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Öğe Acceptor 1s-2p± transitions in GaAs/Ga0.7Al0.3As quantum wells(Wiley-V C H Verlag Gmbh, 1998) Akbas, H; Aktas, S; Okan, SE; Ulas, M; Tomak, MThe valence subband energies in the presence of an electric-field are calculated using the fourth-order Runge-Kutta method. The 1s- and 2p(+/-)-like acceptor states in the presence of a magnetic field are then calculated variationally. The effects of the spatially dependent screening function epsilon(r) on the calculation of the transition energies are specifically investigated. The use of a constant epsilon(o) is shown to yield better agreement with experimental results.Öğe Anomalous polarization in an electric field and self-polarization in GaAs/AlAs quantum wells and quantum well wires(Elsevier Science Bv, 2004) Okan, SE; Erdogan, I; Akbas, HThe response of an electron to an electric field in the infinite quantum wells and the infinite quantum well wires is investigated. Also the self-polarization effect, which can be defined as the influence of the well potential on the impurity electron, is studied for the same structures. The anomalous polarization in the excited states is found to be always exist in the presence of an electric field. Also it is shown that the external electric field polarization is intensified or weakened by an impurity ion depending on its position in the structure. (C) 2003 Elsevier B.V. All rights reserved.Öğe Binding energies of helium-like impurities in parabolic quantum wells under an applied electric field(Academic Press Ltd, 2000) Okan, SE; Akbas, H; Aktas, S; Tomak, MWe present a variational method to compute the binding energies of helium-like impurities in finite parabolic GaAs-Ga1-xAlxAs quantum wells. The effects of band nonparabolicity in the conduction band are taken into account within the effective mass approximation. The dependence of the impurity binding energy on the applied electric field and the impurity position is also discussed together with the polarization effect for all cases. (C) 2000 Academic Press.Öğe Composition of mixed anionic/nonionic surfactant micelles(Amer Oil Chemists Soc A O C S Press, 2000) Akbas, H; Iscan, M; Sidim, TA proposed method of determining the composition of mixed micelles in equilibrium with monomer of known composition is described. The systems were sodium dodecyl sulfate-polyoxyethylene 23 lauryl ether (Brij 35) in water and in 0.1 M sodium chloride solution at 25 degrees C. This technique applies the Gibbs-Duhem equation to the mixed micelle, which is treated as a pseudophase. This proposed methodology, which needs only critical micelle concentration data as a function of monomer composition, is applied to an anionic/nonionic surfactant pair. The calculated monomer-micelle equilibrium is found to be very similar to the much-used regular solution for nonideal systems.Öğe Conductometric studies of hexadecyltrimethylammonium bromide in aqueous solutions of ethanol and ethylene glycol(Pleiades Publishing Inc, 2006) Akbas, H; Kartal, ÇThe effect of cosolvent on micellization of hexadecyltrimethyl ammonium bromide (CTAB) in aqueous solutions was studied. The conductivity of a mixture (cosolvent + water) as function of CTAB concentration was measured at different temperatures. Ethylene glycol and ethanol were used as a cosolvent. The conductivity data were used to determine the critical micelle concentration (CMC) and the effective degree of counterion dissociation of micelle in the temperature range 303.2 to 313.2 K. In all the cases studied, a linear relatoinship between log([CMC](mix)/mol dm(-3)) and the mass fraction of cosolvent in solvent mixture has been observed. The free energy (Delta G(mic)(0)), enthalpy (Delta H-mic(0)), and entropy (Delta S-mic(0)) of micellization were determined using the temperature dependence of CMC. The dependence of these thermodynamic parameters on solvent composition was determined. The standard free energy of micellization was found to be negative in all cases and becomes less negative as the cosolvent content increases. The enthalpy and entropy of micellization are independent of temperature in pure water, while Delta H-mic(0) and Delta S-mic(0) decrease dramatically with temperature in mixed cosolvents. Furthermore, the entropic contribution is larger than the enthalpic one in pure water, while in the mixed solvents, the enthalpic contribution predominates.Öğe Donor binding energies in GaAs quantum wells considering the band nonparabolicity effects and the wavefunction elongation(Academic Press Ltd, 2000) Aktas, S; Okan, SE; Erdogan, I; Akbas, H; Tomak, MThe donor binding energies in finite GaAs/GaxAl1-As-x quantum wells have been calculated by considering the confinement of electrons, which increases as the well width increases. The variational solutions have been improved by using a two-parameter trial wavefunction, and by including the conduction band nonparabolicity. It is shown that the method used gives results in agreement with those obtained in the experiments on the effective mass and the donor binding energy, both of which are strongly dependent on the well width. (C) 2000 Academic Press.