APPLIED PHYSICS LETTERS 96, 253107 共2010兲
Core-shell gold J-aggregate nanoparticles for highly efficient strong coupling applications Diane Djoumessi Lekeufack,1 Arnaud Brioude,1,a兲 Anthony W. Coleman,1 Philippe Miele,1 Joel Bellessa,2 Li De Zeng,2 and Pierre Stadelmann3 1
Laboratoire des Multimatériaux et Interfaces (UMR 5615 CNRS), Université Lyon 1, Université de Lyon, 43 Bd du 11 Novembre 1918, F-69622 Villeurbanne, France 2 Laboratoire de Physique de la Matière Condensée et Nanostructures (UMR CNRS 5620), Université Lyon 1, Université de Lyon, 43 Bd du 11 Novembre 1918, F-69622 Villeurbanne Cedex, France 3 EPFL-SB-CIME, Bâtiment MXC 136, Station 12, CH-1015 Lausanne, Switzerland
共Received 19 April 2010; accepted 27 May 2010; published online 22 June 2010兲 We have developed a straightforward synthetic route to prepare core-shell systems based on gold nanoparticles 共NPs兲 surrounded with J-aggregates molecules. This synthesis allows the direct and efficient coating, at room temperature, of pretreated citrate-stabilized gold NPs with 5, 5⬘, 6, 6⬘-tetrachloro-1-1⬘-diethyl-3, 3⬘-di 共4-sulfobutyl兲-benzimidazolocarbocyanine 共TDBC兲, without supplementary adding of salts and bases during the synthesis. As the size of gold particle is tunable, the precise optimization of the strong coupling between the electronic transitions of organic components 共TDBC兲 and the plasmon modes of the gold NPs is achieved corresponding to a Rabi energy of 220 meV, a value not yet obtained in such a system. © 2010 American Institute of Physics. 关doi:10.1063/1.3456523兴 The coupling of organic molecules and metallic nanostructures brings new opportunities to develop highly efficient photonic devices that combine the best features of those two specific materials. Indeed, the interactions between the electronic transitions of organic components and plasmon modes of noble metallic nanostructures are responsible for emerging mixed states thus creating new optical and electronic properties.1 Complex systems composed of the organic dyes and metal nanoparticles 共NPs兲 are promising for building novel optoelectronic materials, photonics, and sensors.2–4 Nanoscale noble metal structures such as Au NPs have attracted considerable attention because of their unique optical properties arising from localized surface plasmon resonance 共LPs兲. LPs are of interest because they create strong enhancement and spatial confinement of the electromagnetic field around the metallic NPs that plays a key role in surface-enhanced Raman spectroscopy, surfaceenhancement fluorescence.5–7 Molecular organic compounds such as J-aggregates 共JA兲 support excitonic states which are electrically neutral electron/hole pairs created by the absorption of photons. JAs exhibit characteristic optical absorption called the J-band which is redshifted from the monomer band, ultrashort radiative lifetime and nonlinear optical susceptibility. These properties are interesting for applications in imaging materials, optoelectronic devices and dye-sensitive solar cells. Here, we present hybrid plasmonic composites made of gold spherical particles coated with a single monolayer JA of the cyanine dye 5, 5⬘, 6, 6⬘-tetrachloro-1-1⬘-diethyl-3, 3⬘-di 共4-sulfobutyl兲-benzimidazolocarbocyanine 共TDBC兲. Even though a large body of work has been published on the synthesis and spectroscopic properties of dyes bound to
metal nanostructures, only a few describe core-shell systems and study specifically the strong coupling effect for its optimization.2,8 Indeed, different plasmonic structure have been already studied showing an efficient strong coupling: metal voids coated with a molecular excitonic film,9 silica core with gold nanoshell coated with a JA molecular layer,4 dye aggregates deposited on a silver film,10 assembly of aligned Au nanorods as JA,11 and lithographed silver nanodisks.12 We have developed a facile synthetic route to prepare monodisperse Au@TDBC NPs allowing the direct and efficient coating of pretreated citrate-stabilized gold NPs with TDBC, without supplementary addition of salts or bases during the synthesis and carried out at room temperature. This simple preparation process is also reproducible allowing sufficient, high quality Au@TDBC NPs to be made for photonic applications. The only synthesis describing a coating of metallic NPs with TDBC is performed under exacting icecooled conditions.13 The size of gold particle is tuned to optimize the strong coupling between the electronic transitions of organic components 共TDBC兲 and the plasmon modes
TABLE I. Plasmon resonance position vs NP size.
a兲
Gold NPs 共nm兲
Size diameter 共nm兲
518 522 528 532 536 540 548
18 23 28 31 46 54 70
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FIG. 3. TEM images and elemental mapping obtains from EELS measurements. 共a兲 TEM image of Au@TDBC deposited on a holey carbon grid, 共b兲 S-mapping of the same area, and 共c兲 corresponding C-mapping.
