Quantum Dot Radius Research Articles

Theoretical Analysis of Resonant Cavity p-Type Quantum Dot Infrared Photodetector

A theoretical analysis for resonant cavity enhanced p-type quantum dot (QD) infrared photo-detector that uses intervalence subband transitions in In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> As/GaAs QDs is presented. Multiband effective mass k.p model with the strain effect is used to calculate valance subband energy levels. Photocurrent spectra, response wavelength, and dark current density of QD infrared detector have been calculated. The calculations have been performed for a wide range of dot sizes, compositions, dot height, bias voltages, and temperatures. The effect of QD height, radius, and composition on the response of the photodetectors has been analyzed and some criteria for performance improvement have been suggested.

Optical properties of spherical quantum dot with modified Pöschl–Teller potential

In this paper the electronic and hole states in three regimes of size quantization in spherical quantum dot with modified Pöschl–Teller potential made of GaAs are studied. Energy spectrum is compared with the parabolic approximation case for the regime of strong size quantization. Analytical expressions for the particle energy spectrum, absorption coefficient and dependencies of effective threshold frequencies of absorption on the radius of quantum dot are obtained for strong and weak size quantization regimes. The problem solved in the framework of variation method for the intermediate size quantization regime. The selection rules corresponding to different transitions between quantum levels are found. The size dispersion distribution of growing quantum dots by the radius have been taken into account by three experimentally realizing distribution functions. Distribution functions of Gaussian, Lifshits–Slezov and Gamma have been considered.

Photoionization cross-section of donor impurity in spherical quantum dots under electric and intense laser fields

Using a perturbative method we have investigated the behavior of the binding energy and photoionization cross-section of a donor impurity in spherical GaAs–GaAlAs quantum dots under the influence of electric and intense high-frequency laser fields. The dependencies of the binding energy and photoionization cross-section on electric and laser field strength, dot radius and impurity position were investigated. Our results show that the amplitude of photoionization cross-section grows with the dot radius increase and the peak of the cross-section blue shifts with the laser intensity increment. We have found that the binding energy is not a monotonically function of laser intensity: it decreases or increases depending on electric field regime. The studied effects are even more pronounced as the quantum dot radius is smaller.

Theory of confined states of positronium in spherical and circular quantum dots with Kane’s dispersion law

Confined states of a positronium (Ps) in the spherical and circular quantum dots (QDs) are theoretically investigated in two size quantization regimes: strong and weak. Two-band approximation of Kane’s dispersion law and parabolic dispersion law of charge carriers are considered. It is shown that electron-positron pair instability is a consequence of dimensionality reduction, not of the size quantization. The binding energies for the Ps in circular and spherical QDs are calculated. The Ps formation dependence on the QD radius is studied.

Size and Composition Dependent Multiple Exciton Generation Efficiency in PbS, PbSe, and PbSxSe1–x Alloyed Quantum Dots

Using ultrafast transient absorption and time-resolved photoluminescence spectroscopies, we studied multiple exciton generation (MEG) in quantum dots (QDs) consisting of either PbSe, PbS, or a PbSxSe1-x alloy for various QD diameters with corresponding bandgaps (Eg) ranging from 0.6 to 1 eV. For each QD sample, we determine the MEG efficiency, ηMEG, defined in terms of the electron-hole pair creation energy (εeh) such that ηMEG = Eg/εeh. In previous reports, we found that ηMEG is about two times greater in PbSe QDs compared to bulk PbSe, however, little could be said about the QD-size dependence of MEG. In this study, we find for both PbS and PbSxSe1-x alloyed QDs that ηMEG decreases lineally with increasing QD diameter within the strong confinement regime. When the QD radius is normalized by a material-dependent characteristic radius, defined as the radius at which the electron-hole Coulomb and confinement energies are equivalent, PbSe, PbS, and PbSxSe1-x exhibit similar MEG behaviors. Our results suggest that MEG increases with quantum confinement, and we discuss the interplay between a size-dependent MEG rate versus hot exciton cooling.

Effects of electric field and size on the electron-phonon interaction in a spherical quantum dot with impurity

The effects of electric field and size on the electron-phonon interaction with an on-center impurity in a Zn1−xCdxSe/ZnSe spherical quantum dot are studied, taking into account the interactions with confined, half-space and surface optical phonons. In addition, the interaction between impurity and phonons has also been considered. The results show that the electron-confined, electron-half-space, and electron-surface optical phonon interaction energies are all negative. The electron-confined optical phonon interaction energy is weakened by the electric field, but the electron-half-space and electron-surface optical phonon interaction energies are strengthened by it. In particular, the electron-surface optical phonon interaction depends strongly on the electric field, and it will vanish when the electric field is absent. It is also found that the electron-confined optical phonon interaction and electron-impurity “exchange” interaction energies reach a peak values as the quantum dot radius increases and then gradually decrease, but the electron-half-space optical phonon interaction energy exponentially quickly approaches 0 as the quantum dot radius increases.

