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Öğe Characterisation of ZnO nanorod arrays grown by a low temperature hydrothermal method(Taylor & Francis Ltd, 2012) Kahraman, S.; Cetinkara, H. A.; Bayansal, F.; Cakmak, H. M.; Guder, H. S.In this paper, growth steps of well defined ZnO nanorod arrays deposited on seeded substrates were investigated. To obtain ZnO seed layer on glass substrates, a successive ionic layer adsorption and reaction (SILAR) method was used and then ZnO nanorods were grown on seed layer using a chemical bath deposition (CBD) method. The effects of seed layer and deposition time on morphology, crystallographic structure (e.g. grain size, microstrain and dislocation density) and electrical characteristics of ZnO nanorods were studied. From the SEM micrographs, it could be seen that the ZnO nanorods densely covered the substrate and were nearly perpendicular to the substrate surface. The XRD patterns showed that the ZnO nanorod arrays had a hexagonal wurtzite structure with a preferred orientation along the (002) plane. An increase in deposition time resulted in an increase in the intensity of the preferred orientation and grain size, but a decrease in microstrain and dislocation density. Electrical activation energies of the structures were calculated as 0.15-0.85 eV from current-temperature characteristics. It was concluded that the morphologies of the structures obtained in this study via a simple and fast solution method can provide high surface areas which are important in area-dependent applications, such as solar cells, hydrogen conversion devices, sensors, etc.Öğe Characteristics of ZnO thin films doped by various elements(Elsevier Science Bv, 2013) Kahraman, S.; Cakmak, H. M.; Cetinkaya, S.; Bayansal, F.; Cetinkara, H. A.; Guder, H. S.We have investigated the effects of Al, K and Co dopant elements on the properties of ZnO thin films deposited by CBD method on glass substrates. Changing in morphology, structural parameters, ionization energies of impurity levels, absorption behavior and optical band gap values were investigated through scanning electron microscopy (SEM), X-Ray diffraction (XRD), resistance-temperature measurement (R-T) and ultraviolet-visible spectroscopy (UV-vis) techniques. From the SEM observations, various morphologies (rod-like, flower-like and rice-like) were observed. Those morphological variations were attributed to the change in stable growth mechanism of intrinsic ZnO, induced by different atomic radius and different electronegativity of dopants. XRD results indicated that all orientations are well indexed to hexagonal phase crystalline ZnO. The impurity level ionization energy values (Delta E) were estimated as 0.32/0.13/0.07 eV; 0.34/0.15 eV; 0.40/0.13 eV and 0.48/0.22 eV for the Al, K, Co doped samples and i-ZnO, respectively. Optical band gap values were found that the doped samples' were higher than the intrinsic one's. This increasing (blue shift) was attributed to a deterioration which occurred in the lattice of the structures after doping. This effect was also supported by the structural results. (C) 2012 Elsevier B.V. All rights reserved.Öğe Characterization of CBD grown ZnO films with high c-axis orientation(Elsevier Science Sa, 2012) Kahraman, S.; Bayansal, F.; Cetinkara, H. A.; Cakmak, H. M.; Guder, H. S.Highly c-axis oriented ZnO films were deposited on seeded glass substrates. Successive ionic layer adsorption and reaction (SILAR) method and chemical bath deposition (CBD) method were used to obtain seed layers and ZnO films. To see the effects of seed layer and deposition time, structural (e.g. grain size, microstrain and dislocation density), morphological, and electrical (e.g. resistivity, activation energy) properties of the films were investigated by scanning electron microscopy, X-ray diffraction, and four point probe method. From the SEM images, resultant structures were found as well defined nanorods nearly perpendicular to the substrate surfaces and densely cover the substrates. The XRD patterns showed that ZnO films have hexagonal wurtzite structure with a preferred c-axis orientation along (002) plane. C-axis orientation was also supported by texture coefficient calculations. The lattice parameters of the structures were determined as a = 3.2268 angstrom, b = 5.2745 angstrom, alpha = beta = 90 degrees and gamma = 120 degrees. From the XRD patterns, it was revealed that, microstrain and dislocation density values of the structures decreased whereas grain size increased. This was attributed to enhancement occurred in lattice structure of the ZnO films. Activation energy values of the films were found in between 0.12 and 0.15 eV from the dark electrical resistivity-temperature characteristics in a temperature range of 300-500 K. (c) 2012 Elsevier B.V. All rights reserved.