Graphene quantum dots: an emerging material for energy-related applications and beyond

Graphene quantum dots (GQDs) and carbon quantum dots (CQDs) are promising nanomaterials with tunable electronic and optical properties influenced by quantum confinement effects and structural morphology. In this study, the band structures of GQDs and nitrogen-doped CQDs (N-CQDs) were compared to elucidate how particle size, structure, and synthesis methods affect their electronic properties. GQDs were synthesized via electrochemical exfoliation, allowing size control through current density adjustments, while N-CQDs were synthesized hydrothermally with citric acid and urea as precursors to ensure compositional similarity. Transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy, UV-Vis spectroscopy, and synchrotron-based X-ray photoelectron spectroscopy (XPS) were employed to characterize particle sizes, band structures, and semiconductor behavior. Results indicate that GQDs exhibit stronger quantum confinement than N-CQDs, attributed to their sp²-hybridized, two-dimensional structures, which supports lateral electron mobility and increased axial quantum confinement. Furthermore, GQDs exhibited n-type semiconductor behavior, while N-CQDs displayed p-type characteristics, underscoring a fundamental difference in charge transport mechanisms. These findings highlight the critical role of structural and morphological factors in tuning the electronic properties of quantum dots, offering insights into the design of nanomaterials for optoelectronic applications.

Water-soluble graphene quantum dots (GQDs, ca. 10 nm in diameter) that exhibit bright blue photoluminescence (PL) are prepared by hydrothermal (chemical) cutting of oxidized graphene sheets (see figure). The mechanisms of the cutting and luminescence are discussed. This discovery of PL of GQDs may extend the range of application of graphene-based materials to optoelectronics and biological labeling.

Currently, graphene has attracted much attention in the fields of bioimaging, biolabeling and drug delivery. Theoretical and experimental studies have shown that the graphene quantum dots (GQDs) are expected to show good optical properties due to their quantum confinement and edge effect. In this report, using the electrochemical assay the fluorescent GQDs with a diameter between 5 and 10 nm could be obtained via electrolysing graphite in alkaline condition and with hydrazine hydrate as a reducing agent at room temperature. The structure of the GQDs was confimed by means of transmission electron microscope (TEM) and atomic force microscope (AFM). The finding showed that the GQDs have an uniform size, and most of them are separate graphene. The GQDs mainly consist of single layer with less than 1 nm. Their features and properties were characterized by fourier transform infrared spectroscopy (FTIR), photoluminescence spectra (PL), UV-visible spectroscopy (UV-vis) and X-ray diffraction (XRD). The results indicated that the GQDs have bright yellow luminescence with a 14 % quantum yield, which is higher than that of traditional carbon quantum dots reported previously. When they were excited by different excitation wavelengths, the intensity of photoluminescence increased to the maximum, and then decreased gradually. The fluorescent emission peak of the GQDs remained unshifted, suggesting a novel kind of quantum dots different from those of graphene oxide quantum dots depending excitation wavelengths. The luminescence of GQDs arises from the graphene modified with the phthalhydrazide-like groups and hydrazide groups at the edge. The highly fluorescent GQDs have high water solubility, good photostability and biocompatibility, indicating that the GQDs can easily enter the cells. By incorporating the GQDs with A549 (lung cancer) and MCF-7 (breast cancer) cells through MTT assay, the newly obtained GQDs exhibited low cytotoxicity with an advantage of strong photoluminescence in the cells, and thus the GQDs might be used as a bioimaging marker in tumor cell imaging.

Due to pronounced quantum confinement and edge effects, graphene quantum dots (GQDs) demonstrate numerous novel chemical and physical properties and thereby, have potential applications in optoelectronic devices, sensors and biomedical imaging. Doping GQDs with heteroatoms is an effective way to tailor their electronic and optical properties. However, due to different synthesis methods, the widespread use of GQDs doped with heteroatoms has been hindered by the poor understanding of their optical mechanisms. Recently, we studied optical properties of GQDs by employing theoretical calculations and experimental work, which demonstrated that theoretical studies can reveal optical mechanism of GQDs. In this work, the mechanisms underlying the tunable optical properties of GQDs doped with B and S are investigated using density functional theory and time-dependent density functional theory calculations. The electronic structures, optical spectra, molecular orbitals, and electronic density of GQDs are predicted to reveal electron transition processes. Especially, GQDs doped with different B and S patterns are designed to reveal the influence of the existence of O atoms on optical and electronic properties of GQDs. In general, electron-rich S atom increases the HOMO energy, while electron-deficient B atom lowers the LUMO energy, resulting in a decreased HOMO–LUMO gap. Edge doped effect analysis shows that B on GQDs induce a small red shift in absorption spectra whereas a large red shift occurs with surface doped GQDs. However, the absorption spectra of B and O co-doped GQDs exhibit an obvious blue shift and the absorption intensities enhance markedly. For S doped GQDs, the influences of surface and edge doping on absorption spectra of GQDs are almost opposite. Surface doped GQDs induces a small redshift in absorption spectra whereas a large redshift occurs with edge doped GQDs. The recombination of excited, well-separated electron hole pairs can result in enhanced absorption intensity. The heteroatoms doping on the basal plane can transform the sp2 hybridized carbon into the sp3 state. It is expected that this work will provide valuable knowledge for understanding and interpreting the electronic and optical properties of GQDs at atomic scale, and give important insights and guidance for the development of methods to controllably synthesize GQDs with well-defined and desirable properties towards specific purposes.

