Silver Nanoshell Plasmonically Controlled Emission of Semiconductor

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Silver Nanoshell Plasmonically Controlled Emission of Semiconductor Quantum Dots in the Strong Coupling Regime Ning Zhou, Meng Yuan, Yuhan Gao, Dongsheng Li, and Deren Yang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b07400 • Publication Date (Web): 13 Mar 2016 Downloaded from http://pubs.acs.org on March 14, 2016

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Silver Nanoshell Plasmonically Controlled Emission of Semiconductor Quantum Dots in the Strong Coupling Regime Ning Zhou†,‡, Meng Yuan†,‡, Yuhan Gao†,‡, Dongsheng Li†,‡,*, and Deren Yang†

†

State Key Laboratory of Silicon Materials and School of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China

‡

Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China

Corresponding Author *E-mail: [email protected].

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Abstract

Strong coupling between semiconductor excitons and localized surface plasmons (LSPs) giving rise to hybridized plexciton states in which energy is coherently and reversibly exchanged between the components, is vital especially in the area of quantum information processing from fundamental and practical points of view. Here, in photoluminescence spectra, rather than from common extinction or reflection measurements, we report on the direct observation of Rabi splitting of approximately 160 meV as an indication of strong coupling between excited states of CdSe/ZnS quantum dots (QDs) and LSP modes of silver nanoshells under nonresonant nanosecond pulsed laser excitation at room temperature. The strong coupling manifests itself as an anticrossing-like behavior of the two newly formed polaritons when tuning the silver nanoshell plasmon energies across the exciton line of the QDs. Further analysis substantiate the essentiality of high pump energy and collective strong coupling of many QDs with the radiative dipole mode of the metallic nanoparticles for the realization of strong coupling. Our finding opens up interesting directions for the investigation of strong coupling between LSPs and exctions from the perspective of radiative recombination under easily accessible experimental conditions.

KEYWORDS: Rabi splitting, strong coupling, plexciton, localized surface plasmon, silver nanoshell, quantum dot, photoluminescence

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Combining metal and semiconductor nanostructures being resonant with each other opens up unique possibilities for the coupled excitations that can act differently than the optical excitations of the individual components and potentially lead to synergistic properties caused by interactions between the constituents.1 Such exciton−plasmon interactions allow manipulation of absorption2,3 and emission properties,4,5 control of energy-transfer processes,6,7 enhancement of optical nonlinearities,8 and creation of new polaritons in the strong coupling regime.9,10

Generally, exciton−plasmon interactions can be split into two principal regimes, the weak coupling and the strong coupling regime. Strong coupling regime has been realized between emitter and cavity exciton–photon interaction, when their interaction becomes larger than the dipole emitter decay rate and the cavity field decay rate.11-13 However, the diffraction-limited optical cavities place a lower bound on the size of these systems.14 Surface plasmons (SPs) of metal nanoparticles (NPs) are regarded as promising candidates for the investigation of strong coupling effects. The combination of concentrating electromagnetic energy to sub-wavelength scale and enhancing the local density of states of radiation makes it possible to observe the vacuum Rabi splitting for SPs and emitters without the need for a closed cavity at room temperature.15 The resonant exciton–plasmon interactions lead to changes of exciton and SP resonance energies and form new hybrid states called ‘plexciton’16 or ‘excimon’.17 The energy is coherently and reversibly exchanged between the plasmonic and excitonic systems (Rabi oscillations), which is

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fundamentally interesting and creates exciting and attractive opportunities for single-photon sources and transistors,18,19 thresholdless lasing,20 Raman enhancement via polariton states,21 tuning the work-function of materials,22 and modifying chemical reaction landscapes.23 In the context of quantum information technology, achieving the strong coupling regime or, in other words, quantum coherent oscillations between the coupled systems, is a prerequisite for quantum information processing.15

