Thermal Effect of Sulfur Doping for Luminescent Graphene Quantum Dots

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 (similar to 80 degrees C-140 degrees C) with ZnSO4.7H(2)O 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 degrees C and reached to a ID/IG value of similar to 1.14. The photoluminescence measurements of doped GQDs having sulfur containing compounds at temperature of similar to 140 degrees 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 similar to 140 degrees C demonstrated a superior fluorescence quantum yield of 51%. This is, therefore, expected to make S-GQDs more suitable for bioimaging and optoelectronic applications. (C) 2018 The Electrochemical Society.