Coupling between the Mie Resonances of Cu2O Nanospheres and

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Coupling between the Mie Resonances of CuO Nanospheres and the Excitons of Dye Aggregates Qifeng Ruan, Nannan Li, hang Yin, Ximin Cui, Jianfang Wang, and Hai-qing Lin ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00886 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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Coupling between the Mie Resonances of Cu2O Nanospheres and the Excitons of Dye Aggregates Qifeng Ruan,† Nannan Li,† Hang Yin,†,‡ Ximin Cui,† Jianfang Wang,†,* and Hai-Qing Lin§

†

Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China

‡

Institute of Nanostructured Functional Materials, Huanghe Science and Technology College,

Zhengzhou, Henan 450006, China §

Beijing Computational Science Research Center, Beijing 100193, China

ABSTRACT Although the resonance coupling between excitons and surface plasmons has been extensively investigated for decades, the coupling between excitons and the Mie resonances in dielectric/semiconductor nanostructures has remained largely unexplored. Herein we report on the preparation of single-crystalline Cu2O nanospheres with a moderate refractive index and the study of their coupling with organic dye aggregates. Colloidal Cu2O nanospheres of uniform diameters and good sphericity are obtained using a wet-chemistry growth method aided with mild etching. Unlike high-refractive-index dielectrics, Cu2O nanospheres possess spectrally overlapped electric and magnetic dipole modes. Rhodamine aggregates are coated onto the Cu2O nanospheres to investigate the variation of the scattering spectra induced by the resonance coupling. A clear anti-crossing behavior with a mode splitting of 0.30 eV is experimentally observed. It is originated from the coupling between the overlapped dipolar Mie resonances in the Cu2O nanospheres and the excitons in the rhodamine aggregates.

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KEYWORDS: dielectric nanospheres, dye aggregates, excitons, Mie resonances, resonance coupling, scattering

The collective oscillations of free charge carriers endow plasmonic materials with the capability of controlling light at the subwavelength scale and with versatile applications in, to name a few, chemical/biochemical sensing, spectroscopy, photovoltaics and photocatalysis.1–4 Resonance coupling between plasmons and excitons, known as plexcitonic coupling, provides an attractive platform for the realization of strong light–matter interactions.5,6 Excitons in organic chromophores5–8, transition metal dichalcogenides7 and quantum dots5,7,8 have been widely incorporated in various plasmonic nanostructures. Strong coupling between surface plasmons and excitonic systems, even as small as tens of excitons9 and a single molecule10, has been experimentally observed. The plexcitonic coupling can also be actively varied by optical, thermal, electrical and chemical controls.11–13 However, the associated dissipative losses in common plasmonic materials, like Au and Ag, limit to a certain degree the efficiency of these coupling systems.11,14 On the other hand, dielectric nanostructures have recently been investigated to complement plasmonic components in many potential applications owing to their low dissipative losses and optical heating, as well as unique Mie-type resonances.14–16 Dielectric nanostructures with high or moderate refractive indexes, even in simple geometries, can simultaneously support strong magnetic and electric resonances, respectively derived from the circular displacement current and the oscillations of the polarization charges when excited by incident light.17–19 Low-loss dielectric nanostructures with high refractive indexes (larger than 3) have recently been


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employed as alternatives to plasmonic nanoantennas for the coupling with excitons. Pioneer examples include the resonance coupling between the excitons in J-aggregate dye molecules and high-refractive-index nanostructures, such as Si nanospheres,20 Si nanogrooves21 and Ge nanogrooves.22 Photon–exciton coupling in a system consisting of an optically resonant Si nanosphere placed on a WS2 monolayer has been independently studied by two research groups.23,24 However, to the best of our knowledge, there have still been no reports on the coupling between the Mie resonances in dielectric nanoantennas with moderate refractive indexes (~2.5–3) and excitons. High-refractive-index dielectrics, such as Si,25 Ge,26 GaAs27 and GaP,28 possess narrow and spectrally separated electric and magnetic resonances in the visible and near-infrared regions. In contrast, spectrally overlapping electric and magnetic resonances often exist for moderate-refractive-index nanostructures, such as Cu2O nanocrystals,29,30 perovskite nanoparticles,31–33 boron nanospheres,34 and nanodiamonds.35 The unique optical responses of moderate-refractive-index dielectrics enrich the toolbox for constructing functional nanophotonic devices. For example, the spectral overlap of the broad electric and magnetic Mie resonances in moderate-refractive-index nanostructures is conducive to the realization of broadband unidirectional scattering.29,34,36,37 Tunable second harmonic generation has been observed on individual BaTiO3 nanoparticles, which is enabled by the enhanced electromagnetic field over a large spectral range.32 It is therefore of great interest to investigate the optical interaction between the Mie resonances in moderate-refractive-index nanostructures and excitons. Herein we report on the preparation of colloidal Cu2O nanospheres and the resonance coupling between the Mie resonances of single-crystalline Cu2O nanospheres and the excitons of dye aggregates. Cu2O is a p-type semiconductor composed of earth-abundant elements. The real


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part of the refractive index of single-crystalline Cu2O is ~3 in the visible and near-infrared regions.38 Cu2O nanocrystals have been reported to exhibit facet-dependent optical,39,40 electrical41 and photocatalytic properties.39,42 Cu2O nanocrystals in the spherical shape are synthesized and used in this study owing to their perfect geometrical symmetry, which is beneficial for calculating their electromagnetic responses.43 The light extinction and scattering behaviors of the synthesized Cu2O nanospheres are measured and analyzed by electromagnetic mode decomposition. Stronger Mie resonances in the single-crystalline Cu2O nanospheres in comparison with the polycrystalline ones are observed. The resonance coupling between the single-crystalline Cu2O nanospheres and rhodamine dye aggregates is investigated with darkfield scattering spectroscopy, electromagnetic simulations, and a coupled harmonic oscillator model.

