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On-Surface Synthesis of Highly Ordered Covalent Sierpiński Triangle Fractals.

The Sierpiński triangle (ST), a well-known fractal, is predicted to exhibit fascinating electronic, magnetic, optical, and me-chanical properties. Synthesis of stable molecular STs with robust covalent linkages is attractive but challenging. Here, we demonstrate the formation of a series of highly ordered covalent STs via the on-surface dehydration reaction of 1,3-benzenediboronic acid (1,3-BDBA) with the presence of water as an equilibrium regulator at ambient atmosphere. Extended molecular fractals up to 3rd generation are obtained, as disclosed by scanning tunnel microscope. The covalent STs show intriguing bright and dark contrasts irrespective of the fractal generations, which is related with epitaxial relationship to the underlying graphite lattice as supported by theoretical simulations.