Thermal Effect of Sulfur Doping for Luminescent Graphene Quantum Dots

Abstract

This work presents a comprehensive study of quantum yield in doped graphene quantum dots with a series of sulfur containing compounds (S-GQDs). The facile hydrothermal method was used to synthesize S-GQDs at different temperatures (∼80°C–140°C) with ZnSO4.7H2O containing sulfur powder as a reducing agent. High Resolution Transmission Electron Microscope images suggest that the size of S-GQDs vary as a function of temperature during synthesis. Powdered X-Ray Diffraction confirms the crystallinity of all samples. Raman spectroscopy study reveals that the intensity ratio increases with an increase in temperature due to the presence of additional sulfur related defects that create enhanced elastic scattering. Removal of oxygen functional groups was maximized at 140°C and reached to a ID/IG value of ∼1.14. The photoluminescence measurements of doped GQDs having sulfur containing compounds at temperature of ∼140°C attributes to violet shift at lower excitation energy and a blueshift at higher excitation energy within the energy gap of S-GQDs due to the strong interaction of GQDs with high defect concentration of sulfur. The S-GQDs formed at ∼140°C demonstrated a superior fluorescence quantum yield of 51%. This is, therefore, expected to make S-GQDs more suitable for bioimaging and optoelectronic applications.