composite on glass modulates light in the NIR range, and hence it is colorless and nearly transparent, as highlighted in the dashed box.

− particles on various substrates to make layered structures.28 31 charged AgNPs to form layered structures. Poly(methyl In addition, LbL assembly offers exquisite control over the methacrylate) (PMMA), a common plexiglass polymer, is interlayer distance as well as the intralayer density of deposited used as a spacer between the AgNP layers. The plasmonic − species.32 35 polymer nanocomposites contain 2D AgNPs of controlled size, Herein we synergize plasmonic nanoparticles with polymers surface coverage, and interlayer distance and thus have well- to create plasmonic polymer nanocomposites with planarly controlled optical and plasmonic properties. To avoid oriented 2D AgNPs via LbL assembly. In the plasmonic nanoparticle aggregation and undesirable in-plane and out-of- polymer nanocomposites, 2D AgNPs are used as fillers because plane plasmon hybridization in tinted glass, we control the of their tunable LSPR in the visible and NIR.26,27,36 particle−particle distances by tuning AgNP density in each Poly(allylamine hydrochloride) (PAH) is selected because layer and thickness of the PMMA spacer between the layers. the positively charged PAH strongly attracts the negatively Surprisingly, a single layer of AgNPs is able to efficiently

C DOI: 10.1021/acsnano.8b09386 ACS Nano XXXX, XXX, XXX−XXX ACS Nano Article modulate light transmission and reflection. The approach leads The thin-film polymer composite showed minimum T and fi λ ∼ θ to thin- lm polymer nanocomposites that possess exceptional prominent specular R at LSPR of 720 nm and all inc (Figure θ ° ° spectral selectivity across the visible and NIR as well as 1H). As inc was increased from 6 to 75 , T decreased from λ multiwavelength responsiveness. 25% to 5%, while R increased from 19% to 61% at LSPR. Compared to the bulk polymer composites with randomly RESULTS oriented AgNPs, the thin-film polymer composite with planarly fi Orientation of 2D AgNPs. In conventional bulk oriented AgNPs showed signi cantly increased specular R of ° ° plasmonic composites such as the and stained almost 20% at 6 and 61% at 75 , leading to decreased A glass, a large amount of plasmonic nanoparticles are randomly overall. The enhanced specular R was distinctive to the mixed within the matrix.16,37 To design thin-film plasmonic planarly oriented nanoparticles but absent in our bulk polymer fi ff composites as well as in commercial tinted glass with metal polymer nanocomposites, we rst investigated the e ect of 1 AgNP orientation (Figure 1), either randomly oriented (Figure additives. In contrast with the bulk polymer composites, both fi 1A−C) or planarly aligned (Figure 1D−F), on the optical T and R of the thin- lm composites showed strong properties of the nanocomposites. Since the in-plane dipoles of dependence on the p- and s-polarization (Figure S5). the AgNPs are mostly responsible for plasmon resonance in Spectral Selectivity. The characteristic T, R, and A the visible and NIR, the AgNP orientation played an important wavelengths suggest that the plasmonic composites are role. To prepare conventional bulk polymer nanocomposites excellent spectral-selective for tinted glass. To achieve with random AgNP orientation, AgNPs were functionalized spectral selectivity, we employed AgNPs of various sizes and with thiolated PMMA (Scheme S1) so that they were evenly colors across the visible and NIR (Figure 2). Sharp colors were dispersed in the plexiglass PMMA matrix (Figure 1A). The visible for AgNP that resonated in the visible (400− functionalization also prevented the AgNPs from degradation38 700 nm), and light colors were seen for those resonating in the and helped their transfer from aqueous to organic for NIR (Figure 2A,B). The light colors were attributed to the making bulk composites. Via LbL assembly,32,39,40 we prepared weak in-plane quadrupole LSPR of large AgNPs. Representa- thin-film polymer composites with planarly oriented AgNPs tive transmission electron micrographs (TEM) confirmed the (Figure S1). PAH was deposited on negatively charged glass shape and size of the AgNPs (Figure 2C). As the AgNP lateral substrates to initiate the assembly. By dipping the PAH-coated size was increased from 15 to 217 nm, the thickness increased glass in colloidal suspensions, the negatively charged, citrate- from 5.3 to 13.9 nm (Figure S6a−h). The nanoparticles were capped AgNPs were electrostatically attached to the positively 2D and therefore termed as nanoplates. The increase in the charged PAH surface in a planar manner (Figure 1D). The nanoplate size resulted in the formation of various planar AgNPs and PAH strongly attracted each other, and their shapes, including triangles, hexagons, and