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We have continued our fundamental investigations on a newly discovered class of photoactive carbon nanomaterials. These are small particles of neat carbon, which upon surface functionalization by organic or polymeric molecules exhibit interesting optical and electronic properties such as strong photoluminescence and photoinduced electron transfer. We have also continued our project emphases on the measurement and quantification of the photophysical and photochemical properties of these carbon nanomaterials toward an understanding of the related excited state processes and effects associated with the material composition and configuration; the probing and evaluation of the electron transfer characteristics and processes as relevant to the potential development of photochemical energy conversion technologies; and the exploration of strategies with respect to the modification of these carbon nanomaterials that may lead to significant improvements in their desired properties and performance. We have made more significant progress and obtained valuable results in our investigations.

As reported before and further confirmed in this project period, the structure of these photoactive carbon nanomaterials (dubbed “carbon dots”) is rather simple. Basically, the small carbon particle core is essential, and the particle surface is oxidized and then functionalized by organic molecules, which could be oligomeric or polymeric. We have continued the use of the classical amidation reaction for the surface passivation chemistry, with oligomeric polyethylene glycol diamine (PEG_1500N) as an established surface passivation agent. The PEG_1500N-functionalized carbon dots have been found to possess remarkable photophysical properties with, for example, high photoluminescence quantum yields of more than 50% and large radiative rate constants around 10⁸ s^-1 (corresponding to very strong electronic transitions). These and other results have been used in our recent publication to show that the optical properties of carbon dots compare favorably to those of the well-established and currently dominating semiconductor quantum dots.

We have completed a series of investigations on the photoinduced electron transfer behavior of carbon dots. The results from both steady-state and time-resolved luminescence quenching experiments have suggested that the photoluminescence emissions in carbon dots are quenched by either strong electron donor or acceptor molecules, as represented by N,N-diethylaniline and 2,4-dinitrotoluene, respectively, and that the electron transfer is dependent on the redox properties of the quencher molecules, solvent polarity, and other parameters. Some of the major findings have been published.

Mechanistically, we proposed that the observed optical properties must be due to the passivated defects on the carbon particle surface in carbon dots, for which we have continued to pursue experimental verification, indirectly at least. More specifically we hypothesized that there could be phenomenological similarities between the photoluminescence emission mechanisms in traditional semiconductor quantum dots and carbon dots (despite carbon being hardly a member of the semiconductor family), such that the emissions in carbon dots might also be a result of radiative recombination of surface-trapped electrons and holes. Our luminescence quenching results are consistent with the disruption of the electron or hole trapping on the passvated carbon particle surface by the strong electron acceptor or donor molecules, respectively. Also supporting the hypothesis are the available results suggesting that the doping of the carbon particle core by a small amount of inorganic salt, which might act as more effective co-passivation agent in addition to the organic molecules, could dramatically enhance the photophysical properties of the resulting carbon dots.

There have been much concern and also controversies in the literature on the potential toxicity of carbon nanomaterials, including fullerenes and especially carbon nanotubes. The same concern has extended to carbon dots, despite the fact that various forms of carbon nanoparticles are already used in commercial products. Nevertheless, we initiated a systematic toxicity evaluation of carbon dots. The available results, some of which are in the process of being published, have suggested that carbon dots are non-toxic, as expected.

We continue to believe that the photoexcited state processes, especially the electron transfer properties, in carbon dots are particularly relevant to their potential uses in energy conservation. Our effort on thin-film fabrication for photocurrent measurements has continued. We have also started to disperse carbon dots into polymeric hosts, as the initial step toward the development of optoelectronic devices for photoconversion applications. We will continue the exploration both in house and through external collaborations.

Two graduate students participated in the project, and other group members provided substantial technical and experimental assistance.