Spectra Of Semiconductor Quantum Dots Research Articles

Electroabsorption Spectra of Quantum Dots of PbS and Analysis by the Integral Method

Electroabsorption (E-A) spectra of semiconductor quantum dots (QDs) of PbS have been analyzed by the integral method, which is powerful not only to determine the change in electric dipole moment and/or polarizability following absorption precisely but also to confirm the weak absorption bands buried under other strong absorption bands. In the results, two weak absorption bands which induce blue-shift and red-shift, respectively, with application of electric fields have been newly confirmed in PbS QDs to be located just above the intense first exciton band. The energy separation between these two bands becomes smaller, as the applied field strength becomes stronger, suggesting that the interaction between the states excited by these transitions becomes weaker by application of electric fields. The QD size dependence has been reported both for the transition energy and for the magnitude of the change in electric dipole moment and molecular polarizability following the absorption of each band. The assignment of the absorption bands has also been discussed.

Two-photon absorption in CdSe colloidal quantum dots compared to organic molecules.

We discuss fundamental differences in electronic structure as reflected in one- and two-photon absorption spectra of semiconductor quantum dots and organic molecules by performing systematic experimental and theoretical studies of the size-dependent spectra of colloidal quantum dots. Quantum-chemical and effective-mass calculations are used to model the one- and two-photon absorption spectra and compare them with the experimental results. Currently, quantum-chemical calculations are limited to only small-sized quantum dots (nanoclusters) but allow one to study various environmental effects on the optical spectra such as solvation and various surface functionalizations. The effective-mass calculations, on the other hand, are applicable to the larger-sized quantum dots and can, in general, explain the observed trends but are insensitive to solvent and ligand effects. Careful comparison of the experimental and theoretical results allows for quantifying the range of applicability of theoretical methods used in this work. Our study shows that the small clusters can be in principle described in a manner similar to that used for organic molecules. In addition, there are several important factors (quality of passivation, nature of the ligands, and intraband/interband transitions) affecting optical properties of the nanoclusters. The larger-size quantum dots, on the other hand, behave similarly to bulk semiconductors, and can be well described in terms of the effective-mass models.

Integral Method Analysis of Electroabsorption Spectra and Its Application to Quantum Dots of PbSe

The integral method is proposed to analyze the electroabsorption (E-A) spectra, since the change in the electric dipole moment and/or polarizability following absorption can be determined precisely and the bands buried under strong absorption bands can be confirmed. This method, where not only the observed E-A spectra but also each of their first and second integral spectra are fitted using the absorption and their derivative and integral spectra, has been successfully applied to the E-A spectra of semiconductor quantum dots of PbSe. In the results, one absorption band, which is not identified in the absorption spectrum because of the extremely weak intensity and also showing a remarkable blue shift in the presence of an electric field because of the large difference in polarizability between the ground state and the excited state, has been confirmed to be located between the first and second strong exciton bands. The size dependence of PbSe QDs of the peak position of the newly confirmed band as well as ...

Absorption Spectra of Defect-Free Semiconductor Quantum Dots

The presence of accompanying resonances to the longitudinal optical phonon satellites in the optical spectra of semiconductor quantum dots is confirmed theoretically by a non-adiabatic approach of the optical absorption. The theory is applied to simulate features of the optical spectra of spherical quantum dots.

DEPENDENCE OF THE ABSORPTION SPECTRA OF III–V SEMICONDUCTOR QUANTUM DOTS ON THE FUNDAMENTAL PARAMETERS

The absorption spectra of semiconductor quantum dots (QDs) are expected to be a series of δ-function-like discrete lines due to the nature of the density of states. In a realistic III–V QD system, the absorption spectra is the superimposition of the contribution from each individual dot and the overall behavior is modeled by considering a Gaussian size distribution. In this paper, we study and present the dependence of the Gaussian nature of the absorption spectra of In X Ga 1-X N/GaN QD systems on the dot size distribution and some fundamental parameters such as bowing effect of the band gap, band offset ratio, and so on. It is observed that the absorption spectra depend strongly on the dot size distribution. The results presented helps to get a better insight of the optical properties of In X Ga 1-X N/GaN QD systems.

