Mixture Of Quantum Dots Research Articles

Covalent Linking Greatly Enhances Photoinduced Electron Transfer in Fullerene-Quantum Dot Nanocomposites: Time-Domain Ab Initio Study.

Nonadiabatic molecular dynamics combined with time-domain density functional theory are used to study electron transfer (ET) from a CdSe quantum dot (QD) to the C60 fullerene, occurring in several types of hybrid organic/inorganic nanocomposites. By unveiling the time dependence of the ET process, we show that covalent bonding between the QD and C60 is particularly important to ensure ultrafast transmission of the excited electron from the QD photon-harvester to the C60 electron acceptor. Despite the close proximity of the donor and acceptor species provided by direct van der Waals contact, it leads to a notably weaker QD-C60 interaction than a lengthy molecular bridge. We show that the ET rate in a nonbonded mixture of QDs and C60 can be enhanced by doping. The photoinduced ET is promoted primarily by mid- and low-frequency vibrations. The study establishes the basic design principles for enhancing photoinduced charge separation in nanoscale light harvesting materials.

White Hybrid Light-Emitting Devices Based on the Emission of Thermal Annealed Ternary CdSe/ZnS Quantum Dots

We report on a white hybrid light-emitting device employing a mixture of ternary CdSe/ZnS quantum dots (QDs) as an emitting layer (EML) and a small molecular material tris(8-hydroxyquinoline) aluminum (Alq3) as an electron-transporting layer. The film morphology of the spin-coated mixture of QDs is strongly improved via thermal annealing, and therefore a close-packed QD-EML is realized between organic charge-transporting layers. As a result, compared to the device with an unannealed QD-EML, the emission of Alq3 is deeply suppressed. In addition, a maximum luminance of more than 1000 cd/m2 and a maximum luminous efficiency of 2.2 cd/A are achieved.

Fluorescence-profile pre-definable quantum-dot barcodes in liquid-core microcapsules

Microparticles incorporated with quantum-dot (QD) barcodes for multiplexed bioassays attract a great attention due to their potential applications in drug discovery, gene profiling and clinic diagnostics. However, the existing QD barcodes lack a necessary optical stability to ambient fluids or a repeatability of fluorescent profiles. We developed a new QD barcode by loading an aqueous QD mixture as a liquid core into a monodispersed polymer microcapsule by a microfluidic method to avoid those problems. We found that the QDs in the liquid cores were able to maintain their original characteristics, especially the linear relation between photoluminescence intensity and concentration. In addition, we found that the fluorescent profiles of the QD-loaded liquid cores were the same as those of the QD mixtures before being loaded inside the microcapsules. With these two properties, the QD barcodes can be predefined directly by multiplexing the emission peaks and concentrations of the QDs in the liquid cores. Furthermore, the graphical information from fluorescent images of the microcapsules, such as the sizes and numbers of the QD-loaded liquid cores, offers another dimension for barcoding to increase the coding capacity. We also presented a microfluidic method to manufacture the QD-barcoded microcapsules of size ~30 μm. These pre-definable QD barcodes with stable fluorescent profiles can be used as a platform for various high-throughput screening applications in different bioassay buffers.

Effects of nanoscale quantum dots in male Chinese loaches (Misgurnus anguillicaudatus): Estrogenic interference action, toxicokinetics and oxidative stress

Quantum dots (QDs) have more and more attention as a novel example of nanocrystals due to their unique fluorescent characteristics. Recently, the toxicity and the potential environmental effects of QDs have become a research hotspot. In this work, in vivo endocrine disrupting effect, toxicokinetics and oxidative stress of QDs were characterized following the intraperitoneal dosing in Chinese loaches. Vitellogenin (Vtg) levels induced by E2 decreased significantly when administrated with the mixture of QDs and E2, which was consistent with the observations of histopathology in testes. The release of free Cd2+ from QDs and the non-specific adsorption of E2 by QDs might be the joint factors contributing to the inhibition of Vtg expression induced by E2 in the male Chinese loaches. In the muscle, bone, intestines, blood and testis, CdSe QDs reached the maximal concentration (C max) in approximately 1-h postinjection and subsequently presented downtrend with the prolonged time. Whereas, there were even increasing tendencies of CdSe QDs’ concentrations in the liver and kidney. It is educible that CdSe QDs can be persistent at least for 7 days, indicating the overall half-life of CdSe QDs in the fish body is very long. The measurement of hepatic superoxide dismutase (SOD) activity and reduced glutathione (GSH) content indicate that QDs have smaller effects on the antioxidative system of the organisms compared with free Cd2+ due to the effective prevention of the release of Cd by PEG coating of QDs. The comprehensive evaluation of QDs’ toxicity in the present study provides an essential and general framework towards more focused research on the elucidation of the biological effects of QDs in vivo.

