@misc{FischerFoersterAballeetal., author = {Fischer, Simon and Foerster, Michael and Aballe, Lucia and Vonk, Vedran and Falta, Jens and Krisponeit, Jon-Olaf and Flege, Jan Ingo}, title = {Vanadium dioxide thin films on (100)- and (110)-oriented ruthenium dioxide islands}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {330 -- 330}, abstract = {Bulk vanadium dioxide exhibits a metal-insulator transition at 68 ∘C. Because this change in resistivity is linked to a structural transition, the transition temperature can be tuned by epitaxial strain. Due to the high lattice mismatch of RuO2 with VO2, this substrate promises a highly shifted transition temperature, which enables new types of switching devices, smart coatings, and sensors. It is prepared by oxidizing a Ru(0001) crystal, providing micron-sized, (110)- as well as (100)-oriented RuO2 islands simultaneously. One of the challenges in preparing VO2 is to reliably meet the desired stoichiometry. We investigated the vanadium oxidation state using local XAS and XPS measurements taken at a synchrotron-based XPEEM instrument, revealing that VO2 stoichiometry is maintained throughout the entire film. This was found not only on RuO2(100) and RuO2(110), but also on the bare ruthenium substrate. The VO2 film thickness was determined to 3.6 nm by ex situ XRR measurements. Financial support from the DFG is acknowledged.}, language = {en} } @misc{KrisponeitFischerFlegeetal., author = {Krisponeit, Jon-Olaf and Fischer, Simon and Flege, Jan Ingo and Falta, Jens}, title = {Surface reconstructions on VO2(110)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {330 -- 330}, abstract = {Vanadium dioxide features a pronounced metal-insulator transition at 340 K. For thin films, this transition is adjustable via substrate-induced strain, making VO2 highly attractive for oxide electronic applications like switching devices and sensors. In addition, VO2 is considered for smart coatings and as catalytic material. For such applicational efforts, a thorough understanding of the relevant surfaces is an important prerequisite. Among the low-indexed surfaces orientations of VO2, the (110) surface is energetically favorable. We have prepared VO2(110) thin films by means of reactive molecular beam epitaxy on two different substrate types: TiO2(110) single crystals as well as on RuO2(110) islands grown on Ru(0001) crystals. The film stoichiometry was confirmed via XPS, while the surface structure was investigated by µLEED. For both substrate types diffraction patterns of three distinct symmetries have been observed in dependence on temperature. The corresponding surface reconstructions will be discussed in terms of surface oxygen content. Financial support from the DFG and the Institutional Strategy of the University of Bremen, funded by the German Excellence Initiative, is acknowledged.}, language = {en} } @misc{BoehnPenschkeLietal., author = {Boehn, Bernhard von and Penschke, Christopher and Li, Xiaoke and Paier, Joachim and Sauer, Joachim and Krisponeit, Jon-Olaf and Flege, Jan Ingo and Falta, Jens and Marchetto, Helder and Franz, Torsten and Lilienkamp, Gerhard and Imbihl, Ronald}, title = {Reaction dynamics of metal/oxide catalysts: Methanol oxidation at vanadium oxide films on Rh(111) from UHV to 10-2 mbar}, series = {Journal of Catalysis}, volume = {385}, journal = {Journal of Catalysis}, issn = {0021-9517}, doi = {10.1016/j.jcat.2020.03.016}, pages = {255 -- 264}, abstract = {Recent advances in in situ microscopy allow to follow the reaction dynamics during a catalytic surface reaction from ultra-high vacuum to 0.1 mbar, thus bridging a large part of the pressure gap. Submonolayer vanadium oxide films on Rh(111) have been studied during catalytic methanol oxidation in situ with spatially resolving imaging techniques. At 10-6 to 10-4 mbar VOx condenses into macroscopic circular islands that exhibit a substructure, consisting of a reduced island core and an oxidized outer ring. This substructure arises due to an oxygen gradient inside the VOx islands, which results in different coexisting 2D-phases of VOx on Rh(111). This substructure is also responsible for a "breathing-like" oscillatory expansion and contraction that the islands undergo under stationary conditions. Using density functional theory, the 2D-phase diagram of VOx on Rh(111) has been computed. The oscillatory behavior can be understood as a periodic phase transition between two 2D phases of VOx. With a newly developed near ambient pressure - low-energy electron microscope, it was shown that VOx islands disintegrate at 10-2 mbar, resulting in turbulent dynamics.