Spatially defined molecular emitters coupled toplasmonic nanoparticle arraysJianxi Liua,b, Weijia Wangc, Danqing Wangc, Jingtian Hud, Wendu Dingb, Richard D. Schallerb,e, George C. Schatzb,c,1,and Teri W. Odomb,c,d,1

aState Key Laboratory of Solidification Processing, Center of Advanced Lubrication and Seal Materials, Northwestern Polytechnical University, Xi’an, 710072Shaanxi, People’s Republic of China; bDepartment of Chemistry, Northwestern University, Evanston, IL 60208; cGraduate Program in Applied Physics,Northwestern University, Evanston, IL 60208; dDepartment of Materials Science and Engineering, Northwestern University, Evanston, IL 60208; and eCenterfor Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439

Contributed by George C. Schatz, February 2, 2019 (sent for review November 5, 2018; reviewed by Bill Barnes and Paivi Torma)

This paper describes howmetal–organic frameworks (MOFs) confor-mally coated on plasmonic nanoparticle arrays can support exciton–plasmon modes with features resembling strong coupling but thatare better understood by a weak coupling model. Thin films of Zn-porphyrin MOFs were assembled by dip coating on arrays of silvernanoparticles (NP@MOF) that sustain surface lattice resonances(SLRs). Coupling of excitons with these lattice plasmons led to anSLR-like mixed mode in both transmission and transient absorptionspectra. The spectral position of the mixed mode could be tailoredby detuning the SLR in different refractive index environments andby changing the periodicity of the nanoparticle array. Photolumines-cence showed mode splitting that can be interpreted as modulationof the exciton line shape by the Fano profile of the surface latticemode, without requiring Rabi splitting. Compared with pristine Zn-porphyrin, hybrid NP@MOF structures achieved a 16-fold enhance-ment in emission intensity. Our results establish MOFs as a crystal-line molecular emitter material that can couple with plasmonicstructures for energy exchange and transfer.

metal–organic framework | plasmonic nanoparticle arrays | conformalcoating | surface lattice resonance | photoluminescence

Plasmonic nanoparticles (NPs) are driving advances in nano-photonics (1, 2) and photochemistry (3, 4) because of their

ability to concentrate and convert optical excitations into heatproduction, hot-electron generation, and light localization (5). In-teractions between the localized surface plasmons (LSPs) (6) ofisolated metallic NPs and excitons from molecular emitters canboth enhance light absorption and emission (7) and engineer ra-diative and nonradiative decay pathways (8). Compared with LSPs,surface lattice resonances (SLRs), also referred to as lattice plas-mons, are collective modes supported in arrays of NPs that exhibitstronger near-electric field enhancements and higher quality fac-tors Q (9, 10). Because SLRs result from hybridization of LSPs onindividual NPs with Bragg diffraction modes from the array (11,12), their resonance wavelength can be tuned by changing localdielectric environment and lattice periodicity (13, 14). Strongcoupling between excitons and surface plasmons has been dem-onstrated by integrating emitters such as single-walled carbonnanotubes (15), monolayer MoS2 (16), organic dyes (17–19), andlight-harvesting complexes (20, 21) with plasmonic NP arrays.Plasmon–exciton coupling is sensitive to the location of

emitters (22): those very close to the NPs are quenched (8),whereas the ones beyond the near-field region show little effect(23). Reports of coupling between organic dyes and plasmonicNP arrays have limited control over emitter locations, and hence,exciton contributions to the mixed SLR–exciton states cannot betuned (17–19). Although strong exciton–SLR coupling can beobtained at high dye concentrations (24), such densities typicallyresult in aggregation-induced photoluminescence quenching (25).Thus, the ability to control the spatial organization of molecularemitters relative to the nanoscale plasmonic structures is crucial

for enhancing coupling strengths and avoiding unfavorable distance-dependent effects.Metal–organic frameworks (MOFs) are molecular crystalline

structures consisting of organic ligands arranged between metalion nodes and have recently emerged as a functional luminescentmaterial (26–28). The chromophore-based organic ligands of MOFsare electronically isolated with spacings of usually 1‒2 nm (29) andcan serve as organized, but separated, molecular emitters. Unlike Jand H aggregates of dyes that exhibit strong transition dipolemoments that couple to adjacent molecules (30), chromophores inMOFs have weak electronic coupling to each other and do notshow self-quenching (31). In addition, MOFs can be easily pro-cessed on different types of substrates and assembled on variousnanostructures (32, 33). With optical properties derived frommolecular building blocks (28), MOFs can serve as a class ofmolecular emitters in the solid state.Here we show how spatially organizing chromophores via a MOF

