Plasmon-Exciton Interaction at the Nanoscale: Silver Is More "Precious" than Gold!

Predictive understanding of factors affecting plasmon-exciton coupling is crucial for the successful realization of the exciting potentials of plexcitonic nanostructures. Here, we systematically investigate the role of plasmonic metals in controlling the plasmon-exciton coupling strength. We use gold and silver nanoprisms, having identical LSPR maxima, as the plasmonic components and form two plexciton hybrids with the J-aggregates of a cyanine dye. Single-particle spectroscopy is employed to study and compare the abilities of gold and silver in influencing plasmon-exciton interaction at the nanoscale. Despite much faster plasmon dephasing than its gold counterpart, the silver nanoprism exhibits greater Rabi splitting. We reveal that the smaller plasmon mode-volume despite having larger physical volume, superior local electric-field enhancement, and smaller Ohmic losses compared to gold, enables the silver nanoprism to defy the pronounced plasmon decoherence effects and to show stronger plasmon-exciton coupling. These findings suggest that silver nanostructures should be the unequivocal choice over gold when "strong coupling" is desired for any application.

Large Area Film of Highly Crystalline, Cleavable, and Transferable Semi-Conducting 2D-Imine Covalent Organic Framework on Dielectric Glass Substrate.

Two dimensional (2D) imine-based covalent organic framework (COF), 2D-COF, is a newly emerging molecular 2D polymer with potential applications in thin film electronics, sensing, and catalysis. It is considered an ideal candidate due to its robust 2D nature and precise tunability of the electronic and functional properties. Herein, we report a scalable facile synthesis of 2D imine-COF with control over film thickness (ranging from 100 nm to a few monolayers) and film dimension reaching up to 2 cm on a dielectric (glass) substrate. Highly crystalline 2D imine polymer films are formed by maintaining a quasi-equilibrium (very slow, ∼15 h) in Schiff base condensation reaction between p-phenylenediamine (PDA) and benzene-1,3,5-tricarboxaldehyde (TCA) molecules. Free-standing thin and ultrathin films of imine-COF are obtained using sonication exfoliation of 2D-COF polymer. Insights into the microstructure of thin/ultrathin imine-COF are obtained using scanning and transmission electron microscopy (SEM and TEM) and atomic force microscopy (AFM), which shows high crystallinity and 2D layered structure in both thin and ultrathin films. The chemical nature of the 2D polymer was established using X-ray photoelectron spectroscopy (XPS). Optical band gap measurements also reveal a semiconducting gap. This is further established by electronic structure calculation using density functional theory (DFT), which reveals a semiconductor-like band structure with strong dispersion in bands near conduction and valence band edges. The structural characteristics (layered morphology and microscopic structure) of 2D imine-COF show significant potential for its application in thin film device fabrication. In addition, the electronic structure shows strong dispersion in the frontier bands, making it a potential semiconducting material for charge carrier transportation in electronic devices.

Tuning nanoscale plasmon-exciton coupling via chemical interface damping.

Understanding the exact role of each plasmon decay channel in the plasmon-exciton interaction is essential for realizing the translational potential of nanoscale plexciton hybrids. Here, using single-particle spectroscopy, we demonstrate how a particular decay channel, chemical interface damping (CID), influences the nanoscale plasmon-exciton coupling. We investigate the interaction between cyanine dye J-aggregates and gold nanorods in the presence and absence of CID. The CID effect is introduced via surface modification of the nanorods with 4-nitrothiophenol. The relative contribution of CID is systematically tuned by varying the diameter of the nanorods, while maintaining the aspect ratio constant. We show that the incorporation of the CID channel, in addition to other plasmon decay channels, reduces the plasmon-exciton coupling strength. Nanorods' diameter-dependency measurements reveal that in the absence of CID contribution, the plasmon mode-volume factor gradually dominates over the plasmon decoherence effects as the diameter of the nanorods decreases, resulting in an increase in the plasmon-exciton coupling strength. However, the situation is entirely different when the CID channel is active: plasmon dephasing determines the plasmon-exciton coupling strength by outweighing the influence of even a very small plasmon mode-volume. Most importantly, our findings indicate that CID can be used to controllably tune the plasmon-exciton coupling strength for a given plexciton system by modifying the nanoparticle's surface with suitable adsorbates without the need for altering either the plasmonic or excitonic systems. Thus, judicious exploitation of CID can be tremendously beneficial in tailoring the optical characteristics of plexciton hybrid systems to suit any targeted application.

Probing the role of oscillator strength and charge of exciton forming molecular J-aggregates in controlling nanoscale plasmon-exciton interactions.