Graphene Quantum Dots (GQDs) and Carbon Quantum Dots (CQDs) are nanomaterials with rising popularity as an alternative to traditional semiconductor quantum dots and organic dyes.1 In addition to simple fabrication methods and low production costs, GQDs and CQDs exhibit a good chemical stability and solubility, unique photophysical properties, photochemical stability, and biocompatibility. These are remarkable properties for applications in fields such as catalysis, photovoltaic devices, bioimaging or medical diagnosis.2 With the objective of constructing versatile and functional ensembles for nanoelectronics and optoelectronics, we lately embarked in the synthesis of these carbon nanomaterials and their covalent or supramolecular modification with ligands that modify their fundamental properties. For instance, adding a chiroptical response to the semiconductor properties of GQDs will provide the extra value of their potential application in photonics. In this sense, we recently proof the concept that GQDs are able to become chiral and that this property can be transferred to a supramolecular structure built with pyrene molecules, where the chiral-GQDs/pyrene ensembles show a characteristic chiroptical response depending on the configuration of the introduced organic ligands.3 We have also combined GQDs and CQDs materials with p-quinonoid π-extended tetrathiafulvalenes (exTTFs)4 in the search for new electron donor-acceptor systems (Figure). The electronic interactions between the CQDs and exTTF have been investigated in the ground and excited states. The characterization of the obtained GQDs and CQDs nanomaterials by a combination of analytical, microscopic and spectroscopic techniques will be presented and discussed, along with the photophysical properties of some of the aggregates formed.

Graphene quantum dots: Background, synthesis methods, and applications as nanocarrier in drug delivery and cancer treatment: An updated review

Graphene quantum dots (GQDs) have shown broad application prospects in the field of photovoltaic devices due to their unique quantum confinement and edge effects. Here, we prepared GQDs by a photon-Fenton reaction as reported in our previous work, which has great advantage in the preparation scale. The photoelectric properties of the inverted hybrid solar cells based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61 butyric acid methylester (PCBM):GQDs and P3HT:GQDs with different contents of GQDs as the active layers are demonstrated, as well as their morphology and structure by atomic force microscopy images. Then, the different roles of GQDs played in the ternary (P3HT:PCBM:GQDs) and binary (P3HT:GQDs) hybrid solar cells are studied systematically. The results indicate that the GQDs provide an efficient excition separation interface and charge transport channel for the improvement of hybrid solar cells. The preliminary exploration and elaboration of the role of GQDs in hybrid solar cells will be beneficial to understand the interfacial procedure and improve device performance in the future.