Since the first observation of strong coupling between SP modes and excitons in J-aggregates,24 great progress has been made and Rabi splitting even in the ultrastrong coupling regime has been achieved.25-27 Attaining the strong coupling regime was reported for various types of emitters: J-aggregates,26-34 dye molecules,35,36 photochromic molecules,25 quantum wells,37-39 nanowire,40 and quantum dots (QDs),10,41 which interact with SP modes of metal film,9,10,25,35,41 nanoslit array,30-32,38 nanorod array,33,36 nanodisk array,33,37 nanoshells,16,34 nanoprisms,26,29 and graphene.39 As to excitonic materials, most of the researches focus on J-aggregates or dye molecules because they offer a variety of advantages, like relative ease of manipulation and strong dipole moments especially for J-aggregates possessing a relatively narrow absorption line. The disadvantage of organic molecules is that they are prone to bleaching and thus will not endure high optical intensities. QDs, the zero-dimensional excitonic system with engineered electronic levels that can give rise to tunable, highly efficient emission and absorption, on the other hand, are good candidates.42 Rabi splitting has been observed in a single QD embedded in a photonic

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crystal nanocavity,12 a microcavity pillar11 and a microdisk microcavity.43 However, there are few reports about strong coupling between excitons of QDs and SPs except the work made by Gómez’s group utilizing Kretschmann-Raether configuration (prism coupling) and attenuated total reflection technique.10,41

Strong coupling manifests itself as anti-crossing and splitting of energy levels at the resonance frequency in view of emission, extinction or reflection spectra. In photoluminescence (PL) spectra, anti-crossings between the QDs exciton and cavity-mode dispersion relations have been reported.11,12 When it comes to strong coupling between SPs and excitons, PL from the plexciton states has been mentioned.9,28,44 PL spectra of Ag nanoprisms/J-aggregates plasmon–exciton systems were found much broader than the fluorescence of the free J-aggregates in solution.45 Sugawara et al. reported enhanced PL with peak blue-shifted from the strongest plasmon absorption.28 Bellessa et al. found the upper polariton (UP) state is not present in the luminescence spectra,9 which is the case in Ag(I)-carboxylate nanoclusters.44 Fedele et al. found the recreation of a Fano like line shape in PL spectra demonstrating changes in the emission spectral profile under strong coupling.46 Rodriguez et al. investigated the emission angular spectra of an Al nanoantenna array embedded in an organic dye slab waveguide and demonstrated the transition from weak to strong coupling between localized surface plasmons (LSPs) in the nanoantennas and the fundamental guided mode in the slab.47 Eizner et al. also studied the emission properties of the hybrid exciton-LSP states in the complexes of Al nanoantennas coated with

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molecular J-aggregates.33 In the last two systems, due to the large number of uncoupled excitons that contribute to the emission, splitting in the emission from the sample could not been observed directly, and PL enhancement, which is the ratio of the PL from the complex with and without the nanoantenna array, is introduced. Recently, Melnikau et al. obtained clear anticrossing behavior of the hybridized modes not only in the extinction but also in the PL spectra of the hybrid gold nanorods and J-aggregates systems.48 Thus, to our knowledge, there are no definite results about Rabi splitting and anti-crossing realized in direct emission spectra of the hybrid excitons of QDs and LSPs states. Systematic study concerning how the emission from the plexciton state is modified by SPs in the strong coupling regime is necessary and meaningful for quantum information processing.

In this paper, we integrate CdSe/ZnS QDs and silver nanoshells together, and Rabi splitting approximately as large as 160 meV in PL spectra is realized under the nonresonant excitation of nanosecond pulsed laser at room temperature. The Rabi splitting energy is found to increase linearly with the square root of excitation energy. By tuning the silver nanoshell plasmon energies across the exciton line of the QDs, the dispersion lines present an anticrossing-like behavior, which is the signature of strong plasmon–exciton coupling. High pump energy and collective strong coupling of many QDs with the radiative dipole mode of Ag nanoshell are believed vital to the realization of strong coupling in our system. RESULTS AND DISCUSSION Rabi splitting in Ag nanoshells/QDs system under pulsed laser excitation.

