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 - TY - GEN A1 - Kedia, Mayank A1 - Rai, Monika A1 - Phirke, Himanshu A1 - Aranda, Clara A. A1 - Das, Chittaranjan A1 - Chirvony, Vladimir A1 - Boehringer, Stephan A1 - Kot, Małgorzata A1 - Malekshahi Byranvand, Mahdi A1 - Flege, Jan Ingo A1 - Redinger, Alex A1 - Saliba, Michael T1 - Light Makes Right: Laser Polishing for Surface Modification of Perovskite Solar Cells T2 - ACS Energy Letters N2 - Interface engineering is a common strategy for passivating surface defects to attain open circuit voltages (Voc) in perovskite solar cells (PSCs). In this work, we introduce the concept of polishing a perovskite thin-film surface using a nanosecond (ns) pulsed ultraviolet laser to reduce surface defects, such as dangling bonds, undesirable phases, and suboptimal stoichiometry. A careful control of laser energy and scanning speed improves the photophysical properties of the surface without compromising the thickness. Using laser polishing, a Voc of 1.21 V is achieved for planar PSCs with a triple cation composition, showing an improved perovskite/hole transport interface by mitigating surface recombination losses. We measure an efficiency boost from 18.0% to 19.3% with improved stability of up to 1000 h. The results open the door to a new class of surface modification using lasers for interface passivation in well-controllable, automated, scalable, and solvent-free surface treatments. KW - Perovskite solar cells KW - laser polishing KW - perovskite/hole transport interface KW - surface defects KW - recombination losses Y1 - 2023 U6 - https://doi.org/10.1021/acsenergylett.3c00469 SN - 2380-8195 VL - 8 SP - 2603 EP - 2610 ER - TY - GEN A1 - Mazur, Michał A1 - Kapuścik, Paulina A1 - Weichbrodt, Wiktoria A1 - Domaradzki, Jarosław A1 - Mazur, Piotr A1 - Kot, Małgorzata A1 - Flege, Jan Ingo T1 - WO3 Thin-Film Optical Gas Sensors Based on Gasochromic Effect towards Low Hydrogen Concentrations T2 - Materials N2 - Hydrogen gas sensors have recently attracted increased interest due to the explosive nature of H2 and its strategic importance in the sustainable global energy system. In this paper, the tungsten oxide thin films deposited by innovative gas impulse magnetron sputtering have been investigated in terms of their response to H2. It was found that the most favourable annealing temperature in terms of sensor response value, as well as response and recovery times, was achieved at 673 K. This annealing process caused a change in the WO3 cross-section morphology from a featureless and homogenous form to a rather columnar one, but still maintaining the same surface homogeneity. In addition to that, the full-phase transition from an amorphous to nanocrystalline form occurred with a crystallite size of 23 nm. It was found that the sensor response to only 25 ppm of H2 was equal to 6.3, which is one of the best results presented in the literature so far of WO3 optical gas sensors based on a gasochromic effect. Moreover, the results of the gasochromic effect were correlated with the changes in the extinction coefficient and the concentration of the free charge carriers, which is also a novel approach to the understanding of the gasochromic phenomenon. KW - Tungsten oxide (WO3) KW - gas impulse magnetron sputtering KW - thin film KW - gasochromic properties KW - optical properties KW - annealing KW - optical hydrogen gas sensor Y1 - 2023 U6 - https://doi.org/10.3390/ma16103831 SN - 1996-1944 VL - 16 IS - 10 ER - TY - GEN A1 - Pożarowska, Emilia A1 - Pleines, Linus A1 - Ewert, Moritz A1 - Prieto, Mauricio J. A1 - Tănase, Liviu Christian A1 - Souza Caldas, Lucas de A1 - Tiwari, Aarti A1 - Schmidt, Thomas A1 - Falta, Jens A1 - Krasovskii, Eugene A1 - Morales, Carlos A1 - Flege, Jan Ingo T1 - Preparation and stability of the hexagonal phase of samarium oxide on Ru(0001) T2 - Ultramicroscopy N2 - We have used low-energy electron microscopy (LEEM), micro-illumination low-energy electron diffraction (µLEED) supported by ab initio calculations, and X-ray absorption spectroscopy (XAS) to investigate in-situ and in real-time the structural properties of Sm2O3 deposits grown on Ru(0001), a rare-earth metal oxide model catalyst. Our results show that samarium oxide grows in a hexagonal A-Sm2O3 phase on Ru(0001), exhibiting a (0001) oriented-top facet and (113) side facets. Upon annealing, a structural transition from the hexagonal to cubic phase occurs, in which the Sm cations exhibit the +3 oxidation state. The unexpected initial growth in the A-Sm2O3 hexagonal phase and its gradual transition to a mixture with cubic C-Sm2O3 showcases the complexity of the system and the critical role of the substrate in the stabilization of the hexagonal phase, which was previously reported only at high pressures and