TY - GEN A1 - Angrick, Christoph A1 - Henriksen, Annika A1 - Mutzke, Nicole A1 - Reimann, Andre A1 - Ewert, Moritz A1 - Buß, Lars A1 - Falta, Jens A1 - Flege, Jan Ingo A1 - Donath, Markus T1 - Spin-polarized VLEED from single-layer MoS2/Au(111): Investigation of spin-orbit-induced effects T2 - Verhandlungen der DPG - SurfaceScience21 N2 - The influence of spin-orbit interaction on low-energy electron reflection from single-layer MoS2 on Au(111) was studied by VLEED (very-low-energy electron diffraction) [1,2]. The spin-dependent electron reflection was investigated for a wide range of electron incidence angles and kinetic energies. Since the adlayer coverage is about 30%, we studied the Au(111) substrate and a MoS2 bulk sample for comparison. This approach enabled us to distinguish between adlayer and substrate signals. For MoS2/Au(111), we detected a spin asymmetry of the reflected intensities, which shows a characteristic feature with alternating sign in the energy region of a VLEED fine structure [1]. The Au(111) substrate, in contrast, shows qualitatively different spin-asymmetry features, partially with reversed sign compared with MoS2/Au(111). The results of bulk MoS2 confirm that the characteristic feature in the single-layer data originates from MoS2. The influence of the adlayer-substrate interaction on the results will be discussed. [1] Burgbacher et al., Phys. Rev. B 87, 195411 (2013) [2] Angrick et al., J. Phys.: Condens. Matter 33, 115001 (2020) KW - Molybdenum disulfide (MoS2) KW - very-low-energy electron diffraction (VLEED) KW - spin-orbit interaction Y1 - 2021 UR - https://www.dpg-verhandlungen.de/year/2021/conference/surfacescience/part/o/session/78/contribution/3 VL - 2021 PB - Deutsche Physikalische Gesellschaft e.V. CY - Bad Honnef ER - TY - GEN A1 - Fischer, Simon A1 - Flege, Jan Ingo A1 - Foerster, Michael A1 - Aballe, Lucia A1 - Falta, Jens A1 - Krisponeit, Jon-Olaf T1 - Strongly Strained VO2 Thin Film Growth T2 - Verhandlungen der DPG - SurfaceScience21 N2 - The semiconductor-metal transition temperature of VO2 strongly shifts as a result of strain applied along the rutile c axis, making it interesting for various switching applications, as smart coatings and as sensors. In the past, this has been demonstrated, for instance, through the application of epitaxial strain on TiO2 substrates. We extend this tailoring approach by utilizing the much larger lattice mismatch of 8.78 % occurring in the VO2/RuO2 system for orientations where the c axis lies in-plane. Depositing vanadium oxide by atomic oxygen-supported reactive MBE on an oxidized Ru(0001) template, we have grown VO2 thin films on single domain RuO2 islands with distinct orientations. Locally resolved electron spectroscopy was used to ascertain the correct stoichiometry of the grown VO2 films on all template island types. Low energy electron diffraction reveals the VO2 films to grow indeed fully strained on RuO2(110) but fully relaxed on RuO2(100). Hence, the presented template allows for simultaneous access to a remarkable strain window ranging from bulk-like structures to regions of massive tensile strain. KW - vanadium oxide (VO2) KW - strain KW - ruthenium oxide KW - Locally resolved electron spectroscopy KW - Low energy electron diffraction (LEED) KW - MBE Y1 - 2021 UR - https://www.dpg-verhandlungen.de/year/2021/conference/surfacescience/part/o/session/74/contribution/3 VL - 2021 PB - Deutsche Physikalische Gesellschaft e.V. CY - Bad Honnef ER - TY - GEN A1 - Pleines, Linus A1 - Buß, Lars A1 - Menteş, Tevfik Onur A1 - Genuzio, Francesca A1 - Locatelli, Andrea A1 - Flege, Jan Ingo A1 - Falta, Jens T1 - High temperature reduction and reoxidation of cerium oxide on Cu(111) T2 - Verhandlungen der DPG - SurfaceScience21 N2 - Cerium oxide is of great interest due to its technological importance in various electronic, optical and catalytic applications. The inverse model catalyst