  A newly synthesized layered material C2N was investigated based on many- body
perturbation theory using GW plus Bethe-Salpeter equation approach. The
electronic band gap was determined to be ranging from 3.75 to 1.89 eV from
monolayer to bulk. Significant GW quasiparticle corrections, of more than 0.9
eV, to the Kohn-Sham band gaps from the local density approximation (LDA)
calculations are found. Strong excitonic effects play a crucial role in optical
properties. We found large binding energies of greater than 0.6 eV for bound
excitons in few-layer C2N, while it is only 0.04 eV in bulk C2N. All the
structures exhibit strong and broad optical absorption in the visible light
region, which makes C2N a promising candidate for solar energy conversion, such
as photocatalytic water splitting.