In this study, we probe into the roles of exciton oscillator strength and charge of J-aggregates as well as nanoparticle's surface capping ligands in dictating the plasmon-exciton interaction. We systematically compare the plasmon-exciton coupling strengths of two hybrid plexcitonic systems involving CTAB-capped hollow gold nanoprisms (HGNs) and two different cyanine dyes, TDBC and PIC, having very similar J-band spectral positions and linewidths, but different oscillator strengths and opposite charges. Both HGN-PIC and HGN-TDBC systems display large Rabi splitting energies which are found to be extremely dependent on dye-concentrations. Interestingly, for our plexciton systems we find that there is interplay between the exciton oscillator strength and the electrostatic interaction amid dyes and HGN-surfaces in dictating the coupling strength. The oscillator strength dominates at low dye-concentrations resulting in larger Rabi splitting in the HGN-PIC system while at high concentrations, a favorable electrostatic interaction between TDBC and CTAB-capped HGN results in larger exciton population of the HGN-surface and in turn larger Rabi splitting for the HGN-TDBC system than the HGN-PIC system even though TDBC has a lower oscillator strength than PIC. The trend in Rabi splitting is just reversed when the HGN surface is modified with a negatively charged polymer, confirming the role of electrostatic interactions in influencing the plasmon-exciton coupling strength.

Nanoscale plasmon-exciton interaction: the role of radiation damping and mode-volume in determining coupling strength.

Unravelling the exact role of each individual plasmon decay channel in plasmon-exciton coupling is pivotal for successful realization of the exciting potential applications of plexcitonic nanostructures. Here, we successfully demonstrate how exactly one specific plasmon dephasing channel, radiation damping, influences plasmon-exciton coupling in Au nanorod-J-aggregate hybrids. We systematically and selectively varied the contribution of radiation damping, keeping the contributions of other damping channels negligible or invariant, by controllably varying nanorod diameter (above 20 nm) while maintaining the aspect ratio constant and studied the optical response of the corresponding plexcitons using single-particle spectroscopy. Our results show that decreasing radiation damping inversely drives the plasmon-exciton interaction toward a strong coupling regime. However, we find that plasmon mode-volume is a more fundamental parameter in dictating coupling strength than radiation damping. Overall, this comprehensive study provides a significant step toward developing a predictive understanding of how exactly excitation decay channels influence plasmon-exciton coupling.

Modulation of LSPR spectra and enhanced RI-sensitivity through symmetry breaking in hollow gold nanoprism.

Optical responses of plasmonic nanostructures can be tailor-made by judiciously controlling their structural parameters. Here in this article, we describe how symmetry-breaking influences the optical properties of an anisotropic hollow nanostructure, a hollow gold nanoprism (HGN). We find that the introduction of structural asymmetry by shifting the cavity position alters the plasmon hybridization conditions, which, in turn, lifts the degeneracy of bonding plasmon modes and thereby causes mode splitting. The splitting between the nondegenerate bonding modes is directly correlated with the extent of the cavity offset. Interestingly, it is found that a reduced symmetry HGN having a cavity of any arbitrary size does not necessarily show such spectral modulation as a function of the cavity offset. Rather, there is a threshold value of (cavity diameter/edge length) ratio for observing this kind of optical behavior. Symmetry breaking not only leads to spectral modulation but also improves the refractive index (RI) sensitivity as well as the associated figure of merit of the HGN nanosensors tremendously. This comprehensive study develops a predictive understanding of the structure-specificity of the optical properties of HGNs and also suggest that sensible tailoring of the structural parameters can make HGNs as one of the most suitable candidates for RI sensing based applications.

Structure-specific chiroptical responses of hollow gold nanoprisms.

Chiroptical responses of plasmonic chiral nanostructures can be controllably tuned by judicious tailoring of their structural parameters. In this article, the chiroptical properties of a newly designed plasmon-supporting nanostructure, chiral hollow gold nanoprisms (HGNs), has been numerically investigated in detail. The most compelling observation is that the CD response and the dissymmetry factor (g, which is a measure of the strength of chiroptical responses) of the chiral HGNs are large and at the same time, highly structure-specific. Also, we observed finite CD activity not only in absorption and scattering but also in the extinction spectra, which is a signature of a typical 3D chiral structure. We show that the chiroptical responses of HGNs can be exponentially enhanced simply by controlling the cavity-position or cavity size. Our results reveal that the structure-specific chiroptical response is a result of structure-dependent interplay between the non-radiative (Ohmic) and radiative losses. We also show that the CD intensity of a suitably designed chiral HGN is higher than other nanoscale metasurfaces of comparable volume. The insights obtained from this comprehensive study assert that this unique chiral nanostructure has great potential for being used in numerous applications.