cerium oxide on Cu(111) shows a high activity for the production of methanol. Oxygen vacancies, e.g. in form of reduced ceria, are necessary for the production of methanol from CO2 and H2. The reduction of ceria may be achieved by exposure to H2 at elevated temperatures. We studied the interaction of H2 and CO2 with cerium oxide islands on a Cu(111) substrate with low-energy electron microscopy (LEEM) and x-ray absorption spectroscopy (XAS). From earlier studies, the orientation of the cerium oxide is known to be decisive for the catalytic activity. In our experiments, the impact of both orientations are directly compared via growth of (100) and (111) cerium oxide islands side by side. At temperatures around 550 °C, exposure to H2 leads to partial reduction and exposure to CO2 leads to complete reoxidation of the cerium oxide. The (100) and (111) orientations show different reduction and reoxidation behaviors. KW - Cerium oxide KW - H2 interaction KW - CO2 interaction KW - Low-energy electron microscopy (LEEM) KW - X-ray absorption spectroscopy (XAS) Y1 - 2021 UR - https://www.dpg-verhandlungen.de/year/2021/conference/surfacescience/part/o/session/74/contribution/7 VL - 2021 PB - Deutsche Physikalische Gesellschaft e.V. CY - Bad Honnef ER - TY - GEN A1 - Buß, Lars A1 - Falta, Jens A1 - Ewert, Moritz A1 - Shao, Bin A1 - Wehling, Tim Oliver A1 - Flege, Jan Ingo T1 - The role of two-dimensional pressure in sulfur intercalation underneath graphene on ruthenium T2 - Verhandlungen der DPG - SurfaceScience21 N2 - Micrometer-sized single-layer graphene can epitaxially be grown on transition-metal substrates with excellent crystalline quality. However, due to strong binding these substrates have a detrimental influence on the intrinsic properties of the graphene. By lifting the interlayer coupling, e. g., via intercalating foreign atoms, its unique electronic properties can be restored. We have investigated the intercalation of sulfur underneath graphene on Ru(0001) with low-energy electron microscopy (LEEM) and micro-diffraction (µLEED). We find that sulfur deposited at elevated temperatures enters through the edge of the island, leading to wrinkle formation in the decoupled graphene. Interestingly, the presence of the graphene limits the possible S/Ru(0001) reconstructions that may form underneath, preventing less dense reconstructions like the p(2× 2) and (√3× √3) reconstructions. Based on density functional theory calculations, these findings are explained by a 2D pressure exerted by the overlying graphene, which results from the strong graphene-substrate interaction, only rendering the denser reconstructions of the S/Ru phase diagram energetically favorable. KW - Low energy elecron microscopy (LEEM) KW - Low energy electron diffraction LEED KW - graphene KW - ruthenium KW - sulfur intercalation Y1 - 2021 UR - https://www.dpg-verhandlungen.de/year/2021/conference/surfacescience/part/o/session/105/contribution/3 VL - 2021 PB - Deutsche Physikalische Gesellschaft e.V. CY - Bad Honnef ER - TY - GEN A1 - Ewert, Moritz A1 - Buß, Lars A1 - Braud, Nicolas A1 - Kundu, Asish K. A1 - Sheverdyaeva, Polina M. A1 - Moras, Paolo A1 - Genuzio, Francesca A1 - Menteş, Tevfik Onur A1 - Locatelli, Andrea A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - The Transition From MoS2 Single-Layer to Bilayer Growth on the Au(111) Surface T2 - Frontiers in Physics N2 - The transition from single-layer to bilayer growth of molybdenum disulfide on the Au(111) surface is investigated by in situ low-energy electron and photoemission microscopy. By mapping the film morphology with nanometer resolution, we show that a MoS2 bilayer forms at the boundaries of single-layer single-domain MoS2 islands and next to merging islands whereas bilayer nucleation at the island centers is found to be suppressed, which may be related to the usage of dimethyl disulfide as sulfur precursor in the