Öğe Effect of heat treatment on the properties of Cd(OH)2 and CdO films grown by chemical bath deposition(Taylor & Francis Ltd, 2013) Sahin, B.; Bayansal, F.; Cakmak, H. M.; Kahraman, S.; Cetinkara, H. A.Nanostructured cadmium hydroxide (Cd(OH)2) thin films have been fabricated on glass substrates by the chemical bath deposition method and converted into cadmium oxide (CdO) films by heat treatments. Some of the films were studied as-grown and the others were annealed in air at 150, 300 and 450 degrees C. The effects of various annealing processes on the structural, morphological and optical properties of the Cd(OH)2 and CdO films were studied. The band gaps of the films vary from 2.74eV to 2.32eV for different annealing temperatures. It was observed that there is a direct dependence of the structural, morphological and optical properties on the annealing temperature of the films. X-ray diffraction (XRD) measurements showed that as-grown and annealed film at 150 degrees C fully consist of Cd(OH)2 and the annealed films at 300 and 450 degrees C consist of CdO nanoparticles. The size of the nanoparticles, which were calculated from the scanning electron microscopy (SEM) figures by using a pixels program, was found to lie between 460 and 600nm.Öğe The effects of coumarin additive on the properties of ZnO nanostructures(Pergamon-Elsevier Science Ltd, 2013) Kahraman, S.; Cakmak, H. M.; Cetinkaya, S.; Cetinkara, H. A.; Guder, H. S.In this study, we investigated the effects of coumarin presence in the growth solution on the properties of ZnO. Significant changes/improvements were observed in morphology and crystal structure. From the morphological studies, it was seen that the substrates were uniformly covered with rod/rice-like nano-structures and average diameters of the structures decreased with increasing coumarin content in the growth solution. Also coumarin affected the surface morphology, coverage density and uniformity of the shape of the structures. The X-ray diffraction patterns indicated that all diffraction peaks were well indexed to hexagonal phase crystalline ZnO. The greater intensities of (0 0 2) peaks in the patterns revealed c-axis orientation normal to the substrate. The crystallization of the samples was gradually enhanced with increasing coumarin volume up to 5%. When the coumarin volume exceeded 5%, the crystallization was affected negatively. The electrical resistivity values increased with increasing coumarin content and it was attributed to the decrease in carrier concentration originated by carrier traps at the grain boundaries. (C) 2012 Elsevier Ltd. All rights reserved.Öğe Effects of thermal oxidation temperature on vacuum evaporated tin dioxide film(Academic Press Ltd- Elsevier Science Ltd, 2012) Cakmak, H. M.; Cetinkara, H. A.; Kahraman, S.; Bayansal, F.; Tepe, M.; Guder, H. S.; Cipiloglu, M. A.In order to investigate the effect of thermal oxidation temperature on tin dioxide (SnO2), tin dioxide films were obtained on quartz substrates by vacuum evaporation of tin metal. The films were characterized by X-ray diffraction (XRD) analyses, scanning electron microscopy (SEM), temperature dependent electrical resistivity measurement and optical absorption spectroscopy. The SEM images showed that the films are dense, continuous and are composed of nanoparticles and particle sizes are increased after thermal oxidation. From the X-ray measurement results, the films indicated two strong reflection peaks of tetragonal structure in the orientations of (101) and (200) at 20 = 33.8 degrees and 37.95 degrees, respectively. Intensity of the peaks increased with increasing thermal oxidation temperature. We found resistivity values of about 10(-4) Omega-cm. Optical absorption spectra of the films in the UV-Vis spectral range revealed that optical band gap (E-g) value of the films increases with increasing thermal oxidation temperature. (C) 2012 Elsevier Ltd. All rights reserved.