Spherical Semiconductor Quantum Dots Research Articles

Correlating semiconductor nanoparticle architecture and applicability for the controlled encoding of luminescent polymer microparticles

Luminophore stained micro- and nanobeads made from organic polymers like polystyrene (PS) are broadly used in the life and material sciences as luminescent reporters, for bead-based assays, sensor arrays, printable barcodes, security inks, and the calibration of fluorescence microscopes and flow cytometers. Initially mostly prepared with organic dyes, meanwhile luminescent core/shell nanoparticles (NPs) like spherical semiconductor quantum dots (QDs) are increasingly employed for bead encoding. This is related to their narrower emission spectra, tuneability of emission color, broad wavelength excitability, and better photostability. However, correlations between particle architecture, morphology, and photoluminescence (PL) of the luminescent nanocrystals used for encoding and the optical properties of the NP-stained beads have been rarely explored. This encouraged us to perform a screening study on the incorporation of different types of luminescent core/shell semiconductor nanocrystals into polymer microparticles (PMPs) by a radical-induced polymerization reaction. Nanocrystals explored include CdSe/CdS QDs of varying CdS shell thickness, a CdSe/ZnS core/shell QD, CdSe/CdS quantum rods (QRs), and CdSe/CdS nanoplatelets (NPLs). Thereby, we focused on the applicability of these NPs for the polymerization synthesis approach used and quantified the preservation of the initial NP luminescence. The spectroscopic characterization of the resulting PMPs revealed the successful staining of the PMPs with luminescent CdSe/CdS QDs and CdSe/CdS NPLs. In contrast, usage of CdSe/CdS QRs and CdSe QDs with a ZnS shell did not yield luminescent PMPs. The results of this study provide new insights into structure–property relationships between NP stained PMPs and the initial luminescent NPs applied for staining and underline the importance of such studies for the performance optimization of NP-stained beads.

Polaronic Effects on the Electron Raman Scattering in a Spherical Anisotropic Quantum Dot

We study the polaronic effect on the differential cross section (DCS) for an electron Raman scattering process in a spherical anisotropic semiconductor quantum dot in the presence of a homogeneous external electric field. The Fröhlich coupling of electrons with confined and surface polar optical phonon modes for resonance Raman scattering is considered. The influence of electron-phonon coupling on the energy spectrum is taken into account in the framework of perturbation theory. The emission spectra are discussed for different anisotropy cases. Resonant peaks in the spectra are found and interpreted.

Gallium Antimonide Spherical Semiconductor Quantum Dots

The quantum effects at the nano-metric level have been observed in a variety of confined structures, particularly in semiconductor quantum dots. In this contribution, the electronic and optical properties of GaSb spherical semiconductor quantum dots are investigated. For the calculations, the pseudo potential approach was employed. The size dependence of the energy gaps at Г, X and L points, the effective masses of electrons and heavy-holes, the refractive index, and the dielectric function for a studied GaSb spherical quantum dot are analyzed and discussed. When the degree of quantum confinement effect was changed by decreasing the radius of the spherical quantum dots, a striking charge in comparison to the bulk values has been obtained. Our results indicate that as the quantum dot radius is raised, most of properties rapidly decrease. This demonstrates an improvement in the mobility of the material. However, the refractive index and the dielectric constant are increased with increasing the radius of the nano-crystal.

Spontaneous Emission of a Semiconductor Quantum Dot without the Dipole Approximation

The spontaneous emission of a spherical semiconductor quantum dot is considered without using the dipole approximation. It is shown that the dipole-forbidden radiation of a shallow potential well with one electron level becomes more intense than the dipole-permitted radiation of a deeper potential well and that the potential deepening makes the dipole approximation closer. It is also shown that the cylindrically symmetric anisotropic radiation can be obtained from a spherical quantum dot if the direction of the quantization axis remains unchanged for the acts of photon emission.