growth process. This approach, which may open up the possibility of growing continuous films over large areas while delaying bilayer formation, is likely transferable to other transition metal dichalcogenide model systems. KW - molybdenum disulfide (MoS2) KW - bilayer KW - in situ low-energy electron microscopy (LEEM) KW - in situ photoemission microscopy (PEEM) KW - micro-ARPES (angle-resolved photoelectron spectroscopy) KW - Au step bunches Y1 - 2021 U6 - https://doi.org/10.3389/fphy.2021.654845 SN - 2296-424X VL - 9 ER - TY - GEN A1 - Ewert, Moritz A1 - Buß, Lars A1 - Genuzio, Francesca A1 - Menteş, Tevfik Onur A1 - Locatelli, Andrea A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - On the transition from MoS2 single-layer to bilayer growth on the Au(111) surface T2 - Verhandlungen der DPG N2 - 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. KW - angle-resolved photoemission spectroscopy KW - low-energy electron microscopy KW - intensity-voltage KW - micro-spot low-energy electron diffraction KW - step bunches KW - single-terrace growth Y1 - 2022 UR - https://www.dpg-verhandlungen.de/year/2022/conference/regensburg/part/o/session/65/contribution/3 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER - TY - GEN A1 - Pleines, Linus A1 - Buß, Lars A1 - Menteş, Tevfik Onur A1 - Genuzio, Francesca A1 - Locatelli, Andrea A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - In-situ characterization of cyclic reduction and reoxidation of CeOx(111) and CeOx(100) islands on Cu(111) T2 - Verhandlungen der DPG N2 - 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. KW - methanol synthesis KW - low-energy electron microscopy (LEEM) KW - X-ray absorption spectroscopy (XAS) KW - partial reduction KW - reoxidation Y1 - 2022 UR - https://www.dpg-verhandlungen.de/year/2022/conference/regensburg/part/o/session/69/contribution/10 SN - 0420-0195 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER - TY - GEN A1 - Pozarowska, Emilia A1 - Pleines, Linus A1 - Prieto, Mauricio J. A1 - Tănase, Liviu Christian A1 - Souza Caldas, Lucas de A1 - Tiwari, Aarti A1 - Schmidt, Thomas A1 - Falta, Jens A1 - Morales, Carlos A1 - Flege, Jan Ingo T1 - The relation between structure sensitivity and doping of ceria(111) vs. ceria(100) T2 - Verhandlungen der DPG N2 - CeOx-Cu inverse catalysts have been shown to convert CO2 into valuable chemicals through catalytic hydrogenation. The catalytic activity may further be enhanced by alloying ceria with trivalent, catalytically active metals, such as Sm, promoting the formation of Ce3+ active sites. In this work, the structural and chemical properties of (111)- and (100)- oriented CeOx islands alloyed with samarium were explored by low-energy electron microscopy and X-ray photoemission electron microscopy. After Sm deposition on the as-grown CeOx islands, the near-surface region of (100)-oriented CeOx is reduced after exposure to H2 at 470 ∘C, whereas the deeper layers as well as the whole (111)-oriented islands retain the Ce4+ state. Subsequent reoxidation with O2 leads to the complete Ce4+ state recovery, suggesting the healing of oxygen vacancies. Additional annealing at 470 ∘C induces samarium diffusion into the ceria matrix. Yet, subsequent exposure to H2 reduces neither the (111)- nor the (100)-oriented CeSmOx islands, suggesting a quite unexpected stability of this system. KW - catalytic hydrogenation KW - Samarium alloying of cerium oxide KW - low-energy electron microscopy (LEEM) KW - X-ray photoemission electron microscopy (PEEM) KW - healing of oxygen vacancies Y1 - 2022 UR - https://www.dpg-verhandlungen.de/year/2022/conference/regensburg/part/o/session/52/contribution/11 SN - 0420-0195 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER - TY - GEN A1 - Boehn, Bernhard von A1 - Weißbach, Anton A1 - Krisponeit, Jon-Olaf A1 - Flege, Jan Ingo A1 - Falta, Jens A1 - Gregoratti, Luca A1 - Amati, Matteo A1 - Zeller, Patrick