Spontaneous Multielectron Transfer from the Surfaces of PbS Quantum Dots to Tetracyanoquinodimethane

This paper describes an investigation of the interfacial chemistry that enables formation of a multielectron ground-state charge-transfer (CT) complex of oleate-coated PbS quantum dots (QDs) and tetracyanoquinodimethane (TCNQ) in CHCl3 dispersions. Thermodynamically spontaneous electron transfer occurs from sulfur ions on the surfaces of the QDs (radius = 1.6 nm) to adsorbed TCNQ molecules and creates indefinitely stable ion pairs that are characterized by steady-state visible and mid-infrared absorption spectroscopy of reduced TCNQ and by NMR spectroscopy of the protons of oleate ligands that coat the QDs. The combination of these techniques shows that (i) each QD reduces an average of 4.5 TCNQ molecules, (ii) every electron transfer event between the QD and TCNQ occurs at the QD surface, (iii) sulfur ions on the surfaces of the QDs (and not delocalized states within the QDs) are the electron donors, and (iv) some TCNQ molecules adsorb directly to the surface of the QDs while others adsorb upon displacement of oleate ligands.

Two dimensional electron gas mobility limited by scattering of quantum dots with indium composition transition region in quantum wells

A modified quantum dots (QDs) structure model is developed in this study. Compared with the traditional model, the gradual composition transition at the interface between the QDs and matrix is taken into account in the modified model. Two dimensional electron gas (2DEG) mobility limited by QDs scattering is studied based on the modified model. The result is compared with the one based on the traditional model. It is found that the contribution of the gradual composition transition region to the electron mobility is too significant to be negligible, especially in the case of high 2DEG density or small QDs radius.

Confinement of an exciton in a quantum dot: effect of modified Kratzer potential

In the present work, we have considered an exciton confined in a quantum dot with modified Kratzer potential. We have studied the electronic and optical properties of the system by using the numerical diagonalization of the Hamiltonian matrix. For this purpose, we have calculated the binding energies of the ground and first excited states as functions of the quantum dot size. We have also computed the linear, nonlinear and total absorption coefficients between ground and first excited states. It is found that the quantum dot radius has an important role on the binding energy and absorption coefficient.

UV–VIS spectroscopic study of one pot synthesized strontium oxide quantum dots

The properties of drastically change when matter makes transition from 1D, 2D, 3D, to 0D. The quantum dots (QDs) of strontium oxide (SrO) were synthesized by one pot chemical precipitation method using hexamethylenetetramine (HMT). The radius of SrO QDs was calculated from hyperbolic band model (HBM). The direct and indirect band gaps of SrO QDs were estimated from UV–VIS analysis. The particle size was found to be 2.48nm. The quantum confinement effect in SrO QDs is discussed through exciton Bohr radius. The particle size from UV–VIS analysis is in excellent agreement with fluorescence and TEM.

Achiral CdSe quantum dots exhibit optical activity in the visible region upon post-synthetic ligand exchange with d- or l-cysteine

Semiconductor cadmium selenide (CdSe) quantum dots (QDs) exhibited mirror-image circular dichroism (CD) spectra in the visible region (350-570 nm) after replacing the trioctylphosphine oxide/oleic acid ligands on achiral nanocrystals with D- and L-cysteines. Chiroptical properties of cysteine-capped CdSe QDs depend on their size and can be fine-tuned by changing the radius of QDs.

Growth of PbSe Quantum Dots Within High‐Index Lead‐Phosphate Glass for Infrared Saturable Absorbers

PbSe quantum dots (QDs) were grown in high‐refractive‐index low‐melting‐temperature lead‐phosphate glass. The lowest energy exciton transition of the QDs was tuned over a wide range within the infrared spectral region (0.93–2.75 μm) by a controlled heat treatment. The measured QD radius ranged between 2 and 5.3 nm, with a time (t) dependence of t0.29 for long dwelling times during the heat treatment, indicating that the QD growth mechanism tends to follow Lifshitz‐Slyozov‐Wagner theory. The QD saturable absorber behavior at 1.2 μm had a measured saturation fluence of ~2.1 μJ/cm2.

The effect of first order magnetic field in a GaAs/AlAs spherical quantum dot with hydrogenic impurity

In this research, the effect of the first order magnetic field on the ground-state of a centered hydrogenic donor impurity in a GaAs/AlAs spherical quantum dot has been calculated. The perturbation method has been used within the framework of effective mass approximation for these calculations. Overall, the analysis shows that a proper choice of quantum dot radius and magnetic field can significantly change the normalized binding energy. The concluded information may be used to calculate the small changes in quantum dot radius and thus detect different magnetic field strengths.

Use of modified Gaussian potential to study an exciton in a spherical quantum dot

In the present work, we have considered an exciton confined in a spherical quantum dot with the modified Gaussian potential (A. Gharaati, R. Khordad, Superlatt. Microstruct. 48 (2010) 276). We have tried to study the electronic and optical properties of the system by using the numerical diagonalization of the Hamiltonian matrix. For this purpose, we have calculated the binding energy of the ground and first excited states as a function of the quantum dot radius. We have also computed the linear, nonlinear and total absorption coefficient between the ground and the first excited states. It is found that the quantum dot radius has an important role on the binding energy and absorption coefficient.

