Dot Structures Research Articles

Density-functional study of electronic structure of quantum dots in high magnetic fields

Density-functional theory is used to study the electronic structure of quantum dots in magnetic field. New series of magic numbers are found for the total angular momentum of electrons. The empirical formula for the plateau width is obtained. The effect of a charged impurity on the electronic structure of a quantum dot has been studied.

Photoexcited state properties of N-ethylcarbazole-functionalized carbon dots in solution and in PVK polymer matrix

The adducts of N-ethylcarbazole (NEC) to small carbon nanoparticles prepared by microwave-assisted thermal processing are carbon dots (CDots) with NEC for the dot surface functionalization, thus NEC-CDots, which were shown previously for their high stability in the dot structure and properties. In this study, the optical absorptions and photoexcited state properties of NEC-CDots in solution and in poly(N-vinylcarbazole) (PVK) film matrix were evaluated by using optical spectroscopy methods, including those for fluorescence decays and lifetimes. The results suggest that the properties are largely unchanged from solution phase to the polymer matrix environment, very positive to their expected optoelectronic applications.

A Convenient Approach for the Construction of Lewis Structures by Focusing on the Valence Requirements of Outer Atoms

A Convenient Approach for the Construction of Lewis Structures by Focusing on the Valence Requirements of Outer Atoms

Effect of carbon dots nanomaterial concentration on luminance spectral bandwidth via Kirchoff-Bunsen spectroscope

This study aims to determine the effect of the concentration of Carbon Dots on the bandwidth of the Carbon Dots luminescence spectrum. Carbon Dots are produced by ultrasonification method and characterized using UV-Vis spectroscopy, photoluminescence (PL) spectroscopy, scanning electron microscopy (SEM), electron dispersive X-Ray spectroscopy (EDX), X-ray diffraction (XRD), and particle size analyzer (PSA). Measurement of the bandwidth of the Carbon Dots fluorescence spectrum for various concentrations was carried out by irradiating the Carbon Dots sample using a laser with a wavelength of 405 nm and looking at the spectrum of light emitted using a Kirchoff-Bunsen spectroscope.The characterization results show that the resulting Carbon Dots have a light absorption peak at a wavelength of 303 nm, a light wave emission peak at a wavelength of 508.87 nm, the surface structure of the Carbon Dots is in the form of a porous layer, the presence of the dominance of carbon and oxygen atoms in Carbon Dots, an amorphous Carbon Dots structure is observed, and the smallest measured Carbon Dots particle size is 1.12 nm. The results show that increasing the concentration of Carbon Dots causes a tendency to increase the bandwidth of orange, green and blue light spectra emitted by the particles, and in the red color there was no significant effect of increasing the concentration of Carbon Dots on the spectrum. However, increasing the concentration of Carbon Dots actually causes a narrowing of the yellow and violet color spectra.

Investigation of Rod‐Shaped Single‐Graphene Quantum Dot

In recent years, there has been a resurgence of interest in graphene quantum dots (i.e., large polycyclic aromatic hydrocarbons) in particular due to their interesting properties as single‐quantum emitters. Herein, a thorough study of a graphene quantum dot made of 96 carbon atoms in a D2h symmetry is reported. In particular, experiments at the single‐molecule level reveal that this graphene quantum dot structure is a bright and photostable single‐photon emitter at room temperature. Finally, a statistical study of the emission wavelength as a function of the graphene quantum dot concentration highlights the high purity and degree of control on the samples.

