Browsing by Author "Tepehan, Fatma Zehra"

Now showing 1 - 12 of 12

Citation Count: 4
Atomic force microscopy and spectroscopy studies of annealed Ce/Ti/Zr mixed oxide thin films prepared by sol-gel process
(Elsevier Science Bv, 2006) Ghodsi, Farhad E.; Tepehan, Fatma Zehra; Tepehan, Galip Gültekin
Mixed Ce/Ti/Zr oxide thin films with a molar ratio of 0.5:0.25:0.25 were prepared using the sol-gel process and deposited on glass substrates by the dip coating technique. The effect of heat treatment temperature on surface morphology of the films was examined by atomic force microscopy AFM. The optical transmittance and reflectance of the films were measured over the spectral range from 350 to 1000 nm. The refractive index and extinction coefficient and thickness of the films were determined as a function of the heat treatment temperature. The refractive index increased from 1.51 to 2.02 at lambda = 600 nm and the extinction coefficient values increased from 0.006 to 0.094 while the thickness of the films decreased from 81 nm to 45 nm when annealing temperature increased from 100 degrees C to 500 degrees C. The results show that the optical properties and surface morphology of the mixed Cc/Ti/Zr oxide thin films were affected by annealing temperature. (c) 2006 Elsevier B.V. All rights reserved.
Citation Count: 13
Characterization Of Mps Capped Cds Quantum Dots And Formation Self-Assembled Quantum Dots Thin Films On A Glass Substrate
(Natl Inst R&D Materials Physics, 2011) Koç, Kenan; Tepehan, Fatma Zehra; Tepehan, Galip Gültekin
Colloidal powdered and thin film forms of MPS capped CdS quantum dots have been produced by combination of colloidal chemistry and sol-gel method. Nanoparticles were self-assembled directly on a glass substrate using spin coating method without introducing any matrix. Colloidal powdered and thin film forms were characterised by absorbance photolimunescence XRD FT-IR HRTEM and AFM measurements. The HRTEM images give a grain size of 2.5-3.0 nm and this is in agreement with the values found in the absorbance and XRD measurement. Average size of the quantum dots increase with increasing of heat treatment temperature due to Oswald ripening.
Citation Count: 7
Controlling the growth of particle size and size distribution of silica nanoparticles by the thin film structure
(Springer, 2012) Uysal, Bengü Özuğur; Tepehan, Fatma Zehra
Nanostructured silicondioxide thin films were prepared by sol-gel spin coating technique. The SiO2 films were made using a conventional mixture of tetraethoxysilane (TEOS) deionized water and ethanol with various NH3/TEOS ratios. The nanostructured silica films were made using a mixture of the SiO2 sol and regular SiO2 sol to control the enlargement of the particles inside the films. The structural morphological and optical characterizations of the as-deposited and annealed films were carried out using X-ray diffraction (XRD) atomic force microscopy scanning electron microscopy NKD spectrophotometer and ultraviolet-visible (UV-vis) spectroscopy. The transmittance data of the infrared spectra of the films were recorded using an FT-IR Spectrometer. The XRD studies showed that as-deposited films were amorphous and the formation of the alfa-cristobalite phase of the silica film was investigated at annealing temperature close to 1100 A degrees C. Optical properties of the transmittance spectra in the s and p-polarization modes were collected. Refractive indices and extinction coefficients were determined with respect to the NH3/TEOS ratios in the compositions of the films. Optical cut-off wavelength values were investigated from the extrapolation of the absorbance spectra which was estimated from the UV-vis spectroscopy measurements. A red shift in the absorption threshold indicated that the size of silica nanoparticles was increased by an increase in the NH3/TEOS volume ratio from 1:64 to 1:8.
Citation Count: 11
Derivation of the optical constants of spin coated CeO2-TiO2-ZrO2 thin films prepared by sol-gel route
(Pergamon-Elsevier Science Ltd, 2011) Ghodsi, Farhad E.; Tepehan, Fatma Zehra; Tepehan, Galip Gültekin
Ternary thin films of cerium titanium zirconium mixed oxide were prepared by the sol-gel process and deposited by a spin coating technique at different spin speeds (1000-4000 rpm). Ceric ammonium nitrate ce(NO3)(6)(NH4)(2) titanium butoxide Ti[O(CH2)(3)CH3](4) and zirconium propoxide Zr(OCH2CH2CH3)(4) were used as starting materials. Differential calorimetric analysis (DSC) and thermogravimetric analysis (TGA) were carried out on the CeO2-TiO2-ZrO2 gel to study the decomposition and phase transition of the gel. For molecular structural elemental and morphological characterization of the films Fourier Transform Infrared (FTIR) spectral analysis X-ray diffraction (XRD) energy dispersive X-ray spectroscopy (EDS) cross-sectional scanning electron microscopy (SEM) and atomic force microscopy (AFM) were carried out. All the ternary oxide thin films were amorphous. The optical constants (refractive index extinction coefficient band gap) and thickness of the films were determined in the 350-1000 nm wavelength range by using an nkd spectrophotometer. The refractive index extinction coefficient and thickness of the films were changed by varying the spin speed. The oscillator and dispersion energies were obtained using the Wemple-DiDomenico dispersion relationship. The optical band gap is independent of the spin speed and has a value of about E-g approximate to 2.82 +/- 0.04 eV for indirect transition. (C) 2011 Published by Elsevier Ltd.
