Undoped Graphene Quantum Dots Research Articles

Single-sized N-functionality graphene quantum dot in tunable dual-modality near infrared–I/II illumination detection and photodynamic therapy under multiphoton nonlinear excitation

Doping sorted graphene quantum dots (GQDs) with heteroatoms and functionalizing them with amino acid could improve their radiative recombination and two-photon properties—including their excitation-wavelength-independent photoluminescence from the ultraviolet to the near-infrared-I (NIR-I) region, absorption, quantum yield, absolute cross section, lifetime, and radiative-to-nonradiative decay ratio—under two-photon excitation (TPE) at a low excitation energy and short photoexcitation duration, as determined using a self-made optical microscopy system with a femtosecond Ti–sapphire laser. Four types of sorted GQDs were investigated: undoped GQDs, nitrogen-doped GQDs (N-GQDs), amino-functionalized GQDs (amino-GQDs), and N-doped and amino-functionalized GQDs (amino-N-GQDs). Among them, the sorted amino-N-GQDs are effective as a two-photon photosensitizer and generate the highest quantity of reactive oxygen species for the elimination of multidrug-resistant cancer cells through two-photon photodynamic therapy (PDT). Larger amino-N-GQDs result in a greater number of C–N and N–functionalities, leading to a superior photochemical effect and more favorable intrinsic luminescence properties, making the dots effective contrast agents for tracking and localizing cancer cells during in-depth bioimaging in a three-dimensional biological environment under TPE in the NIR-II region. Overall, this study highlights the potential of large amino-N-GQDs as a material for future application to dual-modality two-photon PDT and biomedical imaging.

Synthesis of Nitrogen-Doped Graphene Quantum Dots from Sucrose Carbonization

The synthesis of carbon-based quantum dots has been widely explored in the literature in recent years. However, despite the fact that synthesis processes to obtain highly efficient carbon quantum dots (CQDs) and graphene quantum dots (GQDs) with redshifted photoluminescence (PL) have been improved, few works have exploited sucrose in the synthesis of GQDs with high PL efficiency. In this work, sucrose, which is a widely available non-toxic saccharide, was used as a precursor of GQDs. Initially, sucrose was carbonized in sulfuric acid, and thereafter, the material obtained was treated in dimethyl sulfoxide (DMSO). Nitrogen doping was also performed in this work through an additional step involving the treatment of carbonized sucrose in nitric acid reflux. Nitrogen-doped GQDs (N-GQDs) showed tunable PL dependent on the excitation wavelength. It was also verified that the intensity of the emission in the red region was much higher in the N-GQDs in comparison with that in undoped GQDs. X-Ray Diffraction, Raman, FTIR, TEM, and AFM analyzes were also performed to obtain greater structural details of the obtained GQDs.

Nitrogen-doped graphene quantum dots synthesized by femtosecond laser ablation in liquid from laser induced graphene

In this work, graphene quantum dots (GQDs) were synthesized by femtosecond laser ablation in liquid using laser induced graphene as the carbon source. Nitrogen-doped graphene quantum dots (N-GQDs) were successfully synthesized by adding ammonia water to the graphene suspension. The GQDs/N-GQDs structure consist of a graphitic core with oxygen and nitrogen functionalities with particle size less than 10 nm, as demonstrated by x-ray photoelectron spectroscopy, Fourier infrared spectrometer spectroscopy, and transmission electron microscopy. The absorption peak, PL spectrum, and quantum yield of the N-GQDs were significantly enhanced compared with the undoped GQDs. Further, the possible mechanism of synthesis GQDs was discussed. Furthermore, the N-GQDs were used as a fluorescent probe for detection of Fe3+ ions. The N-GQDs may extend the application of graphene-based materials to bioimaging, sensor, and photoelectronic.

Facile synthesis of heteroatom doped and undoped graphene quantum dots as active materials for reversible lithium and sodium ions storage

Zero-dimensional graphene quantum dots have attractive properties but the synthesis of graphene quantum dots in a simple and scalable technique is tedious, which limits its utilization in different energy storage application. In this study, we present a simple and scalable approach to produce graphene quantum dots and heteroatom doped graphene quantum dots using chemical vapor deposition technique. Graphene quantum dots are prepared using alloy-based catalyst and methane as a carbon source. Boron-doped and nitrogen-doped graphene quantum dots are prepared at low temperature using graphite oxide without the use of dialysis bag. Here, the electrochemical lithium and sodium ion storage properties of doped and undoped graphene quantum dots are studied without being used as a supporting material for the performance enhancement as reported in previous reports. Boron doped GQD (B-GQD) exhibits a high specific capacity of 1097 mAh g−1 and 310 mAh g−1 at a specific current of 50 mA g−1 for lithium and sodium ion batteries respectively. B-GQD exhibits high volumetric energy density of 537 Ah L−1 and 214 Ah L−1 with an average voltage of 0.43 V and 0.57 respectively for lithium ion and sodium ion batteries. Also, the cells observe a satisfactory cyclic performance for 500 cycles with good capacity retention. Detailed investigations show that the edge defects present in GQD and doped GQDs help to enhance the electrochemical storage performance of lithium and sodium ions.

