We have studied the formation of nanostructures on Si(100) surfaces after 1.5 MeV Sb implantation. Scanning Probe Microscopy has been utilized to investigate the ion implanted surfaces. We observe the formation of nanostructures after a fluence of 1 x 10(13) ions/cm(2). These surface structures are elliptical in shape with an eccentricity of 0.86 and their major and minor axes having dimensions of about 11.6 nm and 23.0 nm, respectively. The area of the nanostructure is 210 nm(2) at this fluence. Although the nanostructures remain of elliptical shape, their area increase with increasing fluence. However, after a fluence of 5 x 10(14) ionS/cm(2) a transition in shape of nanostructures is observed. Nanostructures become approximately circular with an eccentricity of 0.19 and a diameter of about 30.1 nm. At this fluence we also observe a large increase in the area of the nanostructures to 726 nm(2). Surface morphology and surface roughness of the ion implanted surfaces has also been discussed. (c) 2006 Elsevier B.V. All rights reserved.