RESULTS Single-crystalline Cu2O nanospheres were synthesized following a one-step growth procedure that was modified from previously reported works.29,44 Scanning electron microscopy (SEM) imaging (Figure 1a,b) revealed that the obtained Cu2O nanospheres have a nearly spherical shape and a narrow size distribution. Figure 1c shows the selected-area electron diffraction pattern of a Cu2O nanosphere from the sample with an average diameter of 181 ± 9 nm (Figure S1, Supporting Information). The diffraction pattern is consistent with the cubic structure of singlecrystalline Cu2O.


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Figure 1. Characterization of the single-crystalline Cu2O nanospheres. (a,b) SEM images of the Cu2O nanosphere samples with diameters of 122 ± 7 and 199 ± 12 nm, respectively. (c) Selectedarea electron diffraction pattern recorded along the [110] axis of a single-crystalline Cu2O nanosphere. The inset is the transmission electron microscopy (TEM) image of the Cu2O nanosphere on which the diffraction pattern was taken. (d) SEM image of the Cu2O nanosphere sample with an average diameter of 245 ± 17 nm. (e,f) SEM images of the Cu2O nanosphere samples with diameters of 235 ± 17 and 215 ± 18 nm, which were obtained by etching the sample shown in (d) with HCl (2.5 mM in ethanol) at 0.3 and 0.6 mL, respectively. (g,h)


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Extinction spectra of the Cu2O samples measured on a spectrophotometer (g) and calculated according to the Mie theory with the diameters being equal to the corresponding experimental average values (h). All of the extinction spectra are normalized for the purpose of comparison.

The sizes of the synthesized Cu2O nanospheres were found to vary from batch to batch (Figure 1a,b,d and Figure S1, Supporting Information), which can be ascribed to the difficulty in separating the nucleation and growth processes in our one-step synthesis approach. A two-step method has been developed for the synthesis of monodisperse Cu2O nanospheres using a mild reducing agent, diethylene glycol.45 The successful separation of the nucleation and growth stages makes the particle diameter precisely controllable in the range of 90–190 nm by simply varying the amount of the Cu precursor. However, only polycrystalline Cu2O nanospheres can be obtained by this approach. Further studies will be required to realize the reproducible and facile growth of uniform single-crystalline Cu2O nanospheres. To overcome the difficulty in directly controlling the size of the grown Cu2O nanospheres, we employed a mild etching method to finely tailor the size of the resultant Cu2O nanospheres. Cu2O nanocrystals have been reported to undergo morphological changes in weakly acidic solutions.46,47 After a dilute HCl solution was injected into the synthesized Cu2O nanosphere solution, the surface of the Cu2O nanospheres experienced dissolution, resulting in a decrease in their size. The injected HCl does not change the chemical composition of the etched Cu2O nanocrystals.46 With the addition of 0.3 and 0.6 mL of a 0.25 mM ethanolic HCl solution, the average diameter of the Cu2O nanospheres was reduced from 245 ± 17 nm (Figure 1d) to 235 ± 17 nm (Figure 1e) and 215 ± 18 nm (Figure 1f), respectively. Cu2O nanospheres with average diameters of 175 ± 9 nm and 160 ± 12 nm (Figure S2, Supporting Information) were also obtained by etching the sample shown in Figure 1b with


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0.6 and 0.9 mL of the HCl solution. We note that a larger amount of the injected HCl solution will break the spherical shape of the Cu2O nanocrystals (Figure S2c, Supporting Information). The extinction spectra of all Cu2O nanosphere samples were recorded on an ultraviolet/visible/near-infrared spectrophotometer (Figure 1g). In general, the extinction peak redshifts and broadens with increasing particle sizes. The extinction spectra were well reproduced in the calculations based on the Mie theory (Figure 1h). Small discrepancies between the measured and calculated extinction spectra are believed to originate from the deviation of the particle shapes from perfect spheres and the size distributions of the Cu2O nanospheres in the colloidal solutions. The small extinction peaks located at ∼460 nm and ∼480 nm are related to the peaks in the dielectric function of single-crystalline Cu2O (Figure S3, Supporting Information), which can be attributed to the ‘indigo’ and ‘blue’ exciton transitions in Cu2O.38,48 In the wavelength range longer than 480 nm, one major peak was observed in the extinction spectra of the 160-nm and 175-nm Cu2O nanospheres, while two peaks were seen for those with diameters larger than ~180 nm. Overall, the optical responses of our single-crystalline Cu2O nanospheres can be tailored over the entire visible region, which is much broader than that of polycrystalline Cu2O nanospheres45 and similar to that of polycrystalline Cu2O nanoshells49. To ascertain the nature of the main extinction peaks, we performed calculations based on the Mie theory with the contributions from different resonances decomposed for two representative Cu2O nanospheres with diameters of 160 nm and 215 nm (Figure 2). The extinction efficiencies were calculated by dividing the extinction cross-sections with the physical cross-sections of the Cu2O nanospheres. For the 160-nm Cu2O nanosphere, the electric and magnetic dipole resonances with comparable strengths contribute to the extinction peak (Figure 2a). The electric and magnetic quadrupole resonances exhibit weak interaction with the incident


