Browsing by Author "Bagiev, V. E."

Now showing 1 - 14 of 14

Citation Count: 10
Effect of Temperature on Band Gap of Pbwo₄ Single Crystals Grown by Czochralski Method
(Iop Publishing Ltd, 2022) Isik, M.; Işık, Mehmet; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
The structural and optical properties of PbWO4 single crystals grown by Czochralski method and investigated by x-ray diffraction (XRD) and transmission experiments. XRD pattern presented well-defined and intensive peaks related with tetragonal scheelite structure. Transmission experiments were accomplished for the first time at various temperatures between 10 and 300 K on PbWO4 single crystals to reveal variation of band gap with temperature. Derivative spectroscopy method presented the experimentally observed band gap energy as increasing from 3.20 to 3.35 eV when the temperature was decreased to 10 K from room temperature. The revealed energy was associated with transition taking place between delocalized and trap levels. Temperature-band gap energy plot was analyzed by Varshni and Bose-Einstein models. The fitting processes under the light of these models revealed optical characteristics of absolute zero experimentally observed band gap, variation rate of gap energy with temperature and Debye temperature of PbWO4 single crystal.
Citation Count: 2
Growth and Characterization of Pbmo0.75w0.25o4 Single Crystal: a Promising Material for Optical Applications
(Elsevier Science Sa, 2023) Isik, M.; Işık, Mehmet; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
The present paper reports the structural and optical properties of PbMo0.75W0.25O4 single crystals grown by Czochralski method. XRD pattern of the crystal indicated well-defined two diffraction peaks associated with tetragonal crystalline structure. Raman and infrared spectra of the grown single crystals were presented to get information about the vibrational characteristics. Observed Raman modes were associated with modes of PbMoO4 and PbWO4. Eight bands were revealed in the infrared spectrum. The bands observed in the spectrum were attributed to multiphonon absorption processes. Transmission spectrum was measured in the 375-700 nm spectral region. The analyses of the spectrum resulted in direct band gap energy of 3.12 +/- 0.03 eV. The compositional dependent band gap energy plot was drawn considering the reported band gap energies of PbMoO4, PbWO4 and revealed band gap of PbMo0.75W0.25O4 single crystal. An almost linear behavior of composition-band gap energy was seen for PbMo1-xWxO4 compounds. Urbach energy was also found from the absorption coefficient analysis as 0.082 +/- 0.002 eV.
Citation Count: 1
Growth and Temperature-Tuned Band Gap Characteristics of Ligd(moo4)2 Single Crystals for Optoelectronic Applications
(Elsevier Sci Ltd, 2023) Delice, S.; Işık, Mehmet; Isik, M.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
LiGd(MoO4)2 has been investigated due to its optoelectronic applications, especially for development of lightemitting diodes. In the present paper, LiGd(MoO4)2 single crystals grown by Czochralski method was studied in terms of structural and temperature dependent optical properties. X-ray diffraction analysis showed that the crystal crystallizes in a single phase tetragonal structure. Raman spectrum exhibited six distinguishable peaks around 207, 319, 397, 706, 756 and 890 cm-1. These peaks correspond to vibrational modes of free rotation, symmetrical stretching, symmetric bending, antisymmetric stretching and antisymmetric bending of (MoO4)2tetrahedron. Infrared transmittance spectrum had eight minima around 2114, 2350, 2451, 2854, 2929, 2960, 3545 and 3578 cm-1 which are due to multiphonon absorptions. Spectral change of transmittance curves at various temperature between 10 and 300 K was utilized to elucidate temperature effect on absorption characteristics. Optical band gap of the material was found using Tauc and spectral derivative methods. The band gap value was obtained as 3.09 eV at room temperature and this value increased to 3.22 eV with decreasing temperature down to 10 K. The detailed analysis on the temperature dependency of the band gap was applied by Varshni model. The band gap at 0 K and change of rate of the band gap were estimated as 3.23 eV and -1.45 x 10-3 eV/K, respectively. Room temperature photoluminescence spectrum of the crystal presented a peak around 709 nm which corresponds to red light emission. LiGd(MoO4)2 is a potential candidate for optoelectronic devices emitting red light.
