Experimental studies on plasmon resonance of Ag nanoparticles on Highly Ordered Pyrolytic Graphite (HOPG)

Abstract

Studies on Ag nanoparticles grown on Highly Ordered Pyrolytic Graphite (HOPG) using HREELS provide different results for smaller and larger particle sizes corresponding to Ag coverages below and above 4 monolayers, respectively. For the larger particles, a positive frequency shift with decreasing particle size and a broadening of the plasmon resonance were observed with decreasing particle size, in line with previous studies on Ag on alumina. For the smaller particles, in contrast, a shift to lower energy with decreasing particle size, and a narrowing of the plasmon resonance with decreasing particle size can be found. The asymmetry of the Ag-features present for Ag coverages above 4 monolayers disappears for Ag coverages below 4 monolayers. The result for the smaller particles can be rationalized in terms of change of the particle growth mode with increasing particle size, which corroborates our STM data, as well as electronic effects due to the metal/support charge transfer.