Properties Of Graphene Quantum Dots Research Articles

Theoretical insights into tunable optical and electronic properties of graphene quantum dots through phosphorization

Doping heteroatoms or covalent bonding with specific groups is an effective route to modify optical and electronic properties of graphene quantum dots (GQDs). In this work, effects of five phosphorous (P) bonding configurations (i.e., C2P, C2PO, C2PO2, C3P, and C3PO) on optical and electronic properties of GQDs are investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). Optical absorption spectra, HOMO–LUMO gaps, and excited states of GQDs doped with different P bonding configurations at various locations are calculated to reveal electron transition processes. Due to the availability of vacant third orbitals, Ρ can have various stereochemistry and bonding. It is demonstrated that P bonding configurations with different geometrical configurations have various influences on the spectra of doped GQDs, and the existence of PO double bond in C2PO2 and C3PO configurations can induce multiple absorption peaks in P-doped GQDs. The calculated HOMO–LUMO gap has a larger gap reduction when P exhibits the sp3 hybridization. According to excited state analysis, P-doping with tetrahedral-like configurations has more evident effect on electronic structure of P-doped GQDs, while pyramidal-like configurations have noticeable charge transfer ability in the absorption process.

G-C3N4/oxygen-deficient BiOCl nanocomposite assisted by distinguished properties of graphene quantum dots for the efficient photocatalytic removal of organic vapors

In this study, photocatalytic properties of a graphene carbon nitride (g-C3N4) oxygen-deficient BiOCl (ODBOC) nanocomposite, which was assisted by the distinguished properties of graphene quantum dots (GQDs/gCN/ODBOC), in the decomposition of harmful organic vapors were investigated. The GQDs/gCN/ODBOC ternary composite was prepared using a facile chemical-mixing method. The presence of oxygen vacancies in ODBOC and the GQDs/gCN/ODBOC nanocomposite was confirmed by electron spin resonance and X-ray photoelectron spectroscopy. Compared to pristine BiOCl, ODBOC exhibited a higher performance, while compared with those of the selected photocatalyst counterparts, GQDs/gCN/ODBOC exhibited a higher performance; particularly, the removal efficiency of hexanal over this catalyst increased up to 95%. The superior performance of the above photocatalyst was related to two distinguished properties of GQDs: improved visible-light absorption by their upconverted photoluminescence and promoted charge-separation ability by their electron attraction properties, as well as the Z-scheme charge transfer at the junctions between g-C3N4 and ODBOC. In addition, the GQDs/gCN/ODBOC fabricated using a GQD solution of 5 mL revealed the highest performance and satisfactory photochemical stability during recycling tests. Finally, the photocatalytic mechanism for the pollutant degradation over GQDs/gCN/ODBOC was proposed on the basis of band-energy structures and hydroxyl radical measurements.

Electronic and optical properties of sulfur and nitrogen doped graphene quantum dots: A theoretical study

Here we present a theoretical study based on density functional theory (DFT) of the electronic and optical properties of graphene quantum dots (GQDs) doped with sulfur and nitrogen atoms. Unlike the actual literature, which focusses mostly in the edge functionalization of GQDs, our report focusses on the effect of sulfur and nitrogen atomic dopants positioned at the center and near the edge of the GQD carbon backbone structure. According to formation energies, we evidence that doping is more favorable for nitrogen than sulfur, and that the near-edge doping is more favorable than center doping. Regarding the electronic properties, there is a clear shift to lower wavelength in the UV–Vis spectra when using the CAM-B3LYP exchange correlation functional, with respect to B3LYP functional for all the structures, showing a better concordance with experimental data. As a general trend, the major transition observed in the nitrogen doped system is situated at higher wavelength, while in the case of the sulfur doped system, the transition is located at lower wavelength when comparing with the undoped GQDs. This observation is also in good agreement with the experimental trend observed for nitrogen and sulfur-doped GQDs for which a redshift and blueshift in the UV–Vis absorption is typically observed respect to the undoped case, respectively. All of this justify the selected methodology for describing the electronic and optical properties of doped GQDs and provide additional insights to correlate with the optical properties observed experimentally.

