Browsing by Author "Sahin, Mehmet"

Now showing 1 - 20 of 25

Citation - WoS: 35
Citation - Scopus: 40
The Photoionization Cross Section of a Hydrogenic Impurity in a Multi-Layered Spherical Quantum Dot
(Amer Inst Physics, 2012) Sahin, Mehmet; Tek, Firdes; Erdinc, Ahmet
In 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]
Citation - WoS: 41
Citation - Scopus: 46
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
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.
Citation - WoS: 2
Citation - Scopus: 2
Cation Exchange Mediated Synthesis of Bright Au@ZnTe Core-Shell Nanocrystals
(IOP Publishing Ltd, 2021) Sadeghi, Sadra; Melikov, Rustamzhon; Sahin, Mehmet; Nizamoglu, Sedat
The synthesis of heterostructured core-shell nanocrystals has attracted significant attention due to their wide range of applications in energy, medicine and environment. To further extend the possible nanostructures, non-epitaxial growth is introduced to form heterostructures with large lattice mismatches, which cannot be achieved by classical epitaxial growth techniques. Here, we report the synthetic procedure of Au@ZnTe core-shell nanostructures by cation exchange reaction for the first time. For that, bimetallic Au@Ag heterostructures were synthesized by using PDDA as stabilizer and shape-controller. Then, by addition of Te and Zn precursors in a step-wise reaction, the zinc and silver cation exchange was performed and Au@ZnTe nanocrystals were obtained. Structural and optical characterization confirmed the formation of the Au@ZnTe nanocrystals. The optimization of the synthesis led to the bright nanocrystals with a photoluminescence quantum yield up to 27%. The non-toxic, versatile synthetic route, and bright emission of the synthesized Au@ZnTe nanocrystals offer significant potential for future bio-imaging and optoelectronic applications.
Citation - WoS: 15
Citation - Scopus: 15
The Electronic Properties of a Two-Electron Multi-Shell Quantum Dot-Quantum Well Heterostructure
(Amer Inst Physics, 2013) Aydin, Rasit; Sahin, Mehmet
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. (C) 2013 AIP Publishing LLC.
Citation - WoS: 9
Citation - Scopus: 10
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, Mehmet
In 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.
Citation - WoS: 26
Citation - Scopus: 29
A Detailed Investigation of Electronic and Optical Properties of the Exciton, the Biexciton and Charged Excitons in a Multi-Shell Quantum Dot Nanocrystal
(IOP Publishing Ltd, 2014) Akturk, Abdurrahman; Sahin, Mehmet; Koc, Fatih; Erdinc, Ahmet
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.
Citation - WoS: 28
Citation - Scopus: 25
Quantum Dot and Electron Acceptor Nano-Heterojunction for Photo-Induced Capacitive Charge-Transfer
(Nature Portfolio, 2021) Karatum, Onuralp; Eren, Guncem Ozgun; Melikov, Rustamzhon; Onal, Asim; Ow-Yang, Cleva W.; Sahin, Mehmet; Nizamoglu, Sedat
Capacitive charge transfer at the electrode/electrolyte interface is a biocompatible mechanism for the stimulation of neurons. Although quantum dots showed their potential for photostimulation device architectures, dominant photoelectrochemical charge transfer combined with heavy-metal content in such architectures hinders their safe use. In this study, we demonstrate heavy-metal-free quantum dot-based nano-heterojunction devices that generate capacitive photoresponse. For that, we formed a novel form of nano-heterojunctions using type-II InP/ZnO/ZnS core/shell/shell quantum dot as the donor and a fullerene derivative of PCBM as the electron acceptor. The reduced electron-hole wavefunction overlap of 0.52 due to type-II band alignment of the quantum dot and the passivation of the trap states indicated by the high photoluminescence quantum yield of 70% led to the domination of photoinduced capacitive charge transfer at an optimum donor-acceptor ratio. This study paves the way toward safe and efficient nanoengineered quantum dot-based next-generation photostimulation devices.
Citation - WoS: 16
Citation - Scopus: 17
Colloidal Aluminum Antimonide Quantum Dots
(Amer Chemical Soc, 2019) Jalali, Houman Bahmani; Sadeghi, Sadra; Sahin, Mehmet; Ozturk, Hande; Ow-Yang, Cleva W.; Nizamoglu, Sedat
AlSb is a less studied member of the III-V semiconductor family, and herein, we report the colloidal synthesis of AlSb quantum dots (QDs) for the first time. Different sizes of colloidal AlSb QDs (5 to 9 nm) were produced by the controlled reaction of AlCl3 and Sb[N(Si(Me)(3))(2)](3) in the presence of superhydride. These colloidal AlSb quantum dots showed excitonic transitions in the UV-A region and a tunable band edge emission (quantum yield of up to 18%) in the blue spectral range. Among all III-V quantum dots, these quantum dots show the brightest core emission in the blue spectral region.
Citation - WoS: 12
Citation - Scopus: 14
Advanced Tunability of Optical Properties of CdS/ZnSe Multi-Shell Quantum Dot by the Band Edge Engineering
(Elsevier, 2023) Koc, Fatih; Kavruk, Ahmet Emre; Sahin, Mehmet
In this study, the advanced manipulability of wave functions in a type-II multi-shell hetero-nanostructure (MS-HNS) and the tunability of radiative exciton lifetime over a wide range with and/or without changing in transition energies has been demonstrated by the band edge engineering. For this purpose, the electronic and optical properties of exciton (X) and biexciton (XX) in a spherical CdS/ZnSe/ZnTe/CdSe HNS have been explored in detail. In the calculations, effects of all Coulombic interactions between the charges have been taken into account on the wave functions. Moreover, in the case of XX, the exchange-correlation potential between the same charged particles has also been considered. The results have been presented as a function of CdS core radius and ZnSe shell thickness and the probable physical reasons have been discussed in detail.
Citation - WoS: 3
Citation - Scopus: 7
The Electronic and Optical Properties of an Exciton, Biexciton and Charged Excitons in CdSe/CdTe-Based Multi-Shell Type-II Quantum Dot Nanocrystals
(Springer Heidelberg, 2019) Koc, Fatih; Sahin, Mehmet
It has been recently reported that multi-shell type-II quantum dot nanocrystals (QDNCs) have higher quantum yields. Besides these higher quantum yields of multi-shell type-II QDNCs, additional second layer has been a critical influence on the formation mechanisms of the excitonic structures. Understanding of bound and unbound cases of the excitonic structures in multi-shell type-II QDNCs gives some important information for applications. In this study, we have investigated the electronic and optical properties of a single exciton (X), biexciton (XX), and positively and negatively charged excitons (X+ and X-) in CdSe/CdTe-based multi-shell type-II QDNCs. In the study, three different structure compositions, i.e., CdSe/ CdTe, CdSe/CdTe/CdS, and CdSe/CdTe/ZnTe, have been considered. We have observed that CdS and ZnTe materials have drastically changed the electronic and optical properties of the bare CdSe/CdTe type-II QDNCs.
Citation - WoS: 24
Citation - Scopus: 25
A Model for the Recombination and Radiative Lifetime of Trions and Biexcitons in Spherically Shaped Semiconductor Nanocrystals
(Amer Inst Physics, 2013) Sahin, Mehmet; Koc, Fatih
In 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.
Citation - WoS: 28
Citation - Scopus: 28
A Detailed Investigation of Electronic and Intersubband Optical Properties of AlxGa1-xAs/Al0.3Ga0.7As Multi-Shell Quantum Dots
(IOP Publishing Ltd, 2014) Kavruk, Ahmet Emre; Sahin, Mehmet; Atav, Ulfet
In 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.
Citation - WoS: 61
Citation - Scopus: 62
Effective Neural Photostimulation Using Indium-Based Type-II Quantum Dots
(Amer Chemical Soc, 2018) Jalali, Houman Bahmani; Aria, Mohammad Mohammadi; Dikbas, Ugur Meric; Sadeghi, Sadra; Kumar, Baskaran Ganesh; Sahin, Mehmet; Nizamoglu, Sedat
Light-induced stimulation of neurons via photoactive surfaces offers rich opportunities for the development of therapeutic methods and high-resolution retinal prosthetic devices. Quantum dots serve as an attractive building block for such surfaces, as they can be easily functionalized to match the biocompatibility and charge transport requirements of cell stimulation. Although indium based colloidal quantum dots with type-I band alignment have attracted significant attention as a nontoxic alternative to cadmium-based ones, little attention has been paid to their photovoltaic potential as type-II heterostructures. Herein, we demonstrate type-II indium phosphide/zinc oxide core/shell quantum dots that are incorporated into a photoelectrode structure for neural photostimulation. This induces a hyperpolarizing bioelectrical current that triggers the firing of a single neural cell at 4 mu W mm(-2), 26-fold lower than the ocular safety limit for continuous exposure to visible light. These findings show that nanomaterials can induce a biocompatible and effective biological junction and can introduce a route in the use of quantum dots in photoelectrode architectures for artificial retinal prostheses.
