Enhancement and suppression effect of molecules on nanocavity plasmon emissions excited by tunneling electrons

We use tunneling electron induced luminescence techniques to investigate the role of adsorbed molecules in nanocavity plasmon (NCP) mediated emissions. Porphyrin molecules directly adsorbed on metals are found to suppress NCP emissions, while molecules on top of an inserted ultrathin oxide layer on the metal substrate yield enhanced NCP emissions. We attribute such difference in enhancement versus suppression to a competing mechanism of two major roles of molecules on the local field enhancement: geometrical spacer and dynamic dipole oscillator. The latter could become dominant when molecules are sufficiently decoupled from the substrate, leading to the overall enhancement of NCP emissions. (C) 2010 American Institute of Physics. [doi:10.1063/1.3525603]