Dots Of Different Sizes Research Articles

Onion-like multicolor thermally activated delayed fluorescent carbon quantum dots for efficient electroluminescent light-emitting diodes

Carbon quantum dots are emerging as promising nanomaterials for next-generation displays. The elaborate structural design is crucial for achieving thermally activated delayed fluorescence, particularly for improving external quantum efficiency of electroluminescent light-emitting diodes. Here, we report the synthesis of onion-like multicolor thermally activated delayed fluorescence carbon quantum dots with quantum yields of 42.3–61.0%. Structural, spectroscopic characterization and computational studies reveal that onion-like structures assembled from monomer carbon quantum dots of different sizes account for the decreased singlet-triplet energy gap, thereby achieving efficient multicolor thermally activated delayed fluorescence. The devices exhibit maximum luminances of 3785–7550 cd m−2 and maximum external quantum efficiency of 6.0–9.9%. Importantly, owing to the weak van der Waals interactions and adequate solution processability, flexible devices with a maximum luminance of 2554 cd m−2 are realized. These findings facilitate the development of high-performance carbon quantum dots-based electroluminescent light-emitting diodes that are promising for practical applications.

Polarized photoluminescence and g-factor in an inhomogeneous ensemble of quantum dots in magnetic fields

In this work, polarized photoluminescence caused by quasi-equilibrium spin orientation of excitons at Zeeman sublevels in an ensemble of quantum dots of different sizes is studied theoretically. The following unexpected results are found: (i) the splitting of photoluminescence bands in a magnetic field in an ensemble of quantum dots is much larger than the Zeeman splitting of exciton levels in a single dot, (ii) contrary to the Boltzmann distribution the low-energy luminescence band has a lower intensity than the high-energy band, (iii) the sign of the circular polarization of photoluminescence changes along the contour of the bands, (iv) a universal formula for the dependence of the exciton g-factor on the quantum dot size is obtained. All of the above features of the polarized luminescence spectra are explained by variations of the exciton g-factor when its quantization energy changes. The results obtained are applicable not only to ensembles of quantum dots, but also to any ensemble of localized states: excitons in quantum wells under conditions of well-width fluctuations, impurity states in quantum wells, etc.

Preparing and properties of films with Ag2S quantum dots in a polyvinyl alcohol matrix

Stable colloidal solutions of Ag2S quantum dots with a size of 2–3 to 28–30 nm were synthesized by a hydrochemical deposition in low-concentration water solutions of silver nitrate, sodium sulfide, and sodium citrate. A large absolute value of the measured zeta potential of colloidal solutions, a small change in the size of quantum dots and zeta potential during long-term storage of solutions for 1000 days after synthesis indicate their high temporal stability. The synthesized colloidal solutions were dispersed in a solution of polyvinyl alcohol. The resulting films of Ag2S quantum dots in a PVA matrix are flexible and have a thickness of no more than 0.03 mm. The use of the PVA matrix to prepare films with Ag2S quantum dots promotes the preservation and increase in the stability of colloidal particles due to their fixation in the polymer matrix. For the first time, the optical properties of Ag2S quantum dots of different sizes in PVA films were determined. Decreasing the size of Ag2S quantum dots to 2.6 nm leads to an increase in the band gap of silver sulfide to 1.85 eV. It is shown that repeated bending of the films has practically no effect on the Eg value of Ag2S quantum dots.

