TY - GEN A1 - Mahmoodinezhad, Ali A1 - Morales, Carlos A1 - Kot, Małgorzata A1 - Naumann, Franziska A1 - Plate, Paul A1 - Henkel, Karsten A1 - Flege, Jan Ingo T1 - A super-cycle approach to atomic layer deposition of indium-gallium-zinc oxide at low temperature T2 - Verhandlungen der DPG N2 - The continuing development of multifunctional devices needs novel multicomponent oxide layers, demanding a high control of both composition and thickness during their preparation. To this end, single metal oxides exhibiting high structural quality and conformity have successfully been grown by atomic layer deposition (ALD). However, the deposition of more complex compounds with specific optical and electrical properties is still challenging. In this work, we follow a bottom-up approach to design an ALD super-cycle to grow mixed indium-gallium-zinc oxide (IGZO) films with a controllable composition. For the formation of the individual indium, gallium, and zinc oxides, we found the use of plasma-enhanced ALD (PEALD) at 150 °C to be favorable when using the organometallic precursors trimethylindium, trimethylgallium, and diethylzinc together with oxygen plasma. The PEALD approach of IGZO films can particularly overcome a nucleation delay within the ZnO sub-cycle known from thermal ALD, achieving a higher growth per cycle and improving the quality and composition homogeneity of the films as shown by in-situ spectroscopic ellipsometry and ex-situ X-ray photoelectron spectroscopy. KW - Indium gallium zinc oxide KW - Transparent conducting oxide KW - Plasma-enhanced atomic layer deposition KW - X-ray photoelectron spectroscopy KW - spectroscopic ellipsometry KW - nucleation delay Y1 - 2023 UR - https://www.dpg-verhandlungen.de/year/2023/conference/skm/part/o/session/40/contribution/3 SN - 0420-0195 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER - TY - GEN A1 - Braud, Nicolas A1 - Buß, Lars A1 - Lundgren, Edvin A1 - Merte, Lindsay R. A1 - Wallander, Harald J. A1 - Krisponeit, Jon-Olaf A1 - Locatelli, Andrea A1 - Mentes, Tevfik Onur A1 - Jugovac, Matteo A1 - Flege, Jan Ingo A1 - Falta, Jens T1 - Cleaning and tailoring the Pt3Sn(111) surface for surface experiments T2 - Surface Science N2 - 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. KW - Pt3Sn KW - Low-energy electron microscopy (LEEM) KW - Platinum KW - Tin KW - Cleaning KW - Alloy Y1 - 2023 U6 - https://doi.org/10.1016/j.susc.2023.122281 SN - 0039-6028 SN - 1879-2758 VL - 732 ER - TY - GEN A1 - Mańkowska, Ewa A1 - Mazur, Michał A1 - Domaradzki, Jarosław A1 - Mazur, Piotr A1 - Kot, Małgorzata A1 - Flege, Jan Ingo T1 - Hydrogen Gas Sensing Properties of Mixed Copper–Titanium Oxide Thin Films T2 - Sensors N2 - Hydrogen is an efficient source of clean and environmentally friendly energy. However, because it is explosive at concentrations higher than 4%, safety issues are a great concern. As its applications are extended, the need for the production of reliable monitoring systems is urgent. In this work, mixed copper–titanium oxide ((CuTi)Ox) thin films with various copper concentrations (0–100 at.%), deposited by magnetron sputtering and annealed at 473 K, were investigated as a prospective hydrogen gas sensing material. Scanning electron microscopy was applied to determine the morphology of the thin films. Their structure and chemical composition were investigated by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The prepared films were nanocrystalline mixtures of metallic copper, cuprous oxide, and titanium anatase in the bulk, whereas at the surface only cupric oxide was found. In comparison to the literature, the (CuTi)Ox thin films already showed a sensor response to hydrogen at a relatively low operating temperature of 473 K without using any extra catalyst. The best sensor response and sensitivity to hydrogen gas were found in the mixed copper–titanium oxides containing similar atomic concentrations of both metals, i.e., 41/59 and 56/44 of Cu/Ti. Most probably, this effect is related to their similar morphology and to the simultaneous presence of Cu and Cu2O crystals in these mixed oxide films. In particular, the studies of surface oxidation state revealed that it was the same for all annealed films and consisted only of CuO. However, in