TY - GEN A1 - Bignardi, Luca A1 - Lizzit, Daniel A1 - Bana, Harsh A1 - Travaglia, Elisabetta A1 - Lacovig, Paolo A1 - Sanders, Charlotte E. A1 - Dendzik, Maciej A1 - Michiardi, Matteo A1 - Bianchi, Marco A1 - Ewert, Moritz A1 - Buß, Lars A1 - Falta, Jens A1 - Flege, Jan Ingo A1 - Baraldi, Alessandro A1 - Larciprete, Rosanna A1 - Hofmann, Philip A1 - Lizzit, Silvano T1 - Growth and structure of singly oriented single-layer tungsten disulfide on Au(111) T2 - Physical Review Materials N2 - A singly oriented, single layer of tungsten disulfide (WS2) was epitaxially grown on Au(111) and characterized at the nanoscale by combining photoelectron spectroscopy, photoelectron diffraction, and low-energy electron microscopy. Fast x-ray photoelectron spectroscopy revealed that the growth of a single crystalline orientation is triggered by choosing a low W evaporation rate and performing the process with a high temperature of the substrate. Information about the single orientation of the layer was obtained by acquiring x-ray photoelectron diffraction patterns, revealing a 1H polytype for the WS2 layer and, moreover, determining the structural parameters and registry with the substrate. The distribution, size, and orientation of the WS2 layer were further ascertained by low-energy electron microscopy. KW - tungsten disulfide (WS2) KW - Au (111) KW - photoelectron spectroscopy KW - photoelectron diffraction KW - low-energy electron microscopy Y1 - 2019 U6 - https://doi.org/10.1103/PhysRevMaterials.3.014003 SN - 2475-9953 VL - 3 IS - 1 ER - TY - GEN A1 - Buß, Lars A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - Intercalation dynamics of sulfur underneath graphene on Ru(0001) T2 - Verhandlungen der DPG N2 - It is known that the binding of epitaxially grown graphene to the substrate has a detrimental effect on its electronic properties. This is especially true for the strong binding to various transition metals and particularly ruthenium. However, via intercalation the interlayer coupling can be lifted and its unique electronic properties can be restored. Therefore, we have investigated the interaction of sulfur with single-layer graphene grown on Ru(0001) via surface segregation and CVD of ethylene under UHV conditions with in situ low-energy electron microscopy (LEEM) and micro-diffraction (µLEED). At elevated temperature and under dimethyl disulfide background pressure, we observe that sulfur intercalates through the open edges of the graphene islands. Prolonged exposure to sulfur induces wrinkling of the graphene islands, consistent with substantial relief of tensile strain after successful sulfur insertion underneath the graphene. It can be seen that the intercalation dynamics are both dependent on the temperature during intercalation and the preparation method of the graphene sheets. Furthermore, darkfield imaging and µLEED of the intercalated graphene reveal a graphene induced improved ordering of sulfur underneath. KW - low-energy electron microscopy (LEEM) KW - micro-diffraction (µLEED) Y1 - 2019 UR - https://www.dpg-verhandlungen.de/year/2019/conference/regensburg/part/o/session/7/contribution/9 SN - 0420-0195 SP - 315 EP - 315 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER - TY - GEN A1 - Ewert, Moritz A1 - Buß, Lars A1 - Moras, Paolo A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - Unraveling the growth mechanism of single-domain molybdenum disulfide on Au(111) T2 - Verhandlungen der DPG N2 - As a transition metal dichalcogenide single-layer molybdenum disulfide (MoS2) is a heavily investigated system. Due to its direct band gap, the electronic properties of single-layer MoS2 have been subject to several surface science methods. A well-known model system is MoS2 on Au(111). We present in situ low-energy electron microscopy (LEEM) and micro-diffraction (LEED) observations of MoS2 growth on Au(111) at elevated temperature using two distinctly different deposition rates. Our investigations reveal similar but different expansion mechanisms of the MoS2 islands changing a balanced distribution of the two mirror domains towards a single domain distribution. Structural characterization by I(V)-LEEM and investigations of the electronic bandstructure using angle-resolved photoelectron spectroscopy both confirm single-layer nature of the MoS2 islands. We could identify step pushing of the growing MoS2 being responsible for this phenomena. KW - low-energy electron microscopy (LEEM) KW - micro-diffraction (LEED) KW - I(V)-LEEM KW - investigations of the electronic bandstructure using angle-resolved photoelectron spectroscopy Y1 - 2019 UR - https://www.dpg-verhandlungen.de/year/2019/conference/regensburg/part/o/session/35/contribution/6 SN - 0420-0195 SP - 329 EP - 329 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER - TY - GEN A1 - Mishra, Neeraj A1 - Forti, Stiven A1 - Fabbri, Filippo A1 - Martini, Leonardo A1 - McAleese, Clifford A1 - Conran, Ben R. A1 - Whelan, Patrick R. A1 - Shivayogimath, Abhay A1 - Jessen, Bjarke S. A1 - Buß, Lars A1 - Falta, Jens A1 - Aliaj, Ilirjan A1 - Roddaro, Stefano A1 - Flege, Jan Ingo A1 - Bøggild, Peter A1 - Teo, Kenneth B. K. A1 - Coletti, Camilla T1 - Wafer-Scale Synthesis of Graphene on Sapphire: Toward Fab-Compatible Graphene T2 - Small N2 - The adoption of graphene in electronics, optoelectronics, and photonics is hindered by the difficulty in obtaining high‐quality material on technologically relevant substrates, over wafer‐scale sizes, and with metal contamination levels compatible with industrial requirements. To date, the direct growth of graphene on insulating substrates has proved to be challenging, usually requiring metal‐catalysts or yielding defective graphene. In this work, a metal‐free approach implemented in commercially available reactors to obtain high‐quality monolayer graphene on c‐plane sapphire substrates via chemical vapor deposition is demonstrated. Low energy electron diffraction, low energy electron microscopy, and scanning tunneling microscopy measurements identify the Al‐rich reconstruction of sapphire to be crucial for obtaining epitaxial graphene. Raman spectroscopy and electrical transport measurements reveal high‐quality graphene with mobilities consistently above 2000 cm2 V−1 s−1. The process is scaled up to 4 and 6 in. wafers sizes and metal contamination levels are retrieved to be within the limits for back‐end‐of‐line integration. The growth process introduced here establishes a method for the synthesis of wafer‐scale graphene films on a technologically viable basis. KW - graphene on insulator KW - interface KW - metal free KW - sapphire KW - wafer scale Y1 - 2019 U6 - https://doi.org/10.1002/smll.201904906 SN - 1613-6810 SN - 1613-6829 VL - 15 IS - 50 ER - TY - GEN A1 - Schmidt, Thomas A1 - Buß, Lars A1 - Ewert, Moritz A1 - Schönhoff, Gunnar A1 - Wehling, Tim Oliver A1 - Falta, Jens T1 - Adsorption of sulfur on Si(111) T2 - Surface Science N2 - The adsorption of S on Si(111)- 7 × 7 has been investigated for different preparation schemes and parameters. S was supplied from an electrochemical Ag2S cell. For room temperature adsorption and subsequent annealing, no ordered S induced reconstruction can be observed with spot profile analysis low-energy electron diffraction (SPALEED). S deposition at temperatures above about 400°C, however, leads to a well-ordered reconstruction. Judging from the LEED pattern, the same reconstruction was already observed by Metzner et al. [Surf. Sci. 377–379(1997) 71–74] who identified it as 4 × 4 reconstruction. The upper temperature limit for the pre-paration of this superstructure depends on S flux, which is needed to compensate for desorption. Prolonged S exposure leads to surface roughening, as observed with SPALEED and scanning tunneling microscopy (STM), pointing to surface etching by S. From our SPALEED data, we can conclude that the observed reconstruction is not a 4 × 4 reconstruction, but a 20/12 superstructure with a rectangular unit cell that exists in three rotational domains, as confirmed by STM. Different structural trial models have been assessed with density functional theory. Among these model structures, a configuration with dimers adsorbed on bridging sites, with a S coverage of 1 monolayer, is most likely, since it is energetically favorable and is in agreement with all experimental results. KW - Passivation KW - Low-energy electron diffraction KW - Scanning tunneling microscopy KW - Density functional theory KW - Auger electron spectroscopy Y1 - 2020 U6 - https://doi.org/10.1016/j.susc.2019.121561 SN - 0039-6028 SN - 1879-2758 VL - 694 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 - 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 - 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 - 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 -