Browsing by Subject "Optical properties"

Now showing 1 - 20 of 90

Open Access
Anharmonicity in GaTe layered crystals
(Wiley-VCH Verlag GmbH & Co. KGaA, 2002) Aydınlı, Atilla; Gasanly, N. M.; Uka, A.; Efeoglu, H.
The temperature dependencies (10-300 K) of seven Raman-active mode frequencies in layered semiconductor gallium telluride have been measured in the frequency range from 25 to 300 cm -1. Softening and broadening of the optical phonon lines are observed with increasing temperature. Comparison between the experimental data and theories of the shift of the phonon lines during heating of the crystal showed that the experimental dependencies can be explained by contributions from thermal expansion and lattice anharmonicity. Lattice anharmonicity is determined to be due to three-phonon processes.
Open Access
Anisotropic electronic, mechanical, and optical properties of monolayer WTe2
(American Institute of Physics Inc., 2016) Torun, E.; Sahin, H.; Cahangirov, S.; Rubio, A.; Peeters, F. M.
Using first-principles calculations, we investigate the electronic, mechanical, and optical properties of monolayer WTe2. Atomic structure and ground state properties of monolayer WTe2 (Td phase) are anisotropic which are in contrast to similar monolayer crystals of transition metal dichalcogenides, such as MoS2, WS2, MoSe2, WSe2, and MoTe2, which crystallize in the H-phase. We find that the Poisson ratio and the in-plane stiffness is direction dependent due to the symmetry breaking induced by the dimerization of the W atoms along one of the lattice directions of the compound. Since the semimetallic behavior of the Td phase originates from this W-W interaction (along the a crystallographic direction), tensile strain along the dimer direction leads to a semimetal to semiconductor transition after 1% strain. By solving the Bethe-Salpeter equation on top of single shot G0W0 calculations, we predict that the absorption spectrum of Td-WTe2 monolayer is strongly direction dependent and tunable by tensile strain.
Open Access
Anisotropic stimulated emission from aligned CdSe/CdS dot-in-rods
(IEEE, 2014-10) Gao, Y.; Ta, V. D.; Zhao, X.; Wang, Y.; Chen, R.; Zhao, Y.; Dang, C.; Sun, X.; Sun, H.; Demir, Hilmi Volkan
Anisotropic optical properties of CdSe/CdS dot-in-rods loaded in a capillary tube are demonstrated, suggesting nanorods' alignment with a microfluidic approach. Polarized emissions from photoluminescence and whispering gallery mode lasing show promising applications for lighting and displays. © 2014 IEEE.
Open Access
Atomic-layer-deposited zinc oxide as tunable uncooled infrared microbolometer material
(Wiley-VCH Verlag, 2014) Battal, E.; Bolat, S.; Tanrikulu, M. Y.; Okyay, Ali Kemal; Akin, T.