view of their crystalline structure, they consisted of Cu and Cu2O nanocrystals in the thin film volume. KW - mixed copper–titanium oxides KW - Cu2O KW - TiO2 KW - (CuTi)Ox KW - hydrogen gas sensing KW - thin films KW - magnetron sputtering Y1 - 2023 U6 - https://doi.org/10.3390/s23083822 SN - 1424-8220 VL - 23 IS - 8 ER - TY - GEN A1 - Baki, Aykut A1 - Abdeldayem, Mohamed A1 - Morales, Carlos A1 - Flege, Jan Ingo A1 - Klimm, Detlef A1 - Bierwagen, Oliver A1 - Schwarzkopf, Jutta T1 - Potential of La-Doped SrTiO3 Thin Films Grown by Metal–Organic Vapor Phase Epitaxy for Thermoelectric Applications T2 - Crystal Growth & Design N2 - La-doped SrTiO3 thin films with high structural quality were homoepitaxially grown by the metal–organic vapor phase epitaxy (MOVPE) technique. Thermogravimetric characterization of the metal–organic precursors determines suitable flash evaporator temperatures for transferring the liquid source materials in the gas phase of the reactor chamber. An adjustment of the charge carrier concentration in the films, which is necessary for optimizing the thermoelectric power factor, was performed by introducing a defined amount of the metal–organic compound La(tmhd)3 and tetraglyme to the liquid precursor solution. X-ray diffraction and atomic force microscopy verified the occurrence of the pure perovskite phase exhibiting a high structural quality for all La concentrations. The electrical conductivity of the films obtained from Hall-effect measurements increases linearly with the La concentration in the gas phase, which is attributed to the incorporation of La3+ ions on the Sr2+ perovskite sites by substitution inferred from photoemission spectroscopy. The resulting structural defects were discussed concerning the formation of occasional Ruddlesden–Popper-like defects. The thermoelectric properties determined by Seebeck measurements demonstrate the high potential of SrTiO3 thin films grown by MOVPE for thermoelectric applications. KW - metal−organic vapor phase epitaxy (MoVPE) KW - SrTiO2 KW - La doping KW - X-ray diffraction (XRD) KW - atomicforce microscopy (AFM) KW - photoemissionspectroscopy KW - Seebeck measurements KW - electrical conductivity Y1 - 2023 U6 - https://doi.org/10.1021/acs.cgd.2c01438 SN - 1528-7483 SN - 1528-7505 VL - 23 IS - 4 SP - 2522 EP - 2530 ER - TY - GEN A1 - Morales, Carlos A1 - Kosto, Yuliia A1 - Tschammer, Rudi A1 - Henkel, Karsten A1 - Flege, Jan Ingo T1 - Reduction by H2 exposure at room temperature of ceria ultrathin films grown by atomic layer deposition T2 - Verhandlungen der DPG N2 - Atomic layer deposition (ALD) exhibits a high potential for integration as a scalable process in microelectronics, allowing well-controlled layer-by-layer deposition and conformal growth on 3D structures. Yet, the ALD technique is also well known to lead to amorphous and defective, non-stoichiometric films, potentially resulting in modified materials properties that, in the case of ultra-thin deposits, can also be affected by film/substrate interaction. Interestingly, initial in situ X-ray photoemission spectroscopy (XPS) measurements of ceria ALD-deposits on Al2O3/Si, sapphire, and SiO2 substrates confirm a Ce3+/Ce4+ mixture dependent on the substrate interaction, deposit thickness, and morphology. Using near-ambient pressure XPS, we have significantly reduced ultrathin (< 10 nm) ceria films grown by ALD by exposing them to different O2/H2 partial pressures at moderate temperatures (< 525K). Notably, the total amount of reduction to Ce3+ is found to depend on the deposit thickness and initial ceria/substrate interaction. Furthermore, the intrinsic defects related to the ALD method seem to play a critical role in the reversible reduction at room temperature. KW - Cerium oxide KW - atomic layer deposition KW - near-ambient pressure X-ray photoemission spectroscopy (NAPXPS) KW - hydrogen detection Y1 - 2023 UR - https://www.dpg-verhandlungen.de/year/2023/conference/skm/part/o/session/92/contribution/6 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 - Sulaiman, Cathy A1 - Schewe, Lukas A1 - Buß, Lars A1 - Ewert, Moritz A1 - Flege, Jan Ingo T1 - In-situ monolayer graphene growth on Ru(10-10): an electron microscopy study T2 - Verhandlungen der DPG N2 - 