  Carbon nitrides have recently come into focus for photo- and thermal
catalysis, both as support materials for metal nanoparticles as well as
photocatalysts themselves. While many approaches for the synthesis of
three-dimensional carbon nitride materials are available, only top-down
approaches by exfoliation of powders lead to thin film flakes of this
inherently two-dimensional material. Here, we describe an in situ on-surface
synthesis of monolayer 2D carbon nitride films, as a first step towards precise
combination with other 2D materials. Starting with a single monomer precursor,
we show that 2,5,8-triazido-s-heptazine (TAH) can be evaporated intact,
deposited on a single crystalline Au(111) or graphite support, and activated
via azide decomposition and subsequent coupling to form a covalent
polyheptazine network. We demonstrate that the activation can occur in three
pathways, via electrons (X-ray illumination), photons (UV illumination) and
thermally. Our work paves the way to coat materials with extended carbon
nitride networks which are, as we show, stable under ambient conditions.