Light storage and retrieval in spherical semiconductor quantum dots with on-center hydrogen impurity in magnetic field

We investigate the light propagation through the medium consisted of spherical semiconductor quantum dots with an on-center hydrogen impurity under the regime of the electromagnetically induced transparency. The weak probe and strong control laser couple to the medium, forming the three-level ladder-type coupling scheme. Maxwell-Bloch equations are solved numerically and analytically to obtain the spatial and temporal dependence of the probe pulse envelope. We observed that the probe pulse can be stored to and retrieved from the medium, with storage times of the order of nanoseconds being obtained. The external static magnetic field is shown to affect the energy level structure of the hydrogen impurity, reshaping the output probe pulse. We demonstrated that exposing the medium to cryogenic temperatures significantly increases the storage efficiency and storage time. These phenomena can greatly contribute to quantum information processing , leading to the construction of optical buffers, optical switches and quantum memories. • Slow light with the group velocity reduced by a factor 15 is achieved. • Storage times of the order of nanoseconds at room temperatures are obtained. • Approximate analytic results accurately describe the storage efficiency. • External magnetic field reshapes the output pulse and acts as an optical switch. • At cryogenic temperatures, storage times are increased to hundreds of nanoseconds.

Quantum beat of excitons in spherical semiconductor quantum dots

Abstract This article reports a study on quantum beat of excitons in spherical semiconductor quantum dots. With renormalized wavefunction formulation for a three-level model including ground state level and two excited ones of exciton, quantum beat of excitons is observed to be twice the electron Rabi frequency while a strong resonant pump laser is connecting two exciton excited levels. In addition, dependences of characteristics of quantum beat of excitons on dot radius and pump laser detuning are examined.

Excitons in spherical quantum dots revisited: analysis of colloidal nanocrystals

The problem of exciton states in spherical semiconductor quantum dots is revisited, employing the finite element method to numerically solve the system of differential equations for the center of mass and relative motion of the interacting electron-hole pair. This process is performed within the effective mass and parabolic bands approximations. The use of a finite confinement together with a parabolic description of the conduction and valence band profiles prevents the two equations from uncoupling. The allowed energies are reported as functions of the quantum dot radius. A comparison of theoretically determined fundamental photoluminescence peak energies with available experimental reports in the cases of CdS, CdSe and CdTe is presented and discussed, showing a good agreement between calculated and measured results.

Electromagnetically induced grating with second field quantization in spherical semiconductor quantum dots

A new approach for diffracting the weak probe beam into higher-order directions is proposed via electromagnetically induced grating in second field quantization formalism, offering a new way for implementations of quantum information with semiconductor quantum dots. The formalism of second field quantization allows describing atoms and photons as a many-body system. An induced diffraction grating is formed based on the electromagnetic induced transparency when a standing-wave coupling field is applied to a spherical quantum dot as a three-level system. Due to phase modulation, the zeroth-order light intensity becomes weak, and the first-order diffraction is improved affectedly. On the contrary, the probe beam is barely diffracted via absorption modulation. The simulation results verify that photon numbers of probe and control fields, as well as other parameters in the QD, can lead to the diffraction efficiency of phase grating to be improved. Phase diffraction grating accompanied with a high transmissivity is demonstrated, and the first-order diffraction efficiency reaches $$30\%$$. Also, the impact of QD dimensions on its optical response is investigated. This model may find potential applications in designing the semiconductor quantum dot-based photonic devices in optical communications and quantum information networks.

The Effect of Relativistic Quantum Corrections on the Thermal Properties of Three-Dimensional Spherical Semiconductor Quantum Dot Under a Magnetic Field

The ground state and excited states energies of 2-electron quantum dot with spherical harmonic oscillator type potential has been determined. The energy spectrum and wave functions for the quantum dot of asymmetric confinement are obtained by analytically solving the eigenvalue equation in the magnetic field. The effect of various relativistic corrections to kinetic energy, Darwin term and spin-orbit for the zero-dimensional structure to the energy eigenvalues and wave functions is also investigated. The thermal properties like internal energy, entropy and free energy are discussed graphically with radius of quantum dot and pressure and are found to have interesting dependence on the radius and pressure parameter with relativistic corrections for our model.