Graphene quantum dots (GQDs) have received great interest in the past few years due to its unique properties such as the quantum confinement and edge effects when their sizes are down to 10 nm. These new physical properties can induce size-dependent bandgap, and unique optical and electronic properties, which make them excellent materials for photovoltaics and photoelectochemical (PEC) devices. In a dye-sensitized solar cell (DSSC), light harvesting capability of dyes could be enhanced by Förster resonance energy transfer (FRET) phenomena, wherein energy is transferred from an excited donor fluorophore to a suitable acceptor dye molecule. The strict requirement that the overlap of the emission spectrum of donor and absorption spectrum of acceptor, which also requires proper band edge alignment between the them, significantly limits the variety of donors and acceptors suitable for the FRET-based DSSCs. In our DSSC work, graphene quantum dots were co-sensitized with N719 dyes and we successfully explored that the GQDs is a suitable donor material for the N719 dyes. From the emission and time decay spectra analysis, the FRET efficiency was measured to be 27 %. The power conversion efficiency (ƞ) of the standard DSSC is 6.12 % with a short-circuit current density (Jsc ) of 12.74 mAcm-2 and open-circuit voltage (Voc ) of 760 mV. The co-sensitization of GQD greatly enhances the ƞ up to 7.96 % with a Jsc of 16.54 mAcm-2 and Voc of 770 mV. The co-sensitization of GQDs improved both the Jsc and ƞvalues by ~ 30 %. The FRET enhanced the overall light absorption capacity of the N719 dye and hence photocurrent generation of the DSSC. Moreover, the co-sensitization of GQDs increase the TiO2/dye/electrolyte interface resistance, which suppresses the charge recombination and leads to increased Voc value. For PEC water splitting, higher light absorption can also be achieved by utilizing light-matter interplaymechanisms, such as, surface plasmon resonance (SPR), plasmonicresonance energy transfer (PRET) and exciton−plasmon interactions (EPI). The optical properties of the semiconductor and metal nanoparticles (SNPs-MNPs) are strongly modified viaEPI comparing to single NPs. Therefore, we have utilized the unique combination of GQDs and AuNPs (SNPs-MNPs) to effectively enhance PEC performance of TiO2 nanorods (TNRs). The AuNP-GQD decorated TNR photoelectrode has achieved a superior PEC performance with a photocurrent density of 1.75 mAcm-2 at 1.23 V vs RHE, which is one of higher value obtained in PEC measurement using TiO2 nanorods due to an efficient utilization of solar light. The experimental analysis has indicated that the AuNPs has contributed to the PRET / SPR-mediated hot electron injection into TiO2 and the GQDs contributed to the improved electron injection into the TiO2. Moreover, the significantly improved PEC water splitting efficiency is attributed to the synergistic effect due to EPI and/or FRET between AuNPs and GQDs in addition to the good catalytic property of the AuNPs and the GQDs.

This endeavor presents state-of-the-art overview on polymer/carbon-based quantum dot nanocomposite. Carbon-based quantum dot (graphene quantum dot, carbon nanodot, and polymer dot) are ∼10nm. Carbon-based quantum dot own exciting features such as tunable optoelectronic and photoluminescence properties, high stability, chemical inertness, low cytotoxicity, and biocompatibility owing to quantum confinement and edge effects. Main emphasis of article was to see the combined effect of polymer and carbon-based quantum dot in nanocomposite. Five major categories have been reviewed in this article including conjugated polymer/carbon-based quantum dot nanocomposite, epoxy/carbon-based quantum dot nanocomposite, polystyrene/carbon-based quantum dot nanocomposite, poly(dimethyl siloxane)/carbon-based quantum dot nanocomposite, and block copolymer/carbon-based quantum dot nanocomposite. The review also refers to cutting edge application areas of polymer/carbon-based quantum dot nanocomposite. Conducting polymer/carbon quantum dot nanocomposite has been integrated in energy storage devices, detectors, and electronic devices. These materials are also promising candidates for bulk heterojunction solar cells and light-emitting diodes. Another important use is the identification and removal of toxic metals. Functional materials have also been used for fluorescence imaging of live cells. Modification of carbon-based quantum dot and incorporation in appropriate polymer matrices can be adopted as powerful future tool enabling desired tailored applicability of nanocomposite in advance high performance technical applications.

Due to quantum confinement and edge effects, graphene quantum dots (GQDs) have demonstrated numerous novel chemical and physical properties. Doping GQDs with heteroatoms is an effective way to tailor their electronic and optical properties. However, due to different synthesis methods, the potential use of GQDs doped with heteroatoms is hindered by the poor understanding of their intrinsic optical properties and mechanisms. In this work, optical spectrum, HOMO–LUMO gap, and electronic density of GQDs doped with sulfur (S) atoms are calculated to reveal electron transition processes. It is found that S=O can reduce the absorption intensity of GQDs, and the calculated absorption spectra tend to be blueshifted when the number of S-containing groups decreases. The S-doped GQDs containing S atom in pentatomic ring has a broad UV−Vis absorption band with a weak shoulder at 300 nm, which is blueshifted by 60 nm with respect to that of pristine GQDs. Moreover, the thioether-S configuration plays remarkable effect on tailoring optical properties of GQDs, and the recombination of electron-hole pairs can result in weakened absorption intensity. This study will provide valuable knowledge for understanding electronic and optical properties of GQDs and give important insights for the development of methods to controllably synthesize GQDs.

