Gram-Scale Room-Temperature Synthesis of Solid-State Fluorescent Carbon Nanodots for Bright Electroluminescent Light Emitting Diodes

Multifunctional Fluorescent Material Based Red-emitting Carbon Dots for Cell Imaging and Photodynamic Sterilization

In this paper, a new carbon dot (R1-CDs) was prepared by one-pot hydrothermal method by using 1,8-diaminonaphthalene and o-phthalic acid (o-PA) as precursors. Due to the high purity of R1-CDs, NMR analysis was performed to identify the types of H and C atoms in their graphene sheets. From our research findings, three important information was disclosed such as (1) five types H atoms are presented in R1-CDs; (2) 18 kinds of C atoms in the graphene sheets are observed, and 8 kinds of them are quaternary atoms, and 10 kinds of carbon atoms as tertiary one; (3) functional groups of -COOH and -NH2 from precursors cannot be inherited into the edges or defect sites of graphene sheet. Obviously, our research findings for the first time revealed the more details of chemical structures of CDs. We believe that our works can supply a general concept to fabricate CDs by selecting specific precursors, also can encourage CDs' development in a more "chemistry" way by employing NMR as a powerful method.

Single Solid-State Emissive Carbon Quantum Dots for Multicolor, Bright and Efficient Electroluminescent Light-Emitting Diodes

Color-tunable electroluminescent light-emitting diodes (LEDs) based on quantum dots (QDs) are rapidly emerging as a key technology for next-generation full-color displays and solid-state lighting. However, achieving broad color tunability in LEDs that utilize a single QD emissive material continues to pose significant challenges. Here, we present the first example of bright, multicolor electroluminescent LEDs with tunable emission peaks spanning from 535 to 640 nm, utilizing a new type of single red solid-state emissive carbon quantum dots (R-SSCQDs). Unlike conventional CQDs, which often exhibit weak or negligible emission in solid-state form, R-SSCQDs demonstrate bright red emission in solid state and green emission in diluted state. Experimental investigations and theoretical calculations reveal that the unique non-planar spatial structure and significant steric hindrance of R-SSCQDs effectively suppress π-π stacking, resulting in efficient solid-state fluorescence emission. By adjusting the weight ratios of R-SSCQDs doped in host materials, we showcase bright, efficient and multicolor CQD-based LEDs that emit green, yellow, orange, and red light, achieving a record-high luminance of 15,834 cd m-2 and current efficiency of 10.3 cd A-1. This work presents a straightforward and universal strategy for creating efficient solid-state emissive CQDs, showcasing significant potential for color-tunable LED applications.

Precursor engineering towards orange- and red-emissive carbon dots for LEDs with tunable emission colors

Carbon dots (CDs) have attracted significant research interest due to their great potential in optoelectronic applications. Although various CDs have been synthesized via the "bottom-up" pathway, few studies have focused on understanding the origins of the structural and optical diversities of CDs. In this study, two benzenoid acids with a slight structural variation (i.e., 9-oxo-9H-fluorene-2,7-dicarboxylic acid (FR) and 4,4'-biphenyl dicarboxylic acid (BP)) are employed as precursors, yielding orange- and red-emissive CDs with quantum yields of 43.1% and 30.9%, respectively. A combined experimental and theoretical study reveals that the structural and optical diversities of the CDs originate from the structural variation of their precursors. Furthermore, we demonstrate that the light-emitting diodes (LEDs) based on the blended emissive layer of poly(N-vinyl carbazole) (PVK) and the synthesized CDs display cyan and yellow lights, respectively, with moderate turn-on voltages of 4.0 and 4.5 V and maximum luminance values of 454 and 276 cd m-2. Such different optoelectronic performances could be attributed to the different energy-level alignments of CDs-FR and CDs-BP, relative to that of PVK. This study thus provides a typical example to understand the precursor-dependent diversities of CDs, which may contribute to the rational screening of precursors towards CDs with desirable optical/optoelectronic properties.