Effect of boundary conditions on the energy spectra of semiconductor quantum dots calculated in the effective mass approximation

We consider semiconductor quantum dots shaped as a rectangular prism and a sphere, in the framework of the effective mass approximation. The Schrödinger equation was solved assuming a specular reflection of a particle from the boundary of the quantum dot, which will set the equivalent values for the particle wave function for an arbitrary original point inside the quantum dot and its image in the quantum dot wall. It is shown that the results obtained in this approximation for some classical problems coincide with those based on traditional approaches using “impenetrable walls” or periodic Born-von Karman boundary conditions. Additionally, several problems that are difficult to treat using the traditional approaches could be easily resolved with the suggested mirror-type boundary conditions. It is also shown that this “mirror walls” approach is favorable for effective mass approximation. The comparison of the calculated energy spectra with existing experimental data shows reasonable agreement between the theory and the experiment.

A spin-polarized bi-exciton in a semiconductor quantum dot

We discuss the role of spin-polarized multi-exciton complexes in the emission spectra ofsemiconductor quantum dots. The spin-polarized bi-exciton complex composed of twospin-polarized electrons and two spin-polarized holes confined in a single, parabolic,self-assembled quantum dot is discussed in detail. The configuration-interaction approach isused to compute the energies, wavefunctions, and emission spectra as a function of themagnetic field. It is shown that the spin-polarized bi-exciton emission spectrumdiffers from the unpolarized spectrum due to the Pauli blocking and exchangeinteractions. The emission from spin-polarized bi-exciton allows for the measurement ofground and excited single-exciton states, and its magnetic field dependence allowsfor the differentiation of the spin-polarized bi-exciton and tri-exciton spectrum.

Investigation of exciton ground state in quantum dots via Hamiltonian diagonalization method

We analyze the electron-hole (exciton) ground state associated with the first peak in the optical absorption spectra of semiconductor quantum dots. We assume the effective mass approximation and a dot radius R on the order of the exciton Bohr radius aB. A Hamiltonian diagonalization method which accounts for the exciton’s kinetic, direct Coulomb, and surface polarization energies is used. We obtain a representation of the exciton ground-state wavefunction and a value for its energy using a basis set consisting of only three composite infinite spherical well wavefunctions. We discuss the precision obtained by this basis set by comparing with results from a much more extended basis set. Our results are used to predict the radius-dependent energy of the first peak in visible-light absorption spectra for CdSe quantum dots. Our analysis accurately describes the experimental data for dots with radii in the range aB<R<2aB. We discuss why our model breaks down for smaller radii.

Frequency degenerate and nondegenerate two-photon absorption spectra of semiconductor quantum dots

The frequency degenerate and nondegenerate two-photon absorption (2PA) spectra of direct band gap semiconductor quantum dots are studied. Measuring the spectra for both cases in samples of CdSe and CdTe with different quantum dot sizes and size distributions, we observe that the 2PA spectra and the 2PA coefficient are size dependent, so that smaller dots have smaller 2PA even after taking into account the volume fraction. Theory considering the mixing of the hole bands, in a $\stackrel{P\vec}{k}∙\stackrel{P\vec}{p}$ model, explains the data quite well except for the smallest dots. A comparison with the parabolic band approximation is also shown.

Optical spectra of quantum dots: A non-adiabatic approach

The presence of accompanying resonances to the longitudinal optical phonon satellites in the optical spectra of semiconductor quantum dots is confirmed theoretically by a non-adiabatic approach of the optical absorption. The theory is applied to simulate features of the optical spectra of small spherical GaAs/AlAs quantum dots. The intensity and the spectral position of the accompanying resonances are influenced by both optically active and dark levels and increase to measurable values with temperature.

Magnetic field effects on the near field spectra of quantum dots

AbstractWe theoretically investigate the linear near field absorption spectra of semiconductor quantum dots under magnetic field of variable orientation. We examine if the application of the magnetic field alone is sufficient to cause – increasing the spot illuminated by the near field probe – “unexpected” features to the absorption spectra. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

NEAR FIELD SPECTROSCOPY OF QUANTUM DOTS UNDER MAGNETIC FIELD

We present the basic steps for the study of the linear near field absorption spectra of semiconductor quantum dots under magnetic field of variable orientation. We show that the application of the magnetic field alone is sufficient to induce -increasing the spot illuminated by the near field probe- interesting features to the absorption spectra.

