Dual Strong Couplings Between TPPS J-Aggregates and Aluminum

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Dual Strong Couplings Between TPPS JAggregates and Aluminum Plasmonic States Jie Li, Kosei Ueno, Hiyori Uehara, Jingchun Guo, Tomoya Oshikiri, and Hiroaki Misawa J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01224 • Publication Date (Web): 06 Jul 2016 Downloaded from http://pubs.acs.org on July 10, 2016

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The Journal of Physical Chemistry Letters

Dual Strong Couplings Between TPPS J-Aggregates and Aluminum Plasmonic States Jie Li,† Kosei Ueno,† Hiyori Uehara,† Jingchun Guo,† Tomoya Oshikiri,† Hiroaki Misawa*,†, ‡

†

Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan

‡

Department of Applied Chemistry & Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan

ACS Paragon Plus Environment

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ABSTRACT We report on the spectral properties of strong coupling between the localized surface plasmon resonances (LSPRs) of aluminum (Al) nanostructures and tetraphenylporphyrin tetrasulfonic acid hydrate (TPPS) J-aggregates. Because of their wide spectral range of LSPR bands from ultraviolet to near-infrared wavelengths by controlling structural size, Al nanodisks can realize strong coupling with different excitons of TPPS J-aggregates. The Rabi splitting energies of the excitons based on Soret and Q bands are 300 meV and 180 meV, respectively. In addition to extinction spectrum, we have also measured an excitation spectrum to determine the essential absorption of the hybrid states and successfully confirmed a shoulder peak corresponding to a lower branch of hybrid states. In Al nanorod systems, strong coupling with two excitons can also be selectively induced by merely rotating the polarization of the incident light, which constituted a simple platform for the dynamic control of exciton/plasmon coupling states. TOC GRAPHICS

KEYWORDS Strong coupling, Aluminum plasmonic state, TPPS J-aggregate, Soret band/Q band.


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Localized surface plasmon resonances (LSPRs) are collective charge oscillations that are confined to the surfaces of metallic nanostructures.1,2 Their unique combination of localization and electromagnetic field enhancement is interesting for a broad range of applications, such as sensing,2−4 energy harvesting,5−9 photochemical reactions,10−12 and artificial photosynthesis.13,14 However, the use of LSPRs in most of these promising applications is limited by substantial challenges associated with the finite conductivity of the metal and radiation damping by the metallic nanostructures.15 The dephasing of LSPRs in gold (Au) nanostructures is known to be less than 10 fs.16 One promising strategy, initially suggested by Bergman et al.,17 is to couple metal nanostructures to active media, such as molecular aggregates. In this case, new hybrid exciton–plasmon states arise when the excitons of molecules resonate with the plasmon modes of metallic nanostructures, thereby altering the dynamics of these hybrid states. The spectral linewidths of hybrid states, which are sharper than that of plasmon, indicate that the dephasing times are longer than that of plasmon.18,19 Therefore, plasmon-exciton hybrid systems are expected to show strong enhancement of electromagnetic fields because of their longer dephasing times.20 As the result, molecules or substances in the vicinity of the hybrid system feel huge electromagnetic field, so that various optical effects such as surface-enhanced Raman scattering, and photochemical reactions are promoted due to the enhancement of light-matter coupling. In previous studies, hybrid systems with energy splitting (Rabi splitting) values of hundreds of meV have been reported based on strong coupling between the plasmon modes and J-aggregates of organic dyes.21−25 However, most work has focused on investigating the properties of the plasmon modes coupling with single exciton state of J-aggregates. Although such strong coupling can lead to strong field enhancement, the spectral range that is suitable for