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light owing to the small size of the 160-nm Cu2O nanosphere. Finite-difference time-domain (FDTD) simulations were carried out to acquire the electric and magnetic field enhancement contours at the peak wavelength (Figure 2b,c). The maximal electric field is located at the surface of the Cu2O nanosphere, while the magnetic field is enhanced inside the nanosphere. The electric field contour shows the characteristic of the electric dipole mode, and the magnetic field contour exhibits the feature of the magnetic dipole mode.32,50 We also note that the field enhancement factors are smaller than those of similarly-sized Si nanospheres with a higher refractive index.50 For the 215-nm Cu2O nanosphere, two peaks at 525 nm and 643 nm were observed in the extinction spectrum (Figure 2d). The low-energy peak contains a dominant contribution from the broad magnetic dipole resonance and a considerable contribution from the electric dipole resonance. In contrast, the high-energy peak originates from the mixed contributions of the electric dipole resonance, magnetic dipole resonance, electric quadrupole resonance and magnetic quadrupole resonance. The near-field contours at the high-energy extinction peak contain the features of the higher-order electric and magnetic resonances (Figure 2e,f).31,32 The near-field contours at the low-energy extinction peak (Figure 2g,h) are similar to those of the 160-nm Cu2O nanosphere (Figure 2b,c), exhibiting the mixed characteristics of the electric and magnetic dipole resonances. The large spectral overlap of the electric and magnetic resonances makes Cu2O optically different from materials with high refractive indexes, which often possess spectrally separated electric and magnetic modes.29,31,34 We also compared the optical properties between the single-crystalline Cu2O nanospheres and the polycrystalline ones at similar sizes (Figures S4 and S5, Supporting Information). The polycrystalline Cu2O nanospheres were synthesized similarly using a NaOH solution at a reduced concentration. Stronger Mie resonances in the single-crystalline Cu2O nanospheres in comparison with the


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polycrystalline ones were observed. In addition, the lack of the accurate data for the dielectric function of the as-grown polycrystalline Cu2O makes it difficult to rigorously elucidate the optical responses of the polycrystalline Cu2O nanospheres by electromagnetic simulations. Therefore, the single-crystalline Cu2O nanospheres were employed in the following investigation on the resonance coupling between Cu2O nanospheres and dye aggregates.

Figure 2. Mie-theory calculations and FDTD simulations. (a) Total extinction efficiency of a 160-nm Cu2O nanosphere and separate contributions from the electric dipole resonance (ED), magnetic dipole resonance (MD), electric quadrupole resonance (EQ) and magnetic quadrupole


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resonance (MQ). (b,c) Simulated electric and magnetic field enhancement contours of the 160nm Cu2O nanosphere at the extinction peak wavelength. (d) Total extinction efficiency of a 215nm Cu2O nanosphere and separate contributions from the electric (magnetic) dipole and quadrupole resonances. (e–h) Simulated electric and magnetic field enhancement contours of the 215-nm Cu2O nanosphere at the peak wavelength of 525 nm (e,f) and 643 nm (g,h), respectively. The dashed circles in (c,f,h) stand for the interfaces between the Cu2O nanospheres and the surrounding ethanol.

An excitonic material, rhodamine 640 (R640, Figure 3a, inset) was employed in our study. A peak at 572 nm was recorded in the absorption spectrum of a dilute ethanolic R640 solution (Figure S6a, Supporting Information). R640 was adsorbed on the surface of an indium tin oxide (ITO)-coated glass slide after immersion and exhibited an extinction peak at 606 nm (Figure S6a, Supporting Information). The redshift is characteristic of the formation of the J-aggregates.51 The R640 aggregates were observed as islands with random shapes and sizes under SEM (Figure S6b, Supporting Information) and atomic force microscopy (AFM) imaging (Figure S6c,d, Supporting Information). R640 islands as large as ~300 nm in width and ~30 nm in height can be found on the ITO substrate (Figure S6b–d). The scattering signals with a 620-nm resonance peak from the large R640 aggregates are strong enough to be clearly recorded using our dark-field optical system (Figure 3a).


