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Spectral-Selective Plasmonic Polymer Nanocomposites Across the Visible and Near-Infrared Assad U. Khan, Yichen Guo, Xi Chen, and Guoliang Liu ACS Nano, Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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Spectral-Selective Plasmonic Polymer Nanocomposites Across the Visible and NearInfrared Assad U. Khan,† Yichen Guo,† Xi Chen,‡ and Guoliang Liu*,†,§,& †

Department of Chemistry, ‡Industrial and Systems Engineering, §Macromolecules Innovation

Institute, and &Division of Nanoscience, Academy of Integrated Science, Virginia Tech, Blacksburg, Virginia 24061, United States * Email: [email protected]

ABSTRACT: State-of-the-art commercial light-reflecting glass 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 (2D) plasmonic silver nanoplates (AgNPs) in polymer nanocomposites. In contrast to conventional bulk polymer nanocomposites where plasmonic nanoparticles are randomly mixed within a polymer matrix, our thin-film polymer nanocomposites comprise a single layer, or any desired number of

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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 nanoparticle, polymer nanocomposite, spectral selectivity, visible, near-infrared Current tinted glass uses metalized films composed of Au, Ag, Cu, Co, Ti, Ce, and Se of various ratios.1 The metal films are sandwiched between,2-4 or stacked with,2,3 two dielectric layers such as TiO2, ZnO2, SnO2, WO3, and ZnS. Such tinted glass modulates light transmission but has a high absorptance, and thus it captures a large amount of heat that re-radiates. Instead of full layers of metals, plasmonic metamaterials actively5 or statically6,7 modulate light. The preparation of metamaterials, however, requires a large amount of metal and metal oxides. Furthermore, it involves complex and expensive top-down fabrication techniques which are time consuming, limited to small areas, and inapplicable to substrates of arbitrary shapes.2-4 Alternatively, researchers have designed electrochromic8 and thermochromic9 glass using tindoped indium oxide (ITO), Nb2O5, and VO2. The electrochromic and thermochromic glass manages light in a certain range of the NIR, but it requires an external energy supply8 or a large temperature gradient from 25 to 100 °C,9 which are impractical under ambient conditions. Recently, colloidal particle suspensions and liquid crystals have been used in tinted glass but they often result in opaque films.1 While all the above glass mitigates the transmittance of light at some wavelengths, the spectral selectivity is severely limited, and there is a need for spectral-


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selective glass that can control the transmittance, reflectance, and filtration of any wavelength across the visible and NIR. In a disparate approach, we investigate polymer nanocomposites as alternatives for modern tinted glass. Polymer nanocomposites comprise polymers mixed with fillers such as carbon black, fumed silica, and clay, which significantly enhance the mechanical strength, flame retardance, and durability of the polymers.10-12 Conventional polymer nanocomposites, however, possess limited optical and aesthetic properties, restricting their use in tinted glass. To prepare spectralselective tinted glass from plasmonic nanocomposites at industrial scales, there are three primary challenges: selection of appropriate fillers, control over filler dispersion and orientation, and suitability for scalable fabrication. Plasmonic nanoparticles are emerging nanomaterials that interact with light of certain wavelengths depending on their size, shape, and composition.13 Colloidal nanoparticles can potentially serve as fillers for constructing spectral-selective polymer nanocomposites via low-cost bottom-up assembly. Compared to the particles prepared via nanofabrication (e.g., chemical or physical vapor deposition followed by etching),14 the colloidal plasmonic nanoparticles prepared by wet-chemistry synthesis have superior crystallinities and thus high-quality optical properties. The use of plasmonic particles in composites dates back to Roman times15-17 and has recently flourished with the incorporation of nanospheres,12,18,19 nanorods,20,21 nanoplates,22,23 nanocubes,24 and nanostars.25 Among these nanoparticles, nanospheres, nanocubes and nanostars have limited ranges of localized surface plasmon resonance (LSPR) wavelengths (λLSPR) and thus limited spectral selectivity; nanorods have tunable λLSPR but are susceptible to percolation. In contrast, two-dimensional (2D) Ag nanoplates (AgNPs) have tunable λLSPR in the visible and NIR26,27 and a high percolation threshold;28 thus they serve as the most promising candidate for addressing the challenge of spectral selectivity.


