Article Cite This: ACS Nano XXXX, XXX, XXX−XXX www.acsnano.org Spectral-Selective Plasmonic Polymer Nanocomposites Across the Visible and Near- Infrared † † ‡ † § ∥ 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 *S Supporting Information 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 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 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 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 dielectric 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. heat that re-radiates. Instead of full layers of metals, plasmonic fi 5 6,7 carbon 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 oxides. 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 tin-doped indium oxide (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 Plasmonic nanoparticles are emerging nanomaterials 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 liquid crystals 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 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. (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 plasmon resonance (LSPR) wavelengths ( LSPR) and λ polymer nanocomposites via low-cost bottom-up assembly. thus limited spectral selectivity; nanorods 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 polyelectrolytes, 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 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 AgNP-polymer 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. − 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