The optical properties of ordered and disordered assemblies of Ag nano-crystals on HOPG have been characterized by photon emission spectroscopy with the STM. The spectra reveal two emission maxima, which are attributed to plasmon modes oscillating parallel and perpendicular to the substrate plane. The interpretation of the emission mechanism is supported by polarization measurements and model calculations. The effect of long-range order in the particle layer on the optical properties is discussed. (c) 2005 Elsevier B.V. All rights reserved.