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Öğe All Solution-Based Fabrication of Copper Oxide Thin Film/Cobalt-Doped Zinc Oxide Nanowire Heterojunctions(WILEY, 2016) Akgul, Funda Aksoy; Akgul, Guvenc; Turan, Rasit; Unalan, Husnu EmrahVersatile and intriguing solution-based processes are utilized to synthesize nanostructured materials for device applications to reduce material production and device fabrication costs. This study presents results on the fabrication and characterization of copper oxide ( CuO) coated cobalt-doped zinc oxide nanowires ( Co-doped ZnO NWs)-based heterojunction diodes prepared by a two-step synthesis route through combined hydrothermal growth and sol-gel spin coating. Highly dense, well-ordered, undoped, and Co-doped ZnO NWs were successfully grown by hydrothermal method. Complementary CuO thin films were synthesized by sol-gel method and subsequently coated onto both undoped and Co-doped ZnO NWs through spin-coating technique. Enhanced diode properties with a rectification ratio of 10(3) at +/-2 V and an ideality factor of n = 2.4 ( in dark) were obtained for Co-doped ZnO NWs-based heterojunction diodes. The obtained results demonstrated that the investigated heterojunction diode structure fabricated by facile and cost-effective solution-based processes can be a promising candidate for the next generation optoelectronic devices.Öğe Deactivation of Ru Catalysts under Catalytic CO Oxidation by Formation of Bulk Ru Oxide Probed with Ambient Pressure XPS(AMER CHEMICAL SOC, 2013) Qadir, Kamran; Kim, Sun Mi; Seo, Hyungtak; Mun, Bongjin S.; Akgul, Funda Aksoy; Liu, Zhi; Park, Jeong YoungThe surface science approach of using model catalysts in conjunction with the development of in situ spectroscopic tools, such as ambient pressure X-ray photoelectron spectroscopy (AP-XPS), offers a synergistic strategy for obtaining a substantially better understanding of deactivation phenomena. In this study, we investigated the nature of Ru oxides on a Ru polycrystalline film under oxidizing, reducing, and catalytic CO oxidation reaction conditions. Thus, bulk Ru oxide was easily formed on such Ru catalysts, the growth of which was dependent on reaction temperature. Once formed, such an oxide is irreversible and cannot be completely removed even under reducing conditions at elevated temperatures (200 degrees C). Our reaction studies showed substantial deactivation of the Ru film during catalytic CO oxidation, and its activity could be partially recovered after reduction pretreatment. Such continuous deactivation of a Ru film is correlated with irreversibly formed bulk Ru oxide, as shown by AP-XPS. Such in situ spectroscopic evidence of the transition of oxides to a catalytically inactive state can enable more effective design of catalysts with less deactivation.Öğe Enhanced diode performance in cadmium telluride-silicon nanowire heterostructures(ELSEVIER SCIENCE SA, 2015) Akgul, Funda Aksoy; Akgul, Guvenc; Gullu, Hasan Huseyin; Unalan, Husnu Emrah; Turan, RasitWe report on the structural and optoelectronic characteristics and photodetection properties of cadmium telluride (CdTe) thin film/silicon (Si) nanowire heterojunction diodes. A simple and cost-effective metal-assisted etching (MAE) method is applied to fabricate vertically oriented Si nanowires on n-type single crystalline Si wafer. Following the nanowire synthesis, CdTe thin films are directly deposited onto the Si nanowire arrays through RF magnetron sputtering. A comparative study of X-ray diffraction (XRD) and Raman spectroscopy shows the improved crystallinity of the CdTe thin films deposited onto the Si nanowires. The fabricated nanowire based heterojunction devices exhibit remarkable diode characteristics, enhanced optoelectronic properties and photosensitivity in comparison to the planar reference device. The electrical measurements revealed that the diodes have a well-defined rectifying behavior with a superior rectification ratio of 10(5) at +/- 5 V and a relatively small ideality factor of n = 1.9 with lower reverse leakage current and series resistance at room temperature in