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.

Facile synthesis of defect-rich nitrogen and sulfur Co-doped graphene quantum dots as metal-free electrocatalyst for the oxygen reduction reaction

Carbon-based metal-free nanomaterials have attracted immense and persistent attention as electrocatalysts for oxygen reduction reaction (ORR) in fuel cells, due to their high electrocatalytic activity, long-term stability and low cost. Here, a facile hydrothermal treatment in the presence of ammonium hydroxide (NH4OH) and sodium sulfide (Na2S), as precursors, has been developed to synthesize heteroatom-doped graphene quantum dots (GQDs). For comparison, undoped graphene quantum dots (GQDs), nitrogen doped graphene quantum dots (N-GQDs), sulfur doped graphene quantum dots (S-GQDs) and nitrogen and sulfur co-doped graphene quantum dots (N,S-GQDs) have been synthesized with the same method. The as-obtained N,S-GQDs possess a high N-doping content up to 9.36% and low S-doping content up to 0.78%, with a uniform size distribution similar to that of GQDs, N-GQDs and S-GQDs. Interestingly, the multi-types of NC or CSC bonding in metal free N,S-GQDs can significantly enhance the electrocatalytic activity for the oxygen reduction reaction (ORR), with the electron transfer number as good as 3.82, which verify the importance of our facile synthesis method to fabricate metal-free electrocatalysts for ORR and other electrochemical applications.

B, N, S, Cl doped graphene quantum dots and their effects on gas-sensing properties of Ag-LaFeO3

P-type Ag-LaFeO3 (AL) is a promising gas-sensing material owing to its large surface area, rich active oxygen lattice, good thermostability, controllable structure and strong reducibility. While the operating temperature of which is high. In this study, B, N, S and Cl doped graphene quantum dots (GQD) were prepared with a simple liquid-phase exfoliation method and with which decorated AL in order to low the operating temperature of AL. The sensors based on the doped GQD operated at room temperature and showed affinity toward different volatile organic compounds (VOCs). B-GQD revealed p-type semiconductor property and reduced the operating temperature of AL markedly form 90 °C to 55 °C when composite with it because of the synergy effect. Undoped GQD and N, S, Cl doped GQD showed n-type semiconductor property and oppositely increased the operating temperature of AL to different extent when composite with AL. That is, the more electrons in the outer layer of the doped element, the higher the temperature of GQD/AL composite rise. Thus the gas-sensing properties of B-GQD/AL composite was further researched, the results showed that the composite possessed high sensitivity, good selectivity, good stability and low operating temperature to low concentration of formaldehyde.

Exploring the Emissive States of Heteroatom-Doped Graphene Quantum Dots

The photoluminescence (PL) emission states of heteroatom-doped graphene quantum dots (GQDs) remain unknown, particularly the assignment of the low-energy excitation band (more than 330 nm). To address these issues, this work synthesized three different types of GQDs: undoped GQDs (UGQDs), nitrogen-doped GQDs (NGQDs), and boron-doped GQDs (BGQDs), with similar sizes, chemical compositions (types and compositions of surface functional groups), and defects using a constant potential electrolysis method. The PL emissive states in these GQDs and the effects of the dopant heteroatom on the PL were revealed based on the combination of spectroscopic methods and theoretical calculations. The results indicated that the GQDs exhibit multiemissive centers for the PL emission mechanism. An excitation-independent PL emission band (band I) results from a high-energy transition originating from the quantum confinement of the carbon core (carbon π–π* transitions in sp2 domain), and an excitation-dependent PL emission band...

Potassium doping: Tuning the optical properties of graphene quantum dots

Doping with hetero-atoms is an effective way to tune the properties of graphene quantum dots (GQDs). Here, potassium-doped GQDs (K-GQDs) are synthesized by a one-pot hydrothermal treatment of sucrose and potassium hydroxide solution. Optical properties of the GQDs are altered as a result of K-doping. The absorption peaks exhibit a blue shift. Multiple photoluminescence (PL) peaks are observed as the excitation wavelength is varied from 380 nm to 620 nm. New energy levels are introduced into the K-GQDs and provide alternative electron transition pathways. The maximum PL intensity of the K-GQDs is obtained at an excitation wavelength of 480 nm which is distinct from the undoped GQDs (375 nm). The strong PL of the K-GQDs at the longer emission wavelengths is expected to make K-GQDs more suitable for bioimaging and optoelectronic applications.

A general solid-state synthesis of chemically-doped fluorescent graphene quantum dots for bioimaging and optoelectronic applications.

Graphene quantum dots (GQDs) have attracted increasing interest because of their excellent properties such as strong photoluminescence, excellent biocompatibility and low cost. Herein, we develop a general method for the synthesis of doped and undoped GQDs, which relies on direct carbonization of organic precursors in the solid state.