Determination of uranium in environmental sample by nanosensor graphene quantum dots

The unique properties of graphene quantum dots (GQDs) i.e. low toxicity, excellent water solubility, low cost, high photostability, biocompatibility, and small size have attracted tremendous attention for new research prospects in the detection of metal ions. The fluorescence properties of GQDs are used for qualitative and quantitative analysis of U(VI) ion. The fluorescence of GQDs is intensively quenched by uranium. Synthesized GQDs were characterized by UV–visible spectroscopy, FTIR, Transmission electron microscopy, fluorescence spectrophotometry. Results indicate a single layer of GQDs, small size. Optimum pH is 7 for uranium detection and up to 1.5 ppb of U(VI) ion is accurately determined.

Laser ablation synthesis of gold nanoparticle to enhance the fluorescence properties of graphene quantum dots

Gold nanoparticles were fabricated in the graphene quantum dots solution using the laser ablation technique. In order to control the particle size and concentration of nanoparticles, the ablation time is changed from 5 to 25 mins. UV–visible spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, and fluorescence spectroscopy were used to characterize the prepared samples. Consequently, the localized surface plasmon resonance absorption peaks appeared in the range of 515.9–520 nm, while the peak which appeared at 319.8 is related to graphene quantum dots. The gold nanoparticles were formed in a spherical shape, which have had interaction with carboxyl and hydroxylic groups. The particle size was in the range of 28.29–11.74 nm, which decreased with an increase in the ablation time. The excitation wavelength was about 300 nm, and the emission wavelength appeared at 432.23 nm. As a result, the intensity of the emission increased with an increase in the ablation time, while the particle size decreased due to the plasmonic property of gold nanoparticles, and the quantum yield is in the range of 38.208%–55.068%.

The Role of Reduced Graphene Oxide Concentration as Ablated Material on Optical Properties of Graphene Quantum Dots

We report the synthesize of Graphene Quantum Dots (GQDs) using ablation method with reduced Graphene Oxide (rGO) solution as a starting material. We have varied the concentration of rGO as following: 0.5, 2, 5 mg/ml and then have ablated them using 800 nm Ti-Sapphire femtosecond laser to obtain GQDs. From the UV-Vis data, we observed that the more concentration of rGO is being ablated, the more secondary absorption peak at 255.1 nm appeared. This secondary absorption peak is a characteristic of n-π* bonding due to the presence of oxygen defect which occurs as a result of the interaction between the laser and the water in rGO solution. We conclude that the population of oxigen defect in GQDs is increasing, following the increase of rGO concentration and could alter the optical properties of GQD. On the other hand, using Tauc’s plot, we confirm that the increase of rGO concentration as the ablated material does not alter GQDs optical band gap. However, it will slightly reduce both, direct and indirect Oxygen defect related optical band gap.

A Practical Guide to Prepare and Synthetically Modify Graphene Quantum Dots

AbstractDue to the favorable properties of graphene quantum dots (GQDs), there has been a rapid development of sensors, devices, and composite materials, which incorporate this 0D nanomaterial. GQDs provide a means to instill superior optical, electronic, mechanical, and adsorptive properties to various platforms and have found use as biosensors, photovoltaic devices, polymer composites, and drug delivery vehicles. One of the key factors to successfully integrating GQD technology is the intelligent choice of synthetic chemical pathways to fabricate, modify, and instill functionality in the developed platform. GQDs are decorated with a variety of functional groups that are amenable to traditional synthetic chemistry transformations; however, the technology for post‐synthetic modifications is only in its infancy. Herein, a comprehensive analysis of the chemistry available for modifying GQDs is provided; starting from methods for GQD fabrication and synthetic chemical pathways for producing functional GQDs, to the advantages, disadvantages, and challenges of specific chemistries, and finally techniques for the appropriate characterization of these functional materials. This review is meant for researchers considering entering the GQD field, as well as those more experienced, providing a practical guide on how to prepare, modify, and characterize GQDs for a broad range of applications.

Optoelectronic properties of graphene quantum dots with molybdenum disulfide

ABSTRACTThe presence of a direct optical bandgap in the transition metal dichalcogenide (TMD) layers leads to promising applications in optoelectronic devices such as phototransistors and photodetectors. These devices are commonly fabricated using few-layer and monolayer MoS2 sheets obtained using mechanical exfoliation or chemical vapor deposition techniques. The hybrid structure of quantum dots (QDs) and 2D materials has been investigated to provide outstanding properties for various applications. Herein we report the fabrication of a hybrid QDs/MoS2 photodetector consisting of graphene quantum dots (GQDs) and multilayer MoS2 sheets. The hybrid GQDs and MoS2 films are characterized by atomic force microscopy (AFM); additionally, the I-V characteristics are measured by two-point probe station.

