Thermal control of oxygen-induced emission states in carbon dots for indoor lighting applications
Introduction
Nanoparticles have unique mechanical, optical, magnetic, and catalytic properties depending on their size and chemical composition. Many studies have investigated the effects of size, heteroatom incorporation (i.e., chemical doping), and surface modification [[1], [2], [3]]. It has been demonstrated that the optical properties of semiconductor nanocrystals (quantum dots) can be widely tuned by varying their composition and size-controlled quantum confinement.
Carbon dots (CDs) are a class of luminescent carbon nanomaterials with quasi-spherical morphology and a characteristic size of less than 10 nm [4,5]. CDs have attracted increasing attention for both fundamental and practical research owing to their attractive properties, particularly their fluorescence (FL), bioavailability, and thermochemical stability [6,7]. Consequently, they have been applied in various areas including biomedicine [8], light-emitting devices [9,10], photochemical sensors [11,12], data security [13,14], photovoltaics [15,16], and photocatalysis [17,18]. Furthermore, improvements in the functionality and diversity of CDs and products containing them have recently been accelerated by the development of simple and efficient methods for their preparation and functionalization [19,20].
The chemical structure of CDs is complex and still not completely known; however, the core is typically described as partially graphitic or polycrystalline with a mixture of graphitic and polymeric structures, and the surface is thought to be covered with heteroatom-containing functional groups and chemical defects [21,22]. Although they have various structures and compositions, CDs share several optical properties: (i) a large Stokes shift [23], (ii) excitation-dependent [24] or even multichromatic fluorescence [25], and (iii) a wavelength-dependent FL lifetime [26]. The contributions of certain physical/chemical structures in CDs to these optical properties are unclear [27,28]; however, incorporating heteroatoms into CDs by doping has been found to considerably enhance the color purity and emission intensity of their FL [29,30].
An understanding of the roles of heteroatoms in CDs is considered critical for designing a robust synthetic scheme and ultimately tailoring the optical properties for specific applications. Heteroatoms with lone electron pairs, such as nitrogen and oxygen, are particularly interesting because they can easily form strong covalent bonds with carbon and generate defect states (or intragap states) [31]. Previous studies have demonstrated that the effect of heteroatoms on the optical band gap of CDs depends strongly on the chemical structure of the precursors and the chemical states of the heteroatoms in CDs [[32], [33], [34]]. Ding et al. synthesized CDs with tunable FL from blue to red, which was attributed to surface oxidation [35]. Wang et al. reported a facile method for the preparation of CDs with full-spectrum emission, which was attributed to the combined effects of particle size and sp2-hybridized carbon [36]. Ding et al. designed a solvothermal method for CDs with tunable FL ranging from 443 nm (blue) to 745 nm (red), which was ascribed to the synergistic effects of the p-electron system, graphitic nitrogen, and surface states [37]. These studies offer valuable insights into the FL mechanism and highlight the potential of CDs for practical applications. However, most previous research has focused on nitrogen because it can significantly change the optical properties of CDs. In contrast to nitrogen, oxygen has generally been regarded as a negative defect that degrades the optical properties, especially the FL color purity. Moreover, the effects of oxygen on CDs have rarely been investigated because the precise control of the oxygen content or the degree of oxidation in CDs is still limited (Table S1, Supplementary Material) [29].
In this paper, we present a new approach to tailoring the optical band gap of CDs by controlling the degree of oxidation using citric acid (CA) as a carbonization source and benzyl alcohol (BzOH) as an oxidizing solvent. The FL color (wavelength) of the CDs can be varied from blue (428 nm) to red (628 nm) by varying the reaction temperature, which affects the oxidizing power of BzOH. The chemical origin of the FL of the CDs was systemically investigated using Stern–Volmer quenching analysis. The correlation between the oxygen-induced structural/chemical changes and carrier dynamics of the CDs was thoroughly studied using pump-probe spectroscopy. Finally, we demonstrated the practical utility of the CDs by fabricating sun-like white light-emitting devices (LEDs) with freestanding and flexible phosphor films based on CDs, which may replace toxic or costly rare-earth phosphors such as yttrium, lanthanide, europium, and terbium.
Section snippets
Reagents
CA (anhydrous, 99.5%), BzOH (anhydrous, 99.8%), toluene (anhydrous, 99.8%), and acetone (anhydrous, 99.5%) were purchased from Sigma-Aldrich. Poly(methyl methacrylate) (PMMA) was purchased from Microchem. All chemicals were used as received, without further purification or modification.
Synthesis of CDs
The CDs were prepared as follows. CA (2660 mg) was soaked in BzOH (9 mL), and the mixture was vigorously stirred under ambient conditions for 24 h. After a predetermined time, the mixture was transferred to a
Synthesis and morphological characterization of CDs
The synthesis of the CDs is illustrated in Fig. 1. Briefly, with increasing synthesis temperature, more BzOH was oxidized and converted into highly oxidative derivatives (HODs) such as benzaldehyde and benzoic acid [38]. These HODs participated in the carbonization reaction to form oxygen-induced defects (OIDs) in the CDs. Because OIDs are known to play an important role in the optical responses of CDs, they are expected to control the FL of the CDs depending on temperature. A detailed
Conclusion
We discovered oxygen-related emission states originating from OIDs in CDs obtained by using a new strategy. The degree of oxidation of the CDs was thermally controlled by varying the oxidizing power of BzOH, which is proportional to the reaction temperature. As the synthesis temperature increased from 200 to 300 °C, more BzOH was converted to HODs; in addition, more OIDs and associated emission states were formed in the CDs, and their FL wavelength was red-shifted by 200 nm, from 428 to 628 nm.
CRediT authorship contribution statement
Yerim Byun: Conceptualization, Methodology, Formal analysis, Investigation, Writing – original draft, Visualization. Chan-Woo Jung: Investigation, Writing – original draft, Visualization. Ji-Hee Kim: Formal analysis, Writing – review & editing, Funding acquisition. Woosung Kwon: Conceptualization, Formal analysis, Data curation, Writing – review & editing, Supervision, Funding acquisition.
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
This work was supported by the Basic Science Research Program (NRF-2022R1A2C4002403) and Nano Material Technology Development Program (2009–0082580) of the National Research Foundation of Korea. J.-H.K acknowledges financial support from the Institute for Basic Science of Korea (IBS-R011-D1).
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