  The bottom-up synthesis of carbon based nanomaterials directly on
semiconductor surfaces allows to decouple their electronic and magnetic
properties from the substrates. However, the lack of reactivity on these
non-metallic surfaces hinders or reduces significantly the yield of these
reactions. Such hurdles practically precludes transferring bottom-up synthesis
strategies onto semiconducting and insulating surfaces. Here, we achieve a high
polymerization yield of terphenyl molecules on the semiconductor TiO$_2$(110)
surface by incorporating cobalt atoms as catalysts in the Ullmann coupling
reaction. Cobalt atoms trigger the debromination of 4,4-dibromo-p-terphenyl
(DBTP) molecules on TiO$_2$(110) and the formation of an intermediate
organometallic phase already at room-temperature (RT). As the debromination
temperature is drastically reduced, the homo-coupling temperature is also
significantly lowered, preventing the desorption of DBTP molecules from the
TiO$_2$(110) surface and leading to a radical improvement on the
poly-para-phenylene (PPP) polymerization yield. The universality of this
mechanism is demonstrated with an iodinated terphenyl derivative (DITP), which
shows analogous dehalogenation and polymerization temperatures with a very
similar reaction yield. Consequently, we propose to use minute amounts of metal
catalyst to drive forward generic bottom-up synthesis strategies on
non-metallic surfaces.
