Spherical Core-shell Quantum Dots Research Articles

Impurity states in non-concentric spherical core-shell quantum dot

It was suggested the model of the non-concentric spherical core-shell quantum dot with hydrogenic impurity. It has been defined that the electron energy spectrum as a function of both an impurity and the core location. The splitting and degeneration of energy levels has been discussed. It is shown that displacing the core of a quantum dot or impurity in opposite directions can lead to alternate splitting and degeneracy of the energy levels of excited states. In certain configurations of arrangements, compensatory effects associated with the partial restoration of spherical symmetry in the field are observed.

Electron energy spectrum of the non-concentric spherical core-shell quantum dot

It was suggested the model of the spherical core-shell quantum dot with shifted core from common center. In the paper the effective mass approximation has been used and the Schrödinger equation has been solved by using the plane wave method. The electron energy spectra have been defined as a function of the both core radius and shell radius in the case of shifted core from the spherical center. Also, the dependence of the electron energy spectrum on the core/shell position has been calculated. The splitting of electron energy levels has been analyzed in the cases of different core position. That is why electron energy transitions will be different in comparison to the case of a no-shifted core.

Interband optical absorption in wurtzite GaN/InxGa1−xN/GaN spherical quantum dots with built-in electric field

Based on the principle of density matrix and the finite element method, the interband optical absorption between the electron and hole has been investigated in a wurtzite [Formula: see text] spherical core–shell quantum dot (CSQD) including a strong built-in electric field (BEF). We have studied the effects of the size and the ternary mixed crystal on the optical absorption coefficients (ACs) and refraction index changes (RICs). The results indicate that the absorption peaks of ACs and RICs decrease rapidly, and show a redshift with the increase of the component [Formula: see text]. It is also found that the absorption peaks of ACs and RICs reduce obviously and depend on the core radius and the well width. When the core radius increases, the positions of the maximum ACs and RICs show a blueshift. At the same time, it presents a redshift when the well width increases. Particularly, the influence of the well width is much stronger than the core radius in the wurtzite [Formula: see text] spherical CSQDs. We hope that these results could provide guidance on both theoretical and experimental study related to the optical properties of spherical CSQDs.

Donor binding energies in a spherical core–shell quantum dot: parabolic and shifted parabolic shell potentials

The effects of parabolic and shifted parabolic potentials on centred hydrogenic impurity-related binding energies are studied. This is theoretically achieved by solving the Schrodinger equation within the effective mass approximation. The hydrogenic impurity situates in the core of a spatially constant electrostatic potential, surrounded by a shell material which has either an intrinsic parabolic potential or shifted parabolic potential. For a given outer radius of the shell, the parabolic potential enhances binding energies (with greater enhancement for small radii of the core), while the shifted parabolic potential dwindles binding energies for small cores, and increases the binding energies for larger cores. An increase in the spatially invariant potential in the core results in reduced binding energies, with appreciable decrement occurring in core–shell structures that have wider cores.

Excitonic nonlinear optical properties in AlN/GaN spherical core/shell quantum dots under pressure

Abstract

Effects of ternary mixed crystals on interface/surface optical phonon in spherical core-shell quantum dots

Within the framework of the dielectric continuum approach and modified random-element-isodisplacement model, the optical vibration mode in a spherical core-shell quantum dot (CSQD) consisting of ternary mixed crystals (TMCs) are investigated. The dispersion relation and electron–phonon interaction Hamiltonian are derived. As a typical case, the numerical results for [Formula: see text] and [Formula: see text] CSQDs are obtained and discussed. Taking the one- and two-mode behaviors of TMCs into account, the effects of TMCs on interface/surface optical (IO/SO) phonon show that there are 3 and 5 branches of IO/SO phonon modes in [Formula: see text] and [Formula: see text] CSQDs for a given component of TMC, respectively. It is also found that the IO/SO phonon frequencies and electron–phonon interactions are strongly dependent on the component of TMCs and the size of CSQDs. We hope this work would be useful for the study of the phonon-related photoelectric properties in CSQDs consisting of TMCs.

Pressure effect on an exciton in a wurtzite AlN/GaN/AlN spherical core/shell quantum dot

Abstract

Impact of spherical nanoparticles on nematic order parameters.

We study experimentally the impact of spherical nanoparticles on the orientational order parameters of a host nematic liquid crystal. We use spherical core-shell quantum dots that are surface functionalized to promote homeotropic anchoring on their interface with the liquid crystal host. We show experimentally that the orientational order may be strongly affected by the presence of spherical nanoparticles even at low concentrations. The orientational order of the composite system is probed by means of polarized micro-Raman spectroscopy and by optical birefringence measurements as function of temperature and concentration. Our data show that the orientational order depends on the concentration in a nonlinear way, and the existence of a crossover concentration χ_{c}≈0.004pw. It separates two different regimes exhibiting pure-liquid crystal like (χ<χ_{c}) and distorted-nematic ordering (χ>χ_{c}), respectively. In the latter phase the degree of ordering is lower with respect to the pure-liquid crystal nematic phase.

