Eco-Friendly Microwave Synthesis of Blue-Graphene Quantum Dots From Bougainvillea glabra and Raman Spectral Analysis

The production of graphene quantum dots (GQDs) with a narrow size distribution is a popular topic in current research. In this work, we developed a single-step synthesis of GQDs from the bio precursor of Bougainvillea glabra flower extract using a power-controlled microwave synthesiser. The Raman studies show that the GQDs are functionalised with oxygen and hydrogen, and a detailed study of the 2D band reveals a bilayer structure. To understand the origin of unknown peaks, the Raman spectrum for the bilayer GQDs is simulated by performing density functional theory (DFT) calculations. All the observed off-resonant peaks in the 200- to 3000-cm⁻¹ range from the theory are compared with the experimental peaks and show good agreement. The evolved peaks in the synthesised GQDs are explained in detail by DFT analysis of the vibrational modes. Additionally, the structural conformation is characterised by X-ray diffraction and high-resolution transmission electron microscopy; the absorption characteristics are examined by UV–visible spectroscopy; the emission characteristics are studied by steady-state and time-resolved photoluminescence spectroscopy; and FTIR identifies functional groups. All studies confirm that the synthesised GQDs have a bilayer structure with a narrow size distribution of 4–5 nm. The emission characteristics show that the synthesised GQDs exhibit excitation-dependent fluorescence emission.