nanodisks. The separation required vigorous agitation at pH > 11 and 40 °C.41 shape , however, imposed negligible influence on the If needed, the planarly oriented AgNPs were fully covered by in-plane dipole plasmon resonance and optical properties, as PAH via LBL deposition or by PMMA via spin-. PAH long as the nanoparticles remained 2D. has a high charge density at neutral pH, and thus it fully Monolayers of AgNPs were integrated with PAH via LbL covered the negatively charged nanoparticles.39 The full assembly to create thin-film polymer composites on glass coverage of nanoparticles with PAH is a well-known (Figure 2D). The AgNPs on the PAH/glass substrates (Figure phenomenon, as shown by the uniform thickness of 5 Å and 2E) were at the interface between PAH and air. Since the the minimal roughness of the PAH layer at pH = 7 in previous of air (nair = 1.00) is lower than that of water 40,42 reports. The thin-film composite exhibited a sharp (nwater = 1.33), the reduced overall refractive index induced λ fi (Figure 1E) compared to the bulk composites (Figure 1B), blueshift of LSPR (Figure 2F), and the thin- lm composites which is attributed primarily to the in-plane dipole plasmon showed different colors from the aqueous colloidal suspensions resonance of the AgNPs. Under an , the (Figure 2A,D). The thin-film composites exhibited reduced T λ fi bulk composite showed randomly oriented and sparsely and enhanced R and A at LSPR, which can be ne-tuned across distributed AgNPs in the matrix (Figure 1C), while the thin- the visible and NIR. The intensity of R depended on the AgNP film composite revealed a monolayer of planarly oriented size and surface density. Due to the large cross- AgNPs (Figure 1F). section, R became dominant as AgNP size was increased, in To evaluate the angular optical properties, we measured the agreement with the prediction of Kondorskiy et al.45 The small transmittance (T), reflectance (R), and absorptance (A) of the AgNPsexhibitedshoulderpeaksatlongerwavelengths, θ 46,47 polymer composites at various incident angles ( inc). The indicative of AgNP overlap. The AgNPs showed relatively substrates were rotated from 6° to 75° in increments of 1° on a broad LSPR peaks, which were associated with the size UV−vis-NIR spectrophotometer (Figure S2). Figure 1G shows variation as shown by TEM. The widths of the LSPR peaks the T, R, and A contour plots of the polymer composites with were comparable or better than those of the nanoparticles randomly oriented AgNPs. At the LSPR wavelength of the prepared via lithography.48,49 If necessary, improved nano- λ AgNPs ( LSPR = 900 nm), the polymer composites exhibited particle synthesis could narrow the peak widths. In this work, minimum T and maximum A. The position of the LSPR peak however, the broad peaks did not obstruct the preparation of θ fl was insensitive to inc. The specular re ectance was weak, and plasmonic polymer nanocomposites with tunable colors. On ∼ λ fi the maximum was 0.2% at LSPR. Incident light was mostly the contrary, the broad peaks were bene cial because they diffusely reflected and then absorbed by the film as indicated blocked heat across a wide range of wavelengths. by the weak specular reflectance and the high absorptance. The A thin layer of PMMA was applied on the assembly to p- and s-polarized light interacted with the bulk polymer protect the AgNPs from mechanical abrasion and oxidation in ff ∼ composite slightly di erently, but the dependence of T, R, and air. The high refractive index of PMMA (nPMMA 1.48) caused fi λ A on the light polarization was insigni cant (Figure S4), LSPR to redshift, and thus the colors changed (Figure S7). The 43,44 Δλ ∼ similar to the previous reports. change in the LSPR wavelength ( LSPR) was 50 nm for

D DOI: 10.1021/acsnano.8b09386 ACS Nano XXXX, XXX, XXX−XXX ACS Nano Article

Figure 3. Plasmonic polymer nanocomposites with tunable surface coverage of AgNPs. (A) Optical photographs and (B) the corresponding SEM images of the glass slides after incubation in AgNP suspensions for various lengths of time. The scale bars apply to all images in the respective panel. (C) T and R spectra of the plasmonic polymer nanocomposites on glass slides at a light incident angle of 6°. (D) Surface coverage and T and R at the LSPR wavelength as a function of incubation time for polymer composites with three different AgNP sizes: (a) 37 nm, (b) 51 nm, and (c) 79 nm. ∼ λ ∼ fi small AgNPs (a) and 174 nm for large AgNPs (h) (Figure LSPR was 1020 nm, the lm was almost transparent to visible S8). In comparison to lithography and Langmuir−Blodgett light, and the NIR transmittance was <30%. − ffi techniques,7,50 52 LbL assembly was applicable to substrates of Light Management E ciency. The light modulation ffi fi arbitrary size and shape; therefore, these plasmonic