dark condition. Moreover, an open circuit voltage of 120 mV is also observed under illumination. Based on spectral photoresponsivity measurements, the nanowire based device exhibits a distinct responsivity (0.35-0.5 A W-1) and high detectivity (6 x 10(12)-9 x 10(12) cm Hz(1/2) W-1) in near-infrared wavelength region. The enhanced device performance and photosensitivity is believed to be due to three-dimensional nature of the interface between the CdTe thin film and the Si nanowires. The device characteristics observed here reveals that fabricated CdTe thin film/Si nanowire heterojunctions are promising for high-performance and low-cost optoelectronic device applications, near-infrared photodetectors in particular. (C) 2015 Elsevier B.V. All rights reserved.Öğe Environment Controlled Dewetting of Rh-Pd Bilayers: A Route for Core-Shell Nanostructure Synthesis(AMER CHEMICAL SOC, 2012) Abrasonis, Gintautas; Wintz, Sebastian; Liedke, Maciej O.; Akgul, Funda Aksoy; Krause, Matthias; Kuepper, Karsten; Gemming, SibylleChemical environment plays a significant role on the size, shape, or surface composition of nanostructures. Here, the chemical environment effects are studied in the context of core-shell nanoparticle synthesis. The environment driven dynamics and kinetics of Rh/Pd bilayers is investigated by in situ ambient pressure X-ray photoelectron spectroscopy. Thin Rh (similar to 1.5 nm)/Pod ( similar to 1.5 nm) bilayers were grown on thermally oxidized Si substrates. The films were heated in CO or NO environments or heated in vacuum with a subsequent NO/CO cycling. This study demonstrates that not the initial stacking sequence but the chemical environment plays a crucial role in controlling the surface composition. Heating in CO results in a surface enrichment of Pd at similar to 200 degrees C and is followed by film dewetting at similar to 300 degrees C. Heating in NO results in progressive oxidation of Rh starting at similar to 150 degrees C, which stabilizes the film continuity up to >similar to 375 degrees C. The film rupture correlates with the thermal destabilization of the surface oxide. Heating in vacuum results in a significant increase in surface Pd concentration, and the following NO/CO cycling induces periodic surface composition changes. The quasi equilibrium states are similar to 50% and similar to 20% of Rh/(Rh + Pd). for NO and CO environments, respectively. Possible surface composition change and dewetting mechanisms are discussed on the basis of the interplay of thermodynamic (surface/oxide energy and surface wetting) and kinetic (surface oxidation and thermally induced and chemically enhanced diffusion) factors. The results open alternative ways to synthesize supported (core-shell) nanostructures with controlled morphology and surface composition.Öğe Fabrication and characterization of copper oxide-silicon nanowire heterojunction photodiodes(IOP PUBLISHING LTD, 2014) Akgul, Guvenc; Akgul, Funda Aksoy; Mulazimoglu, Emre; Unalan, Husnu Emrah; Turan, RasitIn this study, copper oxide (CuO) thin film/silicon (Si) nanowire heterojunctions have been fabricated and their optoelectronic performances have been investigated. Vertically aligned n-type Si nanowires have been fabricated using metal-assisted etching (MAE) technique. CuO thin films were synthesized by the sol-gel method and deposited onto the nanowires through spin-coating. Fabricated nanowire heterojunction devices exhibited excellent diode behaviour compared to the planar heterojunction control device. The rectification ratios were found to be 105 and 101 for nanowire and planar heterojunctions, respectively. The improved electrical properties and photosensitivity of the nanowire heterojunction diode was observed, which was related to the three-dimensional nature of the interface between the Si nanowires and the CuO film. Results obtained in this work reveal the potential of Si nanowire-based heterojunctions for various optoelectronic devices.