@misc{BussFaltaFlege, author = {Buß, Lars and Falta, Jens and Flege, Jan Ingo}, title = {Intercalation dynamics of sulfur underneath graphene on Ru(0001)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {315 -- 315}, abstract = {It is known that the binding of epitaxially grown graphene to the substrate has a detrimental effect on its electronic properties. This is especially true for the strong binding to various transition metals and particularly ruthenium. However, via intercalation the interlayer coupling can be lifted and its unique electronic properties can be restored. Therefore, we have investigated the interaction of sulfur with single-layer graphene grown on Ru(0001) via surface segregation and CVD of ethylene under UHV conditions with in situ low-energy electron microscopy (LEEM) and micro-diffraction (µLEED). At elevated temperature and under dimethyl disulfide background pressure, we observe that sulfur intercalates through the open edges of the graphene islands. Prolonged exposure to sulfur induces wrinkling of the graphene islands, consistent with substantial relief of tensile strain after successful sulfur insertion underneath the graphene. It can be seen that the intercalation dynamics are both dependent on the temperature during intercalation and the preparation method of the graphene sheets. Furthermore, darkfield imaging and µLEED of the intercalated graphene reveal a graphene induced improved ordering of sulfur underneath.}, language = {en} } @misc{EwertBussMorasetal., author = {Ewert, Moritz and Buß, Lars and Moras, Paolo and Falta, Jens and Flege, Jan Ingo}, title = {Unraveling the growth mechanism of single-domain molybdenum disulfide on Au(111)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {329 -- 329}, abstract = {As a transition metal dichalcogenide single-layer molybdenum disulfide (MoS2) is a heavily investigated system. Due to its direct band gap, the electronic properties of single-layer MoS2 have been subject to several surface science methods. A well-known model system is MoS2 on Au(111). We present in situ low-energy electron microscopy (LEEM) and micro-diffraction (LEED) observations of MoS2 growth on Au(111) at elevated temperature using two distinctly different deposition rates. Our investigations reveal similar but different expansion mechanisms of the MoS2 islands changing a balanced distribution of the two mirror domains towards a single domain distribution. Structural characterization by I(V)-LEEM and investigations of the electronic bandstructure using angle-resolved photoelectron spectroscopy both confirm single-layer nature of the MoS2 islands. We could identify step pushing of the growing MoS2 being responsible for this phenomena.}, language = {en} } @misc{JanowitzSchmeisser, author = {Janowitz, Christoph and Schmeißer, Dieter}, title = {Out-of-plane electronic contributions in Bi-cuprates studied by resonant photoelectron spectroscopy at the Cu 2p edge}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {403 -- 403}, abstract = {In high-temperature superconductors with a layered crystal structure out-of-plane contributions are often neglected, while the copper-oxygen planes are commonly considered to dominate the electronic properties around the Fermi energy. Here we report on a resonant photoemission study of (Pb,Bi)2201 and (Pb,Bi)2212 single crystals to unravel the resonant decay mechanisms at the Cu 2p absorption edge. We demonstrate a pronounced polarization dependence caused by two different Auger processes for in-plane and out-of-plane orientations. We deduce that the lowest energy valence state being involved in the two Auger processes, consists of three-dimensional contributions by admixed out-of-plane Sr, Bi, and O 2p states. It also suggests that the doping-induced charge density is dynamic, fluctuating within the Cu-O plane, and spills out perpendicular to it. C. Janowitz and D. Schmeißer, Supercond. Sci. Technol. 31, 045006 (2018)}, language = {en} } @misc{MahmoodinezhadPożarowskaHenkeletal., author = {Mahmoodinezhad, Ali and Pożarowska, Emilia and Henkel, Karsten and Schmeißer, Dieter and Flege, Jan Ingo}, title = {Depth profiling of PEALD-AlN films based on Al2p XPS peak decomposition}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {157 -- 157}, abstract = {AlN has remarkable properties (wide band gap, low electrical and thermal conductivity, high dielectric constant, piezoelectricity) and is attractive for (opto)electronic and sensor applications. However, high oxygen content within nitride films is always a critical issue due to the thermodynamically favorable oxidation against nitridation resulting in deteriorated materials properties. In order to clarify whether the oxidation is a surface-limited or a bulk process elemental depth profiling is essential. In this work XPS in combination with Ar+ sputtering is applied to carry out depth profiling of AlN films prepared by plasma-enhanced atomic layer deposition using different parameters (plasma source, power and pulse duration). Particularly, the Al2p core levels are analyzed where the signals are decomposed into four components, representing weaker contributions of pure AlN and aluminum oxide phases as well as stronger signals of mixed oxygen-rich and nitrogen-rich phases. After sputtering (providing access to the deeper part of the film) the pure AlN phase content increases while the pure aluminum oxide content stays relatively constant. These issues are discussed with regard to the preparation parameters employed and accompanying XRD and electrical measurements.