}, language = {en} } @misc{FischerKrisponeitFoersteretal., author = {Fischer, Simon and Krisponeit, Jon-Olaf and Foerster, Michael and Aballe, Lucia and Falta, Jens and Flege, Jan Ingo}, title = {Massively Strained VO2 Thin Film Growth on RuO2}, series = {Crystal Growth \& Design}, volume = {20}, journal = {Crystal Growth \& Design}, number = {4}, issn = {1528-7483}, doi = {10.1021/acs.cgd.0c00120}, pages = {2734 -- 2741}, abstract = {Strain engineering vanadium dioxide thin films is one way to alter this material's characteristic first order transition from semiconductor to metal. In this study, we extend the exploitable strain regime by utilizing the very large lattice mismatch of 8.78\% occurring in the VO2/RuO2 system along the c axis of the rutile structure. We have grown VO2 thin films on single-domain RuO2 islands of two distinct surface orientations by atomic oxygen-supported reactive molecular beam epitaxy (MBE). These films were examined by spatially resolved photoelectron and X-ray absorption spectroscopy, confirming the correct stoichiometry. Low energy electron diffraction then reveals the VO2 films grow indeed fully strained on RuO2(110), exhibiting a previously unreported (2 × 2) reconstruction. On TiO2(110) substrates, we reproduce this reconstruction and attribute it to an oxygen-rich termination caused by the high oxygen chemical potential. On RuO2(100), on the contrary, the films grow fully relaxed. Hence, the presented growth method allows for simultaneous access to a remarkable strain window ranging from bulk-like structures to massively strained regions.}, language = {en} } @misc{KrisponeitFischerEsseretal., author = {Krisponeit, Jon-Olaf and Fischer, Simon and Esser, Sven and Moshnyaga, Vasily and Schmidt, Thomas and Piper, Louis F. J. and Flege, Jan Ingo and Falta, Jens}, title = {The morphology of VO2/TiO2(001): terraces, facets, and cracks}, series = {Scientific Reports}, volume = {10}, journal = {Scientific Reports}, issn = {2045-2322}, doi = {10.1038/s41598-020-78584-9}, pages = {8}, abstract = {Vanadium dioxide (VO2) features a pronounced, thermally-driven metal-to-insulator transition at 340 K. Employing epitaxial stress on rutile TiO2(001) substrates, the transition can be tuned to occur close to room temperature. Striving for applications in oxide-electronic devices, the lateral homogeneity of such samples must be considered as an important prerequisite for efforts towards miniaturization. Moreover, the preparation of smooth surfaces is crucial for vertically stacked devices and, hence, the design of functional interfaces. Here, the surface morphology of VO2/TiO2(001) films was analyzed by low-energy electron microscopy and diffraction as well as scanning probe microscopy. The formation of large terraces could be achieved under temperature-induced annealing, but also the occurrence of facets was observed and characterized. Further, we report on quasi-periodic arrangements of crack defects which evolve due to thermal stress under cooling. While these might impair some applicational endeavours, they may also present crystallographically well-oriented nano-templates of bulk-like properties for advanced approaches.}, language = {en} } @misc{FischerFlegeFoersteretal., author = {Fischer, Simon and Flege, Jan Ingo and Foerster, Michael and Aballe, Lucia and Falta, Jens and Krisponeit, Jon-Olaf}, title = {Strongly Strained VO2 Thin Film Growth}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {The semiconductor-metal transition temperature of VO2 strongly shifts as a result of strain applied along the rutile c axis, making it interesting for various switching applications, as smart coatings and as sensors. In the past, this has been demonstrated, for instance, through the application of epitaxial strain on TiO2 substrates. We extend this tailoring approach by utilizing the much larger lattice mismatch of 8.78 \% occurring in the VO2/RuO2 system for orientations where the c axis lies in-plane. Depositing vanadium oxide by atomic oxygen-supported reactive MBE on an oxidized Ru(0001) template, we have grown VO2 thin films on single domain RuO2 islands with distinct orientations. Locally resolved electron spectroscopy was used to ascertain the correct stoichiometry of the grown VO2 films on all template island types. Low energy electron diffraction reveals the VO2 films to grow indeed fully strained on RuO2(110) but fully relaxed on RuO2(100). Hence, the presented template allows for simultaneous access to a remarkable strain window ranging from bulk-like structures to regions of massive tensile strain.}, language = {en} } @misc{BoehnWeissbachKrisponeitetal., author = {Boehn, Bernhard von and Weißbach, Anton and Krisponeit, Jon-Olaf and Flege, Jan Ingo and Falta, Jens and Gregoratti, Luca and Amati, Matteo and Zeller, Patrick and Imbihl, Ronald}, title = {Phase Separation within Vanadium Oxide Islands under Reaction Conditions: Methanol Oxidation at Vanadium Oxide Films on Rh(111)}, series = {The Journal of Physical Chemistry C}, volume = {126}, journal = {The Journal of Physical Chemistry C}, number = {45}, issn = {1932-7455}, doi = {10.1021/acs.jpcc.2c04174}, pages = {19101 -- 19112}, abstract = {Submonolayer coverages of V-oxide on Rh(111) condense during catalytic methanol oxidation into a pattern of macroscopic stripes or islands. Under reaction conditions, a phase separation occurs within the VOx islands that has been studied in a pressure range of 10-6-10-4 mbar with photoemission electron microscopy (PEEM), low-energy electron microscopy (LEEM), microspot-low-energy electron diffraction (μLEED), and microspot-X-ray photoelectron spectroscopy (μXPS). An oxidized outer ring with a (√7 × √7)R19.1° structure coexists with an inner (12 × 12) Moir{\´e}-type boundary layer and a reduced core exhibiting a (√3 × √3)R30° Moir{\´e} type pattern. The dependence of the substructure on the reaction conditions, on V coverage, and on island size was investigated. With μXPS, the V coverages of the different phases in the VOx islands were determined.