Van der Waals Effect-Driven Charge–Transfer Excitation in 2D Polymeric Carbon Nitrides: A Vertical Excitation Model

Abstract

Atomically thin, graphene-like two-dimensional (2D) materials have been tailored to exhibit new physical and chemical properties. For g-C₃N₄, a typical 2D semiconductor material, the study of its interlayer coupling effect is still in its early stages, especially in the context of light excitation processes. There are limited studies of excited states. The 2D structures of the monolayer and bilayer of g-C₃N₄ were studied using density functional theory (DFT) and time-dependent density functional theory (TDDFT). Compared with the planar structure, the ripples in the monolayer increase the HOMO–LUMO (H–L) gap due to decreased conjugated coupling but also affect the distribution of electron–hole pairs in excited states, ultimately leading to a red shift in the light absorption peak. It was found that van der Waals (vdW) interlayer coupling affects the distribution of electron–hole pairs in the excited states of the bilayer and promotes charge–transfer excitations. The electron excitations in these excited states of the bilayer structures presented a strong and weak interval distribution between different melon chains, making it easier to form interlayer excitons or in-plane excitons. However, van der Waals forces have little effect on the excitation energy. The S0 → S1 transitions of all four structures are n → π* transitions, with the bilayer structures displaying very low oscillator strengths (<0.0006), indicating a transition barrier for the n → π* transition in the bilayer structures, which makes it challenging to measure using optical instruments in experiments.