Absorption Spectra Of Quantum Dots Research Articles

Investigation of band parameters and electrochemical analysis of multi core-shell CdSe/CdS/ZnS quantum dots

Highly emissive core−multi-shell (CdSe/CdS/ZnS) quantum dots (QDs) within size range of ∼ 2 nm–5 nm were prepared via hot injection method. Capping agent (polymers) were used to increase the lifetime of QDs, without a protective atmosphere. Red shift was observed in an absorption spectrum of QDs peaks at 362 nm, 365 nm, 558 nm, and 556 nm for CdSe/CdS and CdSe/CdS/ZnS with respect to CdSe core. Voltammetry measurements (Eg ∼ 2. 09 eV) for band gaps were in good agreement with the UV–visible spectroscopic data (Eg ∼ 2.65 eV). QDs exhibited three distinguished peaks comparing to (111), (220), and (311) planes of cubic structures of CdSe, CdSe/CdS and CdSe/CdS/ZnS within crystallite size of 20–27 nm as observed by XRD spectra. A larger active surface area 0.06 cm−2, computed by CV increased the catalytic activity of sensors by exposing the electroactive sites. Linear, nonlinear optical susceptibility and absorption coefficients were computed by an effective mass model. Quantum confinement was confirmed by taking electron probability density as function of radial distance of quantum dots.

Polarization-dependent intervalence band absorption in quantum dots

In this work, we compute the electronic band structure of ZnCdSe/ZnS semiconductor quantum dots (QDs) using k. p method, and studied the dependence of QDs parameters on the in-plane polarization anisotropy and degree of polarization (DOP) of intersubband absorption spectra of QDs. Higher energy levels in the valence band show nonlinear dependence on the parabolic confinement strength and exhibit strong anti-crossing. We find that the optical transitions for specific polarization directions are defined by the size, strain, and shape anisotropy of the QDs. We observe that increasing the shape anisotropy, strain, and QDs radius enhanced the in-plane polarization anisotropy, whereas increasing QDs height reduces the in-plane polarization anisotropy. The DOP is found to increase with increasing QDs radius and strain and reducing the shape anisotropy and QDs height. Furthermore, the QDs parameters play a key role in deciding the absorption peak of the specific polarization. The present study suggests that the combination of shape, size, and strain could provide an optimal approach to tune the intraband polarization response from QDs.

Enhancing the FRET by tuning the bandgap of acceptor ternary ZnCdS quantum dots.

In this article, we report the band gap tuning of ternary ZnCdS quantum dots (QDs) by varying the concentration of the capping ligand, mercaptoacetic acid (MAA). The functionalization of QDs leads to the control of their size and band gap due to the quantum confinement effect, causing blue shift in the absorption and photoluminescence (PL) spectra with a gradual change in the concentration of the capping ligand from 0.5 to 2.5 M. Ensulizole (2-phenylbenzimidazole-5-sulfonic acid) is an important organic ultraviolet (UV) filter that is frequently used in sunscreen cosmetics. An effective overlapping of the PL spectrum of ensulizole and the absorption spectrum of QDs with 2.5 M MAA is achieved. A formidable decrease in the PL intensity and the PL lifetime of ensulizole promotes an efficient Förster resonance energy transfer (FRET) from sunscreen ensulizole to the QDs. The magnitude of the FRET efficiency (E) is ∼70%. This very high value of E is the signature of the existence of a very fast energy transfer process from ensulizole to the MAA functionalized ZnCdS QDs. The dyad system consisting of ZnCdS QDs and ensulizole sunscreen can serve as a prototype model to develop a better understanding of the photochemistry of ensulizole and consequently the formulation of more efficient sunscreen cosmetics.

Characterization of the Interfacial Structures of Core/Shell CdSe/ZnS QDs.

Core/shell quantum dots (QDs) have been extensively studied, yet their optical properties widely vary among studies. Such variation may arise from the variation in interfacial structures induced by the subtle difference in each synthetic procedure. Here, we studied the interfacial structures of CdSe/ZnS QDs using the time-of-flight medium energy ion-scattering spectroscopy (TOF-MEIS), which offers the radial elemental distributions as well as the overall elemental compositions of QDs. The TOF-MEIS spectra provided strong evidence for the existence of an alloyed layer at the interface between CdSe and ZnS in typical CdSe/ZnS QDs. On the basis of the emission and absorption spectra of QDs sampled during the synthesis, we conclude that such interfacial alloying is caused by the dissolution of CdSe seeds during the synthesis steps. Such a dissolution mechanism is further corroborated by the observation that the ligand environment of solvent (X or L type) leads to different shapes of interfaces.

