Broadband Absorbing Exciton–Plasmon Metafluids with Narrow

Letter pubs.acs.org/NanoLett

Broadband Absorbing Exciton−Plasmon Metafluids with Narrow Transparency Windows Jihua Yang,† Nicolaas J. Kramer,† Katelyn S. Schramke,† Lance M. Wheeler,† Lucas V. Besteiro,‡ Christopher J. Hogan, Jr.,† Alexander O. Govorov,‡ and Uwe R. Kortshagen*,† †

Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, United States

‡

S Supporting Information *

ABSTRACT: Optical metafluids that consist of colloidal solutions of plasmonic and/or excitonic nanomaterials may play important roles as functional working fluids or as means for producing solid metamaterial coatings. The concept of a metafluid employed here is based on the picture that a single ballistic photon, propagating through the metafluid, interacts with a large collection of specifically designed optically active nanocrystals. We demonstrate water-based metafluids that act as broadband electromagnetic absorbers in a spectral range of 200−3300 nm and feature a tunable narrow (∼100 nm) transparency window in the visibleto-near-infrared region. To define this transparency window, we employ plasmonic gold nanorods. We utilize excitonic boron-doped silicon nanocrystals as opaque optical absorbers (“optical wall”) in the UV and blue-green range of the spectrum. Water itself acts as an opaque “wall” in the near-infrared to infrared. We explore the limits of the concept of a “simple” metafluid by computationally testing and validating the effective medium approach based on the Beer−Lambert law. According to our simulations and experiments, particle aggregation and the associated decay of the window effect are one example of the failure of the simple metafluid concept due to strong interparticle interactions. KEYWORDS: Colloidal boron-doped silicon nanocrystals, gold nanorods, exciton−plasmon metafluid, transparency window

T

metafluids are usually isotropic and can be created via simpler bottom-up self-assembly approaches.18−21 Here, we report on optical metafluids with broadband optical absorption and a narrow transparency window. Such fluids may find utility as working fluids in hybrid solar-thermal/photovoltaic systems, sensing applications, or as media to produce broadband opaque coatings with narrow optical transmission. Recently, Zhang et al. showed computationally that such metafluids could be composed of gold (Au) nanostructures with a wide spectrum of sizes;22 their additive plasmonic response would result in an overall broadband extinction. They proposed that the omission of certain size groups of these nanostructures would lead to a tunable transparency window. In addition to experimentally realizing this type of metafluid, we expand upon and simplify this approach by using a relatively small number of Au nanorods (NRs) to form an optical metafluid with a transmission window and utilize colloidal excitonic silicon nanocrystals (Si NCs) and the suspending solvent (water) to form optically opaque “walls” surrounding the transparency window. These walls absorb all photons with wavelengths outside of the window. To our knowledge, this is the first

uning of the absorptive and emissive properties of materials enables a number of emergent technologies in diverse fields, including optics,1 sensing,2 lighting,3,4 and renewable energy generation.5−7 A relatively established approach to tailoring the optical properties of materials utilizes photonic crystals, which are highly ordered, periodic nanostructures with different refractive indices and a periodicity on the order of the light wavelength.8−10 Such crystals can form photonic band gaps, that is, ranges of photon energies that are not transmitted by the crystal. A second approach utilizes the resonantly enhanced properties of plasmonic nanoparticles, whose surface-localized collective oscillations of the conduction electrons can be tuned over a wide range of the electromagnetic spectrum via control over particle size and shape.11−14 Welldesigned assemblies of such plasmonic nanoparticles can exhibit narrow transparency windows due to plasmonic Fano resonances,15−17 which result from the destructive interference between broad superradiant bright and the narrow subradiant dark plasmonics modes. Such structures have attracted strong interest for applications in nonlinear optics, optical switching, and sensing. The above approaches require engineered nanostructures that are often manufactured by top-down techniques, such as lithography or multilayer stacking, often on rigid substrates. The optical properties of these structures may also be strongly anisotropic (variable with orientation). Conversely, optical © XXXX American Chemical Society