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. KW - photoemission electron microscopy (PEEM) KW - graphene KW - ruthenium KW - low-energy electron microscopy (LEEM) KW - low-energy electron diffraction (LEED) Y1 - 2023 UR - https://www.dpg-verhandlungen.de/year/2023/conference/skm/part/o/session/64/contribution/8 SN - 0420-0195 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER - TY - GEN A1 - Tschammer, Rudi A1 - Buß, Lars A1 - Morales, Carlos A1 - Senanayake, Sanjaya D. A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - In situ characterization of cerium oxide on Au(111) under reducing and oxidizing conditions by low-energy electron microscopy T2 - Verhandlungen der DPG N2 - 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. KW - low-energy electron microscopy KW - cerium oxide KW - reduction KW - reoxidation KW - low-energy electron diffraction (LEED) Y1 - 2023 UR - https://www.dpg-verhandlungen.de/year/2023/conference/skm/part/o/session/78/contribution/9 SN - 0420-0195 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER - TY - GEN A1 - Abdeldayem, Mohamed A1 - Baki, Aykut A1 - Morales, Carlos A1 - Flege, Jan Ingo A1 - Klimm, Detlef A1 - Fiedler, Andreas A1 - Bierwagen, Oliver A1 - Schwarzkopf, Jutta T1 - Potential of La-doped SrTiO3 thin films grown by metal-organic vapor phase epitaxy for thermoelectric applications T2 - Verhandlungen der DPG N2 - Conversion of waste heat energy into electrical energy by exploiting the thermoelectric effect in solids promises a great contribution to energy harvesting concepts. However, most thermoelectric materials use toxic Pb or Te. Recently, La-doped SrTiO3 has gained a lot of interest as a potential candidate for thermoelectric devices for its good thermoelectric properties, chemical and thermal stability. In this paper, we report the homoepitaxial growth of La-doped SrTiO3 thin films by metalorganic vapor phase epitaxy (MOVPE) technique, which works at high oxygen partial pressures and offers upscaling potential for industry. The adjustment of charge carrier concentration, necessary for thermoelectric power factor optimization, was performed by introducing a defined amount of the metal-organic precursor La(tmhd)3 and tetraglyme to the liquid precursor solution. X-ray diffraction and atomic force microscopy verified a pure perovskite phase with high structural quality. The electrical conductivity increases linearly with the La concentration in the gas phase, which is attributed to the substitution of La+3 ions on the Sr+2 sites inferred from photoemission spectroscopy. KW - thermoelectric KW - metalorganic vapor phase epitaxy (MOVPE) KW - X-ray diffraction (XRD) KW - atomic force microscopy (AFM) KW - X-ray photoemission spectroscopy (XPS) Y1 - 2023 UR - https://www.dpg-verhandlungen.de/year/2023/conference/skm/part/ds/session/12/contribution/50 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 - 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 - TY - GEN A1 - Morales, Carlos A1 - Tschammer, Rudi A1 - Gouder, Thomas A1 - Choi, YongMan A1 - Anjum, Dalaver A1 - Baunthiyal, Aman A1 - Krisponeit, Jon-Olaf A1 - Falta, Jens A1 - Flege, Jan Ingo A1 - Idriss, Hicham T1 - Stabilization of Ce3+ cations via U-Ce charge transfer in mixed oxides: consequences on the thermochemical water splitting to hydrogen T2 - Journal of Physics: Energy N2 - 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. KW - cerium uranium mixed oxide KW - charge transfer KW - TCWS KW - In situ x-ray photoelectron spectroscopy (in-situ XPS) KW - DFT KW - statistical entropy Y1 - 2025 U6 - https://doi.org/10.1088/2515-7655/adbad9 SN - 2515-7655 VL - 7 SP - 1 EP - 14 PB - IOP Publishing ER - TY - GEN A1 - Kot, Małgorzata A1 - Gawlińska‐Nęcek, Katarzyna A1 - Henkel, Karsten A1 - Flege, Jan Ingo T1 - Prospects of improving efficiency and stability of hybrid perovskite solar cells by alumina ultrathin films T2 - Small N2 - Over the last few years, the influence of low temperature (≤80 °C) and, in particular, of room temperature, atomic layer deposited alumina (ALD‐Al2O3) on the properties of the underlying hybrid perovskites of different compositions and on the efficiency and stability of the corresponding perovskite solar cells (PSCs) is extensively investigated. The