Investigation of sulfur related defects in graphene quantum dots for tuning photoluminescence and high quantum yield

This paper presents a comprehensive study of the impact of defects on quantum yield in doped graphene quantum dots by having sulfur containing compounds (S-GQDs). The facile and high yielding hydrothermal method was used to process the S-GQDs by selecting two different compounds such as conc. H2SO4 and MgSO4·7H2O containing sulfur. Initially, the synthesized samples were characterized by using High Resolution Transmission Electron Microscope (HRTEM), Raman Spectroscopy, Fourier Transform Infra-Red Spectroscopy (FT-IR), Thermogravimetric and Differential Thermal Analysis (TGA/DTA), UV-vis spectroscopy, and Photoluminescence (PL). HRTEM images suggest that the majority of the both samples were in the narrow range of 5-20 nm in diameter. Optical properties of the GQDs are altered as a result of S-doping with purple tunable PL at shorter wavelengths. As expected, by using the different excitation energy in PL, appearance of peak introduces additional energy levels between π and π* that provide alternative electron transition pathways. The most remarkable finding is that the fluorescence quantum yield (FL QY) of S-doped GQDs is higher than that of the reported doped GQDs. This clearly suggests that the defects states related to S modify the electron density, tailor the PL characteristics and improvements in quantum yield of the GQDs.