Surface-mediated chain reaction through dissociative attachment

Chain reactions on a surface offer an important route to linear nanopatterning. We recently reported cooperative reactions on a silicon surface in which the reaction of one halogen atom with a silicon atom of a silicon dimer induced the halogenation of its neighbouring silicon atom through surface-mediated charge transfer. The reaction was unable to propagate further but here we describe how, by chemically bridging the gaps between the rows of these silicon dimers, this mechanism is able to form extended chains. The agents for chain growth are CH3Cl molecules that dissociatively attach CH3 groups and chlorine atoms to silicon atoms from different dimers. By means of charge transfer through the surface, this gives rise to dangling bonds adjacent to the CH3 groups and chlorine atoms (in effect, 'free radicals') that dissociate further incoming CH3Cl molecules, thereby providing the growing points for chains of indefinite length. This versatile mechanism of chain growth is examined in experiments and using ab initio theory.