Enhanced Triplet–Triplet Annihilation Upconversion in Dual-Sensitizer

Article pubs.acs.org/journal/apchd5

Enhanced Triplet−Triplet Annihilation Upconversion in DualSensitizer Systems: Translating Broadband Light Absorption to Practical Solid-State Materials Anna L. Hagstrom, Fan Deng, and Jae-Hong Kim* Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States S Supporting Information *

ABSTRACT: Photochemical upconversion (UC) of lowenergy photons that would otherwise be wasted could drastically improve the efficiency of solar technologies by allowing them to harness a greater fraction of the solar spectrum. Although UC through the triplet−triplet annihilation (TTA) mechanism operates efficiently under low-power irradiation such as sunlight, its ability to improve solar device efficiencies is limited by the narrow light absorption bands of its sensitizer chromophores. This bottleneck on UC performance can be overcome by employing multiple sensitizers in tandem, but such an approach has thus far been studied exclusively in solution-based TTA-UC systems requiring intensive deoxygenation and sealing procedures. This study presents the first dual-sensitizer TTA-UC system in a solid-state host suitable for practical applications. We fabricate thin polyurethane films containing two benchmark TTA-UC sensitizers in a range of different concentrations and characterize their red-to-blue and green-to-blue UC performance as a function of excitation intensity. The broadband absorption of the dual-sensitizer films significantly enhances their performance under simultaneous low-intensity excitation of the two sensitizers, giving rise to anti-Stokes fluorescence surpassing the combined anti-Stokes fluorescence of the films’ single-sensitizer analogues. We circumvent trade-offs between light absorption and TTA-UC performance at high sensitizer concentrations by harnessing the films’ unique versatility to produce an alternative “multijunction” TTA-UC system comprising overlaid single-sensitizer films, thereby achieving strong broadband light absorption and superior TTA-UC performance. KEYWORDS: triplet−triplet annihilation, upconversion, broadband absorption, solid-state, polymer films

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existing UC systems prevent them from realizing their transformative potential.5,6 Lanthanide-based UC, the first of the two primary UC mechanisms to be integrated with solar devices,7 relies on lanthanide ions to absorb and upconvert low-energy photons.8 The parity-forbidden nature of these ions’ 4f transitions, which gives rise to the long excited state lifetimes necessary for UC, limits them to weak light absorption (i.e., absorption cross sections of ca. 10−20 cm2) over narrow wavelength ranges (1 in Figure S4). Characterization of Dual-Sensitizer TTA-UC Performance in Polymer Films. To analyze the analogous phenomena within a solid-state TTA-UC host, we fabricated thin polyurethane films encased in glass using a simple open-air process in which the polymer precursor solution was doped with chromophores prior to polymerization, enabling relatively precise control over chromophore concentrations.36 The chain mobility that facilitates efficient TTA-UC within rubbery polymers also makes these polymers inherently vulnerable to infiltration by oxygen, which quenches 3S* and 3A* states and drastically lowers TTA-UC efficiency.34 By encasing our films in thin sheets of glass, we provide robust oxygen protection that can be easily reinforced by sealing the film edges with epoxy. We have previously shown this approach to impart superior long-term stability to polyurethane TTA-UC films, enabling them to retain their original UC abilities even after a month of exposure to open-air conditions.23 In the present study, we held the concentration of perylene in the films constant at 18 mM (0.43 wt %), which is comparable to the concentrations that had proven successful in a prior study employing the same polymer.23 Because TTET and TTA are Dexter energy transfers requiring close chromophore proximity,37,55 TTAUC systems in rubbery polymers require relatively high acceptor concentrations to compensate for their restricted chromophore mobility. We employed PtOEP and PdTPBP E