Tracing spatial confinement in semiconductor quantum dots by high-order harmonic generation

We report here on results of systematic experimental-theoretical investigation of high-order harmonic generation (HHG) in layers of CdSe semiconductor quantum dots of different sizes and a reference bulk CdSe thin film. We observe a strong decrease in the efficiency, up to complete suppression of HHG with energies of quanta above the band gap for the smallest dots, whereas the intensity of below band gap harmonics remains weakly affected by the dot size. In addition, it is observed that the suppression of the above gap harmonics is enhanced with increasing the driving wavelength. These systematic investigations allow us to develop a simple physical picture explaining the observed suppression of the highest harmonics: the discretization of electronic energy levels seems to be not the predominant contribution to the observed suppression but rather the confined dot size itself, causing field-driven electrons to scatter off the dot's walls. The reduction in the dot size below the classical electron oscillatory radius and the corresponding scattering limits the maximum acceleration by the laser field. Moreover, this scattering leads to a chaotization of motion, causing dephasing and a loss of coherence, therefore suppressing the efficiency of the emission of highest-order harmonics. Our results demonstrate a regime of intense laser-nanoscale solid interaction, intermediate between the bulk and single-molecule response, and are crucial for nanophotonic platforms aiming at control over high-order harmonic properties and efficiency.

Dye Enhanced Quantum Dot Sensitized Solar Cell

Third generation solar cells, such as dye and quantum dot sensitized solar cells are attracting attention of many research groups. In this investigation we explore the role of dye extract on the enhancement of the photovoltaic properties of quantum dot sensitized solar cells (QDSSCs). Lead sulfide quantum dots of different sizes have been used to sensitize a nanostructured titanium oxide electrode. We compare the open circuit voltage, the short circuit current, and power conversion efficiency of QDSSCs with the same quantities as obtained for dye-enhanced system using pomegranate dye extract. An open circuit voltage of 166 mV, a short circuit current of 0.1 mA, and an efficiency of 0.32% were obtained for 2.4 nm radius lead sulfide quantum dot sensitized solar cells. Using pomegranate dye extract reveals a considerable enhancement of the above characteristics. The combined dye-quantum dot system produces an open circuit voltage of 300 mV, a short circuit current of 0.55 mA, and an efficiency of 3.4%. For 3.2 nm radius QDs, the efficiency is substantially higher, reaching about 7%. A model describing the structure and processes leading to the above enhancement of the assembled solar cell characteristics is presented. In this model we explain the interplay between the transfer of electrons between the dye, quantum dots, and subsequent injection in the wide band gap titanium dioxide semiconductor.

Core-shell type-II spherical quantum dot under externally applied electric field

This study presents a simple one-band model within the effective mass approximation to describe the electric field impact on the energy structure and interband electron quantum transitions in type-II CdSe/ZnTe and ZnTe/CdSe spherical quantum dots. The dependencies of the oscillation strength and the light absorption coefficient on the externally applied electric field are calculated by the diagonalization method for spherical quantum dots of different sizes. It is shown that in the absence of an electric field and due to the spherical symmetry of nanosystem, only quantum transitions with Δl=0 (1se−1sh, 1pe−1ph, 1de−1dh, 1se−2sh,...) are allowed. The oscillator strength of quantum transitions between electron and hole excited states is greater than between the ground states. The electric field breaks the spherical symmetry, therefore, the oscillator strength of the 1se−1ph, 1se−1ph, 1se−1dh,... quantum transitions, which are forbidden for spherical symmetric systems, increase when electric field strength grows. Under electric field effects, the oscillator strength of allowed Δl=0 quantum transitions decreases. Finally, we observed that the absorption peaks in the quantum dot spectra under electric field influence are shifted to the low-energy region.

Electronic, Optical, and Magnetic Properties of Doped Triangular MoS2 Quantum Dots: A Density Functional Theory Approach

A theoretical investigation on electronic states in triangular MoS2 quantum dots of different sizes is performed within density functional theory first‐principles formalism. Herein, the associated interband optical response from real and imaginary parts of the dielectric function is calculated. The study considers both undoped and Al‐, Si‐, and P‐doped systems. Spin‐related magnetic properties are considered through the evaluation of total magnetic moment. It is revealed that small enough structures have a semiconductor character, but evolve to seemingly half‐metallic with increasing dot size. Nonzero magnetic response is attributed to edge effects. Magnetic behavior of doped systems deviates from the linear dependence of the total magnetic moment with size occurring in the undoped case, producing situations with higher total momentum values. It is found that increasing the doping density may have certain influence over total magnetic moment response of the structures, although edge dominance is kept all the way. Optical properties are not as sensitive to incident light polarization, but they actually do with regard to the particular spin orientation. Calculation for the refraction index in the triangular quantum dots shows values still below the accepted in the MoS2 monolayer case, although a correct trend of variation is reported in this sense.