ZnO is an attractive material for both electrical and optical applications due to its wide bandgap of 3.37 eV and tunable electrical properties. Here, we investigate the application potential of atomic-layer-deposited ZnO in uncooled microbolometers. The temperature coefficient of resistance is observed to be as high as-10.4% K-1 near room temperature with the ZnO thin film grown at 120 °C. Spectral noise characteristics of thin films grown at various temperatures are also investigated and show that the 120 °C grown ZnO has a corner frequency of 2 kHz. With its high TCR value and low electrical noise, atomic-layer-deposited (ALD) ZnO at 120 °C is shown to possess a great potential to be used as the active layer of uncooled microbolometers. The optical properties of the ALD-grown ZnO films in the infrared region are demonstrated to be tunable with growth temperature from near transparent to a strong absorber. We also show that ALD-grown ZnO can outperform commercially standard absorber materials and appears promising as a new structural material for microbolometer-based applications. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Band gap and optical transmission in the Fibonacci type one-dimensional A5B6C7 based photonic crystals
(Wiley-VCH Verlag, 2015) Simsek S.; Koc, H.; Palaz S.; Oltulu, O.; Mamedov, A. M.; Özbay, Ekmel
In this work, we present an investigation of the optical properties and band structure calculations for the photonic crystal structures (PCs) based on one-dimensional (1D) photonic crystal. Here we use 1D A5B6C7(A:Sb; B:S,Se; C:I) based layers in air background. We have theoretically calculated photonic band structure and optical properties of A5B6C7(A:Sb; B:S,Se; C:I) based PCs. In our simulation, we employed the finite-difference time domain (FDTD) technique and the plane wave expansion method (PWE) which implies the solution of Maxwell equations with centered finite-difference expressions for the space and time derivatives. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Band Structure and Optical Properties of Kesterite Type Compounds: First principle calculations
(Institute of Physics Publishing, 2017) Palaz S.; Unver H.; Ugur G.; Mamedov, Amirullah; Özbay, Ekmel
In present work, our research is mainly focused on the electronic structures, optical and magnetic properties of Cu2FeSnZ4 (Z = S, Se) compounds by using ab initio calculations within the generalized gradient approximation (GGA). The calculations are performed by using the Vienna ab-initio simulation package (VASP) based on the density functional theory. The band structure of the Cu2FeSnZ4 ( Z = S, Se) compounds for majority spin (spin-up) and minority spin (spin-down) were calculated. It is seen that for these compounds, the majority spin states cross the Fermi level and thus have the metallic character, while the minority spin states open the band gaps around the Fermi level and thus have the narrow-band semiconducting nature. For better understanding of the electronic states, the total and partial density of states were calculated, too. The real and imaginary parts of dielectric functions and hence the optical functions such as energy-loss function, the effective number of valance electrons and the effective optical dielectric constant for Cu2FeSnZ4 (Z = S, Se) compounds were also calculated. © Published under licence by IOP Publishing Ltd.
Open Access
Characteristic equations for the lasing Modes of infinite periodic chain of quantum wires
(IEEE, 2008-06) Byelobrov, V. O.; Benson, T. M.; Altıntaş, Ayhan; Nosich, A.I.
In this paper, we study the lasing modes of a periodic open optical resonator. The resonator is an infinite chain of active circular cylindrical quantum wires standing in tree space. Characteristic equations for the frequencies and associated linear thresholds of lasing are derived. These quantities are considered as eigenvalues of specific electromagnetic-field problem with "active" imaginary part of the cylinder material's refractive index - Lasing Eigenvalue Problem (LEP). ©2008 IEEE.
Open Access
Co doping induced structural and optical properties of sol-gel prepared ZnO thin films
(Elsevier BV, 2014-11) Gungor, E.; Gungor, T.; Caliskan, D.; Ceylan, A.; Özbay, Ekmel
The preparation conditions for Co doping process into the ZnO structure were studied by the ultrasonic spray pyrolysis technique. Structural and optical properties of the Co:ZnO thin films as a function of Co concentrations were examined. It was observed that hexagonal wurtzite structure of ZnO is dominant up to the critical value, and after the value, the cubic structural phase of the cobalt oxide appears in the X-ray diffraction patterns. Every band-edge of Co:ZnO films shifts to the lower energies and all are confirmed with the PL measurements. Co substitution in ZnO lattice has been proved by the optical transmittance measurement which is observed as the loss of transmission appearing in specific region due to Co2+ characteristic transitions. © 2014 Elsevier B.V. All rights reserved.
Open Access
Color vision in humans and computers
(IEEE, 2008) Boyacı, Hüseyin; Akarun L.
Humans and many other species rely on color for object recognition. What are the biological underpinnings of color vision and how can we computationally model human color perception? In this study we briefly summarize recent advences regarding the very early, retinal stages of color vision, as well as recent behavioral models of color perception in three dimensional world within rich context. We also emphasize the recent events on the neuroimaging front that allow the researchers begin to systematically study the cortical processes related to color vision. ©2008 IEEE.
