Study On Fluorescence Mechanism Of Graphene Quantum Dots By Ultrafast Spectroscopy

As an emerging carbon-based fluorescent material, graphene quantum dots(GQDs) possesses low toxicity, high-biocompatibility, strong chemical inertness, and resistance to beach. Compared with traditional toxic metal inorganic semiconductor quantum dots, graphene quantum dots have a broad application prospect in carbon-based LED and biological imaging area.Current researches about the mechanism of fluorescence in GQDs have different opinions. The lack of understanding to the fluorescent mechanism will greatly impede the development of GQDs in application, and increasing the fluorescence yield and fluorescent tuning will be difficult to achieve. In this paper, we investigate a kind of green-fluorescent GQDs by chemical modification and ultrafast spectroscopy. The relationships between fluorescent states and functional groups, and dynamics of fluorescent states have been studied.In the first section, the femtosecond transient absorption system is introduced, including femtosecond laser source, transient absorption light path, and optical path adjusting. In our lab, the light source is an ultrafast laser system with 100 fs pulse width and 250 Hz repetition frequency. The 800 nm laser could generate 400 nm exciting light through the BBO, and it could also generate super continuum white probe light with 380 nm – 850 nm range through the heavy water. The system can be used to measure signals in visible light range with resolution ratio at 100 fs.In the second section, GQDs are investigated with steady spectra. Functional groups are changed with the pH and reductant NaBH4. Comparing the steady fluorescent spectra and PL excitation spectra, we analyze the contribution of –OH,-COOH, and –CO-N(CH3)2 groups to different fluorescent emissions. Then, the single walled carbon nanotubes were used to synthesize GQDs, in order to investigate the effect of element nitrogen. As a result,-CO-N(CH3)2 and –COOH functional groups could form greenfluorescent emission center at 520 nm,-COO- groups could emit 540-550 nm fluorescence, and –OH groups and zigzag edge states could generate blue fluorescent emission at 430 nm.In the third section, GQDs with various functional groups are measured by ultrafast transient absorption spectroscopy. We find that the change of functional groups could influence the fluorescent states in GQDs. If effective fluorescent functional groups decrease, the lifetime of the excited state is shortened. That means different functional groups could form different fluorescent states. Nonradiative states could generate traps that can capture electron in radiative states. Additionally, different polarities of the solvents could slightly change the Stocks shift in fluorescent emission of GQDs, but that cannot change the dynamic processes of fluorescent states.In the last section, GQDs are studied through quenching experiment. Methyl violet(MV) is used as the quencher, and it could quench the fluorescent emission in GQDs. That means GQDs have fine electron donating ability. The quenching process is dominant by dynamic process, which indicates MV could affect the lifetime of fluorescent states in GQDs. The reduction potential of MV is located below the molecule-like states and above the intrinsic state in GQDs.Through above experimental sections to the fluorescent mechanism of GQDs. The relation between functional groups and fluorescent emissions has been investigated, as well as the properties of fluorescent states in GQDs. This make a deep comprehension to the electric states in GQDs, which contributes to the application of GQDs in CLED and photoelectric devices.