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The excitation-detection scheme for the studies on strong coupling between LSPs of Ag nanoshells and excitons of CdSe/ZnS QDs is illustrated in Figure 1. Figure S1 gives the photograph of the configuration of excitation-detection. The QDs solutions suspended with or without Ag nanoshells were loaded in a 10 mm cuvette and pumped with 1.4 ns pulses at λ = 532 nm from a diode pumped passively Q-switched solid state laser FTSS 355-300 (CryLaS GmbH) at repetition rate of 20 Hz. The emission was recorded with an Acton SpectraPro-2500i monochromator coupled to a photomultiplier tube (PMT) at room temperature. The Ag nanoshells prepared by a reported method49 consist of SiO2 core with diameter of 220 nm and Ag shell with thickness of 30 nm as displayed in Figure S2 and are represented as 220 SiO2@30 Ag. Because the continuous shell is achieved by a seed-mediated growth method with Ag nuclei attached on silica spheres serving as nucleation sites for the growth of outer Ag shells, the surfaces of the Ag nanoshells are not smooth.

Then we mixed these Ag nanoshells and CdSe/ZnS QDs together and conduct a series of PL tests. The evolution of emission spectra of QDs_570, QDs_595 and QDs_620 (the number denotes the emission center wavelength of QDs) together with Ag nanoshells as a function of pump energy is depicted in Figure 2a-c, respectively. Under low pump energy, the spectral features resemble that of pure QDs (Figure S3). For pure QDs, only broad spontaneous emission of QDs was identified when excited up to the maximum pump energy. As the pump energy exceeds a threshold, the emission spectra of the mixture systems begin

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to manifest distinctive characteristics and show a doublet of peaks around the exciton resonance frequency which is the direct evidence of Rabi splitting. The higher energy mode on the blue side of the exciton is referred to as the UP branch and the lower energy mode on the red side is referred to as the lower polariton (LP) branch. After analyzing the spectra by fitting with two or three Lorentzian peaks (Figure S4), it is apparent that the UP and LP shift to higher or lower energy with the increase of the pump energy as depicted in Figure 2d-f, meaning that the Rabi splitting energy ∆ERabi increases with the pump energy (∆ERabi=EUP-ELP). As for 220 SiO2@30 Ag/QDs_595 system displayed in Fig. 2b and 2e, remarkable splitting value up to 103 meV has been achieved when the pump energy is 179.6 µJ, the maximum pump energy limited by our laser instrument. When the exciton energy of QDs is close to the pump energy such as QDs_570, even at lower pump energy (130.5 µJ), Rabi splitting value up to 123 meV could be obtained (Fig. 2a and 2d), owing to pump and emission energy matching resultant higher pump efficiency. On the other hand, because the emission wavelength of QDs_620 match the extinction peak of 220 SiO2 @30 Ag (Fig. 3a), the better overlap contributes to easily realized strong coupling (Fig. 2c and 2f).

Further analysis finds that the splitting energy increases being proportional to the square root of the excitation energy as marked in the insets of Figure 2d-f. Taking 220 SiO2@30 Ag/QDs_595 for example, their relation is ∆ERabi = -67.3+12.5×Epum1/2

(1)

where ∆ERabi is the Rabi splitting energy and Epum is the pump energy of the nanosecond

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pulsed laser. The other two systems also follow the relationship and are displayed in Table S1. In addition, we also mix 220 SiO2@30 Ag with CdSSe/ZnS QDs purchased from another corporation. Similar phenomena take place and Rabi splitting value up to 160 meV could be achieved demonstrating that the strong coupling in our system does not only apply to special QDs (Figure S5). The relationship between splitting energy and pump energy has also been investigated in InxGa1-xAs quantum disk focusing on the strong coupling between photon and exciton.50 The pump power (7~46 MW/cm2) in our experiment is far larger than that of quantum disk ( + ⁄2

(4)