Majorana fermions induced fast- and slow-light in a hybrid semiconducting nanowire/superconductor device

We investigate theoretically Rabi-like splitting and Fano resonance in absorption spectra of quantum dots (QDs) based on a hybrid QD-semiconducting nanowire/superconductor (SNW/SC) device mediated by Majorana fermions (MFs). Under the condition of pump on-resonance and off-resonance, the absorption spectrum experiences the conversion from Fano resonance to Rabi-like splitting in different parametric regimes. In addition, the Fano resonances are accompanied by the rapid normal phase dispersion, which will indicate the coherent optical propagation. The results indicate that the group velocity index is tunable with controlling the interaction between the QD and MFs, which can reach the conversion between the fast- and slow-light. Fano resonance will be another method to detect MFs and our research may indicate prospective applications in quantum information processing based on the hybrid QD-SNW/SC devices.

Artificial intelligence approach for calculating electronic and optical properties of nanocomposites

The optical and electronic properties of quantum dots (QDs) are critical points in the design a QD-based optoelectronic device that can be computed using numerical methods, for which surrogate-based optimization procedures can reduce the computational cost and time. In this paper, an artificial neural network-based Multi-Layer-Perceptron (ANN-MLP) approach is developed to calculate the optical and electronic properties of a hybrid nanocomposite. The predictive capability of the methodology was measured in terms of Mean Square Error (MSE), Relative Square Error (RSE), and Correlation Coefficient (R2) on the test dataset. The ANN-MLP model has produced the best RSE, MSE, and R2 values of 6.35 × 10−5, 6 × 10-6, and 0.9999 in predicting energy levels and 0.0116, 3.5 × 10-4, and 0.9887 for the absorption spectrums of QDs, respectively. The comparison of the results shows the high accuracy of the ANN-MLP as a practical algorithm for predicting energy levels and optical absorption spectrum of QDs.

Quantum dot clusters as self-assembled antennae with phycocyanine and phycobilisomes as energy acceptors.

In this study, we investigated an experimental and Monte-Carlo computational characterization of self-assembled antennae built using CdTe colloidal quantum dots (QDs). These clusters provide efficient excitation of phycocyanine (PC) or phycobilisomes (PBSs). PBSs are light-harvesting complexes (LHCs) of cyanobacteria, made of several PC units, organized in disks and rods. Each PC contains three separate cofactors. Therefore, we analyzed variations in multi-donor and multi-acceptor systems. The self-assembled QD clusters were formed mostly by electrostatic interactions, possibly due to the introduction of a positive charge on an originally negatively charged nanoparticle surface. Our results suggest that PC may accept energy from multiple nanoparticles localized at a distance significantly longer than the Förster radius. The excitation transfers between particular nanoparticles with possible delocalization. The maximal energy transfer efficiency was obtained for the PC/PBS : QD ratio from 1 to 20 depending on the QD size. This cannot be fully explained using computational simulations; hence, we discussed the hypothesis and explained the observations. Our self-assembled systems may be considered for possible applications in artificial light-harvesting systems because absorption spectra of QDs are different from the absorption characteristics of PC/PBS. In addition, huge clusters of QDs may effectively increase the optical cross-section of so-created nanohybrids.

Preparation and fluorescence characteristics of CdTe/CdS and CdTe/ZnS core-shell semiconductor quantum dots

Core-shell semiconductor quantum dots materials are being investigated due to their special performance in repairing surface defects for the single quantum dots and greatly improving the optical performance of quantum dots. Instead of a traditional small flask as a reaction vessel to prepare NaHTe, the preparation of the CdTe core using a three-necked flask was achieved. 10 CdTe/CdS and CdTe/ZnS core-shell semiconductor quantum dots with different core sizes, shell thicknesses and shell materials were synthesized. The UV–visible absorption and fluorescence spectrum of 10 kinds of core-shell semiconductor quantum dots materials were measured and analyzed. The absorption spectrum of quantum dots in the UV-visible band shows that with the increase of quantum dots size, the absorption peak is red-shifted. The CdTe/CdS quantum dots differ in fluorescence lifetime and intensity due to the conversion of different core and shell sizes of quantum dots between types I and II. When the shell thickness of CdTe/ZnS increases, the shell thickness of ZnS reduces the number of dangling bonds and defect states on the core surface, which increases the probability of electron-hole pairs recombination and causes the fluorescence peak red-shift.

Nonradiative Energy Transfer in “Colloidal Quantum Dot Nanocluster–Dye” Hybrid Nanostructures: Computer Experiment

A computer model of nonradiative electronic excitation energy transfer from nanoclusters of colloidal quantum dots of cadmium selenide to the meso-tetra(3-pyridyl)porphyrin molecule in hybrid nanostructures nanocluster–dye has been constructed and analyzed. The model uses the experimental luminescence and absorption spectra of quantum dots and the dye, takes into account heterogeneity of the properties of quantum dots in nanoclusters and the different location of the dye in the hybrid structure. It has been shown that, in the ideal case, due to such energy transfer, the intensity of the luminescent signal of the dye can be increased by five orders of magnitude. However, this value is significantly reduced due to nonluminescent particles, the presence of a protective ligand shell, a large size distribution of the particles, and the nonoptimal geometric structure of the hybrid system.

