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

Highlights: • A facile and high yielding hydrothermal method was used to process S-GQDs by selecting MgSO₄.7H₂O (sample-I) and conc. H₂SO₄ (sample-II). • Optical properties of the GQDs were altered as a result of S-doping with purple tunable PL at shorter wavelengths (or larger band gap). • For practical applications in light emitting devices and short wavelengths diode lasers, we need wide band gap semiconductors. • The photoluminescence (PL) quantum yield of the S-GQDs was about 53% in comparison to un-doped graphene quantum dots. 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.