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this enhancement is quite limited, for example, when it is going to be used as an optical antenna of a solar energy conversion system. In this work, we discuss about the hybrid systems in which two different excitons of Jaggregates strongly couple with LSPR modes. For the molecular system, tetraphenylporphyrin tetrasulfonic acid hydrate (TPPS) J-aggregates, as one of well-known molecular J-aggregates, have two energetically different excitons (Soret and Q bands). It is possible that both excitons will show strong coupling with LSPR modes; if so, the field enhancement effect can be realized and controlled over a wider wavelength range of a solar spectrum. However, a type of nanostructure that can enable long-range and coherently tunable LSPR modes is also needed, and aluminum (Al) has been shown to have unique advantages for the development of such nanostructures. In addition to its cost-effectiveness relative to Au and silver (Ag), Al facilitates LSPR stability at shorter wavelengths (down to 150 nm) than Au (500 nm) and Ag (345 nm) because there is no interband transition in the wavelength region shorter than 800 nm.26 Therefore, it can support LSPR over a wide spectral range, exhibits strong optical resonance, and is easy to handle.27−31 In the present study, we fabricated Al nanostructures with various sizes, whose LSPR bands overlap with two different excitons of TPPS J-aggregates. In plasmon-exciton hybrid systems, it is needed to measure not only transmission or photoluminescence (PL) spectrum but also excitation spectrum to confirm the formation of new electronic states based on strong coupling. So far, the formation of hybrid system was discussed only from the measurements of extinction spectrum and PL spectra. However, there is no direct evidence that new hybrid energy states were really formed only from the measurements. Therefore, we explored the formation of hybrid energy states according to investigating not only via extinction spectra but also excitation


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spectra. We discuss about the above-mentioned measurements and analyses for the exploration of the spectral properties of plasmon-exciton hybrid systems in which two different excitons of TPPS J-aggregates strongly couple with plasmon modes of Al nanostructures. Ordered arrays of Al nanodisks with different nanodisk diameters (90-200 nm) were fabricated on a glass substrate to tune the LSPR energy. The thickness of the nanodisks was 30 nm. A scanning electron microscope (SEM) image and a typical size dispersion of nanodisks with a mean diameter of 200 nm are shown in Figure 1a. The period is slightly larger than twice the nanodisk diameter to avoid near-field interactions. Interdisk coupling is known to become negligible when the disk diameter is smaller than the separation distance.32,33 Figure 1b presents the extinction spectra of Al nanodisk arrays with various diameters. The extinction peaks exhibit a monotonous and pronounced red-shift as the size of the nanodisk increases, which enables stable LSPR wavelengths from 400 nm to 700 nm. An absorption spectrum of TPPS J-aggregates deposited on a glass substrate with a thickness of 20 nm is shown in Figure 1c. Notably, both the Q band and Soret band overlap with the LSPR bands, although the diameter of the Al nanodisk overlapping with the absorption spectrum of each exciton is different.


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Figure 1. (a) SEM image of an Al nanodisk array on a glass substrate. (b) Extinction spectra of Al nanodisk arrays with different sizes (diameters from 90 nm to 200 nm). (c) Absorption spectrum of TPPS J-aggregates deposited on a glass substrate. The inset shows the molecular structure of a TPPS monomer and an energy diagram of TPPS J-aggregates.

TPPS molecules were first dissolved in methanol at a concentration of 3 mmol dm-3 and then spin-coated onto the whole sample to form a TPPS J-aggregate film with a thickness of approximately 20 nm. Extinction spectra of the TPPS-covered samples with different nanodisk diameters are presented in Figures 2a and 2b. Three well-defined peaks, denoted by dashed boxes, are clearly evident in the regions of the spectra. The presence of three peaks (two side peaks and one center peak) is a strong indication of plasmon/exciton hybridization, which leads to the formation of two distinct mixed states (two side peaks) at energies differing from those of the bare plasmon and exciton. The center peak is clearly related to the bare, uncoupled exciton, and its energy position (1.76 eV or 2.53 eV) corresponds to the two J-aggregate absorption maxima. The LSPR peaks become red-shifted as the diameter of the nanodisks increases, and


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these peaks can couple not only with the Soret band but also with the Q band of the TPPS Jaggregates.

Figure 2. (a, b) Extinction spectra of the TPPS-covered samples with different nanodisk diameters. The inset shows the diameters of the nanodisks. (c) The plasmon/exciton hybrid dispersion curves for the TPPS-covered samples. The two horizontal green lines show the absorption maxima of the TPPS J-aggregates at 1.76 eV (Q band) and 2.53 eV (Soret band). The red and blue curves are fitting results calculated using equation (1).