composites e ciency of the thin- lm polymer nanocomposites was controlled by AgNP density on the substrates. The AgNP were fabricated on large pieces of glass and used as outdoor density was tuned by controlling the incubation time of the modules, which showed spectral selectivity (Figure 2G). The substrates in the colloidal suspensions (Figure 3). As the fl exibility in the substrate shape and size was challenging for incubation time was increased, the color of the thin-film − 7,50−52 lithographic and Langmuir Blodgett techniques. When composites became intense (Figure 3A), and the AgNP density λ − fi LSPR was in the range of 480 750 nm, the lms selectively on the surface increased (Figure 3B), resulting in reduced T filtered light and exhibited sharp complementary colors. When and enhanced R (Figure 3C). After incubating for >60 min, the

E DOI: 10.1021/acsnano.8b09386 ACS Nano XXXX, XXX, XXX−XXX ACS Nano Article

Figure 4. Plasmonic polymer nanocomposites with multiple layers of AgNPs. Representative SEM images of the polymer composite thin films (A) after depositing one, three, and five layers of AgNPs and (B) after depositing AgNPs followed by a layer of PMMA to cover the AgNPs. The AgNPs appear hazy after being covered by PMMA. (Inset) Schemes depict the number of AgNP layers. (C) Optical properties (T, R, and A) of the plasmonic polymer nanocomposites containing multiple layers of AgNPs.

Figure 5. Plasmonic polymer nanocomposites with multiwavelength responsiveness. (A) Optical photographs and (B) the corresponding λ ∼ λ ∼ SEM images of (a) one monolayer of large AgNPs ( LSPR 1010 nm), (b) one monolayer of small AgNPs ( LSPR 550 nm), and (c) the combined layers of AgNPs (a + b) separated by a thin layer of PMMA. (C) Optical properties (T, R, and A) of a polymer nanocomposite that contains one layer of large AgNPs and one layer of small AgNPs separated by a layer of PMMA (a + b). The peaks (a) and (b) correspond to λ LSPR of the large AgNPs and small AgNPs, respectively. The logo in panel (A) is credited to the University Relations of Virginia Tech. interparticle spacing decreased, and the AgNPs started to shoulder peak in the NIR. As the incubation time was further overlap, leading to plasmonic coupling similar to a previous increased, the AgNP overlapping enhanced plasmon coupling. report.24 The overlapped AgNPs had reduced interparticle 53 λ spacing, resulting in in-plane dipole plasmon coupling and a After incubating for >90 min, the intensities of T and R at LSPR

F DOI: 10.1021/acsnano.8b09386 ACS Nano XXXX, XXX, XXX−XXX ACS Nano Article no longer changed substantially (Figures 3C and S9). Similar heat-blocking capability in the NIR, the multilayer polymer trends were observed for AgNPs of other sizes (Figure S10). nanocomposites exhibited dual functionality, which is limited Image analyses showed that the surface coverage increased in existing polymer nanocomposites. from ∼7% to ∼55% as the incubation time was increased from Previously, the multiwavelength responsiveness required 10 to 300 min for AgNPs of all sizes (Figure 3D). The surface combining multiple that were fabricated coverage as a function of time revealed that the deposition rate separately.59 Recently, multiwavelength nanostructured films − of the AgNPs depended slightly on the size (Figure 3D). As were made using laser printing.60 63 These films acted as the surface coverage reached ∼40%, AgNPs began to overlap, plasmonic reflectors which were unsuitable for tinted glass due which led to plasmon coupling and shoulder peaks in the NIR. to their low transmittance. Differently, our polymer nano- The primary and shoulder peaks engendered the composites composites selectively transmitted or blocked light of certain with plasmonic colors in the visible and heat-blocking ability in wavelengths via multiple separated layers of plasmonic λ the NIR, respectively. The surface coverage, T and R at LSPR nanoparticles. LbL assembly in a single step, however, was plateaued after ∼100 min (Figure 3D). insufficient to make such composites, because nanoparticles of Multilayers and Multiwavelength Responsiveness. different sizes had different deposition rates (Figure S19). The LbL assembly enabled the preparation of plasmonic polymer sequential deposition of nanoparticles of different sizes is a nanocomposites with multiple layers of 2D AgNPs. Via facile method for fabricating multiwavelength responsive repeated cycles of PAH and AgNP deposition, up to 16 layers polymer nanocomposites. Importantly, the introduction of a of AgNPs were deposited (Figure S11). Since PAH strongly spacer layer avoids nanoparticle aggregation and is crucial for attracted the AgNPs, the deposition often led to AgNP constructing multiwavelength-responsive structures. aggregation. In addition, the PAH layer was too thin to