Open Access
Comparison of trimethylgallium and triethylgallium as "ga" source materials for the growth of ultrathin GaN films on Si (100) substrates via hollow-cathode plasma-assisted atomic layer deposition
(AVS Science and Technology Society, 2016-02) Alevli, M.; Haider A.; Kizir S.; Leghari, S. A.; Bıyıklı, Necmi
GaN films grown by hollow cathode plasma-assisted atomic layer deposition using trimethylgallium (TMG) and triethylgallium (TEG) as gallium precursors are compared. Optimized and saturated TMG/TEG pulse widths were used in order to study the effect of group-III precursors. The films were characterized by grazing incidence x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy, and spectroscopic ellipsometry. Refractive index follows the same trend of crystalline quality, mean grain, and crystallite sizes. GaN layers grown using TMG precursor exhibited improved structural and optical properties when compared to GaN films grown with TEG precursor.
Open Access
Cucurbit [7] uril-threaded fluorene-thiophene-based conjugated polyrotaxanes
(Royal Society of Chemistry, 2016) Idris, M.; Bazzar, M.; Guzelturk, B.; Demir, Hilmi Volkan; Tuncel, D.
Here we investigate the effect of cucurbit[7]uril (CB7) on the thermal and optical properties of fluorene-thiophene based conjugated polyelectrolytes. For this purpose, poly(9,9′-bis(6′′-(N,N,N-trimethylammonium)hexyl)fluorene-alt-co-thiophenelene) P1 and poly(9,9′-bis(6′′-(N,N,N-trimethylammonium)propyl)fluorene-alt-co-thiophenelene) P2 and their CB7-based polyrotaxane counterparts, P1CB7 and P2CB7, are synthesized by threading the part of the conjugated backbone of these polymers with CB7 during their synthesis. Threading efficiency in the P1CB7 containing hexyl pendant of as high as 50% is achieved, but in the case of P2, with the propyl pendant, only around 15% is achieved. We observed significant changes in the optical properties of both P1CB7 and P2CB7 with respect to their polymers P1 and P2. Fluorescent quantum yields of P1 and P2 which are 0.11 and 0.35 have increased to 0.46 and 0.55 for P1CB7 (>4 fold) and P2CB7, respectively. Moreover, polyrotaxanes compared to their polymers exhibit longer fluorescence lifetimes in the solution and the solid state thanks to the suppressed overall nonradiative recombination via encapsulation of the conjugated polymer backbone. Thermal analysis also indicates that polyrotaxanes have higher thermal stabilities than their polymer counterparts. In order to demonstrate the applicability of the synthesized materials, we also fabricated proof-of-concept light emitting diodes from P1 and its CB7-based polyrotaxane counterpart P1CB7. The CB7-integrating polymer showed lower turn-on voltages with high electroluminescence colour purity due to balanced charge injection in P1CB7 as compared to the P1 polymer.
Open Access
Defect luminescence in some layered binary chalcogenide semiconductors
(Scientific.Net, 2002) Aydınlı, Atilla; Gasanly, N. M.
A number of semiconductors such as GaS, GaSe, GaSSe show layered structure where intralayer bonding is strong and interlayer bonding quite weak. With bandgaps mostly in the visible and the near infrared and high crystal structure anisotropy, such semiconductors offer interesting possibilities for optoelectronic applications In this review, we will summarize the recent developments on the photoluminescent properties of these materials such as luminescence due to donor-acceptor pair recombination. As these materials are undoped, the observed photoluminescence is attributed mostly to defect states present in these materials.
Open Access
Dependence of the photoluminescence of Tl2InGaS4 layered crystal on temperature and excitation intensity
(Pergamon Press, 1998) Gasanly, N. M.; Serpengüzel, A.; Gürlü, O.; Aydınlı, A.; Yılmaz, I.