Citation Count: 3
Investigation of Optical Characteristics of Pbmoo₄ Single Crystals by Spectroscopic Ellipsometry
(Elsevier Gmbh, 2022) Delice, S.; Işık, Mehmet; Isik, M.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
In this study, we investigated the optical properties of PbMoO4 single crystals grown by Czochralski method. Spectroscopic ellipsometry measurements were carried out in the energy region between 1.0 and 5.5 eV at room temperature. X-ray diffraction measurements were achieved for structural characterization. The resulted pattern exhibited one peak belonging to (200) plane. Spectral variations of complex dielectric function, complex refractive index, absorption coefficient and dissipation function were obtained from the analyses of ellipsometry data. Real part of dielectric function increased up to 4.0 eV and then decreased suddenly at above this value. Zero frequency refractive index and dielectric constant were found to be 2.04 and 4.15, respectively. High frequency dielectric constant was determined to be 4.36. Optical band gap of PbMoO4 crystals was calculated as 3.09 eV. Two critical points with energies of 3.57 and 4.34 eV were estimated from the analyses of second-energy derivative spectra of real and imaginary parts of dielectric function. It was determined that [MoO4]2- complexes and charge transfer from Pb2+ ions into the neighboring Mo groups were responsible for these interband transitions. Dissipation function increased with increasing photon energy.
Citation Count: 17
Investigation of Optical Properties of Bi₁₂geo_{20< Sillenite Crystals by Spectroscopic Ellipsometry and Raman Spectroscopy}
(Elsevier Sci Ltd, 2020) Isik, M.; Işık, Mehmet; Delice, S.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
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.
Citation Count: 16
Optical characteristics of Bi₁₂SiO₂₀ single crystals by spectroscopic ellipsometry
(Elsevier Sci Ltd, 2020) Işık, Mehmet; Delice, S.; Nasser, H.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
Structural and optical characteristics of Bi12SiO20 single crystal grown by the Czochralski method were investigated by virtue of X-ray diffraction (XRD) and spectroscopic ellipsometry measurements. XRD analysis indicated that the studied crystal possesses cubic structure with lattice parameters of a = 1.0107 nm. Spectral dependencies of several optical parameters like complex dielectric constant, refractive index, extinction and absorption coefficients were determined using ellipsometry experiments performed in the energy region of 1.2-6.2 eV. The energy band gap of Bi12SiO20 crystals was found to be 3.25 eV by utilizing absorption coefficient analysis. Moreover, critical point energies were calculated as 3.54, 4.02, 4.82 and 5.58 eV from analyses of the second energy derivative spectra of the complex dielectric constant.
Citation Count: 3
Spectroscopic Ellipsometry Characterization of Pbwo₄ Single Crystals
(Elsevier, 2022) Delice, S.; Işık, Mehmet; Isik, M.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
Optical characterization of PbWO4 single crystals grown by Czochralski method was achieved in virtue of spectroscopic ellipsometry experiments carried out in the energy region of 1.0-5.6 eV at room temperature. Tetragonal scheelite structure with lattice parameters of a = b = 5.4619 & Aring; and c = 12.0490 & Aring; was determined for the bulk crystal utilizing from XRD analysis. Analyses of the ellipsometry data presented the photon energy dependencies of complex dielectric function of the crystal. The real part of the dielectric function exhibited increasing behavior with energy in the below 4.1 eV and then decreased immediately. Zero frequency refractive index and dielectric constant were determined to be 2.02 and 4.08, respectively, using Wemple and DiDomenico oscillator model. High frequency dielectric constant was calculated as 4.30 by Spitzer-Fan model. Optical band gap of PbWO4 was found to be 3.24 eV from the dielectric relaxation time spectrum. Moreover, existence of two critical points with energies of 3.70 and 4.58 eV was revealed from the analyses of extinction coefficient and second derivative of the dielectric function. These levels were considered to be due to creation of cation exciton (Pb2+ 6s(2) - Pb2+ 6s6p) and transitions in the [WO4](2-) group.