Graphene quantum dots obtained by unfolding fullerene

Graphene quantum dots (GQDs) have recently been proposed as fluorescent sensors for metal ions dissolved in water. The physical origin of this fluorescence is still not completely clear, therefore further studies on GQDs and their properties are required. Graphene-like quantum dots were prepared by oxidation and unfolding of commercial buckminsterfullerene. This process was carried out by a modified Hummers method. After dialysis, the solution was diluted with deionized water to obtain the reference solution for optical measurements. Part of the solution was dried to obtain the unfolded fullerene powder. The metal ions were allowed to interact with GQDs by adding the metal-salt water solution to the stock solution of unfolded fullerene. The properties of GQDs before and after incorporation of metal ions (Hg, Cd, Pb, Cu, Ni and As) were characterized by using X-ray photoemission spectroscopy (XPS), Auger electron spectroscopy, UV–vis and fluorescent spectroscopies. In XPS, a particular attention was given to the analysis of photoemission C 1 s and Auger C KVV spectra in order to determine the carbon electronic configuration and its changes after incorporation of metal ions into GQDs. Obtained quantum dots were composed of graphene with oxidized borders. Optical measurements showed that the interaction with metal ions was changing the fluorescence intensity and optical absorbance, with different characteristics for each distinct ionic species. This behavior could be used for a selective multiple sensor detecting different heavy metals in water solutions.

Synthesizes, characterization, measurements and modeling thermal conductivity and viscosity of graphene quantum dots nanofluids

The present study is aimed to measure the thermophysical properties of graphene quantum dots (GQDs) nanoparticles that dispersed in water, ethylene glycol and water-ethylene glycol mixture (60:40) as base fluids. The presence of GQDs enhance the viscosity and thermal conductivity of nanofluids considerably. The maximum thermal conductivity enhancement reaches up 53%, 21% and 18% for the nanofluid containing 0.5% GQDs at 50 °C in water, ethylene glycol and water- ethylene glycol mixture (60:40) as base fluids, respectively. In addition, the viscosity of each solution was measured, and the results show that it increases with increasing volume fractions of GQDs nanoparticles and decreased with increasing of temperature. Besides, an artificial neural network model presents to predict the thermal conductivity and viscosity of nanofluids. The overall predicted thermal conductivities and viscosities of nanofluids are in excellent agreement with experimental data with the AAD of 1.02% and R2 = 0.99929 for thermal conductivity and AAD = 1.09%, R2 = 0.99915 for viscosity.

Separation of Spectroscopically Uniform Nanographenes.

Excitation-dependent photoluminescence (PL) is a well-known property of graphene quantum dots (GQDs). For the development of carbon-based photofunctional materials, GQDs possessing uniform PL properties are in high demand. A protocol has been established to separate spectroscopically uniform lipophilic GQD-1 a from a mixture of GQD-1 mainly composed of GQD-1 a and GQD-1 b. The mixture of GQD-1 was synthesized through the reaction of p-methoxybenzylamine with GQD-2 prepared from graphite by common oxidative exfoliation. Size-exclusion chromatography gave rise to GQD-1 a and GQD-1 b, with diameters of 19.8 and 4.9 nm, respectively. Large GQD-1 a showed that the PL was fairly independent of the excitation wavelengths, whereas the PL of small GQD-1 b was dependent on excitation. The excitation-dependent nature is most likely to be associated with the structures of sp2 domains on the graphene surfaces. The large sp2 -conjugated surface of GQD-1 a is likely to possess well-developed and large sp2 domains, the band gaps of which do not significantly vary. The small sp2 -conjugated surface of GQD-1 b produces small sp2 -conjugated domains that generate band gaps differing with domain sizes.

Tuning the electronic and optical properties of graphene quantum dots by selective boronization

BC3, BCO2, and BC2O show different behaviours in tuning the electronic and optical properties of boron-doped graphene quantum dots.

Environmental effects on the charge transfer properties of Graphene quantum dot based interfaces

AbstractGraphene quantum dots (GQD) are interesting materials due to the confined sizes which allow to exploit their optoelectronic properties, especially when they interface with organic molecules through physisorption. In particular, when interfaces are formed, charge transfer (CT) processes can occur, in which electrons can flow either from the GQD to the absorbed molecule, or vice versa. These processes are accessible by modeling and computational analysis. Yet, the presence of different environments can strongly affect the outcome of such simulations which, in turn, can lead to wrong results if not taken into account. In this multiscale study, we assess the sensibility of the computational approach and compute the CT, calculated at interfaces composed by GQD and amino‐acene derivatives. The hole transfer is strongly affected by dynamic disorder and the nature of the environment, and imposes stringent descriptions of the modeled systems to ensure enhanced accuracy of the transfer of charges.