Received: December 16, 2015 Revised: January 21, 2016

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The wave transmission for our purpose requires a unique feature−a narrow transparency window. The formation of the transparency window is based on the Beer−Lambert law

experimental report of a broadband opaque metafluid with a narrow transparency window. The concept of an electromagnetic metafluid realized in this study is illustrated in Figure 1. It is based on the Beer−Lambert

T=

Itransmitted = 10−OD I0

where the incident and transmitted intensities are denoted as I0 and Itransmitted, respectively. The optical density is defined as OD = Lpath

∑i ni ·σi̅ ln[10]

where ni and σ̅i are the concentration and the extinction cross section of the ith component of a metafluid, and Lpath is the optical path. The extinction σ̅i should be averaged over all orientations, because we typically utilize anisotropic nanoparticles. In Figure 1, each NR is illustrated with a “screen” equal to its extinction cross section. The extinction cross section depends on the wavelength. NRs have strongly resonant cross sections and therefore we can design a solution in which NRs screen all wavelength intervals except for one narrowwindow interval. When the incident wavelength is outside the window wavelength, NRs in the metafluid block all incident photons (the screens of single NRs cover the entire crosssectional plane of incidence and the metafluid does not transmit the light). When the metafluid does not contain NRs with a resonance at λphoton ≈ λwindow, the system is partially transparent. Au NRs are used to define the transparency window because of their dispersibility in water and strong and narrow longitudinal (LO) plasmonic resonance with the wavelength tunable via shape and size.11,12 Figure 2a shows the spectral extinction of four selected types of Au NRs. We first focus on two samples with sizes of 25 nm (diameter) × 36 nm (length) (sample a) and 11 nm × 45 nm (sample b). These Au NRs have surface LO plasmonic resonance bands at 550 and 850 nm, respectively. The transmittance spectrum of a mixture with concentrations of the components of 0.06 mg/mL (sample a) and 0.042 mg/mL (sample b), is shown in Figure 2b, corrected for absorption by water. Combining the two samples leads to a well-pronounced transparency window centered at around 680 nm, referred to as window A, yet there is still significant

Figure 1. Illustration of the concept of the “simple” electromagnetic metafluid with a transparency window. (a) Plasmonic NRs block all incident photons. (b) Metafluid does not contain NRs with the resonance at λphoton ≈ λwindow and the system is spatially transparent.

law and assumes that the elements of the metafluid do not interact directly with each other via Coulomb and electromagnetic forces. However, an incident electromagnetic wave interacts with all elements in an optically thick layer of the fluid. If the resulting transparency obeys the Beer−Lambert law, we consider this a “simple” metafluid. If the elements of a metafluid are aggregated or assembled, the Beer−Lambert law may not apply as it happens for an aggregated state as shown below.

Figure 2. (a) Extinction spectra of four types of Au NRs with various sizes and plasmonic resonance wavelengths: 25 nm (diameter) × 36 nm (length) (sample a), 10 nm × 45 nm (sample b), 10 nm × 58 nm (sample c), 10 nm × 81 nm (sample d). The plasmonic resonances are positioned at 550, 850, 980, and 1200 nm, respectively. (b) Transmittance spectra of three solutions made from mixtures of Au NRs with various plasmonic resonances: window A, Au NRs resonating at 550 nm (0.06 mg/mL) + Au NRs resonating at 850 nm (0.042 mg/mL); window B, window A + Au NRs resonating at 980 nm (0.050 mg/mL); window C, window B + Au NRs resonating at 1200 nm (0.042 mg/mL). (c) TEM image of the four types of Au NRs from a dried droplet of diluted window C solution. B