Graphene quantum dots (GQDs) with quantum confinement and size effect are proposed to be applicable in photovoltaic, nanodevices, and so on, due to extraordinary electronic and optical properties. Here we report a facile approach to synthesize gram-scale GQDs from active carbon atoms, which are obtained via the deflagration reaction of polytetrafluoroethylene (PTFE) and Si, growing from high- to low-temperature zones when traveling through the deflagration flame in a short time with releasing gas as the carrier medium. The prepared GQDs were aggregated into carbon nanospheres; thus, Hummer’s method was utilized to exfoliate the GQD aggregations into individual GQDs. We show that the length of GQDs is ∼10 nm and the exfoliated GQDs solution presents an obvious fluorescence effect with a strong emission peak at 570 at 460 nm excitation. And these GQDs are demonstrated to be excellent probes for cellular imaging. Furthermore, we propose a growth mechanism based on computer simulation, which is well verified by experimental reproduction. Our study opens up a promising route for high-yield and high-quality GQDs, as well as other various quantum dots.

Graphene quantum dots (GQDs), which are edge-bound nanometer-size graphene pieces, have fascinating electronic and optical properties due to their quantum confinement and edge effect. In this paper, GQDs were synthesized by using acid treatment and chemical exfoliation of multi-walled carbon nanotubes (MWCNTs). The structure of the GQDs was investigated by transmission electron microscope. The GQDs have a uniform size distribution, zigzag edge structure and two-dimensional morphology. The results indicated that the GQDs have bright blue emission upon UV excitation. The highly fluorescent GQDs exhibited high water solubility and good stability. It is shown that the acid treatment of MWCNTs leads to the formation of the functional group in zigzag sites, which results in the pH-dependent fluorescence of the GQDs.

Second hyperpolarizability of hexagonal graphene quantum dots: Effects of size and structural defects

The advanced industrial, social, and technological developments in modern society have rendered the production of around 300 million tons of plastic annually, yet only 50% or even less are being recycled. Moreover, the current recycling process is limited to the conversion of plastic wastes (PWs) into hydrocarbon fuels, which also requires the use of high temperatures and energies, long reaction time, and high-cost platinum-based catalysts. Therefore, there is an urgent need to develop a more sustainable and environmentally friendly method to recycle these PWs and possibly into functional materials for diverse applications.We introduce non-equilibrium and low-temperature microplasma technologies to convert PWs into graphene quantum dots (GQDs) at ambient conditions without any additional toxic chemicals, expensive catalysts, and sophisticated vacuum technologies. The reactive species generated by the plasma enables not only rapid deconstruction of PWs into small hydrocarbons moieties, but also simultaneous reconstruction of nanographene domain and nucleation into GQDs. GQDs are zero-dimensional carbon nanomaterials with unique quantum confinement and edge effects, which are hybridized by sp2 carbon in a honeycomb network. The quantum confinement and edge effects bestow GQDs with unique, tunable, and stable photoluminescence (PL) properties. Coupled with the biocompatibility, low toxicity, large surface area, and chemical inertness, GQDs can be employed in various applications, including imaging, sensing, optoelectronics, drug delivery, catalysis, and energy related applications. Nevertheless, the current syntheses of GQDs still need the use of high temperatures, strong acids or reducing agents, long reaction time, and tedious synthesis procedures.Here the utilization of microplasma enables the degradation and conversion of various plastics into functional GQDs with well-controlled structures in one-step at ambient conditions and catalyst-free manner, alleviating the current drawbacks in the GQDs synthesis. Among various PWs, polyethylene terephthalate-derived GQDs (PET-GQDs) exhibit enticing and bright white emission under 365 nm UV irradiation with a Commission Internationale de l’Eclairage 1931 of (0.29, 0.35). Moreover, the colloidal PET-GQDs can be exploited for heavy metal ions detection with a low limit of detection of 8.4 nM, while the composite film state is usable as sensitive temperature tag from 10 – 80 °C and LED panel. Our work provides an insight into the novel technologies for PWs recycling into functional nanomaterials in cost-effective, scalable, and environmentally friendly way.

Owing to their unique fluorescence properties, graphene quantum dots (GQDs), the new zero-dimensional carbon nanomaterials, have intrigued many research interests due to their quantum confinement and edge effects. In addition to luminescence properties, GQDs also possess some of the special properties of graphene, since their structures are mostly similar, consisting of hybridized sp2 carbon. As a result, GQDs can cover a wide variety of possible applications in many fields, including medical, sensing, light-emitting diodes, catalysis, and energy-related fields. Similarly to other nanomaterials, synthesis methods based on top–down or bottom–up approaches are available, with each having their advantages and disadvantages. In this chapter, we will summarize and discuss the current synthesis methods and applications of GQDs.