Electronic and photocatalytic properties of modified MoS2/graphene quantum dots heterostructures: A computational study

Solar energy absorption and photo-induced charge separation and transfer are crucial to enhance photocatalytic properties. Here, the structural, electronic and photocatalytic properties of graphene quantum dots (GQDs), pure and O, N, Sv-modified MoS2 monolayers, GQDs-based heterostructures have been studied by theoretical calculations based on density functional theory. Compared with the GQDs, the obvious red shifts of the absorption peaks can be observed, the absorption intensities increases evidently, and the typical type-II band alignments can be formed after the construction of the pure, O, N, Sv-MoS2/GQDs heterostructures, which is beneficial to promote photo-induced charge transfer and more visible light harvesting. Subsequently, it is worth mentioning that N-MoS2/GQDs heterostructure also has the lower binding energy and higher absorption in the infrared region and impactful photo-induced electron injection from GQDs to N-MoS2 surface. Therefore, this work provides an instrumental and promising approach in designing new GQDs-based heterostructures to enhance the solar energy absorption and conversion.

Preparation and photoluminescence properties of graphene quantum dots by decomposition of graphene-encapsulated metal nanoparticles derived from Kraft lignin and transition metal salts

Graphene quantum dots (GQDs) are nanometer-sized pieces of a graphene sheet possessing a number of fascinating electrical and optical properties. In this work, GQDs were synthesized by a top-down method using lignin as a carbon source due to its high carbon content and high abundance in the nature. GQDs were prepared via an intermediate step, by the synthesis of graphene-encapsulated metal nanoparticles (GEMNs). First the GEMNs were synthesized from lignin and three different transition metal chlorides (Fe, Co, Ni), and then metal nanoparticles were removed, and graphene capsules thus obtained were then decomposed to GQDs by using an alkaline hydrothermal treatment. GQDs have a diameter around 20–25 nm and exhibit a yellowish emission under UV light. Characterization results obtained by using the IR, Raman and UV–vis spectroscopies showed that the GQDs obtained from the three types of GEMNs have a graphene structure with various oxygen-containing functional groups. A comprehensive study on the luminescence properties of the GQDs revealed that the three kinds of GQDs show an excitation-wavelength-dependent photoluminescence. The life times are in the range of 5.2–5.5 ns and the quantum yield values vary within the limits of 11.7–12.4%.

Targeting graphene quantum dots to epidermal growth factor receptor for delivery of cisplatin and cellular imaging

The unique properties of graphene quantum dots (GQDs) which include high loading capacity, excellent physiological stability, strong photoluminescence, biocompatibility, and facile production make them attractive nanomaterials for biomedical applications. In this work, GQDs have been explored as dual-functional targeted drug carriers and cellular bioimaging agents. The GQDs were conjugated to single chain variable fragment of antibody (scFv), which had been engineered with high affinity (B10) to epidermal growth factor receptor (EGFR), via amide covalent linkages (GQDs-scFvB10). The morphology and surface modification of GQDs were characterized by HRTEM, SDS-PAGE, FT-IR, UV–vis and fluorescence spectroscopies. Western blot analysis along with the confocal imaging of EGFR-overexpressing breast cancer cells (MDA-MB-231) demonstrated the targeting functionality of scFvB10 after conjugation to the GQDs, as well as the potential application of GQDs-scFvB10 in targeted bioimaging. The surface of targeted GQDs had a high cisplatin (CDDP) loading capacity of 50% and a pH-dependent release with slower release rate at neutral conditions, which can reduce the commonly observed systemic toxicity of CDDP. The targeted CDDP-loaded nanocarriers ((CDDP)GQDs-scFvB10) exhibited significantly higher toxicity on MDA-MB-231 cells compared to non-targeted ones suggesting their efficient uptake through EGFR. In contrast, cells with saturated EGFR showed lower uptake and cytotoxic effect of (CDDP)GQDs-scFvB10, demonstrating selectivity of the nanocarriers towards EGFR-overexpressing cells. The scFvB10-functionalized GQD is a promising platform for targeted cellular imaging and delivery of CDDP through interactions with EGFRs.