Citation Count: 6
Spectroscopic ellipsometry study of Bi₁₂TiO₂₀ single crystals
(Springer, 2021) Işık, Mehmet; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
Bi12XO20 (X: Si, Ge, Ti, etc.) ternary compounds have attracted attention especially due to their fascinating photorefractive characteristics. The present paper introduces the structural and optical characteristics of Bi12TiO20 single crystals grown by Czochralski method. X-ray diffraction pattern of the compound exhibited sharp and intensive peaks corresponding to parallel planes of cubic crystalline structure. The lattice constant of the cubic structure was determined as a = 1.0118 nm using a diffraction pattern indexing program. The optical characterization of the Bi12TiO20 single crystals was carried through spectroscopic ellipsometry experiments performed in the 1.2-5.0 eV spectral range. The spectral dependencies of refractive index, extinction coefficient, and complex dielectric function were revealed analyzing experimental ellipsometric data under the light of sample-air optical model. The band gap energy of the compound was determined as 3.34 eV from the analyses of absorption coefficient. Three critical points at 3.51, 4.10, and 4.71 eV were obtained from the analyses of components of dielectric function using their second-energy derivative spectra.
Citation Count: 6
Structural and temperature-tuned band gap energy characteristics of PbMoO₄ single crystals
(Elsevier, 2022) Işık, Mehmet; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
PbMoO4 is one of the member of the molybdate materials and has been a significant research interest due to its photocatalytic and optoelectronic applications. In the present paper, the structural and optical properties of PbMoO4 single crystals grown by Czochralski technique were investigated. X-ray diffraction pattern presented well-defined and intensive peaks associated with tetragonal scheelite structure. Energy dispersive spectroscopy analyses presented the atomic compositional ratio of constituent elements as consistent with chemical formula of PbMoO4. Raman and infrared transmittance spectra were reported to give information about the vibrational characteristics of the compound. Room temperature transmission spectrum was analyzed by derivative spectroscopy technique and band gap energy was found as 3.07 eV. Temperature-tuned band gap energy characteristics of the single crystal were investigated by performing transmission measurements at different temperatures between 10 and 300 K. The analyses indicated that band gap energy of the PbMoO4 single crystal increases to 3.24 eV when the temperature was decreased to 10 K. Temperature-band gap energy dependency was studied considering Varshni and Bose-Einstein models. The successful fitting processes under the light of applied models presented various optical parameters like absolute zero band gap energy, variation rate of band gap with temperature and Debye temperature.
Citation Count: 10
Structural and Temperature-Tuned Optical Characteristics of Bi₁₂geo_{20< Sillenite Crystals}
(Elsevier, 2020) Delice, S.; Işık, Mehmet; Isik, M.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
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.
Citation Count: 5
Temperature dependent bandgap in NaBi(WO₄)₂ single crystals
(Elsevier Gmbh, 2022) Işık, Mehmet; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
The double tungstates have been an attractive research interest due to their optoelectronic applications. In the present work, NaBi(WO4)(2), one of the members of double tungstates family, was grown by Czochralski method as single crystal form and optically investigated. X-ray diffraction pattern presented three peaks associated with tetragonal scheelite crystalline structure. Optical properties of the crystal were studied by performing temperature-dependent transmission measurements between 10 and 300 K. The shift of the absorption edge to higher energies was observed with decrease of temperature. The analyses indicated that direct band gap energy increases from 3.50 to 3.60 eV when the temperature was decreased from room temperature to 10 K. The temperature dependency of bandgap was studied considering the Varshni model and fitting of the experimental data under the light of model presented the optical parameters of band gap energy at 0 K, rate of band gap change with temperature and Debye temperature as E-g(0) = 3.61 eV, gamma = 8.83 x 10(-4) eV/K and beta = 456 K, respectively. Urbach energies were also determined from the analyses as 122 and 113 meV for 10 and 300 K experimental data, respectively.