In Figure 2c, the spectral positions of the two side peaks are plotted as a function of the bare plasmon energy. The two horizontal green lines show the absorption maxima of the TPPS Jaggregates at 1.76 eV (Q band) and 2.53 eV (Soret band). The interaction between the LSPRs and the two excitons of the J-aggregates could result in hybridized plexciton states, which exhibit typical anticrossing behavior. Two clear anticrossings are observed when the plasmon resonances are nearly matched to the two molecular excited states. The energies of the upper branch and lower branch plexciton states are calculated using a coupled harmonic oscillator model,34


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‫ܧ‬௎஻,௅஻ =

ாೞ೛ ାா೐ೣ ଶ

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ଶ

ଵ ± ට൫‫ܧ‬௦௣ − ‫ܧ‬௘௫ ൯ + ∆ଶ ଶ

(1)

where Esp and Eex are the LSPR and uncoupled exciton energies, respectively. ∆ is the strongcoupling energy. The red and blue fitting curves in Figure 2c were calculated using equation (1). The Rabi splitting energy is estimated to be approximately 180 meV in the Q band/plasmon coupling of TPPS J-aggregates. In contrast, the Rabi splitting of the Soret band, 300 meV, is much larger than the Q band/plasmon coupling strength. On the other hand, the Rabi splitting energy should be larger than the spectrum widths of the LSPR and the TPPS,

ఊಽೄುೃ ଶ

ఊ

+ ଶబ, in the

strong coupling regime as following,35 Δ>ට

మ ఊಽೄುೃ

ଶ

+

ఊబమ ଶ

(2)

where γLSPR and γ0 are the uncoupled LSPR and TPPS spectrum widths, respectively. ∆ is the మ

మ

ఊ ఊ Rabi splitting energy. In this system, ට ಽೄುೃ + ଶబ were calculated as 280 meV for the Soret ଶ

band/LSPR system and 150 meV for the Q band/LSPR system, respectively. Therefore, the both Rabi splitting energies are larger than these calculated values, and it fulfills the strong coupling conditions. It is noteworthy, furthermore, that the ratio of the Rabi splitting energies of the Soret band and Q band is ~1.67. The Rabi splitting energy is known to be highly dependent on the electric field of the plasmon mode and the dipole moment of the exciton.23,35 Because the electric fields of the LSPR peaks at 1.76 eV and 2.53 eV are almost identical, as confirmed by finite-difference time-domain (FDTD) simulations (Supporting information S1), we speculate that this difference may be directly caused by the different dipole moments of the two excitons. Based on the absorption spectra of TPPS J-aggregates in Figure 1c, the ratio of the dipole moments of the two


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excitons can be determined by calculating the ratio of the square roots of the absorption values. The calculated value, 1.64, is in good agreement with the ratio of Rabi splitting energies, confirming our prediction. The FDTD method was also used to calculate the optical properties of the hybrid samples. The TPPS J-aggregate coating was modeled as two different Lorentzian oscillators (Soret and Q bands). The calculated extinction spectra and dispersion curves are in very good agreement with the experimental results (Supporting Information S2 and S3). Two new peaks were found in the extinction spectra of the hybrid samples, and anticrossing behavior was identified in the dispersion curve. However, it is important to note that even without the formation of hybrid states, the spectral interference between the plasmon and exciton modes can result in spectral splitting and asymmetric line shapes. Ishihara et al.36 proposed in their theoretical study that an excitation spectrum measurement which leads to an understanding of absorption processes is useful to distinguish the energy transparency and Rabi splitting because new bands should be appeared at the hybrid energy levels in the essential absorption spectrum which becomes in evidence of the formation of new hybrid energy states.37 To examine this phenomenon further, we measured the excitation spectrum to obtain an essential absorption of our hybrid states. In the experimental setup, we employed a Ti:sapphire laser (wavelength tuned from 690 nm to 920 nm) to excite the sample. Sample emission was measured by a spectrometer. A monitoring wavelength of 820 nm was selected to measure the emission spectra. The emission intensities at this monitoring wavelength were plotted as a function of the excitation wavelength. Figure 3 shows the excitation spectra of the hybrid (Q band/plasmon coupling) and TPPS J-aggregate samples. In the excitation spectrum of TPPS, only one peak is found at 700 nm because TPPS J-aggregates have only one energy level (S1 state) in the excitation energy range. However, there are two peaks appeared in the excitation spectrum of the