Carboxyl-functionalized multifunctional red-emitting carbon quantum dots as an ideal biomaterial

Carbon quantum dots (CQDs) have been developed into a popular nanomaterial due to their abundant surface state, good biocompatibility, and excellent antimicrobial properties. However, CQDs with multiple functions, such as being red-emitting, having good antibacterial activity, and having excellent pH sensitivity, have rarely been reported. In this work, red-emitting CQDs (R-CQDs) with excellent optical properties and antimicrobial activity were prepared by a simple green hydrothermal method. In antimicrobial applications, the R-CQDs featured good antibacterial activity due to the generation of reactive oxygen species, indicating excellent photodynamic antimicrobial ability. In addition, the R-CQDs showed fine pH sensitivity, giving them potential as pH sensors to monitor the pH of wounds in real time. The promising potential application of R-CQDs for cell imaging was also demonstrated. In summary, we offer R-CQDs with good antibacterial and pH sensitivity as a potential nanomaterial for pH and antimicrobial monitoring of wounds, shedding light on the biomedical field.

Twisted Structure Induced Solid-State Fluorescence and Room-Temperature Phosphorescence from Furan-Based Carbon Dots

Boron doping can effectively induce solid-state fluorescence (SSF) in carbon dots (CDs); however, research on the intrinsic mechanism underlying this phenomenon is lacking. Herein, a design strategy for boron-doped furan-based CDs is proposed, CDs with aggregation-induced emission (AIE) properties are synthesized, and the mechanism by which boron atom dopants induces SSF and room-temperature phosphorescence (RTP) is elucidated. The morphology and structural characterization of the CDs indicate that boron doping leads to structural twisting of the CDs. The AIE phenomenon of CDs arises from the inhibition of the twisted structure motions and a reduction in the nonradiative relaxation rate during the aggregation process. In addition, CDs with twisted structures exhibit a smaller overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), effectively reducing the singlet-triplet splitting energy (ΔEST). CDs embedded in microcrystalline cellulose (MCC) exhibit green RTP because the nonradiative transitions are suppressed, and the excited triplet species remain stable. For the first time, this study reveals the structure-activity relationship between the twisted structure and optical properties of CDs, providing a new approach for the preparation of solid-state light-emitting CDs.

Solid-State Fluorescent Carbon Dots with Hydrophobic Modification Induced Red Emission for White Light-Emitting Diodes

Herein, we developed red solid-state fluorescent carbon dots (SSF-CDs) through a one-step solvothermal method, utilizing acetone as the carbonization solvent. Optical and structural characterization revealed that the sp2 domains in the core of the R-CDs were consistently interrupted and that the oxygen-containing groups on the surface were replaced by alkyl groups. This substitution mitigates excessive π-π interactions, thereby preventing quenching of fluorescence in the solid state. Adjusting the molar ratio of citric acid (CA) and urea yielded solid fluorescent carbon dots (CDs) with panchromatic luminescence, indicating enhanced π-π interactions and more pronounced red shifts in the emission peaks. Furthermore, we found that this strategy is applicable to other carbon sources, including phenylenediamine, salicylic acid, and lignin. This research presents an innovative strategy for the fabrication of solid-state luminescent CDs.

Dimethylene-Cyclopropanide Units as Building Blocks for Fluorescence Dyes

Many organic dyes are fluorescent in solution. In the solid state, however, quenching processes often dominate, hampering material science applications such as light filters, light-emitting devices, or coding tags. We show that the dimethylene-cyclopropanide scaffold can be used to form two structurally different types of chromophores, which feature fluorescence quantum yields up to 0.66 in dimethyl sulfoxide and 0.53 in solids. The increased fluorescence in the solid state for compounds bearing malonate substituents instead of dicyanomethide ones is rationalized by the induced twist between the planes of the cyclopropanide core and a pyridine ligand.

Vacuum-Boosting Precise Synthetic Control of Highly Bright Solid-State Carbon Quantum Dots Enables Efficient Light Emitting Diodes

Carbon quantum dots (C-dots) have emerged as efficient fluorescent materials for solid-state lighting devices. However, it is still a challenge to obtain highly bright solid-state C-dots because of the aggregation caused quenching. Compared to the encapsulation of as-prepared C-dots in matrices, one-step preparation of C-dots/matrix complex is a good method to obtain highly bright solid-state C-dots, which is still quite limited. Here, an efficient and controllable vacuum-boosting gradient heating approach is demonstrated for in situ synthesis of a stable and efficient C-dots/matrix complex. The addition of boric acid strongly bonded with urea, promoting the selectivity of the reaction between citric acid and urea. Benefiting from the high reaction selectivity and spatial-confinement growth of C-dots in porous matrices, in situ synthesize C-dots bonded can synthesized dominantly with a crosslinked octa-cyclic compound, biuret and cyanuric acid (triuret). The obtained C-dots/matrix complex exhibited bright green emission with a quantum yield as high as 90% and excellent thermal and photo stability. As a proof-of-concept, the as-prepared C-dots are used for the fabrication of white light-emitting diodes (LEDs) with a color rendering index of 84 and luminous efficiency of 88.14 lm W-1, showing great potential for applications in LEDs.

