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Photoluminescence of graphene quantum dots doped with different elements

Graphene quantum dots (GQDs), as a new graphene material, possess advantageous chemical-physical properties. Owing to their distinctive photoluminescence (PL) property and low biological toxicity, GQDs have been shown to be good candidates for applications in bioimaging and medical analysis. However, the prepared GQDs have their own slew of problems, including low luminescent efficiency, uncertain luminescent mechanism and lack of effective methods to tuning the luminescence property. Heteroatom doping is a good strategy, which could partially address these above problems. Because the doping atoms will be located in the internal structure of carbon nanomaterials thus change their local electronic configuration, polarizability, defect degree and band structure, etc., the physical and chemical properties of GQDs could be well tuned. At present, GQDs doped with Cl, F, N, S, B, P, etc. have been prepared. These obtained GQDs with heteroatom doping not only exhibited tunable optical properties, but also showed enormous potential applications in the field of photocatalysis. Herein, in order to further improve the PL performance of GQDs and extend their application, we prepared pure GQDs and single elemental-doped GQDs with Cl, N, P and S by using the electrochemical method and hydrothermal method. Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy, (XPS) and Raman measurements were used to characterize their elemental composition, surface element state and structural defect. Based on the experimental results, the position and bonding state of heteroatoms in doped GQDs were analyzed. The doping amounts in these doped GQDs are different, i.e., 1.35% of Cl-GQDs, 7.95% of N-GQDs, 10.08% of P-GQDs and 3.25% of S-GQDs, respectively. The degree of defect state is decreased in the order as follows: P-GQDs>S-GQDs>GQDs>Cl-GQDs>N-GQDs. Meanwhile, the PL performance was tested, and the fluorescent quantum efficiencies were calculated to be 8.2% for Cl-GQDs, 5.3% for N-GQDs, 4.0% for GQDs, 2.8% for S-GQDs, and 0.037% for P-GQDs, respectively. It can be concluded that the diverse doping atoms play different roles for the improvement of PL performance. The doping of Cl and N can form the luminescent center, which improves the fluorescent intensity of GQDs. Especially for the Cl doping, the fluorescent intensity of Cl-GQDs is increased twice compared to the pure GQDs, due to larger atomic radius and more outer shell electron of Cl than that of the others doped elementals. In the S-GQDs, the doped S could become small quenching centers, which decreased the fluorescent intensity slightly compared to that of pure GQDs. However, different from the above doped GQDs, P-GQDs showed the negligible fluorescence. Because the P atoms mainly are present in the surface functional groups of P-GQDs, these P-atoms induced defects may become the large fluorescent quenching center and greatly decrease their PL intensity. Besides the measurement and analysis of luminescence intensity, the emission peak positions and corresponding excitation wavelengths of these doped GQDs were also well studied. The excitation wavelength of pure GQDs is located at 340 nm, while the doped GQDs are all located at 360 nm. The strongest emission peaks of Cl-GQDs and N-GQDs are around 450 nm, while the strongest emission peaks of S-GQDs and pure GQDs are around 430 nm. These results indicate that the doping of heteroatoms could change the band gap of GQDs to some extent.