Calibration of Silver Plasmon Rulers in the 1-25 nm Separation Range: Experimental Indications of Distinct Plasmon Coupling Regimes.

In this manuscript pairs of flexibly linked silver nanoparticles, so called silver plasmon rulers, are synthesized using a rational DNA programmed self-assembly procedure. The plasmon resonance energy (E(res)) versus distance relationship is calibrated for dimers comprising sphere-like silver nanoparticles with diameters of 41.0 ± 4.6 nm and surface-to-surface separations between 1-25 nm. Single dimer Rayleigh scattering spectra are correlated with structural information of the same dimers obtained through transmission electron microscopy with 1 nm spatial resolution. The calibration reveals different plasmon coupling regimes. For larger separations the plasmon resonance energy red-shifts continuously with decreasing center-to-center distance (L) until the L to diameter (D) ratio reaches a value of L/D ≈ 1.05. For shorter interparticle separations E(res) does not further red-shift; instead the slope of E(res)(L/D) levels off, and the measured resonance energies become broadly distributed. Overall, the spectral response of nearly touching dimers indicates that the plasmon coupling does not continue to intensify with decreasing interparticle separation at very short separations. The performed characterization of the distance dependent plasmon coupling forms the quantitative foundation for applications of plasmon rulers in plasmon coupling microscopy and nanoplasmonic devices with defined near- and far-field properties.