The killer of enhanced electromagnetic fields in plasmonic nanoparticle dimers
Moving two plasmonic nanoparticles close together can enhance the electromagnetic field in the gap formed between the two nanoparticles that are illuminated with incident light. The field enhancement represents one of the most remarkable phenomena associated with metal nanoparticles due to its dramatic influence on surface-enhanced Raman scattering, nonlinear harmonic processes, wave mixing, nanolasing, plasmonic sensing, and spontaneous emission.
In principle the field enhancement increases monotonically as the inter-particle gap continuously decreases. However the field enhancement reaches a limit when the inter-particle gap is as small as 0.5 nm because the polarization charge densities are not perfectly localized at the particle surface and are slightly spread over a finite thickness near the surface boundary [see Ciracì et al., Science 2012, 337, 1072-1074]. This nonlocal distribution of charges is a common phenomenon in metal nanoparticles. Even for colloidal metal nanoparticles stabilized with capping molecules, the polarization charges at the particle surfaces can be partially immobilized by the capping molecules through the chemical bonding formed between the capping molecules and the surface metal atoms [see, Peng et al., Proc. Natl. Acad. Sci. USA 2010, 107, 14530-14534]. When the inter-particle gap is smaller than 0.5 nm, electron tunneling between the two nanoparticles becomes predominant to dramatically influence the plasmonic coupling between the two nanoparticles. This electron tunneling significantly decreases the electromagnetic field in the inter-particle gap, in particular when the two nanoparticles move even closer [see Esteban et al., Nat. Commun. 2012, 3, 825; Savage et al., Nature 2012, 491, 574-577]. As a result, quantum tunneling should be carefully considered when nanoparticle dimers are designed and synthesized for field-enhanced applications.