Hypothesis: In recent years, scientists and researchers have been looking for new and advanced materials for use in various fields, including drug delivery and biotechnology. One of the attractive  materials that has been considered in this field is chitosan-based bionanocomposite hydrogel beads. Chitosan-based bionanocomposite hydrogel beads have attracted attention as carriers in drug delivery systems due to their inherent properties such as excellent biocompatibility, high swelling, and high storage capacities.Methods: First graphene quantum dots (GQDs) were prepared by the pyrolysis method, and then their magnetic properties were obtained using iron nanoparticles (MGQD). Next they were coated with chitosan hydrogel (CS-MGQD) and finally loaded with methotrexate (MTX/CS-MGQD). This unique combination of the properties of chitosan hydrogel, the magnetic properties of graphene quantum dots, and the ability to adjust drug release has been able to create an important milestone in the field of drug delivery research and the compatibility of drugs with the biological environment. Through various analyses, including FTIR to analyze the spectra of the functional groups, XRD to identify the crystal structure, and SEM to examine the morphology of the samples, the success of the synthesis and formation of the desired compound was confirmed.Findings: The fabricated CS and CS-MGQD hydrogel beads were loaded with about 84% and 64% MTX, respectively. The results of the swelling behavior and drug release behavior showed that the hydrogel beads experience pH-dependent swelling and release of MTX. In addition, investigating the effect of MGQD concentration on swelling behavior and drug release showed that CS-MGQD has favorable stability and controllable drug release in an acid environment. Also, the Weibull kinetic model was found to be the best-fitting model for the release of MTX from CS-MGQD at pH 5. These findings suggest that the prepared magnetic bionanocomposite hydrogel beads have a good potential for pH-sensitive implantable drug delivery in the treatment of cancerous tissue.

Abstract: Discrete energy levels of quantum dots (QD) have electronic and optoelectronic applications. In this paper, a novel graphene nanoribbon (GNR) field effect transistor (FET) is modeled numerically using the NEGF formalism. In the new device model of this paper, the channel region is composed of one or two QDs, made by only one metallic gate electrode. This model utilizes a semiconductor armchair graphene nanoribbon through which the current may pass. The two highly doped ends of GNR act as source and drain contacts. At this unique model, one or two quantum dots form on GNR channel.  The discreteness of energy levels of the two coupled quantum dots, revealed by applying gate voltage, gives rise to resonant tunneling.  Resonant tunneling through these levels results in negative differential conductance. The coupling between QDs determines the current characteristics of device. Step-wise increment of current by increasing drain voltage manifests QDs discrete energy levels.

This review represents an overview of the graphene quantum dots (GQDs) synthesis and their applications as carriers or probes for the sensor, imaging, drug delivery, and diagnosing of diseases. Furthermore, recent developments in the GQDs for therapy and their potential toxicity for both in vitro and in vivo are reviewed. The recent findings and issues for GQDs and their composites with respect to stability and optimal size and toxicity at various applications are presented.

The most important loss mechanism in single junction solar cells is the inability to convert photons with energies below the bandgap to electricity. Due to quantum confinement, graphene-based quantum dots (QDs) provide a means to create an intermediate band (IB) in the bandgap of semiconductors to absorb sub-bandgap photons. In this work, we introduce a new type-I core/shell-graphene/Si QD for use in all Si-based intermediate band solar cells (IBSCs). Slater-Koster Tight-Binding method is exploited to compute the ground state and the band structure of the graphene/Si QD. The ground state is obtained 0.6 eV above the valance band (VB), which is suitable for creating IB between the conduction band and VB of Si. A superlattice (SL) of this QD is created and the mini-band formation in SL is investigated by varying the inter-dot spacing between QDs. A mini-band with roughly 0.3 eV bandgap is observed in the well-aligned and closely packed SL. This SL is embedded in the intrinsic region of the conventional Si-based solar cell. The mini-band in SL works as an IB in the solar cell and results in increased photon absorption. As a result, carrier generation rate improves from 1.48943×1028 m-3s-1 to 7.94192×1028 m-3s-1 and short circuit current density increases from 211.465 A/m2 to 364.19 A/m2.

Density functional theory (DFT) and thermal DFT (thDFT) calculations were used to evaluate the energy band structure, bandgap, and the total energy of various graphene quantum dots (GQDs). The DFT calculations were performed using local density approximation for the exchange-correlation functional and norm-conserving pseudopotentials. We consider the triangular and hexagonal GQDs with zigzag and armchair edges and 1-3 nm dimensions with many hundred atoms. The simulation results show that all of these GQDs are direct bandgap semiconductors with a flat band structure, and they are suitable for electronics and optoelectronics applications. Analysis of GQDs in which the A and B sublattice symmetries were broken showed degenerate zero-energy shells. Using the thDFT calculations carried out at temperatures up to 1400 K, we evaluated the temperature dependence of the GQDs bandgaps and total energies via entropy-term and electron’ s kinetic energy. The obtained results indicate that the ground-state DFT calculations are valid for determining the electronic properties of GQDs up to room temperature. Moreover, we tune semi-empirical parameters of the tight-binding model by the DFT results in small GQDs to reduce the computational cost of electronic structure calculations for large GQDs, which contained up to thousands of atoms.

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