Öğe Fabrication and Characterization of Ga-doped ZnO / Si Heterojunction Nanodiodes(Amer Inst Physics, 2017) Akgul, Guvenc; Akgul, Funda AksoyIn this study, temperature-dependent electrical properties of n-type Ga-doped ZnO thin film / p-type Si nanowire heterojunction diodes were reported. Metal-assisted chemical etching (MACE) process was performed to fabricate Si nanowires. Ga-doped ZnO films were then deposited onto nanowires through chemical bath deposition (CBD) technique to build three-dimensional nanowire-based heterojunction diodes. Fabricated devices revealed significant diode characteristics in the temperature range of 220 - 360 K. Electrical measurements shown that diodes had a well-defined rectifying behavior with a good rectification ratio of 10(3) +/- 3 V at room temperature. Ideality factor (n) were changed from 2.2 to 1.2 with increasing temperature.Öğe Fabrication of p-type CuO Thin Films Using Chemical Bath Deposition Technique and Their Solar Cell Applications with Si Nanowires(Amer Inst Physics, 2017) Akgul, Funda Aksoy; Akgul, GuvencRecently, CuO has attracted much interest owing to its suitable material properties, inexpensive fabrication cost and potential applications for optoelectronic devices. In this study, CuO thin films were deposited on glass substrates using chemical bath deposition technique and post-deposition annealing effect on the properties of the prepared samples were investigated. p-n heterojunction solar cells were then constructed by coating of p-type CuO films onto the vertically well aligned n-type Si nanowires synthesized through MACE method. Photovoltaic performance of the fabricated devices were determined with current voltage (I-V) measurements under AM 1.5 G illumination. The optimal short-circuit current density, open-circuit voltage, fill factor and power conversion efficiency were found to be 3.2 mA/cm(-2), 337 mV, 37.9 and 0.45%, respectively. The observed performance clearly indicates that the investigated device structure could be a promising candidate for high-performance low-cost new-generation photovoltaic diodes.Öğe Impact of cobalt doping on structural and magnetic properties of zinc oxide nanocomposites synthesized by mechanical ball-milling method(Elsevier, 2022) Akgul, Guvenc; Akgul, Funda AksoyA series of zinc oxide (ZnO) semiconductor nanocomposite samples doped with different concentrations of cobalt (Co) were successfully synthesized via simple and highly effective mechanical ball-milling method using suitable precursors. All doped nanocomposites crystallized in hexagonal wurtzite formation of ZnO. No alteration in the wurtzite structure, with no evidence of any segregated secondary phase(s) was detected with increasing doping concentrations. This result unambiguously verified the substitution of Zn2+ sites by Co2+ ions in the ZnO host lattice for different doping concentrations. Moreover, room temperature ferromagnetic behavior was observed in all doped nanocomposite samples in the reported work. Present study clearly shows that mechanical ball-milling is a facile, inexpensive, eco-friendly method to synthesize crystalline and magnetic Co doped ZnO nano composites for next generation spin-based devices.Öğe Improved diode properties in zinc telluride thin film-silicon nanowire heterojunctions(TAYLOR & FRANCIS LTD, 2015) Akgul, Funda Aksoy; Akgul, Guvenc; Gullu, Hasan Huseyin; Unalan, Husnu Emrah; Turan, RasitIn this study, structural and optoelectronic properties and photodedection characteristics of diodes constructed from p-zinc telluride (ZnTe) thin film/n-silicon (Si) nanowire heterojunctions are reported. Dense arrays of vertically aligned Si nanowires were successfully synthesized on (110)-oriented n-type single crystalline Si wafer using simple and inexpensive metal-assisted etching (MAE) process. Following the nanowire synthesis, p-type ZnTe thin films were deposited onto vertically oriented Si nanowires via radio frequency magnetron sputtering to form three-dimensional heterojunctions. A comparative study of the structural results obtained from X-ray diffraction and Raman spectroscopy measurements showed the improved crystallinity of the ZnTe thin films deposited onto the Si nanowire arrays. The fabricated nanowire-based heterojunction