Citation Count: 16
Temperature-Dependent Band Gap Characteristics of Bi₁₂sio_{20< Single Crystals}
(Amer inst Physics, 2019) Isik, M.; Işık, Mehmet; Delice, S.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
Bi12SiO20 single crystals have attracted interest due to their remarkable photorefractive characteristics. Since bandgap and refractive index are related theoretically to each other, it takes much attention to investigate temperature dependency of bandgap energy to understand the behavior of photorefractive crystals. The present study aims at investigating structural and optical characteristics of photorefractive Bi12SiO20 single crystals grown by the Czochralski method. The structural characterization methods indicated that atomic composition ratios of constituent elements were well-matched with the chemical compound Bi12SiO20, and grown crystals have a cubic crystalline structure. Optical properties of crystals were investigated by room temperature Raman spectroscopy and temperature-dependent transmission measurements between 10 and 300 K. The analyses of transmittance spectra by absorption coefficient and derivative spectrophotometry techniques resulted in energy bandgaps decreasing from 2.61 to 2.48 eV and 2.64 to 2.53 eV as temperature was increased from 10 to 300 K. The Varshni model was applied to analyze temperature-bandgap energy dependency.
Citation Count: 5
Temperature-Tuned Bandgap Characteristics of Bi₁₂tio_{20< Sillenite Single Crystals}
(Springer, 2021) Isik, M.; Işık, Mehmet; Delice, S.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
Bi12MO20 (M: Si, Ge, Ti, etc.) compounds are known as sillenites having fascinating photorefractive characteristics. The present paper reports the structural and optical characteristics of one of the members of this family, Bi12TiO20 single crystals, grown by Czochralski method. X-ray diffraction pattern of the crystal presented sharp and intensive peaks associated with planes of cubic crystalline structure with lattice constant of a = 1.0142 nm. The optical properties were studied by means of room temperature Raman and temperature-dependent transmission experiments at various temperatures between 10 and 300 K. Raman spectrum indicated peaks around 127, 162, 191, 219, 261, 289, 321, 497 and 537 cm(-1). The analyses of transmittance spectra indicated the increase of direct bandgap energy from 2.30 to 2.56 eV as temperature was decreased from room temperature to 10 K. The temperature-dependent bandgap characteristics of Bi12TiO20 were analyzed by means of Varshni and O'Donnell-Chen models. The analyses under the light of these models resulted in absolute zero bandgap energy of E-g(0) = 2.56(4) eV, rate of change of bandgap energy of gamma = - 1.11 x 10(-3) eV/K and average phonon energy of < E-ph & rang; = 8.6 meV.
Citation Count: 2
Trapping Centers in Bi₁₂tio_{20< Single Crystals by Thermally Stimulated Current}
(Elsevier, 2021) Isik, M.; Işık, Mehmet; Delice, S.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics Engineering
Sillenite group compounds have been widely utilized in photocatalytic applications. One of the member of this group, Bi12TiO20 single crystal, was grown by Czochralski method. The structural properties were investigated by x-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. XRD pattern presented well-defined intensive peaks associated with cubic crystalline structure. SEM images indicated the crystal surface as almost uniform and smooth. Thermally stimulated current (TSC) experiments were performed in the 10-280 K temperature range to reveal shallow trapping centers in the Bi12TiO20 single crystal. Two peaks around 112 and 179 K were observed in the TSC glow curve. The analyses of these curves considering the curve fitting and peak shape techniques resulted in presence of two hole centers at 0.09 and 0.14 eV. Heating rate dependencies of peak maximum temperature and current were also investigated throughout the paper.