Nanoscale analysis of the oxidation state and surface termination of praseodymium oxide ultrathin films on ruthenium(0001)

The complex structure and morphology of ultrathin praseodymia films deposited on a ruthenium(0001) single crystal substrate by reactive molecular beam epitaxy is analyzed by intensity-voltage low-energy electron microscopy in combination with theoretical calculations within an ab initio scattering theory. A rich coexistence of various nanoscale crystalline surface structures is identified for the as-grown samples, notably comprising two distinct oxygen-terminated hexagonal Pr2O3(0001) surface phases as well as a cubic Pr2O3(111) and a fluorite PrO2(111) surface component. Furthermore, scattering theory reveals a striking similarity between the electron reflectivity spectra of praseodymia and ceria due to very efficient screening of the nuclear charge by the extra 4f electron in the former case

Surface evolution of 4H-SiC(0001) during in-situ surface preparation and its influence on graphene properties

The evolution of SiC surface morphology during graphene growth process has been studied through the comparison of substrate surface step structure after in-situ etching and graphene growth in vacuum. Influence of in-situ substrate surface preparation on the properties of graphene was studied through the comparison of graphene layers on etched and un-etched substrates grown under same conditions. © (2013) Trans Tech Publications, Switzerland

Untersuchung der Photonen-stimulierten Desorption an Chlor auf Silizium(111) mit stehenden Roentgenwellenfeldern

In this study the X-ray photon stimulated desorption (XPSD) of positive chlorine ions from a Cl/Si(111)-(1 x 1) surface is investigated. The desorbing ions are identified by means of time-of-flight spectroscopy making use of the periodic time structure of the storage ring DORIS III in reduced bunch mode. The combination of X-ray standing waves (XSW) and XPSD allows both the determination of the adsorbate structure and the identification of the dominant desorption mechanisms if difference spectra are taken below and above an absorption edge of the substrate and/or the adsorbate [1]. The in situ experiments were carried out under UHV conditions at the undulator beamline BW1 using a standard non-dispersive setup. After saturation exposure the chlorinated silicon samples were annealed at temperatures between 570 C and 700 C and always exhibited a sharp (1 x 1) LEED pattern. XSW measurements were performed by tuning the incident photon energy through the (111) and (220) Bragg reflections and simultaneously monitoring the Cl"+ and, for the first time, Cl"2"+ ion yields. In some cases Cl 1s photoelectrons were utilized as secondary signals, thus obtaining the amplitude and phase of the (hkl) Fourier component of the adsorbate distribution function. (orig.)SIGLEAvailable from TIB Hannover: RR 8919(2000-044) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Charakterisierung der Photonen-stimulierten Desorption von halogenierten Siliziumoberflaechen mit stehenden Roentgenwellenfeldern

The structure of the adsorbate systems Cl/Si(001), Cl/Si(113) and Cl/Si(111) have been studied using X-ray standing waves (XSW), scanning tunneling microscopy (STM) and X-ray photoemission (XPS). In addition the X-ray induced photodesorption of Cl"+ and Cl"2"+ ions from Si surfaces has been investigated in detail. Ab initio total energy calculations using density functional theory were also performed. The STM measurements revealed the surface morphology of the samples and provided information about surface roughening and etching. The symmetrical dimer model for the Cl/Si(001)-(2 x 1) system was confirmed for majority adsorption sites. In addition various defect sites including dichloride species were calculated using DFT and were found to be consistent with the XSW and XPS results. The combination of XSW with photodesorption detection revealed that direct and indirect desorption processes for Cl"+ ions occur in a 2: 1 ratio. For the Cl/Si(113) adsorbate system the experimental and theoretical results revealed a model comprised of dimers, adatoms and on-top sites (DAO-Model). The model explains the (2 x n)-reconstruction found by STM and is consistent with the XSW results. The XPS data showed that only monochloride species were present. The PSD measurements revealed predominantly direct desorption of Cl"+ and Cl"2"+ ions from majority sites. For an adsorption temperature of 600 C anisotropic surface reorganization with step edges aligned along the [1 anti 10]-direction was observed in STM. For the Cl/Si(111)-(1 x 1) surface preferential stimulated desorption of Cl"+ and Cl"2"+ species from SiCl_3 minority sites was identified from the XSW/PSD experimental data combined with an XSW simulation based on structural models from the DFT studies. The minority sites contributed about 3% to the total Cl-coverage. The site specific desorption cross sections for excitation at the Cl K-edge were 430 barn for Cl"+ ions and 73 barn for Cl"2"+ ions which are 60-80 times higher than the corresponding values for photodesorption from the on-top site. For chlorine adsorption at RT followed by annealing the STM images revealed the formation of etch pits with a depth of two atomic layers and three-fold symmetry with step edges perpendicular to < anti 1 anti 12>-directions. In contrast for chlorine adsorption at 600 C atomically smooth step edges extending over several hundred nm perpendicular to < anti 1 anti 12>-directions were observed. (orig.)Available from TIB Hannover: RA 8919(2004-003) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Are Ni/ and Ni5Fe1/biochar catalysts suitable for synthetic natural gas production? A comparison with g-Al2O3 supported catalysts

