Effects of discrete energy levels on single-electron tunneling in coupled metal particles

We report the measurements of single-electron tunneling (SET) spectra for individual and coupled ultrasmall palladium nanoparticles using scanning tunneling microscopy. Extra fine structures, in addition to the Coulomb blockade and staircases in current–voltage curves, was observed for individual Pd particles of about 2 nm in diameter, which is attributed to the interplay of the SET effect and the effects of discrete energy levels due to significant quantum confinement effects in nanoparticles. The origination of the negative differential resistance effect in series-coupled Pd particles is also attributed to the discreteness of energy levels of both the coupled Pd particles.