Investigation on electrical charging/discharging properties of thin ps/pmma polymeric films by dynamic x-ray photoelectron spectroscopy

Ankara : The Department of Chemistry and the Institute of Engineering and Sciences of Bilkent University, 2008.Thesis (Master's) -- Bilkent University, 2008.Includes bibliographical references leaves 64-72.Electrical charging/discharging properties of thin layers of spin coated pure PS polystyrene (PS), and pure poly(methyl methacrylate), (PMMA) and an immiscible PS/PMMA blend are studied with externally applied DC and dynamic(squarewave stress) X-Ray Photoelectron Spectroscopy (XPS). PS and PMMA exhibit quite different charging properties under DC stress. Due to the different charging properties of PS and PMMA domains within an immiscible PS/PMMA blend film, charge contrast or phase separation is clearly observed between domains. Electrical parameters of PS and PMMA thin films while employing a squarewave pulse stress in a 10-3 - 103 Hz range of frequency are probed by dynamic XPS. A model, which can simulate the dynamical XPS measurements, is shown to lead to obtain effective resistance and capacitance values for each polymer. RC time constants and inflection points(frequency) are also determined. Moreover, thickness also affects the charging/discharging properties and other electrical parameters of polymeric thin films. Furthermore, to mimic certain dielectric properties of thin polymeric films and to validate our model, we have included an externally RC circuit to a conducting graphite sample under both DC and squarewave pulse stresses.Sezen, HikmetM.S

Photo-dynamic XPS for investigating photoinduced voltage changes in semiconducting materials

Ankara : The Department of Chemistry and the Institute of Engineering and Science of Bilkent Univ., 2011.Thesis (Ph. D.) -- Bilkent University, 2011.Includes bibliographical references leaves 90-104.The main motivation of this Ph.D. study is investigation of the photoinduced voltage changes in semiconductive materials with X-ray Photoelectron Spectroscopy (XPS). For this purpose, we have developed a technique for recording the shifts in the positions of the XPS peaks in response to different waveforms of electrical and/or optical stimuli for tracing dynamics of the developed potentials originating from intrinsic or extrinsic factors of the semiconductive materials such as charging/discharging, photoconductivity, surface photovoltage, band-bending/flattening/inversion, etc. Within this purpose, the surface photovoltage behaviors of n- and p-type doped Si and GaN samples are examined with the photo-dynamic XPS, to follow the behavior of the bandbending under photoillumination in both static and dynamic fashions. The band inversion effects are clearly observed on the n- and p-Si samples in the presence of a dielectric silica overlayer and on the p-GaN sample due to variation of the illuminating laser energies Moreover, the extent of the dopant dependent XPS peak shifts of the n- and p-Si samples are assessed after correction of their surface photovoltage values. A laser patterned silicon wafer with a high-power near infrared fiber laser is also investigated. While the patterned silica domains have identical chemical composition with the non-patterned regions, an investigation with dynamic XPS clearly reveals distinct dielectric characteristics of the patterned domains. Electrical parameters of CdS thin film are extracted by dynamic XPS with and without photoillumination. The photo-dynamic XPS technique has also provided useful information by disentanglement of processes; charging/discharging, photoconductivity, and surface photovoltage. Furthermore, location (space) dependent resistance and chemical profile of a CdS based Light Dependent Resistor (LDR) is also probed during realistic operational conditions, by utilizing spatially resolved XPS analysis (in the area mapping mode). In addition, with the XPS mapping analysis defects and malfunctioning sites/domains have been located under various experimental and preparation conditions.Sezen, HikmetPh.D

Intestinal helminths of the white stork (Ciconia ciconia Linnaeus 1758) from an inter-route site in Turkey

In Turkey, a study was conducted during the years 2009–2015 to detect the gastrointestinal helminth species of 18 white storks (Ciconia ciconia Linnaeus, 1758 (Aves: Ciconiiformes) sampled from aquatic or swamp areas of Bursa Province, one of the inter-route sites where storks intensely stay. The results of postmortem examination revealed that 17 (94.44%) white storks harboured one or more helminth species. Eight species of helminths were detected at the following prevalence rates: Dictymetra discoidea (38.88%), Chaunocephalus ferox (37.50%), Schistocephalus solidus (27.77%), Stephanoprora (Monilifer) spinulosa (18.75%), Echinoparyphium sp. (12.50%), Tylodelphys excavata (6.25%), T. clavata (6.25%), and Syncuaria ciconiae (6.25%). This study is the first report on the presence of all the above species except T. excavata from white storks in Turkey. Stephanoprora (M.) spinulosa was found in Ciconiiform birds for the first time in this study

From Growth Surface to Device Interface: Preserving Metallic Fe under Monolayer Hexagonal Boron Nitride

We investigate the interfacial chemistry between Fe catalyst foils and monolayer hexagonal boron nitride (h-BN) following chemical vapour deposition and during subsequent atmospheric exposure, using scanning electron microscopy, X-ray photoemission spectroscopy, and scanning photoelectron microscopy. We show that regions of the Fe surface covered by h-BN remain in a reduced state during exposure to moist air for ~40 hours at room temperature. This protection is attributed to the strong interfacial interaction between h-BN and Fe, which prevents the rapid intercalation of oxidizing species. Local Fe oxidation is observed on bare Fe regions and close to defects in the h-BN film (e.g. domain boundaries, wrinkles, and edges), which over the longer-term provide pathways for slow bulk oxidation of the Fe. We further confirm that the interface between h-BN and reduced Fe can be recovered by vacuum annealing at ~600 °C, although this is accompanied by the creation of defects within the h-BN film. We discuss the importance of these findings in the context of integrated manufacturing and transfer-free device integration of h-BN, particularly for technologically important applications where h-BN has potential as a tunnel barrier such as magnetic tunnel junctions.S.C. and L.D. acknowledge EPSRC Doctoral Training Awards. H.S. acknowledges a research fellowship from the Japanese Society for the Promotion of Science (JSPS). S.H. acknowledges funding from ERC grant InsituNANO (no. 279342). This research was partially supported by the EUFP7 Work Programme under grant GRAFOL (project reference 285275) and EPSRC under grant GRAPHTED (project reference EP/ ACS Applied Materials & Interfaces Research Article K016636/1). R.S.W. acknowledges a Research Fellowship from St. John’s College, Cambridge, and a Marie Skłodowska-Curie Individual Fellowship (Global) under grant ARTIST (no. 656870) from the European Union’s Horizon 2020 research and innovation programme

XPS measurements for probing dynamics of charging

The technique of recording X-ray photoemission data while the sample rod is subjected to +/- 10.0V (dc) or square-wave pulses (ac) with varying frequencies in the range of 10(-3) to 10(3) HZ for probing charging/discharging dynamics of dielectric materials, is reviewed. Application of this technique introduces charging shifts as well as broadening of the peaks, which depend non-linearly on the polarity, as well as on the frequency of the pulses applied. These changes have been measured on: (i) an artificially created dielectric sample consisting of a Au metal strip connected externally to a series resistor of 1 M Omega and a parallel capacitor of 56 nF, and two real dielectric films; (ii) a 20 nm organic polystyrene film spin-coated on a silicon substrate; (iii) a 10 nm SiO(2) inorganic layer thermally grown on silicon. A simple circuit model is introduced to simulate the charging shifts and the peak broadenings. Although this simple model faithfully reproduces the charging shifts in all three cases, and also some of the broadenings for the artificial dielectric and the polystyrene film, the additional broadening in the negatively charged peaks of the SiO(2) dielectric film cannot be accounted for. It is also claimed that these experimental findings can be used for extracting material-specific dielectric properties

Charging/discharging of thin PS/PMMA films as probed by dynamic X-ray photoelectron spectroscopy

Polystyrene/polymethyl methacrylate (PS-PMMA) thin films were analyzed for detecting phase separation as well as probing their electrical responses by XPS. It was also shown that electrical parameters like resistance or capacitance can also be extracted using dynamical XPS measurements. A Kratos ES300 electron spectrometer was used for XPS measurements, and a nearby filament provided low-energy electrons for charge neutralization. The results show that under a positive stress, low-energy electrons are attracted to the sample and yield less positive charge on the sample, due to partial neutralization