Işık, Mehmet

Işık, Mehmet

Name Variants

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.

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Profesör Doktor

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mehmet.isik@atilim.edu.tr

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Scholarly Output

166

Articles

162

Citation Count

1080

Supervised Theses

3 Scholarly Output Search Results

Now showing 1 - 10 of 166

Citation Count: 10
Ellipsometric study of optical properties of GaS_xSe_1-x layered mixed crystals
(Elsevier Science Bv, 2016) Işık, Mehmet; Gasanly, Nizami; Department of Electrical & Electronics Engineering
Spectroscopic ellipsometry measurements were performed on GaSxSe1-x mixed crystals (0 <= x <= 1) in the 1.2-6.2 eV range. Spectral dependence of optical parameters; real and imaginary components of pseudodielectric function, pseudorefractive index and pseudoextinction coefficient were reported in the present work. Critical point (CP) analyses on second-energy derivative spectra of the pseudodielectric function were accomplished to find the interband transition energies. The revealed energy values were associated with each other taking into account the fact that band gap energy of mixed crystals rises with increase in sulfur content. The variation of CP energies with composition (x) was also plotted. Peaks in the spectra of studied optical parameters and CP energy values were observed to be shifted to higher energy values as sulfur concentration is increased in the mixed crystals. (C) 2016 Elsevier B.V. All rights reserved.
Citation Count: 34
Structural and temperature-dependent optical properties of thermally evaporated CdS thin films
(Elsevier Sci Ltd, 2019) Işık, Mehmet; Gullu, H. H.; Güllü, Hasan Hüseyin; Parlak, M.; Gasanly, N. M.; Department of Electrical & Electronics Engineering
In this work, structural and temperature dependent optical properties of thermally evaporated CdS thin films were investigated. X-ray diffraction, energy dispersive spectroscopy and Raman spectroscopy experiments were carried out to characterize the thin films and obtain information about the crystal structure, atomic composition, surface morphology and vibrational modes. Temperature-dependent transmission measurements were performed in between 10 and 300 K and in the spectral range of 400-1050 nm. The analyses of transmittance spectra were accomplished by two different methods called as the absorption coefficient and the derivative spectrophotometry analyses. All evaluated band gap energy values at each studied temperature were in good agreement with each other depending on the applied analyses techniques. Room temperature gap energy values were found around 2.39 eV and 2.40 eV from absorption coefficient and derivative spectrophotometry analyses, respectively. Band gap energy depending on the sample temperature was studied under the light of two different models to investigate average phonon energy, electron phonon coupling parameter and the rate of change of band gap energy with temperature.
Citation Count: 10
Optical constants of layered structured Ga_0.75In_0.25Se crystals from the ellipsometric measurements
(Pergamon-elsevier Science Ltd, 2012) Işık, Mehmet; Cetin, S. S.; Gasanly, N. M.; Ozcelik, S.; Department of Electrical & Electronics Engineering
We have carried out the spectroscopic ellipsometry measurements on Ga0.75In0.25Se single crystals in the 1.2-6.0 eV spectral range at room temperature. The optical constants, real and imaginary parts of the dielectric function, refractive index and extinction coefficient, were found as a result of analysis of ellipsometric data. The critical point analysis of the second derivative spectra of the dielectric function revealed four interband transition structures with critical point energy values of 3.19, 3.53, 4.10 and 4.98 eV. The results of the analysis were compared with those of the ellipsometric studies performed on GaSe which is the main constituent of the Ga0.75In0.25Se crystal. The obtained critical point energies are in good agreement with the energies of the GaSe crystal reported in the literature. (C) 2012 Elsevier Ltd. All rights reserved.
Al-katkılı ZnO nanoparçacıkların düşük sıcaklık termolüminesans özellikleri
(2018) Işık, Mehmet; Işık, Mehmet; Department of Electrical & Electronics Engineering