The impact of lutetium doping on the structure and physical characteristics of aluminum arsenide quantum dots

This report mentioned the synthesis of self-assembly Al1-xLuxAs quantum dot nanocrystals using the L-Glutamine-assisted wet chemical route. The effect of Lu-doping on the structural phase, lattice strain, and deformation lattice energy was studied. The morphology and elemental compositions were emphasized using transmission electron microscopy, selected area of electron diffraction, and energy dispersive X-ray spectroscopy. The effect of Lu-doping on the oxidation state and vibrational modes was studied using X-ray photoelectron spectroscopy and Raman scattering spectroscopy. The influence of Lu-dopant on the optical bandgap and Urbach tail energy of AlAs nanocrystals was investigated. The tendency of Lu doping to create localized levels inside the band gap of AlAs gives rise to an increase in plasma frequency and a decrease in the high-frequency dielectric constant. The Lu-dopant improved the conductivity and the charge carrier mobility of AlAs nanocrystals. The luminescence measurements revealed the ability of Lu-dopant to suppress the crystal defects of the AlAs nanocrystals. The Lu doping improved the quantum yield (84 %) and the luminescent emission spectra of the AlAs QDs by 7-fold. The prepared self-assembly Al1-xLuxAs quantum dots revealed great encouragement for developing Al1-xLuxAs quantum dot diode lasers and optical broadbands.

Tuning the energy gap of graphene quantum dots functionalized by [sbnd]OH and [sbnd]COOH radicals: First principle study

In this work, we performed theoretical calculations based on density functional theory for graphene quantum dots considering three different sizes. We consider graphene quantum dots formed by 7, 19, and 37 rings of C atoms in a hexagonal arrangement. The electronic band structure of graphite and graphene were calculated to evaluate the methodology and parameters used during calculations. The modeled graphene quantum dots structures were initially passivated by H atoms, then the H atoms were gradually replaced by OH or COOH radicals to investigate the influence of oxygen on the chemical stability and the energy gap. Based on the electric dipole moment, the replacement positions were selected. The results demonstrate that all the structures are chemically stable and that the energy gap depends on the number of OH or COOH radicals at the edge of the GQDs. H passivation results in energy gaps of 2.83 eV, 1.87 eV and 1.33 eV, decreasing with increasing OH or COOH radical amount. It was found that the energy gap varies non-monotonically as the OH or COOH radicals increase. To better understand the origin of the energy gap and its changes by OH or COOH radicals, we calculated the DOS and evaluated the HOMO and LUMO by Fermi surfaces.

Modeling of dark current in semispherical quantum dot structures for infrared photodetection

Due to its tunable heterojunction bandgap and great sensitivity to normal incident illumination, the Quantum Dot Infrared Photodetectors (QDIPs) have received a lot of attention for the purpose of infrared sensing. It could be a very promising replacement for conventional infrared photodetectors made with established technology, including mercury cadmium telluride and quantum well infrared photodetectors. In this work, a model for the dark current in semispherical QDIP has been developed, resolves the primary semiconductor Poisson's and continuity equations, where the wave function and the bound states effects are investigated. In this study, Boltzmann transport equation in the photodetector active layer with embedded QDs is solved using the finite difference time domain method to determine the photodetector carrier mobility and its degradation due the quantum dot scattering. The outcomes of the presented have been contrasted with truncated conical QDIPs showing that smaller volume QDs had less noisy dark current. Investigations have been done into how the semispherical QDIP's dark current characteristics are affected by the QD volume, density, and operating temperature.

Carbon quantum dots as a tracer of water seepage sources and pathways in grottoes

Water seepage is one of the main factors leading to the damage of grottoes. The sources and pathways of water seepage need to be identified to relieve it. Although the sources and pathways are investigated using geophysical exploration methods commonly, the results are unsatisfactory due to the limitation of resolution. The tracer method has been widely used to examine water seepage in the natural sciences and engineering. However, most tracers have an impact on grottoes, making this method inapplicable. This study was the first to use the carbon quantum dots as a tracer of water seepage in grottoes. The characteristics of the carbon quantum dots, which was synthesized by various biomass precursors through large-scale synthesis in the field, were analyzed to determine the optimal precursor. The structure, fluorescence intensity, and water solubility of the carbon quantum dots were evaluated. Laboratory tests were designed to examine the transport properties of the carbon quantum dots in rocks and cracks. The results showed that the carbon quantum dots synthesized by Ginkgo biloba were small and had uniform size, excellent fluorescence, good water solubility and transport ability. Furthermore, the carbon quantum dots were successfully used to tracing the source of water seepage at the chest of the Leshan Giant Buddha. The low cost of synthesis, wide precursors, easy and convenient synthesis methods, friendliness to grottoes, and excellent performance of the carbon quantum dots as a tracer suggest the efficacy of this method. These findings could lead to the widespread use of tracer method in studies of water seepage in grottoes.

