Resonance energy transfer in a dense array of II–VI quantum dots

Förster resonance energy transfer in inhomogeneous dense arrays of epitaxial CdSe/ZnSe quantum dots is demonstrated by time- and space-resolved photoluminescence spectroscopy. The specific feature of this process is the dipole–dipole interaction between the ground exciton levels of small quantum dots and the excited levels of large dots. This interaction brings efficient energy collection and spectral selection of a limited number of emitters. Results of theoretical modeling of optical transitions in spheroidal quantum dots with a Gaussian potential profile agree with the observed features of optical spectra induced by the change of the dominant energy transfer mechanism.     

Self-consistent inhomogeneity of quantum wells in II–VI semiconductors

An X-ray diffraction method that uses a slightly diverging (3′) beam and maximally attainable diffraction angles ? _B (as large as 77°) was developed to study quantum wells (QWs) with widths of 5–8 nm separated by wide (100–220 nm) barrier layers. The advantage of this method compared to the use of a parallel beam is an increase by two orders of magnitude in the intensity of the beam incident on the sample and an increase in the probability of diffraction for all QWs as a unified single crystal. It is found that the growth on GaAs substrates misoriented by 10° from the (001) plane in the [111]_II direction brings about monoclinization of crystal lattices of the QW layers and barrier layers in opposite directions. Inhomogeneity of composition over the thickness of each well is observed. In the case of growth of a ZnSe/ZnMgSSe structure in which the layers have a crystal-lattice period close to the lattice period of the GaAs substrate, the QWs are inhomogeneously doped with elements from the composition of the barrier layers. The inhomogeneity of QW composition observed in the growth of mismatched layers in ZnCdSe/ZnSSe and ZnCdS/ZnSSe structures is caused by the fact that mismatch between the lattice parameters of QWs and barriers stimulates the growth of self-consistent compositions; this occurs due to a decrease in the Cd concentration in the Zn_1?x Cd _x Se QW in the initial stages of growth compared to the Cd concentration in the flow of gases and an increase in the Zn concentration in the Cd_1?x Zn _x S QW at small values of x up to the concentration matching GaAs (x = 0.4). The mismatch stresses are partially relaxed via dislocations with the (111)_II glide planes, as a result of which is observed the combination of rotation of the crystal planes of the layers and QW around the [1\(\overline 1 \)0] axis and almost cylindrical bending of the entire sample around the perpendicular [110] axis. Mismatch between lattice parameters of the ZnMgSSe barrier layers and the substrate brings about decomposition of these layers into two phases; this decomposition is caused by thermodynamic instability of the alloy.     

Confined Franz–Keldysh effect in ZnO quantum dots

Within the framework of the effective mass approximation, the confined Franz–Keldysh effect is investigated theoretically in a cylindrical ZnO quantum dot (QD). Numerical results show that the application of an electric field can decrease the strength and the threshold energy of the optical absorption coefficient in ZnO QD. There are additional oscillations in the absorption above the effective band gap, which are due to the Franz–Keldysh effect which occurs in the presence of the electric field. Our results also show that the electric field has a more obviously influence on the optical absorption in cylindrical ZnO QD with larger dot height.     

Nanostructures with Ge–Si quantum dots for infrared photodetectors

In this paper questions of optimization of growth conditions in the method of molecular beam epitaxy for creation of high-efficient quantum dot infrared photodetectors are considered. As a model material system for theoretical investigations, heterostructures with germanium-silicon quantum dots on the silicon surface are chosen. For calculations of the dependencies of quantum dots array parameters on synthesis conditions the kinetic model of growth of differently shaped quantum dots based on the general nucleation theory is proposed. The theory is improved by taking into account the change in free energy of nucleation of an island due to the formation of additional edges of islands and due to the dependence of surface energies of facets of quantum dots on the thickness of a 2D wetting layer during the Stranski–Krastanow growth. Calculations of noise and signal characteristics of infrared photodetectors based on heterostructures with quantum dots of germanium on silicon are done. Dark current in such structures caused by thermal emission and barrier tunneling of carriers, as well as detectivity of the photodetector in the approximation of limitation by generation-recombination noises are estimated. Moreover, the presence of dispersion of quantum dots by size is taken into account in the calculations of the generation-recombination noises. Results of calculations of the properties of structures with quantum dots and their dependencies on growth parameters, as well as the characteristics of quantum dot photodetectors are presented. Comparison of the estimated parameters of quantum dots ensembles and the characteristics of quantum dot photodetectors with experimental data is carried out.     

