The unit cell of graphene contains two carbon atoms, which corresponds to its two-sublattice honeycomb crystal structure. This is a foundational crystallographic fact requiring no specific citation.
The claim states a fundamental, textbook fact about the crystal structure of graphene (a honeycomb lattice with a two-atom basis, namely sublattice A and sublattice B). This is common knowledge in solid-state physics and materials science. None of the retrieved papers specifically test or dispute this definition, making citation unnecessary according to the instructions.