Unveiling the Role of Donor Impurity Position on the Electronic Properties in Strained Type I and Type II Core/Shell Quantum Dots under Magnetic Field.

In this theoretical investigation, we delve into the significant effects of donor impurity position within core/shell quantum dot structures: type I (CdTe/ZnS) and type II (CdTe/CdS). The donor impurity's precise location within both the core and the shell regions is explored to unveil its profound influence on the electronic properties of these nanostructures. Our study investigates the diamagnetic susceptibility and binding energy of the donor impurity while considering the presence of an external magnetic field. Moreover, the lattice mismatch-induced strain between the core and shell materials is carefully examined as it profoundly influences the electronic structure of the quantum dot system. Through detailed calculations, we analyze the strain effects on the conduction and valence bands, as well as the electron and hole energy spectrum within the core/shell quantum dots. The results highlight the significance of donor impurity position as a key factor in shaping the behaviors of impurity binding energy and diamagnetic susceptibility. Furthermore, our findings shed light on the potential for tuning the electronic properties of core/shell quantum dots through precise impurity positioning and strain engineering.

Facile Synthesis of Large Bulk Crystal Based on Perovskite‐Quantum Dots Hybrid Structure

AbstractIn this study, the fabrication of a large bulk crystal based on the hybrid structure of CH3NH3PbBr3 (MAPBr) and PbS quantum dots (QDs) with excellent crystal lattice match using an in situ epitaxial crystallization technique is presented. The hybrid bulk crystal is successfully constructed by uniformly dispersing PbS QDs within the MAPBr matrix. The crystalline growth method is also changed to achieve the desired hybrid structure. The light emission range is modulated and the light emission lifetime is prolonged by constructing a MAPBr‐PbS hybrid structure. Additionally, the hybrid bulk crystal exhibited an up‐conversion effect, which does not appear in MAPBr single crystal. These findings highlight the promising potential of the MAPBr‐PbS QDs hybrid bulk crystal in the fields of optics and photoelectronic, such as all solid‐state lasers, bio‐imaging, optical data storage, and photovoltaic applications. This in situ hybrid crystallization technique also provides a universal pathway for the preparation of other bulk crystals based on perovskite‐QDs hybrid structures.

Study of self- assembly structures of carbon quantum dots

Self-assembly (SA) structures are formed by self-organizing processes in which discrete elements interact spontaneously with one another to produce larger and more complex structures. Compared to disorganized systems, self-assembled nanoparticles with specific functionalities can exhibit enhanced or even novel properties. Among the various nanoparticles capable of forming SAs, we can highlight carbon quantum dots (Cdots). Cdots are photoluminescent core/shell semiconductor nanoparticles with excellent optical properties, such as photo-stability, size-dependent emission energy, and intensity sensitivity to particle aggregation. Thus, the organization of individual Cdots in ordered structures on solid substrates has the potential for possible nanodevices in the area of sensors, catalysis, optoelectronics, and data storage. This study aimed to produce Cdots-based SAs and subsequently study their morphological and optical properties. Cdots were obtained by electrochemical exfoliation of the graphite electrode, and SA structures were obtained by the induced evaporation method under controlled temperature. The effects of the temperature and volume of the deposited Cdots solution in the substrate on the formation of SA were investigated. Optical and fluorescence microscopy images showed the formation of photoluminescent SA structures up to 1 mm in size with different aggregation patterns, such as aggregation by limited diffusion, river-type fractal, fern-leaf-type fractal, films, and bifurcated patterns. The variation of the parameters caused significant changes in some characteristics of the SAs structures, such as an increase in the intensity of the photoluminescence (PL) or its annihilation and change in the self-organization pattern. The results obtained in this work provide a preliminary overview of the different patterns of SA structures that can be obtained using photoluminescent Cdots as building blocks.

