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The structure and ultrafast photodynamics of ~8 nm Au@Pt core-shell nanocrystals with ultrathin (<3 atomic layers) Pt-Au alloy shells are investigated to show that they meet the design principles for efficient bimetallic plasmonic photocatalysis. Photoelectron spectra recorded at two different photon energies are used to determine the radial concentration profile of the Pt-Au shell and the electron density near the Fermi energy, which play a key role in plasmon damping and electronic and thermal conductivity. Transient absorption measurements track the flow of energy from the plasmonic core to the electronic manifold of the Pt shell and back to the lattice of the core in the form of heat. We show that strong coupling to the high density of Pt(d) electrons at the Fermi level leads to accelerated dephasing of the Au plasmon on the femtosecond timescale, electron-electron energy transfer from Au(sp) core electrons to Pt(d) shell electrons on sub-picosecond timescale, and enhanced thermal resistance on the 50 ps timescale. Electron-electron scattering efficiently funnels hot carriers into the ultrathin catalytically active shell at the nanocrystal surface, making them available to drive chemical reactions before losing energy to the lattice via electron-phonon scattering on the 2 ps timescale. The combination of strong broadband light absorption, enhanced electromagnetic fields at the catalytic metal sites, and efficient delivery of hot carriers to the catalyst surface make core-shell nanocrystals with plasmonic metal cores and ultrathin catalytic metal shells promising nanostructures for the realization of high-efficiency plasmonic catalysts.

A series of optical and electron microscopies are utilized in concert to unravel the properties of corrugated metallic tips. While the overall microscopic shapes of the tips dictate their optical resonances and plasmonic field enhancement factors, nanometric structural details govern their tip-enhanced Raman (TER) spectra and images. Using 4thiobenzonitrile (TBN) as a molecular reporter, spatially resolved TER spectra reveal that optical rectification and molecular charging are among the prominent observables in the tip− tip TER geometry. We show the spurious appearance of anions is driven by highly localized resonances that appear as a result of surface corrugation and their manifestation throughout the course of TER nanospectroscopy complicates spectral assignments. Overall, nanoscale spatial variations in the TERS spectra suggest that the tip−tip geometry sustains junction plasmons that appear very different from what may be expected from the hybridization of the bulk tip resonances.

Surface-enhanced coherent anti-Stokes Raman scattering (SECARS) measurements carried out on individual nanosphere dimer nantennas are presented. The ν-domain and t-domain CARS measurements in the few-molecule limit are contrasted as vibrational autocorrelation and cross-correlation, respectively. We show that in coherent Raman spectroscopies carried out with ultrashort pulses, the effect of surface enhancement is to saturate stimulated steps at very low incident intensities (100 fJ in 100 fs pulses), and the principal consideration in sensitivity is the effective quadratic enhancement of spontaneous emission cross sections, σ* = (E L /E o ) 2 σ. Through multicolor femtosecond SECARS measurements we show that beside enhancement factors, an effective plasmon mode matching consideration controls the interplay between coherent electronic Raman scattering on the nantenna and vibrational Raman scattering on its molecular load. Through extensive measurements on individual nantennas, we establish the tolerable average and peak intensities that can be used in ultrafast measurements at nanojunctions, and we highlight a variety of plasmon-driven chemical and physical channels of signal and sample degradation.