The Effect of CdSe and InP Quantum Dots on the Interaction of ZnO with NO2 under Visible Light Irradiation

Nanocomposites based on nanocrystalline ZnO and CdSe and InP nanocrystals (quantum dots) have been synthesized by chemical precipitation and high-temperature colloidal synthesis. The microstructure parameters of the oxide matrix and the size of the CdSe and InP nanocrystals have been determined. A correlation was established between the spectral dependence of the photoconductivity of nanocomposites and the optical absorption spectra of quantum dots. The influence of CdSe and InP quantum dots on the interaction of ZnO with NO2 under visible light irradiation has been studied. It has been shown that the synthesized nanocomposites can be used to detect NO2 under illumination with green light without additional thermal heating.

Nanophotosensitive drugs for light-based cancer therapy: what does the future hold?

Nanophotosensitive drugs for light-based cancer therapy: what does the future hold?

Temperature-Dependent Permeability of the Ligand Shell of PbS Quantum Dots Probed by Electron Transfer to Benzoquinone.

This paper describes an increase in the yield of collisionally gated photoinduced electron transfer (electron transfer events per collision) from oleate-capped PbS quantum dots (QDs) to benzoquinone (BQ) with increasing temperature (from 0 to 50 °C), due to increased permeability of the oleate adlayer of the QDs to BQ. The same changes in intermolecular structure of the adlayer that increase its permeability to BQ also increase its permeability to the solvent, toluene, resulting in a decrease in viscous drag and an apparent increase in the diffusion coefficient of the QDs, as measured by diffusion-ordered spectroscopy (DOSY) NMR. Comparison of NMR and transient absorption spectra of QDs capped with flexible oleate with those capped with rigid methylthiolate provides evidence that the temperature dependence of the permeability of the oleate ligand shell is due to formation of transient gaps in the adlayer through conformational fluctuations of the ligands.

Non-invasive assessment of cutaneous wound healing using fluorescent imaging.

Optical imaging is a very important technique in the biomedical sciences. The purpose of this study was to develop an in vivo optical system for fluorescent imaging and molecular imaging applications using quantum dots (QDs). The in vivo optical system was composed of modular parts, including a light source, light guide, excitation filter wheel, excitation filters, emission filter wheel, emission filters, liquid crystal tunable filter (LCTF), macro lens, dark chamber, and a cooled charged-coupled device (CCD) camera for recording images. Filters were selected based on the excitation and absorption spectra of QDs to allow spectral separation and optimization of the acquired image. In contrast with conventional systems, our system allows selection of the emission bandwidth. The system was tested in an in vivo study using a wound-healing model in nude mice. The healing process was examined after injection of fibroblasts and keratinocytes labeled with two different sets of QDs. The different QD probes were readily detected and distinguished using our system. An in vivo optical system is a very useful tool for the detection of genes, proteins, and small-molecule drugs inside living animals, and this imaging modality can also be adopted for real-time visualization of cancer cell metastasis in live animals.

Control of hot-carrier relaxation for realizing ideal quantum-dot intermediate-band solar cells

For intermediate-band solar cells, the broad absorption spectrum of quantum dots (QDs) offers a favorable conversion efficiency, and photocurrent generation via efficient two-step two-photon-absorption (TS-TPA) in QDs is essential for realizing high-performance solar cells. In the last decade, many works were dedicated to improve the TS-TPA efficiency by modifying the QD itself, however, the obtained results are far from the requirements for practical applications. To reveal the mechanisms behind the low TS-TPA efficiency in QDs, we report here on two- and three-beam photocurrent measurements of InAs quantum structures embedded in AlGaAs. Comparison of two- and three-beam photocurrent spectra obtained by subbandgap excitation reveals that the QD TS-TPA efficiency is improved significantly by suppressing the relaxation of hot TS-TPA carriers to unoccupied shallow InAs quantum structure states.

Tight-binding calculations of the optical properties of HgTe nanocrystals

We propose a tight-binding model of HgTe which gives an accurate band structure in a wide energy range compared to recent ab initio calculations. The inverted band structure near the Fermi level and its temperature dependence are also very well described. Using this parametrization, we study the effects of the quantum confinement on the electronic structure of HgTe quantum dot nanocrystals. We calculate the optical absorption spectra of quantum dots with various shapes and diameters up to 10 nm. We show, using a configuration interaction approach, that excitonic effects are negligible in this range of sizes. Our predictions for the size dependence of the energy gap and for the optical spectra are consistent with recent experimental data.