Optimal mixture of randomly dispersed quantum dots for optical excitation transfer via optical near-field interactions

We theoretically and experimentally investigated a system composed of a mixture of different-sized quantum dots involving optical near-field interactions to effectively induce optical excitation transfer. We demonstrated that the ratio of the number of smaller quantum dots to larger ones can be optimized using a density-matrix formalism so that excitons generated in the smaller ones are efficiently transferred to the larger ones. We also describe experimental demonstrations based on a mixture of 2 nm- and 2.8 nm-diameter CdSe/ZnS quantum dots dispersed on the surface of a silicon photodiode, where the increase in induced photocurrents due to optical excitation transfer is maximized at a certain quantum dot mixture which agrees with theoretical calculations.

Binary metal and semiconductor quantum dot metamaterials with negative optical dielectric constant and compensated loss for small volume waveguides, modulators and switches

We study numerically and analytically a binary mixture of quantum dots exhibiting gain and loss. For a mixture of gain quantum dots and silver nanoparticles, we find conditions when the composite shows negative dielectric constant operation and lossless operation. The composites of this kind may be used for dense integration of photonic components as well as modulation and switching in optical interconnect systems

A metal-wire/quantum-dot composite metamaterial with negative ε and compensated optical loss

Numerical simulations of a binary mixture of quantum dots exhibiting gain with silver nanorods are performed, showing the feasibility of lossless negative ε operation for realistic material structures and parameters.

Semiconductor quantum dot mixture as a lossless negative dielectric constant optical material

Prospects for a lossless negative dielectric constant material for optical devices are studied. Simulations show that, with sufficient gain, a mixture of two semiconductor quantum dots can produce an isotropic effective dielectric constant that is lossless and negative. Both analytical homogenizations based on the work of Maxwell Garnett and frequency-dependent dielectric constants from inversion of numerical computations of scattered fields are used to establish the dielectric constants of a mixture of gain and loss dots. Over length scales where homogenization is meaningful, this material could be used to achieve lossless ``metal-insulator'' optical waveguides and hence a small optical mode volume.

White light emitting diodes realized by using an active packaging method with CdSe/ZnSquantum dots dispersed in photosensitive epoxy resins

White light emitting diodes (LEDs) have been realized using the active packaging(AP) method. The starting materials were bare InGaN LED chips and CdSe/ZnScore–shell quantum dots (QDs) dispersed in photosensitive epoxy resins. Suchhybrid LED devices were fabricated using QD mixtures with one (‘single’), two(‘dual’) or four (‘multi’) emission wavelengths. The AP method allows forconvenient adjustment of multiple parameters such as the CIE-1931 coordinate(x,y),color temperature, and color rending index (CRI). All samples show good white balance, and undera 20 mA working current the luminous efficacies of the single, dual, and multi hybrid devices were8.1 lm W−1,5.1 lm W−1,and 6.4 lm W−1, respectively. The corresponding quantum efficiencies were 4.1%, 3.1%, and 3.1%; the CRIswere 21.46, 43.76, and 66.20; and the color temperatures were 12 000, 8190, and 7740 K.This shows that the CRI of the samples can be enhanced by broadening the QD emissionband, as is exemplified by the 21.46 CRI of the single hybrid LED compared to the 66.20value for the multi hybrid LED. In addition, we were able to increase the CRI of the singlehybrid LED from 15.31 to 32.50 by increasing the working currents from 1 to 50 mA.

Temperature-sensitive microcapsules with variable optical signatures based on incorporation of quantum dots into a highly biocompatible hydrogel

Biocompatible fluorescent microcapsules were constructed by incorporating CdSe quantum dots into an ionotropic hydrogel. The hydrogel formulation used has been shown to be highly resistant to chelators present in biological media, and the resulting composite shows no leakage of entrained quantum dots over test periods of up to 4 months. The optical signature of the microcapsules is controlled by incorporating mixtures of quantum dots having different sizes into the composite, such that microcapsule composites exhibit Forster resonance energy transfer (average estimated Forster radius of 77 A). The resulting microcapsules also display so-called smart behavior, as the optical signature displays a linear dependence on temperature over the 20–45 °C range of interest to biological experiments.

