Rationalizing the phase separation paradox of free-standing pristine honeycomb BCN materials within a thermodynamic framework

Two-dimensional (2D) BCN materials, as an outstanding semiconductor, are of wide interest for their potential applications in electronics and optoelectronics. However, their synthesis encounters a great challenge: how to avoid phase separation of graphene and boron-nitrogen domains, thereby achieving a non-phase-separated structure. Herein, we investigate in depth the structural characteristics and formation thermodynamics of BCN atomic layers (focusing on idealized, pristine honeycomb lattices). Based on the density functional theory calculations, it is found that, under the ideal conditions at 0 K, the BCN structure prefers the phase separation of graphene and h-BN. By comprehensively considering the impacts of temperature, pressure, and precursor chemical potential on Gibbs free energy, this study systematically analyzes the thermodynamic feasibility of synthesizing phase-separation-free BCN atomic layers through precise parameter tuning. Furthermore, by controlling the chemical potentials, the thermodynamic stability of BCN structures with varying B, C, and N stoichiometries can be modulated, offering a viable pathway for the synthesis of BCN materials with specific structural configurations. This study not only reconciles the discrepancies between theoretical predictions and experimental observations regarding the phase separation of BCN materials, but also provides critical insights into their structural features and thermodynamic stability. These findings hold significant implications for advancing the application of BCN materials in electronic and optoelectronic devices.