Öğe Effect of polyoxyethylene chain length and electrolyte on the viscosity of mixed micelles(Scientific Technical Research Council Turkey, 2003) Akbas, H; Sidim, T; Iscan, MThe viscosity measurements of micelles composed of mixtures of anionic/nonionic surfactants were carried out as a function of the mole fraction of surfactants. The systems studied were sodium dodecyl sulfate (SDS)-polyoxyethylene lauryl ether (C(12)POE(n)n = 4, 10 and 23) in water and 0.1 M sodium chloride solution. In the single systems, the relative viscosity of nonionic surfactant is greater than that of SDS, and increases with the increasing level of ethylene oxide. In the mixed surfactant systems, the relative viscosities vary at the mixed molar fraction of SDS, between 0.2 and 0.3, and then the relative viscosities of mixed systems decrease with the increasing mole fraction of SDS. The experimental values show a positive deviation from ideal behavior because of mixed micelle formation and the electroviscous effect. This effect could be suppressed by the addition of an electrolyte (NaCl).Öğe Electric and magnetic field effects on the self-polarization in GaAs/AlAs cylindrical quantum well-wires(Elsevier Science Bv, 2006) Erdogan, I; Akankan, O; Akbas, HIn this work we have calculated the effect of the infinite cylindrical quantum well-wire barrier on the donor electron within a variational scheme. The binding energy and the self-polarization effect are calculated for different location of impurity ion as function of both electric and magnetic fields. We find that, self-polarization effect depends strongly on the applied magnetic field. (c) 2006 Published by Elsevier B.V.Öğe Electric field effect on the binding energy of a hydrogenic impurity in coaxial GaAs/AlxGa1-xAs quantum well-wires(Academic Press Ltd Elsevier Science Ltd, 2001) Aktas, S; Okan, SE; Akbas, HThe ground state binding energy of a hydrogenic impurity in a coaxial cylindirical quantum well wire system subjected to an external electric field applied perpendicular to the symmetry axis of the wire system is studied within a variational scheme. Binding energy calculations were performed as functions of the inner barrier thickness and the electric field for two different impurity positions. The main result is that a sharp decrease in the binding energy, which may be important in device applications, occurs in certain conditions. (C) 2001 Academic Press.Öğe The exciton transition energies in symmetric double GaAs/AlxGa1-xAs quantum wells(Wiley-V C H Verlag Gmbh, 1999) Okan, SE; Aktas, S; Akbas, H; Tomak, MThe exciton transition energies between valence and conduction subbands of symmetric double quantum wells are studied. To study the effect of geometry coupled wells of rectangular, graded and parabolic shapes are considered. A special emphasis is given to the study of crossover between the symmetric light-hole (e1-11) and the antisymmetric heavy-hole (e2-h2) transitions.Öğe Screening effect on the binding energies of shallow donors, acceptors and excitons in finite-barrier quantum wells(Academic Press Ltd, 1998) Akbas, H; Aktas, S; Okan, SE; Ulas, M; Tomak, MThe conduction and valence subband energies in the presence of an electric field are calculated using the fifth-order Runge-Kutta method. The binding energies of shallow donors, accepters and excitons in finite-barrier GaAs/Ga1-xAlx As quantum wells are then obtained variationally in the presence of a magnetic field. The effects of a spatially dependent screening function epsilon(r) On the calculation of binding energies are specifically investigated. The use of epsilon(r) in comparison with the use of a constant epsilon(0) increases the binding energy of accepters as the increase on shallow donors and excitons is quite small. (C) 1998 Academic Press Limited.Öğe Shallow donors in a quantum well wire: Electric field and geometrical effects(Wiley-V C H Verlag Gmbh, 1997) Ulas, M; Akbas, H; Tomak, MSubband and shallow donor binding energies are calculated. The emphasis is given on the electric field effects for wires of different shapes. It is shown that the donor binding energy is sensitive to the interplay of the electric field and geometrical effects.Öğe Spatial electric field effect in GaAs-AlAs quantum wires(Elsevier Science Bv, 2005) Akankan, O; Okan, SE; Akbas, HThe response of an electron to a three-dimensional electric field in an infinite quantum well wire of square cross-section is investigated within a variational scheme. The ionization energy and the polarization are calculated for different locations of impurity ion. It is found that the results for the spatial electric field differ from the previous results found for the electric field applied in the direction perpendicular to the wire axis. The ionization energy weakens rapidly with the axial component of the field as the polarization of the carrier distribution intensifies. (C) 2004 Elsevier B.V. All rights reserved.