This patent discloses an innovative thermal vapor condensation (TVC) method for fabricating high-quality g-CN films. The core innovation involves using a dual-precursor system—solid-phase thiourea and melamine—thermally processed in an air environment. In one embodiment, the precursors are physically separated within a container, with thiourea coated on a support substrate (e.g., a glass strip) and melamine placed at the bottom.