prevent the plasmonic coupling of AgNPs in neighboring layers (Figure DISCUSSION S11). To avoid this problem, we used PMMA as a spacer to ff separate the adjacent AgNP layers. After one deposition cycle We note that the di erence between the maximum and ∼ of PAH and AgNPs, a thin layer of plexiglass PMMA was spin- minimum transmittance ( 30%) was comparable to, or better 59,64 coated on the surface (Figures S12 and S13). The PMMA than, existing fabricated via lithography. In layer was treated with to introduce negative addition, multilayer deposition allowed for controlled surface charge and assist the subsequent deposition of PAH and coverages with AgNPs of various LSPR peaks. As shown in AgNPs. The oxygen plasma treatment also reduced the Figure 3D, the surface density of the AgNPs saturated at thickness of the PMMA spacer down to ∼7 nm and minimized ∼50%. If AgNPs of both sizes were mixed and deposited dielectric effects (Figures S14 and S15). As the number of together, the surface coverage of each type of AgNPs would be AgNP layers was increased, the color of the composites too low. In addition, the AgNPs of different sizes were 24,65,66 became increasingly intense (Figure S16), and the intensities susceptible to overlapping and plasmon coupling. of the LSPR peaks strengthened (Figure 4).54 If no top-layer Therefore, to generate effective plasmonic polymer nano- PMMA was coated, the exposed AgNPs were easily observed composites with multiwavelength responsiveness, we deposited and appeared bright under SEM (Figure 4A). Since SEM has a the multiple layers via multiple steps. The multistep deposition limited probe depth, only the AgNPs in the top layers were allowed for maximum surface coverages and minimum observed. After being fully covered by PMMA, the AgNPs plasmonic coupling of the AgNPs. appeared hazy (Figures 4B and S17), confirming sufficient Compared with conventional lithographic approaches to separation of the adjacent AgNP layers. The sufficient preparing plasmonic nanostructures, the approach in this work separation of the AgNPs layers drastically mitigated the is advantageous in terms of scalability, plasmon coupling and suppressed shoulder peaks in the NIR , and versatility. First, to create plasmonic-colored (Figure 4C), in contrast to the thin-film composites without and NIR-reflective glass at large scales, it is a prerequisite to the PMMA spacers (Figure S11). prepare the AgNPs at large scales. Previously, plasmonic A major advantage of the plasmonic polymer nano- nanostructures were fabricated on surfaces using top-down composites is the multiwavelength responsiveness, which is methods such as electron beam lithography,7 metal deposi- − important and yet lacking in other plasmonic devices.55 58 As a tion,52 and soft lithography.67 The scalability of these methods proof-of-concept, the multilayer plasmonic polymer nano- is limited. In contrast, the bottom-up synthesis of plasmonic composites were extended to the creation of a two-wavelength nanoparticles is scalable to bulk quantities, and LbL assembly construct, which consisted of two layers of AgNPs with is applicable to arbitrarily large surfaces. As demonstrated in ff λ di erent sizes and LSPR (Figure 5). The large AgNPs (a) had a this work, we can prepare plasmonic polymer nanocomposites λ ∼ LSPR of 1010 nm (Figure S18). The second layer of small easily at the inch scale. Second, compared to the patterned λ ∼ AgNPs (b) with a LSPR of 530 nm was separated by a metal nanostructures with rough surfaces and poor - PMMA spacer (Figure S18). With only AgNPs (a) and (b), linities from top-down methods,51,52 the colloidal nanoparticles the polymer composites were colorless and magenta, are highly crystalline and offer superior optical properties.14,65 respectively (Figure 5A,B). When the two types of AgNPs Importantly, colloidal nanoparticles make it possible to use were combined, the composite exhibited a magenta color LbL assembly in aqueous and require no toxic (Figure 5A (a + b)). SEM confirmed that the hybrid film organic solvents. Third, compared to the preparation of contained well-separated layers of large and small AgNPs. The nanoplates using lithography,68 LbL assembly is suitable for UV−vis-NIR spectra revealed two distinct plasmon peaks both rigid and flexible substrates of arbitrary size, curvature, corresponding to the two AgNPs layers in the hybrid film (a + and . For example, LbL is even applicable to substrates b) (Figure 5C). Interestingly, due to the large scattering cross- that degas under high vacuum or are susceptible to high section, the large AgNPs layer exhibited higher R despite a evaporation temperatures. Such versatility is important for lower surface coverage. Considering the plasmonic color and specialty applications such as wearable electronics and sensors.