FlexDotPlot: a universal and modular dot plot visualization tool for complex multifaceted data.

Dot plots are heatmap-like charts that provide a compact way to simultaneously display two quantitative information by means of dots of different sizes and colors. Despite the popularity of this visualization method, particularly in single-cell RNA-sequencing (scRNA-seq) studies, existing tools used to make dot plots are limited in terms of functionality and usability. We developed FlexDotPlot, an R package for generating dot plots from multifaceted data, including scRNA-seq data. It provides a universal and easy-to-use solution with a high versatility. An interactive R Shiny application is also available allowing non-R users to easily generate dot plots with several tunable parameters. Source code and detailed manual are available on CRAN (stable version) and at https://github.com/Simon-Leonard/FlexDotPlot (development version). Code to reproduce figures is available at https://github.com/Simon-Leonard/FlexDotPlot_paper. A Shiny app is available as a stand-alone application within the package. Supplementary data are available at Bioinformatics Advances online.

Unravelling the ultrafast charge dynamics in PbS quantum dots through resonant Auger mapping of the sulfur K-edge.

There is a great fundamental interest in charge dynamics of PbS quantum dots, as they are promising for application in photovoltaics and other optoelectronic devices. The ultrafast charge transport is intriguing, offering insight into the mechanism of electron tunneling processes within the material. In this study, we investigated the charge transfer times of PbS quantum dots of different sizes and non-quantized PbS reference materials by comparing the propensity of localized or delocalized decays of sulfur 1s core hole states excited by X-rays. We show that charge transfer times in PbS quantum dots decrease with excitation energy and are similar at high excitation energy for quantum dots and non-quantized PbS. However, at low excitation energies a distinct difference in charge transfer time is observed with the fastest charge transfer in non-quantized PbS and the slowest in the smallest quantum dots. Our observations can be explained by iodide ligands on the quantum dots creating a barrier for charge transfer, which reduces the probability of interparticle transfer at low excitation energies. The probability of intraparticle charge transfer is limited by the density of available states which we describe according to a wave function in a quantum well model. The stronger quantum confinement effect in smaller PbS quantum dots is manifested as longer charge transfer times relative to the larger quantum dots at low excitation energies.

Low-order optical nonlinearities of PbS quantum dot liquids and films

We demonstrate the variations of the nonlinear optical parameters in the suspensions and thin films containing PbS quantum dots of different sizes (9 and 2.5 nm). We show that these colloidal quantum dots possess the thermal-lens induced negative nonlinear refraction and reverse saturable absorption in the case of the 532-nm, 10-ns probe pulses. The larger nonlinear absorption coefficients of 9-nm quantum dots compared with 2.5-nm species (1 × 10−7 and 8 × 10−9 cm W−1, respectively) in the case of 1064 nm, 10 ns probe pulses were determined. Similar relations between the nonlinear optical parameters of 9- and 2.5-nm species were maintained in the case of twice shorter wavelength (532 nm) of the probe pulses. The almost ten-fold growth of the nonlinear optical parameters in the visible range compared with the near infrared region was attributed to the stronger influence of the excitonic states on the propagation effects in the former case. We show that the thin films containing these quantum dots demonstrate saturable absorption, while the suspensions show mostly the reverse saturable absorption at 532 nm. A significant enhancement of the nonlinear refractive index of films compared with suspensions (−2 × 10−8 cm2 W−1 and -1 × 10−11 cm2 W−1), as well as a large negative nonlinear absorption coefficient (−3.2 × 10−5 cm W−1) in the former case are demonstrated. We discuss the signs of nonlinear refraction and nonlinear absorption of the quantum dots, as well as the role of the sizes of these species in variations of the nonlinear parameters.