temperatures for bulk samaria. Besides, these results highlight the potential interactions that Sm could have with other catalytic compounds with respect to the here gathered insights on the preparation conditions and the specific compounds with which it interacts. KW - Samarium oxide (Sm2O3) KW - low-energy electron microscopy (LEEM) KW - X-ray photoemission electron microscopy (XPEEM) KW - Facets KW - Epitaxy KW - Hexagonal phase Y1 - 2023 U6 - https://doi.org/10.1016/j.ultramic.2023.113755 SN - 0304-3991 SN - 1879-2723 VL - 250 ER - TY - GEN A1 - Wallander, Harald J. A1 - Gajdek, Dorotea A1 - Albertin, Stefano A1 - Harlow, Gary A1 - Braud, Nicolas A1 - Buß, Lars A1 - Krisponeit, Jon-Olaf A1 - Flege, Jan Ingo A1 - Falta, Jens A1 - Lundgren, Edvin A1 - Merte, Lindsay R. T1 - Dynamic Behavior of Tin at Platinum Surfaces during Catalytic CO Oxidation T2 - ACS Catalysis N2 - 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. KW - operando KW - CO oxidation KW - platinum KW - tin KW - oxides KW - alloys KW - stepped surfaces Y1 - 2023 U6 - https://doi.org/10.1021/acscatal.3c04657 SN - 2155-5435 VL - 13 IS - 24 SP - 16158 EP - 16167 ER - TY - GEN A1 - Das, Chittaranjan A1 - Roy, Rajarshi A1 - Kedia, Mayank A1 - Kot, Małgorzata A1 - Zuo, Weiwei A1 - Félix, Roberto A1 - Sobol, Tomasz A1 - Flege, Jan Ingo A1 - Saliba, Michael T1 - Unraveling the Role of Perovskite in Buried Interface Passivation T2 - ACS Applied Materials & Interfaces N2 - Interfaces in perovskite solar cells play a crucial role in their overall performance, and therefore, detailed fundamental studies are needed for a better understanding. In the case of the classical n–i–p architecture, TiO2 is one of the most used electron-selective layers and can induce chemical reactions that influence the performance of the overall device stack. The interfacial properties at the TiO2/perovskite interface are often neglected, owing to the difficulty in accessing this interface. Here, we use X-rays of variable energies to study the interface of (compact and mesoporous) TiO2/perovskite in such a n–i–p architecture. The X-ray photoelectron spectroscopy and X-ray absorption spectroscopy methods show that the defect states present in the TiO2 layer are passivated by a chemical interaction of the perovskite precursor solution during the formation of the perovskite layer and form an organic layer at the interface. Such passivation of intrinsic defects in TiO2 removes charge recombination centers and shifts the bands upward. Therefore, interface defect passivation by oxidation of Ti3+ states, the organic cation layer, and an upward band bending at the TiO2/perovskite interface explain the origin of an improved electron extraction and hole-blocking nature of TiO2 in the n–i–p perovskite solar cells. KW - perovskite solar cells KW - interface KW - defects KW - photoemission spectroscopy Y1 - 2023 U6 - https://doi.org/10.1021/acsami.3c13085 SN - 1944-8244 SN - 1944-8252 VL - 15 IS - 48 SP - 56500 EP - 56510 ER - TY - GEN A1 - Morales, Carlos A1 - Mahmoodinezhad, Ali A1 - Tschammer, Rudi A1 - Kosto, Yuliia A1 - Alvarado Chavarin, Carlos A1 - Schubert, Markus Andreas A1 - Wenger, Christian A1 - Henkel, Karsten A1 - Flege, Jan Ingo T1 - Combination of Multiple Operando and In-Situ Characterization Techniques in a Single Cluster System for Atomic Layer Deposition: Unraveling the Early Stages of Growth of Ultrathin Al2O3 Films on Metallic Ti Substrates T2 - Inorganics N2 - This work presents a new ultra-high vacuum cluster tool to perform systematic studies of the early growth stages of atomic layer deposited (ALD) ultrathin films following a surface science approach. By combining operando (spectroscopic ellipsometry and quadrupole mass spectrometry) and in situ (X-ray photoelectron spectroscopy) characterization techniques, the cluster allows us to follow the evolution of substrate, film, and reaction intermediates as a function of the total number of ALD cycles, as well as perform a constant diagnosis and evaluation of the ALD process, detecting possible malfunctions that could affect the growth, reproducibility, and conclusions derived from data analysis. The homemade ALD reactor allows the use of multiple precursors and oxidants and its operation under pump and flow-type modes. To illustrate our experimental approach, we revisit the well-known thermal ALD growth of Al2O3 using trimethylaluminum and water. We deeply discuss the role of the metallic Ti thin film substrate at room temperature and 200 °C, highlighting the differences between the heterodeposition (<10 cycles) and the homodeposition (>10 cycles) growth regimes at both conditions. This surface science approach will benefit our understanding of the ALD process, paving the way toward more efficient and controllable manufacturing