Browsing by Subject "X-ray photoelectron spectroscopy"
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Item Open Access AC electrowetting modulation of low-volatile liquids probed by XPS: dipolar vs ionic screening(American Chemical Society, 2019) Aydoğan-Göktürk, Pınar; Ülgüt, Burak; Süzer, ŞefikX-ray photoelectron spectroscopic (XPS) data have been recorded for a low-molecular-weight poly(ethylene glycol) microliter-sized sessile liquid drops sitting on a dielectric covered planar electrode while imposing a ±6 V square-wave actuation with varying frequencies between 10–1 and 105 Hz to tap into the information derivable from (AC) electrowetting. We show that this time-varying XPS spectra reveal two distinct behaviors of the device under investigation, below and above a critical frequency, measured as ∼70 Hz for the liquid poly(ethylene glycol) with a 600 Da molecular weight. Below the critical frequency, the liquid complies faithfully to the applied bias, as determined by the constant shift in the binding energy position of the XPS peaks representative of the liquid throughout its entire surface. The liquid completely screens the applied electrical field and the entire potential drop takes place at the liquid/dielectric interface. However, for frequencies above the critical value, the resistive component of the system dominates, resulting in the formation of equipotential surface contours, which are derived from the differences in the positions of the twinned O 1s peaks under AC application. This critical frequency is independent of the size of the liquid drop, and the amplitude of the excitation, but increases when ionic moieties are introduced. The XP spectra under AC actuation is also faithfully simulated using an equivalent circuit model consisting of only resistors and capacitors and using an electrical circuit simulation software. Moreover, a mimicking device is fabricated and its XP spectra are recorded using the Sn 3d peaks of the solder joints at different points on the circuit to confirm the reliability of the measured and simulated AC behaviors of the liquid. These new findings indicate that in contrast to direct current case, XPS measurements under variable frequency AC actuation reveal (through differences in the frequency response) information related to the chemical makeup of the liquid(s) and brings the laboratory-based XPS as a powerful complimentary arsenal to electrochemical analyses of liquids and their interfaces.Item Open Access Analysis of the oxidation state of platinum particles in supported catalysts by double differentiation of XPS lines(M A I K Nauka - Interperiodica, 2016) Smirnov, M. Y.; Kalinkin, A. V.; Vovk, E. I.; Bukhtiyarov, V. I.In the work the double differentiation of functions describing the Pt4f7/2 band in the XPS spectra of model supported Pt/SiO2 catalysts is performed in order to determine the number of different chemical states of platinum particles. The functions for the differentiation are obtained by the deconvolution of the experimental spectral contour into two spin-orbit components. As a result of the performed analysis of the number and position of the minima of the second derivative of the function of Pt437/2 the conditions of the oxidation of platinum particles in the Pt/SiO2 sample on treating in a NO + O2 mixture and the reduction of platinum oxide particles on interacting of the PtOx/SiO2 sample with hydrogen are determined.Item Open Access Chemically specific dynamic characterization of photovoltaic and photoconductivity effects of nanostructures by X-ray photoelectron spectroscopy(2010) Ekiz, Okan ÖnerX-Ray Photoelectron Spectroscopy is a widely used characterization method for chemical analysis of surfaces. In this thesis, We report characterization of photovoltaic and photoconductivity effects on nanostructured surfaces through light induced changes in the X-ray photoelectron spectra (XPS). The technique combines the chemical specificity of XPS and the power of surface photovoltage spectroscopy (SPV), with the addition of the ability to characterize photoconductivity under both static and dynamic optical excitation. A theoretical model that quantitatively describes the features of the observed spectra is presented. We demonstrate the applicability of the model on a multitude of sample systems, including homo- and heterojunction solar cells, CdS nanoparticles on metallic or semiconducting substrates, and carbon nanotube films on silicon substrates. A-Si/c-Si heterojunction solar