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analogous solution-based systems in Figure 2c. These films serve as a valuable point of comparison to their solution-based counterparts in light of their comparable sensitizer-to-acceptor ratio (i.e., 0.05 mM/18 mM ≈ 3 μM/1 mM). Despite their low sensitizer concentrations, they emit robust UC fluorescence under excitation at intensities of the same order of magnitude as the intensities of red and green solar radiation in the ranges of sensitizer absorption (Figure S2b). However, films this dilute capture only a small fraction of incident light (Figure S8a); practical application of TTA-UC films necessitates much higher sensitizer concentrations. Increasing PtOEP and PdTPBP concentrations from 0.05 mM to 0.45 mM in order to better harvest incident radiation (Figure S8b) gave rise to dramatically different trends in TTAUC performance. The power-dependent UC performance of dual- and single-sensitizer films containing 0.45 mM PtOEP and/or 0.45 mM PdTPBP (Figures 4a and S7c,d) echoed that of their solution-based (Figures 2b and S3) and dilute film (Figures 3a and S7a,b) counterparts. Due to the intrinsic dynamics of TTA-UC discussed above, the concentrated dualsensitizer films likewise exhibited enhanced performance under low-power simultaneous excitation (Figure 4a). As a result of their high sensitizer concentrations, the light harvesting of these films surpassed that of their solution-based predecessors (Figures S5 and S8b), giving rise to significantly stronger UC fluorescence emission under equivalent excitation (Figure S10a and b). However, unlike the dual-sensitizer systems characterized thus far, which had both emitted much more TTA-UC fluorescence than their two component single-sensitizer analogues combined (Figures 2c and 3b), the dual-sensitizer films containing 0.45 mM PtOEP and 0.45 mM PdTPBP emitted slightly less UC fluorescence than the single-sensitizer films containing 0.45 mM PtOEP (Figure 4b). Whereas the PtOEP and PdTPBP single-sensitizer controls in the other systems had consistently emitted UC fluorescence similar in magnitude (Figures 2c and 3b), the single-sensitizer films containing 0.45 or 0.90 mM PdTPBP emitted far less UC fluorescence than their PtOEP counterparts (Figure 4b). Moreover, the single-sensitizer films containing 0.90 mM PdTPBP emitted slightly less UC fluorescence than those containing 0.45 mM PtOEP (Figure 4b). These results collectively suggest that high PdTPBP concentrations proved detrimental to the films’ TTA-UC performance. We corroborated this interpretation through a cursory investigation of the effect of sensitizer concentration on the relative performance of dual- and single-sensitizer films under simultaneous red (9.6 mW cm−2) and green (11 mW cm−2) irradiation (Figure S11). As we gradually increased the PtOEP and PdTPBP concentrations in these films from 0.05 mM up to 0.90 mM, the UC fluorescence of the single-sensitizer PtOEP films increased continuously (Figure S11b). The UC fluorescence of the single-sensitizer PdTPBP films and the dual-sensitizer films, on the other hand, only increased for concentrations up to 0.45 mM; beyond this point, higher sensitizer concentrations gave rise to decreased UC fluorescence (Figure S11a and c). As sensitizer concentrations increased, the UC fluorescence emission of the PtOEP films eventually surpassed that of their dual-sensitizer counterparts (Figure S11d−h). We attribute the detrimental effect of high PdTPBP concentrations in these films in part to parasitic reabsorption of UC fluorescence by PdTPBP and/or energy back-transfer from perylene to PdTPBP as a result of the overlap between the

Figure 4. (a) Normalized average peak intensity of UC fluorescence emitted by concentrated dual-sensitizer films under red, green, and simultaneous red and green laser irradiation (powerred/powergreen = 0.9) as a function of combined red and green laser power density. The dashed gray line marks the sum of the emission from red and green excitation individually, and the right axis shows the percent enhancement from simultaneous excitation as defined by eq 1. Each point is the average of the UC emission resulting from irradiation of at least three different spots on each of three different films. (b) Average UC emission spectra of concentrated dual- and single-sensitizer films under simultaneous red (9.6 mW cm−2) and green (11 mW cm−2) laser irradiation. Each spectrum is the average of spectra obtained from irradiation of at least three spots on each of six different films. Inset: Average area of these UC emission spectra. See Figure S9a for photographs of the films. (c) Normalized average Stokes fluorescence from concentrated dual- and single-sensitizer films under direct excitation of perylene at 350 nm using a spectrofluorophotometer. Each spectrum is the average of spectra obtained from a single scan of at least seven different films. Inset: Average area of these emission spectra. All error bars and shaded regions denote one standard deviation. F