main conclusion is that most probably thanks to the presence of intrinsic defect states in the ALD‐Al2O3 and in the perovskite layers, charge transfer and neutralization are possible and the entire lifetime of the PSCs is thus improved. Moreover, the migration of mobile ions between the layers is blocked by the ALD‐Al2O3 layer and thus the occurrence of hysteresis in the current density–voltage characteristics of the PSCs is suppressed. Considering the uniform and nondestructive surface coverage, low thermal budget, small amount of material required, and short duration of the established ALD‐Al2O3 deposition on top of hybrid perovskites, this additional, but fully solar cell technology‐compatible, process step is most likely the most effective, cheapest, and fastest way to improve the efficiency and long‐term stability of PSCs and thus increase their marketability. KW - Perovskite solar cells (PSC) KW - Atomic layer deposition (ALD) KW - Photoelecton spectroscopy (PES) KW - Aluminum oxide Y1 - 2025 U6 - https://doi.org/10.1002/smll.202408435 SN - 1613-6810 VL - 21 IS - 12 PB - Wiley ER - TY - GEN A1 - Sulaiman, Cathy A1 - Buß, Lars A1 - Sánchez-Barquilla, Raquel A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - In-situ growth and characterization of 2D TaSe2 on Au(111) T2 - Verhandlungen der DPG, Regensburg 2025 N2 - Group V dichalcogenides such as TaX2 (X = S, Se, T) have extensively been investigated in recent decades due to their diverse electron correlation effects, including the occurrence of charge density waves and Mott-Hubbard transitions. In 2D, two polytypes, 1T and 1H, exist, which exhibit distinct properties, making selective growth of each polytype crucial. Using low-energy electron microscopy (LEEM), we have successfully observed the growth of two TaSe2 phases on Au(111) in situ after the co-deposition of Ta and Se. At elevated temperature, micron-sized, triangle-shaped islands with bright contrast nucleate first and grow at a higher rate. However, this phase turns out to be meta-stable as it suddenly transitions into a more stable phase (with dark contrast) and continues to grow at a reduced rate. Low-energy electron diffraction shows the presence of TaSe2; bandstructure-sensitive I(V)-LEEM analysis reveals substantial differences in electron reflectivity between both phases. A comparison with TaS2 suggests that the metastable and stable phases are 1T- and 1H-TaSe2, respectively. KW - TMDC KW - LEEM KW - LEED Y1 - 2025 UR - https://www.dpg-verhandlungen.de/year/2025/conference/regensburg/part/o/session/14/contribution/11 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER - TY - GEN A1 - Morales, Carlos A1 - Gertig, Max A1 - Kot, Małgorzata A1 - Alvarado, Carlos A1 - Schubert, Markus Andreas A1 - Zoellner, Marvin Hartwig A1 - Wenger, Christian A1 - Henkel, Karsten A1 - Flege, Jan Ingo T1 - In situ X‐ray photoelectron spectroscopy study of atomic layer deposited cerium oxide on SiO₂ : substrate influence on the reaction mechanism during the early stages of growth T2 - Advanced materials interfaces N2 - Thermal atomic layer deposition (ALD) of cerium oxide using commercial Ce(thd)4 precursor and O3 on SiO2 substrates is studied employing in‐situ X‐ray photoelectron spectroscopy (XPS). The system presents a complex growth behavior determined by the change in the reaction mechanism when the precursor interacts with the substrate or the cerium oxide surface. During the first growth stage, non‐ALD side reactions promoted by the substrate affect the growth per cycle, the amount of carbon residue on the surface, and the oxidation degree of cerium oxide. On the contrary, the second growth stage is characterized by a constant growth per cycle in good agreement with the literature, low carbon residues, and almost fully oxidized cerium oxide films. This distinction between two growth regimes is not unique to the CeOx/SiO2 system but can be generalized to other metal oxide substrates. Furthermore, the film growth deviates from the ideal layer‐by‐layer mode, forming micrometric inhomogeneous and defective flakes that eventually coalesce for deposit thicknesses above 10 nm. The ALD‐cerium oxide films present less order and a higher density of defects than films grown by physical vapor