template on plasmonic NP arrays can produce plasmon-dominatedhybrid modes. Luminescent Zn-porphyrin MOFs were conformallyassembled on silver NPs (AgNPs), and an SLR-like mode wasobserved to span a broad spectral range as the surrounding di-electric environment was varied. Using femtosecond transient ab-sorption (TA) spectroscopy, we showed that the SLR-like mode

Significance

The precise organization of molecular and functional nanoscalebuilding blocks is a long-standing challenge in nanoscience. Inparticular, overlappingmolecular emission with collective opticalexcitations at the nanoscale can result in exquisite control overthe intensity, wavelength, and direction of emitted light. Thiswork describes how positioning molecules that are componentsof a crystalline material called metal–organic frameworks (MOFs)around plasmonic nanoparticle arrays can result in stronglyenhanced emission at wavelengths determined by the lattice,including the splitting of spectral bands by interference ef-fects that do not require strong lattice–molecule coupling fortheir interpretation. This hybrid platform provides a frame-work for both spatially and temporally enhancing the prop-erties of luminescent MOFs.

Author contributions: J.L., W.W., D.W., J.H., W.D., R.D.S., G.C.S., and T.W.O. designedresearch; J.L., W.W., D.W., J.H., and W.D. performed research; J.L., W.W., J.H., W.D.,and R.D.S. analyzed data; and J.L., W.W., D.W., J.H., W.D., R.D.S., G.C.S., and T.W.O. wrotethe paper.

Reviewers: B.B., University of Exeter; and P.T., Aalto University School of Science.

Conflict of interest statement: P.T., W.W., and T.W.O. are coauthors on a 2018review article.

Published under the PNAS license.1To whom correspondence may be addressed. Email: g-schatz@northwestern.edu ortodom@northwestern.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1818902116/-/DCSupplemental.

Published online March 8, 2019.

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appeared immediately (within 50 fs) after femtosecond-scalepumping and persisted over a 1-ns time window, consistent withboth plasmonic and excitonic character. Exciton–SLR couplingproduced modes that persisted even when the SLR was tunedabove and below the excitonic resonances. Interestingly, strongphotoluminescence (PL) enhancement was also observed at wave-lengths close to the SLR. Electrodynamics modeling showed thatthe observed splitting in resonance line shape arose from Fanointerference effects in the SLR rather than strong-coupling of theSLR with molecular excitons.

Results and DiscussionFig. 1A depicts nanoscale confinement of molecular emittersaround plasmonic NP arrays by a conformal MOF coating. First,large-area AgNP arrays (>cm2) were patterned on fused silicasubstrates (refractive index n = 1.46) by a nanofabrication pro-cedure called photolithography, etching, electron-beam deposition,and liftoff (PEEL) (34) (SI Appendix, Fig. S1). A square lattice ofNPs with diameter d = 80 nm, height h = 60 nm, and periodicitya0 = 450 nm was selected so that the SLR would overlap spectrallywith the lowest Q-band exciton of the Zn-porphyrin MOF. Second,5 nm of alumina was deposited by atomic layer deposition on theAgNPs to mitigate oxidation of the Ag and to reduce quenching ofemitters near the metal surface. Finally, luminescent Zn-porphyrinMOF thin films with photoactive porphyrin derivative [5,15-diphenyl-10,20-di(4-carboxyphenyl)porphyrin] organic ligands weresynthesized on the NPs using a layer-by-layer method (35). Differentthicknesses of MOF films were coated on the NP arrays to control

the number of porphyrins by varying deposition cycles (SI Appendix,Methods). We found that the thickness of the Zn-porphyrin film was∼40 nm after 15 cycles (SI Appendix, Fig. S2); a 170-nm-thick MOFfilm was achieved after 60 dipping cycles (SI Appendix, Fig. S3).Fig. 1B shows that the crystallinity of the Zn-porphyrin MOF

