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Article Citation - WoS: 25Citation - Scopus: 25Temperature-Tuned Band Gap Properties of Mos2 Thin Films(Elsevier, 2020) Surucu, O.; Isik, M.; Gasanly, N. M.; Terlemezoglu, M.; Parlak, M.MoS2 is one of the fascinating members of transition metal dichalcogenides and has attracted great attention due to its various optoelectronic device applications and its characteristic as two-dimensional material. The present paper reports the structural and temperature tuned optical properties of MoS2 thin films grown by RF magnetron sputtering technique. It was observed that the atomic composition ratio of Mo:S was nearly equal to 1:2 and the deposited thin films have hexagonal crystalline structure exhibiting Raman peaks around 376 and 410 cm(-1). The band gap energies were determined as 1.66 and 1.71 eV at 300 and 10 K, respectively and temperature dependency of band gap energy was analyzed by means of Varshni and O'Donnell-Chen models. (C) 2020 Elsevier B.V. All rights reserved.Article Citation - WoS: 11Citation - Scopus: 11Structural and Temperature-Tuned Optical Characteristics of Bi12geo20< Sillenite Crystals(Elsevier, 2020) Delice, S.; Isik, M.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.Sillenite compounds exhibit unique photorefractive and electro-optic characteristics providing attractiveness to these materials in various optoelectronic applications. The present paper aims at investigating one of the members of this family. Structural and optical characteristics of Bi12GeO20 (BGO) were studied by means of x-ray diffraction, Raman spectroscopy and temperature-dependent transmittance measurements. Obtained transmission curves in the wavelength range of 350-1100 nm and at different applied temperatures between 10 and 300 K were employed to find out the absorption coefficient dependence on the photon energy. Tauc relation revealed the presence of an energy gap of 2.49 eV at room temperature. Extension of energy gap up to 2.57 eV due to decreased temperature down to 10 K was deduced by the analysis. In order to have reliable results, the energy gap value was corroborated by utilizing derivative spectral method and well consistency between both methods was indicated. Energy gap change with temperature was also discussed in the study using an empirical formula developed by Varshni. Energy gap at absolute zero and rate of band gap alteration with temperature were determined as 2.57 eV and -2.4 x 10(-4) eV K (- 1), respectively. Taking into account the previously reported studies on investigation of band gap characteristics of BGO, intrinsic Bi-Ge(3+) + V-O(+) defect could be responsible for the revealed energy value of 2.49 eV which is much lower than reported band gap energy of similar to 3.2 eV.Article Citation - WoS: 1Citation - Scopus: 1Growth and Optical Properties of (na0.5bi0.5< (x=0.25) Single Crystal: a Potential Candidate for Optoelectronic Devices(Springer, 2024) Guler, I.; Isik, M.; Gasanly, N.Double tungstates (DT) and double molybdates (DM) have significant importance because of their optoelectronic applications. Regarding the importance of DT and DM, we investigated experimentally structural and optical properties of (Na0.5Bi0.5)(Mo1-xWx)O-4 (x = 0.25) crystal that belongs to the NaBi-DT and DM crystals group. Czochralski method was used to grow the single crystals. The structure of the crystal was identified using X-ray diffraction (XRD) measurements. Two sharp peaks associated with tetragonal crystal structure appeared in the pattern. Vibrational modes of the studied crystal were obtained from the Raman experiments. By the help of the Fourier transform infrared spectrophotometer (FTIR) measurements, infrared transmittance spectrum of the studied compound was recorded. Band gap energy wase found around 3.04 eV using two methods, Tauc and derivative analysis, based on transmission spectrum. Based on the analysis of absorption coefficient, Urbach energy was obtained as 0.22 eV. The revealed structural and optical properties of the crystal indicated that the material may be a candidate for optoelectronic devices in which NaBi(MoO4)(2) and NaBi(WO4)(2) materials are utilized.Article Citation - WoS: 14Citation - Scopus: 15Linear and Nonlinear Optical Properties of Bi12geo20 Single Crystal for Optoelectronic