processes. KW - Atomic layer deposition (ALD) KW - in-situ KW - operando KW - X-ray photoelectron spectroscopy KW - ellipsometry KW - quadrupol mass spectrometry (QMS) Y1 - 2023 U6 - https://doi.org/10.3390/inorganics11120477 SN - 2304-6740 VL - 11 IS - 12 ER - TY - GEN A1 - Gonzalez-Castãno, Miriam A1 - Morales, Carlos A1 - Navarro de Miguel, Juan Carlos A1 - Boelte, Jens-H. A1 - Klepel, Olaf A1 - Flege, Jan Ingo A1 - Arellano-García, Harvey T1 - Are Ni/ and Ni5Fe1/biochar catalysts suitable for synthetic natural gas production? A comparison with γ-Al2O3 supported catalysts T2 - Green Energy & Environment N2 - Among challenges implicit in the transition to the post–fossil fuel energetic model, the finite amount of resources available for the technological implementation of CO2 revalorizing processes arises as a central issue. The development of fully renewable catalytic systems with easier metal recovery strategies would promote the viability and sustainability of synthetic natural gas production circular routes. Taking Ni and NiFe catalysts supported over γ-Al2O3 oxide as reference materials, this work evaluates the potentiality of Ni and NiFe supported biochar catalysts for CO2 methanation. The development of competitive biochar catalysts was found dependent on the creation of basic sites on the catalyst surface. Displaying lower Turn Over Frequencies than Ni/Al catalyst, the absence of basic sites achieved over Ni/C catalyst was related to the depleted catalyst performances. For NiFe catalysts, analogous Ni5Fe1 alloys were constituted over both alumina and biochar supports. The highest specific activity of the catalyst series, exhibited by the NiFe/C catalyst, was related to the development of surface basic sites along with weaker NiFe–C interactions, which resulted in increased Ni0:NiO surface populations under reaction conditions. In summary, the present work establishes biochar supports as a competitive material to consider within the future low-carbon energetic panorama. KW - Biochar catalysts KW - Carbon catalysts KW - Ni catalysts KW - NiFe alloy KW - Bimetallic catalysts KW - Synthetic natural gas KW - CO2 methanation Y1 - 2023 U6 - https://doi.org/10.1016/j.gee.2021.05.007 SN - 2468-0257 VL - 8 IS - 3 SP - 744 EP - 756 ER - TY - GEN A1 - Morales, Carlos A1 - Pascual, Antonio A1 - Leinen, Dietmar A1 - Luna-López, Gabriel A1 - Ares, Jose R. A1 - Flege, Jan Ingo A1 - Soriano, Leonardo A1 - Ferrer, Isabel J. A1 - Sanchez, Carlos T1 - Reaction Mechanism and Kinetic Model of the Transformation of Iron Monosulfide Thin Films into Pyrite Films T2 - The Journal of Physical Chemistry C N2 - This work presents a comprehensive reaction and kinetic model of the pyrite thin films formation by sulfuration of Fe monosulfides when a molecular sulfur (S2) atmosphere is used. This investigation completes the results already published on the explanation and interpretation of the sulfuration process that transforms metallic iron into pyrite. It was previously shown that the monosulfide species (i.e., orthorhombic and hexagonal pyrrhotite phases) are intermediate phases in the sulfuration reaction. Based on experimental data we now show that the sulfuration of pyrrhotite to pyrite takes place in two distinct stages: (i) conversion of orthorhombic pyrrhotite to pyrite (Fe1–xSO → FeS2) while the hexagonal pyrrhotite (Fe1–xSH) phase remains unaltered, and (ii) final transformation of hexagonal pyrrhotite to pyrite (Fe1–xSH → FeS2). Both processes occur via interstitial sulfur diffusion through the previously formed pyrrhotite layer. Consequently, the monosulfide is sulfurated at the internal Fe1–xS/FeS2 interface. The reaction mechanism at each stage has been validated using the corresponding kinetic model to fit the experimental data on time evolution of Fe1–xS and FeS2 layers thicknesses and some of the film transport properties. The concluding global reaction mechanism proposed in some of our former papers and completed here (Fe → Fe1–xS → FeS2) can explain the resulting microstructure of the pyrite films (i.e., Kirkendall effect and formation of a porous layer in the film). Simultaneously, it also justifies the presence of intrinsic defects, such as iron and sulfur vacancies, and the accumulation of interstitial sulfur at the film grain boundaries. The conductivity of pyrite films is tentatively explained using a two-band model where the changes in the Seebeck coefficient and the S/Fe ratio during the pyrite recrystallization stage can be successfully explained. KW - Sulfuration KW - Phase Transition KW - Sulfur diffusion KW - Reaction kinetics KW - Intrinsic defects KW - Seebeck coefficient Y1 - 2025 U6 - https://doi.org/10.1021/acs.jpcc.4c08227 SN - 1932-7447 VL - 129 IS - 9 SP - 4724 EP - 4737 PB - American Chemical Society (ACS) ER -