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Figure 3. Resonance coupling between the single-crystalline Cu2O nanospheres and the R640 aggregates. (a) Dark-field scattering spectrum of the R640 aggregates on an ITO substrate. The molecular structure of R640 is shown in the inset. (b–g) Dark-field scattering spectra and corresponding SEM images of the 181-nm (b,c), 210-nm (d,e), and 244-nm (f,g) Cu2O nanospheres before and after the coating of the R640 aggregates. (h) Variations of the energies of the scattering peaks as functions of the resonance energy of the pristine Cu2O nanosphere. E+ and E– are the energies of the high- and low-energy scattering peaks of the rhodamine 640-coated Cu2O nanospheres, respectively. E+ and E– are fitted with the formulas derived from the coupled harmonic oscillator model. The energies of the pristine Cu2O nanospheres and R640 aggregates are also drawn as the diagonal and horizontal lines.


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The colloidal solutions of the synthesized single-crystalline Cu2O nanospheres were dropped on ITO substrates and dried immediately. Sparsely distributed Cu2O nanospheres were found under dark-field and SEM imaging. The morphologies of the individual Cu2O nanospheres were observed under SEM imaging, while their scattering spectra were measured on a dark-field microscope and correlated with the morphologies using a pattern-matching method (Figure 3b– g).52 When the diameter of the Cu2O nanosphere is increased from 181 nm to 244 nm, the major scattering peak undergoes a redshift from 536 nm to 678 nm with an increased peak width. The major scattering peak mainly contains the contribution from the spectrally overlapped electric and magnetic dipole resonances, as shown by the results of the mode decomposition for the individual Cu2O nanospheres embedded in air (Figure S7, Supporting Information). The electric and magnetic dipole resonances with comparable intensities both exhibit redshifts with the enlargement of the Cu2O nanosphere. Since the spectral positions of the Mie resonances of dielectric nanoparticles with moderate or high refractive indexes are not affected considerably by substrates of low refractive indexes,33,53 the ITO substrate employed in our study was neglected in the mode decomposition. After the Cu2O nanospheres deposited on ITO substrates were immersed in an ethanolic R640 solution, R640 aggregates in irregular shapes were adsorbed on the surface of the Cu2O nanospheres (Figure 3c,e,g). The dark-field scattering spectra of the R640-coated Cu2O nanospheres were first recorded and compared with those of the pristine Cu2O nanospheres (Figure 3b,d,f). The morphology of each Cu2O@R640 nanostructure was subsequently inspected by SEM imaging to avoid the influence of the electron beam exposure on the adsorbed R640 aggregates and the scattering measurements. The major peak of the Cu2O nanosphere splits into


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two peaks after the coating of R640 aggregates. Similar peak splitting behaviors were also observed for the differently sized Cu2O nanospheres after being covered with R640 aggregates (Figures S8–S12, Supporting Information), although the shape of the coated dye aggregates varied case by case. The peak splitting behaviors are believed to originate from the coupling between the R640 aggregates and the electric and magnetic dipole resonances in the Cu2O nanospheres. In contrast to the coupling between the excitons in J-aggregates and the magnetic dipole resonance in high-refractive-index Si nanospheres,20 in our study, both of the spectrally overlapped magnetic and electric dipole resonances of the Cu2O nanospheres take part in the coupling with the excitons of the R640 aggregates. The positions of the new peaks are plotted as functions of the energy of the main scattering peak of the naked Cu2O nanosphere in Figure 3h. An anti-crossing behavior of two branches was clearly observed in the energy diagram. The dispersion of these two branches can be fitted by a coupled harmonic oscillator model,

± = ( + ) ± +

(1)

where δ is the energy difference between the scattering peaks of the pristine Cu2O nanosphere and the R640 aggregate.9,11,20 The collective contribution of the spectrally overlapped magnetic and electric dipole resonances of the Cu2O nanospheres is treated as an oscillator resonance for simplicity. A mode splitting value of 2g = 0.30 eV is obtained, which can be used to indicate the coupling strength between the Mie resonances in the Cu2O nanospheres and the exciton resonance in the R640 aggregates. FDTD simulations were also carried out for the Cu2O nanosphere−R640 aggregate heterostructures to ascertain the coupling behaviors (Figure 4). The Cu2O nanospheres with a series of diameters in the range of 150–270 nm were placed on an ITO substrate. For simplicity, light is incident perpendicular to the ITO substrate in the simulations. A uniform dye shell with a


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thickness of 10 nm was used to roughly account for the amount of the R640 aggregates surrounding each Cu2O nanosphere (refer to the SEM images of the Cu2O nanosphere–R640 aggregate heterostructures). The permittivity of the dye shell was described by a one-oscillator Lorentzian model (Figure S13a,b in the Supporting Information), which is commonly employed to express the optical response of J-aggregates.5,6 A larger oscillator strength f corresponds to a stronger exciton resonance in the dye shell. When f is set as 0, the scattering peak of the Cu2O@dye nanostructure exhibits a slight redshift compared to that of the naked Cu2O nanosphere (Figure S13c, Supporting Information). This spectral redshift can be attributed to the increased refractive index of the environment surrounding the Cu2O nanosphere. The dispersionless dielectric shell without exciton resonance cannot induce the experimentally observed peak splitting in the scattering spectra of the Cu2O nanospheres. The exciton resonance is strengthened with the increment in f, resulting in a dip in the scattering spectra of the Cu2O@dye nanostructures. When the oscillator strength f of the Lorentzian-shaped permittivity is adjusted to 0.4, the scattering spectra of the Cu2O@dye nanostructures reproduce well the shapes of the experimental scattering spectra of the Cu2O nanosphere−R640 aggregate heterostructures (Figure 4c). Some discrepancy between the simulated and measured scattering spectra can be ascribed to the less-controllable amount and irregular shape of the R640 aggregates in the experiments as well as the simplified configuration for the light excitation and collection in the simulation.20,31 As a simple example, the amount of the R640 aggregates is reduced by decreasing the thickness of the dye shell (Figure S13d, Supporting Information). The shrinkage of the shell thickness reduces the number of excitons in the dye shell, weakens the resonance coupling, and makes the dip in the scattering spectra shallower. We note that it is difficult to coat the aggregates of the dye molecules in a uniform shape onto the Cu2O


