Investigation of photoluminescence mechanism of graphene quantum dots and evaluation of their assembly into polymer dots
Introduction
Original graphene is a kind of zero-band semiconductor, which is difficult to serve as a fluorescent material. To open up the bandgap and facilitate its applications, researchers usually choose to change the surface chemical groups of original graphene or convert it to 0D graphene quantum dots (GQDs) [1], [2], [3], [4]. GQDs, labeled as a new sort of fluorescent carbon-based material [5], [6], [7], [8], have attracted increasing attention for their good biocompatibility, photoelectric, optical properties as well as convenient surface modification. For the reasons mentioned above, GQDs promise to be used in numerous areas: biomedical applications, optoelectronic devices, sensors and assembly composites, etc.
Graphene quantum dots attracted more attention compared with other kinds of carbon materials, because it is single layered with precise chemical structure and easy chemical modification [12], [13], [14], [15], [16], [17], [18], [19], [20]. As a result, GQDs are ideal model system to investigate the PL mechanism of the carbon-based materials [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. Due to the different chemical structures of the reported GQDs, although the photoluminescence (PL) mechanism of GQDs can be explained from the aspect of size, surface chemical groups and doping atoms, there is no universal agreement on the specific PL mechanism: triple carbene at the zigzag edges, charge transfer/resonance between amino and graphene core, intrinsic state/defect state, edge state, molecule state at the surface of GQDs, high-crystalline carbon core state, additional energy levels by doping and so on [9], [10], [11], [21], [22], [23], [25], [26].
The GQDs were always prepared by the chemical synthetic strategies, which mainly contain cutting different carbon resources and carbonization from small molecules. Besides, the organic synthesis is another efficient route to prepare GQDs [12], [13], [14], [15], [16], [17], [18], [19], [20]. Through organic synthesis, the structure of GQDs can be precisely controlled, leading to comprehending the PL mechesium of GQDs unambiguously. It is not surprising that steady-state spectroscopy has been considered and utilized as efficient mean to reveal the PL origin of the GQDs [33]. However, it cannot obtain enough information, because some crucial details are hidden in the steady-state characterizations. It also implies that these relevant events may happen very fast which are even far beyond instrument resolution of nanosecond time-resolved equipment. Therefore, a requirement for detailed ultrafast dynamics study on GQDs by femtosecond time-resolved spectroscopy becomes indispensable, which could make important contributions to the PL mechanism of GQDs [34], [35].
In this paper, four kinds of GQDs (C42H18, C96H30, C132H34 and C222H42) were synthesized through organic routes [13], [14], [15], [35], [36]. The PL mechanism of the GQDs was investigated by ultrafast spectroscopy. Intrinsic state depends on size, while the energy level offset between intrinsic state and edge state decides their optical properties in these organic synthesized GQDs. As a result, the green fluorescence of the C42H18, C96H30 is not only dependent on the size, but also on the bright edge state. For large GQDs (C132H34 and C222H42), the intrinsic state is lower than the edge state, which lead to the weak photoluminescence. Furthermore, PL polymer dots (PDs) were prepared by assembling GQDs and polymeric surfactant (tween 60) using a modified nanoprecipitation method. The PDs possessed perfect solubility in water and kept the PL behavior of the organic synthesized GQDs.
Section snippets
Materials
Diphenylacetylene, tetraphenylcyclopentadienone and 1,3,5-Triethynylbenzene were purchased from Aldrich. Iron (III) chloride was purchased from Tianjin Huadong Reagents. All the other reagents were purchase from Aldrich or Beijing Chemical Works.
Preparation of C42H18
Diphenylacetylene (1 g) and tetraphenylcyclopentadienone (2.6 g) were dissolved in o-xylene (40 mL) under an argon atmosphere and the resultant mixture was heated for 18 h at 170 °C. After cooling, the solvent was removed by rotary evaporation. The products
Results and discussion
The C42H18, C96H30 were prepared by Diels–Alder reactions as shown in Fig. S1a and b. The molecules were confirmed by matrix-assisted laser desorption/ionization reflect time-of-flight (MALDI-TOF), 1H NMR and IR spectra (Figs. S1c and d, S2 and S3). Firstly, we performed theoretical calculations on these two GQDs to elucidate excited state behaviors. Fig. 1a and b present the optimized electron delocalization MO diagram of these two GQDs. Complete delocalization of the electrons can be observed
Summary
In conclusion, a series of GQDs (C42H18, C96H30, C132H34, C222H42) were synthesized by organic methods. The PL mechanism of the GQDs was investigated by ultrafast spectroscopy [51]. We draw the conclusion that in these organic synthesized GQDs, intrinsic state depends on size, while the energy level offset between intrinsic state and edge state decides their optical properties. As a result, the green fluorescence of these GQDs not only depends on the size, but also results from bright edge
Acknowledgements
This work was supported by the National Science Foundation of China (Grant No. 51373065, 81320108011, 21221063, 21273096), the National Basic Research Program of China (973 Program, Grant No. 2012CB933800 and 2014CB921302), and the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130061130010). The authors also acknowledges the "111" Program under Grant No. B06009.
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These authors contributed equally to this paper.