coating on AgNP arrays was preserved. The Zn-porphyrin filmsshowed a preferred [001] orientation perpendicular to the sub-strate, in good agreement with literature (35), and the NP structuredid not have a significant effect on MOF crystallinity. Similarly, theAgNP features were not changed after coating with 40-nm Zn-porphyrin MOF film (Fig. 1C). The framework contains ∼525fluorescent molecules in a 10 × 10 × 10 nm3 box around the NPs.Fig. 1D shows that the excitation and emission spectra of the 40-nmZn-porphyrin thin film were consistent with that of the molecularporphyrin. The four Q bands in the absorption spectra indicatedthat the free-base porphyrins were immobilized in the Zn-porphyrin MOF structures without metallization of the porphyrinunits (SI Appendix, Fig. S4) (36). All bands were slightly redshiftedcompared with the porphyrin ligands dissolved in ethanol becauseof the extended π conjugation of porphyrins from the molecularcrystals (37). Two PL emission bands were observed at 665 and727 nm (Fig. 1D, red). These results suggest that the Zn-porphyrinMOFs can be used as densely packed molecular emitters in-tegrated onto plasmonic NPs.Fig. 2 demonstrates how coupling between the Q4-band exci-

ton of the MOF and the SLR from the AgNP arrays can be tunedby the surrounding dielectric environment. For bare AgNPs, astrong lattice plasmon mode at λSLR = 665 nm was observed

Fig. 1. Controllable deposition of Zn-porphyrin MOF thin films on NP arrays. (A) Scheme of the MOF coated on AgNP arrays with fused silica (n = 1.46) assupporting substrate. Right shows the crystal structure of Zn-porphyrin. (B) X-ray diffraction patterns of the Zn-porphyrin thin films deposited on AgNPs. The(001), (002), and (003) diffraction peaks appeared at around 3.5°, 7.0°, and 10.5°, respectively, with the intensity showed as counts per second (CPS). (C)Atomic force microscopy analysis of the AgNPs with spacing a0 = 450 nm, diameter d = 80 nm, and height h = 60 nm before and after coating with 40 nm Zn-porphyrin: topographical images (Top) and cross-section analysis (Bottom). (D) Excitation and emission spectra of the 40-nm Zn-porphyrin film deposited onAgNP arrays. A strong Soret band was present at 440 nm followed with less pronounced Q bands at Q1 = 524 nm, Q2 = 560 nm, Q3 = 600 nm, and Q4 = 656 nm.

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when n of the superstrate (n = 1.5) matched the index of thesubstrates (Fig. 2A, black). The SLR was identical in simulationusing finite-difference time-domain (FDTD) modeling, whichshowed strong near-field enhancement near the NPs (SI Appendix,Fig. S5). When AgNPs were coated with the Zn-porphyrin MOF(Fig. 2A, red), an SLR-like mode appeared at 674 nm (SLR-MOF), which we assign as an SLR–exciton mixed state becauseof the coupling between the SLR in AgNPs and the Q4-band ex-citons in MOF. Because only the narrow resonance feature of theSLR overlapped with the Q4 band of the Zn-porphyrin, the Soretand Q bands at shorter wavelengths remained the same as thepristine MOF (Fig. 1D). Although the oscillator strengths of theQ-band excitons in the porphyrin are weak (38), the excitons inspatially organized MOFs within the near-field region of theAgNP arrays contributed to the SLR-dominated mixed state.To investigate the evolution of the mixing, we kept the number