Reactivity in chemistry: the propensity view

We argue for an account of chemical reactivities as chancy propensities, in accordance with the ‘complex nexus of chance’ defended by one of us in the past. Reactivities are typically quantified as proportions, and an expression such as “A + B → C” does not entail that under the right conditions some given amounts of A and B react to give the mass of C that theoretically corresponds to the stoichiometry of the reaction. Instead, what is produced is a fraction α < 1 of this theoretical amount, and the corresponding percentage is usually known as the yield, which expresses the relative preponderance of its reaction. This is then routinely tested in a laboratory against the observed actual yields for the different reactions. Thus, on our account, reactivities ambiguously refer to three quantities at once. They first refer to the underlying propensities effectively acting in the reaction mechanisms, which in ‘chemical chemistry’ (Schummer in Hyle 4:129–162, 1998) are commonly represented by means of Lewis structures. Besides, reactivities represent the probabilities that these propensities give rise to, for any amount of the reactants to combine as prescribed. This last notion is hence best understood as a single case chance and corresponds to a theoretical stoichiometric yield. Finally, reactivities represent the actual yields observed in experimental runs, which account for and provide the requisite evidence for/against both the mechanisms and single case chances ascribed.

Scientific representation and science identity: the case of chemistry

I put forward an inferentialist account of Lewis structures (LSs). In this view, the role of LSs is not to realistically depict molecules, but instead to allow surrogate reasoning and inference in chemistry. I also show that the usage of LSs is a central part of a person’s identity as a chemist, as it is defined within educational identity theory. Taking these conclusions together, I argue that the inferentialist approach to LSs and chemistry identity theory can be studied in parallel, as two complementary sides of the same research programme.

Facile preparation of carbon dots with multicolor emission for fluorescence detection of ascorbic acid, glutathione and moisture content

The nitrogen-doped carbon dots (N-CDs) with multicolor emission (green, yellow, and red) were fabricated with naphthalenetetracarboxylic dianhydride as carbon precursor, and different alkylamines as passive agents. The structures of the as-prepared carbon dots were confirmed by the Fourier transform infrared (FT-IR) spectrum, X-ray photoelectron spectroscopy (XPS) spectra, transmission electron microscopy (TEM), Raman spectra, and X-ray diffraction (XRD) pattern. The characterized results and UV–vis absorption spectrum demonstrated that the luminescence variations of different CDs were probably attributed to the difference of N-related states, oxygen-containing groups, and oxidation degree in the CDs structure. Moreover, the green emitting CDs (G-CDs) could be utilized as fluorescent probe for ascorbic acid (AA) detection with detection limit (LOD) of 5 μM. The yellow emitting CDs (Y-CDs) could be used to detect glutathione (GSH) in the range of 1–70 μM with LOD of 0.07 μM. The red emitting CDs (R-CDs) could detect the water content in THF, DMF, acetone, and EtOH with LODs of 0.44%, 0.31%, 0.55% and 0.55%, respectively. Remarkably, G-CDs and Y-CDs can distinguish AA and GSH respectively with high selectivity and accuracy. Finally, the fluorescent probes based on G-CDs and Y-CDs were successfully applied for the AA and GSH determination in vitamin C tablet, grapefruit juice, and GSH whitening serum samples with high reliability and feasibility, which provided the guidance for the selection of AA and GSH supplements.

Unraveling the Aromatic Rule of Cyclic Superatomic Molecules in π-Conjugated Compounds.

