Colour-tuneable hydrophobic carbon dot aggregates for LEDs applications

Abstract

Solid-state fluorescent carbon dot (CD) aggregates were synthesized by the reaction of 2,2′-dithiosalicylic acid with adenine/4-amino benzene thiol using solvothermal and precipitation methods. Blue-emissive OD and GD CDs were transformed into the orange- and green-emissive solid-state CD aggregates ODA and GDA upon treatment with water, respectively. This water-triggered aggregation-induced emission was responsible for the switching of the emission from blue to orange/green. Interestingly, ODA and GDA revealed reversible emission upon solid–liquid–solid switching. Contact-angle measurements suggested the hydrophobic nature of ODA and GDA. ODA and GDA were utilized to fabricate a fluorescent polymeric cover of a light-emitting diode.

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Carbon dots (CDs) are zero-dimensional eco-friendly nanoparticles. They have garnered interest due to their versatile properties such as fluorescence, nontoxicity, and biocompatibility. CDs have significant applications in various fields such as anticounterfeiting, optoelectronics, energy storage, supercapacitors, batteries, solar energy conversion, and solar cells.^1–4 Although most CDs have been reported to be highly soluble in water and typically found in liquid form, they often encounter significant challenges during drying and can undergo aggregation-caused quenching (ACQ).⁵ In other words, during solidification CDs tend to establish interactions, leading to excessive fluorescence resonance energy transfer and a marked reduction in fluorescence intensity. To overcome ACQ, CDs are incorporated into a polymeric matrix to retain the original fluorescence for a long time, but polymers affect the applicability of CDs.^6,7 Due to these challenges, solid-state CDs have been less explored and utilized compared with liquid-state CDs. However, several research teams have tried to solve ACQ-related problems by varying the substrates, solvents as well as solvothermal reaction conditions. However, it remains challenging to retain the fluorescence properties of CDs in the solid state. Furthermore, using a supramolecular engineering approach, CDs can be triggered to undergo aggregation under the influence of water to achieve aggregation-induced emission (AIE) in the solid state.^8,9

CDs represent a promising class of nanomaterials for the forthcoming generation of lighting and display technologies, which is attributable to their tunable band gap, high quantum yield, and remarkable stability. To achieve the desired fluorescence colour with high emission intensity in solid-state CDs, researchers have explored various strategies, including regulation of size, shape and morphology, surface functionalization, and heteroatom doping (particularly N and S atoms) to attain suitable excited energy levels.

Among fourth-generation lighting technology, light-emitting diodes (LEDs) have emerged as promising energy-saving, durable as well as efficient luminous sources. In addition to CDs, europium-based materials have also been effectively used for white light-emitting diode (WLED) fabrication due to their excellent luminescent properties.^10–12 However, such rare earth metal-based materials are often costly, making them less suitable for large-scale sustainable applications. In recent years, CDs have been employed as smart materials for the fabrication of high-color-purity LEDs following photoluminescence and electroluminescence mechanisms.¹³ The cost-effectiveness, development and utilization of photoluminescence are better than electroluminescence. To follow the photoluminescence mechanism, multicoloured emissive CDs-polymer composites have been used as phosphor-coating materials on ultraviolet chips or on blue chips. However, the design of desired fluorescent CDs followed by incorporation into a polymer matrix to achieve a transparent CDs-polymer composite material for the fabrication of high-color-purity LEDs remains a challenge.

Taking into account the aforementioned challenges to synthesize ACQ-free, AIE-efficient, fluorescent solid-state CDs for LED coating applications, we chose 2,2′-dithiosalicylic acid (DTSA) as the carbon and sulfur source, and adenine/4-aminobenzenethiol served as nitrogen sources (Scheme 1). The presence of a disulfide bond (S–S) may hinder π–π stacking and, in turn, overcome the problem of ACQ in solid-state CDs. In addition, the nitrogen-rich adenine and p-amino benzenethiol may contribute to the regulation of the emission properties, offering a method to synthesize fluorescent CDs. It is also expected that functionalized CDs will undergo further aggregation under the influence of water but also impart fluorescence through AIE in the solid state. The carbon dot OD was synthesized by a solvothermal reaction between DTSA and adenine in acetic acid (Experimental section, ESI†). Furthermore, the obtained CD solution was purified by centrifugation followed by the separation of large particles using a syringe filter.

Scheme 1 Synthesis of the carbon dots aggregates ODA and GDA (schematic).

