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Işık, Mehmet
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Mehmet, Işık
M.,Işık
Isik, Mehmet
Mehmet, Isik
I., Mehmet
I.,Mehmet
Işık,M.
Isik,M.
I.,Mehmet
M.,Isik
Işık, Mehmet
M., Isik
Isik, M.
M.,Işık
Isik, Mehmet
Mehmet, Isik
I., Mehmet
I.,Mehmet
Işık,M.
Isik,M.
I.,Mehmet
M.,Isik
Işık, Mehmet
M., Isik
Isik, M.
Job Title
Profesör Doktor
Email Address
mehmet.isik@atilim.edu.tr
Main Affiliation
Department of Electrical & Electronics Engineering
Status
Former Staff
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ORCID ID
Scopus Author ID
Turkish CoHE Profile ID
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WoS Researcher ID
Sustainable Development Goals
2
ZERO HUNGER
0
Research Products
11
SUSTAINABLE CITIES AND COMMUNITIES
0
Research Products
14
LIFE BELOW WATER
1
Research Products
6
CLEAN WATER AND SANITATION
0
Research Products
1
NO POVERTY
0
Research Products
5
GENDER EQUALITY
0
Research Products
9
INDUSTRY, INNOVATION AND INFRASTRUCTURE
0
Research Products
16
PEACE, JUSTICE AND STRONG INSTITUTIONS
0
Research Products
17
PARTNERSHIPS FOR THE GOALS
0
Research Products
15
LIFE ON LAND
1
Research Products
10
REDUCED INEQUALITIES
0
Research Products
7
AFFORDABLE AND CLEAN ENERGY
11
Research Products
8
DECENT WORK AND ECONOMIC GROWTH
0
Research Products
4
QUALITY EDUCATION
0
Research Products
12
RESPONSIBLE CONSUMPTION AND PRODUCTION
0
Research Products
3
GOOD HEALTH AND WELL-BEING
1
Research Products
13
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0
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This researcher does not have a Scopus ID.
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Scholarly Output
173
Articles
169
Views / Downloads
442/792
Supervised MSc Theses
3
Supervised PhD Theses
0
WoS Citation Count
1802
Scopus Citation Count
1861
WoS h-index
20
Scopus h-index
20
Patents
0
Projects
0
WoS Citations per Publication
10.42
Scopus Citations per Publication
10.76
Open Access Source
11
Supervised Theses
3
Google Analytics Visitor Traffic
| Journal | Count |
|---|---|
| Optical Materials | 17 |
| Physica B: Condensed Matter | 16 |
| Journal of Luminescence | 15 |
| Materials Science in Semiconductor Processing | 14 |
| Journal of Materials Science: Materials in Electronics | 12 |
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29 results
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Now showing 1 - 10 of 29
Article Citation - WoS: 2Citation - Scopus: 2Analysis of Optical Constants and Temperature-Dependent Absorption Edge of Gas0.75se0.25< Layered Crystals(Pergamon-elsevier Science Ltd, 2017) Isik, Mehmet; Gasanly, NizamiGaS0.75Se0.25 single crystals were optically characterized through transmission and reflection measurements in the wavelength range of 450-1000 nm. Derivative spectrophotometry analyses on temperature dependent transmittance spectra showed that band gap energies of the crystal increase from 239 eV (T=300 K) to 2.53 eV (T=10 K). Band gap at zero temperature, average phonon energy, electron phonon coupling parameter and rates of change of band gap energy with temperature were found from the temperature dependences of band gap energies under the light of different models reported in literature. Furthermore, the dispersion of room temperature refractive index was discussed in terms of single effective oscillator model. The refractive index dispersion parameters, namely oscillator and dispersion energies, zero-frequency refractive index, were determined as a result of analyses. (C) 2017 Elsevier Ltd. All rights reserved.Article Citation - WoS: 2Citation - Scopus: 2Temperature-Tuned Band Gap Energy and Oscillator Parameters of Gas0.5se0.5< Single Crystals(Elsevier Gmbh, Urban & Fischer verlag, 2016) Isik, Mehmet; Tugay, Evrin; Gasanly, NizamiTemperature-dependent transmission and room temperature reflection measurements were carried out on GaS0.5Se0.5 single crystal in the wavelength range of 380-1000 nm to investigate its optical parameters. The analysis of the temperature-dependent absorption data showed that direct and indirect band gap energies increase from 2.36 to 2.50 eV and 2.27 to 2.40 eV, respectively, as temperature is decreased from 300 to 10 K. The rates of change of the direct and indirect band gap energies with temperature was found around -7.4 x 10(-4) eV/K from the analysis of experimental data under the light of theoretical relation giving