Applications(Elsevier Sci Ltd, 2023) Isik, M.; Gasanly, N. M.The present paper aims at presenting linear and nonlinear optical properties of Bi12GeO20 single crystals grown by Czochralski method. Transmission and reflection measurements were performed in the 400-1000 nm region. The recorded spectra were analyzed considering well-known optical models. Spectral dependencies of absorption coefficient, skin depth, refractive index, real and imaginary components of dielectric function were presented. The analyses performed on absorption coefficient showed direct bandgap and Urbach energies as 2.56 and 0.22 eV, respectively. The first-and third-order nonlinear susceptibilities and nonlinear refractive index of the crystal were also reported in the present work. The results of the present paper would provide valuable information for optoelectronic device applications of Bi12GeO20.Article Citation - WoS: 23Citation - Scopus: 25Investigation of Optical Properties of Bi12geo20< Sillenite Crystals by Spectroscopic Ellipsometry and Raman Spectroscopy(Elsevier Sci Ltd, 2020) Isik, M.; Delice, S.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.Bi12GeO20 (BGO) compound is one of the fascinating members of sillenites group due to its outstanding photorefractive and photocatalytic characteristics. The present paper aims at investigating optical properties of BGO crystals by means of spectroscopic ellipsometry and Raman spectroscopy measurements. Bi12GeO20 single crystals grown by Czochralski method were structurally characterized by X-ray diffraction (XRD) experiments and the analyses showed that studied crystals have cubic crystalline structure. Raman spectrum exhibited 15 peaks associated with A, E and F modes. Spectroscopic ellipsometry measurement data achieved in the energy region between 1.2 and 6.2 eV were used in the air/sample optical model to get knowledge about complex pseudodielectric constant, pseudorefractive index, pseudoextinction and absorption coefficients of the crystals. Spectral change of real and imaginary part of complex pseudodielectric constant were discussed in detail. Band gap energy of Bi12GeO20 single crystals was calculated to be 3.18 eV using absorption coefficient dependency on photon energy. Critical point energies at which photons are strongly absorbed were determined by utilizing the second energy derivative spectra of components of complex pseudodielectric function. Fitting of both spectra resulted in the presence of four interband transitions with energies of 3.49, 4.11, 4.67 and 5.51 eV.Article Citation - WoS: 20Citation - Scopus: 21Temperature-Tuned Band Gap Characteristics of Inse Layered Semiconductor Single Crystals(Elsevier Sci Ltd, 2020) Isik, M.; Gasanly, N. M.Layered structured InSe has attracted remarkable attention due to its effective characteristics utilized especially in optoelectronic device technology. This point directs researchers to investigate optical properties of InSe in great detail. The temperature dependent band gap characteristics of InSe and analyses performed on this dependency have been rarely studied in literature. Here, temperature-dependent transmission and room temperature reflection experiments were performed on InSe layered single crystals. The band gap energy was found around 1.22 eV at room temperature and 1.32 eV at 10 K. The temperature-gap energy dependency was analyzed using Varshni and O'Donnell-Chen models to reveal various optical parameters of the crystal. The structural characteristics; crystalline parameters like lattice constants, lattice strain, dislocation density and atomic compositions of InSe were also determined from the analyses of XRD and EDS measurements.Article Citation - WoS: 16Citation - Scopus: 16Temperature-Dependent Optical Characteristics of Sputtered Ga-Doped Zno Thin Films(Elsevier, 2021) Gullu, H. H.; Isik, M.; Gasanly, N. M.; Parlak, M.The present paper reports structural and optical properties of gallium (Ga) doped ZnO thin films (GZO) grown by magnetron sputtering technique. The crystalline properties were determined from X-ray diffraction measurements and analyses pointed out the crystalline structure as hexagonal, crystalline size as 43 nm and strain as 6.9 x 10(-5). Derivative spectroscopy analyses showed that band gap energy of GZO thin films decreases from 3.50 eV (10 K) to 3.45 