Giant "Breathing" Proteinosomes with Jellyfish-like Property.

The design and construction of "breathing" self-assemblies has been a rather popular topic due to their potential in materials science and nanotechnology. Inspired by the "breathing" behavior of natural jellyfish, herein, we presented the construction of a giant "breathing" proteinosome through an interfacial self-assembly of proteins and surfactants at the oil/water interface of emulsions: The proteinosome displays "breathing" behavior and can swell and shrink for multiple cycles by protein folding and unfolding through the alternate addition and removal of denaturant; more importantly, when green fluorescent proteins were selected as alternative protein building blocks, the fluorescence of proteinosome can be reversibly switched on/off just like the behavior of jellyfish. Moreover, accompanied by reversible swelling and shrinking and on/off fluorescence, the expanded and shrunk membrane pore can be tuned for distinguishing quantum dots of different sizes. The folding-responsive breathing behavior of intelligent proteinosomes provides a platform for functional biomaterials.

Growth Habits of Bismuth Selenide (Bi2Se3) Layers and Nanowires over Stranski–Krastanov Indium Arsenide Quantum Dots

Bismuth selenide layers and nanowires have been grown by molecular beam epitaxy on self-assembled Stranski–Krastanov InAs quantum dots of different sizes and densities on GaAs substrates. The size ...

Spectroscopy of PbS and PbSe Quantum Dots in Fluorine Phosphate Glasses

Transmission spectra of the narrow-bandgap semiconductor quantum dots PbS and PbSe in the fluorine phosphate glass are experimentally studied at different temperatures. Energies of 1s exciton transitions observed in the absorption spectra are used to determine characteristic sizes of the quantum dots under study. A nontrivial temperature behavior of the ground and excited quantum confined states in the quantum dots of different sizes are observed.

Spectroscopy of PbS and PbSe quantum dots in fluorine phosphate glasses

AbstractTransmission spectra of the narrow-bandgap semiconductor quantum dots PbS and PbSe in the fluorine phosphate glass are experimentally studied at different temperatures. Energies of 1s exciton transitions observed in the absorption spectra are used to determine characteristic sizes of the quantum dots under study. A nontrivial temperature behavior of the ground and excited quantum confined states in the quantum dots of different sizes are observed.

Photoluminescence of Lead Sulfide Quantum Dots of Different Sizes in a Nanoporous Silicate Glass Matrix

The optical properties of lead sulfide quantum dots (QDs) of different sizes embedded in a nanoporous silicate glass matrix (NSM) are investigated by steady-state and transient photoluminescence spectroscopy. The use of this matrix allows the fabrication of samples with reproducible optical characteristics, for both isolated and close-packed QDs. Low-temperature PL analysis of isolated QDs with sizes of 3.7 and 4.5 nm shows that the coefficient of temperature shift of the PL position changes sign with reducing QD size because of size-dependent contributions from thermal expansion, mechanical strain, and electron–phonon coupling. The PL intensity is determined by size-dependent splitting of the lowest energy electronic state.

Sure I'm Sure: Prefrontal Oscillations Support Metacognitive Monitoring of Decision Making.

Monitoring and control of our decision process (metacognition) is a crucial aspect of adaptive decision making. Crucially, metacognitive skills enable us to adjust ongoing behavior and determine future decision making when immediate feedback is not available. In the present study, we constructed a "diagnosis task" that allowed us to assess in what way first-order task performance and metacognition are related to each other. Results demonstrate that the contribution of sensory evidence (size, color, and motion direction) differs between first- and second-order decision making. Further, our results indicate that metacognitive performance specifically is orchestrated by means of prefrontal theta oscillations. Together, our findings suggest a hierarchical model of metacognition.