}, language = {en} } @misc{FischerFoersterAballeetal., author = {Fischer, Simon and Foerster, Michael and Aballe, Lucia and Vonk, Vedran and Falta, Jens and Krisponeit, Jon-Olaf and Flege, Jan Ingo}, title = {Vanadium dioxide thin films on (100)- and (110)-oriented ruthenium dioxide islands}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {330 -- 330}, abstract = {Bulk vanadium dioxide exhibits a metal-insulator transition at 68 ∘C. Because this change in resistivity is linked to a structural transition, the transition temperature can be tuned by epitaxial strain. Due to the high lattice mismatch of RuO2 with VO2, this substrate promises a highly shifted transition temperature, which enables new types of switching devices, smart coatings, and sensors. It is prepared by oxidizing a Ru(0001) crystal, providing micron-sized, (110)- as well as (100)-oriented RuO2 islands simultaneously. One of the challenges in preparing VO2 is to reliably meet the desired stoichiometry. We investigated the vanadium oxidation state using local XAS and XPS measurements taken at a synchrotron-based XPEEM instrument, revealing that VO2 stoichiometry is maintained throughout the entire film. This was found not only on RuO2(100) and RuO2(110), but also on the bare ruthenium substrate. The VO2 film thickness was determined to 3.6 nm by ex situ XRR measurements. Financial support from the DFG is acknowledged.}, language = {en} } @misc{KrisponeitFischerFlegeetal., author = {Krisponeit, Jon-Olaf and Fischer, Simon and Flege, Jan Ingo and Falta, Jens}, title = {Surface reconstructions on VO2(110)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {330 -- 330}, abstract = {Vanadium dioxide features a pronounced metal-insulator transition at 340 K. For thin films, this transition is adjustable via substrate-induced strain, making VO2 highly attractive for oxide electronic applications like switching devices and sensors. In addition, VO2 is considered for smart coatings and as catalytic material. For such applicational efforts, a thorough understanding of the relevant surfaces is an important prerequisite. Among the low-indexed surfaces orientations of VO2, the (110) surface is energetically favorable. We have prepared VO2(110) thin films by means of reactive molecular beam epitaxy on two different substrate types: TiO2(110) single crystals as well as on RuO2(110) islands grown on Ru(0001) crystals. The film stoichiometry was confirmed via XPS, while the surface structure was investigated by µLEED. For both substrate types diffraction patterns of three distinct symmetries have been observed in dependence on temperature. The corresponding surface reconstructions will be discussed in terms of surface oxygen content. Financial support from the DFG and the Institutional Strategy of the University of Bremen, funded by the German Excellence Initiative, is acknowledged.}, language = {en} } @misc{PleinesGenuzioMenteşetal., author = {Pleines, Linus and Genuzio, Francesca and Mente{\c{s}}, Tevfik Onur and Locatelli, Andrea and Falta, Jens and Flege, Jan Ingo}, title = {Reduction and reoxidation of (111) and (100) oriented cerium oxide islands on Cu(111)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {330 -- 330}, abstract = {The conversion of CO2 to methanol is an important process of high industrial potential. It could help solving the greenhouse gas problem (CO2) and further provide methanol which can be used as a synthetic fuel. In order to lower the activation energy barrier of the reactions a catalyst is necessary. We studied the interaction of H2 and CO2 with ultrathin cerium oxide islands on a Cu(111) substrate observed by low-energy electron mircoscopy (LEEM), x-ray absorption spectroscopy (XAS) and resonant photoelectron spectroscopy (RPES). From earlier studies the orientation of the CeOx is known to be decisive for the catalytic activity. In our experiments CeOx(100) is directly compared to CeOx(111) via a side-by-side growth, so that the same conditions prevail during the experiment. At low temperatures no reduction of the CeOx(111) and CeOx(100) were observed. However, at a higher temperature the reduction was activated. Furthermore, we show that the exposure to CO2 leads to the formation of partially reoxidized CeOx and saturation below CeO2 with progressive CO2 exposure.}, language = {en} } @misc{MoralesMahmoodinezhadSchubertetal., author = {Morales, Carlos and Mahmoodinezhad, Ali and Schubert, Andreas Markus and Wenger, Christian and Henkel, Karsten and Flege, Jan Ingo}, title = {Functional ultra-thin oxide films deposited by atomic layer deposition on structured substrates}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {In the last decades, atomic layer deposition (ALD) has gained prominence in the materials and surface science communities owing to its high potential for integration as a scalable process in microelectronics. ALD's largest strengths are its well-controlled layer-by-layer deposition and growth conformity on 3D structures. Yet, the ALD technique is also well known to lead to amorphous and defective, non-stoichiometric thin films, resulting in modified materials properties that may even preferentially be used in certain applications. To study these issues, we have developed an in-situ ALD reactor attached to an X-ray photoelectron spectroscopy (XPS) system, capable of switching between both pump and flow-type operation. This novel tool allows to cover the entire range of compounds and recipes used in ALD, thus clarifying the role of such defects at different deposition stages, growth conditions and film/substrate interfaces. To exemplify these sorts of studies, we show the deposition of Al2O3 5-10 nm films on nanostructured Si, and their use as substrates for functional CeOx ALD deposits.}, language = {en} } @misc{EwertBussGenuzioetal., author = {Ewert, Moritz and Buß, Lars and Genuzio, Francesca and Mente{\c{s}}, Tevfik Onur and Locatelli, Andrea and Falta, Jens and Flege, Jan Ingo}, title = {On the transition from MoS2 single-layer to bilayer growth on the Au(111) surface}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, abstract = {MoS2 is well known for changing from an indirect to a direct band-gap semiconductor as a single layer. Here, for the model system MoS2/Au(111), we present in-situ studies of the continued growth of micron-size single-layer MoS2 islands including the first formation of bilayer patches. We have used angle-resolved photoemission spectroscopy from micrometer sized regions to investigate the local band structure of the islands' rims and centers, showing a prevalence for bilayer and single-layer formation at the rims and centers, respectively. The bilayer patches can clearly be identified locally on the few nanometer scale employing intensity-voltage low-energy electron microscopy as a fingerprinting method. Astonishingly, micro-spot low-energy electron diffraction hints toward the nucleation of the second layer of the MoS2 between the single layer MoS2 and the Au(111) substrate when the step bunches formed by the single-terrace growth mechanism become sufficiently high.}, language = {en} } @misc{PleinesBussMenteşetal., author = {Pleines, Linus and Buß, Lars and Mente{\c{s}}, Tevfik Onur and Genuzio, Francesca and Locatelli, Andrea and Falta, Jens and Flege, Jan Ingo}, title = {In-situ characterization of cyclic reduction and reoxidation of CeOx(111) and CeOx(100) islands on Cu(111)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {Cerium oxide (CeOx) is of special interest due to its catalytic activity and various other electronic and optical applications. The inverse model catalyst CeOx on Cu(111) has a high activity for methanol synthesis from H2 and CO2. For the activation of CO2, Ce3+ sites have to be present at the surface, which means that the CeOx has to be reduced to some extent. This may be achieved by exposure to H2 at elevated temperatures. We studied the interaction of H2 and CO2 with CeOx islands on Cu(111) with low-energy electron microscopy (LEEM) and X-ray absorption spectroscopy (XAS). From earlier studies, the orientation of the CeOx is known to be decisive for its catalytic activity. In our experiments (100) and (111) CeOx islands are grown side by side on the metal substrate, so that identical reaction conditions prevail during the experiment. At a high temperature of 550 ∘C, exposure to H2 leads to partial reduction, and exposure to CO2 leads to reoxidation of the CeOx. The differences observed for the two island orientations regarding structure and composition will be discussed.}, language = {en} } @misc{KotDasKegelmannetal., author = {Kot, Małgorzata and Das, Chittaranjan and Kegelmann, Lukas and K{\"o}bler, Hans and Vorokhta, Mykhailo and Escudero, Carlos and Albrecht, Steve and Abate, Antonio and Flege, Jan Ingo}, title = {Application of atomic layer deposition and x-ray photoelectron spectroscopy in perovskite solar cells}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {In this work we have utilized near-ambient pressure and ultra-high vacuum X-ray photoelectron spectroscopy as well as atomic layer deposition to investigate perovskite solar cells (PSCs). We have demonstrated that ultrathin room temperature atomic layer-deposited aluminium oxide on the perovskite surface very effectively suppresses iodine migration[1] and improves the long term stability and efficiency of PSCs [2,3]. Furthermore, exposure to light proves more detrimental to the perovskite film than exposure to water vapor.[2] Absorbed photons create Frenkel defects in the perovskite crystal and their number strongly depends on the used illumination. The higher the photon flux, the higher the concentration of Frenkel defects, and thus the stronger the degradation of power conversion efficiency and the stronger the hysteresis in the J-V characteristics. [1] C. Das, M. Kot et al., Cell Reports Physical Science 2020, 1, 100112. [2] M. Kot et al., ChemSusChem 2020, 13, 5722. [3] M. Kot et al., ChemSusChem 2018, 11, 3640.}, language = {en} }