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Nano Letters transmission in the blue-green and infrared range. The spectrum of window A is essentially the superposition of spectra of the two Au NRs components, which indicates that Coulomb and electromagnetic interactions between Au NRs is negligible due to the large interparticle distance in solution. The lack of Au NR plasmonic interactions can be utilized to design an absorption spectrum as superposition of component spectra. Figure 2b demonstrates how near-infrared absorption can be improved by subsequently adding Au NRs resonating at 980 (sample c) and 1200 nm (sample d) from Figure 2a. Window B is formed by combining Au NRs of sample c (0.050 mg/mL) with window A. The transmission in the longwavelength region is clearly reduced. By additionally adding Au NRs of sample d (0.042 mg/mL) the transmittance in the longwavelength region is completely suppressed except for a small tail (window C). A transition electron microscope (TEM) image of the Au NRs used to produce window C is shown in Figure 2c. Currently, Au NRs are available with strong LO resonances at wavelengths larger ∼525 nm. However, Au NRs of any size do not provide efficient absorption at shorter wavelengths; this results in residual transmission at short wavelength (Figure 2b). It is thus desirable to utilize another type of optical absorber for the UV-blue-green range. This material must be codispersed with Au NRs in water, that is, the two material types must be stabilized from aggregating with one another. For this purpose, we apply excitonic Si NCs, as they have strong absorption in the UV and blue spectral ranges with an almost exponential decay in absorption with increasing wavelength, which is due to silicon’s indirect bandgap character.23,24 While typically Si NCs are not easily dispersed in water, the Si NCs used in this study were produced by nonthermal plasma synthesis (depicted in Figure S1); nonthermal plasmas can be used to produce both intrinsic as well as phosphorus (P) and boron (B) doped Si NCs.25−27 Recently, Sugimoto et al. revealed that codoping of Si NCs with B and P aids in dispersing Si NCs in polar solvent with long-time stability achievable.28 We have additionally found that nonthermal plasma-synthesized 10% (nominally) Bdoped Si NCs can be well dispersed in dimethyl sulfoxide (DMSO) and that the Si NC-DSMO suspension can then be mixed with aqueous suspensions without appreciable aggregation of the Si NCs or the Au NRs. We attribute the colloidal stability of B-doped Si NCs to the ability of B at the Si NC surface to undergo Lewis acid-base interaction with DMSO, similar to a mechanism that we had previously found for chlorine terminated Si NCs.29 As both Au NRs and B-doped Si NCs are able to form stable colloidal solution in water, water itself can be used as infrared absorbing “wall” material. Figure 3a shows an image of a clear colloidal solution of 10 nm B-doped Si NCs in deionized water (0.2 mg/mL). Figure 3b shows the transmittance spectra of the B-doped Si NC colloidal water solutions at various selected concentrations. Bdoped Si NCs have excellent transparency in green-to-near IR region (>97%) but strong extinction in the UV-blue region. The rapid increase of the transmission in the visible range indicates that there is a negligible amount scattering and hence minimal Si NC aggregation. We also compared the transmittance spectra of the B-doped Si NCs in pure DMSO and in DMSO/water at concentrations of 0.04 and 0.008 mg/mL but did not find any differences, supporting the lack of aggregation in DMSO/water. The B-doped Si NCs exhibit a high optical extinction coefficient, such that a solution with a concentration of 0.2 mg/mL almost completely absorbs light below 420 nm.

Figure 3. (a) Image of a B-doped Si NC colloidal water solution with a concentration of 0.2 mg/mL. The solution was formed by adding 10 μL of DMSO colloidal solution of B-doped Si NC (50 mg/mL) into 2940 μL of water. (b) Transmittance spectra of B-doped Si NCs colloidal water solutions with various concentrations: black solid line, 0.008 mg/mL; red solid line, 0.04 mg/mL; red dashed line, 0.04 mg/ mL after 2 days in air; blue solid line, 0.2 mg/mL. Inset: TEM image of B-doped Si NCs dried from a droplet of colloidal water solution.

