Electromagnetic field enhancement in the gap between two Au nanoparticles: the size of hot site probed by surface-enhanced Raman scattering

We have estimated the apparent size of the 'hot' site for surface-enhanced Raman scattering (SERS) located within the gap between two spherical Au nanoparticles. Initially, 55-nm sized Au nanoparticles are laid on a thiol-group terminated silane film, and then 1,4-phenylenediisocyanide (1,4-PDI) molecules are self-assembled onto the Au nanoparticles. 1,4-PDI is bonded to Au by forming one Au-CN bond, with another isocyanide group being pendent with respect to the Au surface. Up to this point, no Raman scattering is detected at all for 1,4-PDI. Upon attaching new Au nanoparticles onto the pendent isocyanide groups of 1,4-PDI, a Raman signal is distinctly observed. In agreement with the finite-difference time-domain (FDTD) calculation, this is understood in terms of the electromagnetic hot sites formed at the gaps between two Au nanoparticles. The Raman signal does not increase further, however, even after the adsorption of additional 1,4-PDI onto the vacant surfaces of the second Au nanoparticles. To a rough estimate, about 400 molecules residing only within a 10-nm diameter area of the center of the gap, adsorbed in the first stage, are then conjectured to have contributed most of the measured Raman signal of 1,4-PDI. This apparently indicates that the size of 'hot' site is very limited.