How to Control Luminescence of C-Dots

Cuvettes with blue-luminescent carbon dots, optimized with added nitrogen. (Source: S. Bhattacharyya)

Thanks to their unusual optical pro­perties, carbon particles with dia­meters on the order of a few nano­meters – C-dots – show great promise for a wide range of techno­logical appli­cations, as diverse as energy conversion and bio-imaging. Moreover, C-dots have several practical advan­tages over comparable materials insofar as they are easy to fabricate, stable and contain no toxic heavy metals. Their versa­tility is largely due to the fact that – depending on their chemical compo­sition and aspects of their complex structure – they can either act as emitters of light in the form of photo­luminescence or function as photo­catalysts by absorbing light energy and triggering chemical reactions, such as water splitting.

However, the factors that determine these disparate capa­bilities are not well understood. Now physicists from the Munich Uni­versity LMU led by Jacek Stolarczyk have taken a closer look at the mechanisms under­lying these very different pro­perties. Their study shows that the physico­chemical charac­teristics of these nano­materials can be simply and precisely tuned by intro­ducing nitrogen atoms into their complex chemical structure in a controlled manner. “Up until now, C-dots have typically been optimized on the basis of the principle of trial and error,” says Stolarczyk. “In order to get beyond this stage, it is essential to obtain a detailed under­standing of the mechanisms that underlie their diverse optical charac­teristics.”

C-dots are made up of networks of poly­cyclic aromatic carbon compounds, whose complex inter­actions determine how they react to light. In the new study, the Munich researchers synthesized their C-dots by com­bining citric acid as a carbon skeleton with a branched, nitrogen-con­taining polymer, and irra­diating the mixture with micro­waves. In a series of experiments, they systema­tically varied the concen­tration of the polymer in the mixture, such that different amounts of nitrogen were incor­porated into the carbon networks. They found that the precise conditions used had a critical impact on mode of inco­rporation of nitrogen into the carbon lattices that make up the C-dots, i.e. whether it sub­stituted for one of the carbon atoms forming the inter­linked 6-membered carbon rings resemb­ling tiny graphene flakes, or in the 5- and 6-membered rings found on the free edges of the aromatic structures.

“Our inves­tigation showed that the chemical environ­ment of the nitrogen atoms incor­porated has a crucial influence on the pro­perties of the resulting C-dots,” says Santanu Bhatta­charyya, Alexander-von-Humboldt fellow in the research group of Jochen Feldmann. Incor­poration inside the graphene-like structures, found at inter­mediate polymer concen­trations, led to the dots which predo­minantly emit blue photo­luminescence when irradiated with light of a suitable wave­length. On the other hand, incor­poration at edge positions, found for either very high or very low amounts of the polymer, suppressed photo­luminescence and resulted in C-dots that photo­catalyti­cally reduced water to hydrogen instead. In other words, the optical properties of the C-dots can be modified at will by varying the details of the procedure used to synthesize them. The members of the team believe that their latest insights will stimu­late further work on the use of these intriguing nano­materials, both as photo­luminescent light sources and as photo­catalysts in energy conversion processes. (Source: LMU)

Reference: S. Bhattacharyya et al.: Effect of nitrogen atom positioning on the trade-off between emissive and photocatalytic properties of carbon dots, Nat. Commun. 8, 1401 (2017); DOI: 10.1038/s41467-017-01463-x

Link: Photonics und Optoelektronics, Ludwig-Maximilians-University LMU, Munich, Germany

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