the band gap energy as a function of temperature. The absolute zero value of the band gap energies were also found from the same analysis as 2.50 eV (for direct) and 2.40 eV (for indirect). Wemple-DiDomenico single effective oscillator model, Sellmeier oscillator model and Spitzer-Fan model were used for the room temperature reflection data to find optical parameters of the crystal. (C) 2016 Elsevier GmbH. All rights reserved.Article Citation - WoS: 140Citation - Scopus: 140CaXH3 (X = Mn, Fe, Co) perovskite-type hydrides for hydrogen storage applications(Wiley, 2020) Surucu, Gokhan; Gencer, Aysenur; Candan, Abdullah; Gullu, Hasan H.; Isik, MehmetHydrogen storage is one of the attractive research interests in recent years due to the advantages of hydrogen to be used as energy source. The studies on hydrogen storage applications focus mainly on investigation of hydrogen storage capabilities of newly introduced compounds. The present paper aims at characterization of CaXH3 (X: Mn, Fe, or Co) perovskite-type hydrides for the first time to understand their potential contribution to the hydrogen storage applications. CaXH3 compounds have been investigated by density functional theory studies to reveal their various characteristics and hydrogen storage properties. CaXH3 compounds have been optimized in cubic crystal structure and the lattice constants of studied compounds have been obtained as 3.60, 3.50, and 3.48 angstrom for X: Mn, Fe, and Co compounds, respectively. The optimized structures have negative formation enthalpies pointing out that studied compounds are thermodynamically stable and could be synthesized experimentally. The gravimetric hydrogen storage densities of X: Mn, Fe, and Co compounds were found in as 3.09, 3.06, and 2.97 wt%, respectively. The revealed values for hydrogen storage densities indicate that CaXH3 compounds may be potential candidates for hydrogen storage applications. Moreover, various mechanical parameters of interest compounds like elastic constants, bulk modulus, and Poisson's ratio have been reported throughout the study. These compounds were found mechanically stable with satisfying Born stability criteria. Further analyses based on Cauchy pressure and Pugh criterion, showed that they have brittleness nature and relatively hard materials. In addition, the electronic characteristics, band structures, and associated partial density of states of CaXH3 hydrides have been revealed. The dynamic stability behavior of them was verified based on the phonon dispersion curves.Article Citation - WoS: 10Citation - Scopus: 11Low Temperature Thermoluminescence of Gd2o3< Nanoparticles Using Various Heating Rate and tmax< - texc< Methods(Elsevier, 2019) Delice, Serdar; Isik, Mehmet; Gasanly, Nizami M.Thermoluminescence (FL) measurements for Gd2O3 nanoparticles were carried out for various heating rates between 0.3 and 0.8 K/s at low temperatures (10-280 K). TL spectrum exhibited two observable and one faint peaks in the temperature region of 10-100 K, and four peaks in the temperature region of 160-280 K. Heating rate analysis was achieved to understand the behaviors of trap levels. It was seen that the peak maximum temperatures and TL intensities of all peaks increase with increasing heating rate. This behavior was ascribed to anomalous heating rate effect. T-max - T(exc )analysis was accomplished for TL, peaks at relatively higher temperature region to reveal the related traps depths. T-max - T-exc plot presented a staircase structure indicating that the TL glow curve is composed of well separated glow peaks. Mean activation energies of trapping centers corresponding to these separated peaks were found as 0.43, 0.50, 0.58 and 0.80 eV.Article Citation - WoS: 7Citation - Scopus: 6Tunable Nonlinear Absorption and Optical Limiting Behavior of Nabi(moxw1-x< Single Crystals With Ratio of Molybdenum/Tungsten(Iop Publishing Ltd, 2023) Pepe, Yasemin; Yildiz, Elif Akhuseyin; Isik, Mehmet; Karatay, Ahmet; Gasanly, Nizami; Elmali, AyhanThe compositional effect of Mo/W ratio on linear, nonlinear absorption and optical limiting behavior of the NaBi(MoxW1-xO4)(2) single crystals grown by Czochralski technique was investigated. X-ray diffraction patterns of the studied crystals presented well-defined peaks associated with the tetragonal crystalline structure. The nonlinear absorption performance and optical limiting threshold were determined using an open-aperture