nn9b09520_si_001.pdf (24.54 MB)
Anatomy of On-Surface Synthesized Boroxine Two-Dimensional Polymers
journal contribution
posted on 2020-02-03, 16:29 authored by Nerea Bilbao, Cristina Martín, Gaolei Zhan, Marta Martínez-Abadía, Ana Sanz-Matı́as, Aurelio Mateo-Alonso, Jeremy N. Harvey, Mark Van der Auweraer, Kunal S. Mali, Steven De FeyterSynthetic
two-dimensional polymers (2DPs) obtained from well-defined
monomers via bottom-up fabrication strategies are
promising materials that can extend the realm of inorganic 2D materials.
The on-surface synthesis of such 2DPs is particularly popular, however
the pathway complexity in the growth of such films formed on solid
surfaces is poorly understood. In this contribution, we present a
straightforward experimental protocol which allows the synthesis of
large-area, defect-free 2DPs based on boroxine linkages at room temperature.
We focus on unravelling the multiple pathways available to the polymerizing
system for the spatial extension of the covalent bonds. Besides the
anticipated 2DP, the system can evolve into self-assembled monolayers
of partially fused monodisperse reaction products that are difficult
to isolate by conventional synthetic methods or remain in the monomeric
state. The access to each pathway can be controlled via monomer concentration and the choice of the solvent. Most importantly,
the unpolymerized systems do not evolve into the corresponding 2DP
upon annealing, indicating the presence of strong kinetic traps. Using
high-resolution scanning tunneling microscopy, we show reversibility
in the polymerization process where the attachment and the detachment
of monomers to 2DP crystallites could be monitored as a function of
time. Finally, we show that the way the 2DP grows depends on the choice
of the solvent. Using UV–vis absorption and emission spectroscopy,
we reveal that the dominant pathway for 2DP growth is via in-plane self-condensation of the monomers, whereas in the case
of an aprotic solvent, the favored growth mode is via π stacking of the monomers.