A facile synthesis route for preparing aqueous colloidal CdS quantum dots with size-tunable optical properties

A simple one-pot route for the chemical synthesis of CdS colloidal quantum dots in aqueous medium was developed. In this wet chemical preparation, the anionic surfactant SDS (sodium dodecylsulfate) was used as the organic stabilizer added to the solution of chemical precursors containing the appropriate sources of the metallic cation (Cd2+) and the chalcogenide anion (S2-). The optical characterization was performed by means of ultraviolet-visible absorption and photoluminescence spectroscopy. Particle size distributions were obtained from the statistical analysis of atomic force microscopy (AFM) images generated for several quantum dot samples. The maximum of each AFM histogram was correlated to the size determined from absorption onset measurements combined with a well-established effective mass model for the size-dependent bandgap of spherical semiconductor quantum dots. An alternative theoretical model capable of providing improved size estimates was also proposed and implemented. From a simple experimental protocol based on the variation of the chalcogenide precursor concentration in the reaction medium, CdS quantum dots with size-tunable optical absorption and emission were synthesized.

Resonant electronic excitation energy transfer by exchange mechanism in the quantum dot system

A microscopic theory of nonradiative resonance energy transfer between spherical A3B5 semiconductor quantum dots by the exchange mechanism is suggested. The interdot Coulomb interaction is taken into consideration. It is assumed that the quantum dot-donor and the quantum dot-acceptor are made from the same A3B5 compound and are embedded in the matrix of another material that produces potential barriers for electrons and holes. The dependences of the energy transfer rate on the quantum-dot system parameters are found in the frame of the Kane model that provides the most adequate description of the real spectra of A3B5 semiconductors. The analytical treatment is carried out with using the density matrix method, which enabled us to perform an energy transfer analysis both in the weak-interaction approximation and in the strong-interaction approximation. The numerical calculations showed the saturation of the energy transfer rate at the distances between the donor and the acceptor approaching the contact one. The contributions of the exchange and direct Coulomb intractions can be of the same order at the small distances and can have the same value in the saturation range.

Optical manipulation of photo-induced current in spherical semiconductor quantum dots by optical vortices

In this work, we have investigated the existence of photo-induced current in spherically symmetric semiconductor quantum dots by orbital angular momentum carrying twisted light. For the quantum dot structure, the core and shell materials are and semiconductor, respectively. Because, the thickness of semiconductor layer is in the scale of nanometre, the structure has quantum mechanical properties. And, the total radius of quantum dot is comparable to the spot size of highly focused twisted laser beam. In that case, we investigate the effect of orbital angular momentum and radial node of twisted light on the induced current and magnetic field in the structure. Furthermore, we calculate dependence of induced current on frequency of the light. The results show that magnitude and direction of the induced current can be manipulated by tuning the parameters: radial node, orbital angular momentum, frequency of light and polarisation vector.

Electro-magnetic weak coupling optical polaron and temperature effect in quantum dot

We investigate the influence of the electric field and magnetic fields on the ground state energy of a polaron in a spherical semiconductor quantum dot (QD) using the modified Lee Low Pines (LLP) method. The numerical results show the increase of the ground state energy with the increase of the electric field and the electron–phonon coupling constant, and the decrease with the magnetic field and the longitudinal confinement length. It is also seen that the temperature is an increasing function of the cyclotron frequency and the coupling constant whereas it decreases with the electric field strength. The modulation of the electric field, the magnetic field and the confinement length leads to the control of decoherence in the system.