eV (300 K). Temperature-band gap energy dependency was analyzed using Varshni and O'DonnellChen models. The absolute zero band gap energy, the rate of change of band gap energy with temperature and phonon energy were found as 3.50 eV, -2.8 x 10(-4) eV/K and 15 meV, respectively. The room temperature band gap and Urbach energies were also determined as 3.43 eV and 102 meV, respectively, from the absorption analysis.Article Citation - WoS: 21Citation - Scopus: 24Temperature Dependence of Band Gaps in Sputtered Snse Thin Films(Pergamon-elsevier Science Ltd, 2019) Delice, S.; Isik, M.; Gullu, H. H.; Terlemezoglu, M.; Surucu, O. Bayrakli; Parlak, M.; Gasanly, N. M.; Bayrakli Surucu, O.Temperature-dependent transmission experiments were performed for tin selenide (SnSe) thin films deposited by rf magnetron sputtering method in between 10 and 300 K and in the wavelength region of 400-1000 nm. Transmission spectra exhibited sharp decrease near the absorption edge around 900 nm. The transmittance spectra were analyzed using Tauc relation and first derivative spectroscopy techniques to get band gap energy of the SnSe thin films. Both of the applied methods resulted in existence of two band gaps with energies around 1.34 and 1.56 eV. The origin of these band gaps was investigated and it was assigned to the splitting of valence band into two bands due to spin-orbit interaction. Alteration of these band gap values due to varying sample temperature of the thin films were also explored in the study. It was seen that the gap energy values increased almost linearly with decreasing temperature as expected according to theoretical knowledge.Article Citation - WoS: 5Citation - Scopus: 5Structural and Optical Properties of Thermally Evaporated (gase)0.75-(gas)0.25 Thin Films(Elsevier Gmbh, 2021) Isik, M.; Işık, Mehmet; Emir, C.; Gasanly, N. M.; Işık, Mehmet; Department of Electrical & Electronics Engineering; Department of Electrical & Electronics EngineeringGaSe and GaS binary semiconducting compounds are layered structured and have been an attractive research interest in two-dimensional material research area. The present paper aims at growing (GaSe)0.75 - (GaS)0.25 (or simply GaSe0.75S0.25) thin film and investigating its structural and optical properties. Thin films were prepared by thermal evaporation technique using evaporation source of its single crystal grown by Bridgman method. The structural properties were revealed using x-ray diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. XRD pattern and EDS analyses indicated that thin films annealed at 300 ?C were successfully deposited and its structural characteristics are well-consistent with its single crystal form. Surface morphology was studied by means of SEM and AFM measurements. Optical properties were investigated by transmission and Raman spectroscopy techniques. Raman spectrum exhibited three peaks around 172, 242 and 342 cm-1. Analyses of transmission spectrum revealed the direct band gap energy as 2.34 eV. The mixed compounds of GaSe0.75S0.25 were prepared for the first time in a thin film form and the results of the present paper would provide valuable information to research area in which layered compounds have been studied in detail.Article Citation - WoS: 7Citation - Scopus: 7Interband Critical Points in Tlgax< Layered Mixed Crystals (0 ≤ x ≤ 1)(Elsevier Science Sa, 2013) Isik, M.; Işık, Mehmet; Gasanly, N. M.; Işık, Mehmet; Department of Electrical & Electronics Engineering; Department of Electrical & Electronics EngineeringThe layered semiconducting TlGaxIn1-xS2 mixed crystals (0 <= x <= 1) were studied by spectroscopic ellipsometry measurements in the 1.2-6.2 eV spectral range at room temperature. The spectral dependence of the components of the complex dielectric function, refractive index and extinction coefficient were revealed using an optical model. The interband transition energies in the studied samples were found from the analysis of the second-energy derivative spectra of the complex dielectric function. The variation of the obtained energies with composition were plotted to see the effect of the substitution of indium with gallium. Moreover, a simple diagram showing the revealed transitions in the available electronic band structure was given for TlGaS2 single crystals. (C) 2013 Elsevier B.V. All rights
