Optical Properties Prediction for Red and Near-Infrared Emitting Carbon Dots Using Machine Learning.

Functional nanostructures build up a basis for the future materials and devices, providing a wide variety of functionalities, a possibility of designing bio-compatible nanoprobes, etc. However, development of new nanostructured materials via trial-and-error approach is obviously limited by laborious efforts on their syntheses, and the cost of materials and manpower. This is one of the reasons for an increasing interest in design and development of novel materials with required properties assisted by machine learning approaches. Here, the dataset on synthetic parameters and optical properties of one important class of light-emitting nanomaterials - carbon dots are collected, processed, and analyzed with optical transitions in the red and near-infrared spectral ranges. A model for prediction of spectral characteristics of these carbon dots based on multiple linear regression is established and verified by comparison of the predicted and experimentally observed optical properties of carbon dots synthesized in three different laboratories. Based on the analysis, the open-source code is provided to be used by researchers for the prediction of optical properties of carbon dots and their synthetic procedures.

Expanding the Horizons of Machine Learning in Nanomaterials to Chiral Nanostructures.

Machine learning holds significant research potential in the field of nanotechnology, enabling nanomaterial structure and property predictions, facilitating materials design and discovery, and reducing the need for time-consuming and labor-intensive experiments and simulations. In contrast to their achiral counterparts, the application of machine learning for chiral nanomaterials is still in its infancy, with a limited number of publications to date. This is despite the great potential of machine learning to advance the development of new sustainable chiral materials with high values of optical activity, circularly polarized luminescence, and enantioselectivity, as well as for the analysis of structural chirality by electron microscopy. In this review, an analysis of machine learning methods used for studying achiral nanomaterials is provided, subsequently offering guidance on adapting and extending this work to chiral nanomaterials. An overview of chiral nanomaterials within the framework of synthesis-structure-property-application relationships is presented and insights on how to leverage machine learning for the study of these highly complex relationships are provided. Some key recent publications are reviewed and discussed on the application of machine learning for chiral nanomaterials. Finally, the review captures the key achievements, ongoing challenges, and the prospective outlook for this very important research field.

Green and Red Emissive N,O-Doped Chiral Carbon Dots Functionalized with l-Cysteine.

Although chirality plays an important role in the natural world, it has also attracted much scientific attention in nanotechnology, in particular, spintronics and bioapplications. Chiral carbon dots (CDs) are promising nanoparticles for sensing and bioimaging since they are biocompatible, ecofriendly, and free from toxic elements. Herein, green and red emissive chiral CDs are fabricated via surface modification treatment of achiral CDs at room temperature. After modification with l-cysteine molecules, the treated CDs demonstrate an intense chiral signal in the region of 200-300 nm with a dissymmetry factor up to 2.3 × 10-4 and high photoluminescence quantum yields of 19% and 15% for green and red emission bands, respectively. These CDs preserve their chiral signal in different ion systems, such as those with pH changes or in the presence of metal ions, along with remarkably low cytotoxicity, making them potential candidates for use as photoluminescent labels for biological objects.

Red-Emissive Center Formation within Carbon Dots Based on Citric Acid and Formamide.

The formation of red-emissive optical centers in carbon dots based on citric acid and formamide was investigated by varying the synthesis parameters with focus on finding optimal─necessary and sufficient─amount of precursors to decrease byproduct amount and to increase the chemical yield of red-emissive carbon dots. The emission is observed at 640 nm excited at 590 nm and quantum yield reaches up 19%. A high chemical yield of carbon dots of 26% was achieved at an optimal molar ratio of citric acid to formamide of 1:4.

Energy-level engineering of carbon dots through a post-synthetic treatment with acids and amines.