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To fully utilize the in-plane LSPR but minimize the out-of-plane LSPR, one must control the planar orientation and uniform dispersion of the 2D AgNPs in the polymer nanocomposites. To this end, layer-by-layer (LbL) assembly is suitable because it has shown excellent capability of depositing polyelectrolytes, graphene, and nanoparticles on various substrates to make layered structures.28-31 In addition, LbL assembly offers exquisite control over the interlayer distance, as well as the intralayer density of deposited species.32-35 Herein we synergize plasmonic nanoparticles with polymers to create plasmonic polymer nanocomposites with planarly oriented 2D AgNPs via LbL assembly. In the plasmonic polymer nanocomposites, 2D AgNPs are used as fillers because of their tunable LSPR in the visible and NIR.26,27,36 Poly(allylamine hydrochloride) (PAH) is selected because the positively charged PAH strongly attracts the negatively charged AgNPs to form layered structures. Poly(methyl methacrylate) (PMMA), a common plexiglass polymer, is used as a spacer between the AgNP layers. The plasmonic polymer nanocomposites contain 2D AgNPs of controlled size, surface coverage, and interlayer distance, and thus have well-controlled optical and plasmonic properties. To avoid nanoparticle aggregation and undesirable in-plane and out-of-plane plasmon hybridization in tinted glass, we control the particle-particle distances by tuning AgNP density in each layer and thickness of the PMMA spacer between the layers. Surprisingly, a single layer of AgNPs is able to efficiently modulate light transmission and reflection. The approach leads to thin-film polymer nanocomposites that possess exceptional spectral selectivity across the visible and NIR, as well as multi-wavelength responsiveness. RESULTS Orientation of 2D AgNPs. In conventional bulk plasmonic composites such as the Lycurgus cup and stained glass, a large amount of plasmonic nanoparticles are randomly mixed within the


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matrix.16,37 To design thin-film plasmonic polymer nanocomposites, we first investigated the effect of AgNP orientation (Figure 1)—either randomly oriented (Figure 1A-C) or planarly aligned (Figure 1D-F)—on the optical properties of the nanocomposites. Since the in-plane dipoles of the AgNPs are mostly responsible for plasmon resonance in the visible and NIR, the AgNP orientation played an important role. To prepare conventional bulk polymer nanocomposites with random AgNP orientation, AgNPs were functionalized with thiolated PMMA (Scheme S1) so that they were evenly dispersed in the plexiglass PMMA matrix (Figure 1A). The functionalization also prevented the AgNPs from degradation38 and helped their transfer from aqueous to organic solvents for making bulk composites. Via LbL assembly,32,39,40 we prepared thin-film polymer composites with planarly oriented AgNPs (Figure S1). PAH was deposited on negatively charged glass substrates to initiate the assembly. By dipping the PAHcoated glass in colloidal suspensions, the negatively charged, citrate-capped AgNPs were electrostatically attached to the positively charged PAH surface in a planar manner (Figure 1D). The AgNPs and PAH strongly attracted each other, and their separation required vigorous agitation at pH>11 and 40 °C.41 If needed, the planarly oriented AgNPs were fully covered by PAH via LBL deposition or by PMMA via spin-coating. PAH has a high charge density at neutral pH, and thus it fully covered the negatively charged nanoparticles.39 The full coverage of nanoparticles with PAH is a well-known phenomenon, as shown by the uniform thickness of 5 Å and the minimal roughness of the PAH layer at pH=7 in previous reports.40,42 The thin-film composite exhibited a sharp color (Figure 1E) compared to the bulk composites (Figure 1B), which is attributed primarily to the in-plane dipole plasmon resonance of the AgNPs. Under an electron microscope, the bulk composite showed randomly oriented and sparsely distributed


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AgNPs in the matrix (Figure 1C), while the thin-film composite revealed a monolayer of planarly oriented AgNPs (Figure 1F).