Ω is the normal-mode splitting (corresponding to the vacuum Rabi splitting). Although the condition required for strong coupling is often worded as ‘the splitting has to be larger than the widths of the modes’, this should be understood more as an order of magnitude and the actual measured splitting can be slightly smaller than the average width.15 In our experiments, although the Rabi splitting is smaller than both the linewidths of QDs and Ag nanoshells when the pump energy is low, the emission spectra of the mixture systems as displayed in Figure 2, Figure S9 and Figure S10 indeed show a doublet of peaks around the exciton resonance frequency when the pump energy exceeds a threshold, and these should be the direct evidence of Rabi splitting. Calculated from equation (1), when the pump energy Epum >29.0 µJ, the Rabi splitting energy ∆ERabi >0 and thus Rabi splitting should be observed in the emission spectra. However, only one peak is observed in Figure 2c under low pump energy (the Rabi splitting threshold is 90.7 µJ). This is in analogue with the case of strong coupling between photon and exciton in a single QD–semiconductor microcavity system, in which the broadening of each peak is too important as compared to their splitting although in the strong coupling regime.11,52 Moreover, the doublets peaks are not symmetric (Figure 2a-c and Figure S4) due to internal coupling and propagation effects as proposed in Mollow triplets.53,54

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In the total Hamiltonian of the hybrid system consisting of a QD and a metal NP, one term describes the interaction of the external driving field with the transition of QDs. Hakami et al. predicted that the spontaneous emission spectra of the QDs strong coupling to SPs could be manipulated via the external coherent control.55 The external driving field in their calculation was applied at a different frequency from that of the QDs and SPs to avoid overheating effects which could degrade the performance. This is practically essential for designing devices such as surface plasmon amplification by stimulated emission of radiation (SPASER)56 or dipole nanolasers. However, so far there are no experimental reports about external driving field controlled emission from the plexciton states and our report provides a simple method to realize this.

The interaction between SPs and the excitons of QDs results in hybridized plexciton states which exhibit typical anti-crossing behavior. Thus we also prepared different Ag nanoshells with different LSP properties by changing geometrical parameters (Figure S8). Figure 3a demonstrates that the dipolar plasmon peak of Ag nanoshells can be tuned from 566 nm to 945 nm by tuning the SiO2 core diameter and the Ag shell thickness. Rabi splitting is reproduced in all these systems (Figure S9). The pump threshold for Rabi splitting becomes larger when the LSP resonance detunes largely from the exciton energy as depicted in Figure 3b. When pumped at the same energy, the solid and open circle dots in Figure 3b standing for UP and LP energy exhibit anticrossing-like behavior (red solid line) when LSP resonance energy is tuned across the exciton energy. This is an evidence of strong

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coupling. We didn’t obtain standard anticrossing behavior perhaps because of the pump-emission energy matching dependent coupling strength or the latter mentioned collective strong coupling of many QDs with the radiative dipole mode of the metallic NPs.

It is noteworthy to emphasize the importance of Ag nanoshells compared to other shape of metallic NPs. SPs and their electro-optical properties can be intentionally and effectively regulated by the size and shape of nanostructures.57,58 The plasmonic properties of the nanoshells can be understood as the hybridization of the SPs on the inner and outer surfaces of the metallic shell layer corresponding to sphere-like and cavity-like plasmonic modes.59 Consequently, the LSP resonance can be conveniently modulated by the relative dimensions of their core and shell layers. Metallic nanoshells have been extensively researched in many fields including solar cells,3 stimulated emission depletion fluorescence lifetime imaging microscope,60 enhancement of fluorescence intensity and photochemical robustness,61-63 and low-threshold plasmonic lasing.64,65 Meanwhile, metallic nanoshells turn out to be a suitable platform to investigate the strong-coupling of LSP−exciton interaction. Alpeggiani et al. gave a theoretical formulation of the interaction between a radiating dipole and a metal nanoshell in the quasistatic approximation.59 They found the most favorable situation for strong coupling is when the dipole is located inside the nanoshell. Cacciola et al. also theoretically demonstrated that ultrastrong coupling regime can be reached in nanoshells constituted by a squaraine core with broad optical transitions surrounded by a silver or gold shell.66 Halas et al. experimentally observed coherent coupling between the excitons of