Chemically synthesized carbon dots (CDs) have attracted a lot of attention as an eco-friendly and cost-efficient light-emitting material, and functionalization of CD surfaces with additives of different natures is a useful way to control their properties. In this study, we show how a post-synthetic treatment of CDs with citric acid, benzoic acid, urea and o-phenylenediamine changes their chemical composition and optical properties. In particular, it results in the formation of carboxyl/imide/carbonyl groups at the CD surface, leading to the appearance of additional blue (or for CDs treated with phenylenediamine, blue and green) emissive optical centers on top of the remaining emission from the original CDs. Most importantly, the increased oxidation degree alongside a decreased relative amount of carbon and nitrogen in such treated CDs decreases their highest occupied molecular orbital (HOMO) energy level by up to 0.9 eV (the maximal value was observed for CDs treated with o-phenylenediamine). Moreover, the Fermi energy level shifted above the lowest unoccupied molecular orbital (LUMO) energy level for some of the treated CD samples. Thus, the energy structure of CDs can be tuned and optimized for further applications through the functionalization of their surface with organic additives.

Ytterbium-Doped Lead-Halide Perovskite Nanocrystals: Synthesis, Near-Infrared Emission, and Open-Source Machine Learning Model for Prediction of Optical Properties.

Lead-halide perovskite nanocrystals are an attractive class of materials since they can be easily fabricated, their optical properties can be tuned all over the visible spectral range, and they possess high emission quantum yields and narrow photoluminescence linewidths. Doping perovskites with lanthanides is one of the ways to widen the spectral range of their emission, making them attractive for further applications. Herein, we summarize the recent progress in the synthesis of ytterbium-doped perovskite nanocrystals in terms of the varying synthesis parameters such as temperature, ligand molar ratio, ytterbium precursor type, and dopant content. We further consider the dependence of morphology (size and ytterbium content) and optical parameters (photoluminescence quantum yield in visible and near-infrared spectral ranges) on the synthesis parameters. The developed open-source code approximates those dependencies as multiple-parameter linear regression and allows us to estimate the value of the photoluminescence quantum yield from the parameters of the perovskite synthesis. Further use and promotion of an open-source database will expand the possibilities of the developed code to predict the synthesis protocols for doped perovskite nanocrystals.

Carbon Dot Emission Enhancement in Covalent Complexes with Plasmonic Metal Nanoparticles.

Carbon dots can be used for the fabrication of colloidal multi-purpose complexes for sensing and bio-visualization due to their easy and scalable synthesis, control of their spectral responses over a wide spectral range, and possibility of surface functionalization to meet the application task. Here, we developed a chemical protocol of colloidal complex formation via covalent bonding between carbon dots and plasmonic metal nanoparticles in order to influence and improve their fluorescence. We demonstrate how interactions between carbon dots and metal nanoparticles in the formed complexes, and thus their optical responses, depend on the type of bonds between particles, the architecture of the complexes, and the degree of overlapping of absorption and emission of carbon dots with the plasmon resonance of metals. For the most optimized architecture, emission enhancement reaching up to 5.4- and 4.9-fold for complexes with silver and gold nanoparticles has been achieved, respectively. Our study expands the toolkit of functional materials based on carbon dots for applications in photonics and biomedicine to photonics.

Assignment of Core and Surface States in Multicolor-Emissive Carbon Dots.

It is important to reveal the luminescence mechanisms of carbon dots (CDs). Herein, CDs with two types of optical centers are synthesized from citric acid in formamide by a solvothermal method, and show high photoluminescence quantum yield reaching 42%. Their green/yellow emission exhibits pronounced vibrational structure and high resistance toward photobleaching, while broad red photoluminescence is sensitive to solvents, temperature, and UV-IR. Under UV-IR, the red emission is gradually bleached due to the photoinduced dehydration of the deprotonated surface of CDs in dimethyl sulfoxide, while this process is hindered in water. From the analysis of steady-state and time-resolved photoluminescence and transient absorption data together with density functional theory calculations, the green/ yellow emission is assigned to conjugated sp2 -domains (core state) similar to organic dye derivatives stacked within disk-shaped CDs; and the broad red emission-to oxygen-containing groups bound to sp2 -domains (surface state), whereas energy transfer from the core to the surface state can happen.

Dual-Purpose Sensing Nanoprobe Based on Carbon Dots from o-Phenylenediamine: pH and Solvent Polarity Measurement.