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


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randomly oriented AgNPs on a glass slide, C) TEM image of a thin slice of the polymer nanocomposite. (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. (GH) 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. To evaluate the angular optical properties, we measured the transmittance (T), reflectance (R), and absorptance (A) of the polymer composites at various incident angles (θinc). The substrates were rotated from 6° to 75° in increments of 1° on a UV-vis-NIR spectrophotometer (Figure S2). Figure 1G shows the T, R, and A contour plots of the polymer composites with randomly oriented AgNPs. At the LSPR wavelength of the AgNPs (λLSPR = 900 nm), the polymer composites exhibited minimum T and maximum A. The position of the LSPR peak was insensitive to θinc. The specular reflectance was weak, and the maximum was ~0.2% at λLSPR. Incident light was mostly diffusely reflected and then absorbed by the film as indicated by the weak specular reflectance and the high absorptance. The p- and s-polarized light interacted with the bulk polymer composite slightly differently, but the dependence of T, R, and A on the light polarization was insignificant (Figure S4), similar to the previous reports.43,44 The thin-film polymer composite showed minimum T and prominent specular R at λLSPR of ~720 nm and all θinc (Figure 1H). As θinc was increased from 6° to 75°, T decreased from 25% to 5% while R increased from 19% to 61% at λLSPR. Compared to the bulk polymer composites with


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randomly oriented AgNPs, the thin-film polymer composite with planarly oriented AgNPs showed significantly increased specular R of almost 20% at 6° and 61% at 75°, leading to decreased A overall. The enhanced specular R was distinctive to the planarly oriented nanoparticles but absent in our bulk polymer composites, as well as in commercial tinted glass with metal additives.1 In contrast with the bulk polymer composites, both T and R of the thinfilm composites showed strong dependence on the p- and s-polarization (Figure S5). Spectral Selectivity. The characteristic T, R, and A wavelengths suggest that the plasmonic composites are excellent spectral-selective coatings for tinted glass. To achieve spectral selectivity, we employed AgNPs of various sizes and colors across the visible and NIR (Figure 2). Sharp colors were visible for AgNP colloids that resonated in the visible (400–700 nm), and light colors were seen for those resonating in the NIR (Figure 2A and 2B). The light colors were attributed to the weak in-plane quadrupole LSPR of large AgNPs. Representative transmission electron micrographs (TEM) confirmed the shape and size of the AgNPs (Figure 2C). As the AgNP lateral size was increased from 15 to 217 nm, the thickness increased from 5.3 to 13.9 nm (Figure S6, a to h). The nanoparticles were two-dimensional (2D) and therefore termed as nanoplates. The increase in the nanoplate size resulted in the formation of various planar shapes, including triangles, hexagons, and nanodisks. The shape dispersity, however, imposed negligible influence on the in-plane dipole plasmon resonance and optical properties, as long as the nanoparticles remained two-dimensional.


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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


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photographs of the monolayer AgNP-polymer composites on glass. As the AgNP size increases from (a) to (h), the polymer composites show characteristic colors. E) Representative SEM images of the monolayer AgNPs. F) Optical properties (T, R, and A) of the monolayer AgNPpolymer composites. G) A photograph of selected polymer composite thin-films that contain monolayers 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. Monolayers of AgNPs were integrated with PAH via LbL assembly to create thin-film polymer composites on glass (Figure 2D). The AgNPs on the PAH/glass substrates (Figure 2E) were at the interface between PAH and air. Since the refractive index of air (nair=1.00) is lower than that of water (nwater=1.33), the reduced overall refractive index induced blueshift of λLSPR (Figure 2F), and the thin-film composites showed different colors from the aqueous colloidal suspensions (Figure 2A and 2D). The thin-film composites exhibited reduced T and enhanced R and A at λLSPR, which can be fine-tuned across the visible and NIR. The intensity of R depended on the AgNP size and surface density. Due to the large scattering cross-section, R became dominant as AgNP size was increased, in agreement with the prediction of Kondorskiy et al.45 The small AgNPs exhibited shoulder peaks at longer wavelengths, indicative of AgNP overlap.46,47 The AgNPs showed relatively broad LSPR peaks, which were associated with the size variation as shown by TEM. The widths of the LSPR peaks were comparable or better than those of the nanoparticles prepared via lithography.48,49 If necessary, improved nanoparticle synthesis could narrow the peak widths. In this work, however, the broad peaks did not obstruct