G DOI: 10.1021/acsnano.8b09386 ACS Nano XXXX, XXX, XXX−XXX ACS Nano Article

Thin-film LbL assembly signifies a completely different similar to, if not more complicated than, the metalized films approach to preparing plasmonic polymer nanocomposites. In used in current commercial tinted glass. Nonetheless, these contrast to the conventional approach of randomly mixing metamaterials are based on the mechanism of extraordinarily fillers in polymers, LbL assembly offers superb control over the low transmission (ELT) and are nontransparent to visible distribution and orientation of the AgNPs in each layer. light; thus they cannot be used as tinted glass but rather Because of LbL assembly, nanoparticle aggregation and plasmonically colored displays.72 Our thin-film plasmonic separation12,69 are easily avoided by controlling nanoparticle polymer nanocomposites, however, use bottom-up assembly orientation and confining them to a thin layer. In addition, and make it unnecessary to deposit full layers of metal. The polyelectrolytes naturally adsorb on the nanoparticle surface thin-film nanocomposites are solely based on the plasmon and prevent nanoparticle aggregation, especially at high resonance of presynthesized colloidal AgNPs and exhibit color nanoparticle volume fractions. For polymer-grafted anisotropic in the visible range that is significantly sharper than that of the nanorods in polymer nanocomposites, the nanoparticle volume metastructures prepared by soft lithography.51,52 Compared fraction reaches 16.1% before aggregation.70 In our thin-film with these metamaterials, the thin-film polymer composites polymer composites, however, the surface coverage of AgNPs have one or multiple layers of sparsely assembled AgNPs, easily reaches ∼55%, and the estimated volume fraction is which allow for independent modulation of multiple LSPR ∼32.4%. The high AgNP surface coverage and volume fraction wavelengths. impart the thin-film polymer nanocomposites with intense The dispersion of nanoparticles in polymers has always been plasmonic colors and high tint levels, which are unattainable challenging due to nanoparticle aggregation and phase via the random mixing approach in conventional polymer separation.73,74 In our bulk polymer composites, uniform composites. Moreover, the planar orientation fully exploits the distribution is achieved by grafting the AgNPs with PMMA ∼ in-plane dipole resonance of the AgNPs and thus provides (Mn 60 kDa) and mixing the PMMA-grafted AgNPs in a ∼ outstanding optical and plasmonic properties. The tunable matrix PMMA (Mn 75 kDa). Because the degree of intralayer and interlayer distances harness the plasmon of the matrix PMMA, P, is less than twice the coupling among the AgNPs. The spectral selectivity of our of the grafted PMMA, N, the grafted plasmonic polymer nanocomposites, enabled by precise and matrix PMMA have good physical interaction.65 In this control over AgNP size and orientation in the composites, is work, the P/N ratio is approximately 1.25, which results in a exceptional and tunable across the entire visible and NIR so-called wet brush and leads to uniform distribution of region. nanoparticles in the polymer matrix. The grafted PMMA on The thin-film polymer composites are advantageous the AgNPs is relatively sparse,75 and therefore the AgNPs compared to existing chromic glass. For example, the state- interact favorably with the matrix PMMA, similar to previous of-the-art electrochromic glass uses expensive ITO nanocryst- reports about silica nanoparticles.76,77 As a result, the AgNPs als in niobium oxide and shows a high transmittance of ∼80% are uniformly distributed in the PMMA matrix, suggesting that 78 in the NIR with limited tunability in the visible.8 the composites are in the well-dispersed regime. If the Thermochromic VO is yet impractical because (1) it requires AgNPs are grafted with a dense layer of polymers, they might 2 79,80 a huge temperature gradient from room temperature to ∼100 potentially be used for creating advanced photonic crystals. °C to reduce the NIR transmittance; and (2) even with a large temperature gradient, the transmittance remains high (i.e., 45% CONCLUSIONS at 1500 nm). In contrast, our thin-film plasmonic polymer In conclusion, we have created spectral-selective plasmonic composites (e.g., Figure 2G) show a low NIR transmittance of polymer nanocomposites using -assisted LbL 28% at 1020 nm. Moreover, the