Nanooptics of Molecular-Shunted Plasmonic Nanojunctions

Felix Benz NanoPhotonics Centre, Department of Physics, University of Cambridge, Cambridge, UK Christos Tserkezis Donostia International Physics Center (DIPC) and Centro de Física de Materiales, Centro Mixto CSIC-UPV/EHU, Donostia-San Sebastián, Spain Lars Herrmann NanoPhotonics Centre, Department of Physics, University of Cambridge, Cambridge, UK Bart de Nijs NanoPhotonics Centre, Department of Physics, University of Cambridge, Cambridge, UK Laurynas Pukenas Molecular and Nanoscale Physics, School of Physics and Astronomy, University of Leeds, Leeds, UK Stephen Evans Molecular and Nanoscale Physics, School of Physics and Astronomy, University of Leeds, Leeds, UK Javier Aizpurua Donostia International Physics Center (DIPC) and Centro de Física de Materiales, Centro Mixto CSIC-UPV/EHU, Donostia-San Sebastián, Spain Jeremy J. Baumberg NanoPhotonics Centre, Department of Physics, University of Cambridge, Cambridge, UK

Investigating the interaction of nano-plasmonic systems and conductive organic materials is key for combining organic electronics and plasmonics. Being able to optically probe the conductance of just a few molecules would open possibilities for novel tiny ultra-fast switches providing an interface between optics and organic electronics.

We investigate individual gold nanoparticles on a gold film separated only by nanometre-thick organic self-assembled monolayers. These narrow gaps allow the surface plasmons to couple to their image charges creating a tightly confined coupled plasmon mode. For completely insulating gap materials charges are built up in every optical cycle on either side of the gap, similar to a plate capacitor. Introducing a slight conductivity across the gap discharges the plasmon creating a screened coupled mode which is blue-shifted with respect to the coupled mode in the non-conductive case. [1]

a Geometry utilised: a gold nanoparticle is placed on a gold mirror separated only by a thin self assembled molecular monolayer. b Scattering spectra of individual gold nanoparticles on conductive and non-conductive spacer layers differing by 1 atom.

Figure 1: a Geometry utilised: a gold nanoparticle is placed on a gold mirror separated only by a thin self assembled molecular monolayer. b Scattering spectra of individual gold nanoparticles on conductive and non-conductive spacer layers differing by 1 atom.

We use pure and mixed organic self-assembled monolayers of a conducting and a non-conducting molecule. Both molecules are chemically equivalent consisting of a biphenyl unit with either one or two thiol groups. The thiol groups ensure good overlap of the π-electrons of the conductive biphenyl backbone with the respective gold surface. [2] Gradually exchanging the non-conducting molecule for the conducting one continuously blue-shifts the coupled plasmon mode. Analysis of this blue-shift allows determination of the single molecule conductivity as well as the total number of molecules in the plasmonic hotspot. [3] Raman spectrscopy in real time shows how the molecules respond to this confinement.

[1] O. Pérez-González, et al. Nano Lett., 10, 3090–3095 (2014).

[2] M. Bürkle, et al. Phys. Rev. B, 85, 075417 (2012).

[3] F. Benz, et al. Nano Lett. accepted, doi:10.1021/nl5041786 (2014).

fb400@cam.ac.uk