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hybrid sample. The peak located at 700 nm is the same as that of the TPPS J-aggregates sample and corresponds to the uncoupled TPPS J-aggregates. Another small shoulder peak located at 725 nm (highlighted by the dashed circle) shows good agreement with the location of the peak of the hybrid system’s extinction spectrum. Here, the small shoulder peak in the excitation spectrum is attributed to the formation of lower branch of hybrid states (inset in Figure 3). Therefore, the excitation spectra provide strong evidence that a new hybrid state really exists in our hybrid sample. In order to clarify more in detail, we performed additional experiments about an excitation spectrum measurement of the hybrid sample (Soret band/plasmon coupling) with a different Al disk (Supporting information S4). Even in the hybrid sample (Soret band/plasmon coupling), two shoulder peaks derived from upper and lower branches of the hybrid state, which are corresponding to the wavelengths of the hybrid state in extinction spectrum, were obviously observed.


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Figure 3. Excitation spectra of the hybrid sample (red) and TPPS J-aggregates (black) and the corresponding extinction spectra. The energy level diagram of the hybrid sample indicates the origin of the peak in the excitation spectrum (denoted by the gray dashed circle). The extinction spectra of TPPS-J aggregates and the hybrid system were taken from Figures 1(c) and 2(b) (diameter: 185 nm), respectively.

In addition to Al nanodisks, Al nanorod arrays were also investigated to realize strong coupling with the two excitons of TPPS J-aggregates. Al nanorods can support two LSPR modes: a longitudinal plasmon mode (L-mode) with polarization parallel to the long axes of the nanorods and a transverse plasmon mode (T-mode) with polarization parallel to the short axes of the nanorods.29 Therefore, by carefully designing the Al nanorod array, we successfully obtained the desired L-mode and T-mode by simply rotating the polarization of the incident light. Figure 4a shows the SEM image of the Al nanorod array, which had a thickness of 30 nm, a length of 195 nm, a width of 105 nm, and array periods of Px = 300 nm and Py = 230 nm. The hybrid


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samples were prepared by spin-coating following the same method as that used to deposit on Al nanodisks.

Figure 4. (a) SEM micrograph of the Al nanorod array on glass. (b, c) Extinction spectra of TPPS J-aggregates, Al nanorods, and TPPS-covered Al nanorods. The polarization of the incident light are θ = 0° (T-mode) and θ = 90° (L-mode), respectively. (d) Extinction spectra of the TPPS-covered nanorod samples under different polarization angles of the incident light (θ = 0°−90°). (e) Polarized microscope transmission images for incident light polarized from 0° to 90°. The left column shows the transmission images of a bare Al nanorod array (yellow to dark green), and the right column shows those of the hybrid sample (orange to green).


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In this system, TPPS J-aggregates coupled to LSPRs become polarization dependent. Figures 4b and 4c show the extinction spectra of hybrid samples (red line) at two different polarizations of incident light (θ = 0° and 90°). It must be noted that the TPPS coating also changes the background dielectric constant, which red-shifts the LSPR mode. Thus, the bare plasmon extinction peak is located at a shorter wavelength compared to the exciton. In the hybrid sample’s extinction spectrum, the presence of two new peaks is an indication of plasmon/exciton hybridization. Finally, Figure 4d shows the experimental extinction spectra of the TPPS-covered nanorod samples under different polarizations of incident light. The transition from Soret band/plasmon coupling to Q band/plasmon coupling can be clearly observed by simply rotating the polarization of the incident light. The polarized transmission images through the Al nanorod array with and without the TPPS J-aggregate coating are shown in Figure 4e. When the polarization is rotated from 0° to 90°, vivid colors are observed in the hybrid sample owing to the LSPRs of Al nanorods in the visible spectrum and coupling with the TPPS J-aggregates. Thus, these materials may have potential applications as color filters. In conclusion, we studied the strong coupling between the plasmonic states of Al nanostructures and two different excitons of TPPS J-aggregates. Strong coupling was successfully induced between the Al nanodisks and both the Q band and the Soret band of TPPS J-aggregates by controlling the structural size of the Al nanodisks. The Rabi splitting energy of the Soret band/plasmon was 300 meV, which is much larger than the Q band splitting energy of 180 meV. We speculated that the different strengths of the strong coupling were attributable to the different strengths of the dipole moments of the two excitons. To confirm that strong coupling was really induced, not only the extinction spectra but also excitation spectra were measured in order to obtain the essential absorption of the hybrid states. The results confirmed


