Yazar "Sahin, Mehmet" seçeneğine göre listele
Listeleniyor 1 - 16 / 16
Sayfa Başına Sonuç
Sıralama seçenekleri
Öğe A detailed investigation of electronic and intersubband optical properties of AlxGa(1-x)As/Al0.3Ga0.7As/AlyGa1-yAs/Al0.3Ga0.7As multi-shell quantum dots(IOP PUBLISHING LTD, 2014) Kavruk, Ahmet Emre; Sahin, Mehmet; Atav, UlfetIn this study, we have investigated the electronic and intersubband optical properties of Al x Ga1-x As/Al0.3Ga0.7As/Al y Ga1-y As/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.Öğe Effect of a buffer layer between the shell and ligand on the optical properties of an exciton and biexciton in type-II quantum dot nanocrystals(TAYLOR & FRANCIS LTD, 2017) Koc, Fatih; Koksal, Koray; Sahin, MehmetIn this study, we have investigated the effect of the buffer layers on the electronic and optical properties of an exciton (X) and a biexciton (XX) in a type-II CdTe/CdSe quantum dot nanocrystal. In an experimental study, it has been reported that when a CdTe/CdSe quantum dot nanocrystal is coated by a ZnTe material as a buffer layer, the photoluminescence quantum yield is growing from 4 to 20%. We have confirmed theoretically this improvement and extended the calculations to an XX structure. In the calculations, two different semiconductor materials, CdS and ZnTe, have been considered for the buffer layer. We have theoretically shown that the buffer layer causes an increase in the radiative oscillator strength of both X and XX. When the ZnTe is used as the buffer layer, the oscillator strength becomes stronger when compared to CdSe buffer material because of higher conduction band offset between CdSe and ZnTe.Öğe Electronic and optical properties of single excitons and biexcitons in type-II quantum dot nanocrystals(AMER INST PHYSICS, 2014) Koc, Fatih; Sahin, MehmetIn 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 approximation proposed by Sahin and Koc [Appl. Phys. Lett. 102, 183103 (2013)] for the recombination oscillator strength of the biexciton for bound and unbound cases. The results obtained have been presented comparatively as a function of the shell thicknesses and probable physical reasons in behind of the results have been discussed in a detail. (C) 2014 AIP Publishing LLC.Öğe The electronic properties of a core/shell/well/shell spherical quantum dot with and without a hydrogenic impurity(AMER INST PHYSICS, 2012) Tas, Hatice; Sahin, MehmetIn this study, we have performed a detailed investigation of the electronic properties of a core/shell/well/shell multilayered spherical quantum dot, such as energy eigenvalues, wave functions, electron probability distribution, and binding energies. The energy eigenvalues and their wave functions of the considered structure have been calculated for cases with and without an on-center impurity. For this purpose, the Schrodinger equation has been numerically solved by using the shooting method in the effective mass approximation for a finite confining potential. The electronic properties have been examined for different core radii, barrier thicknesses, and well widths in a certain potential. The results have been analyzed in detail as a function of the layer thicknesses and their physical reasons have been interpreted. It has been found that the electronic properties are strongly dependent on the layer thicknesses. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3702874]Öğe The electronic properties of a two-electron multi-shell quantum dot-quantum well heterostructure(AMER INST PHYSICS, 2013) Aydin, Rasit; Sahin, MehmetA 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. (C) 2013 AIP Publishing LLC.Öğe Excitonic condensation under spin-orbit coupling and BEC-BCS crossover(AMERICAN PHYSICAL SOC, 2007) Hakioglu, T.; Sahin, MehmetThe condensation of electron-hole pairs is studied at zero temperature and in the presence of a weak spin-orbit coupling (SOC) in coupled quantum wells. Under realistic conditions, a perturbative SOC can have observable effects in the order parameter of the condensate. First, the fermion exchange symmetry is absent. As a result, the condensate spin has no definite parity. Additionally, the excitonic SOC breaks the rotational symmetry yielding a complex order parameter in an unconventional way; i.e., the phase pattern of the order parameter is a function of the condensate density. This is manifested through finite off-diagonal components of the static spin susceptibility, suggesting a new experimental method to confirm an excitonic condensate.