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 - 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 - Buß, Lars A1 - Braud, Nicolas A1 - Ewert, Moritz A1 - Jugovac, Matteo A1 - Menteş, Tevfik Onur A1 - Locatelli, Andrea A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - Unraveling van der Waals epitaxy: A real-time in-situ study of MoSe2 growth on graphene/Ru(0001) T2 - Ultramicroscopy N2 - In the present work we investigate the growth of monolayer MoSe2 on selenium-intercalated graphene on Ru(0001), a model layered heterostructure combining a transition metal dichalcogenide with graphene, using low energy electron microscopy and micro-diffraction. Real-time observation of MoSe2 on graphene growth reveals the island nucleation dynamics at the nanoscale. Upon annealing, larger islands are formed by sliding and attachment of multiple nanometer-sized MoSe2 flakes. Local micro-spot angle-resolved photoemission spectroscopy reveals the electronic structure of the heterostructure, indicating that no charge transfer occurs within adjacent layers. The observed behavior is attributed to intercalation of Se at the graphene/Ru(0001) interface. The unperturbed nature of the proposed heterostructure therefore renders it as a model system for investigations of graphene supported TMD nanostructures. KW - Graphene KW - transition metal dichalcogenides (TMDs) KW - angle-resolved photoemission spectroscopy (ARPES) KW - Heterostructures KW - low energy electron microscopy (LEEM) KW - low energy electron diffraction (LEED) Y1 - 2023 U6 - https://doi.org/10.1016/j.ultramic.2023.113749 SN - 0304-3991 SN - 1879-2723 VL - 250 ER - TY - GEN A1 - Buß, Lars A1 - Sulaiman, Cathy A1 - Sánchez-Barquilla, Raquel A1 - Cojocariu, Iulia A1 - Szpytma, Marcin A1 - Menteş, Tevfik Onur A1 - Locatelli, Andrea A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - Rise and fall of 1T-TaS₂ : epitaxial growth of monolayer TaS₂ on Au(111) T2 - Physical review materials N2 - Monolayer tantalum disulfide epitaxially grown on Au(111) is studied in real time during molecular beam epitaxy using low-energy electron microscopy and microdiffraction. Complementary x-ray photoelectron emission microscopy provides insight into the chemical and electronic structure of the grown layers. Our study reveals a previously unreported growth mechanism where the formation of 2⁢H−TaS2 proceeds via a transient 1⁢T−TaS2 phase. The 1⁢T−TaS2 phase exhibits a significantly higher growth rate than the 2⁢H−TaS2 phase, with growth proceeding mainly in the 1T phase. By comparison with TaSe2 on Au(111), we find that this growth mechanism is common to other Ta-based transition-metal dichalcogenides on Au(111). Furthermore, we find spectroscopic evidence for the presence of charge-density-wave order in 1⁢T−TaS2 on Au(111). These findings provide perspectives on the growth dynamics and phase control of TaS2, opening up avenues for tailoring its electronic properties through substrate interaction and phase engineering. KW - LEEM KW - Low-energy electron microscopy Y1 - 2025 U6 - https://doi.org/10.1103/1bxg-yvw2 SN - 2475-9953 VL - 9 IS - 7 SP - 1 EP - 9 PB - American Physical Society (APS) CY - College Park, MD ER -