Today, the development of nanomaterials with sensing properties attracts much scientific interest because of the demand for low-cost nontoxic colloidal nanoprobes with high sensitivity and selectivity for various biomedical and environment-related applications. Carbon dots (CDs) are promising candidates for these applications as they demonstrate unique optical properties with intense emissions, biocompatibility, and ease of fabrication. Herein, we developed synthesis protocols to obtain CDs based on o-phenylenediamine with a variety of optical responses depending on additional precursors and changes in the reaction media. The obtained CDs are N-doped (N,S-doped in case of thiourea addition) less than 10 nm spherical particles with emissions observed in the 300−600 nm spectral region depending on their chemical composition. These CDs may act simultaneously as absorptive/fluorescent sensing probes for solvent polarity with ∆S/∆ENT up to 85, for ∆ENT from 0.099 to 1.0 and for pH values in the range of 3.0−8.0, thus opening an opportunity to check the pH in non-pure water or a mixture of solvents. Moreover, CDs preserve their optical properties when embedded in cellulose strips that can be used as sensing probes for fast and easy pH checks. We believe that the resulting dual-purpose sensing nano probes based on CDs will have high demand in various sensing applications.

Revealing the nature of optical activity in carbon dots produced from different chiral precursor molecules.

Carbon dots (CDs) are light-emitting nanoparticles that show great promise for applications in biology and medicine due to the ease of fabrication, biocompatibility, and attractive optical properties. Optical chirality, on the other hand, is an intrinsic feature inherent in many objects in nature, and it can play an important role in the formation of artificial complexes based on CDs that are implemented for enantiomer recognition, site-specific bonding, etc. We employed a one-step hydrothermal synthesis to produce chiral CDs from the commonly used precursors citric acid and ethylenediamine together with a set of different chiral precursors, namely, L-isomers of cysteine, glutathione, phenylglycine, and tryptophan. The resulting CDs consisted of O,N-doped (and also S-doped, in some cases) carbonized cores with surfaces rich in amide and hydroxyl groups; they exhibited high photoluminescence quantum yields reaching 57%, chiral optical signals in the UV and visible spectral regions, and two-photon absorption. Chiral signals of CDs were rather complex and originated from a combination of the chiral precursors attached to the CD surface, hybridization of lower-energy levels of chiral chromophores formed within CDs, and intrinsic chirality of the CD cores. Using DFT analysis, we showed how incorporation of the chiral precursors at the optical centers induced a strong response in their circular dichroism spectra. The optical characteristics of these CDs, which can easily be dispersed in solvents of different polarities, remained stable during pH changes in the environment and after UV exposure for more than 400 min, which opens a wide range of bio-applications.

Carbon Dots with an Emission in the Near Infrared Produced from Organic Dyes in Porous Silica Microsphere Templates.

Carbon dots (CDs) with an emission in the near infrared spectral region are attractive due to their promising applications in bio-related areas, while their fabrication still remains a challenging task. Herein, we developed a template-assisted method using porous silica microspheres for the formation of CDs with optical transitions in the near infrared. Two organic dyes, Rhodamine 6G and IR1061 with emission in the yellow and near infrared spectral regions, respectively, were used as precursors for CDs. Correlation of morphology and chemical composition with optical properties of obtained CDs revealed the origin of their emission, which is related to the CDs' core optical transitions and dye-derivatives within CDs. By varying annealing temperature, different kinds of optical centers as derivatives of organic dyes are formed in the microsphere's pores. The template-assisted method allows us to synthesize CDs with an emission peaked at 1085 nm and photoluminescence quantum yield of 0.2%, which is the highest value reported so far for CDs emitting at wavelengths longer than 1050 nm.

White Light Afterglow in Carbon Dots Achieved via Synergy between the Room-Temperature Phosphorescence and the Delayed Fluorescence.

Carbon dot (CD) based long-lived afterglow emission materials have attracted attention in recent years, but demonstration of white-light room-temperature afterglow remains challenging, due to the difficulty of simultaneous generation of multiple long-lived excited states with distinct chromatic emission. In this work, a white-light room-temperature long-lived afterglow emission from a CD powder with a high efficiency of 5.8% and Commission International de l'Eclairage (CIE) coordinates of (0.396, 0.409) is realized. The afterglow of the CDs originates from a synergy between the phosphorescence of the carbon core and the delayed fluorescence associated with the surface CN moieties, which is accomplished by matching the singlet state of the surface groups of the CDs with the long-lived triplet state of the carbon core, resulting in an efficient energy transfer. It is demonstrated how the long-lived afterglow emission of CDs can be utilized for fabrication of white light emitting devices and in anticounterfeiting applications.

Optical Properties of Carbon Dots in the Deep-Red to Near-Infrared Region Are Attractive for Biomedical Applications.

Carbon dots (CDs) represent a recently emerged class of luminescent materials with a great potential for biomedical theranostics, and there are a lot of efforts to shift their absorption and emission toward deep-red (DR) to near-infrared (NIR) region falling in the biological transparency window. This review offers comprehensive insights into the synthesis strategies aimed to achieve this goal, and the current approaches of modulating the optical properties of CDs over the DR to NIR region. The underlying mechanisms of their absorption, photoluminescence, and chemiluminescence, as well as the related photophysical processes of photothermal conversion and formation of reactive oxygen species are considered. The already available biomedical applications of CDs, such as in the photoacoustic imaging and photothermal therapy, photodynamic therapy, and their use as bioimaging agents and drug carriers are then shortly summarized.

Chiral carbon dots based on L/D-cysteine produced via room temperature surface modification and one-pot carbonization.

Since chirality is one of the phenomena often occurring in nature, optically active chiral compounds are important for applications in the fields of biology, pharmacology, and medicine. With this in mind, chiral carbon dots (CDs), which are eco-friendly and easy-to-obtain light-emissive nanoparticles, offer great potential for sensing, bioimaging, enantioselective synthesis, and development of emitters of circularly polarized light. Herein, chiral CDs have been produced via two synthetic approaches using a chiral amino acid precursor l/d-cysteine: (i) surface modification treatment of achiral CDs at room temperature and (ii) one-pot carbonization in the presence of chiral precursor. The chiral signal in the absorption spectra of synthesized CDs originates not only from the chiral precursor but from the optical transitions attributed to the core and surface states of CDs. The use of chiral amino acid molecules in the CD synthesis through carbonization results in a substantial (up to 8 times) increase in their emission quantum yield. Moreover, the synthesized CDs show two-photon absorption which is an attractive feature for their potential bioimaging and sensing applications.

The influence of thermal treatment conditions (solvothermal versus microwave) and solvent polarity on the morphology and emission of phloroglucinol-based nitrogen-doped carbon dots.

The optical properties of chemically synthesized carbon dots (CDs) can be widely tuned via doping and surface modification with heteroatoms such as nitrogen, which results in a range of potential applications. Herein, two most commonly used synthesis approaches, namely, solvothermal and microwave-assisted thermal treatments, have been used for the preparation of CDs from phloroglucinol using three different nitrogen containing solvents, namely, ethylenediamine, dimethylformamide, and formamide. Based on the analysis of the morphology and optical properties, we demonstrate the tenability of the CD appearance from amorphous or well-carbonized spherical particles to onion-like ones, which is controlled by solvent polarity, whereas the thermal treatment conditions mostly influence the degree of N-doping and the nature of emissive centers of CDs formed. The findings of this study expand the toolkit of the available CDs with variable morphology and energy structure.

Applications of Carbon Dots in Optoelectronics.

Carbon dots (CDs) are an attractive class of nanomaterials due to the ease of their synthesis, biocompatibility, and superior optical properties. The electronic structure of CDs and hence their optical transitions can be controlled and tuned over a wide spectral range via the choice of precursors, adjustment of the synthetic conditions, and post-synthetic treatment. We summarize recent progress in the synthesis of CDs emitting in different colors in terms of morphology and optical properties of the resulting nanoparticles, with a focus on the synthetic approaches allowing to shift their emission to longer wavelengths. We further consider formation of CD-based composite materials, and review approaches used to prevent aggregation and self-quenching of their emission. We then provide examples of applications of CDs in optoelectronic devices, such as solar cells and light-emitting diodes (LEDs) with a focus on white LEDs.