Alüminyum katkılı ZnO nanoparçacıklar (AZO nanoparçacıklar) düşük sıcaklık termolüminesans (TL) ve yapısal karakterizayon deneyleri kullanılarak incelendi. Yapısal özellikleri x-ışını kırınımı ve tarayıcı elektron mikroskobu deneyleri kullanılarak çalışıldı. AZO nanoparçacıkların kristal yapısı hegzagonal yapı olarak bulundu. X-ışını deseni katkılanan elemente ait herhangi bir pik göstermedi. Bu katkılamanın başarılı bir şekilde gerçekleştiğini gösterdi. Tarayıcı elektron mikroskobu ölçümleri nanoparçacıkların yüzek morfolojisini gösterdi ve nanoparçaıkların boyutlarının 100 nm civarında olduğu görüldü. Düşük sıcaklık TL deneyleri AZO nanoparçacıkları üzerinde 10-300 K sıcaklık aralığında gerçekleştirildi. TL eğrisi 123 K civarında şiddetli bir pik ve bu pike çakışık 85 K ve 150 K civarında iki pik gösterdi. Eğri fit, ilk yükselme, pik şekli ve farklı ısıtma oranı metotları ilgili tuzak merkezlerinin aktivasyon enerjilerini bulmak için kullanıldı. Analizler pik maksimum sıcaklıları 86.2, 121.5 ve 147.1 K olan 44, 76 ve 165 meV seviyelerinde üç merkezin varlığını gösterdi. Pik şekli metodu analizleri, gözlemlenen pikler için µg parametresinin genel kinetik merterbesini işaret eden 0.42 ve 0.52 arasında olduğunu gösterdi. TL deneyleri 0.1 K/s ve 0.7 K/s arasında farklı ısıtma oranları kullanılarak genişletildi. Tuzak merkezlerinin aktivasyon enerjileri pik maksimum sıcaklığının ısıtma hızına bağımlılığı kullanılarak eld edildi.
Citation Count: 40
Gd-doped ZnO nanoparticles: Synthesis, structural and thermoluminescence properties
(Elsevier Science Bv, 2019) Işık, Mehmet; Gasanly, N. M.; Department of Electrical & Electronics Engineering
Structural and thermoluminescence (TL) properties of Gd-doped ZnO nanoparticles were investigated in the present study. TL glow curves recorded in the 10-300 K range presented three overlapped peaks around 56, 110 and 155 K for undoped ZnO nanoparticles. The Gd-doping to the ZnO nanoparticles resulted in shift of peak maximum temperatures to lower values and decrease of activation energies associated with the observed peaks. Activation energies of trapping centers were also reported. Taking into account the previously reported studies on defect characterization of ZnO compound, revealed centers were associated with interstitial defects. Structural characteristics of the synthesized nanoparticles were investigated by x-ray diffraction and scanning electron microscopy measurements.
Citation Count: 3
Optical characterization of Ga₂SeS layered crystals by transmission, reflection and ellipsometry
(World Scientific Publ Co Pte Ltd, 2015) Işık, Mehmet; Gasanly, Nizami; Department of Electrical & Electronics Engineering
Optical properties of Ga2SeS crystals grown by Bridgman method were investigated by transmission, reflection and ellipsometry measurements. Analysis of the transmission and reflection measurements performed in the wavelength range of 400-1100 nm at room temperature indicated the presence of indirect and direct transitions with 2.28 eV and 2.38 eV band gap energies. Ellipsometry measurements were carried out in the 1.2-6.0 eV spectral region to get information about optical constants, real and imaginary parts of the pseudodielectric function. Moreover, the critical point (CP) analysis of the second derivative spectra of the pseudodielectric constant in the above band gap region was accomplished. The analysis revealed the presence of five CPs with energies of 3.87, 4.16, 4.41, 4.67 and 5.34 eV.
Citation Count: 9
Effect of heating rate on thermoluminescence characteristics of Y₂O₃ nanoparticles
(Elsevier, 2019) Işık, Mehmet; Isik, M.; Gasanly, N. M.; Department of Electrical & Electronics Engineering
The present paper reports the results of heating rate dependencies of thermoluminescence (TL) peaks observed for Y2O3 nanoparticles in the below room temperature region. TL glow curve presented six peaks around 62.5, 91.3, 114.5, 162.7, 196.0 and 214.9 K for heating rate of 0.4 K/s. The increase of heating rate resulted in increase in peak maximum temperature and decrease in peak maximum intensity as expected according to theoretical information. Peak maximum temperature-heating rate dependencies of observed peaks were analysed according to exponential dependency relation. Curve fit and initial rise methods were applied on thermally cleaned individual peaks and activation energies of associated trap centers, frequency factors and order of kinetics were obtained from the analyses. Activation energy values of the revealed trapping centers found from both methods were in good agreement with each other. Moreover, lattice parameters, crystalline size and micro-strain of nanoparticles were investigated by means of x-ray diffraction measurements.
Photo-transferred thermoluminescence characteristics of beta-irradiated MgO nanopowders