Citation Count: 0
Determination of growth kinetics and size dependent structural morphological optical characteristics of sol-gel derived silica nanoparticles in silica matrix
(De Gruyter Poland Sp Zoo, 2019) Uysal, Bengü Özuğur; Tepehan, Fatma Zehra
Nanocomposite silica thin films made using the sol-gel method were studied. The nano-silica films were prepared using a mixture of tetraethyl orthosilicate (TEOS) deionized water ethanol and ammonia solution. To control the growth of the particles inside the film the nanocomposite silica film was prepared using a mixture of the nano-silica sol and the silica sol. The change in the particle size with the heat treatment temperature ranging from 450 degrees C to 1100 degrees C was investigated. X-ray diffraction (XRD) atomic force microscopy (AFM) scanning electron microscopy (SEM) NKD (refractive index-N extinction coefficient-K and thickness-D) and ultraviolet-visible (UV-Vis) spectrophotometry were used for characterization purposes. The XRD studies showed that the nano-silica thin films were amorphous at all annealing temperatures except for 1100 degrees C. The alpha-cristobalite crystal structure formed at the annealing temperature of 1100 degrees C. Optical parameters such as refractive indices and extinction coefficients were obtained using the NKD analyzer with respect to the annealing temperature of the films. The activation energy and enthalpy of the nanocomposite silica film were evaluated as 22.3 kJ/mol and 14.7 kJ/mol respectively. The cut-off wavelength values were calculated by means of extrapolation of the absorbance spectra estimated using the UV-Vis spectroscopy measurements. A red shift in the absorption threshold of the nanocomposite silica films indicated that the size of the silica nanoparticles increased with an increase of the annealing temperatures from 450 degrees C to 900 degrees C and this confirms the quantum confinement effect in the nanoparticles.
Citation Count: 28
Electrochromic properties of heat-treated thin films of CeO2-TiO2-ZrO2 prepared by sol-gel route
(Elsevier Science Bv, 2008) Ghodsi, Farhad E.; Tepehan, Fatma Zehra; Tepehan, Galip Gültekin
CeO2-TiO2-ZrO2 thin films were prepared using the sol-gel process and deposited on glass and ITO-coated glass substrates via dipcoating technique. The samples were heat treated between 100 and 500 degrees C. The heat treatment effects on the electrochromic performances of the films were determined by means of cyclic voltammetry measurements. The structural behavior of the film was characterized by atomic force microscopy and X-ray diffraction. Refractive index extinction coefficient and thickness of the films were determined in the 350-1000nm wavelength using nkd spectrophotometry analysis. Heat treatment temperature affects the electrochromic optical and structural properties of the film. The charge density of the samples increased from 8.8 to 14.8 mC/cm(2) with increasing heat-treatment temperatures from 100 to 500 degrees C. It was determined that the highest ratio between anodic and cathodic charge takes place with increase of temperature up to 500 degrees C. (c) 2007 Elsevier B.V. All rights reserved.
Citation Count: 8
Growth kinetics of MPS-capped CdS quantum dots in self-assembled thin films
(Springer, 2012) Koç, Kenan; Tepehan, Fatma Zehra; Tepehan, Galip Gültekin
For this study we prepared colloidal CdS quantum dots using 3-mercaptopropyltrimethoxysilane as capping agent. Colloidal CdS quantum dots were directly deposited on glass substrates by a spin-coating process. Coated substrates were heat-treated between 225A degrees C and 325A degrees C for various heat treatment time intervals to investigate the growth kinetics of the quantum dots. Results showed that sizes of the CdS quantum dots grew approximately from 2.9 to 4.6 nm and the E (1s1s) energy values shifted approximately from 3.3 to 2.7 eV. Results showed that the average size of quantum dots increase by thermal treatment due to Ostwald ripening. The thermal process used to grow the size of quantum dots was examined according to the Lifshitz-Slyozov-Wagner theory. The activation energy of CdS quantum dots in thin films was calculated at approximately 44 kJ/mol.