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molecular J-aggregates and the LSP of a nanoshell associated with both the dipolar plasmon mode and the quadrupolar plasmon mode of the nanoshells.16 Furthermore, the ultrafast optical dynamics of the hybridized system was also revealed.34 However, the aforementioned experimental results were all characterized by extinction spectra. Our system promises an uncustomary opportunity for the quantum control of light and application in quantum information processing by achieving Ag nanoshell plasmonically controlled emission of QDs in the strong coupling regime. Although the broad extinction of Ag nanoshells imposes a large threshold for strong coupling, on the other hand, this concurrently makes LSPs of Ag nanoshells strongly interact with excitons with a large range of emission energy as seen in Figure 2. Thus our system is a flexible, accessible, and universal platform to investigate plasmon controlled emission in the strong coupling regime.

Collective strong coupling of many QDs with the radiative dipole mode of the metallic NPs.

There have been theoretical examinations of the strong coupling between excitons and LSPs based on quantum mechanical framework taking into consideration higher order modes, all of which study quantum optical interactions between a QD emitter and a single metallic NP.51,53,55,67,68 Due to their small effective volumes, higher order modes are most likely to give rise to strong-coupling effects.59 In the weak coupling regime, higher order modes are all nonradiative and act as an effective path for QDs decay. When considering a collection of QDs interacting with metal NPs in the strong coupling regime, higher order 14


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modes do not behave as a featureless continuum, but as a far-detuned pseudomode.69 In this case, collective strong coupling of many QDs with the radiative dipole mode of the metallic NPs builds up even at very short distances. Moreover, the PL intensity of the hybrid system is enhanced compared with that of pure QDs, which is consistent with the results in Figure 2a-c and Figure S3. We also prepared SiO2 shell with thickness of 1.5 nm isolated Ag nanoshell (Figure S10a), and the threshold for Rabi splitting becomes larger and the splitting energy is reduced compared with that of Ag nanoshell directly touching the QDs (Figure S9b) due to the increase of the effective distance between QDs and Ag nanoshells and the resultant decrease of the coupling strength.70,71 This demonstrates that short distance even direct touching makes collective strong coupling of many QDs with the radiative dipole mode of metallic NPs easily realizable.

To systematically study the collective strong coupling of many QDs with the radiative dipole mode of metallic NPs, we change the concentration of QDs (Figure 4a) and Ag nanoshells (Figure 4b) while maintaining the same pump energy. The polariton peaks especially the UP exhibit a redshift with increasing the concentration of QDs due to the self-absorption of QDs. When the concentration is below 4 µM, no apparent Rabi splitting is observed, because the aforementioned broadening of the extinction peak of Ag nanoshell prevails the splitting and there are not enough QDs available in the near-field regime of metallic NPs. Increasing the concentration of QDs above 8 µM leads to the reduction of splitting energy, which is consistent with the prediction that the splitting energy between the

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hybrid modes does not increase with the number of QDs.69 Additionally, high concentrations of QDs also make it difficult for the pump energy to propagate to the surface of the Ag nanoshells and consequently severely reduce the splitting energy.42

As shown in Figure 4b, increasing the amount of Ag nanoshells gives rise to no apparent Rabi splitting due to the increased metal dissipation. Meanwhile, the effect of electron tunneling from the emitter into metallic nanostructures has been theoretically predicted to prevent the observation of strong coupling.72 These two factors synergistically lead to the reduction of Rabi splitting energy. It is worthy to note that Rabi splitting can still be observed even if the nanoshell concentration is as low as c = 8.3 ×108 mL-1, thus strong coupling in the hybrid system exists spanning three orders of magnitude of Ag nanoshell concentration.

The mechanism of strong coupling realized in our hybird system. Firstly, pump energy absorbed by the QDs and by the Ag nanoshells was measured. The energy of the input and output pulsed laser before and after the solution loaded in a 2×10 mm cuvette was recorded. The cuvette with optical length of 2 mm rather than 10 mm is used because the absorbance of the solution is high leading to the output pulsed laser so weak that the output energy is below the lower detection limit of the pyroelectric energy sensors. The cuvette is held in a substrate with the front face perpendicular to the direction of the puled laser.