Energy levels of magneto-optical polaron in spherical quantum dot — Part 1: Strong coupling

We investigate the influence of a magnetic field on the ground state energy of a polaron in a spherical semiconductor quantum dot (QD) using the modified LLP method. The ground state energy is split into sub-energy levels and there is a degeneracy of energy levels. It is also observed that the degenerate energy increase with the electron–phonon coupling constant and decrease with the magnetic field. The numerical results show that, under the influence of magnetic field and the interaction with the total momentum along the z-direction, the split energy increases and decreases with the longitudinal and the transverse confinement length, respectively.

Electric and magnetic optical polaron in quantum dot—Part 1: strong coupling

We investigated the influence of electric field and magnetic field on the ground state energy of polaron in spherical semiconductor quantum dot (QD) using a modified Lee Low Pines (LLP) method. The numerical results show the increase of the ground state energy with the increase of the electric field and the decreasing with the magnetic field. The modulation of the electric field, magnetic field and the confinement lengths lead to the control of the decoherence of the system.

Energy and binding energy of donor impurity in quantum dot with Gaussian confinement

Using the method of the shifted 1/N expansion, we investigate the problem of hydrogenic-like donor impurity, located at the center of a spherical semiconductor quantum dot. We have calculated the energy eigenvalues for both ground and first excited sates under the assumption of Gaussian confining potential. The binding energies for three dimensional (3D) and two dimensional (2D) quantum dots are calculated. We show their dependence on dimensionality, dot radius and potential confinement. Our present numerical results show quantitative and qualitative very good agreement with those results obtained by diagonalization, Numerov’s integration, and Hartree–Fock methods.

Modified Lee-Low-Pines Polaron in Spherical Quantum Dot in an Electric Field

In this paper, we investigated the influence of e lectric field on the ground state energy of polaron in spherical semiconductor quantum dot (QD) using modified Lee Low Pines (LLP) method. The numerical results show the increase of the ground state energy with the increase of the electric field and the co nfinement lengths. The modulation of the electric and the confinement len gths lead to the control of the decoherence of the system.

Modified Lee-Low-Pines Polaron in Spherical Quantum Dot in an Electric Field. Part 2: Weak Coupling and Temperature Effect

In this paper, we investigate the influence of an electric field on the ground state energy of a polaron in a spherical semiconductor quantum dot (QD) using the modified Lee Low Pines (LLP) method. The numerical results show an increase of the ground state energy with the increase of the electric fi eld and the confinement lengths. The modulation of the electric and the confinement lengths enables the control of the decoherence of the system. It is also seen that the temperature is a decreasing function of the electron - phonon coupling constant and the longitudinal confinement length, whereas it increases with the electric field strength. Original Research Article

Energy Levels of Weak Coupling Magneto-Optical Polaron and Temperature Effect in Spherical Quantum Dot

In this paper, we investigate the influence of the magnetic field and the temperature on the ground state energy of a weak coupling polaron in a spherical semiconductor quantum dot (QD) using the modified LLP method. The ground state energy of a weak coupling magneto-optical polaron is split into sub-energy levels and there is the degeneracy of the energy levels. It is also seen that the split energies are increasing functions of the electron-phonon coupling constant and decreasing functions of the magnetic field while the temperature is an increasing function of the cyclotron frequency for very low values of the longitudinal confinement length.

The control of the nonlinear optical response of semiconductor quantum dots

In the present work, we investigate the nonlinear optical properties emerged from excitonic features in an experimentally realized spherical parabolic semiconductor quantum dot (QD). The lowest exciton states together with relevant wave functions are calculated through the expansion method with direct matrix diagonalization method within the effective mass approximation. The effect of the size of QD and confinement potential in exciton state is studied in details. Results show that with increasing the size of the QD the energy of exciton decreases because of decreasing of the effect of coulomb potential. Using the compact density matrix formalism second order nonlinear optical rectification (χ(2)) are obtained. By means of the applied electric and magnetic field we manipulate the exciton states and control the nonlinear optical response in a typical GaAs, InAs, CdSe QDs. Our model system presents a way to control the performance of excitonic optoelectronic devices based on semiconductor nanostructures.