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nanospheres in our experiment. The challenge of the reproducible and accurate positioning of excitonic materials close to plasmonic and dielectric nanostructures remains to be overcome.7,8

Figure 4. Simulated scattering behaviors of the single-crystalline Cu2O nanospheres without and with an R640 shell. (a) Schematic illustration of the models used in the FDTD simulations. (b) Calculated scattering spectra of the single-crystalline Cu2O nanospheres with different diameters. (c) Calculated scattering spectra of the Cu2O@R640 nanostructures with the shell thickness of 10 nm. (d) Variations of the energies of the scattering peaks as functions of the resonance energy of the pristine Cu2O nanosphere. E+ and E– are the energies of the high- and low-energy scattering peaks of the Cu2O@R640 nanostructures, respectively. E+ and E– are fitted by the formulas derived from the coupled harmonic oscillator model. The energies of the pristine


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Cu2O nanospheres and rhodamine 640 aggregates are also drawn as the diagonal and horizontal lines.

For the Cu2O nanospheres with diameters smaller than 180 nm, the positions of their main scattering peaks largely deviate from the resonance energy of the excitons. The weak coupling strength between these spectrally detached resonances results in two hybridized peaks with their energies close to those of the resonances of the pristine Cu2O nanospheres and the dye aggregates, respectively (Figure 4c). For the diameters ranging from 180 nm to 210 nm, the resonance energy of the dipolar Mie modes in the Cu2O nanospheres gets closer to the exciton resonance in the R640 aggregates. The Cu2O nanosphere−R640 aggregate coupling leads to the occurrence of two branches with energies larger than the Mie resonance energy in the Cu2O nanospheres and smaller than the exciton resonance energy in the R640 aggregates, respectively. When the diameter is further increased, the scattering spectra of the Cu2O nanospheres become more and more undisturbed by the dye shell. The high-energy hybridized scattering peak gradually vanishes into a shoulder near the exciton resonance peak, which hampers the direct readout of the spectral position of the high-energy hybridized peak. The positions of the clearly identified hybridized peaks of all Cu2O@dye nanostructures are plotted in Figure 4d. A similar anti-crossing behavior of the hybridized branches is displayed on the energy diagram. The fitting of the two branches based on the coupled harmonic oscillator model gives rise to a mode splitting of 2g = 0.28 eV, which is close to the experimental one. Although the achieved mode splitting in our Cu2O@R640 nanostructures is larger than the splitting energy in the previously reported Si nanosphere−J-aggregate heterostructures (0.10 eV)20 and Si@exciton nanospheres (0.20 eV)54, we still have not reached the strong coupling regime. The linewidths of the main


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scattering peak of the Cu2O nanospheres (γCu2O) with diameters ranging from 150 to 270 nm are ~0.38–0.55 eV. The linewidth of the exciton resonance peak of the R640 aggregates (γR640) is 0.17 eV. The criterion 2g > γCu2O + γR640 for achieving strong coupling is not yet satisfied in our Cu2O nanosphere−R640 aggregate coupling system.7,23 We note that the interaction regime of a coupled plasmon–exciton system can be further identified with the help of the absorption spectra.55,56 In order to better clarify the coupling regime of our Cu2O nanosphere–R640 aggregate heterostructures, we also calculated the absorption cross-sections of the Cu2O@R640 nanostructures. Two representative Cu2O@R640 nanostructures with different core sizes are taken as the examples. The scattering spectra of the Cu2O@R640 nanostructures with the 180-nm and 210-nm cores possess a dip at 577 nm (purple curve, Figure 4c) and 568 nm (black curve, Figure 4c), respectively. These dips spectrally correspond to a strong peak located at 573 nm in the absorption spectra of the nanostructures (Figure S14, Supporting Information). Therefore, the resonance coupling between the Cu2O nanospheres and the R640 aggregates can be assigned to the interaction regime of enhanced absorption.55,56 In contrast, both of the absorption and scattering spectra of a strong-coupling system exhibit a clear dip and two split peaks. Dye aggregates with a large numerical extent of permittivity, realized by increasing the oscillator strength f and/or decreasing the linewidth of the exciton transition γ0, are desirable to increase the coupling strength.56 The largest numerical extent of permittivity reported for J-aggregate has an f value of 0.4 and γ0 value of 0.052 eV.56 We therefore attempted to adopt dye shells with narrow exciton transitions to increase the coupling with the Cu2O nanospheres. The scattering and absorption spectra of a representative Cu2O@dye nanosphere with an f value of 0.4 and varying γ0 values were calculated based on the Mie theory (Figure S15, Supporting Information). For γ0 = 0.17 eV, a scattering dip at 616 nm


