Scalable Synthesis and Characterization of Multilayer γ‑Graphyne, New Carbon Crystals with a Small Direct Band Gap

γ-Graphyne
is the most symmetric sp2/sp1 allotrope of carbon,
which can be viewed as graphene uniformly expanded
through the insertion of two-carbon acetylenic units between all the
aromatic rings. To date, synthesis of bulk γ-graphyne has remained
a challenge. We here report the synthesis of multilayer γ-graphyne
through crystallization-assisted irreversible cross-coupling polymerization.
A comprehensive characterization of this new carbon phase is described,
including synchrotron powder X-ray diffraction, electron diffraction,
lateral force microscopy, Raman spectroscopy, infrared spectroscopy,
and cyclic voltammetry. Experiments indicate that γ-graphyne
is a 0.48 eV band gap semiconductor, with a hexagonal a-axis spacing of 6.88 Å and an interlayer spacing of 3.48 Å,
which is consistent with theoretical predictions. The observed crystal
structure has an aperiodic sheet stacking. The material is thermally
stable up to 240 °C but undergoes transformation at higher temperatures.
While conventional 2D polymerization and reticular chemistry rely
on error correction through reversibility, we demonstrate that a periodic
covalent lattice can be synthesized under purely kinetic control.
The reported methodology is scalable and inspires extension to other
allotropes of the graphyne family.