@misc{BussFaltaEwertetal., author = {Buß, Lars and Falta, Jens and Ewert, Moritz and Shao, Bin and Wehling, Tim Oliver and Flege, Jan Ingo}, title = {The role of two-dimensional pressure in sulfur intercalation underneath graphene on ruthenium}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {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.}, 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 = {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{AngrickHenriksenMutzkeetal., author = {Angrick, Christoph and Henriksen, Annika and Mutzke, Nicole and Reimann, Andre and Ewert, Moritz and Buß, Lars and Falta, Jens and Flege, Jan Ingo and Donath, Markus}, title = {Spin-polarized VLEED from single-layer MoS2/Au(111): Investigation of spin-orbit-induced effects}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {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)}, 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{EwertBussLauritsenetal., author = {Ewert, Moritz and Buß, Lars and Lauritsen, Jeppe V. and Falta, Jens and Flege, Jan Ingo}, title = {Growth Mechanism of Single-Domain Monolayer MoS2 Nanosheets on Au(111) Revealed by In Situ Microscopy: Implications for Optoelectronics Applications}, series = {ACS Applied Nano Materials}, volume = {5}, journal = {ACS Applied Nano Materials}, number = {12}, issn = {2574-0970}, doi = {10.1021/acsanm.2c03584}, pages = {17702 -- 17710}, abstract = {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.}, language = {en} } @misc{MerteBraudBussetal., author = {Merte, Lindsay R. and Braud, Nicolas and Buß, Lars and Bisbo, Malthe Kj{\ae}r and Wallander, Harald J. and Krisponeit, Jon-Olaf and Flege, Jan Ingo and Hammer, Bj{\o}rk and Falta, Jens and Lundgren, Edvin}, title = {Oxygen Storage by Tin Oxide Monolayers on Pt3Sn(111)}, series = {The Journal of Physical Chemistry. C}, volume = {127}, journal = {The Journal of Physical Chemistry. C}, number = {6}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.2c09041}, pages = {2988 -- 2994}, abstract = {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.}, language = {en} } @misc{BussBraudEwertetal., author = {Buß, Lars and Braud, Nicolas and Ewert, Moritz and Jugovac, Matteo and Mentes, Tevfik Onur and Locatelli, Andrea and Falta, Jens and Flege, Jan Ingo}, title = {In-situ growth characterization of 2D heterostructures: MoSe2 on intercalated graphene/Ru(0001)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {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.}, language = {en} } @misc{ScheweSulaimanBussetal., author = {Schewe, Lukas and Sulaiman, Cathy and Buß, Lars and Ewert, Moritz and Flege, Jan Ingo}, title = {In-situ photoemission electron microscopy investigation of mono- and bilayer graphene growth on Ru(10-10)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {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).}, language = {en} } @misc{AngrickThiedeReimannetal., author = {Angrick, Christoph and Thiede, Christian and Reimann, Andre and Henriksen, Annika and Mutzke, Nicole and Ewert, Moritz and Buß, Lars and Falta, Jens and Flege, Jan Ingo and Donath, Markus}, title = {Spin-polarized very-low-energy electron diffraction from spin-orbit- and/or exchange-influenced targets}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {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).}, language = {en} } @misc{BussBraudEwertetal., author = {Buß, Lars and Braud, Nicolas and Ewert, Moritz and Jugovac, Matteo and Mente{\c{s}}, Tevfik Onur and Locatelli, Andrea and Falta, Jens and Flege, Jan Ingo}, title = {Unraveling van der Waals epitaxy: A real-time in-situ study of MoSe2 growth on graphene/Ru(0001)}, series = {Ultramicroscopy}, volume = {250}, journal = {Ultramicroscopy}, issn = {0304-3991}, doi = {10.1016/j.ultramic.2023.113749}, pages = {7}, abstract = {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.}, language = {en} } @misc{WallanderGajdekAlbertinetal., author = {Wallander, Harald J. and Gajdek, Dorotea and Albertin, Stefano and Harlow, Gary and Braud, Nicolas and Buß, Lars and Krisponeit, Jon-Olaf and Flege, Jan Ingo and Falta, Jens and Lundgren, Edvin and Merte, Lindsay R.}, title = {Dynamic Behavior of Tin at Platinum Surfaces during Catalytic CO Oxidation}, series = {ACS Catalysis}, volume = {13}, journal = {ACS Catalysis}, number = {24}, issn = {2155-5435}, doi = {10.1021/acscatal.3c04657}, pages = {16158 -- 16167}, abstract = {Platinum-tin surfaces are active for CO oxidation, but their activity and the effects of tin oxide phases that form under reaction conditions are poorly understood. We have studied surface alloys of tin prepared on platinum single crystals during catalytic CO oxidation using near-ambient-pressure X-ray photoemission spectroscopy. On the flat terraces of Sn/Pt(111), a wetting layer of Sn(II) surface oxide forms, while on the stepped Sn/Pt(223) surface, 3D clusters of Sn(IV) oxide are formed. Oxidation of tin by O2 competes with the reduction of the oxides by CO under reaction conditions. Oxides that do not completely cover the surface can be reduced to metallic tin, while a fully covering layer of Sn(II) oxide cannot, showing the importance of oxide edge sites for the reduction process. The samples where 2D oxide layers are formed show a higher CO oxidation activity than for pure platinum at low temperatures, while the Sn(IV) oxide clusters on the stepped surfaces do not affect the measured CO oxidation rate. We therefore identify 2D Sn(II) oxide as an active phase for CO oxidation. While oxide island edges appear to make only minor contributions to conversion under these conditions, reactions at these sites play a major role in determining the phases present and their transformations.