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than those containing 0.45 mM PdTPBP (Figure S16). We compared the extent to which the perylene in these films quenched the phosphorescence of PtOEP and PdTPBP and concluded that the simultaneous presence of both sensitizers in the dual-sensitizer films did not have a measurable effect on the efficiency of their TTET to perylene (Figure S14). However, if significant back-transfer from PdTPBP to perylene occurs within the dual-sensitizer films, this PtOEP−PdTPBP interaction could still conceivably interfere with the films’ TTA-UC performance. The relative intensity of PdTPBP phosphorescence was significantly higher in the dual-sensitizer TTA-UC films as compared to their counterparts without perylene (Figure S13), providing further evidence that PdTPBP causes UC fluorescence reabsorption and/or energy back-transfer within the concentrated dual-sensitizer films. Comparison with TTA-UC Performance of Overlaid Single-Sensitizer Films. With the goal of maintaining the strong broadband light absorption abilities of the dual-sensitizer films while minimizing the detrimental effects of high PdTPBP concentrations and precluding potentially unfavorable interactions between PtOEP and PdTPBP, we herein propose an alternative approach employing overlaid PtOEP and PdTPBP single-sensitizer films. This approach capitalizes on the configurational flexibility unique to solid-state TTA-UC systems; thin polymer films, unlike sealed solution chambers, can be overlaid very easily. In these layered film assemblies, which operate under the same principle as multijunction solar cells, the individual layers target distinct wavelength ranges. As the photographs in Figure 5a illustrate, the PdTPBP films emit UC fluorescence under red irradiation and the PtOEP films emit UC fluorescence under green irradiation, so all layers emit UC fluorescence under simultaneous irradiation. To test the efficacy of this approach, we compared the UC fluorescence emitted by pairs of 0.45 and 0.90 mM single-sensitizer films under simultaneous red (9.6 mW cm−2) and green (11 mW cm−2) irradiation to the UC fluorescence emitted by a pair of dual-sensitizer films containing 0.45 mM PtOEP and 0.45 mM PdTPBP (Figures 5b and S9). The paired 0.90 mM singlesensitizer films, which contained the same total sensitizer concentration as the paired dual-sensitizer films, emitted the most UC fluorescence, followed by the paired 0.45 mM singlesensitizer films (Figure 5c), which outperformed the paired dual-sensitizer films despite containing only half of their total sensitizer concentration. The order of the two single-sensitizer films significantly impacted the UC fluorescence emission of these pairs, and the superior performance of the pairs with the PdTPBP film placed behind the PtOEP film (i.e., farther from the light source) supported our hypothesis that high concentrations of PdTPBP give rise to significant reabsorption of UC fluorescence. In a previous study, we found that TTA-UC films provided the greatest enhancement in a solar cell’s photocurrent production when placed behind the solar cell in conjunction with a reflector.23 To test the performance of our paired singlesensitizer films in a configuration consistent with this optimized architecture, we herein collected photoluminescence from the side of the films closest to the light source (Figures 5b and S1). We measured the UC fluorescence emission of both possible arrangements of the 0.45 and 0.90 mM single-sensitizer films under simultaneous irradiation (Figure 5b). In both cases, the pair with the PdTPBP film placed in front of the PtOEP film emitted less UC fluorescence than its counterpart with the PtOEP film placed in front of the PdTPBP film (Figure 5c).