Z-scan technique. A theoretical model including one photon absorption (OPA), two photon absorption (TPA) and free carrier absorption was used to determine the nonlinear absorption parameters. All of the results showed that defect states, which strongly affect nonlinear absorption (NA) and optical limiting behaviors, can be tuned with the Mo/W ratio, enabling NaBi(MoxW1-xO4)(2) single crystals to be used in desired optoelectronic applications. Linear optical absorption analysis revealed that bandgap energy and defect states can be tuned by changing the Mo/W ratio in the crystal structure. The obtained results showed that all the studied crystals had NA behavior and the nonlinear absorption coefficient decreased with increasing Mo/W ratio. Sequential TPA is the main NA mechanism for these crystals due to the fact that the incident light energy is lower than the bandgap energies and the existence of the real intermediate state around 2.32 eV.Article Citation - WoS: 21Citation - Scopus: 21The Role of Defects on the Transition From Saturable Absorption To Nonlinear Absorption of Bi12geo20< Single Crystal Under Increasing Laser Excitation(Elsevier, 2022) Pepe, Yasemin; Isik, Mehmet; Karatay, Ahmet; Yildiz, Elif Akhuseyin; Gasanly, Nizami; Elmali, AyhanThis work reports defect and input intensity dependent nonlinear optical behaviors of Bi12GeO20 (BGO) single crystal. Open aperture (OA) Z-scan experiments were performed with 532 nm excitation wavelength under 4 ns and 100 fs pulsed laser irradiation. Obtained data were fitted with a theoretical model considering one-photon, two-photon and free carrier absorption contributions to nonlinear absorption due to longer lifetime of localized defect states than that of used laser pulse durations. At low input intensities, the BGO single crystal showed saturable absorption (SA) behavior and transition to nonlinear absorption (NA) behavior observed with further increase of the input intensities both of pulse durations. At low input intensity, the OPA mechanism is dominant and results in SA by filling of the defect states due to defect state at around one photon energy (2.32 eV). At higher input intensity, multi-photon, two-photon and free carrier absorption become dominant mechanisms, and nonlinear absorption behavior was observed. The lowest saturation threshold was found as 1.36 x 1010 W/cm2 with nanosecond pulses. We have revealed the mechanisms contributing both SA and NA, and determined saturation intensity threshold and effective nonlinear absorption coefficients. Our findings indicate that the tails of defect states overlap in the energy band gaps especially in sufficiently disordered crystal. With this way, the spectral range for saturable absorption and nonlinear absorption can be broadened.Article Citation - WoS: 1Citation - Scopus: 1Analysis of Glow Curve of Gas0.5se0.5< Single Crystals(Elsevier Science Bv, 2015) Isik, Mehmet; Delice, Serdar; Gasanly, NizamiCharacterization of shallow trapping centers in GaS0.5Se0.5 crystals grown by a Bridgman method was carried out in the present work using thermoluminescence (TL) measurements performed in the low temperature range of 10-300 K. The activation energies of the trapping centers were obtained under the light of results of various analysis methods. The presence of three trapping centers located at 6, 30 and 72 meV was revealed. The analysis of the experimental glow curve gave reasonable results under the model that assumes slow retrapping which states the order of kinetics as b=1. Heating rate dependence of the observed TL peaks was studied for the rates between 0.4 and 1.0 K/s. Distribution of the traps was also investigated using an experimental technique based on the thermal cleaning of centers giving emission at lower temperatures. The distributed levels with activation energies increasing from 6 to 136 meV were revealed by increasing the stopping temperature from 10 to 52 K. (C) 2015 Elsevier B.V. All rights reserved.Article Citation - WoS: 24Citation - Scopus: 23Revealing the Effects of Defect States on the Nonlinear Absorption Properties of the Tlinsse and Tl2in2< Crystals in Near-Infrared Optical Limiting Applications(Amer Chemical Soc, 2024) Dogan, Anil; Karatay, Ahmet; Isik, Mehmet; Yildiz, Elif Akhuseyin; Gasanly, Nizami Mamed; Elmali, AyhanThe present study represents the effect of defect states on the nonlinear absorption and optical limiting performances of