Öğe Spectrophotometric studies of anionic dye-cationic surfactant interactions in mixture of cationic and nonionic surfactants(Pergamon-Elsevier Science Ltd, 2005) Akbas, H; Kartal, CThe interaction between an anionic dye C.I. Reactive Orange 16 (RO16) and a cationic surfactant dodecylpyridinium chloride (DPC) in mixtures of DPC and nonionic surfactants poly(oxyethylene)ethers (CmPOEn; m = 12, 16 and 18, n = 4, 10 and 23) are investigated spectrophotometrically in a certain micellar concentration range. The spectrophotometric measurements of dye-surfactant systems are carried out as function of mole fraction of surfactant at four different temperatures. For this reason, a typical system was occurred at 1.0 x 10(-2) mol l(-1) for surfactants and at 1.0 x 10(-4) mol l(-1) for dye concentrations. The formation of DPC-RO16 complex in the CmPOEn solutions of different mole fractions in its micellar concentration range have been determined and compared to those obtained in the binary mixtures. From the spectrophotometric measurements has been observed that the addition of nonionic surfactant in to the mixture of DPC-RO16, causes a significant increase of the value of absorbance. This increase explains that the stability of DPC-RO16 complex is reduced in the presence of nonionic surfactant micelles. It can be seen from results; in mixed surfactant solutions, there are DPC-CmPOEn and RO16-CmPOEn interactions in addition to DPC-RO16 interaction. Since the solubilizaton of the DPC-RO16 complex has been appeared in the CmPOEn, solution, our results support the conclusion that adding CmPOEn influences the hydrophobic-hydrophylic balance of the studied complex. Furthermore effect of the alkyl chain length and the number of poly(oxyethylene) in nonionic surfactant on values of absorbance have been investigated. (C) 2004 Elsevier B.V. All rights reserved.Öğe Study on the interaction of anionic dye-nonionic surfactants in a mixture of anionic and nonionic surfactants by absorption spectroscopy(Elsevier Sci Ltd, 2005) Kartal, Ç; Akbas, HThe interaction between an anionic dye C.I. Reactive Orange 16 (RO16) and different surfactants. i.e.. the anionic surfactant sodium dodecylsulfate (SDS) and nonionic surfactants polyoxyethylene ethers (CmPOEn; m = 12, 16 and 18, n = 4, 10 and 23) has been researched spectrophotometrically in a certain micellar concentration range. Light absorbances in the visible spectral range are measured as a function of mole fraction of surfactant at four different temperatures and at 24 h over the periodic time intervals. For this reason, a typical system occurred at 1.0 x 10(-2) mol/l for surfactants and at 1.0 x 10(-4) mol/l for dye concentrations. It is evident from the experimental measurements that while nonionic surfactants form a complex with the anionic dye RO16 in aqueous solution in studied concentration range, anionic surfactant does not. The formation of CmPOEn-RO16 complex in the SDS solutions of different mole fractions in its micellar concentration range has been determined and compared to those obtained in the dye-surfactant binary mixtures. Spectrophotometric measurements indicated that the value of absorbance of SDS-RO16 is greater than that of nonionic surfactant-RO16 in the binary systems. This state explains that the ionic and hydrophobic interactions between SDS-RO16 are less than that of nonionic surfactant-RO16 interactions. The addition of SDS to the mixture of CmPOEn-RO16 causes an increase in the value of absorbance. This increase is due to changed hydrophobic-hydrophilic balance of the studied mixtures. Furthermore, we observed that the alkyl chain length and the number of polyoxyethylene in nonionic surfactants do not have an enormous effect on the values of absorbance. (C) 2004 Elsevier Ltd. All rights reserved.Öğe The viscous properties of anionic/cationic and cationic/nonionic mixed surfactant systems(Pleiades Publishing Inc, 2005) Akbas, H; Sidim, TThe behavior of mixed cationic/anionic and cationic/nonionic surfactants solutions have been studied by viscosimetry. The systems studied were sodium dodecyl sulfate (SDS)/cetyltrimethylammonium bromide (CTAB) and CTAB/Brij (polyoxyethylene lauryl ether, n = 10 and 23) in aqueous and sodium chloride solutions. The relative viscosity of single nonionic surfactant solutions is larger than that of SDS or CTAB solutions. It increases with the number of ethylene oxide groups. In the mixed systems, viscosity deviates from ideal behavior. The deviation results from electrostatic interactions. The surfactant mixture composition affects the self-assembled microstructure and rheology. A new mixed system that forms clear micellar solution above CMC was detected. In CTAB/Brij systems, the experimental data also deviate from ideal behavior due to mixed micelle formation and electroviscous effect. This effect is less pronounced than that of SDS/CTAB system and could be suppressed by adding an electrolyte (NaCl).