cell is fabricated to characterize photovoltage generation in a nanostructured solar cell. Cadmium Sulfide (CdS) nanoparticles were synthesized by a solvothermal route. Both Multi Wall Carbon Nanotube (MWCNT) and Single Wall Carbon Nanotube (SWCNT) films were characterized with different substrates by XPS. Raman Microscopy, EDS, SEM, XRD, SAXS are used to characterize the samples and solar cells.Item Open Access Chemically specific dynamic characterization of photovoltaic and photoconductivity effects of surface nanostructures(American Chemical Society, 2010) Ekiz, O. Ö.; Mizrak, K.; Dâna, A.We report characterization of photovoltaic and photoconductivity effects on nanostructured surfaces through light induced changes in the X-ray photoelectron spectra (XPS). The technique combines the chemical specificity of XPS and the power of surface photovoltage spectroscopy (SPV), with the addition of the ability to characterize photoconductivity under both static and dynamic optical excitation. A theoretical model that quantitatively describes the features of the observed spectra is presented. We demonstrate the applicability of the model on a multitude of sample systems, including homo- and heterojunction solar cells, CdS nanoparticles on metallic or semiconducting substrates, and carbon nanotube films on silicon substrates.Item Open Access Coarse-grained electrostatic model including ion-pairing equilibrium that explains DC and AC X-ray photoelectron spectroscopy measurements on ionic liquids(American Chemical Society, 2019) Uzundal, Can Berk; Aydoğan-Göktürk, Pınar; Süzer, Şefik; Ülgüt, BurakThe dynamics of the electrochemical double layer in ionic liquids can be experimentally probed by a number of experimental techniques. Earlier, we reported on the results of an X-ray photoelectron spectroscopic investigation under applied square-wave bias of two distinct frequencies. Our studies had revealed counterintuitive properties involving the physical and temporal progression of the effect of the electrochemical double layer that could not be modeled with conventional equivalent-circuit approaches. Herein, we present a new coarse-grained modeling methodology that accounts for particle diffusion, migration, and ion-association equilibrium. Our model is computationally efficient enough to be used to predict and match the results at extended time scales and distances of the experiment. Modeling efforts predict that a crucial component of the behavior is controlled by the ion-pairing equilibrium, an often overlooked aspect of ionic liquids.Item Open Access Comparative operando XPS and SEM spatiotemporal potential mapping of ionic liquid polarization in a coplanar electrochemical device(American Chemical Society, 2021-09-21) Suzer, Sefik; Strelcov, E.; Kolmakov, A.The polarization response of a coplanar electrochemical capacitor covered with an ionic liquid as the electrolyte has been examined using a combination of two powerful analytic techniques, X-ray photoelectron spectroscopy (XPS) and scanning electron microcopy (SEM). Spatiotemporal distribution of the ionic liquid surface potential, upon DC or AC (square wave) biasing, has been monitored via chemical element binding energy shifts using XPS and secondary electron intensity variations using SEM. SEM’s high spatial resolution and speedy imaging together with application of a data mining algorithm made mapping of the surface potential distribution across the capacitor possible. Interestingly, despite the differences in the detection principles, both techniques yield similar polarization relaxation time constants. The results demonstrate the power of a synergistic combination of the two techniques with complementary capabilities and pave the way to a deeper understanding of liquid/solid interfaces and for performance evaluation and diagnostics of electrochemical devices.Item Open Access Composition of γ-ray induced triethoxyvinylsilane-methyl methacrylate copolymers determined by XPS(Elsevier, 1998) Çaykara, T.; Güven, O.; Süzer, Ş.Methyl methacrylate (MMA) was copolymerized with triethoxyvinylsilane (TEVS) using 60Co-γ radiation at varying masses of the liquid monomers in the feed. Their homopolymers PMMA and PTEVS were also prepared by the same method. Thin copolymer and homopolymer blend films were prepared by dissolving the polymers in tetrahydrofuran and casting on clean Teflon or glass substrates. Analysis of the surface composition of these films using XPS indicated that the surfaces of the blend films