The avenue to synthesize eco-friendly and high-performing warm-white light emitting diodes (WLEDs) using quantum-dots for color conversion is challenging. Here, the graphene quantum dots (GQDs) are synthesized from Moringa oleifera leaves without the need of any organic solvents or reducing agents by a one-pot hydrothermal method and utilized for the design of efficient warm WLEDs. The photoluminescence of the obtained GQDs is found to be red-shifted as the excitation wavelength increases. This is ascribed to an excitation of multiple transitions due to various surface traps related to surface amino and oxygen functionalized groups as revealed from X-ray-photoelectron–spectroscopy and FTIR results. Three different concentrations of GQDs are embedded in polyvinyl-alcohol matrix acting as color-converters for the design of WLED devices. By increasing the GQDs concentration, the color correlated temperatures are tuned from 3804 to 2593 K and the luminous efficacy from 39.3 to 71.69 lm/W. Moreover, the chromaticity coordinates of the devices are shifted from (0.3825, 0.3665) to (0.4807, 0.4478). The brightness of the fabricated devices based on these green-GQDs are comparable with those of warm LEDs prepared from chemically synthesized graphene and carbon dots and can be suitable for indoor lighting applications.

Bright white light emitting diodes based on sustainable graphene quantum dots derived from Moringa oleifera leaves as photo-converter layer
    Abstract
        Graphical abstract
    1 Introduction
    2 Experimental section
        2.1 Materials
        2.2 Synthesis of GQDs
        2.3 Fabrication of GQDs films and LED devices
        2.4 Characterization
    3 Results and discussion
    4 Conclusions
    Acknowledgements 
    References

• F. S. Abd El‑kawy(Energy Materials Department, Faculty of Basic and Applied Science, Institute of Basic and Applied Science, Egypt-Japan University of Science and Technology, Alexandria 21934, Egypt, Physics Department, Faculty of Science, Ain Shams University, Cairo 11566, Egypt)
• M. Ghali(Energy Materials Department, Faculty of Basic and Applied Science, Institute of Basic and Applied Science, Egypt-Japan University of Science and Technology, Alexandria 21934, Egypt, Physics Department, Faculty of Science, Kafrelsheikh University, Kafrelsheikh 33516, Egypt) Corresponding author
• S. Hammad(Egypt-Japan University of Science and Technology, Alexandria 21934, Egypt)
• H. Talaat(Physics Department, Faculty of Science, Ain Shams University, Cairo 11566, Egypt)
• G. Bacher(Werkstoff der Electrotechnik, Duisburg-Essen University, 47057 Duisburg, Germany)