Abstract
Carbon dots (CDs) are an emerging class of carbon based nanoparticles, which are easy to synthesize, have bright luminescence and high chemical stability, and possess a broad range of applications ranging from light emitting diodes (LED) to biolabeling and drug release. The choice of precursors, doping atoms and synthetic conditions strongly influence the properties of CDs. Brightest emission and highest quantum yield (QY) values have been observed for citric acid based CDs, however, the origin of their emission and the related processes still remain under discussion. In this work we systematically explored absorption, emission and photoluminescence (PL) lifetime of bottom-up synthesized citric acid based CDs in dependence of synthetic conditions, precursors and doping atoms used. This allowed us to deduct a model, where molecular fluorophores dominate the optical properties of CDs. Under certain conditions, purely aggregated fluorophores, ‘Fluorophore-Dots’, are a sufficient description for the observed optical properties of CDs. The main contributions of this thesis are summarized as follows:First, we evaluated the specific influence of solvent and temperature on the nanoparticles. We studied the impact of precursors in connection with nitrogen and sulfur heteroatom doping on the emissive properties and derived their impact on a molecular level. We analysed solvatochromic shifts to identify internal emission processes of CDs. Three emission bands were identified belonging to the sp2 hybridized core, the edge and the functional surface groups of CDs. A band specific solvatochromism was discovered with edge and surface band transitions showing opposite solvatochromic shifts depending on the dopant used. In combination with band specific emission quenching we were able to identify extrinsic emission from surface attached fluorophores. From the detailed analysis we derived a model of the emission processes in such common hybrid CD particles. We assigned charge transfer as dominating process in the edge band and identified hydrogen bonding between CDs and the solvent as influential interaction for surface states. We further applied our model successfully to explain the emission processes in highly graphitic CDs.
Second, we varied the N-doping source to modulate the reaction pathways during the synthesis of CDs. We found that the precursors ethylenediamine and hexamethylenetetramine are able to react to derivatives of the citrazinic acid fluorophore through intermolecular condensation and decomposition to ammonia. Triethanolamine was found unable to form fluorophores due to its tertiary amine nature. We tracked characteristic fluorophore features in the optical spectra and PL lifetime measurements revealing the significant influence of molecular fluorophores in citric acid based CDs. In reference to the first chapter of this work, we similarly proposed that the fluorophores are (surface) attached to the carbonized elements in CDs.
Third, we conducted a novel ammonothermal synthesis of CDs using citric acid and pure ammonia as precursor. In previous studies, we already linked ammonia to the formation of fluorophores. Without the presence of water, however, a new kind of CD, a ‘Fluorophore-Dot’, has been formed. We found optical properties that are characteristic of aggregated fluorophores by comparing them to film samples of different concentration of pure citrazinic acid fluorophore. We therefore proposed that many of the observed characteristic optical features of citric acid based CDs are an actual result of aggregated fluorophores. In water and other solvents the amount and size of aggregation is not so large, but enough to dominate the observed absorption and emission processes.
Fourth, we studied the impact of the different reaction parameters such as synthesis time, temperature and carbonization / graphitization and successfully applied our aggregated fluorophore model. From the optical traces we deducted formation and aggregation of molecular fluorophores within 30 minutes of reaction time. After 2 hours, the CDs reached a steady state with only minor changes to their emissive properties. We therefore concluded that aggregated fluorophores are stable with only small alterations to surface groups. Carbonized material could function as a matrix holding the carbon ‘fluorophore’ dots together. In our temperature dependent studies, we compared CDs synthesized below (140 °C) and over (200 °C) the carbonization temperature threshold. We surprisingly found that the optical properties of both samples are very similar, demonstrating that carbonization to larger sp2/sp3 carbon domains can only play a minor role in CDs. To enforce carbonization and to study the actual impact of sp2/sp3 carbon, we exchanged the citric acid precursor with graphene oxide sheets. The synthesis therefore started with an already strongly carbonized, graphitic core with subsequently only the surface left for possible chemical reactions. The resulting CDs showed no presence of fluorophores and significantly different optical properties. In fact, we found that their optical properties match with those predicted by theoretical studies assuming a multi layered graphitic core. Lifetime spectra match with our previously proposed model of charge transport and low energy surface states in the hybrid model of graphitic carbon core with surface attached fluorophores.
In conclusion, the results of this work suggest the need of careful consideration of precursors and synthetic conditions when considering optical properties of CDs. The general assumption of emission from sp2/sp3 may not be applicable if a reaction pathway allows for the formation of fluorophores. However, the presence of aggregated fluorophores enables particles of high PL QYs in combination with good chemical stability. Knowledge of the fluorophore formation and their respective emission processes is key to the use of CDs in a broad range of applications ranging from ion detection to LEDs and solar cells.
| Date of Award | 31 Aug 2016 |
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| Original language | English |
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| Supervisor | Andrey ROGACH (Supervisor) |