We explore plasmon coupling between silver nanoparticles (AgNPs) as two AgNPs approach each other within a subnanometer distance. We prepare AgNP dimers with two 21-nm AgNPs separated by alkanedithiol linkers in high yield. Changing the length of the alkanedithiol linkers enables us to control the interparticle distance down to the subnanometer level on the molecular scale. We observe that the longitudinal plasmon coupling band, which is sensitive to the interaction between AgNPs, gradually redshifts as the interparticle distance decreases. This observation is fully consistent with the classical electromagnetic model. The redshift of the plasmon coupling, however, undergoes a drastic change when the interparticle distance reaches 1 nm. The longitudinal plasmon coupling band vanishes and a new intense band appears at a shorter wavelength. This band redshifts as the nanogap further narrows, but crosses over to a blueshift at 0.7 nm. A comparison of our observation with finite-difference timedomain simulations reveals that this band arises from quantum effects. Controlled assembly of AgNP dimers in combination with simulations allows us to observe the transition of the plasmon coupling from the classical to the quantum regime at the ensemble level. (C) 2015 Elsevier Inc. All rights reserved.