Pressure coefficients of the photoluminescence of the II–VI semiconducting quantum dots grown by molecular beam epitaxy

Luminescence properties of CdTe and CdSe quantum dots have been studied under high hydrostatic pressure. The luminescence pressure coefficients of the II–VI quantum dots appear to be very similar to the pressure coefficients of the band-gap of bulk CdTe and CdSe, respectively. In contrary to that, the luminescence pressure coefficients of the III–V quantum dots are significantly lower than pressure coefficients of energy gaps of the appropriate dot materials. The discrepancy can be explained by the theoretical model, which takes into account effects of strain on pressure coefficients in thin strained layers. The experimentally observed pressure-induced quenching of the QDs luminescence is attributed to the “zinc-blende–cinnabar” phase transition in CdTe QDs and to the “zinc-blende–rock-salt” phase transition in CdSe QDs.     

Thermopower in parallel double quantum dots with Rashba spin-orbit interaction

Based on the Green’s function technique and the equation of motion approach,this paper theoretically studies the thermoelectric effect in parallel coupled double quantum dots (DQDs),in which Rashba spin-orbit interaction is taken into account.Rashba spin-orbit interaction contributions,even in a magnetic field,are exhibited obviously in the double quantum dots system for the thermoelectric effect.The periodic oscillation of thermopower can be controlled by tunning the Rashba spin-orbit interaction induced phase.The interesting spin-dependent thermoelectric effects will arise which has important influence on thermoelectric properties of the studied system.     

Magnetization of interacting electrons in anisotropic quantum dots with Rashba spin–orbit interaction

Magnetization of anisotropic quantum dots in the presence of the Rashba spin–orbit interaction has been studied for three and four interacting electrons in the dot for non-zero values of the applied magnetic field. We observe unique behaviors of magnetization that are direct reflections of the anisotropy and the spin–orbit interaction parameters independently or concurrently. In particular, there are saw-tooth structures in the magnetic field dependence of the magnetization, as caused by the electron–electron interaction, that are strongly modified in the presence of large anisotropy and high strength of the spin–orbit interactions. We also report the temperature dependence of magnetization that indicates the temperature beyond which these structures due to the interactions disappear. Additionally, we found the emergence of a weak sawtooth structure in magnetization for three electrons in the high anisotropy and large spin–orbit interaction limit that was explained as a result of merging of two low-energy curves when the level spacings evolve with increasing values of the anisotropy and the spin–orbit interaction strength.     

Direct spin–phonon coupling of spin-flip relaxation in quantum dots

Within the frame of the Pavlov–Firsov spin–phonon coupling model, we study the spin-flip assisted by the acoustical phonon scattering between the first-excited state and the ground state in quantum dots. We analyze the behaviors of the spin relaxation rates as a function of an external magnetic field and lateral radius of quantum dot. The different trends of the relaxation rates depending on the magnetic field and lateral radius are obtained, which may serve as a channel to distinguish the relaxation processes and thus control the spin state effectively.     

Optical investigation of CdS quantum dots in Langmuir–Blodgett films

Structures with CdS quantum dots produced by the Langmuir–Blodgett (LB) technique were investigated by Raman, IR, and UV spectroscopies. The confinement effect of longitudinal optical (LO) phonons in CdS quantum dots was investigated by Raman spectroscopy. Surface vibrational modes of CdS quantum dots were observed in IR spectra. It was shown experimentally that the frequency of the surface vibrational modes depends on the properties of the surrounding media. An average size of CdS quantum dots of about 3–6.4 nm was obtained from the analysis of UV measurements. Received: 1 February 1999 / Accepted: 1 April 1999 / Published online: 19 May 1999     

Size effects of an exciton–acceptor complex in quantum dots

In this study, a detailed investigation of the size effects of an exciton–acceptor complex in a disc-like quantum dot has been carried out by using the matrix diagonalization method and the compact density-matrix approach. We calculate the binding energy and the oscillator strength of intersubband quantum transition from the ground state into the first excited state as a function of the dot radius. Based on the computed energies and wave functions, the linear, third-order and total optical absorption coefficients as well as the refractive index have been examined between the ground and the first excited states. We find that the all absorption spectra and refractive index changes are strongly affected by the quantum dot size. However, for two cases of a smaller dot and a larger dot, the results of quantum size effects on the optical absorptions are opposite.     

Quantum entanglement transition in vertically coupledtwo single-electron quantum dots with charged impurity

Effects of a charged impurity on the ground state of two vertically coupled identical single-electron quantum dots with and without applied magnetic field are investigated. In the absence of the magnetic field, the investigations of the charged impurity effect on the quantum entanglement (QE) in some low-lying states are carried out. It is found that, both the positive charged impurity (PCI) and the negative charged impurity (NCI)reduce the QE in the low-lying states under oonsideration except that the QE in the ground state is enhanced by the NCI. Additionally, in the domain of B from 0 Tesla to 15 Tesla, the ground state energy E, the ground state angular momentum L and the ground state QE entropy S are worked out. As far as the ground state are concerned, the PCI (NCI) blocks (induces) the angular momentum phase transition and the QE phase transition besides the known fact (i. e., the PCI/NCI decreases/increases the energy) in the magnetic field.     