Intermediate aminophenol enables hectogram-scale synthesis of highly bright red carbon quantum dots under ambient conditions

Carbon quantum dots (C-dots) have developed into potential nanomaterials for lighting, catalysis and bioimaging because of their excellent optical properties and good biocompatibility. However, it is still a challenge to produce efficient red emitting carbon quantum dots (R-C-dots) due to their obscure formation mechanism. This work offered a method to reveal the formation process from the precursor o-phenylenediamine (o-PDA) to R-C-dots. Different from traditional hydrothermal reactions, R-C-dots were synthesized at relatively low temperature and ambient pressure. The pre-oxidation intermediate aminophenol played an important role in the synthesis of R-C-dots, which further cross-linked and polymerized with o-PDA in an acid environment to form R-C-dots. The obtained R-C-dots had a photoluminescence quantum yield of up to 33.26% and excellent two-photon fluorescence properties. A white light-emitting diode (WLED) based on R-C-dots as the red phosphor exhibited standard white light CIE color coordinates of (0.33, 0.33) with a correlated color temperature of 5342 K and a high color rendering index (CRI) of 94.5. The obtained rendering index is the highest value among WLEDs with color coordinates of (0.33, 0.33) based on C-dots. This work provides a new perspective for the controllable large-scale synthesis of red C-dots.

Solid-State Red Carbon Dots Based on Biomass Furan Derivatives

In the preparation of carbon dots (CDs), precursors are crucial, and abundant precursors endow CDs with various structures and fluorescence characteristics. Furan (FU) and its derivatives are considered excellent carbonization materials due to their π conjugated structures and active functional groups, such as hydroxyl and aldehyde groups. Herein, we prepare FU-derivative-based CDs by a solvothermal method and investigate the influences of the precursor structure on the fluorescence characteristics. Surprisingly, CDs prepared from 5-hydroxymethylfurfural (HMF) with both aldehyde and hydroxyl groups exhibit red-shifted fluorescence characteristics in the solid state. We postulate that this solid-state fluorescence characteristic is due to the enhancement of supramolecular cross-linking fluorescence between CDs. The unique precursor structure leads to carboxyl groups on the surface of HMF-CDs that are conducive to the hydrogen bond formation. As the concentration of CDs increases, the hydrogen bonding effect increases, leading to a red-shift in the fluorescence wavelength. Therefore, basically full-color CDs/poly(vinyl alcohol) (PVA) phosphor-based light-emitting diodes can be achieved by controlling the degree of supramolecular cross-linking of CDs in PVA. This research provides a new approach for the preparation of solid-state luminescent CDs.

Crystallization-induced emission from F-doped carbon dots

Herein, F-doped CDs with bright red SSF were synthesized by a solvothermal method using trifluoroethanol as the solvent and m-hydroxybenzaldehyde as the carbon source. Strong F-F interactions are vital for inducing crystallization, and solid luminescence is achieved by blocking the nonradiative energy dissipation pathways of crystalline organizations.

Matrix-Free Thermally Activated Delayed Fluorescent Carbon Dots-Based Electroluminescent Light-Emitting Diodes Exceeding 5.6% External Quantum Efficiency

Carbon dots (CDs) are promising luminescent emission layer materials for next generation electroluminescent light emitting diodes (EL-LEDs) due to their many advantages, such as environmental friendliness, low cost, and high stability. However, limited by the spin-forbidden properties of the triplet transition, it is difficult to improve the external quantum efficiency (EQE) of fluorescent CDs-based EL-LEDs. Meanwhile, traditional thermally activated delayed fluorescent (TADF) CDs prepared using coating strategies are difficult to utilize in EL-LEDs due to the nonconductivity of the coating agent. Herein, we successfully developed matrix-free TADF CDs with yellow emission and achieved a device EQE of 5.68%, which is the highest value reported in CDs-based EL-LEDs. In addition, we also developed white EL-LEDs with an EQE of 1.70%. This study highlights the importance of interactions between precursors in modulating the electroluminescence properties of TADF emitters and provides an effective design principle for matrix-free TADF CDs.