Coulomb effects in semiconductor quantum dots

Coulomb correlations in the optical spectra of semiconductor quantum dots are investigated using a full-diagonalization approach. The resulting multi-exciton spectra are discussed in terms of the symmetry of the involved states. Characteristic features of the spectra like the nearly equidistantly spaced s-shell emission lines and the approximately constant p-shell transition energies are explained using simplified Hamiltonians that are derived taking into account the relative importance of various interaction contributions. Comparisons with previous results in the literature and their interpretation are made.

Optical spectra of quantum dots: effects of non‐adiabaticity

It is shown that in many cases an adequate description of optical spectra of semiconductor quantum dots requires a treatment beyond the commonly used adiabatic approximation. We have developed a theory of phonon-assisted optical transitions in semiconductor quantum dots, which takes into account non-adiabaticity of the exciton–phonon system. Effects of non-adiabaticity lead to a mixing of different exciton and phonon states that provides a key to the understanding of surprisingly high intensities of phonon satellites observed in photoluminescence spectra of quantum dots. A breakdown of the adiabatic approximation gives an explanation also for discrepancies between the serial law, observed in multi-phonon optical spectra of some quantum dots, and the Franck–Condon progression, prescribed by the adiabatic approach.

Role of Coulomb Correlations in the Optical Spectra of Semiconductor Quantum Dots

We present a consistent theoretical description of few-particle effects in the optical spectra of semiconductor quantum dots, based on a direct-diagonalization approach. We show that, because of the strong Coulomb interaction among electrons and holes, each configuration of the confined few-particle system leads to its characteristic signature in the optical spectra. We discuss quantitative predictions and comparison with experiments for both absorption and luminescence.

Excitonic and Biexcitonic States in Semiconductor Quantum Dots

We analyze excitonic and biexcitonic effects in the coherent optical spectra of semiconductor quantum dots using a density-matrix approach, that explicitly accounts for exciton–exciton interactions. A consistent description of additional peaks appearing at higher photoexcitation density is given.

Excitonic and biexcitonic effects in the coherent optical response of semiconductor quantum dots

We analyze few-particle effects in the coherent optical spectra of semiconductor quantum dots using a density-matrix approach that explicitly accounts for exciton–exciton interactions. A consistent description of additional peaks appearing at high photoexcitation density is given.

Few-particle effects in the optical spectra of semiconductor quantum dots

We analyze few-particle effects in the optical spectra of semiconductor quantum dots using a density-matrix approach that explicitly accounts for exciton–exciton, as well as exciton–carrier interactions. We give a consistent description of additional peaks appearing at high photoexcitation density, and predict that a strong polarization dependence should be characteristic of few-particle features.

Few-Particle Effects in Nonlinear Optical Spectra of Semiconductor Quantum Dots

ABSTRACTWe present a density-matrix approach for the description of nonequilibrium carrier dynamics in optically excited semiconductor quantum dots, that explicitly accounts for exciton-exciton as well as exciton-carrier interactions. Within this framework, we analyze few-particle effects in the optical spectra and provide a consistent description of additional peaks appearing at high photoexcitation density. We discuss possible applications of such optical nonlinearities in future coherent-control experiments.

Multiphonon photoluminescence and Raman scattering in semiconductor quantum dots

Multiphonon optical spectra of semiconductor quantum dots, in particular, of nanocrystals with structure imperfections, are studied in the framework of a non-adiabatic approach. The phonon modes and the amplitudes of the electron–phonon interaction are found taking into account both electrostatic and mechanical boundary conditions, as well as the finite number of vibrational degrees of freedom in quantum dots. Selection rules for Raman scattering are deduced for quantum dots of semiconductor materials with a degenerate valence band. The effects of non-adiabaticity of the exciton–phonon system are shown to lead to a significant enhancement of phonon-assisted transition probabilities and to a substantial difference of the optical spectra from the Franck–Condon progression. Calculated optical spectra compare well with experimental data on photoluminescence and Raman scattering in CdSe and PbS quantum dots.