Formation and coalescence of linear chains are studied in low-energy atom-by-atom deposition of nano-structured sp-sp(2) amorphous carbon films. At the energy of 1 eV, the adsorption of incident atoms on the top of the surface atoms and on the top of existing chain-tips prevails, which explains the initial nucleation and the elongation of sp linear chain structures. During the whole growth process, the film experiences a transition from a sp-dominated linear chain structure to a sp(2)-dominated network structure. This transition happens when linear chains get instable beyond the critical length limit, and coalesce into the three-dimensional network structure. (C) 2008 Elsevier B.V. All rights reserved.