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 - Transitions from single-layer MoS2 to bilayer growth: A LEEM study T2 - Verhandlungen der DPG - SurfaceScience21 N2 - Molybdenum disulfide (MoS2) is well-known to change from an indirect to a direct semiconductor as a single layer. We present insights from in-situ low-energy electron microscopy (LEEM) on the extended growth of MoS2 on the Au(111) surface at elevated temperatures of 720°C. Our continuous growth method leads to the formation of micron-sized single-layer MoS2 islands. The single-domain character of these islands is confirmed by employing dark-field imaging and micro-diffraction (LEED). This also reveals the distribution of 90:10 of the two expected MoS2 mirror domains on Au(111). Selected-area angle-resolved photoelectron spectroscopy (ARPES) measurements of these mirror domains underline the threefold symmetry of the two mirror domains and indicate the presence of MoS2 bilayer. Using X-ray photoemission electron microscopy (XPEEM) and intensity-voltage LEEM (I(V))-LEEM we identify the bilayer nucleation areas at nearly full surface coverage and propose a model pathway for their formation. KW - Molybdenum disulfide (MoS2) KW - Low energy electron microscopy (LEEM) KW - Lowenergy electron diffraction KW - Angle-resolved photoelecton spectroscopy Y1 - 2021 UR - https://www.dpg-verhandlungen.de/year/2021/conference/surfacescience/part/o/session/105/contribution/12 VL - 2021 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 - 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 - Jugovac, Matteo A1 - Menteş, Tevfik Onur A1 - Genuzio, Francesca A1 - Lachnitt, Jan A1 - Feyer, Vitaliy A1 - Flege, Jan Ingo A1 - Locatelli, Andrea T1 - Sensitivity to crystal stacking in low-energy electron microscopy T2 - Applied Surface Science N2 - In this work we demonstrate the general characteristics of hcp and fcc stacking in low-energy electron reflectivity for transition metal surfaces, by following the restacking during homoepitaxial growth in real-time. For this purpose, the stacking of a model system, single-crystalline Ag islands during layer-by-layer growth at high temperature on O/W(110), is chosen. Multiple scattering calculations are used to model the relation between electron reflectivity and the crystal geometry. The changes in the electron reflectivity are shown to derive from the changes in the stacking sequence of the topmost surface layers. The results allow to distinguish between the hcp and fcc crystalline arrangements at a surface based on typical differences in the reflectivity curves, making the Ag results relevant for a variety of materials with hexagonal surface geometry. In particular, the multiplet structure within the first Bragg peak in the very low electron energy regime is identified with the fcc structure and thus it can be utilized as a fingerprint to determine the stacking sequence. KW - LEEM-I(V) KW - Stacking fault KW - fcc KW - hcp KW - Ag(111) Y1 - 2021 U6 - https://doi.org/10.1016/j.apsusc.2021.150656 SN - 0169-4332 SN - 1873-5584 VL - 566 ER -