Capacitive coupling induced Kondo–Fano interference in side-coupled double quantum dots

We report capacitive coupling induced Kondo–Fano(K–F) interference in a double quantum dot(DQD) by systematically investigating its low-temperature properties on the basis of hierarchical equations of motion evaluations. We show that the interdot capacitive coupling U12 splits the singly-occupied(S-O) state in quantum dot 1(QD1) into three quasi-particle substates: the unshifted S-O0 substate, and elevated S-O1 and S-O2. As U12 increases, S-O2 and S-O1 successively cross through the Kondo resonance state at the Fermi level(ω = 0), resulting in the so-called Kondo-I(KI), K–F, and Kondo-II(KII) regimes. While both the KI and KII regimes have the conventional Kondo resonance properties, remarkable Kondo–Fano interference features are shown in the K–F regime. In the view of scattering, we propose that the phase shift η(ω)is suitable for analysis of the Kondo–Fano interference. We present a general approach for calculating η(ω) and applying it to the DQD in the K–F regime where the two maxima of η(ω = 0) characterize the interferences between the Kondo resonance state and S-O2 and S-O1 substates, respectively.     

Control of electron spin–orbit anisotropy in pyramidal InAs quantum dots

We investigate the electron spin–orbit interaction anisotropy of pyramidal InAs quantum dots using a fully three-dimensional Hamiltonian. The dependence of the spin–orbit interaction strength on the orientation of externally applied in-plane magnetic fields is consistent with recent experiments, and it can be explained from the interplay between Rashba and Dresselhaus spin–orbit terms in dots with asymmetric confinement. Based on this, we propose manipulating the dot composition and height as efficient means for controlling the spin–orbit anisotropy.     

Spin-polarized zero-energy states in BN/C core–shell quantum dots

Boron-nitride (BN) domains in graphene or graphene domains in BN monolayer offer additional freedoms for tuning the electronic properties of these BN/C nanostructures, which is quite crucial for the applications in nanoscale devices. Based on first-principles calculations combined with a simple Hubbard model, we show that the electron zero-energy states (ZESs) of BN/graphene core–shell quantum dots (QDs) in triangular shapes can be well tuned by varying the size and topology of each domain. The net spin of the systems is dominated by the graphene segment which can be described by a Lieb?s theorem. We also demonstrated that a π-electron Hubbard model within a mean-field approximation is implementable in dealing with the electron spin-polarization of BN/C hetero-structured graphene-like materials. This provides an efficient theoretical approach for the BN/C systems where electron spin-polarization is involved.     

Quantum entanglement transition in vertically coupled two single-electron quantum dots with charged impurity

Effects of a charged impurity on the ground state of two vertically coupled identical single-electron quantum dots with and without applied magnetic field are investigated. In the absence of the magnetic field, the investigations of the charged impurity effect on the quantum entanglement (QE) in some low-lying states are carried out. It is found that, both the positive charged impurity (PCI) and the negative charged impurity (NCI)reduce the QE in the low-lying states under consideration except that the QE in the ground state is enhanced by the NCI. Additionally, in the domain of B from 0 Tesla to 15 Tesla, the ground state energy E, the ground state angular momentum L and the ground state QE entropy S are worked out. As far as the ground state are concerned, the PCI (NCI) blocks (induces) the angular momentum phase transition and the QE phase transition besides the known fact (i. e., the PCI/NCI decreases/increases the energy) in the magnetic field.     

Control of excitons in a bent bunch of molecular aggregates by dipole–dipole interaction with quantum dots

The nonlocal dipole–dipole interaction is studied between excitations in chromophores forming a bunch or a tube of J-aggregates and closely spaced quantum dots (QDs). Equations describing the evolution of exciton pulses in a quasi-one-dimensional medium are derived taking into account the interaction with the transition resonant to nanoparticles. It is shown that the efficient controllable resonance energy transfer can occur in the system between QDs and an exciton pulse. The efficiency of this process significantly increases if the bunch of aggregates is deformed to bend nanoparticles round. It is shown that the interaction of permanent dipole moments of QDs and chromophores leads to the formation of a potential barrier or a well. It is found that the combined influence of these factors can be used to efficiently control the dynamics of pulses in aggregates.     

Magnetic tuning of exciton–photon resonance in II–VI microcavities

We demonstrate the effectiveness of the giant Zeeman effect in II–VI semimagnetic semiconductors to tune the exciton resonance of quantum wells onto the Fabry–Pérot resonance of a microcavity. A large oscillator strength of 3 × 10¹³cm⁻²per quantum well is deduced from the measured 10.6 meV vacuum Rabi splitting.