The formation of nanostructures on SiGe surfaces by erosion using mixed beams of isobaric species (Cs/Xe) is shown to depend on the Cs/Xe ratio. The nanostructures exhibit different wavelengths (longer wavelengths for higher Cs concentrations) contrary to the present theoretical understanding. Moreover, experiments with pure Cs and Xe beams also demonstrate that such differences are enhanced at lower bombarding energies. Such effects are primarily due to the fact that the retentivity and mobility of cesium at the sample surface gets enhanced at lower bombarding energies. The phenomenon could be explained theoretically by including an additional diffusion term in the growth equation describing the mobility of the primary ions on the irradiated surface. Semi-empirical calculations done in this direction also confirm this phenomenon. (C) 2011 Elsevier B.V. All rights reserved.