Öğe Growth of homogenous CuO nano-structured thin films by a simple solution method(Elsevier Science Sa, 2011) Bayansal, F.; Kahraman, S.; Cankaya, G.; Cetinkara, H. A.; Guder, H. S.; Cakmak, H. M.This paper describes a simple, low temperature and cost effective solution method to synthesize homogenous cupric oxide (CuO) nano-structured thin films. By this method dense and continuous CuO films with good crystallinity can be prepared in a short time, e.g., in 15 min. The scanning electron microscopy illustrated that the synthesized CuO plate-like nanostructures have RMS thicknesses of 100 nm. The XRD measurements showed that the synthesized CuO nanostructures have a high crystallinity with monoclinic crystal structure preferentially in ((1) over bar 1 1) and (1 1 1) directions. From the temperature dependant dark electrical resistivity measurements the ionization energies of the impurity levels and thermal band gap energies of the films were calculated. (C) 2010 Elsevier B.V. All rights reserved.Öğe Nano-structured CuO films prepared by simple solution methods: Plate-like, needle-like and network-like architectures(Elsevier Sci Ltd, 2012) Bayansal, F.; Cetinkara, H. A.; Kahraman, S.; Cakmak, H. M.; Guder, H. S.Two simple, safe and environmental friendly chemical solution methods which are suitable to mass production, applicable at low temperatures and cost effective, were used to prepare homogenous nano-structured cupric oxide (CuO) films. By using different types of substrates (glass, quartz, Cu foil and Si) and techniques various types of CuO nanostructures (plate, needle and network like) were produced. The products were characterized by scanning electron microscopy, X-ray diffraction and temperature dependant dark electrical resistivity measurements in order to investigate the effects of substrate on morphology, crystallographic structure and electrical properties of the films. From these characterizations it was seen that there are significant differences in the morphological, crystallographic, structural and electrical properties of the nanostructures that were produced by different techniques and/or on different substrates. Besides, growth mechanisms of CuO films have been investigated. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.Öğe A novel study on ZnO nanostructures: coumarin effect(Taylor & Francis Ltd, 2012) Cakmak, H. M.; Kahraman, S.; Bayansal, F.; Cetinkaya, S.This novel study reports the behaviour of coumarin during the ZnO chemical bath deposition (CBD) process. Scanning electron microscopy studies revealed that grain sizes of the ZnO nanostructures get smaller and become more uniform in size with increasing coumarin content. Length-to-diameter ratio of the structures increased with the incorporation of coumarin. From the X-Ray diffraction results, with increasing coumarin content, significant decrease in grain sizes was determined. It was found that resistivity values of the structures increased with increasing coumarin content. This increase is attributed with a decrease in carrier concentration originated by carrier traps at the grain boundaries. Through slopes of lnR versus 1/T curves, impurity level ionization energy values were calculated as 0.16, 0.06 eV; 0.11, 0.02 eV; and 0.08, 0.06 eV for the structure growth in the solutions containing 0, 1 and 5 at.% coumarin, respectively.Öğe Synthesis and characterization of undoped and tin-doped ZnO nanostructures(Springer Heidelberg, 2012) Kahraman, S.; Bayansal, F.; Cakmak, H. M.; Cetinkara, H. A.; Guder, H. S.In this paper, undoped and tin-doped ZnO nanostructures were grown onto non-conductive substrates by a simple solution method. Structural, morphological, optical and electrical properties of the structures were investigated with respect to tin concentration. From XRD studies, all the ZnO nanostructures were found as hexagonal wurtzite type structures growing preponderantly oriented with c-axis normal to the substrate. An increase in tin content resulted in a decrease in grain size, whereas the dislocation density increases. SEM observations indicated that all the structures were textured throughout the substrates without any cracks or pores. The influence of incorporation of tin on surface morphology of the samples was clearly seen. Average diameter of the nanostructures decreased with increasing tin content. Absorption spectra of the structures revealed that the band gap of the films increases with increasing tin concentration. It is found that the tin-doped samples have higher average transmittance than the undoped one. The 1 % tin-doped sample exhibited similar to 80 % average transparency, which was the best transparency among the doped samples. Electrical measurements showed that resistivity of the structures increased with increasing dopant concentration. This increasing was attributed due to a decrease in carrier concentration caused by carrier traps at the grain boundaries.