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the preparation of plasmonic polymer nanocomposites with tunable colors. On the contrary, the broad peaks were beneficial because they blocked heat across a wide range of wavelengths. A thin layer of PMMA was applied on the assembly to protect the AgNPs from mechanical abrasion and oxidation in air. The high refractive index of PMMA (nPMMA~1.48) caused λLSPR to redshift, and thus the colors changed (Figure S7). The change in the LSPR wavelength (ΔλLSPR) was ~50 nm for small AgNPs (a) and ~174 nm for large AgNPs (h) (Figure S8). In comparison to lithography and Langmuir-Blodgett techniques,7,50-52 LbL assembly was applicable to substrates of arbitrary size and shape; therefore, these plasmonic composites were fabricated on large pieces of glass and used as outdoor modules, which showed spectral selectivity (Figure 2G). The flexibility in the substrate shape and size was challenging for lithographic and LangmuirBlodgett techniques.7,50-52 When λLSPR was in the range of 480–750 nm, the films selectively filtered light and exhibited sharp complementary colors. When λLSPR was ~1020 nm, the film was almost transparent to visible light, and the NIR transmittance was 60 min, the interparticle spacing decreased, and the AgNPs started to overlap, leading to plasmonic coupling similar to a previous report.24 The overlapped AgNPs had reduced interparticle spacing, resulting in in-plane dipole plasmon coupling53 and a shoulder peak in the NIR. As the incubation time was further increased, the AgNP overlapping enhanced plasmon coupling. After incubating for >90 min, the intensities of T


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and R at λLSPR no longer changed substantially (Figure 3C and S9). Similar trends were observed for AgNPs of other sizes (Figure S10). Image analyses showed that the surface coverage increased from ~7% to ~55% as the incubation time was increased from 10 to 300 min for AgNPs of all sizes (Figure 3D). The surface coverage as a function of time revealed that the deposition rate of the AgNPs depended slightly on the size (Figure 3D). As the surface coverage reached ~40%, AgNPs began to overlap, which led to plasmon coupling and shoulder peaks in the NIR. The primary and shoulder peaks engendered the composites with plasmonic colors in the visible and heat-blocking ability in the NIR, respectively. The surface coverage, T and R at λLSPR plateaued after ~100 min (Figure 3D).


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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


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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. Multilayers and Multi-Wavelength Responsiveness. LbL assembly enabled the preparation of plasmonic polymer nanocomposites with multiple layers of 2D AgNPs. Via repeated cycles of PAH and AgNP deposition, up to 16 layers of AgNPs were deposited (Figure S11). Since PAH strongly attracted the AgNPs, the deposition often led to AgNP aggregation. In addition, the PAH layer was too thin to prevent the plasmonic coupling of AgNPs in neighboring layers (Figure S11). To avoid this problem, we used PMMA as a spacer to separate the adjacent AgNP layers. After one deposition cycle of PAH and AgNPs, a thin layer of plexiglass PMMA was spin-coated on the surface (Figure S12 and S13). The PMMA layer was treated with oxygen plasma to introduce negative charge and assist the subsequent deposition of PAH and AgNPs. The oxygen plasma treatment also reduced the thickness of the PMMA spacer down to ~7 nm and minimized dielectric effects (Figure S14 and S15). As the number of AgNP layers was increased, the color of the composites became increasingly intense (Figure S16), and the intensities of the LSPR peaks strengthened (Figure 4).54 If no top-layer PMMA was coated, the exposed AgNPs were easily observed and appeared bright under SEM (Figure 4A). Since SEM has a limited probe depth, only the AgNPs in the top layers were observed. After being fully covered by PMMA, the AgNPs appeared hazy (Figure 4B and S17), confirming sufficient separation of the adjacent AgNP layers. The sufficient separation of the AgNPs layers drastically


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mitigated the plasmon coupling and suppressed shoulder peaks in the NIR (Figure 4C), in contrast to the thin-film composites without the PMMA spacers (Figure S11).

