Trends in excitonic, vibrational and polaronic properties of graphitic carbon nitride polymorphs

Although graphitic-carbon nitride (gCN) is a highly investigated inorganic semiconductor, especially in the field of photocatalysis, it is still the object of many controversial discussions. The possibility to easily synthesize a homogeneous heterostructure through the condensation and the polymerization of simple molecules allows the growth of a variety of structures with different electronic and optical properties. With the aim of driving the development of the catalyst toward improved performances or newer applications, it is paramount to understand the optically-driven excitation process within each polymer. In this work, we focus on two models of melem-based gCN, i.e. the fully and the partially polymerized structures, and we perform a computational investigation, based on hybrid density functional theory calculations, of their optical properties in terms of vibrational and electronic excitations. First, we determine the normal modes of the ground state and we interpret the IR absorption spectrum. Then, we simulate the electronic excited state with an electron–hole pair model and we determine the exciton binding energy, the self-trapping energy and the photo-emission energy. We compare these numerical results with the experimental data available in literature and in addition we discuss the role of the different polymeric arrangements.