A1 - Imbihl, Ronald T1 - Phase Separation within Vanadium Oxide Islands under Reaction Conditions: Methanol Oxidation at Vanadium Oxide Films on Rh(111) T2 - The Journal of Physical Chemistry C N2 - Submonolayer coverages of V-oxide on Rh(111) condense during catalytic methanol oxidation into a pattern of macroscopic stripes or islands. Under reaction conditions, a phase separation occurs within the VOx islands that has been studied in a pressure range of 10–6–10–4 mbar with photoemission electron microscopy (PEEM), low-energy electron microscopy (LEEM), microspot-low-energy electron diffraction (μLEED), and microspot-X-ray photoelectron spectroscopy (μXPS). An oxidized outer ring with a (√7 × √7)R19.1° structure coexists with an inner (12 × 12) Moiré-type boundary layer and a reduced core exhibiting a (√3 × √3)R30° Moiré type pattern. The dependence of the substructure on the reaction conditions, on V coverage, and on island size was investigated. With μXPS, the V coverages of the different phases in the VOx islands were determined. KW - Methanol Oxidation KW - Vanadium Oxide KW - Photoemission electron microscopy (PEEM) KW - Low-energy electron microscopy (LEEM), KW - Microspot-low-energy electron diffraction (μLEED) KW - Microspot-X-ray photoelectron spectroscopy (μXPS) Y1 - 2022 U6 - https://doi.org/10.1021/acs.jpcc.2c04174 SN - 1932-7455 SN - 1932-7447 VL - 126 IS - 45 SP - 19101 EP - 19112 ER - TY - GEN A1 - Ewert, Moritz A1 - Buß, Lars A1 - Lauritsen, Jeppe V. A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - Growth Mechanism of Single-Domain Monolayer MoS2 Nanosheets on Au(111) Revealed by In Situ Microscopy: Implications for Optoelectronics Applications T2 - ACS Applied Nano Materials N2 - The nucleation and growth of single-layer molybdenum disulfide single-domain nanosheets is investigated by in situ low-energy electron microscopy. We study the growth of micrometer-sized flakes and the correlated flattening process of the gold surface for three different elevated temperatures. Furthermore, the influence of surface step edges on the molybdenum disulfide growth process is revealed. We show that both nanosheet and underlying terrace grow simultaneously by pushing the surface step in the expansion process. Our findings point to an optimized growth procedure allowing for step-free, single-domain, single-layer islands of several micrometers in size, which is likely transferable to other transition-metal dichalcogenides (TMDs), offering a very fine degree of control over the TMD nanosheet structure and thickness. KW - in situ methods KW - low-energy electron microscopy and diffraction KW - molybdenum disulfide KW - 2D material KW - epitaxial growth mechanism KW - Au(111) Y1 - 2022 U6 - https://doi.org/10.1021/acsanm.2c03584 SN - 2574-0970 VL - 5 IS - 12 SP - 17702 EP - 17710 ER - TY - GEN A1 - Buß, Lars A1 - Zamborlini, Giovanni A1 - Sulaiman, Cathy A1 - Ewert, Moritz A1 - Cinchetti, Mirko A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - Hexagons on rectangles: Epitaxial graphene on Ru(10-10) T2 - Carbon N2 - Ruthenium is emerging as a promising candidate to replace copper in highly integrated electronics by enabling barrierless metallization in ultrathin interconnects. From this perspective, the study of graphene growth on such surface templates is of paramount importance as a platform for graphene integration in electronic devices. In particular, graphene growth on the Ru (10-10) surface allows selective growth of different graphene orientations, one-dimensional structures, and reduced substrate interaction compared to the well-established hexagonal Ru(0001) substrate. Real-time growth observations using low-energy electron microscopy and micro-diffraction highlight the influence of substrate symmetry on graphene growth, leading to the formation of rectangular islands with distinct zigzag- or armchair-terminated edges. Bilayer