Mehmet Sahin

Linear and nonlinear optical properties of GaAs/AlxGa1−xAs/GaAs/AlyGa1−yAs multi-shell spherical quantum dot

In this work, the optical properties of GaAs/AlxGa1−xAs/GaAs/AlyGa1−yAs multi-shell quantum dot heterostructure have been studied as a function of Al doping concentrations for cases with and without a hydrogenic donor atom. It has been observed that the absorption coefficient strength and/or resonant absorption wavelength can be adjusted by changing the Al content of inner-barrier and/or outer-barrier regions. Besides, it has been shown that the donor atom has an important effect on the control of the electronic and optical properties of the structure. The results have been presented as a function of the Al contents of the inner-barrier x and outer-barrier y regions and probable physical reasons have been discussed.

Electronic and optical properties of single excitons and biexcitons in type-II quantum dot nanocrystals

In this study, a detailed investigation of the electronic and optical properties (i.e., binding energies, absorption wavelength, overlap of the electron-hole wave functions, recombination oscillator strength, etc.) of an exciton and a biexciton in CdTe/CdSe core/shell type-II quantum dot heterostructures has been carried out in the frame of the single band effective mass approximation. In order to determine the electronic properties, we have self-consistently solved the Poisson-Schrodinger equations in the Hartree approximation. We have considered all probable Coulomb interaction effects on both energy levels and also on the corresponding wave functions for both single exciton and biexciton. In addition, we have taken into account the quantum mechanical exchange-correlation effects in the local density approximation between same kinds of particles for biexciton. Also, we have examined the effect of the ligands and dielectric mismatch on the electronic and optical properties. We have used a different appro...

A detailed investigation of electronic and intersubband optical properties of AlxGa1−xAs/Al0.3Ga0.7As/AlyGa1−yAs/Al0.3Ga0.7As multi-shell quantum dots

In this study, we have investigated the electronic and intersubband optical properties of AlxGa1−xAs/Al0.3Ga0.7As/AlyGa1−yAs/Al0.3Ga0.7As multi-shell quantum dot heterostructures as a function of the Al doping concentrations in both the core (x) and the well (y) regions for cases with and without a hydrogenic donor impurity. Our results reveal how resonant absorption wavelengths and absorption coefficient strengths are changed by variation of the Al content in the core (x) and well (y) regions. Besides this, how the electronic and intersubband optical properties of this structure are affected by the existence of the impurity has also been shown. The physical reasons for this relationship between the electronic and optical properties of this structure and the Al content in the core and well regions have been discussed in detail.

A detailed investigation of electronic and optical properties of the exciton, the biexciton and charged excitons in a multi-shell quantum dot nanocrystal

In the present study, the electronic and optical properties of the exciton (X), the biexciton (XX) and charged excitons (X− and X+) in a multi-shell quantum dot nanocrystal have been systematically explored in detail. The electronic properties have been determined in the framework of the single-band effective mass approximation. For this purpose, the Poisson–Schrodinger equations have been solved self-consistently in the Hartree approximation. In the electronic structure calculations for XX, X− and X+, the quantum mechanical exchange–correlation potentials between particles of the same type have been taken into account in the local density approximation. Some optical parameters, such as the overlap integrals, recombination oscillator strengths, radiative lifetimes, etc, have been determined by using the single-particle energy levels and wavefunctions obtained. A different approximation, reported in Sahin and Koc 2013 Appl. Phys. Lett. 102 183103, has been used in the recombination oscillator strength calculations. The results have been presented comparatively as a function of the shell thicknesses, and the well widths and probable physical reasons underlying them have been discussed in detail.

The electronic properties of a two-electron multi-shell quantum dot-quantum well heterostructure

A detailed investigation of the electronic properties of a double electron in a core/shell/well/shell quantum dot heterostructure has been systematically studied for cases with and without an on-center donor impurity. For this purpose, the Poisson-Schrodinger equations have been solved self-consistently in the frame of the single band effective mass approximation and Hartree treatment. The variation of the binding energies of negatively charged donor impurity (D−) have been examined for different core radii, shell thicknesses, and well widths. The results obtained have been presented comparatively as a function of layer thicknesses and probable physical reasons behind in their behavior have been discussed.

A model for the recombination and radiative lifetime of trions and biexcitons in spherically shaped semiconductor nanocrystals

In this letter, we propose a new model to determine the recombination oscillator strength of trions and biexcitons for bound and unbound cases in the effective mass approximation. The validity of our model has been confirmed by the radiative lifetime of the trion and biexciton in a spherical quantum dot. The results show that the model works sufficient accuracy in comparison with results of more complex methods such as quantum monte carlo techniques and atomistic calculations.

Motivation Levels of Gifted Students and Their Metaphorical Perceptions of School.

The aim of this study is to analyze the school motivation levels of gifted students and their metaphorical perception of school. For this purpose, quantitative and qualitative approach was used as the mixed method. The sample for both methods consisted of 96 gifted students at secondary school level. For quantitative data, School Motivation Scale was used and the students were also asked to form their metaphors about school as the qualitative data collection tool. Based on the descriptive statistics of School Motivation Scale, scores which indicated +/-1 standard deviation from mean and the metaphors of these students were compared depending on their motivation levels. The metaphors formed were collected under three main headings; positive metaphors, negative metaphors and neutral metaphors. According to the results, students who had high school motivation score stated mostly positive metaphors about school. The students who had low school motivation score tend to state mostly negative metaphors. Key words: Gifted students, Metaphor, mixed method, school motivation.