Morphology Control of Luminescent Carbon Nanomaterials: From Dots to Rolls and Belts.

In this work, we report a successful extension of the family of light-emitting colloidal carbon nanostructures to a number of different shapes and morphologies, namely, carbon nanorolls (CNRs) and carbon nanobelts (CNBs). Near infrared (NIR)-emissive CNRs were synthesized via a solvothermal fusion of carbon dots (CDs) triggered by a dehydration process of their surface functional groups. They appear in a form of short cylinders, with diameters ranging from 20 to 40 nm and cylinder lengths ranging from 7 to 20 nm. In ethanol solution, CNRs have a maximum absorption peak at 665 nm and a NIR emission band extending from 650 to 800 nm, with a photoluminescence quantum yield of 9.2%. Intriguingly, the rolled structure of CNRs can be uncoiled under 655 nm laser irradiation (power density 1 W·cm-2) of their solution in ethanol, forming CNBs with a width of 7-20 nm and lengths reaching several hundreds of nanometers, which is accompanied by a considerably decreased absorption band at 665 nm and a decreased NIR emission. This unfolding is ascribed to the decrease of the strength of interlayer hydrogen bonding, owing to the photothermally induced dehydration and further carbonization of the CNRs. Alongside the decreased NIR emission, CNBs exhibit enhanced green and red emissions under UV and green light excitation, respectively, which allows us to demonstrate multiple-level luminescence encryptions on a paper stamped with CNR- and CNB-inks.

Enhanced Near-Infrared Emission from Carbon Dots by Surface Deprotonation.

Carbon dots (CDs) with efficient excitation and emission in deep-red/near-infrared (NIR) spectral range are important for bioimaging applications. Herein, we develop a simple and effective method to significantly enhance both the absorption and emission of CDs in deep-red/NIR by suppressing nonradiative charge recombination via deprotonation of the CD surface. As compared to aqueous solutions at room temperature, NIR emission of CDs in N,N-dimethylformamide and glycerol experience a 50- and 70-fold increase at -20 °C, respectively, due to enhanced deprotonation ability and viscosity. On the basis of the adjustable NIR fluorescence intensity of CDs, multilevel data encryption in the NIR region is realized by controlling the humidity and the temperature of a CD-ink stamped paper.

Strongly Luminescent Composites Based on Carbon Dots Embedded in a Nanoporous Silicate Glass.

Luminescent composites based on entirely non-toxic, environmentally friendly compounds are in high demand for a variety of applications in photonics and optoelectronics. Carbon dots are a recently developed kind of luminescent nanomaterial that is eco-friendly, biocompatible, easy-to-obtain, and inexpensive, with a stable and widely tunable emission. Herein, we introduce luminescent composites based on carbon dots of different chemical compositions and with different functional groups at the surface which were embedded in a nanoporous silicate glass. The structure and optical properties of these composites were comprehensively examined using electron microscopy, Fourier transform infrared transmission, UV-Vis absorption, and steady-state and time-resolved photoluminescence. It is shown that the silicate matrix efficiently preserved, and even enhanced the emission of different kinds of carbon dots tested. The photoluminescence quantum yield of the fabricated nanocomposite materials reached 35-40%, which is comparable to or even exceeds the values for carbon dots in solution.

Room-Temperature Lasing from Mie-Resonant Nonplasmonic Nanoparticles.

Subwavelength particles supporting Mie resonances underpin a strategy in nanophotonics for efficient control and manipulation of light by employing both an electric and a magnetic optically induced multipolar resonant response. Here, we demonstrate that monolithic dielectric nanoparticles made of CsPbBr3 halide perovskites can exhibit both efficient Mie-resonant lasing and structural coloring in the visible and near-IR frequency ranges. We employ a simple chemical synthesis with nearly epitaxial quality for fabricating subwavelength cubes with high optical gain and demonstrate single-mode lasing governed by the Mie resonances from nanocubes as small as 310 nm by the side length. These active nanoantennas represent the most compact room-temperature nonplasmonic nanolasers demonstrated until now.