Band Structure Engineering Induced Ultraviolet–Visible Emission Bulk Carbon Nitride with Near‐Unity Quantum Yield for Information Security

Abstract In the past few years, the photoluminescence (PL) efficiency of carbon nitride materials (CNM) in solution has been greatly improved. However, the PL efficiency of bulk CNM is still very limited. Here a bulk CNM of 2,5,8‐triamino‐tri‐ s ‐triazine (Melem) with efficient ultraviolet–visible emission is obtained via band structure engineering by simply modifying the calcining atmosphere in the solid‐state reaction. The as‐obtained Melem exhibits much enhanced PL with a near‐unity quantum yield of up to 98.3%, which, as far as it is known, is the highest among all CNM in bulk or solution. The enhanced PL is ascribed to the emission origin evolution from the vacancy‐defect‐dominated emission to the bandgap transition‐dominated emission. Benefiting from the high quantum yield and ultraviolet–visible emission, the as‐obtained Melem powder is successfully used for information security via silk‐screen printing. The information encrypted can be easily decrypted by a 254 nm ultraviolet lamp, while not decryptable by a 365 nm ultraviolet lamp. This research proposes a facile method for the development of highly efficient metal‐free Melem and will also provide significant guidance to the band structure engineering of carbon nitride‐based luminescent materials.