wavelengths of perylene emission and PdTPBP Soret band absorption (ca. 450 nm, Figure 1a). The wavelengths of PtOEP Q-band absorption overlap with perylene emission to a much smaller extent (ca. 500 nm, Figure 1a). To probe the effects of this spectral overlap, we compared the Stokes fluorescence emission of the concentrated dual- and single-sensitizer films under direct excitation at 350 nm, where perylene alone absorbs significantly, to that of control films containing only perylene. While high concentrations of either sensitizer gave rise to somewhat decreased perylene emission, the impact of high concentrations of PdTPBP was far more severe (Figure 4c), which was consistent with the relative intensities of the PtOEP and PdTPBP films’ UC fluorescence in Figure 4b. This was likely a consequence of secondary inner filter effects; due to the overlap between perylene emission and PdTPBP Soret band absorption, we expect greater reabsorption of perylene fluorescence to occur in films containing high concentrations of PdTPBP (see absorption spectra in Figure S8b). The observed trend in perylene fluorescence could likewise reflect energy back-transfer from perylene to PdTPBP due to resonance between perylene fluorescence and PdTPBP absorption. Acceptor-to-sensitizer back-transfer is known to impair the performance of TTA-UC systems containing high sensitizer concentrations.35,38 Past studies have attributed diminished UC fluorescence in such systems to Förster-type back-transfer between resonant acceptor and sensitizer singlet states33,46,47 and to TTET from acceptors to sensitizers with triplet states close in potential,38 both of which could conceivably occur within our concentrated TTA-UC films. The detrimental effect of high PdTPBP concentrations on our films’ TTA-UC performance is likely also due to quenching between neighboring PdTPBP 3S* states. PdTPBP, like most TTA-UC chromophores,35 tends to aggregate when doped into solid materials in high concentrations, thereby hindering TTET to surrounding acceptors. A prior study employing high concentrations of PdTPBP in polyphenylenevinylene films,42 one of only a handful of studies to employ this sensitizer in a solid matrix rather than in solution,23,42,56,57 found quenching processes within PdTPBP aggregates to consume most PdTPBP 3S* states before they could interact with nearby acceptors. Analogous aggregation and quenching could potentially have occurred in our polyurethane films, interfering with TTET to perylene and contributing to the unexpectedly poor performance of films containing high PdTPBP concentrations. Examination of the phosphorescence of the concentrated TTA-UC films provided evidence of interaction between PtOEP and PdTPBP within the dual-sensitizer films. Under green irradiation, the dual-sensitizer films emitted considerable PdTPBP phosphorescence (Figures S12−S14), but the singlesensitizer PdTPBP films emitted none (Figure S12f), which indicates that a transfer of energy took place from PtOEP to PdTPBP. As the same phenomenon occurred in control films containing identical sensitizer concentrations but no perylene (Figures S13 and S14), this intersensitizer interaction did not appear to require perylene as a mediator. It likely consisted of either TTET between the triplet excited states of PtOEP and PdTPBP or reabsorption of PtOEP phosphorescence by the Qband of PdTPBP (Figure S15). Either scenario could explain why, under simultaneous red and green irradiation, the films containing 0.45 mM PtOEP and 0.45 mM PdTPBP emitted slightly less PtOEP phosphorescence than those containing 0.45 mM PtOEP and slightly more PdTPBP phosphorescence G

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Figure 5. (a) Schematic and photographs of four overlaid single-sensitizer TTA-UC polymer films under red (ca. 830 mW cm−2), green (ca. 910 mW cm−2), and simultaneous laser excitation. Photographs were taken through a 600 nm shortpass filter and a 532 nm notch filter to exclude the laser wavelengths. (b) Schematic illustrating the configurations of the five different film pairings whose UC fluorescence emission was compared. The films labeled “0.45 mM dual” contain 0.45 mM PtOEP and 0.45 mM PdTPBP. (c) Average UC emission spectra of the film pairs from (b) under simultaneous red (9.6 mW cm−2) and green (11 mW cm−2) laser irradiation. Each spectrum is the average of spectra obtained from irradiation of at least three spots on each of six different film pairs. Inset: Integrated area of these average UC emission spectra. (d) Normalized average area of the UC emission spectra achieved by overlaying increasing numbers of individual films (left panel) and single-sensitizer film pairs (right panel) under simultaneous red (9.6 mW cm−2) and green (11 mW cm−2) laser irradiation. Each point is the average of the UC emission resulting from irradiation of at least three spots on at least three different film combinations. Error bars and shaded regions denote one standard deviation. All films contain 18 mM perylene.

the poor performance of the dual-sensitizer films and that additional factors like back-transfer from perylene to PdTPBP are most likely at play, though further study is necessary to confirm the relative contributions of these phenomena. We note that for other applications, collecting photoluminescence from the side of the films opposite the light source could be preferable; our results suggest that, in this case, placing the PdTPBP film closer to the light source would provide better TTA-UC performance. Layering progressively greater numbers of films approximated the effect of increasing the films’ thickness, thereby increasing their light absorption and UC fluorescence emission without any need to further increase their sensitizer concentrations. We measured the UC fluorescence emission of each individual type of concentrated dual- and singlesensitizer film under simultaneous red (9.6 mW cm−2) and green (11 mW cm−2) irradiation as a function of the number of layered films (Figure 5d, left panel). We did the same for the 0.45 and 0.90 mM single-sensitizer film pairs with the PtOEP film placed in front, measuring UC fluorescence emission as a function of the number of layered film pairs (Figure 5d, right panel). Because each film already absorbs a significant fraction of the incident laser radiation (Figure S8b), increasing the number of overlaid films gave rise to diminishing gains in