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Figure S11 gives the energy of the input and output pulsed laser at 532 nm and the ratio of the pump energy absorbed by the solutions ( =

! "#$ !

). It is shown that the ratio of the

pump energy absorbed by the QDs has little change with the pump energy and that by the Ag nanoshells is reduced with the increase of the pump energy. However, in the 220 SiO2@30 Ag/QDs_595, the ratio of the pump energy absorbed by the system firstly increases with the pump energy and then reaches a steady value when the pump energy exceeds a threshold. When strong coupling is reached, the resonant exciton–plasmon interactions lead to changes of exciton and SP resonance energies and form new hybrid states. The energy is coherently and reversibly exchanged between the plasmonic and excitonic systems (Rabi oscillations). It is hard to distinguish them. The results on the other hand demonstrate that coupling is indeed obtained in our system. So the ratio of the pump energy absorbed by the system is not simply the sum of the ratio of QDs and Ag nanoshells. The ratio could be used to evaluate the extinction change as a function of pump power. The strong coupling energy % is given by the spatial overlap of the excitonic transition dipole moment µ and the induced surface plasmon electric field E as in equation (5): ∆'()* ∝ % ∝ ,- ∙ '/- ∝ 012 ∙E

(5)

Here 2 is the oscillator strength of dipoles and 1 is the concentration of the dipoles involved in strong coupling.21 Thus in our system the splitting energy is proportional to the induced surface plasmon electric field E, the square root of the oscillator strength f of a QD and the concentration of excitons n, i.e. the number density of QDs in the solution. Below

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we will analyze the mechanism of strong coupling from these three aspects.

To model the electric field surrounding rough Ag nanoshells, we distributed randomly small nanospheres with diameters following Gaussian distribution on a smooth nanosphere using FDTD Solutions (Lumerical Solutions Inc.). The schematic diagram of a rough SiO2@Ag nanoshell is displayed in Figure 5a as a simulation of a real nanoshell with many lumps protruding out on the surface with height up to 15 nm as displayed in the inset of Figure 5a. By optimizing parameters, the distribution of electric field intensity across the center (Figure 5b) are calculated. Compared with smooth nanoshell (Figure S12), it is obvious that the electric field is highly intensified and focuses on the protruding particles and the spaces between neighbouring particles, which makes strong coupling easily accessible and the PL intensity enhanced.33

The pump-generated exciton density n will saturate the excitonic oscillator strength, which, in thermal quasi-equilibrium, can be approximated as 2 = 25 /(1 + 1/19 ), where f0 is the excitonic oscillator strength in the absence of the pump and ns is the exciton saturation density.30,73 Thus increasing pump density will accordingly reduce the excitonic oscillator strength and consequently lessen the splitting energy. Meanwhile, more participated QDs will lead to larger splitting energy. From the evolution of emission spectra as a function of pump energy for pure QDs with emission center at 595 nm in Figure S3b. We observe PL intensity saturation which may be the manifestation of the number saturation of contributing QDs, but this is not the case in the other two systems in Figure S3. 18


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As we mentioned before, the pump power density (7~46 MW/cm2) from the pulsed laser in our experiment is far larger than that of normally used laser (usually continuous laser). The induced surface plasmon electric field E, which is proportional to square root of the excitation energy, is predominant among the splitting energy related three factors. This is consistent with linear relationship as displayed in Figure 2d-f. Further investigation is necessary to prove this convincingly.

Therefore, Figure 5c gives the representation of collective strong coupling of many QDs with the radiative dipole mode of an Ag nanoshell with strongly amplified electric fields around under pulsed laser excitation ΩL. The QD considered here has generic three discrete states, and the transition |3= ↔ |1= is resonantly driven by an external field of pulsed laser ΩL, while the transition|2= ↔ |1= is nearly resonant to the plasmon mode of the Ag nanoshell. The electric field is largely intensified and focused on and around the rough Ag nanoshells, which makes strong coupling easily accessible. In the solution, many QDs surround and some of them even touch the Ag nanoshells, collective strong coupling of many QDs with the radiative dipole mode of the NPs make strong coupling build up and Rabi splitting appears in the emission spectra.

