DySi2 monolayers were prepared by thermal evaporation of Dy at room temperature followed by annealing to 500°C on Si(111), which yields a perfect (1×1) low-energy electron diffraction pattern. These monolayers of DySi2 were investigated by electron energy loss spectroscopy with both high energy and momentum resolution. A low-energy acoustic-like dispersion was found with very small anisotropy in reciprocal space, consistent with the characteristic losses due to a two-dimensional plasmon in the hexagonal monolayer structure of DySi2, and effective hole densities of N2d =4.1× 1013 cm-2 with an effective mass of m* =0.37 me. Deviations of the expected dispersion due to single-particle excitations in the Si substrate were found above q∥ =0.08 Å-1. On (111) facets with terrace widths of 12 lattice constants, these properties change little along the terraces, but no plasmon wave propagation across step edges is possible, leading to strong suppression of characteristic loss signals in the direction normal to the steps. © 2008 The American Physical Society.