Carbon Dots for Next-generation Artificial Lighting Devices

Abstract

Energy conservation has become a major concern globally in today’s world and artificial lighting (AL) accounts to ~50% of total energy consumption. With increase in population accompanied by modern civilization there is a tremendous rise in the use of artificial lighting sources leading to energy crisis in near future. So there rises a high demand for the fabrication of energy efficient sources for artificial lighting. Traditional AL sources involve the use of Incandescent lamps (ILs)/Halogen Lamps (HLs) and Compact fluorescent lamps (CFLs) for lighting and the commercially used AL sources use white LEDs. Recent advancements in the fabrication of white LEDs involve the use of Quantum dots as down converting phosphor material coated on blue emitting InGaN/GaN LEDs. Quantum dots (QDs) are semiconductor nanoparticles with confinement of charge carriers and exhibit size/surface dependent emission properties. Semiconductor quantum dots as phosphor material for WLEDs exhibit size-tuneable emissions, high photoluminescence quantum yields, low scattering compared to traditional phosphors. White LEDs reported using semiconductor QDs, show potentially toxic behaviour to be used for indoor and outdoor lightening due to the toxicity of precursors involved. Further these dots show self-absorption loses effecting the color rendering index and efficiency of the fabricated device. Due to these shortcomings, there is a high demand for the synthesis of benign nanomaterial with similar or better optical properties.

The emerging carbon dots (CDs) seems to be an alternative to semiconductor dots for white LEDs due to the wide availability of carbon in nature and broad emissive behaviour of CDs with least toxicity increasing their potential applications in fabricating white LEDs. Compared to semiconductor QDs CDs have drawn a great attention among researches across the globe due to its good bio-compatibility, non-toxicity, photo bleaching, photo blinking, excellent water solubility, chemical stability, ease of surface functionalization and large two photon cross section areas, which makes them highly beneficial for applications in fabricating white LEDs. Also, photoluminescence properties of carbon dots can be controlled by modifying the size and surface of carbon dots fabricated. This surface modification of CDs enhances the intensity of PL emission from CDs that has a direct impact on increasing efficiency of white LED fabricated. Despite these advantages of CDs, they have few drawbacks in excitation dependent and dilution dependent emissive behaviour that can alter the CRI of the WLED in the long run. Also, although few reports have been published using CDs for white LED fabrication, However, the luminance, color-rendering index (CRI) doesn’t yet meet the requirements for practical application. Therefore, it is still urgent; to develop novel CD phosphor with enhanced properties for white LEDs. This thesis focuses on the fabrication of highly stable single system white light emitting carbon nanoparticles. The fabricated CDs show non photo bleachable behaviour and dilution independent emissive behaviour. Further luminescence from CDs is tailored using surface passivation and functionalization routes. Photoluminescence mechanism in CDs and reaction parameters effecting quantum yield of CDs are explored. In our next work plastic waste generated during covid-19 in the form of face masks, face shield, gloves, syringes and other plastics are recycled to white light emitting nanoparticles. This work of generating light from waste will reduce environmental plastic pollution (raised beyond limited due to Covid-19). Further we use the fabricated nanoparticles as fluorescent markers, active emitters in LEDs and in bio imaging.