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 - 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 - 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 - 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 - Braud, Nicolas A1 - Buß, Lars A1 - Lundgren, Edvin A1 - Merte, Lindsay R. A1 - Wallander, Harald J. A1 - Krisponeit, Jon-Olaf A1 - Locatelli, Andrea A1 - Mentes, Tevfik Onur A1 - Jugovac, Matteo A1 - Flege, Jan Ingo A1 - Falta, Jens T1 - Cleaning and tailoring the Pt3Sn(111) surface for surface experiments T2 - Surface Science N2 - The cleaning process of the bimetallic Pt3Sn(111) surface has been studied by means of low-energy electron microscopy (LEEM), microspot low-energy electron diffraction (-LEED), and X-ray photoemission electron microscopy (XPEEM). Different cleaning procedures, performed under ultra-high vacuum conditions (UHV), including sputtering with argon ions and repeated cycles of annealing up to 1500 K were investigated. In this work, we show that a clean Pt3Sn(111) surface of high structural quality with a sharp and brilliant (2 × 2) bulk reconstruction in LEED as well as a perfectly smooth surface with terraces of micron size can be achieved by sputtering, annealing at very high temperatures, followed by a subsequent slow (0.09 K/s) and careful cooling procedure. Additionally, we show the possibility of tailoring the Sn concentration in the topmost layers of Pt3Sn(111) as a function of annealing temperature and subsequent cooling rate. Structural changes of the surface are induced by Sn segregation combined with a surface order–disorder transition at 1340 K. Moreover, two new surface reconstructions depending on the cooling rate are reported. KW - Pt3Sn KW - Low-energy electron microscopy (LEEM) KW - Platinum KW - Tin KW - Cleaning KW - Alloy Y1 - 2023 U6 - https://doi.org/10.1016/j.susc.2023.122281 SN - 0039-6028 SN - 1879-2758 VL - 732 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 - 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 - TY - GEN A1 - Pleines, Linus A1 - Genuzio, Francesca A1 - Menteş, Tevfik Onur A1 - Locatelli, Andrea A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - Reduction and reoxidation of (111) and (100) oriented cerium oxide islands on Cu(111) T2 - Verhandlungen der DPG N2 - 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. KW - conversion of CO2 to methanol KW - low-energy electron mircoscopy (LEEM) KW - x-ray absorption spectroscopy (XAS) KW - resonant photoelectron spectroscopy (resPES) Y1 - 2019 UR - https://www.dpg-verhandlungen.de/year/2019/conference/regensburg/part/o/session/37/contribution/10 SN - 0420-0195 SP - 330 EP - 330 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER -