![]() Our study represents the first demonstration of active switching between these two coupling regimes on individual nanoparticle pairs in a completely nanoscopic system, as well as the first reported switchable and tunable CTP. Finally, by electrochemically bridging the interparticle gap, we demonstrate fully reversible transitions between capacitive and conductive coupling regimes, as evidenced by the dynamic evolution of the charge transfer plasmon (CTP) mode. We then reversibly tune the hybridized bonding plasmon mode of strongly coupled individual Au/Ag nanoparticle dimers. Here, the well-known Ag-AgCl redox chemistry ( 26) allows for reversible resonance energy shifts and intensity modulation that are one and three orders of magnitude larger than charge density tuning, respectively. We initially demonstrate reversible tuning of the localized surface plasmon resonance of individual core-shell Au/Ag nanoparticles through Ag redox electrochemistry, the fundamentals of which have only recently been demonstrated ( 10, 25) (Au/Ag denotes gold core with silver shell). We tune the optical and electronic properties by electrochemically controlling the nanostructure morphology, chemical composition, electronic coupling strength, and, ultimately, fundamental coupling mechanism. Large changes in nanoparticle optical spectra are typically irreversible, but here we demonstrate a simple and robust chemical mechanism that allows large reversible changes in nanoparticle optical properties. In this report, we demonstrate reversible, active plasmon control through electrochemical reduction and chloridation of Ag metal on both isolated Au nanoparticles and strongly coupled Au nanoparticle dimers. Significant challenges still remain for active nanophotonic control over a broad optical range. To date, reversible tuning between capacitive and conductive plasmon coupling has only been achieved for one nanostructure at a time using nanopositioners ( 14). Transitions from capacitive to conductive coupling produce enormous spectral changes, but so far all in situ fully nanoscopic methods have been irreversible ( 22– 24). If the gap is further decreased until the structures are within tunneling contact, optical-frequency electric currents can flow through the entire joined structure, resulting in the emergence of new plasmon modes ( 14– 17, 21). For strongly coupled nanoparticles (gaps <1 nm), quantum effects strongly influence the optical response ( 14– 18), and electron tunneling conductance between nanostructures depends exponentially on the gap width ( 18– 20). Modest tunability of the capacitive coupling between nanoparticles was achieved with lithographically prepared dimers on a stretchable elastomeric substrate ( 12, 13). One strategy toward achieving larger plasmon resonance shifts is tuning the plasmon coupling strength between adjacent nanostructures. Recent electrochemical efforts have resulted in either small reversible modulations ( 7, 8) or large irreversible plasmon shifts ( 10, 11). Researchers in pursuit of in situ active control have used electronic ( 1, 2), chemical ( 3), and electrochemical ( 4– 9) approaches. Optical tunability of metallic nanoparticles can be achieved by exploiting the sensitivity of the localized surface plasmon. ![]() Our study represents a highly useful approach to the precise tuning of the morphology of narrow interparticle gaps and will be of value for controlling and activating a range of properties such as extreme plasmon modulation, nanoscopic plasmon switching, and subnanometer tunable gap applications.Īctive control of the optical and electronic properties of nanoparticles is critical for many future technological applications, yet remains largely elusive. Dynamic single-particle spectroelectrochemistry also gave an insight into the reaction kinetics and evolution of the charge transfer plasmon mode in an electrochemically tunable structure. We demonstrated reversible formation of the charge transfer plasmon mode by switching between capacitive and conductive electronic coupling mechanisms. We achieved plasmon tuning by oxidation-reduction chemistry of Ag-AgCl shells on the surfaces of both individual and strongly coupled Au nanoparticle pairs, resulting in extreme but reversible changes in scattering line shape. By electrochemical modification of individual nanoparticles and nanoparticle pairs, we induced equally dramatic, yet reversible, changes in their optical properties. The optical properties of metallic nanoparticles are highly sensitive to interparticle distance, giving rise to dramatic but frequently irreversible color changes.
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