Optical absorption in semiconductor quantum dots coupling to dispersive phonons of infinite modes

Optical absorption spectrum of semiconductor quantum dot is investigated by means of an analytical approach based on the Green's function for different forms of coupling strength in an unified method by using the standard model with valence and conduction band levels coupled to dispersive quantum phonons of infinite modes. The analytical expression of the optical absorption coefficient in semiconductor quantum dots is obtained and by this expression the line shape and the peak position of the absorption spectrum are procured. The relation between the properties of absorption spectrum and the forms of coupling strength is clarified, which can be referenced for choosing the proper form of the coupling strength or spectral density to control the features of absorption spectrum of quantum dot. The coupling and confinement induced energy shift and intensity decrease in the absorption spectrum are determined precisely for a wide range of parameters. The results show that the activation energy of the optical absorption is reduced by the effect of exciton-phonon coupling and photons with lower frequencies could also be absorbed in absorption process. With increase of the coupling constant, the line shape of optical absorption spectrum broadens and the peak position moves to lower photon energy with a rapid decrease in intensity at the same time. Both the coupling induced red shift and the confinement induced blue shift conduce to decrease in the intensity of absorption spectrum. Furthermore, this method may have application potential to other confined quantum systems.

Quantum‐Dot‐Sensitized Solar Cells

Quantum-dot-sensitized solar cells (QDSCs) are a promising low-cost alternative to existing photovoltaic technologies such as crystalline silicon and thin inorganic films. The absorption spectrum of quantum dots (QDs) can be tailored by controlling their size, and QDs can be produced by low-cost methods. Nanostructures such as mesoporous films, nanorods, nanowires, nanotubes and nanosheets with high microscopic surface area, redox electrolytes and solid-state hole conductors are borrowed from standard dye-sensitized solar cells (DSCs) to fabricate electron conductor/QD monolayer/hole conductor junctions with high optical absorbance. Herein we focus on recent developments in the field of mono- and polydisperse QDSCs. Stability issues are adressed, coating methods are presented, performance is reviewed and special emphasis is given to the importance of energy-level alignment to increase the light to electric power conversion efficiency.

Theoretical and experimental studies of stressed nanoparticles of II-VI semiconductors

A theoretical and experimental study of isolated nanoparticles of II-VI semiconductor materials has been done. Using the framework of the effective mass model, the optical absorption spectrum of distributions of spherical quantum dots, freestanding, and under compressive or tensile stress, has been examined theoretically. The theoretical results allow one to foresee the absorption spectra of quantum dots made of a series of materials and having any size. The syntheses of colloidal quantum dots of CdS and CdSe has also been performed through wet chemical routes and characterized by means of optical techniques. The values of the strains in the synthesized quantum dots were inferred from a correlation established between the theoretical and the experimental results.

Design and analysis of a new structure of InAs/GaAs QDIP for 8–12 μm infrared windows with low dark current

Based on the effective mass Schrödinger equation and eight-band k.p method, we study optical absorption in the 8–12 µm infrared window through the conduction subbands of self-assembled InAs/GaAs pyramidal shape quantum dots (QDs) using the finite difference method. Considering homogeneous and inhomogeneous broadening effects, the absorption spectra of QDs are calculated. Regarding the simulation results, we design a QD infrared photodetector (QDIP) with a double barrier resonant tunnelling (DBRT) at λ = 10 µm. We calculate the responsivity, detectivity and dark current for the device, considering the coverage factor of QDs. To enhance the performance of the presented DBRT especially at near room temperature, we propose a modified QDIP with asymmetric multiple barrier resonant tunnelling structure. With the modified structure, the dark current reduces to about half an order of magnitude compared with DBRT–QDIP, and detectivity increases to 1.4 × 109 cm Hz1/2/W at 200 K.

An inhomogeneous theoretical model for analysis of PbSe quantum-dot-doped fiber amplifier

Optical fiber doped with lead salts quantum dots (QDs) of different radii, has previously been proposed as a wide-band amplifier in the optical communication range. The emission and absorption spectra of QDs are size-dependent. Optical fiber amplifier doped with QDs of different sizes is an inhomogeneous gain medium. This inhomogeneity is the principle of operation of wide-band quantum-dot-doped fiber amplifiers (QDFAs). To study the effects of inhomogeneity such as spectral hole burning on the amplifier behavior, a fiber amplifier with an arbitrary size distribution of QDs is taken into consideration. The light propagation equations and rate equations for the inhomogeneous medium are presented. To investigate the gain spectrum, the amplifier bandwidth and the spectral hole burning effect on the amplifier behavior, the system of governing equations for the steady-state regime of operation are solved and numerical results are presented.