OUTLOOK AND CONCLUSIONS In summary, Rabi splitting energy as large as 160 meV and anticrossing-like behavior in emission spectra are realized in hybrid CdSe/ZnS QDs and Ag nanoshells system upon the excitation of nonresonant nanosecond pulsed laser at room temperature. Further analysis 19


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comes to the conclusion that high pump energy and collective strong coupling of many QDs with the radiative dipole mode of the metallic NPs are essential to the realization of strong coupling. We believe that our finding provides a simple method for deeply studying the strong coupling between LSPs of metallic NPs and excitons of QDs from the perspective of radiative recombination, which promises applications in quantum control of light and quantum information processing.

Meanwhile, there are still many things to explore and improve such as adapting strategies to reduce the strong coupling threshold like using Ag nanoplates with easily identified LSP mode and concentrated electric field at the edges,26,27 attaining standard anticrossing behavior, verifying the essence of collective strong coupling of many QDs with the radiative dipole mode of the metallic NPs, effectively sensing based on the relative intensity of plexcitonic peaks,74 observing ultrafast Rabi oscillations in the time resolved PL spectra, making continuous laser excited strong coupling possible by using epitaxially grown silver film,75 and ultimately obtaining electrically pumped strong coupling.76

EXPERIMENTAL METHODS

Materials. Polyvinylpyrrolidone (PVP, k30), silver nitrate, formaldehyde (37%), absolute

ethanol,

ammonia

(28%),

hydrochloric

acid,

n-hexane,

SnCl2·2H2O,

Na2SiO3·9H2O and NaOH were supplied by Sino-pharm Chemical Reagent Co. Ltd.

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Oleylamine, (3-Aminopropyl) trimethoxysilane (APS) and tetraethoxysilane (TEOS) were provided by Sigma-Aldrich. The CdSe quantum dots were purchased from Wuhan Jiayuan Quantum Dots Co. Ltd and Najing Technology Co. Ltd.

Preparation of SiO2@Ag nanoshells. Silica microspheres with diameter of about 220 nm were prepared by the StÖber method.77 Ag nanoshells with SiO2 as cores were prepared by a reported method.49 Firstly, 0.1 g of as-prepared silica microspheres were sonically dispersed in 10 mL of deionized water, and 10 mL of SnCl2·2H2O (3wt%) aqueous solution (containing 100 µL of hydrochloric acid to avoid hydrolyzation of SnCl2) was added. The mixture was stirred for 30 min. To remove unabsorbed Sn2+ the mixture was centrifuged for 5 times and re-dispersed in 5 mL of deionized water. Secondly, the Sn2+ functioned silica colloids were added into the equal volume of ammonia silver nitrate (0.35 mM) solution under ultra-sonication. The reaction lasted for 20 min, then the mixture was centrifuged for 4 times and dispersed in 10 mL of water. Finally, silver nuclei grew into complete silver shells. Different amount of silver nuclei decorated silica sphere solution (0.2-0.8 ml) was dispersed in 200 mL of silver nitrate (0.25 mM) aqueous solution containing 0.25 wt% PVP (11 mM in term of PVP monomer), then 0.2 ml of formaldehyde (37 wt%) and 0.4 mL of ammonia (28 wt%) were added in sequence. The whole reaction was carried out at 30 oC. The SiO2@Ag core-shell NPs were obtained in a few seconds. After 5 min the products was centrifuged for 4 times and dispersed in a solution containing 5 mL ethanol and 5mL of oleylamine. The solution was stirred for 8 h at 80 oC and the precipitate was isolated by

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centrifugation and redispersed in n-hexane.

