A Question Of Location

As LMU physicists show, the light-emitting and photocatalytic properties of tiny carbon nanospheres can be precisely tuned by precisely positioning nitrogen atoms.

Carbon nanoparticles, known as C-dots, have unusual optical properties that make them highly interesting for a range of technological applications, from solar energy conversion to medical imaging. In addition, compared to similar materials, C-dots have the advantage that they are stable and easy to manufacture and contain no toxic heavy metals. Whether or not certain C-dots show the light emission relevant to imaging, or rather possess the photocatalytic properties that are important for energy conversion, depends on their chemical composition and complex internal structure. However, the underlying mechanisms are poorly understood. LMU Physicist Jacek Stolarczykhave investigated these relationships and show that the properties of the C-dots can be easily influenced by chemical modifications. The scientists report on their findings in the journal Nature Communications.

"Previously, C-Dots were typically optimized using the trial-and-error principle," says Stolarczyk. "To improve this, a deeper understanding of the mechanisms underlying the optical properties of the C-Dots is essential." The scientists carried out their study as part of the interdisciplinary project "Solar Technologies Go Hybrid" (SolTEch), funded by the Free State Bavaria is promoted generously. "SolTech's goal is to explore innovative concepts for converting solar energy into non-fossil fuels, ideally using abundant and non-toxic materials," said Professor Jochen Feldmann , head of the SolTech project. C-Dots are ideally suited for such applications.

The nanospheres consist of various polycyclic hydrocarbon compounds whose complex interaction determines their optical properties. For their study, the scientists made C-dots by irradiating a mixture of citric acid and a nitrogen-containing branched polymer with microwaves. They varied the concentration of the polymer so that different amounts of nitrogen were incorporated into the nanospheres. In particular, the way in which the nitrogen was incorporated varied with how much nitrogen was available.

"Our investigations showed that the chemical environment of the incorporated nitrogen atoms has a decisive influence on the properties of the C-dots," says Dr. Santanu Bhattacharyya, the first author of the publication and Alexander-von-Humboldt Fellow at the Chair of Jochen Feldmann. Incorporation in the inner regions of graphene-like structures, as found at moderate polymer concentrations, resulted in nanospheres that show predominantly fluorescence in the blue spectral region upon appropriate excitation. In contrast, incorporation into edge positions, as occurred for very high and very low levels of polymer, led to the suppression of light emission and, instead, to effective photocatalytic reduction of water to hydrogen. Due to small variations of the synthesis procedure, these properties can therefore be finely controlled.