This effect was particularly pronounced for the pair of 0.90 mM single-sensitizer films (Figure 5c). Placing the PdTPBP film in front (1) required laser irradiation to pass through the PdTPBP film in order to reach the PtOEP film and (2) required UC emission from the PtOEP film to pass back through the PdTPBP film in order to reach our photoluminescence detection setup (Figure S1). While the PdTPBP films’ marginal absorption at 532 nm (Figure S8b) could slightly decrease the amount of excitation reaching the underlying PtOEP films, the resulting impact on UC fluorescence emission would be minor. The drastically decreased UC fluorescence emission of the 0.90 mM pair with the PdTPBP film placed in front most likely resulted primarily from strong reabsorption of the UC fluorescence of the PtOEP film by the PdTPBP film (Figure S8b). In Figure 5c, evidence of this reabsorption can be seen in the slight red-shifting of the peak (ca. 470 nm) and the highenergy edge (ca. 450 nm) of the UC fluorescence emission from the film pairs with the PdTPBP film placed in front (teal spectra) with respect to that of their counterparts with the PtOEP film placed in front (orange spectra). Interestingly, the 0.45 mM single-sensitizer film pair with the PdTPBP film placed in front emitted far more UC fluorescence than a single dual-sensitizer film. This suggests that the secondary inner filter effect of PdTPBP reabsorption is probably not the lone cause of H

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performance and the UC fluorescence emission of each individual type of film began to plateau at four films or fewer (Figure 5d, left panel). As more pairs of single-sensitizer films were overlaid and light harvesting was maximized, the UC fluorescence emission of the 0.45 mM pairs eventually overtook that of the 0.90 mM pairs, marginally surpassing the highest UC fluorescence emitted by layered 0.90 mM PtOEP singlesensitizer films (Figure 5d). This overlay approach has the added benefit of allowing for independent optimization of the PtOEP and PdTPBP concentrations within their respective single-sensitizer films, a simpler task than the optimization of their concentrations within the dual-sensitizer films. Overlaying single-sensitizer films containing optimized chromophore concentrations should ultimately yield even greater improvements in TTA-UC performance.

Preparation of TTA-UC Films. Polyurethane precursor Clear Flex 50 (CLRFLX) was purchased from Smooth-on, Inc. in the form of two components: component A, 4,4′methylenebis(cyclohexyl isocyanate), and component B, a polyester polyol. Precursor mixtures for the TTA-UC films were prepared by adding appropriate aliquots of the chromophore stock solutions to a mixture of CLRFLX components A and B (A:B = 1:2 by volume). To enable comparison between films with different sensitizer concentrations within the sets of dilute and concentrated films, pure THF was added to compensate for disparities in the requisite sensitizer stock solution volumes and ensure that all mixtures contained equal polymer precursor concentrations. The resulting mixtures were mixed thoroughly through vortexing and ultrasonication (ca. 5 min). Individual films were then fabricated by casting 30 μL of precursor mixture onto a round microscopy cover glass (Harvard Apparatus, diameter 12 mm) and carefully placing a second cover glass on top to produce a flat, level surface. Films were left to cure at room temperature in the dark for ca. 12 h and subsequently heated at 60 °C for ca. 48 h to remove residual THF. Spectroscopic Characterization of TTA-UC Samples. Static absorption spectra (PtOEP/PdTPBP/perylene) and Stokes emission spectra (PtOEP/perylene) were obtained using a UV−visible spectrophotometer (Agilent 8453) and a spectrofluorophotometer (Shimadzu, RF-5301), respectively. Stokes and anti-Stokes emission spectra for TTA-UC solutions and films were obtained using commercial laser diodes (532 and 635 nm) as excitation sources. To avoid photobleaching due to oxygen quenching under extended irradiation, TTA-UC solutions were sealed within a 10 mm quartz cuvette equipped with a cap and septum (Starna) and degassed with argon for at least 60 min prior to laser experiments. During laser experiments, solutions were kept under a positive flow of argon and stirred continuously with a magnetic stir bar (Figure S6). For an illustration of the laser setups employed, see Figure S1. Shortpass and longpass filters with cut-off wavelengths of 600 nm were placed in front of the green and red lasers, respectively, and the beam diameter of the green laser was increased using a series of focusing lenses. The red and green laser beams were combined using a dichroic mirror with a cuton wavelength of 593 nm (Edmund Optics) and then passed through a continuously variable iris diaphragm (Newport) to yield a uniform beam. Laser power was adjusted using continuously variable neutral density filters (Thorlabs) and measured using a power meter (Ophir, Nova II) connected to a photodiode sensor (Ophir, PD300) placed directly in front of the sample location. Photoluminescence emission was collected normal to laser excitation for TTA-UC solutions and at an angle of approximately 45° for TTA-UC films. Emission was modulated by an optical chopper (120/160 Hz), directed to a monochromator (Oriel Cornerstone) using a series of focusing lenses, detected by a photomultiplier tube (Oriel), and processed by a radiometry detection system (Oriel Merlin). Notch filters with central stop-band wavelengths of 532 and 632.5 nm were placed in front of the monochromator to remove scattered laser light. Photoluminescence magnitude was quantified either by monitoring signal magnitude at the wavelength of peak emission as a function of time or by collecting emission spectra as a function of wavelength and calculating the integrated area of the desired emission peak.