Preparation of SiO2@Ag @SiO2. The silver nanoshells with 1.5 nm SiO2 capping layer were prepared by a reported method.78 Basically, 0.1 ml portion of freshly prepared 1 mM APS was added to 25 ml of Ag nanoshell aqueous solution under vigorous stirring for 15 min. Then 0.4 ml of 0.54 wt% Na2SiO3 and 0.5 ml of 0.1 M NaOH solution were injected sequentially into the sol. The reaction was kept at 90 oC under stirring for 1h. Finally, the product was collected by centrifugation.

Characterization. The morphology of SiO2@Ag particles was characterized by a transmission electron microscope (TEM, JEM-2100F), and a scanning electron microscope (SEM, Hitachi S-4800). The extinction spectra were measured on an Ocean Optics spectrophotometer with the optical path of 2 mm over the range of 200-1100 nm. The QDs solution suspended with or without Ag nanoshells was loaded in a 10 mm cuvette and pumped with 1.4 ns pulses at λ = 532 nm from a diode pumped passively Q-switched solid state laser FTSS 355-300 (CryLaS GmbH) at repetition rate of 20 Hz. The pump energy was regulated by a beamsplitter/attenuator (VBA05-532, Thorlabs) and detected by a pyroelectric energy sensors (ES111, Thorlabs) connecting to a compact power and energy meter console (PM100D, Thorlabs). The pump energy ranged from 30~180 µJ and the excitation area was about 0.4 mm in diameter. The pump power density to solution spanned from 7~46 MW/cm2. The emission was recorded with an Acton SpectraPro-2500i monochromator coupled to a photomultiplier tube (PMT) at room temperature. 22


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ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Conflict of Interest: The authors declare no competing financial interest.

Acknowledgements. This work was supported by the National Natural Science Foundation of China (No. 61176117), and the 973 Program (Grant No. 2013CB632102).

Supporting Information Available: The photograph of the configuration of excitation-detection, SEM and TEM images of CdSe/ZnS QDs and Ag nanoshells, the linewidth, FDTD simulation methods and results, and evolution of emission spectra as a function of pump energy in different systems. This material is available free of charge via the Internet at http://pubs.acs.org/.

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TOC Graphic

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Figure 1. Schematic of the excitation-detection configuration. The solution consisting of Ag nanoshells and CdSe/ZnS QDs is pumped by pulsed 532 nm laser light at room temperature and the emission is collected by PMT.

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Figure 2. Evolution of emission spectra as a function of pump energy for 220 SiO2@30 Ag mixed with QDs with different emission centers (a) 570 nm, (b) 595 nm, (c) 620 nm. The emission lines have been smoothed and offset vertically by 500 counts in sequence from bottom to top for clarity. (d-f) Evolution of UP and LP resonance energies as a function of pump energy deduced from a-c, respectively. The insets in d-f are fits of splitting energy of polariton emission as a function of pump energy.

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Figure 3. (a)The extinction spectra of five types of Ag nanoshells with different dimension of core and shell; (b) Dispersions of UP and LP resonance energy (solid and open red circles) pumped at 140 µJ and the Rabi splitting thresholds (blue diamonds) plotted against the corresponding bare LSPR positions, where the magenta dashed cross represents the uncoupled QDs exciton energy [2.08 eV (595 nm)] and the red solid and blue dotted lines are guides for the eyes.

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Figure 4. Evolution of emission spectra as a function of the concentrations of QDs (a) and Ag nanoshells (b) under the same excitation energy.

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Figure 5. (a) Schematic diagram of a rough SiO2@Ag nanoshell with Ag NPs having diameter with Gaussian statistics attached on the shell surface. The inset is the TEM image of a single Ag nanoshell. (b) Distribution of electric field intensity at 595 nm for a rough SiO2@Ag nanoshell obtained by means of FDTD simulation. (c) Schematic representation of collective strong coupling of many QDs with the radiative dipole mode of an Ag nanoshell. The right part gives the energy diagram and optical transitions in the hybrid system, and the Rabi oscillation ΩR describes the transfer of an excitation between the LSP mode of and the QDs emitters at Rabi frequency.

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