Among challenges implicit in the transition to the post–fossil fuel energetic model, the finite amount of resources available for the technological implementation of CO2 revalorizing processes arises as a central issue. The development of fully renewable catalytic systems with easier metal recovery strategies would promote the viability and sustainability of synthetic natural gas production circular routes. Taking Ni and NiFe catalysts supported over g-Al2O3 oxide as reference materials, this work evaluates the potentiality of Ni and NiFe supported biochar catalysts for CO2 methanation. The development of competitive biochar catalysts was found dependent on the creation of basic sites on the catalyst surface. Displaying lower Turn Over Frequencies than Ni/Al catalyst, the absence of basic sites achieved over Ni/C catalyst was related to the depleted catalyst performances. For NiFe catalysts, analogous Ni5Fe1 alloys were constituted over both alumina and biochar supports. The highest specific activity of the catalyst series, exhibited by the NiFe/C catalyst, was related to the development of surface basic sites along with weaker NiFe–C interactions, which resulted in increased Ni0:NiO surface populations under reaction conditions. In summary, the present work establishes biochar supports as a competitive material to consider within the future low-carbon energetic panorama.Peer reviewe

Are Ni/ and Ni5Fe1/biochar catalysts suitable for synthetic natural gas production? A comparison with γ-Al2O3 supported catalysts

Among challenges implicit in the transition to the post–fossil fuel energetic model, the finite amount of resources available for the technological implementation of CO2 revalorizing processes arises as a central issue. The development of fully renewable catalytic systems with easier metal recovery strategies would promote the viability and sustainability of synthetic natural gas production circular routes. Taking Ni and NiFe catalysts supported over γ-Al2O3 oxide as reference materials, this work evaluates the potentiality of Ni and NiFe supported biochar catalysts for CO2 methanation. The development of competitive biochar catalysts was found dependent on the creation of basic sites on the catalyst surface. Displaying lower Turn Over Frequencies than Ni/Al catalyst, the absence of basic sites achieved over Ni/C catalyst was related to the depleted catalyst performances. For NiFe catalysts, analogous Ni5Fe1 alloys were constituted over both alumina and biochar supports. The highest specific activity of the catalyst series, exhibited by the NiFe/C catalyst, was related to the development of surface basic sites along with weaker NiFe–C interactions, which resulted in increased Ni0:NiO surface populations under reaction conditions. In summary, the present work establishes biochar supports as a competitive material to consider within the future low-carbon energetic panorama

Less strain energy despite fewer dislocations: The Impact of Ordering

The average strain state of Ge films grown on Si(111) by surfactant mediated epitaxy has been compared to the ordering of the interfacial misfit dislocation network. Surprisingly, a smaller degree of average lattice relaxation was found in films grown at higher temperature. On the other hand, these films exhibit a better ordered dislocation network. This effect energetically compensates the higher strain at higher growth temperature, leading to the conclusion that, apart from the formation of misfit dislocations, their ordering represents an important channel for lattice-strain energy relaxation

In situ Near Ambient Pressure X ray Photoelectron Spectroscopy Reveals the Influence of Photon Flux and Water on the Stability of Halide Perovskite

For several years, scientists have been trying to understand the mechanisms that reduce the long term stability of perovskite solar cells. In this work, we examined the effect of water and photon flux on the stability of CH3NH3PbI3 perovskite films and solar cells using in situ near ambient pressure X ray photoelectron spectroscopy NAP XPS , field emission scanning electron microscopy FESEM , and current density voltage J V characterization. The used amount of water vapor up to 1 mbar had a negligible impact on the perovskite film. The higher the photon flux, the more prominent were the changes in the NAP XPS and FESEM data; also, a faster decline in power conversion efficiency PCE and a more substantial hysteresis in the J V characteristics were observed. Based on our results, it can be concluded that the PCE decrease originates from the creation of Frenkel pair defects in the perovskite film under illumination. The stronger the illumination, the higher the number of Frenkel defects, leading to a faster PCE decline and more substantial hysteresis in the J V sweep