The formation of Si dots by chemical vapor deposition is studied from the very early stages of the dot formation up to about 25% of substrate coverage. Structural characterization is mainly performed by means of energy filtered transmission electron microscopy, which couples chemical information to very high spatial resolution. The dots are shown to be surrounded by Si-free regions and this is attributed to the Si adatom capture mechanism from each nucleus. The data are discussed in the framework of a self-similar model, which takes into account the dot local environment, the adatom diffusion and the continuous nucleation of new islands. From the fit to the data the correlation between the dot size and the capture area is obtained and the number of deactivated nucleation sites is quantified.