Formation mechanism and structure-property relationships of carbon dots for applications in light-emitting devices
Creators: name: Javed, Nasir
Publisher: name: No Publisher Supplied
Year: 2023

description: Fluorescent carbon nanoparticles are a class of light-emitting materials that were discovered in 2004. They are spherical particles that are usually composed of considerable amounts of oxygen, nitrogen and hydrogen along with carbon, and are often referred to as carbon dots (CDs). One of the most attractive properties of CDs is their bright fluorescence. Their photoluminescence (PL) emission wavelength can be tuned anywhere from blue to red, which makes them appealing for a variety of applications. Besides this, they have many other advantages such as high biocompatibility, excellent electronic properties and an abundance of raw materials to synthesize them. Therefore, CDs have been widely researched since their discovery as an alternative to conventional semiconductor quantum dots. One of the main reasons for the widespread research into the synthesis of CDs is to investigate the underlying mechanism of PL emission. Even though numerous reports on the synthesis and application of CDs are available in published literature, the PL emission mechanism of CDs is still debated, and researchers have explained it in different ways. In most reports, PL emission from CDs is attributed to surface electronic states caused by surface functional groups. On the other hand, some researchers attribute PL emission to inter-band electronic transitions of crystalline carbon caused by quantum confinement. In recent years, CDs with ultra-high PL quantum yield (PLQY) have been reported. The high PLQY is attribute to the presence of fluorescent molecules on the surface of CDs or free floating in the CD dispersion. It appears that many factors like the presence of molecule-like fluorophores, surface functional groups and the structure of CD core play important roles in the optical properties of CDs. The main source of the controversy about PL emission mechanism is variety of synthesis methods used in the literature and lack of understanding of structure-properties relationships of CDs. In this dissertation we synthesize CDs with stable fluorophores having high PLQY and use them for the fabrication of light emitting device (LEDs). We first study the formation mechanism of the fluorophores and the CDs during the bottom-up synthesis at mild conditions. It is demonstrated that during the chemical reaction at mild condition small molecular precursor form fluorescent molecules. These fluorophores then aggregate together to form spherical particles, i.e., CDs. The size of unpurified CDs changes significantly over time due to adsorption of the fluorophores on them. It is also explained that the PL emission mostly originates from the molecular fluorophores, which slowly decomposed and attach to the CDs and caused an increase in particle size. We also demonstrate that PLQY is higher when fluorophores are floating freely in the solvent and not attached on the CDs, and PL emission also depends on the solvents. By reporting PL emission from relatively larger particles and showing the dependence of PL emission on local environment (solvent) of the particles we rule out the possibility of PL emission due to quantum size effects. Then we investigate the structure of the fluorophores and structure-property relationships of the CDs. After comprehensive structural and optical characterizations, it is proved that the more than one type of fluorophores can form during the synthesis which make spherical particles. Based on electron microscopy and structural characterization, we prove that the spherical particles can be perceived to be similar to CDs; however, they lack an extended carbon network. We demonstrate the separation of different types of fluorophores to further investigate their structure and optical properties of both types of fluorophores. By thorough chemical and structural characterization and density functional theory calculation we investigate the structure of the molecular fluorophores and show that the building block of the carbon-dot like particles is molecule with fused rings that is identified as N4,N11-dimethyldibenzo[a,h]phenazine-4,11-diamine (BPD). We further studied the detailed optical and photophysical properties of these molecules. It is shown that the BPD is one of the most stable fluorophores ever reported in the context of CDs. This study helps explaining the formation and PL emission mechanism for CDs synthesized at mild temperature conditions using bottom-up approaches. In order to utilize these fluorophores, i.e., BPD in LEDs studying the solid-state properties is very important. Therefore, we investigate the optical and photophysical properties of BPD in the solid state, and study the effects of concentration and aggregation of BPD in the polymer matrices on its optical properties. It is observed that the PLQY of BPD films strongly depends on the concentration of BPD in the solid matrices. We demonstrate that the reduction in PLQY observed at higher concentration is due to intermolecular charge transfer, nonradiative pathways created by aggregation and increased reabsorption of PL emission by surrounding molecules. Finally, we demonstrate the application of BPD in solid-state lighting. We utilize BPD in the color converting layer of optically pumped LEDs. We demonstrate control on the color of fabricated LEDs through selection of appropriate pump wavelength, concentration of BPD and thickness of color converting layer. Fabrication of almost pure white light emitting devices with luminous intensity significantly higher than reported values in the literature on CD-based white light-emitting devices is demonstrated.