Citation Count: 18
Influence of pH on the optical and structural properties of spin coated CeO2-TiO2 thin films prepared by sol-gel process
(Elsevier Science Bv, 2007) Ghodsi, Farhad E.; Tepehan, Fatma Zehra; Tepehan, Galip Gültekin
Optical and structural properties of mixed CeO2-TiO2 thin films have been investigated by varying the pH of the coating sol. The films were prepared by sol-gel process using spin-coating technique with a spin speed of 2500 rpm. The optical and structural properties of films were examined by a spectrophotometer AFM and XRD. Optical constants and thickness of the CeO2-TiO2 thin films have been determined with respect to the pH of the coating bath before deposition. The refractive index at 550 nm wavelength increases from 1.49 to 1.60 when the pH of the sol increases from 2.22 to 2.92. The thickness increases from 46.6 nm to 96.1 nm in this pH range. The XRD measurements show that the films have amorphous structure. The AFM analysis shows that the pH of sol prior to deposition changes the uniformity and porosity of the films. (C) 2007 Elsevier B.V. All rights reserved.
Citation Count: 17
Modeling the optical properties of WO(3) and WO(3)-SiO(2) thin films
(Elsevier Science Bv, 2008) Saygin-Hinczewski, Dursen; Hinczewski, Michael; Sorar, İdris; Tepehan, Fatma Zehra; Tepehan, Galip Gültekin
The optical properties and surface morphology of sol-gel spin coated WO(3) and WO(3)-SiO(2) composite films annealed at 250 and 400 degrees C are investigated. For the purpose of extracting the optical parameters of the films a novel form for the dielectric function is introduced consisting of two Tauc-Lorentz oscillators and an Urbach tail component which is suited for amorphous multi-transition materials with substantial subgap absorption. The evolution of the refractive indices transmittances and band gaps with doping is marked by sizable shifts at 2.0-2.5% SiO(2) doping for the 250 degrees C films and 4.0-4.5% doping for the 400 degrees C films. In addition pronounced changes in the surface roughness of the films occur at these doping values. (c) 2008 Elsevier B.V. All rights reserved.
Citation Count: 12
Optical and structural properties of sol-gel made Ce/Ti/Zr mixed oxide thin films as transparent counter electrode for electrochromic devices
(Elsevier Science Bv, 2008) Ghodsi, Farhad E.; Tepehan, Fatma Zehra; Tepehan, Galip Gültekin
Ce/Ti/Zr mixed oxide thin films were prepared using sol-gel process with mole ratios from 45/5 to 5/45 of Ti/Zr and 50 of Cc and deposited by dip coating technique. Optical electrochromic and structural properties of such films were investigated. The thickness refractive index and extinction coefficient of the films were calculated through transmission and reflection measurement by an nkd spectrophotometer. The surface morphology and structural behaviors of the films were characterized by atomic force microscopy and X-ray diffraction. Cyclic voltammetry measurements also were used to study electrochromic properties of these films. The best counter electrode Ce/Ti/Zr oxide thin film is achieved for the sample with a mole ratio of 40/10 of Ti/Zr. The ratio between anodic and cathodic charge is about 0.95 for this sample with a surface roughness of 1.8 nm. (C) 2008 Elsevier B.V. All rights reserved.
Citation Count: 19
Optical and structural properties of Ta2O5-CeO2 thin films
(Elsevier Science Bv, 2007) Saygın-Hinczewski, Dursen; Koç, Kenan; Sorar, İdris; Hinczewski, Michael; Tepehan, Fatma Zehra; Tepehan, Galip Gültekin
In this study the sol-gel spin-coating method has been used to make Ta2O5-CeO2 thin films. These films have been prepared in various composition ratios to observe changes in their optical and structural properties. Reflectance and transmittance spectra were collected in the spectral range of 300-1000 nm and were accurately fit using the Tauc-Lorentz model. Film thicknesses refractive indices absorption coefficients and optical band gaps were extracted from the theoretical fit. The highest refractive index value was found at 5% CeO2 doping. The structure of the films was characterized by X-ray diffractometry and Fourier transform infrared spectrometry while the surface morphology was examined through atomic force microscopy. (C) 2007 Elsevier B.V. All rights reserved.
Citation Count: 8
Preparation and characterization of self-assembled thin film of MPS-capped ZnS quantum dots for optical applications
(Hindawi Publishing Corporation, 2012) Koç, Kenan; Tepehan, Fatma Zehra; Tepehan, Galip Gültekin
For this study we prepared colloidal ZnS quantum dots using 3-mercaptopropyltrimethoxysilane (MPS) as the capping agent. Colloidal ZnS quantum dots were directly deposited on glass substrates by a spin coating process. Therefore self-assembled films made of ZnS quantum dots in a SiO2 network were obtained using only one production step. The films were heat-treated at 100 degrees 125 degrees 150 degrees 175 degrees and 200 degrees C in an N-2 atmosphere. The results showed that the dimension of quantum dots changed from 2.8 nm to 3.2 nm by heat treatment. The refractive index extinction coefficient thickness and dielectric coefficient values of the films were calculated. The present study showed that size and the refractive indices of films can be controlled by the heat treatment. Therefore such films can be a good candidate in optical filter applications.