The emission band spectra of Tl2InGaS4 layered crystals were investigated in the 10-120 K temperature range and in the 540-860 nm wavelength range using photoluminescence (PL). The peak energy position of the emission band is located at 1.754 eV (707 nm) at 10 K. The emission band has a half-width of 0.28 eV and an asymmetric Gaussian lineshape. The increase of the half-width of the emission band, the blue shift of the emission band peak energy and the quenching of the PL with increasing temperature is explained using the configuration coordinate model. The blue shift of the emission band peak energy and the sublinear increase of the emission band intensity with increasing excitation intensity is explained using the inhomogenously spaced donor-acceptor pair recombination model. © 1998 Elsevier Science Ltd. All rights reserved.
Open Access
Dynamic nonlinear optical processes in some oxygen-octahedra ferroelectrics: first principle calculations
(Taylor & Francis Inc., 2015) Simsek S.; Koc, H.; Palaz S.; Oltulu, O.; Mamedov, A. M.; Özbay, Ekmel
The nonlinear optical properties and electro-optic effects of some oxygen-octahedra ferroelectrics are studied by the density functional theory (DFT) in the local density approximation (LDA) expressions based on first principle calculations without the scissor approximation. We present calculations of the frequency- dependent complex dielectric function and the second harmonic generation response coefficient over a large frequency range in tetragonal and rhombohedral phases. The electronic linear electro-optic susceptibility is also evaluated below the band gap. These results are based on a series of the LDA calculation using DFT. The results for are in agreement with the experiment below the band gap and those for are compared with the experimental data where available. © 2015 Taylor & Francis Group, LLC.
Open Access
The effect of cucurbit[n]uril on the solubility, morphology, and the photophysical properties of nonionic conjugated polymers in an aqueous medium
(2010) Tuncel, D.; Artar, M.; Hanay, S. B.
The effects of cucurbit[n]uril on the dissolution and the photophysical properties of nonionic conjugated polymers in water are described. For this purpose, a fluorine-based polymer, namely, poly[9,9-bis{6(N,N-dimethylamino) hexyl}fluorene-co-2,5-thienylene (PFT) was synthesized and characterized by spectroscopic techniques including 1D and 2D NMR, UV-vis, fluorescent spectroscopy, and matrix-assisted laser desorption mass spectrometry (MALDI-MS). For the first time, it was demonstrated that a nonionic conjugated polymer can be made soluble in water through an inclusion complex formation with CB8. The structure of the complex was elucidated by NMR experiments including 1H and selective 1D-NOESY. This complex emits green and is highly fluorescent with fluorescent quantum yield of 35%. In contrast, CB6 or water-soluble CB7 although they are chemically identical to CB8 do not have any effect on the dissolution and photophysical properties of PFT. By preparing a protonated version of PFT, the optical properties of PFT in methanol, protonated PFT and PFT@CB8 in water have been studied and compared. It was also observed that the morphology of the polymer PFT was affected by the presence of CB8. Thus CB8-assisted self-assembly of polymer chains leads to vesicles formation; these structures were characterized by DLS, AFM, SEM, and TEM fluorescent optical microscopy.
Open Access
Effect of O2/Ar flow ratio and post-deposition annealing on the structural, optical and electrical characteristics of SrTiO3 thin films deposited by RF sputtering at room temperature
(Elsevier, 2015) Goldenberg, E.; Bayrak, T.; Ozgit Akgun, C.; Haider A.; Leghari, S.A.; Kumar, M.; Bıyıklı, Necmi
SrTiO3 (STO) thin films have been prepared by reactive RF magnetron sputtering on Si (100) and UV fused silica substrates at room temperature. The effect of oxygen flow on film characteristics was investigated at a total gas flow of 30 sccm, for various O2/O2 + Ar flow rate ratios. As-deposited films were annealed at 700 °C in oxygen atmosphere for 1 h. Post-deposition annealing improved both film crystallinity and spectral transmittance. Film microstructure, along with optical and electrical properties, was evaluated for both as-deposited and annealed films. Abroad photoluminescence emission was observed within the spectral range of 2.75–3.50 eV for all STO thin films irrespective of their deposition parameters. Upon annealing, the optical band gap of the film deposited with 0% O2 concentration slightly blue-shifted, while the other samples grown at higher oxygen partial pressure did not show any shift. Refractive indices (n) (at 550 nm) were in the range of 2.05 to 2.09, and 2.10 to 2.12 for as-deposited and annealed films, respectively. Dielectric constant values (at 100 kHz) within the range of 30–66 were obtained for film thicknesses less than 300 nm, which decreased to ~30–38 after postdeposition annealing.
