The greatly enhanced fields near metal nanoparticles have demonstrated remarkable optical properties and are promising for applications from solar energy to biosensing. However, direct experimental study of these light-matter interactions at the nanoscale has remained difficult due to the limitations of optical microscopy. Here, we use single-molecule fluorescence imaging to probe how a plasmonic nanoantenna modifies the fluorescence emission from a dipole emitter. We show that the apparent fluorophore emission position is strongly shifted upon coupling to an antenna and that the emission of dyes located up to 90 nm away is affected by this coupling. To predict this long-ranged effect, we present a framework based on a distance-dependent partial coupling of the dye emission to the antenna. Our direct interpretation of these light-matter interactions will enable more predictably optimized, designed, and controlled plasmonic devices and will permit reliable plasmon-enhanced single-molecule nanoscopy.

Coupling to metal nanoparticles can increase the fluorescence intensity and photostability of fluorescent probes, and this plasmon-enhanced fluorescence is particularly promising for the dimmer fluorescent proteins common in biological imaging. Here, we measure the intensity distribution of single Cy3.5 dye molecules and mCherry fluorescent proteins one at a time as they adsorb on a conformal surface 4.8−61.0 nm thick over a gold nanorod (NR). The emission intensities for both types of fluorophores depend nonmonotonically on the spacer thickness, and an optimal spacer thickness of ∼10 nm is observed for both fluorophores using two different spacer layer materials. Emission from fluorophores coupled to metal nanoparticles is affected by two competing processes: an enhanced spontaneous decay rate and quenching via nonradiative antenna modes. After averaging over a conformal surface, the product of the simulated enhanced local electric field intensity and the quantum efficiency modification reproduces the experimental 10 nm ideal spacer thickness. Overall, up to a 3.4-fold average enhancement in fluorescence intensity was achieved despite the simple geometry, based on biocompatible, tunable, and economic colloidal gold NRs. This study of the distance dependence of single-molecule plasmon-enhanced fluorescence shows promise for super-resolving cellular membrane proteins naturally positioned above an extracellular substrate.

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Optical measurements and first-principles calculations of the band structure and exciton states in direct-gap bulk and few-layer PbI 2 indicate that the n = 1 exciton is Frenkel-like in nature in that its energy exhibits a weak dependence on thickness down to atomic-length scales. Results reveal large increases of the gap and exciton binding energy with decreasing number of layers, and a transition of the fundamental gap, which becomes indirect for 1-2 monolayers. Calculated values are in reasonable agreement with a particle-in-a-box model relying on the Wannier-Mott theory of exciton formation. General arguments and existing data suggest that the Frenkel-like character of the lowest exciton is a universal feature of wide-gap layered semiconductors whose effective masses and dielectric constants give bulk Bohr radii that are on the order of the layer spacing.