Figure 4. Plasmonic polymer nanocomposites with multiple layers of AgNPs. Representative SEM images of the polymer composite thin films A) after depositing 1, 3, and 5 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.


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A major advantage of the plasmonic polymer nanocomposites is the multi-wavelength responsiveness, which is important and yet lacking in other plasmonic devices.55-58 As a proof-ofconcept, the multi-layer plasmonic polymer nanocomposites were extended to the creation of a two-wavelength construct, which consisted of two layers of AgNPs with different sizes and λLSPR (Figure 5). The large AgNPs (a) had a λLSPR of ~1010 nm (Figure S18). The second layer of small AgNPs (b) with a λLSPR of ~530 nm was separated by a PMMA spacer (Figure S18). With only AgNPs (a) and (b), the polymer composites were colorless and magenta, respectively (Figure 5A and 5B). When the two types of AgNPs were combined, the composite exhibited a magenta color (Figure 5A (a+b)). SEM confirmed that the hybrid film contained well-separated layers of large and small AgNPs. The UV-vis-NIR spectra revealed two distinct plasmon peaks corresponding to the two AgNPs layers in the hybrid film (a+b) (Figure 5C). Interestingly, due to the large scattering cross-section, the large AgNPs layer exhibited higher R despite a lower surface coverage. Considering the plasmonic color and heat-blocking capability in the NIR, the multilayer polymer nanocomposites exhibited dual functionality, which is limited in existing polymer nanocomposites. Previously, the multi-wavelength responsiveness required combining multiple nanostructures that were fabricated separately.59 Recently, multi-wavelength nanostructured films were made using laser printing.60-63 These films acted as plasmonic reflectors which were unsuitable for tinted glass due to their low transmittance. Differently, our polymer nanocomposites selectively transmitted or blocked light of certain wavelengths via multiple separated layers of plasmonic nanoparticles. LbL assembly in a single step, however, was insufficient to make such composites, because nanoparticles of different sizes had different deposition rates (Figure S19). The sequential deposition of nanoparticles of different sizes is a facile method for fabricating multi-


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wavelength responsive polymer nanocomposites. Importantly, the introduction of a spacer layer avoids nanoparticle aggregation and is crucial for constructing multi-wavelength-responsive structures.

Figure 5. Plasmonic polymer nanocomposites with multi-wavelength responsiveness. A) Optical photographs and B) the corresponding SEM images of (a) one monolayer of large AgNPs (λLSPR ~1010 nm), (b) one monolayer of small AgNPs (λLSPR ~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 λLSPR of the large