significantly reduced NIR assembly of colloidal 2D AgNPs. By controlling the size, transmittance does not interfere with visible light trans- orientation, surface density, and interlayer spacing of the 2D mittance, which remains 50−80% in the range of 400−700 AgNPs, the thin-film polymer nanocomposites exhibit well- nm. Independent control over visible and NIR light allows for controlled plasmonic colors and NIR reflectivity. The planar the preparation of multilayer thin-film composites with dual orientation of the AgNPs plays a critical role in the optical function of color modulation and NIR reflection, which are properties and contributes to the strong light reflectance of the unattainable by other chromic materials. nanocomposites. Depending on the AgNP size, the thin-film The plasmonic colors generated by our thin-film polymer polymernanocompositesshowcolorssuchasyellow, nanocomposites are shown in the chromaticity CIE 1931 plot chocolate, pink, violet, blue, dodger blue, and steel blue. (Figure S20). These colors resemble aluminum-based Polymer nanocomposites with AgNPs of large sizes block NIR plasmonic pixels6 but differ drastically from most metamate- but transmit visible light, suggesting their potential application rials.5,67,71 Most plasmonic metamaterials are nontransparent in heat-reflecting windows. Our polymer nanocomposites are and absorb heat because they are based on light reflection and responsive to multiple wavelengths, and such multiwavelength absorption. For example, by depositing a full metal layer responsiveness is a feature that is unattainable by existing followed by an insulating layer and then patterning another polymer nanocomposites. We anticipate the plasmonic layer of metal particles or holes, the metamaterials enhance polymer nanocomposites to be applied in energy-efficient scattering and absorb a large amount of energy that re-radiates buildings and vehicles,1 color filters,72 optical coatings,81 − as heat.7,56 Moreover, the plasmonic metamaterials often plasmonic printings,7 photovoltaics,82 84 and advanced − require expensive lithographic techniques and are limited to plasmonic constructs.85 88 small areas. Scalable approaches such as nanoimprinting52 and soft interference lithography67 can prepare metastructures at MATERIALS AND METHODS fl relatively large scales, but these methods are restricted to at Materials. Silver nitrate (≥99.9999%) (204390), sodium borohy- substrates. Additionally, the deposition of full metal layers, dride (≥99.99%) (480886), sodium citrate tribasic dihydrate followed by patterning and etching into nanostructures, is (≥99.0%) (S4641), ascorbic acid (≥99.0%) (A5960),

H DOI: 10.1021/acsnano.8b09386 ACS Nano XXXX, XXX, XXX−XXX ACS Nano Article ≥ ∼ ( 99.9%), poly(allylamine hydrochloride) (PAH, average Mw Synthesis of -Terminated Poly(methyl methacrylate) · −1 ∼ · − 17,500 g mol ) (283215), poly() (PAA, Mv 450,000 g (PMMA-SH). PMMA-SH was synthesized via reversible addition mol−1), 2-phenyl-2-propyl benzodithioate (CDB, ≥ 99%), 2,2′- fragmentation (RAFT) polymerization followed by azobis(2-methylpropionitrile) (AIBN, ≥ 98%), and poly(sodium 4- subsequent reduction of thiocarbonylthio to thiol (Scheme S1). ∼ · −1 fl styrenesulfonate) (PSSS, average Mw 1000 kg mol ) (434574) Brie y, MMA (0.230 mol), CDB (0.297 mmol), AIBN (0.148 mmol), were purchased from Sigma-Aldrich and used as received. Plain glass and benzene (58.5 mL) were mixed in a Schlenk flask (100 mL) using microscope slides (25 × 75 × 1 mm) (cat. no. 12-544-4) were a stir bar. The mixture was then then subjected to three freeze− − fi fl purchased from Fisher Scientific and used as received. Nanoparticle pump thaw cycles. After being lled with N2, the Schlenk ask was synthesis was carried out in ultrapure deionized (DI) water obtained kept in an oil bath at 65 °C under constant stirring. After 24 h, the from Thermo Scientific Barnstead GenPure Pro water purification reaction was quenched by cooling down to 0 °C. The resulting CDB- system at 17.60 MΩ-cm. terminated PMMA (PMMA-CDB) was precipitated in methanol Synthesis of Ag Nanoplates (AgNPs). AgNPs were synthesized twice and dried in vacuum oven for 24 h. fi 26,27 ∼ following a seed-mediated method with slight modi cations. The PMMA-CDB had a number-average molecular weight (Mn)of 60 Ag seeds were synthesized as follows. First, 0.25 mL of PSSS (2.6 kDa and a polydispersity index (PDI) of 1.2, as characterized