Doctor of Philosophy
Luminescent materials play increasingly important roles in our lives. Improvements in these materials’ quantum efficiencies (QEs), costs and toxicity can greatly reduce the power consumption, price and environmental damage to the planet, respectively. Carbon quantum dots (CQDs) and luminescent materials exhibiting thermally activated delayed fluorescence (TADF) are two interesting materials in these aspects. Furthermore, measurements on electrochemiluminescence (ECL) and electroluminescence of these luminescent materials can quickly evaluate their performance for many applications. In this thesis, controlled CQD syntheses are revealed beneficial to luminescent materials applications. Their film ECL demonstrated relatively stable anionic and cationic radicals leading to high emission at solid/solution interfaces. It has been found that small sizes of CQDs produce efficient bright white film emissions. It has been discovered that film ECL is similar to the light emitting layer in an LED and can be used to find surface states in these CQDs that are predominantly excited to generate a wavelength-potential applied relationship. Combining this knowledge, small emitting CQDs have been analyzed using synchrotron spectroscopy, where thiophene states were uniquely found and likely caused the solid-state emissions and high cationic radical stability, both through cation-pi interactions. For the first time, CQD light emitting devices have been constructed to achieve 1 cd/m2 blue emissions for 3 h at a constant potential of 7.0 V. Electrochemical impedance spectroscopy has been utilized to display the possibility that device failure is due to small ions reacting at the highest energy interface. Organic long-persistent ECL has been observed for the first time with several TADF molecules. This phenomenon has occurred likely because of an exciplex being formed with a compound that had a small singlet-triplet energy gap. Two simple methods have been developed in this thesis to determine the absolute ECL QE of compounds along with the common commercial emitter Ru(bpy)32+. Finally, an analytical strategy has been established for measuring QEs of indicator and lighting LEDs in high accuracy and precision.
Summary for Lay Audience
Luminescent materials play increasingly important roles in our lives. Improvements in these materials’ efficiencies, costs and toxicity can greatly reduce the power consumption, price and environmental damage to the planet, respectively. Carbon quantum dots (CQDs) and luminescent materials exhibiting thermally activated delayed fluorescence (TADF) are two of the interesting materials in these aspects. Furthermore, measurements on electrochemiluminescence (ECL) and electroluminescence of these luminescent materials can quickly evaluate their performance for many applications. This thesis explores an organic light emitter that can be used in the applications described above with a small organic nanoparticle called a carbon quantum dot (CQD). CQDs were chosen because of low toxicity, simple syntheses and tunable optoelectronic properties. This thesis has an emphasis on the CQDs emission in the solid state and understanding how to increase this solid state emission. This thesis also investigated the best ways to measure absolute emissions to provide comparisons between CQDs and other luminescent material standards. The results of this thesis found high yield and simple CQD syntheses and CQD purification techniques. The CQD syntheses can also be tailored to achieve unique emission properties such as specific colors, stable radicals or solid state emissions. The nature of these emission properties were also investigated to find chemical moieties that are likely responsible for these emission properties. Finally, combining all previous knowledge, the CQDs were successfully incorporated into a bright blue lighting device that emitted visibly for 3 hours. The work presented here contributed to the knowledge of organic and nanoparticle luminescent materials. This thesis also found novel absolute measurement methods that allows researchers to easily compare research results and raise the impact of their research.
Adsetts, Jonathan Ralph, "Developing Carbon Quantum Dots as a Luminescent Material and Revisiting ECL and LED Absolute Measurement Methods" (2022). Electronic Thesis and Dissertation Repository. 9002.
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