were completely covered by PTEVS after 10% composition by weight. Similar analysis on the copolymer films, however, revealed that the surfaces of the copolymers contain comparable amounts of PMMA and PTEVS in agreement with the bulk analysis using infrared spectrometry. Hence, use of PMMA/PTEVS copolymers for stone preservation seems feasible by radiation induced polymerization. © 1998 Elsevier Science Ltd. All rights reserved.Item Open Access Effects of rapid thermal annealing on the structural and local atomic properties of ZnO: Ge nanocomposite thin films(A I P Publishing LLC, 2015) Ceylan, A.; Rumaiz, A. K.; Caliskan, D.; Ozcan, S.; Özbay, Ekmel; Woicik, J. C.We have investigated the structural and local atomic properties of Ge nanocrystals (Ge-ncs) embedded ZnO (ZnO: Ge) thin films. The films were deposited by sequential sputtering of ZnO and Ge thin film layers on z-cut quartz substrates followed by an ex-situ rapid thermal annealing (RTA) at 600 °C for 30, 60, and 90 s under forming gas atmosphere. Effects of RTA time on the evolution of Ge-ncs were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), hard x-ray photoelectron spectroscopy (HAXPES), and extended x-ray absorption fine structure (EXAFS). XRD patterns have clearly shown that fcc diamond phase Ge-ncs of sizes ranging between 18 and 27 nm are formed upon RTA and no Ge-oxide peak has been detected. However, cross-section SEM images have clearly revealed that after RTA process, Ge layers form varying size nanoclusters composed of Ge-ncs regions. EXAFS performed at the Ge K-edge to probe the local atomic structure of the Ge-ncs has revealed that as prepared ZnO:Ge possesses Ge-oxide but subsequent RTA leads to crystalline Ge structure without the oxide layer. In order to study the occupied electronic structure, HAXPES has been utilized. The peak separation between the Zn 2p and Ge 3d shows no significant change due to RTA. This implies little change in the valence band offset due to RTA. © 2015 AIP Publishing LLC.Item Open Access Electrical circuit modeling of surface structures for X-ray photoelectron spectroscopic measurements(Elsevier BV * North-Holland, 2008) Tasci, T. O.; Atalar, Ergin; Demirok, U. K.; Süzer, ŞefikWe model the X-ray photoelectron spectrometer and the sample with lumped electrical circuit elements, and simulate various types of conditions using a widely used computer program (PSpice) and compare the results with experimental measurements. By using the electrical model simulations, the surface voltage and the spectrum can be estimated under various types of external voltage stimuli, and the zero potential condition can be predicted accurately for obtaining a truly uncharged spectrum. Additionally, effects of several charging mechanisms (taking place during XPS measurements) on the surface potential could easily be assessed. Finally, the model enables us to find electrical properties, like resistance and capacitance of surface structures, under X-ray and low-energy electron exposure.Item Open Access A photoelectron spectroscopic investigation of conducting polypyrolle-polyamide composite film(Elsevier, 1995-04) Süzer, Şefik; Toppare, L.; Allen, G. C.; Hallam, K. R.X-ray photoelectron spectrum of the electrochemically prepared polypyrrole and polypyrrole-polyamide composite films exhibit an additional strong high binding energy peak at 402.0 eV corresponding to N+ moieties. Intensity of this peak is significantly reduced upon electrochemical reduction. Atomic concentrations derived from the observed N+ and F (stemming from the dopant BF4-) peaks reveal a slightly higher cation/anion ratio for this composite and suggest that the composite has a different chemical composition than the corresponding polymers. © 1995 Elsevier Science B.V.Item Open Access Preparation of Au and Au-Pt nanoparticles within PMMA matrix using UV and X-ray irradiation(2009) Ozkaraoglu, E.; Tunc, I.; Süzer, ŞefikAu and Au-Pt alloy nanoparticles are prepared and patterned at room temperature within the PMMA polymer matrix by the action of 254 nm UV light or X-rays. The polymer matrix enables us to entangle the kinetics of the photochemical reduction from the nucleation and growth processes, when monitored by UV-vis spectroscopy. Accordingly, increase of the temperature to 50 °C of the reaction medium increases the nucleation and growth rates of the nanoparticle formation by more than one order of magnitude, due to enhanced diffusion and nucleation at the higher temperature, but has no effect on the photochemical reduction process. Presence of Pt ions also increases the same rate, but by a factor two only. Similar photochemical reduction and particle growth take also place within the PMMA matrix, when these metal ions are subjected to prolonged exposure to X-rays, as evidenced by XPS analysis. Both angle-resolved and charge-contrast measurements using XPS reveal that the resultant Au and Pt species are in close proximity to each other, indicating the Au-Pt alloy formation to be the most likely case.Item Open Access Probing hot-electron effects in plasmonic surfaces using X-ray photoelectron spectroscopy(2014) Çupallari, AndiHot-electron effects in plasmonic structures have been recently investigated as potential alternative mechanisms for solar energy harvesting and photodetection. [1][2][3] Hot-electron effects provide a semiconductor free route for the conversion of photons into electrical power. Here we investigate plasmonic hot electron effects in Metal-Insulator-Metal (MIM) structures using X-ray photoelectron spectroscopy (XPS). XPS has been previously used to investigate optoelectronic effects in semiconductors and nanocomposite surfaces. [4][5][6] Here, a similar approach is used to characterize the plasmonic and hot electron effects in MIM Junctions. Monochromatic Laser excitation with 450, 532 and 650 nm wavelengths are employed to illuminate the plasmonic surfaces fabricated using thermal evaporation, atomic layer deposition and electron beam lithography. The top metal of the MIM structures act as the plasmonic antenna (metal nanodiscs and gratings/stripes) that provide wavelength selective or wide band optical absorption. Plasmonic enhancement at the interface between the top metal and the insulator enhances the absorption of light in the device and leads to excitation of a larger number of hot electrons from the metal. Hot electron effects are characterized through studying the metal-insulator-metal junction and comparing shifts of binding energy belonging to the top metal islands for dark and illuminated conditions. XPS spectrum provides important information regarding the plasmonic and hot electron effects in the interface between top metal and the dielectric. A systematic study of the dependence of the XPS spectra on excitation wavelength, light intensity, polarization, insulator thickness and nanostructure geometry is presented. Effects of using different metals and insulator materials are also studied in symmetric and asymmetric tunnel junctions. Keywords:Item Open Access Probing the dynamics of non-faradaic processes in Ionic liquids at extended time and length scales using XPS with AC modulation(American Chemical Society, 2021-05-06) Aydoğan Göktürk, Pınar; Süzer, ŞefikCharging dynamics of ionic liquid (IL) electrolytes play important roles in various aspects of electrochemical processes. However, the precise understanding of such processes at extended time and length scales is incomplete due to the experimental difficulties in probing the electrochemical potential and other relevant parameters. In principle, such shortcomings should not apply to theoretical/computational approaches; however, existing works have mostly concentrated on or around electrode/electrolyte interfaces and short timescales due mostly to prohibitive demands on computational efforts. To fill this gap, we have utilized X-ray photoelectron spectroscopy to study the charging dynamics of ILs in contact with two wire electrodes under AC square wave excitation, with frequencies ranging from hundreds of kHz down to the mHz region. Using the changes in the binding energy position of the IL-related core-level peaks, electrical potential profiles along the lines in between the electrodes and on the entire surface of the electrolyte have been investigated in situ. From these results, we identify two widely different time constants. The timescale of the fast process was shown to be on the order of RC time constant, while the slow process takes place on a timescale of seconds. Our method in the present study is expected to open up a new way for extracting novel dynamic information for gaining a better understanding of such processes and designing efficient IL-based electrochemical devices with a novel perspective on the charging.Item Open Access Radiochemical and spectroscopic studies of cesium, barium, and cobalt sorption on some natural clays(2000-08) Shahwan, TalalThe wide growth in the nuclear activities results in an increasing subsequent influx of radioactive wastes into the environment. This problem has manifested a great deal of interest aiming at finding out ways through which those wastes can be harmlessly isolated from the human environment. Geological disposal is considered as one of the most promising solutions that ensures a safe storage of radioactive wastes as long as their activities are above the accepted levels. Clay minerals are proposed as backfill buffering materials in the geological repositories that can delay the migration of the radionuclides through sorption and thus decrease the contamination of underground waters. The extent of retardation of the radionuclide migration is dependent on factors like time of contact, pH and Eh of groundwater, concentration, temperature and grain size of the mineral particles. In this study radiochemical, spectroscopic (ToF-SIMS, XPS), and X-ray diffraction techniques were applied to examine different aspects of the sorption behavior of cesium, barium and cobalt on three natural clay minerals containing primarily kaolinite, illite-chlorite, and bentonite. The elements cesium (Z=55), barium (Z=56), and cobalt (Z=27) have the radioactive isotopes superscript 137 Cs (half-life=30.17 years), superscript 140 Ba (half-life=12.79 days), and superscript 60 Co (half-life=5.3 y) which are important in radioactive waste management. The first two radionuclides are produced in high yields in nuclear fission, whereas the third is an activation product. The natural clay samples that were used in this study originated from natural mineralogical beds at Sindırgı, Afyon, and Giresun regions in Turkey. The characterization of these clay samples showed that the primary clay minerals were kaolinite in Sındırgı clay, chlorite and illite in Afyon clay, and montmorillonite in Giresun clay. Each of these clays possesses different structural properties that result in different sorption capabilities. Radiochemical batch experiments were carried out to examine the effects of time, concentration, and temperature on the sorption of cesium, barium and cobalt on clays. Solutions of these cations spiked with several microliters of the radionuclides 137 CS (half-life=30.1 y), 133 Ba (half-life=10.7 y), and 60 Co (half-life=5.3 y) were monitored using gamma-ray spectroscopy prior to and after each sorption experiment. These results showed that equilibrium is achieved within two days in all cases. The sorption data was adequately described by Freundlich and Dubinin-Radushkevich isotherm models. Based on the parameters of those isotherm models, it was found that sorption was nonlinear, and that bentonite showed the highest sorption affinity and sorption capacity towards the sorbed ions. The thermodynamic parameters indicated that while sorption of cesium and barium on the three clays is exothermic that of cobalt is endothermic. The obtained values of Gibbs free energy change, Delta G degrees, were generally in the 8-16 (kJ/mol) energy range that corresponds to ion exchange type sorption mechanism. Since sorption is mainly a surface phenomenon, part of our sorption studies were carried out using the surface sensitive techniques; Time of Flight- Secondary Ion Mass Spectroscopy (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS). In addition, depth profiling up to 70 angstroms was performed using ToF-SIMS to investigate cesium, barium and cobalt concentrations through the clay surface. ToF-SIMS and XPS studies were helpful in figuring out the surface composition of different clays prior to and after sorption. Quantification of the depletion of different alkali and alkaline-earth metals initially contained within the analyzed clay surface showed that ion exchange plays a primary role in the sorption process. In addition, X-Ray Diffraction (XRD) technique was applied to figure out the mineralogical composition of the clay minerals used and examine any structural change a accompanying the sorption process. XRD spectra of the clay samples after sorption showed that -apart from some intensity reductions in some clay features-, no primary changes were detected in the sorption cases of cesium and cobalt. In barium sorption, however, features belonging to barium carbonate were present in the spectra corresponding to sorption on chlorite-illite and bentonite.Item Open Access Size effect in the oxidation-reduction processes of platinum particles supported onto silicon dioxide(Maik Nauka Publishing / Springer SBM, 2015) Smirnov, M.Yu.; Kalinkin, A.V.; Vovk, E.I.; Bukhtiyarov