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spectrally corresponds to an absorption peak, demonstrating the occurrence of the enhancedabsorption interaction. When γ0 is gradually decreased, the dip in the scattering spectra becomes deeper and narrower, indicating the increase in the coupling strength. As γ0 reaches 0.03 eV, both of the scattering and absorption spectra possess a dip around 620 nm. Considering the small peak splitting of 0.056/0.212 eV in the absorption/scattering spectrum, the Cu2O@dye nanosphere has not yet entered the strong coupling regime even when a large numerical extent of permittivity is used to represent the dye shell. The design and preparation of Mie–exciton hybrid systems toward strong coupling will require further studies. To gain further insights into the resonance coupling between the Cu2O nanospheres and the dye aggregates, the near-field optical responses of the Cu2O@dye nanostructures and the pristine Cu2O nanospheres were simulated by FDTD (Figure 5). A Cu2O nanosphere with a diameter of 210 nm was taken as an example. The scattering spectrum of the 210-nm Cu2O nanosphere is shown in Figure 4b (black curve). The electric field enhancement contour at its scattering peak shows the feature of the electric dipole mode, while the magnetic field enhancement contour exhibits mixed characteristics of the electric and magnetic dipole modes (Figure 5a).32,50 Therefore, the scattering peak of the 210-nm Cu2O nanosphere is constructed from the collective contribution of the electric and magnetic dipole resonances. When a 10-nm-thick R640 shell is coated on the Cu2O nanosphere, two spectral peaks (549 and 647 nm) and a dip (568 nm) were observed in the scattering spectrum (black curve in Figure 4c). The field intensity and charge density distribution contours were calculated at the scattering peaks and dip for the Cu2O@R640 nanostructure (Figure 5b–d). The electric and magnetic field enhancement contours at the 549nm peak resemble a typical electric dipole mode (Figure 5b).50 On the other hand, the features of the electric and magnetic dipole modes can be found in the electric field and magnetic field


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contours, respectively, at the 647-nm peak (Figure 5d). As shown in the second column of Figure 5b–d, the magnetic field enhancement contour at the 568-nm dip carries a mixed nature of the electric and magnetic dipole resonances, separately seen at the 549-nm and 647-nm peaks. Moreover, the charge densities at the Cu2O−R640 and R640−air interfaces at the 568-nm dip are anti-phase and possess comparable amplitudes. The anti-phase coupling between the Mie resonances in the Cu2O nanosphere and the exciton resonance in the R640 shell cancels each other and produces a dip in the scattering spectrum. In this way, the resonance coupling between the collective electromagnetic dipolar resonances and exciton resonance gives rise to the peak splitting in the scattering spectra of the Cu2O nanosphere−R640 aggregate heterostructures. Here, the simulations were performed using normally incident light. Considering that the excitation light was incident on the samples at an angle of 64° in our dark-field optical system, we also performed FDTD simulations for a 210-nm Cu2O nanosphere using the angled light sources with s- and p-polarization (Figure S16a,b in the Supporting Information). The scattering spectra collected under the s- and p-polarized excitation are close to that under normal incidence, especially at the spectral range of 500–700 nm (Figure S16c, Supporting Information). The features of the electric and magnetic dipole modes are also presented in the field contours (Figure S16d,e in the Supporting Information). However, the near-field distributions are more complicated than those in the case of normal incidence. For example, the electric field is concentrated at the top semi-sphere for s-polarized incidence, while a stronger electric field is found in the gap between the Cu2O nanosphere and the ITO substrate for p-polarized incidence. Therefore, the position of the dye aggregates on the surface of the Cu2O nanosphere will also affect the coupling strength. Since the R640 aggregates are randomly adsorbed on the Cu2O nanospheres in our experiment, the Cu2O–dye interaction is far more complicated than the


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simulations given here. Nevertheless, our simplified models help the further understanding on the Mie–exciton coupling.

Figure 5. Near-field optical responses of a representative Cu2O nanosphere and Cu2O@R640 nanostructure under normal incidence. (a) Simulated electric and magnetic field enhancement contours and charge density distributions at the scattering peak of a single-crystalline Cu2O nanosphere on an ITO substrate. (b–d) Simulated electric and magnetic field enhancement contours and charge density distributions at the high-energy scattering peak, dip and low-energy peak of an R640 coated-Cu2O nanosphere, respectively. The diameter of the single-crystalline


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Cu2O nanosphere is 210 nm in the simulations. The thickness of the R640 shell is 10 nm. The monitors were placed in the planes across the center of the nanostructures perpendicular (left panel) and parallel to (right panel) the direction of the incident light. The dashed circles stand for the surfaces of the Cu2O nanospheres.

