Colloidal Bimetallic Nanorings for Strong Plasmon Exciton Coupling

Abstract

Nobel-metal nanostructures strongly localize and manipulate light at nanoscale dimension by supporting surface plasmon polaritons. In fact, the optical properties of the nobel-metal nanostructures strongly depend on their morphology and composition. Until now, various metal nanostructures such as nanocubes, nanoprisms, nanorods, and recently hollow nanostructures have been demonstrated. In addition, the plasmonic field can be further enhanced at nanoparticle dimers and aggregates because of highly localized and intense optical fields, which is known as "plasmonic hot spots". However, colloidally synthesized and circular-shaped nanoring nanostructures with plasmonic hot spots are still lacking. We, herein, show for the first time that colloidal bimetallic nanorings with plasmonic nanocavities and tunable plasmon resonance wavelengths can be synthesized via colloidal synthesis and galvanic replacement reactions. In addition, in the strong coupling regime, plasmons in nanorings and excitons in J-aggregates interact strongly and nanoring-shaped colloidal plexcitonic nanoparticles are demonstrated. The results reveal that the optical properties of the nanoring and the onset of strong coupling can be tamed by the galvanic replacement reaction. Further, the plasmonic nanocavity in the nanorings has immense potential for applications in sensing and spectroscopy because of the space, enclosed by the plasmonic nanocavity, is empty and accessible to a variety of molecules, ions, and quantum dots.

Laser assisted synthesis of anisotropic metal nanocrystals and strong light-matter coupling in decahedral bimetallic nanocrystals

“…In fact, the galvanic replacement reaction between silver atoms and gold ions is spontaneous since the free energy of the reaction is negative. 7 Figure 3a shows photos (from 1 to 6) of the Ag nanodecahedral colloids treated with varying amounts of gold ions. The first and second photos from left to right in Figure 3a NPs may contain very small voids as shown in our previous study.…”

“…smaller than around 10 nm, plasmonic nanocavities supporting new hybrid plasmonic modes with small mode volume and extremely large local electric field intensity are created. [6][7][8] The strong interaction is resonant at a frequency that strongly depends on the size, shape, composition (monometallic or bimetallic), and dielectric constant of the surrounding environment of the metal nanoparticles. 9 Although the optical properties of metal NPs slightly change with the size of the NPs, the shape anisotropy enormously modifies plasmonic properties of metal NPs.…”

“…19 Recently, we have demonstrated plasmonic nanocavities in the bimetallic nanorings, which have empty space that is accessible to a variety of molecules, ions, and quantum dots. 7 In the strong coupling regime, metal NPs interact strongly with a molecular transition or electron-hole pairs, namely excitons, and two new hybrid modes (plexcitons) are formed. [20][21] In this regime, the exchange rate of energy between plasmon and exciton exceeds their individual dephasing rates and hence the coupled system shows new hybrid eigenstates, which are half-plasmonic and half-excitonic.…”

“…22 At zero detuning, the energy separation between the upper and lower polariton branches representing the strength of the coupling is called Rabi splitting energy, ћΩ=2g where g stands for the coupling constant indicating the degree of the strong coupling. Until now, many studies on plasmon-exciton hybrid nanoparticles focused on plasmonic nanostructures with a variety of shapes, 23 e.g., core-shell nanoparticles, 20 nanoprisms, hollow gold nanoprism, 24 nanorods, nanoparticle dimers, 25 nanorings, 7 and recently nanodisks. 13 However, in order to boost plasmon-exciton interaction at nanoscale dimension and expand the application of plexcitonic nanoparticles in a variety of fields such as solar cells, 26 light emitting diodes, and nanolasers, new plexcitonic nanoparticles with outstanding optical and chemical properties remains a key goal and challenge.…”

Large Rabi splitting of mixed plasmon–exciton states in small plasmonic moiré cavities

“…A decrease of the Rabi splitting energy, while the cavity size increases is due to a decrease of the mode volume of the cavity. The Rabi splitting energy, = 2g ∼ 1/(V ) 1/2 where g is the coupling strength, is inversely proportional to the square root of the mode volume of the plasmonic cavity; hence, large enhancement of light matter interaction in small plasmonic cavities is expected [17,18]. In addition, in small cavities, the plasmonic mode confinement perpendicular to the surface might be another reason affecting stronger plasmon-exciton interaction in small cavities; see the Supporting Information.…”