Furthermore, transmittance of the B-doped Si NC colloid did not change after 2 days in air, as evidenced by the dotted curve in Figure 3b. Shown in the inset of Figure 4a is the image of a clear colloidal solution mixture of B-doped Si NCs at a concentration of 0.2 mg/mL and four types of Au NRs at the same concentrations as those in window C, as well as an overview TEM image of the B-doped Si NCs and four types of Au NRs. Figure 4a is the transmittance spectrum of this colloid, referred to as window D. The spectrum includes the extinction of water as a solvent, via using an empty quartz cuvette as the reference. The addition of B-doped Si NCs eliminates the residual transmission of window C in the UV-blue-green range. Water acts as an opaque wall in the near IR range of the spectrum. The transparency window has a transmittance of 39% at 681 nm and a window width of 0.38 eV. The position and width of the transparency window can be modified by selective use of different Au NRs (Figure S2). In Figure 4b, we display the transmission spectrum of a metafluid with almost complete opacity in the visible range of the spectrum and a narrow near IR transparency window. The infrared transparency window (window G) is made from a mixture of B-doped Si NCs (0.2 mg/mL) and five types of Au NRs resonating at 550, 662, 678, 780, and 1200 nm. Guided by the superposition of the absorption of the individual components (inset in Figure 4b), the resulting window has a maximum transmittance of 40% at 890 nm and a narrow window width of 0.22 eV. The small residual signals at 600, 710, and 1294 nm could be eliminated by finely adjusting the spectra of single Au NRs or introducing a small amount of Au NRs with plasmonic resonance peaks at those positions. An alternative way to realizing a high-transmittance IR window could be developing semiconductor NCs with their excitonic absorption shifted toward the IR region. We finally study the limitations of the concept of the simple metafluid. Aggregation of NRs leads to interparticle plasmonic interactions that cause deviations from the Beer−Lambert law. As shown in Figure 5a, the aggregation of Au NRs, for instance, by extensive centrifugation to remove some of the CTAB ligands stabilizing the Au NRs, obviously diminishes the C

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Figure 4. (a) Transmittance spectrum of window D made from the mixture solution of four types of Au NRs (at the same concentrations as those in window C) and B-doped Si NCs (at a concentration of 0.2 mg/mL). Insets: Image of window D solution and overview TEM image of B-doped Si NCs and four types of Au NRs from a dried droplet of diluted window D solution. (b) Transmittance spectrum of an infrared solution transparency window (window G) made from a mixture of B-doped Si NCs (0.2 mg/mL) and five types of Au NRs resonating at 550 nm (25 nm × 36 nm, 0.06 mg/mL), 662 nm (25 nm × 64 nm, 0.06 mg/mL), 678 nm (10 nm × nm 31 nm, 0.042 mg/mL), 780 nm (10 nm × 38 nm, 0.042 mg/mL) and 1200 nm (10 nm × 81 nm, 0.042 mg/mL). Insets: Image of window G solution and single transmittance spectra of the five types of Au NRs for window G.

Figure 5. (a) Effect of aggregation of Au NRs on transmittance efficiency of solution window. (b) Computational transmissions of the Au-NR clusters which are shown in the inset. The cluster system is composed of eight Au-NRs with the sizes equal to the experimental ones as given in the caption of Figure 2. In one cluster, we included two Au-NRs of each size. The optical path was assumed to be 1 cm and the concentration of NRs of one kind was assumed 3.7 × 10−10 M.

transparency window effect because the elements do not act independently in the aggregated state. Computationally, we investigate the interparticle interaction as shown in Figure 5b. We generate clusters of NRs with random distributions and vary the NR density in a cluster. The nonaggregated state has broadly dispersed NRs and a well-defined optical window, however, the aggregated state exhibits a strong interaction between NRs and leads to significant reduction of the window effect. Interestingly, we observe in our calculations that the Beer−Lambert law is very robust and breaks down only for very small interparticle separations or for touching NRs. This tells us that the window effect in a metafluid can be rather robust unless the nanocrystal elements start to aggregate or touch. It is interesting to compare our concept of the window effect with traditional band-pass filters based on the interferometry or the Bragg reflection. Our metafluid is isotropic and threedimensional, while a band-pass filter is a layered, strongly anisotropic system. The technology of fabricating band-pass filters obviously involves a set of reflecting planes or lenses, whereas a metafluid with a window effect can be fabricated simply by mixing of nanocrystals. In summary, we demonstrated here a concept of forming simple metafluids with isotropic broadband optical absorption and narrow, tunable transparency windows. This concept is