of molecular emitters constant (40-nm Zn-porphyrin film thick-ness) and changed the energy (wavelength) of the SLR levels bychanging the dielectric environment (n =1.35–1.65) (SI Appen-dix, Fig. S6). Stronger SLR-like modes (SLR-MOF) at energiesdifferent from either the Q4-band exciton or the SLR of the bareNP arrays were observed. Fig. 2B depicts the change of the SLR-MOF mode as a function of dielectric environment. Bare AgNParrays supported SLRs between 645 and 684 nm with n rangingfrom 1.40 to 1.55 (Fig. 2B, black). The hybrid system, however,supported SLR-MOF modes over a broader wavelength range(654‒723 nm) that tolerated significant refractive index differencesbetween the superstrate (n =1.35–1.65) and substrate (n = 1.46).The sustained high-quality SLR-MOF mode [assuming a MOFindex of n = 1.73, similar to other porphyrins (39)] was confirmedwith FDTD simulations of the transmission spectra; however, thecalculated SLR-MOF mode tolerance was suppressed with asmaller MOF refractive index (n = 1.52) (SI Appendix, Fig. S7). Wealso calculated transmission spectra of the AgNPs@MOF systemusing a Drude–Lorentz model for the dielectric function of the Zn-porphyrin MOF (SI Appendix, Fig. S8). The simulated transmissiondid not show any significant difference between Drude and theFDTD results, which means that n = 1.73 is reasonable for the realpart of the MOF dielectric function, and the imaginary part of thedielectric function is unimportant. Note that the porous Zn-porphyrin MOF layer extended the range of dielectric mismatcharound the NPs that could support a Fano resonance line shapecharacteristic of SLRs with a distinct dip (or peak in the case oftransmission) (40). To validate that the SLR-MOF mode involvedcontributions from excitons in the Zn-porphyrin, we coated the AgNPs with a different MOF (HKUST-1) with no exciton resonanceat the SLR wavelength (41, 42); no mixed mode excitation wasobserved. Compared with porphyrin-based MOFs, HKUST-1 only

allowed tuning of AgNP SLRs within a narrow range (646‒684 nm)as n increased from 1.35 to 1.65 (SI Appendix, Figs. S9–S11). Thus,the large extension of the plasmon-dominated SLR-MOF mode inAgNPs@Zn-porphyrin MOF resulted from coupling of the por-phyrin emitters to the AgNPs.To understand excited-state interactions of the mixed modes, we

conducted fs TA measurements (SI Appendix, Methods). Fig. 3Ashows that bare AgNP arrays (a0 = 450 nm) excited at 400 nmexhibited a broad bleaching signal at 525 nm associated with pho-toinduced absorption near 480 and 600 nm corresponding to theLSP of individual Ag NPs (43, 44). The SLR of the AgNP arrays(669 nm) showed only a very weak bleaching signal with light scat-tering extending to 100 ps from the arrays. For the Zn-porphyrinthin film (t ∼ 40 nm) only, ground-state bleaching of the Soretband was at 443 nm along with induced absorption at 481 nm,followed by the Q bands at Q1 = 526 nm, Q2 = 562 nm, Q3 =602 nm, and Q4 = 658 nm (Fig. 3B). For the hybrid AgNP-s@MOF system (Fig. 3C), we observed a strong bleaching signalat λ = 676 nm with absorption at 664 nm that persisted over the1-ns window without decaying to zero. These spectral featuresverified a mixed SLR–exciton state that exhibited the sharplinewidth of the lattice resonance and the long lifetime of theporphyrin excitons. Notably, the SLR-MOF mode was formedwithin 50 fs after photon excitation of the Zn-porphyrin (λ =400-nm laser); no Q-band signals were observed within the wave-length range of 550–650 nm on this same timescale (SI Appendix,Fig. S12), which suggests that SLR-MOF modes can be excitedfaster than MOF-only excitons.To determine the generality of Q-band and SLR interactions,

we tested coupling between theQ3-band excitons in the Zn-porphyrinand SLRs in AgNP arrays with a different spacing (a0 = 400 nm)(SI Appendix, Figs. S13 and S14). The same thickness (∼40 nm) ofZn-porphyrin was conformally coated on the AgNP arrays inwhich the particle size (d = 70 nm and h = 60 nm) was optimizedto support a high-quality SLR at shorter wavelengths. Similar tothe AgNP arrays with larger periodicity, we observed a weakbleaching signal at 575 nm from the SLR of the smaller periodicityarrays (SI Appendix, Fig. S15). For the hybrid AgNPs@MOF, theSLR-MOF mode showed a strong bleaching signal that persistedover 1 ns and indicated a mixed state (SI Appendix, Fig. S15C).Our findings verified that the hybrid SLR-MOF mode can berealized and controlled over a wide wavelength range by couplingthe energetically different excitons of Zn-porphyrin MOF to thetunable SLR modes in the NP arrays. In contrast, for porphyrinmolecules in solution (dimethyl sulfoxide) around the AgNP ar-rays, no exciton–SLR coupling was observed (SI Appendix, Fig.S16), most likely because (i) the density of emitters was too small(the highest concentration of 0.5 mM had only 0.3 porphyrin in a

Fig. 2. Coupling of the SLR in AgNP arrays with Q4-band excitons in Zn-porphyrin with different refractive index environments. (A) Transmission spectra ofAgNP arrays before and after coating with the Zn-porphyrin in a refractive index environment of n = 1.50. (B) Peak wavelength of SLR for bare AgNP arraysand peak wavelength of the SLR-like mode (SLR-MOF) for the coupled AgNPs@MOF system as a function of indices ranging from n = 1.35 to 1.65, which wereextracted from the transmission spectra in SI Appendix, Fig. S6. Periodicity of AgNP arrays is a0 = 450 nm, and thickness of MOF is t = 40 nm.