Surfactant-Controlled Polymerization of Semiconductor Clusters to Quantum Dots through Competing Step-Growth and Living Chain-Growth Mechanisms

This article reports control of the competition between step-growth and living chain-growth polymerization mechanisms in the formation of cadmium chalcogenide colloidal quantum dots (QDs) from CdSe(S) clusters by varying the concentration of anionic surfactant in the synthetic reaction mixture. The growth of the particles proceeds by step-addition from initially nucleated clusters in the absence of excess phosphinic or carboxylic acids, which adsorb as their anionic conjugate bases, and proceeds indirectly by dissolution of clusters, and subsequent chain-addition of monomers to stable clusters (Ostwald ripening) in the presence of excess phosphinic or carboxylic acid. Fusion of clusters by step-growth polymerization is an explanation for the consistent observation of so-called "magic-sized" clusters in QD growth reactions. Living chain-addition (chain addition with no explicit termination step) produces QDs over a larger range of sizes with better size dispersity than step-addition. Tuning the molar ratio of surfactant to Se(2-)(S(2-)), the limiting ionic reagent, within the living chain-addition polymerization allows for stoichiometric control of QD radius without relying on reaction time.

Electric field effect on the nonlinear and linear intersubband absorption spectra in CdTe/ZnTe spherical quantum dot

Linear and nonlinear absorption spectra of neutral (D 0 ) hydrogenic impurity located at the center of the CdTe/ZnTe spherical quantum dot (QD) were investigated after assuming a spherically symmetric confining potential of finite depth. Calculations were performed under the effective mass approximation on the basis of exact solution of the Schrodinger and Poisson equations. Eigenfunctions were expressed in terms of the Whittaker and Coulomb wave functions. Results for D 0 impurity energies of ground 1s , and excited 2p , 3d , and 2s states strongly depend on QD radius if it does not exceed a few effective Bohr radius. Wave functions and Stark shift energy levels in external electric field were determined from a variational-calculus approach for states labeled m=0 . The absorption spectra for intersubband transitions were found to depend strongly on the QD radius. Whether or not the impurity is present, the peak energy of absorption decreases with increasing QD radius. An external electric field increases the transition energy but does not significantly change absorption characteristics.

Degradation of quantum dots under nuclear irradiation and its influence on exciton spectra in semimagnetic semiconductors

AbstractThe paper studies radiation‐induced modification of energetic spectra of excitons in CdTe quantum dots (QDs) in a (Cd, Mn)Te matrix. Exciton levels in such system are very sensitive to the changes in Mn concentration due to the exchange interaction between magnetic Mn ions and charge carriers (the giant splitting effect). Radiation‐enhanced diffusion causes Mn penetration into the QD that leads to the changes in the exciton level energy. We found the spatial distribution of Mn ions for different durations of the irradiation and performed quantum calculations of the energy of exciton states with the obtained Mn concentration profiles. The dependences of the position of the exciton levels and the splitting between the energy of exciton states with σ+ and σ– polarizations on the irradiation parameters and the QD's radius have been investigated. The calculations show that the irradiation causes a large shift of exciton levels. The splitting between levels with σ+ and σ– polarizations may be increased in the order of value due to the irradiation. The radiation effects are stronger for QD with smaller radius.

Control of growth and the processes of energy transfer from CdSe quantum dots for Nd3+ ions in a vitreous system: Thermal annealing time

The authors report clear evidence of radiative and nonradiative energy transfer from CdSe quantum dots to Nd3+ ions in a glass system synthesized by melting method. An efficient control of energy transfer can be obtained by controlling the mean radius of the CdSe quantum dots. Increase of about 100% of the near infrared Nd3+ emission (4F3/2 → 4I9/2) was observed as a function of thermal annealing time.

Excitonic properties of a spherical semiconductor quantum dot: The role of phonons

The present work investigates the exciton–LO phonon interaction in a semiconductor spherical quantum dot. Through a variation method, a fully detailed analysis of contributions is given. The exciton–LO phonon interaction is studied in two different cases: (i) with a finite confining potential surrounding the quantum dot and (ii) with a infinite confining potential. The calculation for different material such as CdSe/ZnSe and GaAs/AlxGa1−x As, shows that the interaction energy is increasing with increasing the quantum dot radius. Also it has been observed that the interaction energy for QDs with finite confining potential is higher than these with infinite confining potential, furthermore because of confinement potential variation with Al molar fraction in GaAs/AlxGa1−x As QDs a different behavior for interaction energy has been achieved at different Al molar fraction.

Effect of doping and in-composition on gain of long wavelength III-nitride QDs

In this work, we calculate material gain for long wavelength III-nitride InN and AlInN quantum dot (QD) structures. Strain and QD inhomogeneity are included in the calculations. The study covers (800-2300 nm) wavelength range which is important in optical communications. While p-doping is shown to be efficient to increasing gain, changing QD size (especially QD radius) is more efficient to vary wavelength. The results predicted that n-doped QD structures are promises for broad band laser applications.