formation on Ru(10-10) occurs by nucleation of graphene nanoribbons under the monolayer. Micro-spot angle-resolved photoemission spectroscopy shows significantly less charge-transfer doping in these freestanding, zigzag-terminated bilayer graphene nanoribbons, indicating reduced graphene-substrate interaction and hence more effective decoupling as compared to graphene/Ru(0001). Our results show that the growth of graphene on non-hexagonal substrates opens new pathways for tailoring the graphene-substrate interaction at the interface, and thus the properties of graphene beyond the limits imposed by hexagonal substrates. KW - low-energy electron microscopy (LEEM) KW - angle-resolved photoelectron spectroscopy (ARPES) KW - Nanoribbons KW - Graphene KW - Ruthenium Y1 - 2025 U6 - https://doi.org/10.1016/j.carbon.2024.119600 SN - 0008-6223 VL - 231 PB - Elsevier BV ER - TY - GEN A1 - Merte, Lindsay R. A1 - Braud, Nicolas A1 - Buß, Lars A1 - Bisbo, Malthe Kjær A1 - Wallander, Harald J. A1 - Krisponeit, Jon-Olaf A1 - Flege, Jan Ingo A1 - Hammer, Bjørk A1 - Falta, Jens A1 - Lundgren, Edvin T1 - Oxygen Storage by Tin Oxide Monolayers on Pt3Sn(111) T2 - The Journal of Physical Chemistry. C N2 - The high performance of platinum–tin catalysts for oxidation reactions has been linked to the formation of tin oxides at the metal surface, but little is known about the structure of these oxides or the chemical behavior that determines their catalytic properties. We show here how surface oxides on Pt3Sn(111) incorporate oxygen at the metal interface, which may be subsequently removed by reaction with CO. The storage mechanism, where oxygen uptake occurs without loss of interfacial Pt–Sn bonds, is enabled by the peculiar asymmetrical coordination state of Sn2+. O atoms are bound at pocket sites in the 2D oxide sheet between these outward-buckled Sn atoms and metallic Sn in the alloy surface below. KW - Oxygen storage KW - low-energy electron microscopy (LEEM) KW - microprobe low-energy electron diffraction (μ-LEED) KW - scanning tunneling microscopy (STM) KW - GOFEE algorithm Y1 - 2023 U6 - https://doi.org/10.1021/acs.jpcc.2c09041 SN - 1932-7447 SN - 1932-7455 VL - 127 IS - 6 SP - 2988 EP - 2994 ER - TY - GEN A1 - Buß, Lars A1 - Braud, Nicolas A1 - Ewert, Moritz A1 - Jugovac, Matteo A1 - Mentes, Tevfik Onur A1 - Locatelli, Andrea A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - In-situ growth characterization of 2D heterostructures: MoSe2 on intercalated graphene/Ru(0001) T2 - Verhandlungen der DPG N2 - Despite the great fundamental interest in 2D heterostructures, most of the investigated 2D heterostructures were realized by mechanical exfoliation or chemical vapor deposition in the millibar range, preventing true in-situ characterization of the growth process. Here, we have investigated the growth of MoSe2 on single-layer graphene on Ru(0001) via real-time in-situ low-energy electron microscopy and micro-diffraction. After preparation of the graphene by standard procedures from an ethylene precursor, MoSe2 has been prepared via co-deposition of Mo and Se. Prior Se intercalation of the graphene appears to enhance the subsequent growth of MoSe2 on the graphene. At elevated temperatures, rotational ordering of the MoSe2 is facilitated by the strongly enhanced mobility of single-domain MoSe2 islands that align with the high symmetry orientations of the underlying graphene, indicating a non-negligible interaction between the two van-der-Waals materials. Micro-spot angle-resolved photoemission proves the monolayer nature of the as-grown MoSe2 as well as the free-standing character of the Se-intercalated graphene underneath. KW - MoSe2 KW - low-energy electron microscopy (LEEM9 KW - micro-diffraction Y1 - 2023 UR - https://www.dpg-verhandlungen.de/year/2023/conference/skm/part/o/session/74/contribution/1 SN - 0420-0195 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER -