@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 = {Transitions from single-layer MoS2 to bilayer growth: A LEEM study}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, address = {Bad Honnef}, abstract = {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.}, language = {en} } @misc{PleinesBussMenteşetal., author = {Pleines, Linus and Buß, Lars and Mente{\c{s}}, Tevfik Onur and Genuzio, Francesca and Locatelli, Andrea and Flege, Jan Ingo and Falta, Jens}, title = {High temperature reduction and reoxidation of cerium oxide on Cu(111)}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {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.}, language = {en} } @misc{EwertBussBraudetal., author = {Ewert, Moritz and Buß, Lars and Braud, Nicolas and Kundu, Asish K. and Sheverdyaeva, Polina M. and Moras, Paolo and Genuzio, Francesca and Mente{\c{s}}, Tevfik Onur and Locatelli, Andrea and Falta, Jens and Flege, Jan Ingo}, title = {The Transition From MoS2 Single-Layer to Bilayer Growth on the Au(111) Surface}, series = {Frontiers in Physics}, volume = {9}, journal = {Frontiers in Physics}, issn = {2296-424X}, doi = {10.3389/fphy.2021.654845}, abstract = {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.}, 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} }