F, N co-doped carbon nanomaterials for multi-level information encryption: Room-temperature phosphorescence and solid-state fluorescence

Abstract

The non-radiative transition of triplet excitons limits the emission of room-temperature phosphorescence (RTP). In this paper, F, N co-doped carbon nanomaterials including carbon dot solutions (L-CDs) with cyan fluorescence and solid-state carbon nanowires (S-CNWs) with yellow fluorescence were successfully prepared by the one-pot hydrothermal method. Interestingly, L-CDs@PVA composites with green RTP were obtained by mixing L-CDs with PVA. The RTP production was attributed to F, N related heteroatomic functional groups on the surface of L-CDs. The C-N, CN bonds form lone electron pairs, which promote the electron transition from n→π∗. The F doping reduces the energy gap from the single to the triplet state, which facilitates the inter-system crossing (ISC) process and increases the radiative transition of the triplet exciton to produce RTP. In addition, PVA forms abundant hydrogen bonds with L-CDs to effectively immobilize the luminescence center and isolate atmospheric oxygen, so that L-CDs@PVA has stable phosphorescence emission at room temperature and this phosphorescence is reversible in dry and wet states. In contrast, S-CNWs displayed solid-state yellow fluorescence, but no phosphorescence even though doped into the PVA matrix (S-CNWs@PVA). Therefore, based on the different characteristics of fluorescence and phosphorescence of L-CDs and S-CNWs, a complex, multi-level information encryption method is proposed and the potential applications of S-CNWs powder for fingerprint recording are explored.