}, language = {en} } @misc{SulaimanScheweBussetal., author = {Sulaiman, Cathy and Schewe, Lukas and Buß, Lars and Ewert, Moritz and Flege, Jan Ingo}, title = {In-situ monolayer graphene growth on Ru(10-10): an electron microscopy study}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {During the last decade, the controlled growth of monolayer (MLG) and bilayer graphene has extensively been studied on the hexagonal Ru(0001) surface, which is a system that is known to form strong chemical bonds at the metal-graphene interface. Yet, little attention was paid to the influence of the substrate orientation that was demonstrated to have a significant impact for graphene growth on the Ir(001) and Ir(111) surfaces, the latter exhibiting a weak coupling between the graphene and the support. Therefore, in this study we have grown graphene on the rectangular Ru(10-10) surface by segregation and ethylene-supported chemical vapor deposition. A photoemission and low-energy electron microscope (PEEM \& LEEM) has been utilized to directly characterize the MLG growth process with respect to variations in substrate temperature and step orientation. The expansion of the MLG islands is compared to the well-established carpet-growth mode on the Ru(0001) surface. These results have been complemented by probing of the occupied and unoccupied electronic structure of the islands using PEEM and intensity-voltage LEEM. Furthermore, the existence of two preferential surface reconstructions is identified via micro-illumination low-energy electron diffraction (LEED), whose spatial distribution is revealed by employing dark-field LEEM imaging.}, language = {en} } @misc{TschammerBussMoralesetal., author = {Tschammer, Rudi and Buß, Lars and Morales, Carlos and Senanayake, Sanjaya D. and Falta, Jens and Flege, Jan Ingo}, title = {In situ characterization of cerium oxide on Au(111) under reducing and oxidizing conditions by low-energy electron microscopy}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {The development of novel catalysts for a variety of applications is a key challenge for modern catalysis. Inverse metal oxide catalysts consisting of oxide nanoparticles dispersed on a metal support have recently attracted much attention, showing higher activity and selectivity compared to traditional catalytic systems, harnessing synergistic effects attributed to the so-called metal-support interaction. To gain further insights, we deposited cerium oxide nanoparticles on Au(111) and studied this system by low-energy electron microscopy (LEEM) and low-energy electron diffraction (LEED). The prepared samples demonstrate a distinct correlation between the deposition temperature and the structural order of the nanoparticles. This has been expanded upon by exploring the changes induced by reduction with H2 and reoxidation with O2 or CO2, again exhibiting a connection between structural order and activity, while also showing the influence of the oxide-metal interaction on the stability of cerium oxide under reducing conditions.}, language = {en} } @misc{BraudBussLundgrenetal., author = {Braud, Nicolas and Buß, Lars and Lundgren, Edvin and Merte, Lindsay R. and Wallander, Harald J. and Krisponeit, Jon-Olaf and Locatelli, Andrea and Mentes, Tevfik Onur and Jugovac, Matteo and Flege, Jan Ingo and Falta, Jens}, title = {Cleaning and tailoring the Pt3Sn(111) surface for surface experiments}, series = {Surface Science}, volume = {732}, journal = {Surface Science}, issn = {0039-6028}, doi = {10.1016/j.susc.2023.122281}, abstract = {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.}, 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} } @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{SchmidtBussEwertetal., author = {Schmidt, Thomas and Buß, Lars and Ewert, Moritz and Sch{\"o}nhoff, Gunnar and Wehling, Tim Oliver and Falta, Jens}, title = {Adsorption of sulfur on Si(111)}, series = {Surface Science}, volume = {694}, journal = {Surface Science}, issn = {0039-6028}, doi = {10.1016/j.susc.2019.121561}, pages = {11}, abstract = {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.}, language = {en} } @misc{BignardiLizzitBanaetal., author = {Bignardi, Luca and Lizzit, Daniel and Bana, Harsh and Travaglia, Elisabetta and Lacovig, Paolo and Sanders, Charlotte E. and Dendzik, Maciej and Michiardi, Matteo and Bianchi, Marco and Ewert, Moritz and Buß, Lars and Falta, Jens and Flege, Jan Ingo and Baraldi, Alessandro and Larciprete, Rosanna and Hofmann, Philip and Lizzit, Silvano}, title = {Growth and structure of singly oriented single-layer tungsten disulfide on Au(111)}, series = {Physical Review Materials}, volume = {3}, journal = {Physical Review Materials}, number = {1}, issn = {2475-9953}, doi = {10.1103/PhysRevMaterials.3.014003}, pages = {8}, abstract = {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.}, language = {en} } @misc{EwertBussMorasetal., author = {Ewert, Moritz and Buß, Lars and Moras, Paolo and Falta, Jens and Flege, Jan Ingo}, title = {Unraveling the growth mechanism of single-domain molybdenum disulfide on Au(111)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {329 -- 329}, abstract = {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.}, language = {en} }