Graphene quantum dots have recently gained significant attention for their potential application in the development of optoelectronic materials. The present study focused on the ultrasonic method to synthesize white-light-emitting graphene quantum dots from coal soot in just 2 min at room temperature. The white-light emission was achieved in solution and polymeric film with good Commission Internationale del’Eclairage index (0.28, 0.33) and (0.25, 0.30), respectively. The graphene quantum dots cover a significant fraction of the visible region in the emission spectrum with two prominent bands at 475 and 635 nm at 380 nm photoexcitation, corresponding to monomer and J-aggregate emission. The strong reducing and basic nature of the ethylene diamine facilitated the preparation of self-assembled J-aggregate graphene quantum dots through hydrogen bonding and electrostatic interaction. The mechanism of origin J-aggregate emission in the prepared graphene quantum dots was studied using UV–visible absorption, steady-state, lifetime fluorescence spectroscopy, and zeta potential. The as-synthesized graphene quantum dots are successfully coated on the UV-LEDs' surface and emit white light on the applied voltage. The colours of red, green, blue, and yellow balls appear significantly in the lighting of prepared white LEDs.

White-light emitting self-assembled graphene quantum dots from coal soot
    Abstract
        Graphical abstract
    1 Introduction
    2 Experimental details
        2.1 Materials
        2.2 Instrumentation
        2.3 Preparation of graphene quantum dots (GQDs) from coal soot
        2.4 Preparation of PVA film
    3 Results and discussion
        3.1 Morphological characterization
        3.2 Photophysical study
            3.2.1 White light emission from self-assembled GQDs
        3.3 Mechanism of J-aggregation in GQDs
        3.4 WLE in GOQDs-coated PVA film and UV-LEDs
    4 Conclusion
    Acknowledgements 
    References

• Rabi Narayan Senapati(Environment Emission and CRM Section, CSIR-Central Institute of Mining and Fuel Research, Dhanbad, Jharkhand 828108, India)
• Shiv Rag Mishra(Environment Emission and CRM Section, CSIR-Central Institute of Mining and Fuel Research, Dhanbad, Jharkhand 828108, India, Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India)
• Tuhin Mandal(Environment Emission and CRM Section, CSIR-Central Institute of Mining and Fuel Research, Dhanbad, Jharkhand 828108, India, Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India)
• Vikram Singh(Environment Emission and CRM Section, CSIR-Central Institute of Mining and Fuel Research, Dhanbad, Jharkhand 828108, India, Coal to Hydrogen Energy for Sustainable Solutions, CSIR-Central Institute of Mining and Fuel Research, Dhanbad, Jharkhand 828108, India, Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India) Corresponding author