Highly stable and efficient solid-state solar cells based on methylammonium lead bromide (CH3NH3PbBr3) perovskite quantum dots

Highly stable and efficient solid-state solar cells have been made based on perovskite quantum dots of methylammonium lead bromide (MAPbBr3). Sawanta Mali, Chang Su Shim and Chang Kook Hong of Chonnam National University in South Korea fabricated highly crystalline MAPbBr3 quantum dots by an ex situ process using dimethyl sulphoxide as a solvent and then examined their performance in perovskite solar cells having two different hole-transporting materials. They also produced quantum dots of different sizes by varying the solution processing conditions and found that the solar-cell performance depended significantly on the quantum dot size. The team obtained a maximum conversion efficiency of 11.46% when poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] was used as the hole-transporting material and MAPbBr3 quantum dots with diameters smaller than three nanometres were used. They found that the solar cells were stable over a four-month period.

Two Photon Absorption in II-VI Semiconductors: The Influence of Dimensionality and Size.

We report a comprehensive study on the two-photon absorption cross sections of colloidal CdSe nanoplatelets, -rods, and -dots of different sizes by the means of z-scan and two-photon excitation spectroscopy. Platelets combine large particle volumes with ultra strong confinement. In contrast to weakly confined nanocrystals, the TPA cross sections of CdSe nanoplatelets scale superlinearly with volume (V(∼2)) and show ten times more efficient two-photon absorption than nanorods or dots. This unexpectedly strong shape dependence goes well beyond the effect of local fields. The larger the particles' aspect ratio, the greater is the confinement related electronic contribution to the increased two-photon absorption. Both electronic confinement and local field effects favor the platelets and make them unique two-photon absorbers with outstanding cross sections of up to 10(7) GM, the largest ever reported for (colloidal) semiconductor nanocrystals and ideally suited for two-photon imaging and nonlinear optoelectronics. The obtained results are confirmed by two independent techniques as well as a new self-referencing method.

Charge-noise tolerant exchange gates of singlet-triplet qubits in asymmetric double quantum dots

In the semi-conductor double quantum dot singlet-triplet qubit architecture, the decoherence caused by the qubit's charge environment poses a serious obstacle in the way towards large scale quantum computing. The effects of the charge decoherence can be mitigated by operating the qubit in the so called sweet spot regions where it is insensitive to electrical noise. In this paper, we propose singlet-triplet qubits based on two quantum dots of different sizes. Such asymmetric double dot systems allow the implementation of exchange gates with controllable exchange splitting $J$ operated in the doubly occupied charge region of the larger dot, where the qubit has high resilience to charge noise. In the larger dot, $J$ can be quenched to a value smaller than the intra-dot tunneling using magnetic fields, while the smaller dot and its larger splitting can be used in the projective readout of the qubit.

Self-organization of colloidal PbS quantum dots into highly ordered superlattices.

X-ray structural analysis, together with steady-state and transient optical spectroscopy, is used for studying the morphology and optical properties of quantum dot superlattices (QDSLs) formed on glass substrates by the self-organization of PbS quantum dots with a variety of surface ligands. The diameter of the PbS QDs varies from 2.8 to 8.9 nm. The QDSL's period is proportional to the dot diameter, increasing slightly with dot size due to the increase in ligand layer thickness. Removal of the ligands has a number of effects on the morphology of QDSLs formed from the dots of different sizes: for small QDs the reduction in the amount of ligands obstructs the self-organization process, impairing the ordering of the QDSLs, while for large QDs the ordering of the superlattice structure is improved, with an interdot distance as low as 0.4 nm allowing rapid charge carrier transport through the QDSLs. QDSL formation does not induce significant changes to the absorption and photoluminescence spectra of the QDs. However, the luminescence decay time is reduced dramatically, due to the appearance of nonradiative relaxation channels.