deposition techniques, likely affecting their reactivity in oxidizing and reducing conditions. KW - Atomic Layer Deposition (ALD) KW - Cerium oxide KW - In-situ X-ray Photoelectron spectroscopy (in-situ XPS) KW - Growth model KW - Substrate influence Y1 - 2025 U6 - https://doi.org/10.1002/admi.202400537 SN - 2196-7350 VL - 12 IS - 5 SP - 1 EP - 13 PB - Wiley CY - Weinheim ER - TY - GEN A1 - Ferbel, Letizia A1 - Veronesi, Stefano A1 - Mentes, Tevfik Onur A1 - Buß, Lars A1 - Rossi, Antonio A1 - Mishra, Neeraj A1 - Coletti, Camilla A1 - Flege, Jan Ingo A1 - Locatelli, Andrea A1 - Heun, Stefan T1 - Rubidium intercalation in epitaxial monolayer graphene T2 - Nanoscale N2 - Alkali metal intercalation of graphene layers has been of particular interest due to potential applications in electronics, energy storage, and catalysis. Rubidium (Rb) is one of the largest alkali metals and among the least investigated as an intercalant. Here, we report a systematic investigation, with a multi-technique approach, of the phase formation of Rb under epitaxial monolayer graphene on SiC(0001). We explore a wide phase space with two control parameters: the Rb density (i.e., deposition time) and sample temperature (i.e., room and low temperature). We reveal the emergence of (2 × 2) and [sqrt(3) x (sqrt3)] R30° structures formed by a single alkali metal layer intercalated between monolayer graphene and the interfacial C-rich reconstructed surface, also known as the buffer layer. Rb intercalation also results in strong n-type doping of the graphene layer. Upon progressively annealing to higher temperatures, we first reveal the diffusion of Rb atoms, which results in the enlargement of intercalated areas. As desorption sets in, intercalated regions progressively shrink and fragment. Eventually, at approximately 600 °C, the initial surface is retrieved, indicating the reversibility of the intercalation process. KW - Graphene KW - N-type doping KW - Micro-spot low-energy electron diffraction (µLEED) KW - Scanning tunneling microscopy (STM) KW - Low-energy electron microscopy (LEEM) KW - Density functional theory (DFT) Y1 - 2025 U6 - https://doi.org/10.1039/D5NR00417A SN - 2040-3364 VL - 17 IS - 19 SP - 12465 EP - 12472 PB - Royal Society of Chemistry (RSC) CY - London ER - TY - GEN A1 - Tschammer, Rudi A1 - Buß, Lars A1 - Pożarowska, Emilia A1 - Morales, Carlos A1 - Senanayake, Sanjaya D. A1 - Prieto, Mauricio J. A1 - Tănase, Liviu C. A1 - de Souza Caldas, Lucas A1 - Tiwari, Aarti A1 - Schmidt, Thomas A1 - Niño, Miguel A. A1 - Foerster, Michael A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - High-temperature growth of CeOx on Au(111) and behavior under reducing and oxidizing conditions T2 - The journal of physical chemistry C N2 - Inverse oxide–metal model catalysts can show superior activity and selectivity compared with the traditional supported metal–oxide architecture, commonly attributed to the synergistic overlayer–support interaction. We have investigated the growth and redox properties of ceria nanoislands grown on Au(111) between 700 and 890 °C, which yields the CeO2–Au(111) model catalyst system. We have observed a distinct correlation between deposition temperature, structural order, and oxide composition through low-energy electron microscopy, low-energy electron diffraction, intensity–voltage curves, and X-ray absorption spectroscopy. Improved structural order and thermal stability of the oxide have been achieved by increasing the oxygen chemical potential at the substrate surface using reactive oxygen (O/O2) instead of molecular O2 during growth. In situ characterization under reducing (H2) and oxidizing atmospheres (O2, CO2) indicates an irreversible loss of structural order and redox activity at high reduction temperatures, while moderate temperatures result in partial decomposition of the ceria nanoislands (Ce3+/Ce4+) to metallic cerium (Ce0). The weak interaction between Au(111) and CeOx would facilitate its reduction to the Ce0 metallic state, especially considering the comparatively strong interaction between Ce0 and Au0. Besides, the higher reactivity of atomic oxygen promotes a stronger interaction between the gold and oxide islands during the nucleation process, explaining the improved