Blue Emission Enhancements And Their Mechanisms Of Carbon Quantum Dots

Carbon quantum dots(CQDs), sometimes named carbon dots(CDs), are quasispherical nanocarbons with the crystal or amorphous structure and the diameters of less than 10 nm that possess stable photoluminescence(PL), which can be obtained by bottom-up routes and up-bottom routes.Moreover, CDs with lattice structure of graphene can be called graphene quantum dots(GQDs). Due to the unique optical and electronic properties, such as excellent biocompatibility, low toxicity and resistance to photobleaching, CDs have been widely researched in many fields since its discovery in2004. Advances in this area are appearing frequently, but some problems associated with CDs with high quantum yield(QY) are still unsolved. In this paper, we taked CDs which were synthesised by different kinds of methods as the research object to explore the way to enhance the QY of CDs from the two aspects: one is to enhance the radiative transition; the other one is to inhibit the non-radiative transition.Moreover, we want to reveal the influence of defects and molecule-like localized states on the PL enhanced of CDs. The main results are summarized as follows:In order to investigate the formation mechanism of CDs prepared by bottom-up routes, citric acid(CA) and ethylenediamine(EDA) or its derivatives were selected as raw materials to produce CDs by hydrothermal methods. By the characterization of the structure and optical properties, we found that both the photoluminescence quantum yield(QY) and the optical gaps were related to N content of CDs. By optimizing the synthesis conditions, the carbon quantum dots with quantum yield as high as(97.4 ±4.2) % were obtained, denoted as CDs-DETA. Then we examined the effect of concentration, solution,pH and ionic strength on the fluorescence of CDs-DETA. By using semi-preparative liquid chromatography to purify CDs-DETA, the enhanced PL of CDs-DETA originating from AEOIP with pyridone structure was elucidated. Besides, we also found that there may be a strong electrostatic interaction between AEOIP, CA and DETA, which made them to form ionic liquid with good fluidity at room temperature. Moreover, because of the electrostatic interaction, they could form aggregates in the concentrated solution of CDs-DETA. The fluid of CDs-DETA exhibits the behavior of Newtonian fluids.Besides, we explored the application of CDs-DETA in Fe3+ detection.By chemical oxidation of resin-based carbon fibers, the greenish-luminescent GQDs were obtained with a size of 3-5 nm. Followed by a single step of moderately reducing GQDs with NaBH4, the blue-luminescent GQDs(rGQDs) were obtained with almost the same dimension. By the comprehensive structure analyses and spectral analyses, we got some results. PL of GQDs and rGQDs were composed of two distinct PL features. The weak green emission was associated with disorder-induced localized states formed during acidic oxidation of carbon sources. As the reduction degree increases, the QY of GQDs increased dramatically from 2.6% to 10.1%. In the meantime, the PL peak position moved to blue region rapidly and full width at half maximum(FWHM) becomed narrower. Thus we infered that graphenol topological defects were gradually formed during reduction, which resulted in the enhanced blue tuning PL from rGQDs. Moreover, graphenol defect related PL featured a longer fluorescence lifetime,excitation wavelength dependence but less pH sensitivity.We produced graphene quantum dots with graphite as raw materials. By treating graphene quantum dots with ammonia under hydrothermal conditions, we obtained amino modified graphene quantum dots(N-GQDs). Atomic force microscope(AFM) showed that ammonia could cut oxidized graphene sheets toform GQDs or porous oxidized graphene sheets. Fourier transform infrared spectrometer demonstrated that ammonia could react with epoxy groups to form primary amine and alcohol by nucleophilic substitution and react with carboxyls to form amide. Compared with re-GQDs, the QY of N-GQDs increased from 0.3% to 9.6%. Time resolved photoluminescence spectra were also used to investigate amino-modified graphene quantum dots. The PL lifetimes were found to be dependent on the emission wavelength and coincident with the PL spectrum, which are different from most fluorescent species. This result shows the synergy and competition between defects derived photoluminescence and amino passivation. Compared to oxygen related defects, nitrogen related localized electronic states are expected to have longer lifetime.