Perspectives on development of optoelectronic materials in artificial intelligence age

Optoelectronic devices, such as light-emitting diodes, have been demonstrated as one of the most demanded forthcoming display and lighting technologies because of their low cost, low power consumption, high brightness, and high contrast. The improvement of device performance relies on advances in precisely designing novelty functional materials, including light-emitting materials, hosts, hole/electron transport materials, and yet which is a time-consuming, laborious and resource-intensive task. Recently, machine learning (ML) has shown great prospects to accelerate material discovery and property enhancement. This review will summarize the workflow of ML in optoelectronic materials discovery, including data collection, feature engineering, model selection, model evaluation and model application. We highlight multiple recent applications of machine-learned potentials in various optoelectronic functional materials, ranging from semiconductor quantum dots (QDs) or perovskite QDs, organic molecules to carbon-based nanomaterials. We furthermore discuss the current challenges to fully realize the potential of ML-assisted materials design for optoelectronics applications. It is anticipated that this review will provide critical insights to inspire new exciting discoveries on ML-guided of high-performance optoelectronic devices with a combined effort from different disciplines.

The emergence and prospects of carbon dots with solid-state photoluminescence for light-emitting diodes

The significant features of carbon dots (CDs), such as bright and tunable photoluminescence, high thermal stability, and low toxicity, endow them with tremendous potential for application in next generation optoelectronics. Despite great progress achieved in the design of high-performance CDs so far, the practical applications in solid-state lighting and displays have been retarded by the aggregation-caused quenching (ACQ) effect ascribed to direct π-π interactions. This review provides a comprehensive overview of the recent progress made in solid-state CD emitters, including their synthesis, optical properties and applications in light-emitting diodes (LEDs). Their triplet-excited-state-involved properties, as well as their recent advances in phosphor-converted LEDs and electroluminescent LEDs, are mainly reviewed here. Finally, the prospects and challenges of solid-state CD-based LEDs are discussed with an eye on future development. We hope that this review will provide critical insights to inspire new exciting discoveries on solid-state CDs from both fundamental and practical standpoints so that the realization of their potential in optoelectronic areas can be facilitated.

One-step hydrothermal method synthesized pH-dependent carbon dots for multistage anti-counterfeiting

Work to combat counterfeiting has always been crucial to defending the interests of the public. The usual anti-counterfeiting marks are now fundamental and easy to imitate. Therefore, it is more beneficial to anti-counterfeiting work to develop an anti-counterfeiting mark with more variations to make forgery more difficult. Due to its exceptional stability and fluorescence variability, carbon dots (CDs), a newly developed fluorescent material, offer a wide range of potential applications in anti-counterfeiting. However, there currently needs to be more CD applications in multi-level anti-counterfeiting, and additional issues include high cost and environmental contamination. Therefore, considering the problems of green environmental protection and cost, CDs with excellent green (530 nm) and blue (475 nm, 486 nm) luminescence properties were prepared by a one-step reaction of m-phenylenediamine and glucose. The average fluorescence lifespan is longer than 5 ns, and the optimal quantum yield can reach 37%. Due to the large number of protonated amino groups and surface carboxyl functional groups, the prepared carbon dots exhibit green and blue fluorescence emission modes under acidic and alkaline conditions, respectively. Based on this situation, we produced CD ink and successfully used it for multi-level anti-counterfeiting.

Synergism of Photo-Induced Electron Transfer and Aggregation-Induced Quenching Mechanisms for Highly Sensitive Detection of Silver Ion and Captopril

Carbon-based nanoprobes, with excellent physicochemical performance and biocompatibility, are a kind of ideal nanomaterial for biosensing. Herein, we designed and prepared novel oxygen-doped nitrogen-enrichment carbon nanoribbons (ONCNs) with an excellent optical performance and uniform morphology, which could be used as a dual-mode fluorescence probe for the detection of Ag⁺ ion and captopril (Ctl) based on the synergism of photo-induced electron transfer and aggregation-induced quenching mechanisms. By recording the changes in fluorescent intensities of ONCNs, the Ag⁺ ion and Ctl concentrations can be easily tested in real samples. The results displayed that two good linear relationships existed between the change in fluorescent intensity of ONCNs and the concentrations of Ag⁺ ion and Ctl in the ranges of 3 μM to 30 μM and 1 μM to 30 μM, with the detection limit of 0.78 µM and 74 nM, respectively. The proposed sensing platform has also been successfully applied for the Ctl analysis in commercial tablet samples based on its high selectivity, proving its value in practical applications.