CONCLUSION In summary, single-crystalline Cu2O nanospheres with good shape and size uniformity have been synthesized by a wet-chemistry growth method. The size of the directly grown Cu2O nanospheres can be finely modified using a mild etching process. The employed chemical synthesis method favors the large-scale production of uniform Cu2O nanospheres for practical applications. The optical responses of the moderate-refractive-index Cu2O nanospheres have been experimentally measured and electromagnetically analyzed. The spectral overlap between the electric and magnetic dipole resonances of the single-crystalline Cu2O nanospheres has been revealed by mode decomposition based on the Mie theory. The splitting of the scattering peaks has been observed upon the coating of rhodamine aggregates on the individual Cu2O nanospheres. The peak splitting is ascribed through electromagnetic simulations to the resonance coupling between the excitons in the dye aggregates and the overlapped dipolar Mie resonances in the Cu2O nanospheres. A mode splitting of 0.30 eV is determined from the anti-crossing branches of the hybridized modes based on a coupled harmonic oscillator model. The effects of the oscillator strength and thickness of the dye shell on the scattering spectra of the Cu2O@dye nanostructures have also been studied. This work extends our understanding on the coupling between Mie resonances, especially in moderate-refractive-index nanostructures, and molecular excitons.


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METHODS Growth of the Single-Crystalline Cu2O Nanospheres. The single-crystalline Cu2O nanospheres were prepared following previously reported methods with slight modification.29,44 Specifically, cupric acetate (0.05 g) and poly(vinylpyrrolidone) (0.05 g, molecular weight: ~10000) were dissolved in water (100 mL), forming a blue solution. Deionized water with a resistivity of 18.2 MΩ cm obtained from a Direct-Q 5 UV water purification system was used in all experiments. NaOH solution (20 mL, 0.25 M) was subsequently added dropwise into the solution at room temperature under vigorous stirring, followed by the addition of ascorbic acid (15 mL, 0.05 M) under vigorous stirring. After the resultant solution was kept under stirring for 30 min, the product was centrifuged immediately, then washed with ethanol three times, and finally stored in ethanol. The Cu2O nanospheres with diameters of 81 ± 7, 122 ± 7, 181 ± 9, 199 ± 12 and 245 ± 17 nm were obtained from different batches. The batch to batch variation in the size of the produced Cu2O nanospheres can be explained by the difficulty in separating the nucleation and growth stages in our one-step synthesis approach. Mild Etching of the Cu2O Nanospheres. HCl aqueous solution (5 M) was diluted with ethanol and used as an etching agent (2.5 mM). Different amounts of the etching agent were added into 2 mL of the as-prepared Cu2O nanosphere samples. The resultant solution was vigorously shaken on a vortex mixer for 30 min. The etched nanospheres were collected by centrifugation and redispersed in ethanol. Growth of the Polycrystalline Cu2O Nanospheres. The procedure for the synthesis of the polycrystalline Cu2O nanospheres is similar to that for the growth of the single-crystalline ones,


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except that the concentration of the used NaOH solution was reduced to 0.025 M and that the added amount of cupric acetate is 0.018 g. Scattering Measurements. 20 µL of the prepared Cu2O colloidal solution was dropcast onto an ITO-coated glass slide (Shenzhen Nanbo Display Technology Co., Ltd., STN-SI-10) and blown dry immediately with nitrogen gas, leaving sparsely distributed Cu2O nanospheres on the substrate. A pattern-matching method was employed to correlate the morphology of each nanosphere observed under SEM imaging and its scattering spectrum recorded on a dark-field optical microscope. The dark-field optical system was built on an Olympus BX60 optical microscope, which was integrated with a quartz–tungsten–halogen lamp (100 W), a monochromator (Acton SpectraPro 2360i), and a charge-coupled device camera (Princeton Instruments, Pixis 400, thermoelectrically cooled to −70 °C). A dark-field objective (100×, numerical aperture: 0.9) was employed for exciting the target nanostructure with white light and collecting the scattered light. The collected scattering spectrum of the individual nanostructure was corrected by first subtracting the background spectrum taken from the adjacent region without any nanostructures and then dividing with the calibrated, normalized response curve of the entire optical system. In order to coat the Cu2O nanospheres with R640, the ITO slide deposited with the single-crystalline Cu2O nanospheres was immersed in a 3-mM solution of R640 perchlorate (Exciton, Inc., also known as rhodamine 101) in ethanol. After 10 h, the slide was taken out and blown dry immediately with nitrogen gas. The same Cu2O nanospheres were located using the pattern-matching method under the optical imaging. The morphology of each R640-coated Cu2O nanosphere was recorded by SEM after the measurement of its dark-field scattering spectrum. A bare ITO slide was also immersed in the 3-mM R640 solution for 10 h for the characterization of the dye aggregates.