TlInSSe and Tl2In2S3Se single crystals with near-infrared excitations. The band gap energies were 2.2 and 2.22 eV, and the Urbach energies were 0.049 and 0.034 eV for TlInSSe and Tl2In2S3Se, respectively. The trapping time of localized defect states was found to be 8 ns by femtosecond transient absorption measurements. The analysis of open-aperture Z-scan data depends on two different fitting models to determine the effect of defect states on the nonlinear absorption (NA) properties of the studied crystals. Model 1 only considers two-photon absorption (TPA), while model 2 includes one-photon absorption (OPA), TPA, and free carrier absorption (FCA). The NA coefficients (ss(eff)) obtained from model 2 are higher than the values (ss) obtained from model 1 at the same intensities, revealing that defect states contribute to NA through OPA. The optical limiting properties of the TlInSSe and Tl2In2S3Se crystals were examined under 1064 nm wavelength excitation. The limiting thresholds were found to be 1.16 and 0.27 mJ/cm(2) at 29.8 GW/m(2) and 99.5 GW/m(2) input intensities, respectively. The results show that TlInSSe and Tl2In2S3Se crystals have promising potential for near-infrared optical limiting applications.Article Citation - WoS: 8Citation - Scopus: 9Revealing Photoluminescence and Nonlinear Optical Absorption Characteristics of Pbmo0.75w0.25< Single Crystal for Optical Limiting Applications(Iop Publishing Ltd, 2024) Dogan, Anil; Karatay, Ahmet; Isik, Mehmet; Yildiz, Elif Akhuseyin; Gasanly, Nizami; Elmali, AyhanNonlinear absorption properties of PbMo0.75W0.25O4 single crystal fabricated by the Czochralski method were studied. The band gap energy of the crystal was determined as 3.12 eV. Urbach energy which represents the defect states inside the band gap was found to be 0.106 eV. PbMo0.75W0.25O4 single crystal has a broad photoluminescence emission band between 376 and 700 nm, with the highest emission intensity occurring at 486 nm and the lowest intensity peak at 547 nm, depending on the defect states. Femtosecond transient absorption measurements reveal that the lifetime of localized defect states is found to be higher than the 4 ns pulse duration. Open aperture (OA) Z-scan results demonstrate that the PbMo0.75W0.25O4 single crystal exhibits nonlinear absorption (NA) that includes two-photon absorption (TPA) as the dominant mechanism at the 532 nm excitations corresponding to 2.32 eV energy. NA coefficient (beta(eff)) increased from 7.24 x 10(-10) m W-1 to 8.81 x 10(-10) m W-1 with increasing pump intensity. At higher intensities beta(eff) tends to decrease with intensity increase. This decrease is an indication that saturable absorption (SA) occurred along with the TPA, called saturation of TPA. The lifetime of the defect states was measured by femtosecond transient absorption spectroscopy. Saturable absorption behavior was observed due to the long lifetime of the localized defect states. Closed aperture (CA) Z-scan trace shows the sign of a nonlinear refractive index. The optical limiting threshold of PbMo0.75W0.25O4 single crystal at the lowest intensity was determined as 3.45 mJ/cm(2). Results show that the PbMo0.75W0.25O4 single crystal can be a suitable semiconductor material for optical limiting applications in the visible region.Article Citation - WoS: 5Citation - Scopus: 5Optical Characterization of Cuin5s8< Crystals by Ellipsometry Measurements(Pergamon-elsevier Science Ltd, 2016) Isik, Mehmet; Gasanly, NizamiOptical properties of CuIn5S8 crystals grown by Bridgman method were investigated by ellipsometry measurements. Spectral dependence of optical parameters; real and imaginary parts of the pseudodielectric function, pseudorefractive index, pseudoextinction coefficient, reflectivity and absorption coefficients were obtained from the analysis of ellipsometry experiments performed in the 1.2-6.2 eV spectral region. Analysis of spectral dependence of the absorption coefficient revealed the existence of direct band gap transitions with energy 1.53 eV. Wemple-DiDomenico and Spitzer-Fan models were used to find the oscillator energy, dispersion energy, zero-frequency refractive index and high-frequency dielectric constant values. Structural properties of the CuIn5S8 crystals were investigated using X-ray diffraction and energy dispersive spectroscopy analysis. (C) 2015 Elsevier Ltd. All rights reserved.
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