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the formation of hybrid energy states. Furthermore, strong coupling with two excitons of the TPPS J-aggregates could also be selectively induced in Al nanorod systems by simply rotating the polarization of the incident light, which constitutes a simple platform for the dynamic control of exciton/plasmon coupling. Because of the wide spectral range and strong optical resonance of LSPRs in Al nanostructures, strong coupling with different excitons can be easily realized in a molecular system. These Al nanostructures make up a good platform and could significantly affect our ability to enhance and control the efficiency of photochemical processes over a wide range of wavelengths.

EXPERIMENTAL METHODS To fabricate the Al nanostructures, electron-beam lithography was applied, followed by thermal evaporation of the metal and lift-off techniques. Patterns of Al nanostructures were formed on glass substrates designed using a high-resolution electron-beam lithography system (ELS-F125; Elionix, Tokyo, Japan) operating at 125 kV. A conventional copolymer resist (ZEP520A; Zeon Chemicals, Louisville, USA) diluted with a ZEP thinner (1:1) was spin-coated onto the substrate at 1000 r.p.m. for 10 s and 4000 r.p.m. for 90 s and was then prebaked on a hot plate for 2 min at 150°C. The electron-beam lithography was conducted at an electrical current of 50 pA. After development, a 30-nm-thick Al layer was deposited by thermal evaporation (SVC700, Sanyu Electron). Finally, the residual resist was removed by lift-off in an ultrasonic bath of ZDMAC (Zeon) solvent and then rinsed with acetone, methanol, and deionized water. Fieldemission scanning electron microscopy (JSM-6700FT, JEOL, Tokyo, Japan) was used to characterize the morphologies of the Al nanostructures. The far-field spectral properties were measured by a transmission microscope coupled to a spectrophotometer (PMA-11, Hamamatsu).


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The light source was a halogen lamp equipped with an optical microscope (Olympus, BX-51). To generate the molecular system, TPPS molecules were dissolved in methanol at a concentration of 3 mmol dm-3 and then spin-coated (3000 r.p.m. for 90 s) onto the whole sample to form a TPPS J-aggregate film with thickness of approximately 20 nm. A Ti:Sapphire laser (Tsunami, SpectraPhysics, 690-920 nm tunable, un-mode-locked mode) was used for the excitation of the hybrid (Q band/plasmon coupling) and TPPS J-aggregate samples, and the emission spectra were measured with a spectrometer (SpectraPro-2300i, Roper Scientific). The excitation spectra of the hybrid (Soret band/plasmon coupling) and TPPS J-aggregate samples were also investigated by a fluorescence spectrometer (F-4500, Hitachi).

ASSOCIATED CONTENT Supporting Information. The FDTD simulations and an excitation spectrum of the Soret band/plasmon hybrid sample are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION *To whom correspondence should be addressed. Phone: +81-11-706-9358. Fax: +81-11-7069359. E-mail: [email protected].

ACKNOWLEDGMENT We gratefully acknowledge financial support from JSPS KAKENHI Grant Numbers JP23225006, JP15K00856, JP15H01073, JP15K04589, the Nanotechnology Platform (Hokkaido University), and Nano-Macro Materials, Devices and System Research Alliance of MEXT.


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