Browsing by Author "Koksal, Koray"

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The angular electronic band structure and free particle model of aromatic molecules: High-frequency photon-induced ring current
(WORLD SCIENTIFIC PUBL CO PTE LTD5 TOH TUCK LINK, SINGAPORE 596224, SINGAPORE, 2017) Oncan, Mehmet; Koc, Fatih; Sahin, Mehmet; Koksal, Koray; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü
This work introduces an analysis of the relationship of first-principles calculations based on DFT method with the results of free particle model for ring-shaped aromatic molecules. However, the main aim of the study is to reveal the angular electronic band structure of the ring-shaped molecules. As in the case of spherical molecules such as fullerene, it is possible to observe a parabolic dispersion of electronic states with the variation of angular quantum number in the planar ring-shaped molecules. This work also discusses the transition probabilities between the occupied and virtual states by analyzing the angular electronic band structure and the possibility of ring currents in the case of spin angular momentum (SAM) or orbital angular momentum (OAM) carrying light. Current study focuses on the benzene molecule to obtain its angular electronic band structure. The obtained electronic band structure can be considered as a useful tool to see the transition probabilities between the electronic states and possible contribution of the states to the ring currents. The photoinduced current due to the transfer of SAM into the benzene molecule has been investigated by using analytical calculations within the frame of time-dependent perturbation theory.
A detailed investigation of the electronic properties of a multi-layer spherical quantum dot with a parabolic confinement
(ELSEVIER, 2012) Akgul, Selcuk; Koksal, Koray; Sahin, Mehmet; 0000-0002-9419-1711; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü; Sahin, Mehmet
In this work, we aim a detailed investigation of the electronic properties of a spherical multi-layer quantum dot with and without a hydrogenic impurity. The structure is introduced in the form of core/ shell/well/shell layers. The core and well layers are defined by the parabolic electronic potentials. We carry out the effect of the core radius and layer thickness on the energy levels, their wave functions, binding energies of the impurity and the probability distributions. In order to determine the sublevel eigenvalues and eigenfunctions, the Schrodinger equation is solved full numerically by shooting ¨ method in the frame of the effective mass approximation. The results are analyzed in detail as a function of the layer thicknesses and their probable physical reasons are tried to be explained. It is found that the electronic properties and impurity binding energies are strongly depending on the layer thicknesses.
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 LTD2-4 PARK SQUARE, MILTON PARK, ABINGDON OR14 4RN, OXON, ENGLAND, 2017) Koc, Fatih; Koksal, Koray; Sahin, Mehmet; 0000-0002-9419-1711; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü; Sahin, Mehmet
In 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.
The effect of dilute nitrogen on nonlinear optical properties of the InGaAsN/GaAs single quantum wells
(SPRINGER, 2012) Sahin, Mehmet; Koksal, Koray; 0000-0002-9419-1711; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü; Sahin, Mehmet
In this study, we investigate the linear and third order nonlinear optical properties of InGaAsN/GaAs depending on nitrogen content and laser dressing parameter. As theoretical models, band anticrossing and model solid theory are used. In order to obtain the electronic properties of the quantum well, the finite difference method is used. The laser beam affects the electronic properties of the quantum well by changing the shape of the confinement potential. This modification of the potential is determined by laser dressing parameter. By using dilute amount of nitrogen, conduction band and the depth of quantum well can be controlled. The strain which is introduced due to the presence of nitrogen can be compensated by using indium atoms. The electronic and the linear and third order nonlinear optical properties of InGaAsN/GaAs quantum well structure are obtained.
The electronic and optical properties of a triexciton in CdSe/ZnS core/shell quantum dot nanocrystals
(TAYLOR & FRANCIS LTD2-4 PARK SQUARE, MILTON PARK, ABINGDON OR14 4RN, OXON, ENGLAND, 2016) Akturk, Abdurrahman; Tas, Hatice; Koksal, Koray; Sahin, Mehmet; 0000-0002-9419-1711; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü; Tas, Hatice; Sahin, Mehmet
In the study, we aim to investigate the electronic and optical properties of single excitons, biexcions and triexcitons in a CdSe/ZnS core/shell quantum dot nanocrystal. The electronic structure has been determined by solving of the Poisson-Schrodinger equations self-consistently. In calculations, the exchange-correlation effects between identical particles have been taken into account in the frame of the local density approximation. We have demonstrated that the optical properties of triexciton systems are remarkably different from the single and biexciton systems. Absorption peaks or transition energies of the triexciton system are well separated from those of single- and bi-exciton systems. We have observed that the core-radius dependent transition energy variations of triexcitons are higher when compared with single- and bi-excitonic systems. The transition energy shifts of double and triple excitons with respect to the single exciton have been calculated as a function of the core radius and we have shown that the energy shifts are inversely proportional with the radius. We have also investigated the radius-dependent changes in binding energies and lifetimes of the structures and the comparative results have been discussed in a detail manner.
The linear optical properties of a multi-shell spherical quantum dot of a parabolic confinement for cases with and without a hydrogenic impurity
(IOP Publishing Ltd, 2012) Sahin, Mehmet; Koksal, Koray; 0000-0002-9419-1711; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü; Sahin, Mehmet
Throughout this work, we aim to explore the linear optical properties of a semiconductor multi-shell spherical quantum dot with and without a hydrogenic donor impurity. The core and well layers are defined by the parabolic electronic potentials in the radial direction. The energy levels and corresponding wavefunctions of the structure are calculated by using the shooting technique in the framework of the effective-mass approximation. We investigate the intersublevel absorption coefficients of a single electron and the hydrogenic donor impurity comparatively as a function of the photon energy. In addition, we carry out the effect of a donor impurity and the layer thickness on the oscillator strengths and magnitude and position of absorption coefficient peaks. We illustrate the electron probability distribution and variation of the energy levels in cases with and without the impurity for different thicknesses of layers. This kind of structure gives an opportunity to tune and control the absorption coefficient of the system by changing three different thickness parameters. Also it provides a possibility to separate 0s and 1p electrons in different regions of the quantum dot.