Open Access
Effect of solvent refractive index on the surface plasmon resonance nanoparticle optical absorption
(2007) Ertas, G.; Süzer, Şefik
Optical properties of plasmon coupled silver and gold nanoparticles were studied as a function of the refractive index of the surrounding medium. Our studies confirmed that the effect of changes in the refractive index of the surrounding medium was more difficult to demonstrate from an experimental point of view, because of the very high susceptibility of nanoparticles to aggregate in aqueous and organic solvents. Whereas the position of the absorption bands of triiodide in these solvents shows a clear dependence on medium's refractive index, the surface plasmon band position of silver and gold nanoparticles do not exhibit the same dependence. This is attributed to a non-negligible interaction of these solvents with nanoparticle surfaces. Copyright © 2007 American Scientific Publishers All rights reserved.
Open Access
Effect of substrate temperature and Ga source precursor on growth and material properties of GaN grown by hollow cathode plasma assisted atomic layer deposition
(IEEE, 2016) Haider, Ali; Kizir, Seda; Deminskyi, P.; Tsymbalenko, Oleksandr; Leghari, Shahid Ali; Bıyıklı, Necmi; Alevli, M.; Gungor, N.
GaN thin films grown by hollow cathode plasma-assisted atomic layer deposition (HCPA-ALD) at two different substrate temperatures (250 and 450 °C) are compared. Effect of two different Ga source materials named as trimethylgallium (TMG) and triethylgallium (TEG) on GaN growth and film quality is also investigated and reviewed. Films were characterized by X-ray photoelectron spectroscopy, spectroscopic ellipsometery, and grazing incidence X-ray diffraction. GaN film deposited by TMG revealed better structural, chemical, and optical properties in comparison with GaN film grown with TEG precursor. When compared on basis of different substrate temperature, GaN films grown at higher substrate temperature revealed better structural and optical properties.
Open Access
The effects of surface treatment on optical and vibrational properties of stain-etched silicon
(Pergamon Press, 1995) Kalem, Ş.; Göbelek, D.; Kurtar, R.; Mısırlı, Z.; Aydınlı, A.; Ellialtioǧlu, R.
The effects of surface treatment on optical and vibrational properties of porous silicon. (por-Si) layers grown on p-type Si wafers by electroless etching technique were studied by FTIR spectroscopy and photoluminescence (PL). The results indicate a correlatiora between the PL intensity and the strength of the absorption bands induced by mulltihydride complexes (SiHn, n ≥ 2). However, similar correlation was also established for monohydride species as evidenced from the layers containing no multihydrides. Furthermore, a new band is observed at 710 cm-1 and assigned to multihydrides suggesting a ne it, local bonding environment in these layers. © 1995.
Open Access
Elastic and optical properties of sillenites: First principle calculations
(Taylor & Francis, 2020-04) Koç, H.; Palaz, S.; Şimşek, Ş.; Mamedov, Amirullah M.; Özbay, Ekmel
In the present paper, we have investigated the electronic structure of some sillenites - Bi12MO20 (M = Ti, Ge, and Si) compounds based on the density functional theory. The mechanical and optical properties of Bi12MO20 have also been computed. The second-order elastic constants have been calculated, and the other related quantities have also been estimated in the present work. The band gap trend in Bi12MO20 can be understood from the nature of their electronic structures. The obtained electronic band structure for all Bi12MO20 compounds is semiconductor in nature. Similar to other oxides, there is a pronounced hybridization of electronic states between M-site cations and anions in Bi12MO20. Based on the obtained electronic structures, we further calculate the frequency-dependent dielectric function and other optical functions.