based on solutions of excitonic semiconductor NCs, plasmonic metal NRs, and the optical properties of the solvent itself. We demonstrated that at sufficiently low concentrations, the electronic coupling among the individual components is weak and that the optical properties can be designed by the superposition of the transmittance spectra of the individual components. In our practical implementation, we used aqueous solutions of B-doped Si NCs and Au NRs to realize the metafluids. Au NRs were used to define a narrow transparency window. B-doped Si NCs served as short-wavelength opaque “wall” material. The absorption of water in the IR range provided the needed “wall” at long wavelengths. We demonstrated tunable narrow visible-near IR transparency windows with a broadband extinction from 200 to 3300 nm (instrument limits). Obviously, this concept can be extended to other excitonic and plasmonic materials as well as solvents to realize broadband absorbing metafluids with narrow transparency windows over wide spectral ranges. Methods. Materials. The CTAB-caped Au NRs in water solutions were purchased from Nanopartz. To control the concentrations of the Au NRs for the preparation of mixtures with transparency windows and optical measurements, the Au NR solutions were centrifuged at 8000 rpm for 20 min, followed by the redispersion of the Au NRs into water or D

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Nano Letters mixing at desirable concentrations for the fabrication of solution transparency windows. B-doped Si NCs were synthesized in a nonthermal plasma as described in refs 26 and 27. A mixture of argon (35 standard cubic centimeters per minute (sccm)), silane (0.5 sccm), and diborane (10% in hydrogen) (0.3 sccm) was introduced into a quartz tube reactor (see Supporting Information Figure S 1), with a power of 110 W at 13.56 Hz applied to a pair of 1 cm-separated ring electrodes. The pressure in the plasma reactor was maintained at 133 Pa. The 10% (nominal) B doping concentration leads to excellent dispersibility of the Si NCs in polar solvents such as DMSO. The parent B-doped Si NC colloidal solutions were prepared at a NC concentration of 50 mg/mL by adding DMSO to the as-synthesized B-doped Si NC powders. For spectral measurements, different concentrations of B-doped Si NC colloidal solutions can be made from this parent solution via dilution by which, for example, a colloidal solution of 10 nm B-doped Si NCs (0.2 mg/mL) in deionized water was made by dispersing B-doped Si NCs/DMSO solution in water at a volume ratio of 1:249. To disperse B-doped Si NCs into water at a concentration of 0.2 mg/mL, 10 μL of the B-doped Si NC parent colloidal solution was added into 2490 μL of water and a clear colloidal solution of B-doped Si NCs in water (with trace amount of DMSO) was obtained after slight agitation by simple shaking of the vial. Lower concentration of the B-doped Si NCs colloidal water solution can be obtained by diluting the 0.2 mg/ mL solution in respectively larger amounts of water. Window Fabrication. The solution transparency windows were fabricated by mixing a B-doped Si NC colloidal solution predispersed in water with a mixture solution of one-five types of Au NCs selected with desirable plasmonic resonances for transparency at certain wavelength. Characterization. The optical measurements were carried out using a Cary 5000 UV/vis/NIR Spectrophotometer (Agilent Technologies) for which the solutions were loaded in a quartz cuvette with a 1 cm light path. For the single Au NRs or their mixtures, the transmittance spectra were measured with deionized water as a reference sample. The transmittance spectra of B-doped Si NCs in DMSO or water (including trace amount of DMSO) were measured with DMSO or deionized water including the same trace amount of DMSO as a reference sample. For characterization of solution transparency windows, the transmittance spectra were measured with an empty cuvette as reference. TEM measurements were performed on a FEI Tecnai T12 at an operating voltage of 120 kV. For TEM measurements, samples are prepared by dropping a droplet of the dilute solution onto a copper TEM grid covered with a holey carbon film. The samples were then transferred into a vacuum for drying before measurements. Calculations. To compute nonaggregated and aggregated complexes in Figure 5, we used the standard COMSOL software and the empirical dielectric function of gold. The matrix was assumed to be water with the optical dielectric constant 1.77.

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NCs); transmittance spectra of solution transparency windows made from the mixtures of B-doped Si NCs (0.2 mg/mL) and various gold nanorods (Au NRs). (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was a part of the Multidisciplinary University Research Initiative (MURI) program grant. It was primarily supported by the Army Office of Research under MURI Grant W911NF-12-1-0407. Part of this work was carried out in the College of Science and Engineering Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program under Award Number DMR-1420013. L.V.B. and A.O.G. also thank the Volkswagen Foundation for its support.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b05142. Schematic and image of the nonthermal plasma reactor for synthesis of boron (B)-doped silicon nanocrystals (Si E

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