The blue-emissive pure CD solution was further treated with hot DI water and, in turn, CDs precipitated out from the solution. This water-assisted precipitation was helpful in obtaining pure solid-state CDs but also triggered AIE to produce solid-state ODA-CDs (Scheme 1). In other words, the blue-emissive OD solution was transformed into orange-emissive solid-state ODA under the influence of water. Interestingly, the substitution of adenine with 4-aminobenzenethiol produced the green-emissive aggregated solid-state CDs GDA under similar reaction conditions to ODA, which signified the importance of heteroatoms to tune the emission properties in CDs. Notably, the dissolution of ODA and GDA into methanol produced a blue-emissive solution but, upon drying, it turned orange and green, respectively. In other words, ODA and GDA having reversible emission upon solid–liquid–solid switching may have been due to aggregation–segregation–aggregation, respectively. ODA and GDA were characterized by FTIR spectroscopy, FESEM, XRD, and spectrophotometry. The structural properties of ODA and GDA were confirmed by powder X-ray diffraction (PXRD) within a scanning range of 3–80° (Fig. 1). The PXRD pattern of aggregated CDs powder revealed peaks at 25° and 41° corresponding to the (002) and (100) planes in both ODA and GDA, thereby confirming the formation of a graphitic carbon core in CDs.⁹

Fig. 1 Powder X-ray diffraction pattern of aggregated (a) ODA and (b) GDA.

To analyse the morphology of aggregated CDs, the ODA and GDA samples were subjected to field emission scanning electron microscopy (FESEM) (Experimental section, ESI†). FESEM revealed uniform spherical aggregated structures of average size 0.89 μm and 0.68 μm for ODA and GDA, respectively (Fig. 2 and Fig. S1, S2, ESI†). Interestingly, these spherical aggregated structures were interconnected to each other through a stick-like structure. Adaptation of the spherical shape of CDs during aggregation was perhaps because of the minimization of contact with water or the hydrophobic effect of the molecular assembly involved in CDs. The hydrophobic nature of CDs can be attributed to the surface functional group originating from the selection of precursors used during synthesis. The contact angle was found to be 137.7° for ODA mixed with the binder polyvinylidene fluoride (PVDF) and coated over mild steel. Conversely, GDA showed a compromised contact angle of 117.8° under similar experimental conditions to ODA (Fig. S4 and S5, ESI†).¹⁴ The significant difference in the contact angle of ODA and GDA may have been due to the availability of hydrophobic atoms over the surface of aggregated CDs.¹⁵ Additionally, the compact structure and lack of polar surface groups can further enhance hydrophobicity. However, the morphology of OD and GD could not be captured due to presence of acetic acid and their hygroscopic nature, as well as the compatibility of the FESEM instrument to solid-state samples.

Fig. 2 FESEM images of (A) ODA and (B) GDA along with their average particle size histogram.

The FT-IR spectra of aggregated CDs were recorded to identify the surface functional groups and chemical bonds present in ODA and GDA (Fig. S3, ESI†). A prominent peak at 1676 cm⁻¹ and 1666 cm⁻¹ for ODA and GDA, respectively, was attributed to the C=O stretching vibration of amide groups, confirming the amidation of precursor materials. Peaks at 1586, 1560, 1460, and 1416 cm⁻¹ for ODA and 1586, 1535, 1492, and 1394 cm⁻¹ for GDA were associated with the symmetric and antisymmetric stretching vibrations of aromatic ring structures, indicating that the aromatic framework remained intact during carbonization.^6,16 Additional peaks at 1259, ∼1034, and 695 cm⁻¹ were assigned to the stretching vibrations of C–N, C–O, and C–S bonds, respectively. Notably, the peak present in ODA and GDA at 550 cm⁻¹ confirmed the presence of the S–S bond. The S–S bond is well known for playing a critical part in imparting stable fluorescence properties to CDs (Fig. S3, ESI†).

To explore the optical properties of ODA and GDA, detailed UV-visible spectroscopy and fluorescence experiments were performed, as demonstrated in Fig. 3. The solid-state UV-vis spectra of ODA displayed a broader absorption spectrum extending from the UV to the visible region, with a prominent peak around 345 nm (attributed to π–π* transitions of C=C), a shoulder around 450 nm (suggesting n–π* transitions of C–N/S) and another smaller shoulder at 557 nm (due to n–π* of C=O and C=O/S). Furthermore, freshly prepared solid-state ODA was subjected to fluorescence spectrophotometry. ODA showed an excitation peak at 557 nm, and an emission peak centred at 600 nm upon excitation at 557 nm (Fig. 3).¹⁷

Fig. 3 UV-vis absorbance with the excitation–emission plot of (a) ODA and (b) GDA.