devices exhibited remarkable diode characteristics and enhanced optoelectronic properties and photosensitivity in comparison to the planar reference. The electrical measurements revealed that the diodes with nanowires had a well-defined rectifying behaviour with a rectification ratio of 10(4) at +/- 2V and a relatively small ideality factor of n=1.8 with lower reverse leakage current and series resistance at room temperature in dark condition. Moreover, an open-circuit voltage of 100mV was also observed under illumination. Based on spectral photoresponsivity measurements, the nanowire-based device exhibited a distinct responsivity and high detectivity in visible and near-infrared (NIR) wavelength regions. The device characteristics observed here offer that the fabricated ZnTe thin film/Si nanowire-based p-n heterojunction structures will find important applications in future and will be a promising candidate for high-performance and low-cost optoelectronic device applications, NIR photodedectors in particular.Öğe Influence of thermal annealing on microstructural, morphological, optical properties and surface electronic structure of copper oxide thin films(ELSEVIER SCIENCE SA, 2014) Akgul, Funda Aksoy; Akgul, Guvenc; Yildirim, Nurcan; Unalan, Husnu Emrah; Turan, RasitIn this study, effect of the post-deposition thermal annealing on copper oxide thin films has been systemically investigated. The copper oxide thin films were chemically deposited on glass substrates by spin-coating. Samples were annealed in air at atmospheric pressure and at different temperatures ranging from 200 to 600 degrees C. The microstructural, morphological, optical properties and surface electronic structure of the thin films have been studied by diagnostic techniques such as X-ray diffraction (XRD), Raman spectroscopy, ultraviolet-visible (UV-VIS) absorption spectroscopy, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The thickness of the films was about 520 nm. Crystallinity and grain size was found to improve with annealing temperature. The optical bandgap of the samples was found to be in between 1.93 and 2.08 eV. Cupric oxide (Cuo), cuprous oxide (Cu2O) and copper hydroxide (Cu(OH)(2)) phases were observed on the surface of as-deposited and 600 degrees C annealed thin films and relative concentrations of these three phases were found to depend on annealing temperature. A complete characterization reported herein allowed us to better understand the surface properties of copper oxide thin films which could then be used as active layers in optoelectronic devices such as solar cells and photodetectors. (C) 2014 Elsevier B.V. All rights reserved.Öğe Influence of Ti doping on ZnO nanocomposites: Synthesis and structural characterization(ELSEVIER SCI LTD, 2016) Akgul, Funda AksoyIn this study, pure phase titanium (Ti) doped zinc oxide (ZnO) nanocomposites were successfully synthesized through facile and effective high energy ball-milling method using suitable precursors. Different Ti concentrations (Zn1-xTixO, x = 0, 0.03, 0.05 and 0.1) were doped into ZnO. Detailed crystal and local atomic structures of synthesized samples were characterized via X-ray diffraction (XRD) and non-destructive synchrotron based X-ray absorption spectroscopy (XAS). Obtained results revealed that substitution of Zn sites by Ti ions in Zn1-xTixO did not cause any alteration in the hexagonal wurtzite phase of ZnO with increasing doping concentrations. This study clearly suggests that ball-milling is a quite simple, cost effective, eco-friendly method to synthesize highly crystalline nanosized Ti doped ZnO composites for future nanotechnological applications, spintronic devices in particular. (C) 2016 Elsevier Ltd. All rights reserved.Öğe Investigation of thickness dependence on electronic structures of iron and nickel thin films by L-edge X-ray absorption spectroscopy(PERGAMON-ELSEVIER SCIENCE LTD, 2014) Akgul, Guvenc; Akgul, Funda Aksoy; Ufuktepe, YukselWe have studied the effect of the film thickness on the electronic structure of pure nickel and iron thin films. Series of the thin films were evaporated by e-beam evaporation on SiN substrates. The electronic structure of the thin films was investigated using X-ray absorption near edge structure (XANES) spectroscopy. We have showed the thickness dependent variation of the experimental branching ratio (BR) and full-width at half-maximum (FWHM) at the L-3 and L-2 edges for both thin films. A strong thickness dependence of the L-2,L-3 BR and FWHM was found. We have also focused on the deviation of L-3 to L-2 ratio from its statistical value. The average L-3/L-2 white-line intensity ratio was calculated to be 3.4 and 3.0 from peak height and integrated area under each L-3 and L-2 peaks, respectively for iron. Additionally, a theoretical L-2,L-3 edge calculation for nickel was presented. The obtained results were consistent with the general view of the L-2,L-3 BR and FWHM of iron and nickel transition metals. (c) 2013 Elsevier Ltd. All rights reserved.Öğe Microstructural properties and local atomic structures of cobalt oxide nanoparticles synthesised by mechanical ball-milling process(Taylor & Francis Ltd, 2016) Akgul, Funda Aksoy; Akgul, Guvenc; Kurban, MustafaIn this study, facile preparation of pure and nano-sized cobalt oxides particles was achieved using low-cost mechanical ball-milling synthesis route. Microstructural and morphological properties of synthesised products were characterised by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. XRD results indicated that the fabricated samples composed of cubic pure phase CoO and Co3O4 nanocrystalline particles with an average crystallite size of 37.2 and 31.8nm, respectively. TEM images showed that the resulting samples consisted of agglomerates of particles with average diameter of about 37.6nm for CoO and 31.9nm for Co3O4. Phase purity of the prepared samples was further investigated due to their promising technological applications. Local atomic structure properties of the prepared nanoparticles were probed using synchrotron radiation-based X-ray absorption spectroscopy (XAS) including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). EXAFS data analysis further confirmed the formation of single-phase CoO and Co3O4 nanoparticles. In addition, structural properties of cobalt oxide nanoparticles were investigated by performing density functional theory calculations at B3LYP/TZVP level and Born-Oppenheimer molecular dynamics. Theoretical calculations for both prepared samples were found to be consistent with the experimental results derived from EXAFS analysis. Obtained results herein reveals that highly crystalline and pure phase CoO and Co3O4 nanoparticles can be synthesised using simple, inexpensive and eco-friendly ball-milling method for renewable energy applications involving fuel cells and water splitting devices.Öğe Observation of Substrate Orientation-Dependent Oxygen Defect Filling in Thin WO3-delta/TiO2 Pulsed Laser-Deposited Films with in Situ XPS at High Oxygen Pressure and Temperature(AMER CHEMICAL SOC, 2012) Braun, Artur; Akgul, Funda Aksoy; Chen, Qianli; Erat, Selma; Huang, Tzu-Wen; Jabeen, Naila; Zhang, XiaojunSubstoichiometric tungsten oxide films of approximately 10 nm thickness deposited with pulsed laser ablation on single-crystal TiO2 substrates with (001) and (110) orientation show defect states near the Fermi energy in the valence-band X-ray photoelectron spectroscopy (XPS) spectra. The spectral weight of the defect states is particularly strong for the film grown on the (001) surface. In situ XPS under an oxygen pressure of 100 mTorr shows that the spectral weight of the defect states decreases significantly at 500 K for the film on the (110) substrate, whereas that of the film grown on the (001) substrate remains the same at a temperature up to 673 K. Furthermore, diffusion of titanium from the substrate to the film surface is observed on the (110) substrate, as is evidenced by the sudden appearance of the Ti 2p core level signature above 623 K and below 673 K. The film grown on the (001) surface does not show such an interdiffusion effect, which suggests that the orientation of the substrate can have a significant influence on the high-temperature integrity of the tungsten oxide films. Quantitative analysis of the O Is core level XPS spectra shows that chemisorbed water from sample storage under ambient conditions is desorbed during heating under oxygen exposure.Öğe Photovoltaic performance of Gallium-doped ZnO thin film/Si nanowires heterojunction diodes(TAYLOR & FRANCIS LTD, 2016) Akgul, Guvenc; Akgul, Funda Aksoy; Unalan, Husnu Emrah; Turan, RasitIn this work, photovoltaic performance of Ga-doped ZnO thin film/Si NWs heterojunction diodes was investigated. Highly dense and vertically well-aligned Si NW arrays were successfully synthesised on a p-type (1 0 0)-oriented Si wafer through cost-effective metal-assisted chemical etching technique. Ga-doped ZnO thin films were deposited onto Si NWs via radio frequency magnetron sputtering to construct three-dimensional heterostructures. Photovoltaic characteristics of the fabricated diodes were determined with current density (J)-voltage (V) measurements under simulated solar irradiation of AM 1.5G. The optimal open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency were found to be 0.37V, 3.30mAcm(-2), 39.00 and 0.62%, respectively. Moreover, photovoltaic diodes exhibited relatively high external quantum efficiency over the broadband wavelengths between 350 and 1100nm interval of the spectrum. The observed photovoltaic performance in this study clearly indicates that the investigated device structure composed of Ga-doped ZnO thin film/Si NWs heterojunctions could facilitate an alternative pathway for optoelectronic applications in future, and be a promising alternative candidate for high-performance low-cost new-generation photovoltaic diodes.Öğe Structural and electronic properties of SnO2(ELSEVIER SCIENCE SA, 2013) Akgul, Funda Aksoy; Gumus, Cebrail; Er, Ali O.; Farha, Ashraf H.; Akgul, Guvenc; Ufuktepe, Yuksel; Liu, ZhiHighly transparent polycrystalline thin film of SnO2 (tin dioxide) was deposited using a simple and low cost spray pyrolysis method. The film was prepared from an aqueous solution of tin tetrachloride (stannic chloride) onto glass substrates at 400 degrees C. A range of diagnostic techniques including X-ray diffraction (XRD), UV-visible absorption, atomic force microscopy (AFM), scanning electron microscopy (SEM), and synchrotron-based X-ray photoelectron spectroscopy (XPS) were used to investigate structural, optical, and electronic properties of the resulting film. Deposited film was found to be polycrystalline. A mixture of SnO and SnO2 phases was observed. The average crystallite size of similar to 21.3 nm for SnO2 was calculated by Rietveld method using XRD data. The oxidation states of the SnO2 thin film were confirmed by the shape analysis of corresponding XPS O 1s, Sn 3d, and Sn 4d peaks using the decomposition procedure. The analysis of the XPS core level peaks showed that the chemical component is non-stoichiometric and the ratio of oxygen to tin (O/Sn) is 1.85 which is slightly under stoichiometry. (c) 2013 Elsevier B.V. All rights reserved.Öğe Structural properties of zinc oxide and titanium dioxide nanoparticles prepared by chemical vapor synthesis(ELSEVIER SCIENCE SA, 2013) Akgul, Guvenc; Akgul, Funda Aksoy; Attenkofer, Klaus; Winterer, MarkusTransition metal (TM) oxides provide a wide range of functional materials especially when nanostructured. Titanium dioxide (TiO2) and wurtzite type zinc oxide (ZnO) nanostructured materials were fabricated by chemical vapor synthesis (CVS). Crystal and local structures of the prepared nanosamples were ascertained using X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and extended Xray absorption fine structure (EXAFS) techniques. Based on the XRD data, a second phase(s) was not found in both samples. A single wurtzite and anatase type structures were observed in ZnO and TiO2 nanosamples, respectively. Ti K pre-edge features of XANES spectrum indicated the presence of sixfold coordinated Ti in TiO2 nanosamples. The results showed that CVS is quite useful method to produce high crystalline nanoparticles. (C) 2012 Elsevier B.V. All rights reserved.