(2018) Işık, Mehmet; Işık, Mehmet; Department of Electrical & Electronics Engineering
MgO nanotozlarındaki sığ tuzak merkezleri foto-transfer termolüminesans ölçümleri kullanılarak karakterize edildi. Deneyler sabit ısıtma hızı ile 10-300 K düşük sıcaklık aralığında gerçekleştirildi. Sığ tuzak merkezleri ilk olarak oda sıcaklığında S90/Y90 kaynağı ile ışıma yaptırarak ve daha sonra 10 K sıcaklığında mavi LED kullanılarak ışıklandırarak yük taşıyıcıları ile dolduruldu. Termolüminesans eğrisi 150 K civarında bir pik gösterdi. Eğri fit analizleri bu pikin maksimum sıcaklıkları 149.0 ve 155.3 K olan iki birbirinden ayrı pikten oluştuğunu gösterdi. Piklere karşılık gelen tuzak merkezlerinin aktivasyon enerjileri 0.70 ve 0.91 eV olarak ortaya çıkarıldı. TL süreçlerindeki baskın mekanizma, hızlı geri tuzaklanmanın bant boşluğu içerisinde gerçekleşen geçişlerde etkili olduğunu ifade eden ikinci dereceden mertebe olarak bulundu. Analizler ayrıca ilk yükselme ve pik şekli metotları kullanılarak yapıldı. Tüm sonuçlar birbirleri ile uyumluluk gösterdi. MgO nanotozların yapısal karakterizasyonu x-ışını kırınımı, tarayıcı elektron mikroskobu ve Fourier dönüşümlü kızılötesi spektroskobi ölçümleri ile incelendi. Deneysel gözlemlerin analizleri MgO nanotozların 20-30 nm civarında parçacık boyutuna sahip iyi kristallik özelliği gösterdiğini işaret etti.
Citation Count: 49
First-principle investigation for the hydrogen storage properties of NaXH₃ (X= Mn, Fe, Co) perovskite type hydrides
(Pergamon-elsevier Science Ltd, 2019) Işık, Mehmet; Candan, Abdullah; Gencer, Aysenur; Isik, Mehmet; Department of Electrical & Electronics Engineering
In the present study, NaXH3 (X = Mn, Fe, Co) perovskite type hydrides have been investigated by performing first-principles calculation. The results of the structural optimizations show that all these compounds have negative formation energy implying the thermodynamic stability and synthesisability. The mechanical stability of these compounds has been studied with the elastic constants. Moreover, the polycrystalline properties like bulk modulus, Poisson's ratio, etc. have been obtained using calculated elastic constants of interest compounds. The electronic properties have been studied and band structures have been drawn with the corresponding partial density of states. These plots indicated that NaXH3 hydrides show metallic characteristics. The charge transfer characteristics in these compounds have been studied with the Bader partial charge analysis. The phonon dispersion curves and corresponding density of states indicated that NaXH3 compounds are dynamically stable compounds. The investigation on hydrogen storage characteristics of NaXH3 compounds resulted in hydrogen storage capacities of 3.74, 3.70 and 3.57 wt% for X = Mn, Fe and Co, respectively. The present study is the first investigation of NaXH3 perovskite type hydrides as known up to date and may provide remarkable contribution to the future researches in hydrogen storage applications. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Citation Count: 0
Annealing Effect on Dark Electrical Conductivity and Photoconductivity of Ga-In-Se Thin Films
(Polish Acad Sciences inst Physics, 2018) Işık, Mehmet; Gullu, H. H.; Güllü, Hasan Hüseyin; Department of Electrical & Electronics Engineering
Dark-conductivity and photoconductivity properties of thermally evaporated Ga-In-Se (GIS) thin films were investigated in the temperature range of 80-430 K. All measurements were performed on as-grown and annealed GIS thin films at 300 and 400 degrees C to get information about the effect of the annealing temperature on the conductivity properties. Room temperature conductivity was obtained as 1.8 x 10(-8) Omega(-1) cm(-1) for as-grown films and increased to 3.6 x 10(-4) Omega(-1) cm(-1) for annealed films at 400 degrees C. Analysis of the dark-conductivity data of as-grown films revealed nearly intrinsic type of conductivity with 1.70 eV band gap energy. Temperature dependent dark conductivity curves exhibited two regions in the 260-360 and 370-430 K for both of annealed GIS films. Conductivity activation energies were found as 0.05, 0.16 and 0.05, 0.56 eV for films annealed at temperatures of 300 and 400 degrees C, respectively. The dependence of photoconductivity on illumination intensity was also studied in the range from 17 to 113 mW/cm(2).

Işık, Mehmet

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Scholarly Output

166

Articles

162

Citation Count

1080

Supervised Theses

3

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