Öğe The inter-sublevel optical properties of a spherical quantum dot-quantum well with and without a donor impurity(AMER INST PHYSICS, 2012) Tas, Hatice; Sahin, MehmetIn this study, we have investigated the inter-sublevel optical properties of a core/shell/well/shell spherical quantum dot (QD) with the form of quantum dot-quantum well heterostructure. In order to determine the energy eigenvalues and corresponding wave functions, the Schrodinger equation has been solved full numerically by using shooting method in the effective mass approximation for a finite confining potential. The inter-sublevel optical absorption and the oscillator strength between ground (1 s) and excited (1 p) states have been examined based on the computed energies and wave functions. Also, the effect of a hydrogenic donor impurity, located at the center of the multi-shell spherical quantum dot (MSQD), has been researched for different core radii (R-1), shell thicknesses (T-s), and well widths (T-w) in certain potential. It is observed that the oscillator strengths and the absorption coefficients are strongly depend on the core radii and layer thicknesses of the MSQD. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4751483]Öğe The intersubband optical properties of a two-electron quantum dot-quantum well heterostructure(ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD, 2015) Aydin, Rasit; Tas, Hatice; Sahin, MehmetIn this paper, both linear and third-order nonlinear optical properties of two-electron in a semiconductor core/shell/well/shell quantum dot (QD) heterostructure for cases with and without a hydrogenic donor impurity have been investigated in a detailed manner as depending on the structure parameters. For this purpose, first, the energy eigenvalues and corresponding wave functions of the structure have been computed as a function of the layer thicknesses by means of the self-consistent solution of the Poisson and Schrodinger equations in envelope function effective mass approximation. Second, using these energy eigenvalues and their wave functions obtained from the calculations, both linear and third-order nonlinear optical properties of the multi-shell QD (MSQD) with two-electron have been determined as a function of the photon energies and shell thicknesses. Also, all procedures mentioned above have been repeated for negatively charged donor impurity (D-) located in the center of the same structure. Finally, obtained results have been presented comparatively for cases with and without the impurity. (C) 2015 Elsevier Ltd. All rights reserved.Öğe Linear and nonlinear optical properties of GaAs/Al(x)Ga1-xAs/GaAs/Al(y)Ga1-yAs multi-shell spherical quantum dot(AMER INST PHYSICS, 2013) Kavruk, Ahmet Emre; Sahin, Mehmet; Koc, FatihIn 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. (c) 2013 AIP Publishing LLC.Öğe A model for the recombination and radiative lifetime of trions and biexcitons in spherically shaped semiconductor nanocrystals(AMER INST PHYSICS, 2013) Sahin, Mehmet; Koc, FatihIn this letter, we propose a different 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 with sufficient accuracy in comparison with results of more complex methods such as quantum Monte Carlo techniques and atomistic calculations. (C) 2013 AIP Publishing LLC.Öğe Photoionization cross section and intersublevel transitions in a one- and two-electron spherical quantum dot with a hydrogenic impurity(AMER PHYSICAL SOC, 2008) Sahin, MehmetA detailed investigation of the optical properties of a spherical quantum dot (QD) containing one and two electrons has been performed for cases with and without a hydrogenic impurity. First, the photoionization cross section of both D(0) and D(-) impurities in the QD has been calculated for an on-center impurity. Second, the intersublevel optical absorption and oscillator strength between the ground and excited states have been examined based on the computed energies and wave functions. The full numeric matrix diagonalization technique has been employed in determining sublevel energy eigenvalues and their wave functions. The Poisson-Schrodinger equations have been solved self-consistently in the Hartree approximation. In addition, quantum-mechanical many-body effects have been investigated in the local density approximation. The results are presented as a function of quantum dot radii and photon energies.Öğe The photoionization cross section of a hydrogenic impurity in a multi-layered spherical quantum dot(AMER INST PHYSICS, 2012) Sahin, Mehmet; Tek, Firdes; Erdinc, AhmetIn this study, we have investigated the photoionization cross section of an on-center hydrogenic impurity in a multi-layered spherical quantum dot. The electronic energy levels and their wave functions have been determined fully numerically by shooting method. Also, we have calculated the binding energy of the impurity by using these energy values. The photoionization cross section has also been computed as a function of the layer thickness and normalized photon energies. We have discussed in detail the possible physical reasons behind the changes in the binding energies and photoionization cross section. It is observed that both the binding energies and the photoionization cross sections depend strongly on the layer thickness and photon energies. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4705410]Öğe Reordering orbitals of semiconductor multi-shell quantum dot-quantum well heteronanocrystals(AMER INST PHYSICS, 2012) Sahin, Mehmet; Nizamoglu, Sedat; Yerli, Ozan; Demir, Hilmi VolkanBased on self-consistent computational modeling of quantum dot-quantum well (QDQW) heteronanocrystals, we propose and demonstrate that conduction-electron and valence-hole orbitals can be reordered by controlling shell thicknesses, unlike widely known core/shell quantum dots (QDs). Multi-shell nanocrystals of CdSe/ZnS/CdSe, which exhibit an electronic structure of 1s-1p-2s-2p-1d-1f for electrons and 1s-1p-2s-2p-1d-2d for holes using thin ZnS and CdSe shells (each with two monolayers), lead to 1s-2s-1p-1d-1f-2p electron-orbitals and 1s-2s-1p-1d-2p-1f hole orbitals upon increasing the shell thicknesses while keeping the same core. This is characteristically different from the only CdSe core and CdSe/ZnS core/shell QDs, both exhibiting only 1s-1p-1d-2s-1f-2p ordering for electrons and holes. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3678585]Öğe Self-consistent computation of electronic and optical properties of a single exciton in a spherical quantum dot via matrix diagonalization method(AMER INST PHYSICS, 2009) Sahin, Mehmet; Nizamoglu, Sedat; Kavruk, A. Emre; Demir, Hilmi VolkanIn this study, we develop and demonstrate an efficient self-consistent calculation schema that computes the electronic structure and optical properties of a single exciton in a spherical quantum dot (QD) with an interacting pair of electron and hole wave functions. To observe modifications on bands, wave functions, and energies due to the attractive Coulomb potential, the full numeric matrix diagonalization technique is employed to determine sublevel energy eigenvalues and their wave functions in effective mass approximation. This treatment allows to observe that the conduction and valance band edges bend, that the electron and hole wave functions strongly localize in the QD, and that the excitonic energy level exhibits redshift. In our approach for the Coulomb term between electron and hole, the Poisson-Schrodinger equations are solved self-consistently in the Hartree approximation. Subsequently, exciton binding energies and associated optical properties are computed. The results are presented as a function of QD radii and photon energies. We conclude that all of these numerical results are in agreement with the experimental studies. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3197034]Öğe Third-order nonlinear optical properties of a one- and two-electron spherical quantum dot with and without a hydrogenic impurity(AMER INST PHYSICS, 2009) Sahin, MehmetIn this study, a detailed investigation of the nonlinear optical properties of a spherical quantum dot (QD) containing one and two electrons has been carried out. First, intersublevel linear, third-order, and total absorptions of both single- and double-electron QDs have been investigated for cases with (i.e., D-0 and D- donor ) and without impurity. Second, linear, third-order, and total refractive indices have been examined in the same structure. In the calculation of optical properties, for the density of electrons in the QD considered, some symbolic values chosen are not similar to those in previous studies. The full numeric matrix diagonalization technique has been employed in determining sublevel energy eigenvalues and their wave functions. The Poisson-Schrodinger equations have been solved self-consistently in the Hartree approximation. In addition, the quantum mechanical many-body effects have been taken into account in local density approximation. The results are presented as a function of QD radii, photon intensities, and photon energies. It is found out that although magnitudes of the linear absorption coefficients are the same for cases with and without impurity, the third-order ones are larger, and so the total absorption coefficients are smaller in the case without impurity. It is also observed that the size of the QD and the intensity of the illumination have drastic effects on nonlinear optical properties. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3225100]Öğe Virtual Training Centre For CNC: a Sample Virtual Training Environment(BUCHAREST UNIVERSITY PRESS, 2007) Sahin, Mehmet; Yaldiz, Suleyman; Unsacar, Faruk; Yaldiz, Y. Burak; Bilalis, Nikolaos; Maravelakis, Emmanuel; Antoniadis, AristomenisIt is a fact that virtual training has been a scope of interest for vocational training for a very long time. However, it needs more time to be more common in all specific training fields. This paper presents the development of a Virtual Training Centre (VTC) to promote and reinforce Vocational Training in Computer Numerical Control (CNC) Machines. "Virtual Training Centre for CNC" is a multilateral LdV project and of high importance in terms of virtual teaching and learning in vocational education and training. This virtual training environment is based on a common curriculum developed by the project partners. Since it is an interactive training tool, accessibility is the most important advantage over other training tools in VET.