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AgNPs and small AgNPs, respectively. The logo in panel (A) is credited to the University Relations of Virginia Tech. DISCUSSION We note that the difference between the maximum and minimum transmittance (~30%) was comparable to, or better than, existing materials fabricated via lithography.59,64 In addition, multilayer deposition allowed for controlled surface coverages with AgNPs of various LSPR peaks. As shown in Figure 3D, the surface density of the AgNPs saturated at ~50%. If AgNPs of both sizes were mixed and deposited together, the surface coverage of each type of AgNPs would be too low. In addition, the AgNPs of different sizes were susceptible to overlapping and plasmon coupling.24,65,66 Therefore, to generate effective plasmonic polymer nanocomposites with multiwavelength responsiveness, we deposited the multiple layers via multiple steps. The multi-step deposition allowed for maximum surface coverages and minimum plasmonic coupling of the AgNPs. Compared with conventional lithographic approaches to preparing plasmonic nanostructures, the approach in this work is advantageous in terms of scalability, nanostructure crystallinity, and versatility. First, to create plasmonic-colored and NIR-reflective glass at large scales, it is a prerequisite to prepare the AgNPs at large scales. Previously, plasmonic nanostructures were fabricated on surfaces using top-down methods such as electron beam lithography,7 metal deposition,52 and soft lithography.67 The scalability of these methods is limited. In contrast, the bottom-up synthesis of plasmonic nanoparticles is scalable to bulk quantities, and LbL assembly is applicable to arbitrarily large surfaces. As demonstrated in this work, we can prepare plasmonic polymer nanocomposites easily at the inch-scale. Second, compared to the patterned metal nanostructures with rough surfaces and poor crystallinities from top-down methods,51,52 the


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colloidal nanoparticles are highly crystalline and offer superior optical properties.14,65 Importantly, colloidal nanoparticles make it possible to use LbL assembly in aqueous solutions and require no toxic organic solvents. Third, compared to the preparation of nanoplates using lithography,68 LbL assembly is suitable for both rigid and flexible substrates of arbitrary size, curvature, and material. For example, LbL is even applicable to substrates that degas under high vacuum or are susceptible to high evaporation temperatures. Such versatility is important for specialty applications such as wearable electronics and sensors. Thin-film LbL assembly signifies a completely different approach to preparing plasmonic polymer nanocomposites. In contrast to the conventional approach of randomly mixing fillers in polymers, LbL assembly offers superb control over the distribution and orientation of the AgNPs in each layer. Because of LbL assembly, nanoparticle aggregation and phase-separation12,69 are easily avoided by controlling nanoparticle orientation and confining them to a thin layer. In addition, polyelectrolytes naturally adsorb on the nanoparticle surface and prevent nanoparticle aggregation, especially at high nanoparticle volume fractions. For polymer-grafted anisotropic nanorods in polymer nanocomposites, the nanoparticle volume fraction reaches 16.1% before aggregation.70 In our thin-film polymer composites, however, the surface coverage of AgNPs easily reaches ~55%, and the estimated volume fraction is ~32.4%. The high AgNP surface coverage and volume fraction impart the thin-film polymer nanocomposites with intense plasmonic colors and high tint levels, which are unattainable via the random mixing approach in conventional polymer composites. Moreover, the planar orientation fully exploits the in-plane dipole resonance of the AgNPs and thus provides outstanding optical and plasmonic properties. The tunable intralayer and interlayer distances harness the plasmon coupling among the AgNPs. The spectral selectivity of our plasmonic polymer nanocomposites, enabled by precise control


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over AgNP size and orientation in the composites, is exceptional and tunable across the entire visible and NIR region. The thin-film polymer composites are advantageous compared to existing chromic glass. For example, the state-of-the-art electrochromic glass uses expensive ITO nanocrystals in niobium oxide and shows a high transmittance of ~80% in the NIR with limited tunability in the visible.8 Thermochromic VO2 is yet impractical because 1) it requires a huge temperature gradient from room temperature to ~100 °C to reduce the NIR transmittance; and 2) even with a large temperature gradient, the transmittance remains high (i.e., 45% at 1500 nm). In contrast, our thin-film plasmonic polymer composites (e.g., Figure 2G) show a low NIR transmittance of 28% at 1020 nm. Moreover, the significantly reduced NIR transmittance does not interfere with visible light transmittance, which remains 50-80% in the range of 400–700 nm. Independent control over visible and NIR light allows for the preparation of multi-layer thin-film composites with dual function of color-modulation and NIR-reflection, which are unattainable by other chromic materials. The plasmonic colors generated by our thin-film polymer nanocomposites are shown in the chromaticity CIE 1931 plot (Figure S20). These colors resemble aluminum based plasmonic pixels6 but differ drastically from most metamaterials.5,67,71 Most plasmonic metamaterials are non-transparent and absorb heat because they are based on light reflection and absorption. For example, by depositing a full metal layer followed by an insulating layer and then patterning another layer of metal particles or holes, the metamaterials enhance scattering and absorb a large amount of energy that re-radiates as heat.7,56 Moreover, the plasmonic metamaterials often require expensive lithographic techniques and are limited to small areas. Scalable approaches such as nanoimprinting52 and soft interference lithography67 can prepare metastructures at