stability. Thus, we propose that by driving the nucleation and growth of the ceria/Au system in a highly oxidizing regime, novel chemical properties can be obtained. KW - Inverse oxide-metal model catalysts KW - Ceria nanoislands KW - Growth KW - Redox properties KW - Low-energy electron microscopy (LEEM) KW - Low-energy electron diffraction (LEED) KW - Intensity–voltage curves KW - X-ray absorption spectroscopy (XAS) Y1 - 2025 U6 - https://doi.org/10.1021/acs.jpcc.4c08072 SN - 1932-7447 VL - 129 IS - 7 SP - 3583 EP - 3594 PB - American Chemical Society (ACS) CY - Washington, DC ER - TY - GEN A1 - Richter, Jana A1 - Rachow, Fabian A1 - Israel, Johannes A1 - Roth, Norbert A1 - Charlafti, Evgenia A1 - Günther, Vivien A1 - Flege, Jan Ingo A1 - Mauß, Fabian T1 - Reaction Mechanism Development for Methane Steam Reforming on a Ni/Al2O3 Catalyst T2 - Catalysts N2 - In this work, a reliable kinetic reaction mechanism was revised to accurately reproduce the detailed reaction paths of steam reforming of methane over a Ni/Al2O3 catalyst. A steady-state fixed-bed reactor experiment and a 1D reactor catalyst model were utilized for this task. The distinctive feature of this experiment is the possibility to measure the axially resolved temperature profile of the catalyst bed, which makes the reaction kinetics inside the reactor visible. This allows for understanding the actual influence of the reaction kinetics on the system; while pure gas concentration measurements at the catalytic reactor outlet show near-equilibrium conditions, the inhere presented temperature profile shows that it is insufficient to base a reaction mechanism development on close equilibrium data. The new experimental data allow for achieving much higher quality in the modeling efforts. Additionally, by carefully controlling the available active surface via dilution in the experiment, it was possible to slow down the catalyst conversion rate, which helped during the adjustment of the reaction kinetics. To assess the accuracy of the revised mechanism, a monolith experiment from the literature was simulated. The results show that the fitted reaction mechanism was able to accurately predict the experimental outcomes for various inlet mass flows, temperatures, and steam-to-carbon ratios. KW - kinetic reaction mechanism development KW - 1D modeling KW - reaction rates KW - methane steam reforming KW - fixed-bed reactor experiments KW - nickel catalyst Y1 - 2023 U6 - https://doi.org/10.3390/catal13050884 SN - 2073-4344 VL - 13 IS - 5 ER - TY - GEN A1 - Buß, Lars A1 - Sulaiman, Cathy A1 - Sánchez-Barquilla, Raquel A1 - Cojocariu, Iulia A1 - Szpytma, Marcin A1 - Menteş, Tevfik Onur A1 - Locatelli, Andrea A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - Rise and fall of 1T-TaS₂ : epitaxial growth of monolayer TaS₂ on Au(111) T2 - Physical review materials N2 - Monolayer tantalum disulfide epitaxially grown on Au(111) is studied in real time during molecular beam epitaxy using low-energy electron microscopy and microdiffraction. Complementary x-ray photoelectron emission microscopy provides insight into the chemical and electronic structure of the grown layers. Our study reveals a previously unreported growth mechanism where the formation of 2⁢H−TaS2 proceeds via a transient 1⁢T−TaS2 phase. The 1⁢T−TaS2 phase exhibits a significantly higher growth rate than the 2⁢H−TaS2 phase, with growth proceeding mainly in the 1T phase. By comparison with TaSe2 on Au(111), we find that this growth mechanism is common to other Ta-based transition-metal dichalcogenides on Au(111). Furthermore, we find spectroscopic evidence for the presence of charge-density-wave order in 1⁢T−TaS2 on Au(111). These findings provide perspectives on the growth dynamics and phase control of TaS2, opening up avenues for tailoring its electronic properties through substrate interaction and phase engineering. KW - LEEM KW - Low-energy electron microscopy Y1 - 2025 U6 - https://doi.org/10.1103/1bxg-yvw2 SN - 2475-9953 VL - 9 IS - 7 SP - 1 EP - 9 PB - American Physical Society (APS) CY - College Park, MD ER - TY - GEN A1 - Morales, Carlos A1 - Tschammer, Rudi A1 - Pożarowska, Emilia A1 - Kosto, Julia A1 - Villar‐Garcia, Ignacio J. A1 - Pérez‐Dieste, Virginia A1 - Favaro, Marco A1 - Starr, David