}, 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{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{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{MoralesTschammerGouderetal., author = {Morales, Carlos and Tschammer, Rudi and Gouder, Thomas and Choi, YongMan and Anjum, Dalaver and Baunthiyal, Aman and Krisponeit, Jon-Olaf and Falta, Jens and Flege, Jan Ingo and Idriss, Hicham}, title = {Stabilization of Ce3+ cations via U-Ce charge transfer in mixed oxides: consequences on the thermochemical water splitting to hydrogen}, series = {Journal of Physics: Energy}, volume = {7}, journal = {Journal of Physics: Energy}, publisher = {IOP Publishing}, issn = {2515-7655}, doi = {10.1088/2515-7655/adbad9}, pages = {1 -- 14}, abstract = {The work's objective is to enhance the generation of H2 via the thermochemical water splitting (TCWS) reaction over nanocrystalline mixed oxide Ce1-xUxO2. While CeO2 is the most active and stable known reducible oxide for the TCWS reaction, it is below par to make it practical. This has motivated many works to enhance its reduction capacity and therefore increase its activity. In this work the presence of both metal cations (Ce4+ and U4+) has allowed for the charge transfer reaction to occur (Ce4+ + U4+ → Ce3+ + U5+) and therefore increased its capacity to generate oxygen vacancies, VO (2 Ce3+ + VO), needed for the TCWS reaction. Test reactions on the polycrystalline mixed oxides indicated that small atomic percentages of U (\<10 \%) were found to be optimal for H2 production due to a considerable increase of Ce3+ states. Further studies of the Ce-U interaction were performed on thin epitaxial Ce1-xUxO2 (111) films of about 6 nm deep. In situ X-ray photoelectron spectroscopy showed clear evidences of charge transfer at low U content. Moreover, it was found that while increasing the content of U decreased the charge transfer efficiency it protected reduced Ce3+ from being oxidized. Our computational results using the DFT + U method gave evidence of charge transfer at 3.5 and 6.2 at.\% of U. In agreement with experiments, theoretical calculations also showed that the charge transfer is sensitive to the distribution of U4+ around the Ce4+ cations, which in turn affected the creation of VO needed for water splitting. Our results point out to the important yet often neglected effect of statistical entropy (cations distribution in the lattice), in addition to composition, in increasing the density of reduced states and consequently enhancing H2 production from water.}, language = {en} } @misc{BraudBussMerteetal., author = {Braud, Nicolas and Buß, Lars and Merte, Lindsay Richard and Wallander, Harald and Krisponeit, Jon-Olaf and Schmidt, Thomas and Lundgren, Edvin and Flege, Jan Ingo and Falta, Jens}, title = {Growth and oxidation of ultra-thin Pt-Sn layers on Pt(111) by molecular and atomic oxygen}, series = {Ultramicroscopy}, volume = {278}, journal = {Ultramicroscopy}, publisher = {Elsevier BV}, address = {Amsterdam}, issn = {0304-3991}, doi = {10.1016/j.ultramic.2025.114243}, pages = {1 -- 11}, abstract = {The preparation of ultra-thin PtSn-alloyed layers by molecular beam epitaxy was studied using low-energy electron microscopy (LEEM) and micro-diffraction (-LEED). Deposition at a sample temperature of 435 °C initially results in the formation of a PtSn/Pt(111) layer showing a (2 × 2) reconstruction. With continued Sn deposition, a PtSn/Pt(111) layer develops, showing a ()R30° reconstruction. An ultra-thin tin oxide was formed from the (2 × 2) surface by exposure to molecular oxygen at temperatures of 500 °C and 590 °C, respectively. LEED shows the evolution of a new surface structure, which could be identified as an incommensurate rectangular reconstruction with lattice parameters of a = (6.4 ± 0.1) {\AA} and b = (8.6 ± 0.1) {\AA} present in three domains rotated by 120° with respect to each other. This structure can be related to the zigzag reconstructions found for similar ultra-thin oxide systems. Contrarily, the ()R30° structure showed no oxide formation even after extensive exposure to molecular oxygen. The usage of atomic oxygen, however, allows for oxidation of this surface and the growth of thicker oxides on both types of overlayers. At 500 °C this process is accompanied by substantial roughening of the surface.}, language = {en} }