Fanyao Qu

The optical Stark effect in semiconductor quantum wires

Abstract A new approach for controlling the optical emission wavelength of semiconductor quantum wires is proposed. The wavelength control resides upon the effect of an intense, long-wavelength laser field radiation applied to the semiconductor structure. Under such condition a strong optical Stark effect leads to optical tunability. Calculation of the optical Stark effect was carried out in the frame of the nonperturbative theory and finite difference method. Different geometries concerning the size of GaAs–AlGaAs quantum wires as well as the polarization direction and the strength of the applied laser field with respect to the quantum structure were considered.

Synthesis process controlled magnetic properties of Pb1−xMnxS nanocrystals

Mn-doped PbS nanocrystals (NCs) in an oxide glass matrix have been synthesized by the fusion method. Two kinds of Mn2+ sites, located inside and on the surface of NCs, are observed by electron paramagnetic resonance (EPR) spectroscopy in the X band and at room temperature. The proportion of their contribution to the hyperfine structure in the EPR spectrum depends strongly on thermal annealing time. The authors illustrate how thermal annealing process manifests itself in engineering the magnetic properties of NCs.

Energy levels in metal oxide semiconductor quantum dots in water-based colloids

The three-dimensional Schrodinger and Poisson’s equations are used to calculate the conduction band profile, energy levels, and Fermi energy of negatively charged semiconductor quantum dots. The calculation is carried out self-consistently within the frame of the finite-difference method. Assuming the effective mass of the proton at the semiconductor–electrolyte interface, we found the conduction band profile for the spherical ZnO quantum dots dispersed as aqueous colloids very similar to the conduction band profile of symmetric modulation-doped semiconductor quantum wells. The energy levels and Fermi energy of the spherical ZnO quantum dots are obtained as a function of the band offset at the semiconductor–electrolyte interface. A comparison of the energy levels for negatively charged and uncharged quantum dots is used as an alternative explanation of the observed reversible blue shift in the absorption spectrum of semiconductor colloids under illumination.

The pH dependence of the surface charge density in oxide-based semiconductor nanoparticles immersed in aqueous solution

A new approach to calculate the surface charge density in semiconductor metal-oxide quantum dots (QDs) dispersed in alkaline aqueous colloids, at different pH values, is presented. Surface charge density up to about 0.3 C/m/sup 2/ was obtained for spherical nanoparticles in the diameter range of 8 to 14 nm, at pH 10 and 12. The calculated surface charge density, in very good agreement with the average value reported in the literature, is obtained as long as a proton transfer mechanism through the semiconductor-electrolyte interface takes place. The data reported here explain the stability found in ionic magnetic fluids at the usual lower/higher pH values and particle sizes.

Experimental study of absorption and luminescence properties of Er3+ in lead silicate glass

Abstract The optical absorption and temperature dependence of photoluminescence (PL) in the temperature range of 10 to 300 K have been measured for Er3+-doped lead silicate glass. A strong PL band that originates from 4S13/2→4I15/2 transition centered at 562 nm was observed even at room temperature. A very weak temperature quenching compared with that of Er3+ in crystal Si and the peak wavelength remaining constant with temperature are the main characteristics of this PL emission band. They are explained by the competition of thermal population effect and multiphonon relaxation between the 4S3/2 and 2H11/2 levels of Er3+.

Optical Properties of PbSe and PbS Quantum Dots Embedded in Oxide Glass

PbSe and PbS quantum dots (QDs) embedded in the oxide glass matrixes (SiO 2 -Na 2 CO 3 -ZnO-Al 2 O 3 -PbO 2 -B 2 O 3 ) were synthesized by means of fusion method. Their optical properties have been studied both theoretically and experimentally. We found that the optical absorption is strongly dependent upon thermal-treatment processes, for instance annealing temperature and time. An increase of the annealing time leads to a red shift of the optical features at the higher energy side, suggesting an increase of QD-size. An up-conversion photoluminescence emission band at about 2.476 eV was observed in the PbS QDs. It is attributed to a resonant Raman process.

Hydrogenic Impurity States in n-Doped and Undoped Quantum Wells of GaAs-AlxGa1-xAs Embedded in Intense Laser Fields.

The ground state binding energy of hydrogenic impurities in n-doped and undoped GaAs–Al x Ga 1- x As quantum wells, under intense high-frequency polarized laser field is investigated. The calculation has been performed by means of a nonperturbative theory and variational approach which takes into account the “dressed” confinement barrier potential, the “dressed” Coulomb potential, and the screening effect due to the two-dimensional electron gas. We found that the impurity binding energy in a n-doped quantum well is dramatically reduced by both, the screening effect and the intense high-frequency laser field, thus leading, under certain circumstances, to a metal-insulator transition.

A quantum dot model for the surface charge density in ferrite-based ionic magnetic fluids

Calculation of the surface charge density in semiconductor metal-oxide quantum dots (QDs) dispersed in aqueous colloids, in a wide range of pH values (1–13), is presented. Surface charge density in the range 70.4 C/m 2 was obtained for spherical nanoparticles in the typical diameter range. The shape of the calculated surface charge density curve as well as the range of the values obtained, in excellent agreement with the data reported in the literature, are obtained as long as a proton transfer mechanism through the semiconductor–electrolyte interface takes place. The data reported here explain the stability found in ionic magnetic fluids at the usual lower/higher pH values. r 2002 Elsevier Science B.V. All rights reserved.

Electrical control of singlet-triplet entanglement in lateral quantum dot molecules

We study effects of spin-orbit coupling and electron-electron correlation, which can be electrically tuned, on the singlet-triplet entanglement in double lateral quantum dots charged with two electrons. We have found that the magnetic character of a double-dot system can be changed from diamagnetic to paramagnetic, only by adjusting the strength of spin-orbit coupling. Coulomb interaction leads to an unusual singlet-triplet energy splitting, accompanied by a shift of this transition energy to the lower magnetic field side in the energy spectrum.

Synthesis and characterization of PbS quantum dots embedded in oxide glass

The fusion method was used to produce PbS quantum dots (QDs) embedded in S-doped glass matrix (SiO2-Na2CO3-Al2O3-PbO2-B2O3:S). Measurements of optical absorption (OA), photoluminescence (PL) and atomic force microscopy (AFM) have been carried out in order to characterize the produced QDs. A strong red-shift observed in the optical features with an increase of the annealing time indicates an increase in QD-size. The QD sizes predicted by k.p theoretical results were confirmed by AFM observation.