by gel mM) and 0.3 mL of ice-cold NaBH4 (10 mM) aqueous solutions permeation chromatography. To reduce PMMA-CDB to PMMA-SH, were added to 5 mL of sodium citrate (2.5 mM) under 7.02 g of the polymer was dissolved in 175 mL of THF. In a separate constant stirring. Afterward, 5 mL of AgNO3 (0.5 mM) was added to vial, 220 mg (50 mol equiv) of NaBH4 was dissolved in 35 mL of the solution at a rate of 2 mL/min using a Cole-Parmer pump. water. The polymer and NaBH4 were then mixed together and stirred The seed solution was then immediately covered in an Al foil to avoid vigorously at room temperature for 24 h (Scheme S1).89 The resulting exposure to light. After 5 min, the stirring was stopped. thiol terminated PMMA (PMMA-SH) was precipitated in methanol To synthesize AgNPs, 1.5 mL of 10 mM ascorbic acid solution was twice and dried in a vacuum oven at room temperature for 48 h. added to 254 mL of water under vigorous stirring, followed by the Random Orientation of AgNPs. Aqueous suspensions of AgNPs addition of the seed solution (ranged from 60 to 2000 μL) to prepare were centrifuged at 10,000 rpm for 30 min and then dispersed in AgNPs of various sizes. Afterward, 6 mL of AgNO3 (5 mM) solution DMF. The AgNPs were functionalized with PMMA by mixing the was added to the mixture at a rate of 2 mL/min. Finally, 10 mL of AgNPs in DMF with a solution of PMMA-SH in (1 wt %). sodium citrate (25 mM) solution was added to the product solution After 24 h, the mixture was centrifuged at 10,000 rpm for 30 min. The to stabilize the AgNPs. To obtain large AgNPs with a plasmon supernatant was removed, and the AgNPs at the bottom of the tube resonance wavelength of more than 800 nm, the Ag seeds were used were redispersed in a solution of PMMA in chloroform (5 wt %). To within 5−10 min after the seed preparation. Using seeds that were obtain a polymer nanocomposite film with randomly oriented AgNPs, aged more than 10 min may cause instability of the synthesis, which the solution was casted on a glass slide and kept at room temperature was discussed in our previous report.27 for 24 h to evaporate the . To avoid the interference of the Layer-by-Layer Deposition of AgNPs and Polymers. Thin glass, the thin films of polymer composite were detached from the films of plasmonic polymer composites were prepared via layer-by- glass slides prior to the evaluation of the optical properties. layer (LbL) using a nanostrata dipping robot. First, two beakers were Transmission Electron Microscopy (TEM). Aliquots of AgNPs respectively filled with 100 mL of PAH solution (10 mM, pH = 7) (5−10 μL) were drop-casted on copper grids (Formvar/Carbon 200 and 100 mL of the as-synthesized AgNP colloidal solution. Glass Mesh, Copper, Electron Microscopy Sciences). The grids were dried slides were treated with oxygen plasma and dipped in the PAH overnight at room temperature. The AgNPs were imaged using a solution for 5 min, allowing for the positively charged PAH to adsorb Philips EM420 TEM at an accelerating voltage of 120 keV (Figure 2 onto the glass via electrostatic interactions. As a result, a monolayer of and S6). The bulk AgNPs-PMMA composite film was embedded in 32,33 PAH formed on the glass substrates. The glass substrates were an and cured at room temperature for 24 h (Figure rinsed in DI water three times. After rinsing, the glass substrates were S3A,B). The composite was then microtomed into ∼100 nm-thick immersed in colloidal solutions of AgNPs for various lengths of time films and transferred onto a TEM grid for imaging (Figure S3). (10−300 min). To assist the deposition of AgNPs, the glass substrates Field Emission Scanning Electron Microscopy (FE-SEM). The were rotated at a speed of 600 rpm. The AgNPs were negatively polymer composites on the glass slides were sputtered with a thin charged due to the sodium citrate adsorbed on the surfaces, layer of iridium (thickness, 1.5−2nm)toensuresufficient and therefore they were easily deposited on the glass substrates that conductivity. The glass slides were then loaded into a LEO (Zeiss) had a monolayer of positively charged PAH through electrostatic 1550 field-emission SEM and imaged at an EHT of 10 kV using an interaction. Afterward, the glass substrates were rinsed in DI water InLens detector. three times. UV−vis Near-Infrared (NIR) Spectroscopy with Universal To deposit multilayers of AgNPs in the polymer composites, two Measurement Accessory (UMA). Optical properties