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CONCLUSION This study presents the first solid-state TTA-UC systems employing more than one sensitizer in order to achieve broadband light absorption. Unlike their solution-based predecessors, the thin polymer film systems can operate open to the atmosphere without intensive deoxygenation and sealing procedures, which makes them far more practical for real-world applications. Under broadband excitation, the ability of the PtOEP/PdTPBP dual-sensitizer systems to harvest both red and green wavelengths allows them to absorb more photons than their single-sensitizer counterparts, which enables them to emit stronger UC fluorescence, i.e., to reach higher external quantum efficiencies. We demonstrate that simultaneous excitation of both sensitizers elevates the TTA quantum yield (ϕTTA) of the dual-sensitizer systems, greatly enhancing their UC fluorescence emission. This enhancement is most pronounced under low-power excitation, making multisensitizer TTA-UC particularly well suited to operation under solar radiation. We leverage the remarkable versatility of these polymer films to explore promising alternative architectures for solid-state TTA-UC materials; by overlaying PtOEP and PdTPBP single-sensitizer films in a multijunction fashion, we realize the high sensitizer concentrations necessary for effective light harvesting while circumventing the complications arising from high PdTPBP concentrations. Analogous multisensitizer solid-state systems hosting new chromophore combinations could enable TTA-UC with even broader light harvesting and a variety of different anti-Stokes shifts, providing an adaptable framework for the production of practical, broadly absorbing, and highly performing TTA-UC materials capable of drastically increasing solar device efficiencies.

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EXPERIMENTAL METHODS Preparation of TTA-UC Solutions. Sensitizer and acceptor chromophores were used as received without additional purification. Stock solutions of PdTPBP (Frontier Scientific/ Lumtec), PtOEP (Frontier Scientific), and perylene (Aldrich) were prepared in tetrahydrofuran (THF) and stored in the dark. UC solutions were prepared by adding appropriate aliquots of the stock solutions to a 4 wt % solution of polyisobutylene (Polysciences, MW = 1350) in mineral oil (light MO; Aldrich). After removal of residual THF through overnight heating in a convection oven at 70 °C, the resulting solutions were cooled to room temperature and stored in the dark. I

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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/acsphotonics.6b00679. Laser setup schematics, AM1.5G spectrum, powerdependent performance of single-sensitizer solutions and films, UV/vis absorption spectra of solutions and films, additional photographs of solutions and films, comparison between the UC fluorescence intensity of films and solutions, photoluminescence of dual- and single-sensitizer films with additional sensitizer concentrations, phosphorescence of concentrated films with and without perylene under red, green, and simultaneous irradiation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1-203-432-4386. Fax: +1-203-432-4387. ORCID

Jae-Hong Kim: 0000-0003-2224-3516 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CBET-1335934).

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DOI: 10.1021/acsphotonics.6b00679 ACS Photonics XXXX, XXX, XXX−XXX