Research ArticleDisclosing the emissive surface traps in green-emitting carbon nanodots
Graphical abstract
Introduction
Fluorescent carbon dots (CDs) have attracted considerable scientific attention thanks to their remarkable optical and functional characteristics, such as high emission efficiency in the visible range, low toxicity, high biocompatibility [[1], [2], [3], [4]], ease of synthesis and versatile surface passivation [[5], [6], [7], [8], [9]]. Beginning from the very first work on CDs [10], and as later confirmed by countless studies, it was shown that the surface structure of these nanosystems is crucial to determine their optical properties. On one hand, a well passivated surface seems to be the key to activate efficient fluorescence [6,10] on the other hand, any modifications of the surface structure reflect into strong changes of the optical spectra [11]. Thus, the need of a properly functionalized surface to activate CD light emission is a cornerstone of our phenomenological understanding of CDs [11,12]. Nevertheless, neither the underlying reason is entirely understood, nor the specific role of different surface groups is clear. To explain the role of the surface, it was long hypothesized that CDs emission origins from the recombination of photoexcited charges which are temporarily trapped on certain surface sites. Although this idea has received a certain success [13], it has not been possible so far to identify the specific moieties which behave as surface traps. Here, we provide strong evidence that these traps can be identified as carboxylic groups.
Indeed, CD optical properties are highly debated and synthesis-dependent. Different authors hypothesize various types of electronic transitions: transitions within the carbogenic core [14], surface chromophores [[15], [16], [17]], molecular-like emission [18], or excitonic processes [19]. This variability is probably related to the many different types of CDs characterized by variable core and surface structures and/or aggregation states (J-, H-aggregates) [20,21]. Despite of this, one can pinpoint some common features which are frequently encountered in the literature. In fact, the truly “archetypal” traits of CD optical spectra are two: broad and unstructured absorption bands covering the visible range, and a tunable fluorescence which, in the vast majority of cases, peaks in the green. In fact, although high emission quantum yield are often obtained for blue-emitting CDs, the reliability of these studies is often highly debated, considering the variety of blue emitting aromatic molecules that can be generated as side products from the synthesis, contaminating the true emission of CDs [18]. As for the structure, typical CDs have very high surface/volume ratios, due to the small size (<5 nm), and a disordered surface covered by a few types of polar functional groups: mostly –OH, –NH2, –COOH, –CONH2, which, in bottom-up synthesis routes, are left on the surface as residues of the reagents. Considering this, it makes sense to look for a common mechanism for the optical response of CDs even if the variability of their structures is quite large. In particular, it is reasonable to think that the emission directly involves one of these few simple moieties which are so ubiquitous on CD surfaces. On the other hand, high levels of nitrogen doping are often found essential to increase emission efficiency, but the underlying reasons are still unclear.
This study is founded on the synergy between experimental and theoretical studies of a carefully chosen model CD system. These dots were strategically selected because they display the archetypal optical response of CDs within a particularly small size (1.5 nm) and a simple structure, which makes them computationally feasible. This choice is very important in that it allows conducting a combined experimental-theoretical study on essentially the same system, without having to compare experimental data to unrealistic, excessively idealized theoretical models. Thereby, combining theoretical investigations with a comprehensive photophysical study (from femtosecond to nanosecond range) and using a well-controlled chemical passivation route, it is possible to clarify the typical photocycle of CDs. Our results directly highlight the electron transfer character of the electronic transition, while demonstrating the important role of nitrogen atoms in the core and key role of carboxylic groups as surface traps.
Section snippets
Synthesis, purification and functionalization of CDs
As previously reported [22] CDs have been obtained through a solvothermal synthesis in autoclave, mixing urea (6 g) and citric acid (3 g) in a DMF solution (30 mL) and heating to 160 °C for 4 h. When the solution cooled down, 60 mL of ethanol were added. After centrifugation procedures at 10,000 rpm for 10 min and ethanol washing, a dark powder was collected and dispersed in water. Size exclusion chromatography (SEC) was performed to purify the sample by using a glass column (100 cm length,
Results & discussion
CD fluorescence has been often related to the presence of nitrogen in the structure of the dot [35,36]. In fact, several works have found that a substantial degree of nitrogen doping is crucial to increase the emission efficiency above an acceptable threshold. For this reason, the CDs we used in this study were synthesized by a standard protocol that leads to green-emitting CDs with high nitrogen content and quantum efficiency >10%. Based on previous studies [22,26], they are certainly expected
Conclusions
In conclusion, we unravelled the identity of the surface charge traps responsible of the typical green fluorescence of carbon nanodots, which were recognized in the carboxylic groups.
Experiments and theoretical calculations conclusively agree on the character of the lowest-energy electronic transition of these nano-systems, which involves an electron transfer between the nitrogen atoms of the core and the carboxylic groups attached on the surface, both crucial for a bright emission. The
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
A. Sciortino was supported by the “L’Oréal Italia Per le Donne e la Scienza” Program (17th edition) and N. Mauro was supported by Fondazione Umberto Veronesi (2020 FUV Fellowship). The authors thank G. Napoli for technical assistance in low temperature measurements. The authors thank the LABaM group at University of Palermo for support and stimulating discussions.
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