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ACS Nano

relatively large scales, but these methods are restricted to flat substrates. Additionally, the deposition of full metal layers, followed by patterning and etching into nanostructures, is similar to, if not more complicated than, the metalized films used in current commercial tinted glass. Nonetheless, these metamaterials are based on the mechanism of extraordinarily low transmission (ELT) and are non-transparent to visible light; thus they cannot be used as tinted glass but rather plasmonically colored displays.72 Our thin-film plasmonic polymer nanocomposites, however, use bottom-up assembly and make it unnecessary to deposit full layers of metal. The thin-film nanocomposites are solely based on the plasmon resonance of presynthesized colloidal AgNPs and exhibit color in the visible range that is significantly sharper than that of the metastructures prepared by soft lithography.51,52 Compared with these metamaterials, the thin-film polymer composites have one or multiple layers of sparsely assembled AgNPs, which allow for independent modulation of multiple LSPR wavelengths. The dispersion of nanoparticles in polymers has always been challenging due to nanoparticle aggregation and phase separation.73,74 In our bulk polymer composites, uniform distribution is achieved by grafting the AgNPs with PMMA (Mn ~60 kDa) and mixing the PMMA-grafted AgNPs in a matrix PMMA (Mn ~75 kDa). Because the degree of polymerization of the matrix PMMA, P, is less than twice the degree of polymerization of the grafted PMMA, N, the grafted and matrix PMMA have good physical interaction.65 In this work, the P/N ratio is approximately 1.25, which results in a so-called wet brush and leads to uniform distribution of nanoparticles in the polymer matrix. The grafted PMMA on the AgNPs is relatively sparse,75 and therefore the AgNPs interact favorably with the matrix PMMA, similar to previous reports about silica nanoparticles.76,77 As a result, the AgNPs are uniformly distributed in the PMMA matrix, suggesting that the composites are in the well-dispersed regime.78 If the AgNPs are grafted with a


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dense layer of polymers, they might potentially be used for creating advanced photonic crystals.79,80 CONCLUSIONS In conclusion, we have created spectral-selective plasmonic polymer nanocomposites using polyelectrolyte-assisted LbL assembly of colloidal 2D AgNPs. By controlling the size, orientation, surface density, and interlayer spacing of the 2D AgNPs, the thin-film polymer nanocomposites exhibit well-controlled plasmonic colors and NIR reflectivity. The planar orientation of the AgNPs plays a critical role in the optical properties and contributes to the strong light reflectance of the nanocomposites. Depending on the AgNP size, the thin-film polymer nanocomposites show colors such as yellow, chocolate, pink, violet, blue, dodger blue, and steel blue. Polymer nanocomposites with AgNPs of large sizes block NIR but transmit visible light, suggesting their potential application in heat-reflecting windows. Our polymer nanocomposites are responsive to multiple wavelengths, and such multi-wavelength responsiveness is a feature that is unattainable by existing polymer nanocomposites. We anticipate the plasmonic polymer nanocomposites to be applied in energy-efficient buildings and vehicles,1 color filters,72 optical coatings,81 plasmonic printings,7 photovoltaics,82-84 and advanced plasmonic constructs.85-88 MATERIALS AND METHODS Materials. Silver nitrate (≥99.9999%) (204390), sodium borohydride (≥99.99%) (480886), sodium citrate tribasic dihydrate (≥99.0%) (S4641), ascorbic acid (≥99.0%) (A5960), benzene (≥99.9%), poly(allylamine hydrochloride) (PAH, average Mw~17,500 g⋅mol-1) (283215), poly(acrylic acid) (PAA, Mv~450,000 g mol-1), 2-phenyl-2-propyl benzodithioate (CDB, ≥99%), 2,2'-azobis(2-methylpropionitrile) (AIBN, ≥98%), and poly(sodium 4-styrenesulfonate) (PSSS,