Enhancement of photoluminescence efficiency in binary quantum dot arrays in hybrid organic/inorganic materials

The photoluminescence (PL) quantum efficiency of dense semiconductor colloidal quantum dot (QD) arrays is significantly reduced by the quenching of the optical excitons via defect-induced nonradiative decay channels. This luminescence quenching is facilitated by the rapid migration of excitons between neighboring QDs via resonant Förster transfer processes. We propose to mitigate this quenching by using nonradiative “spacer” quantum dots (SQDs) to separate radiative primary quantum dots (PQDs). We have identified the maximum and minimum values of the PL quantum efficiency (PLQE) for different compositions of spacer-primary QD arrays. Using the kinetic Monte Carlo technique, we have found that for a given composition, the PLQE is highest for randomly distributed spacer and primary QDs, decreasing dramatically when clusters of SQDs and PQDs are formed. We have modeled cluster formation of QDs in binary QD arrays and determined the resultant PL properties. By comparing our simulation results with those we obtained experimentally, we have estimated the magnitude of the attractive pairwise van der Waals interaction between the QDs which is the driving force for QD cluster formation. We have also explored possible strategies to achieve the highest PLQE of the PQDs in hybrid organic/inorganic materials for photoelectronics and we found that lowering the mixing time of the binary QD mixture can improve the PLQE.

Spectral Bar Coding of Polystyrene Microbeads Using Multicolored Quantum Dots

This paper focuses on encoding polystyrene microbeads, 10-100 microm in diameter, with a luminescent spectral bar code composed of mixtures of quantum dots (QDs) emitting at different wavelengths (colors). The QDs are encapsulated in the bead interior during the bead synthesis using a suspension polymerization, and the bar code is constructed by varying both the number of colors included in the bead and, for each color, the number of QDs of that color. Confocal laser scanning microscopy images of the beads demonstrate that the multicolored QDs are pushed together into inclusions within the bead interior. The encoded bead emission spectrum indicates that the peak position of the included colors does not shift relative to the corresponding peaks of the spectra recorded for the nonaggregated QDs at identical loading concentrations. Due to the spatial proximity of the QDs in the inclusions, electronic energy transfer from the lower wavelength emitting QDs to the higher emitting QDs changes the relative intensities of the colors compared to the values in the nonaggregated spectra. We show that this energy transfer does not obscure the spectral uniqueness of the different codes. Ratiometric encoding, in which the bar code is read as relative color intensity, is shown to remove the dependence of the code on the bead size.

Optical Properties of Fluorescent Mixtures: Comparing Quantum Dots to Organic Dyes

Using eye catching demos and the size-dependent properties of quantum dots (QDs) we can appeal to a broad range of students, from high school and general chemistry students to those in more advanced chemistry and physics courses. This experiment is easily tailored to meet the requirements of the instructor and equipment availability. Visually observed fluorescent colors of QDs in conjunction with spectroscopic data show students the additive emission of QD mixtures. Although some mixtures appear to emit white light, their fluorescence peaks remain spectroscopically resolved; this is not possible using organic dyes. More advanced topics on inorganic synthesis and energy transfer can be discussed for students of higher levels.

Correlation of Photoluminescence and Optical Absorption Spectra of Porous Silicon

Using a quantum confinement based-PL model, PS was modelled as a mixture of Quantum Dots (QDs) and Quantum Wires (QWs) having different concentrations and sizes. It was shown that in the optical absorption edge the PL peak energy and the Optical Absorption (OA) exhibit the same trend, depending on preparation conditions. The spectral behaviours of PL and OA are analysed and correlated throughout the shapes and the size distribution of the nanocrystallites forming PS. Using the quantum confinement formalism, the value of the effective band-gap energy determined from the lowest PL energy almost corresponds to that estimated from the optical absorption coefficient. These results suggest that the lowest radiative transition between the valence band and the conduction band corresponds to the largest luminescent wires, and that the radiative recombination process leading to the PL emission occurs in the c-Si crystallite core.

Origin of the photoluminescence shifts in porous silicon

The origin of the photoluminescence (PL) shifts in Porous Silicon (PS) is discussed according to a quantum confinement – based model, in which we modelize the PS layer as a mixture of quantum dots and wires. It was shown that a PL blueshift or redshift may occur during laser irradiation of PS, depending on preparation conditions. No PL shift was observed for some PS samples, even after a long ageing in air, due to the presence of an amorphous silicon phase detected from Raman spectroscopy measurements. It was found that the presence of the amorphous phase plays an important role in the PL behaviour of oxidised PS.