Citation - WoS: 3
Citation - Scopus: 5
Elliptical Quantum Rings With Variable Heights and Under Spin-Orbit Interactions
(MDPI, 2023) Mora-Ramos, Miguel E.; Vinasco, Juan A.; Radu, A.; Restrepo, Ricardo L.; Morales, Alvaro L.; Sahin, Mehmet; Duque, Carlos A.
We investigate the electronic properties of a semiconductor quantum ring with an elliptical shape and non-uniform height, allowing for distributed quantum-dot-like bulges along its perimeter. The adiabatic approximation and the finite element method are combined to calculate the allowed electron states in the structure under the effective mass approximation, considering the contributions from Rashba and Dresselahaus spin-orbit interactions and the Zeeman effect in the presence of an applied magnetic field. We discuss the features of the calculated spectra for two different ring geometries: a symmetric one with four dot-like bulges, and an asymmetric one with three hilled protuberances. The information about those states allows us to evaluate the linear optical absorption response associated with interlevel transitions between the ground and lowest excited states. This phenomenon takes place at resonant energies of only a few milielectronvolts. It is observed that spin-orbit interactions tend to quench this response under zero-field conditions in the case of symmetric confinement.
Citation - WoS: 3
Citation - Scopus: 3
The Angular Electronic Band Structure and Free Particle Model of Aromatic Molecules: High-Frequency Photon-Induced Ring Current
(World Scientific Publ Co Pte Ltd, 2017) Oncan, Mehmet; Koc, Fatih; Sahin, Mehmet; Koksal, Koray
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.
Citation - WoS: 4
Citation - Scopus: 4
Double Quantum Ring Under an Intense Nonresonant Laser Field: Zeeman and Spin-Orbit Interaction Effects
(MDPI, 2023) Mora-Ramos, Miguel E.; Vinasco, Juan A.; Radu, Adrian; Restrepo, Ricardo L.; Morales, Alvaro L.; Sahin, Mehmet; Duque, Carlos A.
We theoretically investigate the properties of an electron energy spectrum in a double GaAs-Al0.3Ga0.7As quantum ring by using the effective mass and adiabatic approximations, together with a realistic description of the confining potential profile, which is assumed to be deformed due to the application of an intense nonresonant laser field. The effects of the applied magnetic field and spin-orbit interaction are included. We discuss the features of the lowest confined energy levels under a variation of magnetic field strengths and intense laser parameters. The influence of this external probe on the linear optical absorption response associated with interlevel transitions is analyzed by considering both the presence and absence of spin-orbit effects.
Citation - WoS: 54
Citation - Scopus: 61
The Electronic Properties of a Core/Shell Spherical Quantum Dot With and Without a Hydrogenic Impurity
(Amer Inst Physics, 2012) Tas, Hatice; Sahin, Mehmet
In 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]
Citation - WoS: 68
Citation - Scopus: 75
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, Mehmet
In 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]
Citation - WoS: 40
Citation - Scopus: 40
Linear and Nonlinear Optical Properties of GaAs/AlxGa1-xAs Multi-Shell Spherical Quantum Dot
(Amer Inst Physics, 2013) Kavruk, Ahmet Emre; Sahin, Mehmet; Koc, Fatih
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. (c) 2013 AIP Publishing LLC.
Citation - WoS: 3
Citation - Scopus: 3
The Electronic and Optical Properties of a Triexciton in CdSe/ZnS Core/Shell Quantum Dot Nanocrystals
(Taylor & Francis Ltd, 2016) Akturk, Abdurrahman; Tas, Hatice; Koksal, Koray; 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.