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Characterization. The extinction spectra were measured on a Hitachi U-3501 ultraviolet/visible/near-infrared spectrophotometer. SEM imaging was carried out on an FEI Quanta 400 FEG microscope. The diameters of the Cu2O nanospheres were measured on their respective SEM images, with ~100 particles measured per sample. TEM imaging and electron diffraction were performed on an FEI Tecnai Spirit microscope, which was operated at 120 kV. AFM imaging was conducted in air on a Veeco Metrology system (Model No. 920-006-101) that was operated at the contact mode using a super-sharp silicon nitride AFM tip (Bruker). ICP-OES measurements were carried out on an Agilent ICP-MS 7500a system. 1 mL of the Cu2O nanosphere solution was centrifuged. Aqua regia (20 µL) was then added to digest the Cu2O nanospheres. (Caution! Aqua regia is highly corrosive and extremely dangerous.) The volume of the digested solution was subsequently adjusted to 1 mL with deionized water. The Cu concentration of the resultant solution (diluted 10000 times before measurements) was determined with ICP-OES against a pre-calibrated linear relationship between the optical emission intensity and the Cu concentration. Electrodynamic Simulations. The calculations based on the Mie theory for the extinction and scattering spectra of the Cu2O nanospheres, as well as the mode decomposition, were performed using a self-made code.29,43 The refractive indexes of the surrounding environment were set at 1.36 for calculating the extinction spectra of the Cu2O nanospheres dispersed in ethanol, and 1 for calculating the scattering spectra of the Cu2O nanospheres in air. FDTD calculations were performed using commercial software, FDTD Solutions 8.7 (Lumerical Solutions). In the simulations, a total-field scattered-field source was launched into a box containing a target nanostructure deposited on an ITO substrate (refractive index = 1.9) or dispersed in ethanol. Planar monitors were placed in the backward direction to simulate the dark-


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field scattering measurements. The nanostructure and its surrounding medium inside the box were divided into meshes of 1 nm in size. The dielectric function of Cu2O was calculated according to a previously reported formula38 = + ∑!=C,D

+ ∑!=C,D

C C &D % (' )

ln (

( )

) ('

)

+ ∑!=,+

+

C

+

(2)

,-

.- /

where the five terms on the right side correspond to the contribution from the high-frequency dielectric constant, three-dimensional discrete excitons, three-dimensional continuum excitons, two-dimensional discrete excitons and E2 transition, respectively. All of the parameter values in the above dielectric function of Cu2O were taken from the previous work.38 The dispersion of the dye layer was described by a Lorentz line shape (0) = −

23

3 3 '3/

(3)

where the high-frequency component ε∞ was fixed at 2.0, the exciton transition frequency of the R640 aggregates ω0 was set at 2.0 eV, the linewidth of the exciton transition γ0 was configured to be 0.17 eV according to the measured extinction spectrum of the R640 aggregates, and the oscillator strength f was adjusted to be 0.4.

ASSOCIATED CONTENT Supporting Information SEM images and extinction spectra of the as-grown and etched single-crystalline Cu2O samples; refractive index of single-crystalline Cu2O; comparison between the single-crystalline and polycrystalline Cu2O nanospheres; linear calibration for the ICP-OES measurements; characterization of the rhodamine 640 aggregates; mode decomposition of the electric and


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magnetic modes for the Cu2O nanospheres; single-particle dark-field scattering spectra and SEM images of more Cu2O nanospheres before and after the coating of the rhodamine 640 aggregates; effects of the oscillation strength f and shell thickness on the resonance coupling; FDTDsimulated absorption spectra of two Cu2O nanospheres coated with a 10-nm R640 shell; Miecalculated absorption and scattering spectra of the Cu2O@dye nanospheres with various exciton linewidth γ0; effect of the angled incidence. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT J.F.W. acknowledges financial support from Hong Kong Research Grants Council (GRF, 14320916). H.Q.L. acknowledges financial support from NSAF (U1530401), computational resources from Beijing Computational Science Research Center, and National Natural Science Foundation of China (91630313). Q.F.R. acknowledges financial support from CUHK Postdoctoral Fellowship Scheme.

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(52) Chen, H. J.; Shao, L.; Man, Y. C.; Zhao, C. M.; Wang, J. F.; Yang, B. C. Fano resonance in (gold core)−(dielectric shell) nanostructures without symmetry breaking. Small 2012, 8, 1503– 1509. (53) Markovich, D. L.; Ginzburg, P.; Samusev, A. K.; Belov, P. A.; Zayats, A. V. Magnetic dipole radiation tailored by substrates: numerical investigation. Opt. Express 2014, 22, 10693– 10702. (54) Tserkezis, C.; Gonçalves, P. A. D.; Wolff, C.; Todisco, F.; Busch, K.; Mortensen, N. A. Mie-excitons: understanding strong coupling in dielectric nanoparticles. arXiv preprint arXiv:1805.00788 2018, na. (55) Zengin, G.; Gschneidtner, T.; Verre, R.; Shao, L.; Antosiewicz, T. J.; Moth-Poulsen, K.; Käll, M.; Shegai, T. Evaluating conditions for strong coupling between nanoparticle plasmons and organic dyes using scattering and absorption spectroscopy. J. Phys. Chem. C 2016, 120, 20588–20596. (56) Antosiewicz, T. J.; Apell, S. P.; Shegai, T. Plasmon–exciton interactions in a core–shell geometry: from enhanced absorption to strong coupling. ACS Photonics 2014, 1, 454–463.


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Title: Coupling between the Mie Resonances of Cu2O Nanospheres and the Excitons of Dye Aggregates Authors: Qifeng Ruan, Nannan Li, Hang Yin, Ximin Cui, Jianfang Wang, and Hai-Qing Lin


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Coupling between the Mie Resonances of Cu2O Nanospheres and

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