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We further extended our fundamental investigations on carbon dots as a new platform of photoactive nanomaterials. Carbon dots are small particles of neat carbon with the surface functionalized by organic or polymeric molecules to exhibit interesting optical and electronic properties, such as strong photoluminescence and efficient photoinduced electron transfers. We continued our project emphases on the measurement and quantification of the photophysical and photochemical properties of carbon dots toward a clear understanding of their related excited state processes and effects associated with the material composition and configuration; on the probing and evaluation of the electron transfer characteristics and processes as relevant to the potential development of photochemical energy conversion technologies; and on the exploration of strategies with respect to the modification of the existing carbon dots platform for significant improvements in their desired properties and performance. We made more significant progress and obtained valuable results in our investigations.

As reported previously and further confirmed in this project period, the structure of carbon dots is indeed rather simple. Basically, the carbon nanoparticle core is necessarily small (less than 10 nm). In fact, surface-oxidized small carbon nanoparticles in a suspension are found to be absorptive in the visible spectral region, where the photoexcitation results in weak luminescence emissions. Upon surface functionalization by organic molecules (oligomeric or polymeric), however, the absorption is increased and the photoluminescence is enhanced dramatically. We 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 were found to possess remarkable photophysical properties with, for example, high photoluminescence quantum yields of 60% and large radiative rate constants (>10⁸ s^-1, corresponding to very strong electronic transitions). Beyond our recently published paper showing that the optical properties of carbon dots compare favorably to those of the well-established and currently dominating semiconductor quantum dots, we obtained additional results that suggest consistency and stability in the performance of carbon dots.

We continued the investigation on the photoinduced electron transfer behavior of carbon dots. New results from both steady-state and time-resolved luminescence quenching experiments suggested an interesting quencher concentration dependence scheme, with luminescence emission intensities actually increasing at very low quencher concentrations (or “reversed quenching”) and then normal quenching at higher quencher concentrations. The former was attributed mechanistically to surface passivation effects provided by the quencher molecules at those low concentrations, thus consistent with the proposed photoluminescence mechanism for carbon dots. In fact, there has been increasing experimental evidence supporting the same mechanistic picture that the observed optical properties of carbon dots must be due to the passivated defects on the core carbon particle surface in the dots. Our recent publication pointed out some 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 could also be a result of radiative recombination of surface-trapped electrons and holes. We performed more experiments on the doping of core carbon nanoparticles with various inorganic salts (such as nanoscale silicon oxide and titanium dioxide) as a co-passivation agent, in addition to the organic passivation molecules, which all resulted in dramatically improved photophysical properties.

The concern and controversies have persisted in the literature on the potential toxicity of carbon nanomaterials (fullerenes and especially carbon nanotubes). For carbon dots, we reported biological evaluation results suggesting no meaningful toxicity. We recently extended our evaluation to include the precursor carbon nanoparticles from various sources. The results from their evaluation again suggested no significant toxicity.

We believe that the photoexcited state processes, especially the electron transfer properties, in carbon dots are particularly relevant to their potential uses in energy conservation. However, our experiments on device fabrication and photocurrent measurements encountered some technical difficulties due largely to the limitation in resources. We will continue the exploration in house, and pursue more vigorously for external collaborations.

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