The aromaticity of π-conjugated compounds has long been a confusing issue. Based on a recently emerged two-dimensional (2D) superatomic-molecule theory, a unified rule was built to decipher the aromaticity of cyclic superatomic molecules of π-conjugated compounds from the chemical bonding perspective. Herein, a series of planar [n]helicenes and [n]circulenes, composed of benzene, thiophene, or furfuran, are systemically studied and seen as superatomic molecules ◊On-2◊F2 or ◊On, where superatoms ◊F and ◊O denote π-conjugated units with 5 and 4 π electrons, respectively. The ascertained superatomic Lewis structures intuitively display aromaticity with each basic unit meeting the superatomic sextet rule of benzene, similar to classical valence bond theory, which is favored by the synthesized complex π-conjugated compounds comprising different numbers and kinds of subrings. The evolutionary trend of ring currents and chemical bonding suggests a local ribbon-like aromaticity in these π-conjugated compounds. Moreover, nonplanar helical π-conjugated compounds have the potential to evolve into spring-like periodic materials with excellent physical properties.

Optical gain of CdxZn1−xTe quantum dot structures

AbstractGain spectra of undoped and doped quantum dot (QD) structures are studied under the inhomogeneous broadening assumption at four mole fractions . For the QD structure, two peaks appear due to the excited state (ES) and ground state (GS) transitions. The gain for the doped structures doubles the undoped ones. The gain increases while the wavelength is reduced with increasing Cd content due to the broader band discontinuity between the QD states. The discontinuity of the bands for each structure (mole fraction) is calculated, which is one of the merits of this work. While the structure offers the peak wavelengths 470 and 630 nm, other mole fractions offer the wavelengths between them. These visible bands are essential in different applications. The effect of QD size effect is also examined. The wavelength is extended by 20 nm for each 1 nm QD height increment.

(Third Place Poster Award) Computational Assessment of the Optoelectronic Structure and Biomolecular Complexation of Graphene Quantum Dots

With novel materials getting smaller and their size falling to the nanometer scale, it becomes harder to fully characterize them by only using the experimental apparatus at hand. Therefore, taking advantage of computational methods proves to be trustworthy in filling those gaps and in aiding our experimental data to get a better understanding of the nanomaterials’ structural and electronic properties. Graphene quantum dots (GQDs) have recently become one of the flagships of carbon nanotechnology due to their remarkable physical properties and, when functionalized, an ability to become water soluble, biocompatible and capable of fluorescence in the visible and near-infrared. This makes them perspective carriers for therapeutic delivery and image-tracking. In order to assess the advantages of their utilization for a variety of bioapplications, we have investigated optical properties of doped GQDs and their interactions with biomolecules using a variety of molecular simulation approaches. True atomic ground state of the N-GQD is achieved via performing first-principle calculations based on density functional theory (DFT). DFT calculations also unrevealed the contributions of each functional group within the structure to HOMO–LUMO band edges. The adsorption of biomolecules and genes on the GQD surface has been further investigated with regards of the GQD structure, complementing experimental results that verify gene and drug complexation. Figure 1

Tuning of binding energy, polarizability and photoionization cross-section of a hydrogenic impurity in CdTe/ZnTe core-shell nanostructure by the electric field and capped matrix

The current work describes in detail the effect of polarization charges on the electronic properties of core/shell spherical quantum dots (CSQDs) structures composed of CdTe/ZnTe semiconductors. In the calculations, energy levels and wave functions matching these levels were calculated by using the effective mass approach (EMA). Furthermore, the binding energy of a hydrogenic impurity was calculated under the variational approach. The matrix effects surrounding the CSQDs structure, the quantum dot size, the impurity, and how the applied external electric field affects the impurity binding energy (Eb), polarizability (Pa) and photoionization cross-section (PICS) are calculated and discussed in detail. The obtained results revealed that the Eb, the PICS and Pa are not only affected by the quantum parameter size but also by the capped matrix, the impurity and external electric field EF actions.

On the usage of anomalous SAXS to analyze the structure and composition of bimetallic nanoparticles and quantum dots

On the usage of anomalous SAXS to analyze the structure and composition of bimetallic nanoparticles and quantum dots