including T, approaches were used. In the first approach, only PAH and AgNPs R, and A were measured using an Agilent Cary 5000 UV−vis-NIR were used. Typically, the deposition of PAH for 5 min and the spectrophotometer with a universal measurement accessary (UMA) subsequent deposition of AgNPs for 10 min were repeated as unit. The polymer composites were mounted on the stage. The required. The deposition of PAH, however, occurred mostly on the incident light angle was controlled by rotating the samples on the prior layer of AgNPs, leading to nanoparticle aggregation (Figure stage. The position of the detector was controlled to collect the T and S11). In the second approach, PMMA was added as a spacer between R spectra at various angles (Figure S2). To collimate the incident light the adjacent AgNP layers. Briefly, after the deposition of PAH and beam, the incident aperture was set at 3°. To maximize the signal, the AgNPs, a PMMA layer was applied on top of the film by spin-coating detector aperture was set at 5°. T and R were directly measured by the a PMMA solution in chloroform (0.5 wt %). The PMMA layer was detector. A was indirectly calculated using the relationship A = 100% subject to oxygen plasma treatment (PC-200 South Bay Technologies − R − T. Inc.). The thickness of the PMMA layer was tuned by the spin-coating conditions (e.g., solution concentration and spin speed) and the oxygen plasma etching time. After oxygen plasma etching, the ASSOCIATED CONTENT thickness of the PMMA thin layer decreased to <7 nm (Figure S15). *S Supporting Information In addition, oxygen plasma created negative charges on the surface, which facilitated the subsequent deposition of PAH and AgNPs. The The Supporting Information is available free of charge on the above deposition process was repeated to obtain the desired number ACS Publications website at DOI: 10.1021/acsnano.8b09386. of AgNPs and polymer layers. In the last deposition cycle, the AgNPs Supplementary scheme (S1) and figures (S1−S20) may or may not be covered by the PMMA layer. If the last PMMA layer was applied, it was not subject to oxygen plasma treatment. (PDF)

I DOI: 10.1021/acsnano.8b09386 ACS Nano XXXX, XXX, XXX−XXX ACS Nano Article

AUTHOR INFORMATION (12) Balazs, A. C.; Emrick, T.; Russell, T. P. Nanoparticle Polymer Composites: Where Two Small Worlds Meet. Science 2006, 314, Corresponding Author − *E-mail: [email protected]. 1107 1110. (13) Jin, R.; Cao, Y. C.; Hao, E.; Metraux,́ G. S.; Schatz, G. C.; ORCID Mirkin, C. A. Controlling Anisotropic Nanoparticle Growth through Assad U. Khan: 0000-0001-6455-3219 Plasmon Excitation. Nature 2003, 425, 487−490. Yichen Guo: 0000-0002-1637-4440 (14) Chen, T.; Reinhard, B. M. Assembling Color on the Nanoscale: Xi Chen: 0000-0001-7965-9198 Multichromatic Switchable Pixels from Plasmonic and Molecules. Adv. Mater. 2016, 28, 3522−3527. Guoliang Liu: 0000-0002-6778-0625 (15) Freestone, I.; Meeks, N.; Sax, M.; Higgitt, C. The Lycurgus Author Contributions Cupa Roman . Bull. 2007, 40, 270−277. G.L. and X.C. conceived the idea. G.L. and A.U.K. designed (16) Mulvaney, P. Spectroscopy of Nanosized research. G.L., A.U.K., and Y.G. performed research. G.L., Metal Particles. Langmuir 1996, 12, 788−800. X.C., A.U.K., and Y.G. analyzed data. The manuscript was (17) Odom, T. W. Colours at the Nanoscale: Printable Stained written and approved by all authors. Glass. Nat. Nanotechnol. 2012, 7, 550−551. Notes (18) Orlicki, J. A.; Zander, N. E.; Martin, G. R.; Kosik, W. E.; Derek The authors declare no competing financial interest. Demaree, J.; Leadore, J. L.; Rawlett, A. M. Self-Segregating Hyperbranched Polymer/ Hybrids in Thermo- − ACKNOWLEDGMENTS Films. J. Appl. Polym. Sci. 2013, 128, 4181 4188. (19) Kao, J.; Thorkelsson, K.; Bai, P.; Rancatore, B. J.; Xu, T. This material is based upon work supported by the National Toward Functional Nanocomposites: Taking the Best of Nano- Science under grant no. DMR-1752611. The particles, Polymers, and Small Molecules. Chem. Soc. Rev. 2013, 42, authors acknowledge Prof. James R. Heflin and Dr. Jonathan S. 2654−2678. Metzman for discussion and use of instrumentation. The (20) Jiang, G.; Hore, M. J.; Gam, S.; Composto, R. 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K DOI: 10.1021/acsnano.8b09386 ACS Nano XXXX, XXX, XXX−XXX ACS Nano Article

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L DOI: 10.1021/acsnano.8b09386 ACS Nano XXXX, XXX, XXX−XXX