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ACS Nano

average Mw~1,000 kg⋅mol-1) (434574) were purchased from Sigma-Aldrich and used as received. Plain glass microscope slides (25 × 75 × 1mm) (Cat. No. 12-544-4) were purchased from Fisher Scientific and used as received. Nanoparticle synthesis was carried out in ultrapure deionized (DI) water obtained from Thermo Scientific™ Barnstead™ GenPure™ Pro water purification system at 17.60 MΩ-cm. Synthesis of Ag Nanoplates (AgNPs). AgNPs were synthesized following a seed-mediated method with slight modifications.26,27 The Ag seeds were synthesized as follows. First, 0.25 mL of PSSS (2.6 mM) and 0.3 mL of ice-cold NaBH4 (10 mM) aqueous solutions were added to 5 mL of sodium citrate solution (2.5 mM) under constant stirring. Afterwards, 5 mL of AgNO3 (0.5 mM) was added to the solution at a rate of 2 mL/min using a Cole-Parmer syringe pump. The seed solution was then immediately covered in an Al foil to avoid exposure to light. After 5 min, the stirring was stopped. To synthesize AgNPs, 1.5 mL of 10 mM ascorbic acid solution was added to 254 mL of water under vigorous stirring, followed by the addition of the seed solution (ranged from 60 to 2000 µL) to prepare AgNPs of various sizes. Afterwards, 6 mL of AgNO3 (5 mM) solution was added to the mixture at a rate of 2 mL/min. Finally, 10 mL of sodium citrate (25 mM) solution was added to the product solution to stabilize the AgNPs. To obtain large AgNPs with a plasmon resonance wavelength of more than 800 nm, the Ag seeds were used within 5–10 min after the seed preparation. Using seeds that were aged more than 10 min may cause instability of the synthesis, which was discussed in our previous report.27 Layer-by-Layer Deposition of AgNPs and Polymers. Thin films of plasmonic polymer composites were prepared via layer-by-layer (LbL) using a nanostrata dipping robot. First, two beakers were respectively filled with 100 mL of PAH solution (10 mM, pH = 7) and 100 mL of


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the as-synthesized AgNP colloidal solution. Glass slides were treated with oxygen plasma and dipped in the PAH solution for 5 min, allowing for the positively charged PAH to adsorb onto the glass via electrostatic interactions. As a result, a monolayer of PAH formed on the glass substrates.32,33 The glass substrates were rinsed in DI water three times. After rinsing, the glass substrates were immersed in colloidal solutions of AgNPs for various lengths of time (10–300 min). To assist the deposition of AgNPs, the glass substrates were rotated at a speed of 600 rpm. The AgNPs were negatively charged due to the sodium citrate molecules adsorbed on the surfaces, and therefore they were easily deposited on the glass substrates that had a monolayer of positively charged PAH through electrostatic interaction. Afterwards, the glass substrates were rinsed in DI water three times. To deposit multilayers of AgNPs in the polymer composites, two approaches were used. In the first approach, only PAH and AgNPs were used. Typically, the deposition of PAH for 5 min and the subsequent deposition of AgNPs for 10 min were repeated as required. The deposition of PAH, however, occurred mostly on the prior layer of AgNPs, leading to nanoparticle aggregation (Figure S11). In the second approach, PMMA was added as a spacer between the adjacent AgNP layers. Briefly, after the deposition of PAH and AgNPs, a PMMA layer was applied on top of the film by spin-coating a PMMA solution in chloroform (0.5 wt.%). The PMMA layer was subject to oxygen plasma treatment (PC-200 South Bay Technologies 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 thickness of the PMMA thin layer decreased to

Spectral-Selective Plasmonic Polymer ... - ACS Publications

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