E. A1 - Kapuścik, Paulina A1 - Mazur, Michał A1 - Wojcieszak, Damian A1 - Domaradzki, Jarosław A1 - Alvarado, Carlos A1 - Wenger, Christian A1 - Henkel, Karsten A1 - Flege, Jan Ingo T1 - Hydrogen sensing via heterolytic H₂ activation at room temperature by atomic layer deposited ceria T2 - ChemSusChem : chemistry, sustainability, energy, materials N2 - Ultrathin atomic layer deposited ceria films (<20 nm) are capable of H2 heterolytic activation at room temperature, undergoing a significant reduction regardless of the absolute pressure, as measured under in‐situ conditions by near ambient pressure X‐ray photoelectron spectroscopy. ALD‐ceria can gradually reduce as a function of H2 concentration under H2/O2 environments, especially for diluted mixtures below 10 %. At room temperature, this reduction is limited to the surface region, where the hydroxylation of the ceria surface induces a charge transfer towards the ceria matrix, reducing Ce4+ cations to Ce3+. Thus, ALD‐ceria replicates the expected sensing mechanism of metal oxides at low temperatures without using any noble metal decorating the oxide surface to enhance H2 dissociation. The intrinsic defects of the ALD deposit seem to play a crucial role since the post‐annealing process capable of healing these defects leads to decreased film reactivity. The sensing behavior was successfully demonstrated in sensor test structures by resistance changes towards low concentrations of H2 at low operating temperatures without using noble metals. These promising results call for combining ALD‐ceria with more conductive metal oxides, taking advantage of the charge transfer at the interface and thus modifying the depletion layer formed at the heterojunction. KW - Atomic Layer Deposition KW - Ceria KW - Hydrogen Sensing KW - X-Ray photoelectron spectroscopy KW - Raman spectroscopy KW - Resitive sensor Y1 - 2025 U6 - https://doi.org/10.1002/cssc.202402342 SN - 1864-5631 VL - 18 IS - 13 SP - 1 EP - 13 PB - Wiley-VCH CY - Weinheim ER - TY - GEN A1 - Braud, Nicolas A1 - Buß, Lars A1 - Merte, Lindsay Richard A1 - Wallander, Harald A1 - Krisponeit, Jon-Olaf A1 - Schmidt, Thomas A1 - Lundgren, Edvin A1 - Flege, Jan Ingo A1 - Falta, Jens T1 - Growth and oxidation of ultra-thin Pt-Sn layers on Pt(111) by molecular and atomic oxygen T2 - Ultramicroscopy N2 - 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) Å and b = (8.6 ± 0.1) Å 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. KW - Tin KW - Platinum KW - Tin oxide KW - Platinum-tin KW - Oxidation KW - SnOx KW - Ultra-thin films KW - LEED KW - LEEM Y1 - 2025 U6 - https://doi.org/10.1016/j.ultramic.2025.114243 SN - 0304-3991 VL - 278 SP - 1 EP - 11 PB - Elsevier BV CY - Amsterdam ER - TY - GEN A1 - Franken, Tim A1 - Rachow, Fabian A1 - Charlafti, Evgenia A1 - Flege, Jan Ingo A1 - Jenssen, Martin A1 - Verma, Rakhi A1 - Günther, Vivien A1 - Mauss, Fabian T1 - Numerical investigation of oxy-methane combustion for stationary engines T2 - 40th International Symposium on Combustion N2 - This work presents a numerical investigation of turbulent oxyfuel combustion of methane in a gas engine with passive pre-chamber. The experimental data of a motored operating point at 1600 rpm and natural gas fired operating point at 2450 rpm, 6 bar IMEP and λ=1.5 are provided by TU Freiberg to validate the simulation model. The performance of the detailed chemistry model of Shrestha et al. predicting laminar burning velocity of premixed methane-oxygen flames is evaluated using the experiments of Mouze-Mornettas et al. The detailed chemistry model predicts the laminar flame speed within an accuracy range of ±10% for elevated pressure, temperature, and different equivalence ratios. For predicting the turbulent combustion in the gas engine, a three-dimensional (3D) Large Eddy Simulation (LES) with G Equation model and laminar flame speed look-up tables is used. The chemistry in the unburnt and burnt gas is solved using a constant volume detailed chemistry solver. The 3D LES model shows a good match of the motored and natural gas fired in-cylinder pressure profile. Subsequently the fuel is switched to methane and oxygen is used as oxidizer. The 3D LES results show an increase of maximum cylinder pressure up to 100 bar for