Systhesis And Optical Properties Of Carbon Dots And Their Composites

Carbon dots (CQDs or CDs) are a new kind of carbon nanomaterials followed by graphene, carbon nanotubes, and fullerenes, whose sizes usually are below 10 nm with strong quantum confinement effect. Because of simple preparation method, low synthesis cost, good optical stability, low toxicity and easy surface functionalization, CQDs have been intensively studied over the past decades in many applications. However, CQDs have obvious limitations in their practical applications. First, CQD production yields are painfully low.The yields of CQDs are in gram scale, which is too low for industrial-scale production. Second, most CQDs are used alone or in solutions and thus have limited applications in optical devices. Third, although fluorescence from CQDs has been widely documented and studied, the phosphorescence development of CQDs remains inchoate due to the nonradiative deactivation of the highly active excited states. According to the above mentioned problems, we obtained three kinds of carbon dots with different properties by selecting and optimizing of synthetic strategies. In addition, we also dispersed the obtained carbon dots into different matrices to synthesize the fluorescence/phosphorescence dual emission carbon dots based composites.Specifically, there are three parts introducing in detail in the following paragraphes.In the second chapter, we synthesized novel oil-soluble N-doped CQDs by using IPDI as a single carbon source under microwave irradiation. Notably,the yield of raw N-doped CQDs was about 83% and suitable for industrial-scale production. A detailed mechanism for the formation of N-doped CQDs from IPDI was proposed. Moreover, the obtained N-doped CQDs can be homogeneously dispersed in organic monomers and can use the"monomer as solvent" approach to synthesize CQD composites, avoiding the use of toxic organic solvents as dispersing agents. Surprisingly, obvious phosphorescence and delayed fluorescence were discovered at room temperature when the CQD/PU composites were excited with UV light, in addition to fluorescence. The PU matrixes play a key role in suppressing the nonradiation transitions of triplet excitons. In addition, we are the first to find that the phosphorescence of CQD/PU composites is sensitive to oxygen and makes the obtained N-doped CQDs have potential applications in oxygen sensors.In the third chapter, we have provided an easy and facile technique for preparing ACDs by one-pot hydrothermal treatment of oil soluble N-doped CQDs. In this method, no strong acids, bases or surface passivation agents were used. The obtained ACDs exhibit remarkable PL stability, low cytotoxicity, and excellent solubility and stability in various solvents. They were successfully employed as fluorescent sensors for Fe3+ ions, displaying a wide detection range and high accuracy. Besides, the UC PL of ACDs is observed and successfully applied in one- and two- photon cell imaging.Moreover, ACDs can be homogeneously dispersed in PU and PVA matrices and RTP can be detected in these composites. Importantly, the ACD/PVA composite films show an ultralong phosphorescence lifetime (450 ms), which is much longer than those of conventional organic RTP materials.In the fourth chapter, a new strategy of achieving long lifetime and high efficiency RTP CDs-based composites was proposed, where the efficient RTP is attributed to the intermolecular hydrogen bonds and the deprotonation of carboxyl groups on the surface of CDs. The CANa matrix as an ideal proton donor and acceptor can effectively form hydrogen bonds with CDs to rigidify phosphorescent chromophores (C=O groups) and ensure the triplet emission.The fluorescence and RTP of CDs-based composites can effectively tuned by adjusting the pH value of the reaction system. At pH=11.5, the bright fluorescence with a QY reaching 51% and persistent RTP with the longest lifetime of 0.7 s and the highest QY of 15% in an aqueous environment are realized. Based on the fluorescence/phosphorescence dual emission feature in water medium, the obtained CD/CApH=7.3 composites were used as Fe3+ ion detection. Fluorescence/phosphorescence dual independent results give much more reliable and accurate measurements. Furthermore, the stable/effective dual emission endow our CD/CApH=11.5 composites with great potential in LEDs.