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10 × 10 × 10 nm3 box around the NPs) and (ii) there was a lack ofporphyrin orientation relative to the NPs. Thus, MOF excitons indifferent energy states can couple with SLRs in plasmonic NParrays and form mixed exciton–SLR states.Compared with excitons in the Zn-porphyrin MOF and lattice

plasmons in the AgNP arrays, the SLR-MOF mode exhibitedunique dynamics with new decay channels. A biexponential fitrevealed that the decay lifetimes of the excited Q4 for the pristineMOF (τ1 = 25.2 ps and τ2 = 208.3 ps) were considerably slowerthan that of the mixed system at the same wavelength (τ1 = 1.2 psand τ2 = 54.4 ps) (Fig. 4A). Significant differences appeared forthe excited mixed mode, where ΔA signal was negative becauseof strong bleaching (Fig. 4B). The decay lifetimes of the SLR-MOF mode (τ1 = 4.9 and τ2 = 104.8 ps) were longer than theSLR but shorter than the Q4 band of the pristine MOF at thesame wavelength. For bare AgNP arrays, the ΔA signal atthe LSP resonance yielded a lifetime of 1.4 ps by fitting with amonoexponential decay function (SI Appendix, Fig. S17), typicalfor electron–phonon scattering (43). The decay lifetime of thebleaching signal at the SLR wavelength (2.3 ps) was longer thanthat of the LSP because of suppressed radiative loss from dif-fractive coupling (45). Compared with the mixed states for the

AgNPs@MOF system (Fig. 4), both SLR and LSP decayed rapidlyin the bleaching signal.Fig. 5 depicts how the coupling can be modulated by varying

the dielectric environment above the MOF. For a0 = 450 nmAgNPs lattices, the TA spectral maps showed identical long-livedbleaching signals with same decay dynamics in different super-strate index environments (n = 1.45, 1.50, and 1.55) (SI Appen-dix, Fig. S18 A–C). Importantly, the high-intensity SLR-MOFmodes evolved from λ = 667 to 688 nm as the SLR shifted aboveand below the Q4 band (Fig. 5A). In contrast, the Q1–Q3 bands ofthe Zn-porphyrin did not change. Note that there was nobleaching signal between 600 and 750 nm for the Zn-porphyrinin different dielectric environments (SI Appendix, Fig. S18 D andE). Also, we observed evolution of the mixed states with strongbleaching for the Q3-band exciton–SLR as the SLR was tunedacross the Q3 band; however, the Q1, Q2, and Q4 bands of Zn-porphyrin were unchanged (Fig. 5B and SI Appendix, Fig. S19).Significantly, local dielectric environment of AgNPs@MOF canbe used to manipulate the energy levels of the mixed states.PL emission from the exciton–plasmon coupling system is of

interest because it can provide information about radiative re-combination pathways and energy transfer dynamics. We ob-served strong PL enhancement from the hybrid AgNPs@MOF

Fig. 3. TA provides evidence of electron population of the SLR-like state for coupling of the Zn-porphyrin with AgNP arrays. (A–C) TA spectral maps (Top)and cross-section at ∼700 fs delay time when the ΔA (absorbance difference) reaches maximal (Bottom) for (A) bare AgNP arrays, (B) Zn-porphyrin thin film,and (C) the hybrid AgNPs@MOF system pumped at 400 nm with the fluence of 64 μJ/cm2. Q1–Q4 indicate the position of the four Q bands of Zn-porphyrin.Spectra were recorded with the index of the environment at n = 1.50.

Fig. 4. Temporal evolution of TA dynamics for the exciton–SLR hybridized state. Decay of ΔA signal at a probe wavelength of (A) 658 nm (Q4) and (B) 676 nm(SLR-MOF) for the hybrid system in Fig. 3C. The decay of the ΔA signal for the pristine MOF was probed at the same wavelength for comparison and plotted atthe bottom. The black lines represent biexponential decay fitting of the kinetics.