In contrast, solid-state GDA showed a broader absorption peak around 250 nm (which could be assigned to the π–π* electronic transitions of C=C) and a weaker shoulder at 450 nm (indicating the n–π* transitions of C=O or C–N/S). Furthermore, the excitation profile obtained from fluorescence spectrophotometry confirmed suitable excitation at 450 nm. Considering 450 nm as the excitation value, an emission peak was found at 500 nm. The Stokes shift was calculated to be 43 and 50 nm corresponding to ODA and GDA, respectively.^9,18 The narrow excitation profile compared with emission for ODA and GDA was a consequence of the Franck–Condon principle.¹⁹

To confirm ODA and GDA were aggregated CDs, fluorescence titration studies between CDs (OD/GD) and water were performed (Fig. 5). The original acetic acid-mediated OD or GD showed emission at 445 nm and 473 nm (at fixed excitation (λ_ex) of 365 nm), respectively. Interestingly, upon aliquot addition of 70% water, a new peak appeared at 605 and 500 nm for OD and GD, respectively. Remarkably, the spectral change observed was found to be consistent with the visual results obtained under UV light (λ_ex = 365 nm) and by the naked eye (Fig. 5). The blue-emissive OD solution turned orange-pink upon maintaining 90% water content, indicating the formation of aggregated ODA. Conversely, GD turned into a blue–green turbid solution of GDA at addition of 90% water. The excitation-independent experiments also supported the emission at 605 and 500 nm for ODA and GDA, respectively (Fig. 4). The excitation-independent behaviour of ODA and GDA suggested fewer surface defects and uniform size distribution.²⁰ In other words, aliquot addition of water (90%) produced red-shifted peaks at 605 and 500 nm at the expense of diminishing the original peaks corresponding to OD and GD, suggesting that aggregation-induced emission caused colour changes in ODA and GDA.^21,22

Fig. 4 Photoluminescence spectra at different excitation wavelengths along with CIE coordinates of (a) and (c) ODA and (b) and (d) GDA CDs powder.

Fig. 5 (a) and (c) Demonstration of water-triggered aggregation-induced emission of an acetic acid OD and GD CDs solution and formation of ODA and GDA under UV light (λ_ex = 365 nm). (b) and (d) Aliquot addition of water to OD and GD solutions (λ_ex = 550, OD and 450 nm, GD).

The emission at 600, 500 nm for ODA and GDA supported their solid-state orange and green fluorescence, respectively (Fig. 4). In order to support the aforementioned observation about emission color, the CIE coordinates were acquired for ODA and GDA using the same set of data as demonstrated in Fig. 4. The emission point with CIE coordinates found to be at (0.54, 0.45) lay in the orange region, showcasing orange fluorescence for ODA, while the emission point labelled at (0.22, 0.35) fell in the region of blue-green emission, indicating a material emitting light in the blue green, but predominantly towards the green, spectrum.²³ The inclusion of two-colour emission might have been the reason behind the broader spectrum of GDA compared with ODA.

The shift in chromaticity coordinates between the two figures highlighted the tuneable optical properties of the materials, potentially influenced by variations in synthetic conditions or chemical modifications. These findings are significant for applications in colour-specific technologies such as LEDs and fluorescence-tuneable lenses.

To fabricate high-colour-purity UV LEDs following photoluminescence mechanisms, ODA or GDA powders wee mixed with epoxy resin to acquire a mould over a UV LED, as demonstrated in Fig. 6 (Experimental section, ESI†). After 1 h of setting time, LEDs of 365 nm were illuminated by a power supply at 3.2 V. The ODA-polymer composite produced orange LEDs, while the GDA-based composite emitted a green colour. The glowing fluorescence satisfied the aforementioned CIE coordinates.

Fig. 6 LED fabrication and illumination at 3.2 V (a) without CDs, (b) with an ODA cover and (c) a GDA cover.

Conclusions

The aggregated solid-state CDs ODA and GDA were synthesized using solvothermal and precipitation methods. These aggregate powders were characterized using FT-IR spectroscopy, UV-vis spectroscopy, fluorescence studies, PXRD and FESEM. The presence of AIE was established by fluorescence titration followed by FESEM. ODA was more hydrophobic in nature than GDA according to contact-angle measurements. Furthermore, owing to excellent fluorescence properties and photostability, we fabricated multicolour LEDs by applying ODA and GDA CDs aggregates. The fabricated LEDs possessed decent device performances by following a photoluminescence mechanism. Various types of colour-tuneable LEDs could be fabricated by following this simple and cost-effective approach.

Author contributions

MD: methodology, project administration, resources, conceptualization, funding acquisition, supervision, visualization, and writing (original draft). DK: data curation, methodology, validation, visualization, and figure creation. BKS: Helped DK in instrumentation, data analysis, and figure creation AK: visualization, discussion, review and editing.

Data availability

All the original experimental data for this study (FT-IR spectroscopy, UV-vis spectroscopy, fluorescence studies, contact-angle measurement and FESEM) have been provided in the main text, and ESI† is available upon reasonable request via email.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

DK acknowledges IIT Indore for financial assistance. MD thanks to Anusandhan National Research Foundation, Government of India, for funding under the TARE scheme (TAR/2022/000362). AK appreciates to the Deanship of Research and Graduate Studies at King Khalid University through number RGP 2/345/45 We acknowledge SIC and IIT Indore for instrumental facilities.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Materials and methods, synthesis, sample preparation, and characterization. See DOI: https://doi.org/10.1039/d5nj01351h

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