The effect of magnetic field and donor impurity on electron spectrum in spherical core-shell quantum dot

The effect of homogeneous magnetic field and location of donor impurity on the electron energy spectrum and distribution of its probability density in spherical core-shell quantum dot is investigated. In the framework of the effective mass approximation and rectangular infinitely deep potential well, the solutions of the Schrodinger equation are found using the matrix method. The wave functions are expanded over the complete set of exact functions obtained without the magnetic field and impurity.It is shown that when the induction of magnetic field increases, the ground state of electron in the nanostructure without impurity or on-center impurity is successively formed by the states with m = 0, −1, −2, … (Aharonov-Bohm effect). When donor impurity is located in the shell of the nanostructure the Aharonov-Bohm effect vanishes.The dependences of electron energy spectrum and its wave functions on the location of impurity, placed along the direction of magnetic field or perpendicularly to it, are studied. It is shown, that in the first case, the quantum states are characterized by the certain value of magnetic quantum number m and the expansion contains the wave functions of the states with it only. In the second case, the cylindrical symmetry of the problem is broken and the new quantum states are formed from the states with different values of all three quantum numbers n, l, m and electron energy spectrum weakly depends on the magnetic field induction.

Intersubband optical absorption between multi energy levels of electrons in InGaN/GaN spherical core-shell quantum dots

The intersubband optical absorption between multi energy levels of electrons in InxGa1-xN/GaN spherical core-shell quantum dots (CSQDs) and ternary mixed crystal and size effects have been investigated by using the principle of density matrix. Electronic eigenstates under the effect of built-in electric field (BEF) have been calculated by a finite element method. The results show that optical absorption between intersubbands with main quantum numbers n = 1 and n = 2 are as important as that between ones with n = 1 and different angular quantum numbers when the BEF is taken into account. In consideration of BEF, the saturation of total optical absorption coefficients (ACs) and secondary peaks of refractive index changes (RICs) appear when incident light intensity I surpasses a certain value. For a given I, the maximum ACs and zero RICs positions in InxGa1-xN/GaN CSQDs with a fixed shell size have a blue-shift when x increases or the core InxGa1-xN radius R1 decreases from 5 nm. However, when R1 > 5 nm, ACs and RICs tend to be stable. The results indicate that effective adjustment of ACs and RICs in CSQDs with BEFs by size is in a limited scale range. The saturation of ACs or secondary peaks of RICs appear more likely in CSQDs with smaller x or larger R1. These results are expected to be helpful both in the further theoretical and experimental study on optic devices consisting of CSQDs.

Effect of confining potential shape on energy levels, binding energy and diamagnetic susceptibility of a spherical core–shell quantum dot

In this paper, we have studied the effect of confining potential shape on the energy levels, binding energy and diamagnetic susceptibility of a spherical core–shell quantum dot. For this goal, we have used a finite element method to solve the single electron Schrodinger equation in the effective mass approximation with and without impurity in the center. Also we have calculated the energy levels and wave functions, biding energy and diamagnetic susceptibility as a function of core and shell radius. We have found that the effect of confinement potential shape on the energy levels and wave functions, binding energy and diamagnetic susceptibility is considerable.

OPTICAL VIBRATION MODES IN SPHERICAL CORE-SHELL QUANTUM DOTS

Using a dielectric continuum approach, the optical vibration modes in a spherical core-shell quantum dots (QDs) imbedded in a host nonpolar material are studied. The dispersion relation and the corresponding electron–phonon interaction Hamiltonian are derived. The numerical calculations for the CdSe/ZnS system are performed. The results reveal that there are three branches frequencies of interface/surface optical phonon in the system. A detailed discussion of the combined effects of the spatial confinement and dielectric mismatch between the dot and the host medium is given.

Analytical approach for type-II semiconductor spherical core–shell quantum dots heterostructures with wide band gaps

A one-band model within the effective mass approximation is adopted to characterize the energy structure and oscillator strength of type-II semiconductor spherical core–shell quantum dots. The heteroepitaxial strain of the core–shell heterostructure is modeled by the elastic continuum approach. The model is applied to ZnTe/ZnSe core–shell, a wide band gap type-II heterostructure. The simulated absorption spectra are in fair agreement with available experimental results.

Electronic and optical properties of CaCO3 calcite, and excitons in Si@CaCO3 and CaCO3@SiO2 core-shell quantum dots

Density functional theory calculations of the electronic and optical properties of the CaCO3 calcite polymorph were performed within both the local density (LDA) and generalized gradient (GGA) approximations, respectively. The carriers effective masses are estimated, and the energy gap is shown to be indirect, with and (for comparison, the experimental value is 6.0 ± 0.35 eV). Two optical absorption regimes are predicted, and the dielectric function does not change considerably with the light polarization. The confinement features of excitons in Si@CaCO3 and CaCO3@SiO2 spherical core-shell quantum dots are also presented.

Third-order nonlinear optical susceptibilities associated with intersubband transitions in CdSe/ZnS core–shell quantum dots

The nonlinear susceptibilities associated with intersubband transitions in the conduction band are theoretically calculated for CdSe/ZnS spherical core–shell quantum dots. The confined wave functions and eigenenergies of electrons in quantum dots have been calculated under the effective-mass approximation by solving a three-dimensional Schrödinger equation. And the third-order susceptibilities for quadratic electro–optic effects as a function of core, shell radii, relaxation time and pump photo energy involving the position of energy levels also have been analyzed.