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with intensity 7.2, 16.4, and 16.2 times higher than the pristineZn-porphyrin for emission normal to the substrate (Fig. 1A,Right), when the dielectric environment was varied from n = 1.45to n = 1.55 (Fig. 6A and SI Appendix, Fig. S20). Although theemission peak at ∼727 nm was unaffected by the AgNP arrays, theemission at shorter wavelengths split into two new peaks. At n =1.50, the bluer peak (658 nm) can be assigned as a partial feature ofthe enhanced MOF emission centered at 665 nm, and the redderpeak (673 nm) can be attributed to the SLR-MOF mode. Thehighest PL enhancement appeared at the SLR-MOF resonance atall n tested (SI Appendix, Fig. S20). As a control to eliminate NPstructure effects, we tested Zn-porphyrin MOFs coated on Ti NParrays and found only a small PL enhancement with intensity 1.6times higher than theMOF (SI Appendix, Figs. S21 and S22), whichrevealed that the enhancement was mainly mediated by surfaceplasmons and not diffraction effects. Absorption contributions tothe PL enhancement were negligible because the plasmonic reso-nances were not at the wavelengths of both excitation beam (λ =400 nm) and Soret band of Zn-porphyrin (440 nm). Therefore, thespectrally modified PL emission of AgNPs@MOF originated fromthe SLR coupled with the emissive Q4-band excitons.We developed a semiclassical model to predict the emission

enhancement and splitting due to MOF exciton–SLR coupling.The emission rate was calculated under the framework ofFermi’s golden rule, in which the exciton emission spectrum wasmodified by the SLR density of states. Note that this model doesnot include Rabi splitting due to plasmon–exciton interactionsbut does account for the influence of photon state density onexciton emission. Following FDTD calculations (46, 47) alsodescribed in SI Appendix, Eqs. S2–S6, the influence of this statedensity was determined by evaluating the electric field at themolecule position (SI Appendix, Fig. S23). Our model showedthat strong enhancement of the emission rate was achieved onlyfor those emitters close to the Ag NPs and whose transition di-pole moments were aligned with the lattice array. Maximumcoupling was achieved when molecular dipoles were parallel toin-plane dipolar lattice plasmons (SI Appendix, Fig. S23). Themodel suggests that the splitting of the PL emission (Fig. 6A)arises from a combination of three factors: (i) strong near-fieldenhancement associated with the SLR of AgNPs, which leads tothe most intense emission; (ii) the effect of the Fano resonanceline shape (40) dip to the blue of the SLR in combination with aless-enhanced portion of the emission spectrum at bluer wave-lengths from the dip; and (iii) the effect of averaging over

different molecule locations and orientations. Experiment (SIAppendix, Fig. S20) and theory (SI Appendix, Fig. S25) are inqualitative agreement and support these conclusions.The dispersive property of lattice plasmons allows for addi-

tional tailoring of light emission from the SLR–exciton mixedstate in the same hybrid nanostructure by changing only thedetection angle (9). The coupling between Q4-band excitons inZn-porphyrin and the propagating lattice modes of the SLR inAgNP arrays followed the dispersion of SLR wavelength; hence,a range of tunable mixed states at different PL emission wave-lengths is possible (Fig. 6 B and C). We observed that the (0, ±1)modes of the SLRs in AgNP arrays blue-shifted from 664 to654 nm as the incident angle increased from 0° to 18° (SI Ap-pendix, Fig. S26). For AgNPs@MOF, the strong plasmon-likemode at ∼674 nm for transmission at normal incidence

Fig. 5. The evolution of the excited states by coupling SLR to different Q-band excitons in Zn-porphyrin. (A) SLR and Q4-band exciton coupling (a0 = 450 nm):TA spectra (at ∼600 fs) of the hybrid AgNPs@MOF system with index values n = 1.45, 1.50, and 1.55 and the pristine Zn-porphyrin MOF spectrum (with n =1.50), which were extracted from SI Appendix, Fig. S18. The pump wavelength was 400 nm, and the fluence was 27 μJ/cm2. (B) SLR and Q3-band excitoncoupling (a0 = 400 nm): TA spectra (at ∼600 fs) of the hybrid AgNPs@MOF system with index values n = 1.45, 1.50, and 1.55 and the pristine Zn-porphyrin MOFspectrum (with n = 1.50), which were extracted from SI Appendix, Figs. S15 and S19. The pump wavelength was 400 nm, and the fluence was 32 μJ/cm2. Notethat the curves are shifted up from the MOF spectrum for clarity.

