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 - 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 - 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 - Ewert, Moritz A1 - Buß, Lars A1 - Lauritsen, Jeppe V. A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - Growth Mechanism of Single-Domain Monolayer MoS2 Nanosheets on Au(111) Revealed by In Situ Microscopy: Implications for Optoelectronics Applications T2 - ACS Applied Nano Materials N2 - The nucleation and growth of single-layer molybdenum disulfide single-domain nanosheets is investigated by in situ low-energy electron microscopy. We study the growth of micrometer-sized flakes and the correlated flattening process of the gold surface for three different elevated temperatures. Furthermore, the influence of surface step edges on the molybdenum disulfide growth process is revealed. We show that both nanosheet and underlying terrace grow simultaneously by pushing the surface step in the expansion process. Our findings point to an optimized growth procedure allowing for step-free, single-domain, single-layer islands of several micrometers in size, which is likely transferable to other transition-metal dichalcogenides (TMDs), offering a very fine degree of control over the TMD nanosheet structure and thickness. KW - in situ methods KW - low-energy electron microscopy and diffraction KW - molybdenum disulfide KW - 2D material KW - epitaxial growth mechanism KW - Au(111) Y1 - 2022 U6 - https://doi.org/10.1021/acsanm.2c03584 SN - 2574-0970 VL - 5 IS - 12 SP - 17702 EP - 17710 ER - TY - GEN A1 - Merte, Lindsay R. A1 - Braud, Nicolas A1 - Buß, Lars A1 - Bisbo, Malthe Kjær A1 - Wallander, Harald J. A1 - Krisponeit, Jon-Olaf A1 - Flege, Jan Ingo A1 - Hammer, Bjørk A1 - Falta, Jens A1 - Lundgren, Edvin T1 - Oxygen Storage by Tin Oxide Monolayers on Pt3Sn(111) T2 - The Journal of Physical Chemistry. C N2 - The high performance of platinum–tin catalysts for oxidation reactions has been linked to the formation of tin oxides at the metal surface, but little is known about the structure of these oxides or the chemical behavior that determines their catalytic properties. We show here how surface oxides on Pt3Sn(111) incorporate oxygen at the metal interface, which may be subsequently removed by reaction with CO. The storage mechanism, where oxygen uptake occurs without loss of interfacial Pt–Sn bonds, is enabled by the peculiar asymmetrical coordination state of Sn2+. O atoms are bound at pocket sites in the 2D oxide sheet between these outward-buckled Sn atoms and metallic Sn in the alloy surface below. KW - Oxygen storage KW - low-energy electron microscopy (LEEM) KW - microprobe low-energy electron diffraction (μ-LEED) KW - scanning tunneling microscopy (STM) KW - GOFEE algorithm Y1 - 2023 U6 - https://doi.org/10.1021/acs.jpcc.2c09041 SN - 1932-7447 SN - 1932-7455 VL - 127 IS - 6 SP - 2988 EP - 2994 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 - Schewe, Lukas A1 - Sulaiman, Cathy A1 - Buß, Lars A1 - Ewert, Moritz A1 - Flege, Jan Ingo T1 - In-situ photoemission electron microscopy investigation of mono- and bilayer graphene growth on Ru(10-10) T2 - Verhandlungen der DPG N2 - Epitaxial graphene growth has often been studied on close-packed transition metal substrates, e. g., the Ru(0001) surface, which is a well-studied model system for strong graphene-support interaction. Here, we focus on a Ru surface with different symmetry, i. e., the Ru(10-10) surface, to investigate the influence of the presumably modified graphene-substrate interaction on the growth of epitaxial monolayer and bilayer graphene (MLG, BLG) islands. The structural and chemical differences of the graphene on the two different surfaces are investigated by photoemission electron microscopy (PEEM), delivering information on both morphology and electronic structure. In-situ PEEM observation of graphene growth on the Ru(10-10) substrate by ethylene decomposition reveals the growth characteristics of MLG and BLG, the latter showing second layer nucleation via surface segregation of carbon. Furthermore, depending on growth temperature and relative orientation of the growing islands and surface steps, different growth characteristics are observed, in contrast to previous studies of the graphene/Ru(0001) system whereas similar electronic properties seem to prevail. Yet, when the MLG is decoupled from the Ru(10-10) substrate via intercalation of oxygen a distinct shift in work function is identified, slightly different from the resulting shift on Ru(0001). KW - photoemission electron microscopy (PEEM) KW - graphene KW - ruthenium KW - growth characteristics Y1 - 2023 UR - https://www.dpg-verhandlungen.de/year/2023/conference/skm/part/o/session/93/contribution/7 SN - 0420-0195 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER - TY - GEN A1 - Angrick, Christoph A1 - Thiede, Christian A1 - Reimann, Andre A1 - Henriksen, Annika A1 - Mutzke, Nicole A1 - Ewert, Moritz A1 - Buß, Lars A1 - Falta, Jens A1 - Flege, Jan Ingo A1 - Donath, Markus T1 - Spin-polarized very-low-energy electron diffraction from spin-orbit- and/or exchange-influenced targets T2 - Verhandlungen der DPG N2 - Exchange (XC) or spin-orbit (SOC) interaction cause electron scattering from surfaces to be spin dependent. The resulting spin filtering of the scattered electron beam can be used in spin-polarization analyzers. These analyzers are implemented in, for instance, photoemission setups to obtain spin resolution. Therefore, for promising targets, electron reflectivity and resulting spin asymmetry of very-low-energy electrons are measured for a wide range of incident electron energies and angles. By this, the investigated target is put to a test regarding the usability as a scattering target in a spin-polarization analyzer. Here, several results of SOC- as well as XC-influenced targets are presented. The results of the SOC-influenced targets Au(111), single-layer MoS2/Au(111) and W(110) [1] are compared with the results of the XC-influenced target Fe(001)-p(1x1)O [2]. Additionally, the influence of SOC interaction in the case of the XC-influenced target is investigated. The spin asymmetry caused by SOC is found to be one order of magnitude smaller than the spin asymmetry caused by XC. [1] Angrick et al., J. Phys.: Condens. Matter 33, 115001 (2020). [2] Thiede et al., Phys. Rev. Applied 1, 054003 (2014). KW - spin-polarization analysis KW - very-low-energy electron diffraction Y1 - 2023 UR - https://www.dpg-verhandlungen.de/year/2023/conference/skm/part/o/session/67/contribution/8 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 -