V.I.The interaction of the Pt/SiO2 model catalysts as thin films on the surface of tantalum supports with a mixture of NO + O2 (1: 1) was studied by X-ray photoelectron spectroscopy. The pressure of the reaction mixture was varied from 6 to 64 mbar, and the temperature was varied from room temperature to 500°C. Two types of the catalysts, in which the Pt/Si atomic ratios were ~0.1 and ~0.3 (0.1-Pt/SiO2 and 0.3-Pt/SiO2, respectively) according to the XPS data, were studied. In 0.1-Pt/SiO2, the particles of platinum predominantly had a size from 1 to 2.5 nm; a wide Pt particle size distribution in a range from 1 to 15 nm with a maximum at ~4 nm was characteristic of 0.3-Pt/SiO2. The interaction of all of the samples with NO + O2 at room temperature led to the dissolution of oxygen atoms in the bulk of platinum metal particles. As the reaction temperature was increased, PtO x platinum oxide particles were formed: from small Pt particles in 0.1-Pt/SiO2 at 300°C and from larger particles in 0.3-Pt/SiO2 at 400-500°C. It was established that the reactivity of platinum oxide particles toward hydrogen also depended on the particle size. The small particles of platinum oxide were converted into platinum metal under the action of hydrogen (16 mbar) at 300°C. The coarse particles of PtO x in the samples of 0.3-Pt/SiO2 were reduced much more easily starting with room temperature. © 2015 Pleiades Publishing, Ltd.Item Open Access X-ray photoelectron spectroscopy with electrical modulation can be used to probe electrical properties of liquids and their interfaces at different stages(American Chemical Society, 2019) Uzundal, Can Berk; Şahin, Özgür; Aydoğan-Göktürk, Pınar; Wu, H.; Mugele, F.; Ülgüt, Burak; Süzer, ŞefikOperando X-ray photoelectron spectroscopy (o-XPS) has been used to record the binding energy shifts in the C 1s peak of a pristine poly(ethylene glycol) (PEG) liquid drop in an electrowetting on dielectric (EWOD) geometry and after exposing it to several high-voltage breakdown processes. This was achieved by recording XPS data while the samples were subjected to 10 V dc and ac (square-wave modulation) actuations to extract electrical information related to the liquid and its interface with the dielectric. Through analysis of the XPS data under ac actuation, a critical frequency of 170 Hz is extracted for the pristine PEG, which is translated to a resistance value of 14 MΩ for the liquid and a capacitance value of 60 pF for the dielectric, by the help of simulations using an equivalent circuit model and also by XPS analyses of a mimicking device under similar conditions. The same measurements yield an increased value of 23 MΩ for the resistance of the liquid after the breakdown by assuming that the capacitance of the dielectric stays constant. In addition, an asymmetry in polarity dependence is observed with respect to both the onset of the breakdown voltage and also the leakage behavior of the deteriorated (PEG + dielectric) system such that deviations are more pronounced at positive voltages. Both dc and ac behaviors of the postbreakdown system can also be simulated, but only by introducing an additional element, a diode or a polarity- and magnitude-dependent voltage source (VCVS), which might be attributed to negative charge accumulation at the interface. Measurements for a liquid mixture of PEG with 8% ionic liquid yields an almost 2 orders of magnitude smaller resistance for the drop as a result of the enhanced conductivity by the ions. Coupled with modeling, XPS measurements under dc and ac modulations enable probing unique electrochemical properties of liquid/solid interfaces.Item Open Access XPS analysis with pulsed voltage stimuli(2006) Karabudak, E.; Demirok, U. K.; Süzer, ŞefikWe record XPS spectra while applying 0 to +10 V or 0 to -10 V square pulses to the sample rod, which normally results in twinning of all peaks at correspondingly increased (for +10 V) or decreased (for -10 V) binding energies. For poorly conducting samples, like silicon oxide layer on a silicon substrate, the twinned peaks appear at different energies due to differential charging, which also vary with respect to the frequency of the applied pulses. Moreover, the frequency dependence varies with the thickness and can be correlated with the capacitance of the oxide layer. The technique is simple and can lead to extract important information related with dielectric properties of surface structures in a totally non-contact fashion. © 2005 Elsevier B.V. All rights reserved.