FIG. 1. 共Color online兲 Extinction spectra of bare gold NPs with different sizes from 10 to 45 nm 共full lines兲. The resonance band of the cyanine dye centered at 586 nm corresponds to the aggregate band while the blueshift and broad band correspond to that of the monomer 共dotted line兲.
of gold NPs 共Table I兲. Those gold NPs prepared according to the Turkevich method are stable in aqueous solutions for a long time owing to surface-adsorbed citrate ions.14 So as to obtain tunable sizes of NPs, smaller NPs are used as seeds for the preparation of larger ones. See supplementary material at Ref. 15 for the experimental details of the gold NPs synthesis and of the TDBC coating. Figure 1 illustrates the ground-state extinction spectra of bare gold NPs with different sizes and of pure TDBC. The optical response of noble metal based NPs is known to redshift with increasing size. Dye coated NPs are prepared by replacing the citrate ions by the TDBC molecules on the surface of gold NPs 共Fig. 2兲. This was realized by a simple ligand exchange reaction between the citrate and TDBC, during the mixing of the NP colloidal solution and the TDBC solution. Since the citrate is linked to the gold NPs surface by the oxygen, it can be easily replaced by the TDBC molecules whose adsorption on the gold NPs will be via the
FIG. 2. 共Color online兲 共a兲 Citrate ions molecules, 共b兲 TDBC molecules, and 共c兲 TEM images of resulting NPs of different sizes coated with TDBC molecules.
nitrogen atoms. The aqueous solution of TDBC with a concentration of 5 ⫻ 10−5 M is pre-prepared with a small amount of NaOH so as to adjust the pH above 7. For higher concentration, the TDBC molecules aggregate and form linear chains.16 The solution is then mixed under magnetic stirring for 5 min and then placed in an ultrasonic bath for 15 min. The mixture is kept from light and can be used directly for the coating of gold NPs. Figure 2共c兲 shows transmission electron microscopy 共TEM兲 of gold NPs coated with TDBC. We can observe that few particles are aggregated. This clearly indicates that the coating is not perfect but does prevent the strong coupling from occurring. Contrary to previous studies, we did not manage to improve the quality of the coating by adding salts 共NaCl, KCl兲 that are known to promote adsorption of dye molecules on the surface of metallic molecules. The reason is that additive salts amount in the TDBC solution can modify the colloidal solution stability and the citrate ions adsorption on gold NPs. This leads to the NPs aggregation prior to the direct TDBC coating. In order to check the local coating of TDBC, an electron energy loss spectroscopy and imaging 共EELSI兲 study has been conducted on NPs deposited on a holey carbon grid. The atomic resolution and single atom sensitivity allows us to obtain the elemental mapping of S atoms brought by the TDBC molecules located around the gold NP 共Fig. 3兲. The gold NPs are preliminary coated with citrate ions molecules free of S atoms. A JEOL 2200FS transmission electron 共fitted with omega filter and field emission gun兲 was employed for atomic resolution imaging and EELSI with an energy resolution better than 1 eV. Figure 3共a兲 shows TEM images of Au@TDBC NPs obtained with 10 s exposure time and a 5 eV energy slit centered at the S L2,3 共160 eV兲 and C K 共285 eV兲 core loss edges. The S 关Fig. 3共b兲兴 can be seen to be evenly distributed around gold NPs 共though some residual diffraction contrast affects the S-TEM image of the strongly diffracting NPs兲. It is not surprising to find preferentially the C 关Fig. 3共c兲兴 elements between the NPs corresponding to the carbon grid. This observation confirms that the coating of TDBC molecules on gold NPs has been efficient. In Fig. 4 are given the extinction spectra of gold NPs with different sizes coated with TDBC molecules. Four interesting contributions can be distinguished for these spectra. The first denoted by the dotted line I correspond to the bare Au NPs absorption as represented on Fig. 1. It clearly shows that some particles are not coated with TDBC. The second noted II existing for larger wavelength over 750 nm is broad
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FIG. 4. 共Color online兲 Extinction spectra of Au NPs with different sizes coated with TDBC. The spectra are arbitrary translated for the clarity of the figure.
and correspond to the NPs aggregation. Those remarks confirm the observations made by electron microscopy. A dip in the extinction spectra centered at 586 nm matches with the extinction wavelength of the TDBC. On both sides of this dip, two extinction peaks represented by the full lines can be distinguished. These resonances present an energy anticrossing characteristic of the plasmon/exciton hybridization.10 A minimum wavelength separation of 61 nm has been calculated on the blue curve in Fig. 4 corresponding to Rabi energy of 220 meV. This value is higher than those evaluated in previous works for similar nanostructures.4,8 It has to be noted that the extinction wavelength values of the coated NPs extinction peaks are larger than the uncoated ones 共I兲 for each size except for the larger NPs 共54 and 70 nm兲 showing an efficient coating process. This is the key point for the strong coupling efficiency in those systems. In conclusion, we have developed a straightforward synthetic route to prepare monodisperse Au@TDBC NPs allowing the direct and efficient coating of pretreated citratestabilized gold NPs with TDBC, without supplementary adding of salts and bases during the synthesis and at room temperature. As the size of gold particle is tunable, the precise optimization of the strong coupling between the electronic transitions of organic components 共TDBC兲 and the plasmon modes of the gold NPs is achieved corresponding to a Rabi energy of 220 meV, value not yet obtained in such a system.
1
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