Single photon emission from graphene quantum dots at room temperature

Graphene being a zero-gap material, considerable efforts have been made to develop semiconductors whose structure is compatible with its hexagonal lattice. Size reduction is a promising way to achieve this objective. The reduction of both dimensions of graphene leads to graphene quantum dots. Here, we report on a single-emitter study that directly addresses the intrinsic emission properties of graphene quantum dots. In particular, we show that they are efficient and stable single-photon emitters at room temperature and that their emission wavelength can be modified through the functionalization of their edges. Finally, the investigation of the intersystem crossing shows that the short triplet lifetime and the low crossing yield are in agreement with the high brightness of these quantum emitters. These results represent a step-forward in performing chemistry engineering for the design of quantum emitters.

Exciton Self-Trapping in sp2 Carbon Nanostructures Induced by Edge Ether Groups.

Recent experiments have suggested that exciton self-trapping plays an important role in governing the optical properties of graphene quantum dots (GQDs) and carbon dots (CDs), while the molecular structures related to this phenomenon remain unclear. This theoretical study reports exciton self-trapping induced by edge-bonded ether (C-O-C) groups in graphene nanosheets. Density functional theory (DFT) and time-dependent DFT calculations show that the initially delocalized electron and hole are trapped in the vicinity of the edge ether groups on graphene nanosheets upon excited-state (S1) relaxation, accompanied by structural planarization of the seven-membered cyclic ether rings in the same region. The resulted significant structural deformation leads to large Stokes shift energies, which are comparable to the magnitudes of the notably large emission shift observed in experiments. This study provides a feasible explanation of the origin of exciton self-trapping and offers guidance for experiments to investigate and engineer exciton self-trapping in relevant materials.

Graphene quantum dots as charge trap elements for nonvolatile flash memory

In this study, we investigated the charge storage properties of graphene quantum dots (GQDs) by embedding them between poly(methylmethacrylate) (PMMA) sheets as control and tunneling dielectric layers in a silicon metal-oxide-semiconductor-based nonvolatile memory device (Ag-Pd/PMMA/GQDs/PMMA/Si). The conduction mechanism through the gate dielectric stack embedded with GQDs could be explained by Fowler Nordheim (F–N) tunneling. The GQDs trapped electrons to yield a maximum memory window of ∼5.45 V (at ± 7 V) with a low leakage current density. A control experiment without GQDs obtained very low hysteresis (∼7 mV) even when the gate voltage was swept from +8.5 V to –8.5 V. Thus, the memory window obtained in the present study originated from the GQDs and not from the trapped states in the PMMA/Si interface. The maximum charge density was determined as 12.1 × 1011 cm−2, which is comparable to or even better than that found in conventional semiconductor devices. The endurance and charge retention characteristics were examined to assess the reliability of device operation when the devices were put under a constant stress of ±12 V. The flat-band voltage shifts were determined as 0.16 V and 0.23 V during the programming and erasing operations, respectively, and the charge loss was only ∼14.9% even after 105 s, thereby demonstrating the excellent charge confinement property of the GQDs.

Exploring the excellent photophysical and electrochemical properties of graphene quantum dots for complementary sensing of uranium

Graphene quantum dots (GQDs) possessing excellent photophysical and electrochemical properties have been exposed here, for the first time, to develop the low cost, non-toxic, metal-free, sustainable real-life adaptable sensors for strategically important and environmentally concerned metals such as uranium(U). The Förster resonance energy transfer involving f-orbitals of U as well as the nonspecific electrostatic association between GQDs and ions of U quench the photoluminescence of the GQDs resulting into photophysical detection of U in a pure solution down to 0.56 μg L−1. The electrochemically reduced GQDs are also found to be more electrocatalytically active than gold and it could detect U down to 2 μg L−1 in aqueous solution having high ionic strengths. This complementary dual-purpose sensor is applicable to determine U in pure solution as well as in the presence of other metal ions in the ground water. Further, we have re-evaluated the kinetic models viz. Nicholson and Klingler-Kochi equations from their respective roots and unambiguously showed neither of the equations is suitable for universal kinetic analysis in electrochemistry. We have persuasively proposed a universal electrodic equation eliminating the perplexities of the traditional kinetic equations. This article paves the unconventional way for environmentally clean synthesis of GQDs possessing excellent photophysical and electrochemical properties expected from the complementary sensor.