Fig. 6. PL enhancement from the exciton–SLR coupled system indicated fastradiative recombination and precise control of emission wavelength. (A) PLemission spectra of Zn-porphyrin deposited on AgNP arrays with index val-ues n = 1.45, 1.50, and 1.55 and PL emission of the pristine MOF depositedon plane fused silica (with n = 1.50). The detection angle was fixed normal tothe substrate (θem = 0°). (B) Angle-dependent transmission spectra and (C)angle-dependent PL emission of the AgNPs@MOF hybrid system (Zn-porphyrin deposited on AgNP arrays) with the index of n = 1.50. Periodicityof AgNP arrays is a0 = 450 nm, and the thickness of MOF is t = 40 nm.

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blue-shifted with increased incident angle θin (Fig. 6B). Fig. 6C showssimilar dispersion behavior of the PL emission with a maximumpeak wavelength that evolved from 674 to 662 nm as a functionof angle. Interestingly, we also observed spectral splitting in PLemission from propagating lattice modes, as the shoulder peakblue-shifted as angle increased (SI Appendix, Fig. S27 A and B).PL emission of pristine Zn-porphyrin showed no change in eitherwavelength or intensity as a function of angle (SI Appendix, Fig.S27C). Thus, strong enhancement of PL emission is possiblefrom molecular emitters coupled with propagating SLRs, withemission wavelengths tunable by angle.In summary, we demonstrated a SLR-MOF mixed mode from

coupling between excitons in Zn-porphyrin MOF and SLRs inAgNP arrays. The nanoscale confinement of molecular emittersvia a MOF scaffold provides a strategy to arrange transition dipolesby their fixed orientations in the near-field enhancement region ofplasmonic nanocavities. TA measurements revealed that the mixedexciton–SLR mode was dominated by plasmonic character basedon its narrow spectral width and had excitonic character from thelong-decay lifetime. Spectral splitting in the PL emission was ob-served even though MOF excitons were only weakly coupled withthe SLRs, which is different from the strong coupling behaviornormally associated with plasmon–exciton interactions (48). Al-though the measurements and model showed clear evidence ofmixed mode character, any Rabi splitting must be small comparedwith the SLR linewidth because we could model transmission and

emission line shapes without considering this splitting. We believethe spatial organization of functional MOFs around plasmonicnanostructures provides a general strategy to develop their in-tegration in nonlinear optics applications as solid-state gain mediaand in photovoltaics as photoelectric conversion materials.

Materials and MethodsDetailed information for the materials and methods can be found inSI Appendix.

ACKNOWLEDGMENTS. This work was supported by the Air Force Office ofScientific Research under Grant FA9550-14-1-0274 (to J.L., J.H., and T.W.O.).W.W., D.W., T.W.O., and G.C.S. were supported by NSF Grant DMR-1608258.Theory development by W.D. and G.C.S. was supported by US Department ofEnergy (DOE) Grant DE-SC0004752. We are grateful for the use of EPIC(Electron Probe Instrumentation Center), Keck-II, and SPID (Scanned ProbeImaging and Development) facilities of NUANCE Center (NorthwesternUniversity Atomic and Nanoscale Characterization Experimental Center),which has received support from the Soft and Hybrid NanotechnologyExperimental Resource (Grant NSF NNCI-1542205), the MRSEC (MaterialsResearch Science and Engineering Center) program (Grant NSF DMR-1121262) at the Materials Research Center, the International Institute forNanotechnology (IIN), the Keck Foundation, and the State of Illinois,through the IIN. This research was also supported in part by computationalresources and staff contributions provided for the Quest high-performancecomputing facility at Northwestern University, which is jointly supported bythe Office of the Provost, the Office for Research, and Northwestern UniversityInformation Technology. Use of the Center for Nanoscale Materials, an Officeof Science user facility, was supported by the US DOE, Office of Science, Officeof Basic Energy Sciences, under Contract DE-AC02-06CH11357.

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