λ=1.5, and the turbulent flame regime is shifted towards high Damköhler numbers compared to combustion with air. Diluting the cylinder gas with 50 mole-% CO2 or 65 mole-% H2O shows a significant reduction of peak cylinder pressure, and lower Damköhler and higher Karlovitz numbers compared to methane-oxygen combustion. KW - Oxyfuel KW - Simulation KW - Engines Y1 - 2024 ER - TY - GEN A1 - Kalra, Amanpreet A1 - Alvarado Chavarin, Carlos A1 - Nitsch, Paul-Gregor A1 - Tschammer, Rudi A1 - Flege, Jan Ingo A1 - Ratzke, Markus A1 - Zoellner, Marvin Hartwig A1 - Schubert, Markus Andreas A1 - Wenger, Christian A1 - Fischer, Inga Anita T1 - Deposition of CeOₓ/SnOₓ-based thin films via RF magnetron sputtering for resistive gas sensing applications T2 - Physica B, Condensed matter N2 - Cerium oxide-tin oxide (CeOx/SnOx) thin films with varying Sn content were deposited using RF magnetron sputtering and investigated for hydrogen sensing applications. Structural, compositional, and morphological properties were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX). Gas sensing measurements showed effective hydrogen detection at room temperature, with the sensitivity strongly influenced by Sn content and oxygen vacancy concentration. Higher Sn concentration enhanced the sensing response, which was correlated with microstructural features obtained from AFM and EDX, as well as with the presence of Ce3+ and Ce4+ oxidation states identified by XPS. This study highlights the potential of CeOx/SnOx thin films for possible back-end-of-line integration and provides proof-of-principle for room-temperature hydrogen sensing. KW - RF magnetron sputtering KW - CeOx/SnOx thin film KW - Room temperature KW - Hydrogen sensing Y1 - 2026 U6 - https://doi.org/10.1016/j.physb.2025.418098 SN - 0921-4526 VL - 723 SP - 1 EP - 7 PB - Elsevier BV CY - Amsterdam ER - TY - GEN A1 - Weichbrodt, Wiktoria A1 - Domaradzki, Jaroslaw A1 - Obstarczyk, Agata A1 - Kot, Malgorzata A1 - Flege, Jan Ingo A1 - Mazur, Michal T1 - Influence of thermal modification on the gasochromic properties of WO₃ thin films fabricated by electron beam evaporation T2 - Applied optics N2 - This paper describes the effect of post-deposition annealing on the structural and gasochromic properties of WO3 thin films deposited by electron beam evaporation and additionally decorated with a Pd catalyst layer of varying thickness. The WO3 layers were annealed at 800°C, which led to a phase transition from an amorphous to a monoclinic crystal structure, accompanied by an increase in surface roughness from 1.3 to 66 nm and the formation of a discontinuous island-like morphology with grain sizes up to 3 µm. The structural changes had a significant effect on the optical response of the layers to hydrogen. For the annealed samples, the absolute change in light transmission reached 20.2–20.9% for the 1.5 nm thick Pd catalyst and 7.7–9.2% for the 5 nm thick Pd catalyst at a wavelength of 850 nm. The corresponding optical response was 184–186% for the thin Pd layer and 353–396% for the thick Pd layer, depending on the hydrogen concentration (25–1000 ppm). The response time was reduced from 10 min at 25 ppm to < 4 min at 1000 ppm, while the recovery time to the original state in air remained below 66 s under all conditions. XPS studies confirmed the reduction of W6+ to W5+ under the influence of hydrogen and reversible transition PdO - Pd, which correlates with the observed optical changes. The results show that annealing increases crystallinity and modifies porosity, which, in combination with the Pd catalyst directly affects the kinetics and magnitude of the gasochromic response. Y1 - 2025 U6 - https://doi.org/10.1364/AO.574918 SN - 1559-128X VL - 65 IS - 5 SP - A58 EP - A67 PB - Optica Publishing Group CY - Washington, DC ER - TY - GEN A1 - Gawlińska−Nęcek, Katarzyna A1 - Dąbczyński, Paweł A1 - Nuckowski, Paweł A1 - Starowicz, Zbigniew A1 - Kot, Małgorzata A1 - Panek, Piotr A1 - Flege, Jan Ingo T1 - Crystallographic changes mediated by copper migration from hole transporting layer into CsPbBr₃ perovskite T2 - The journal of physical chemistry C N2 - Copper oxides, due to their low cost and high ambient stability, are promising candidates for use as a hole-transporting layer (HTL) in perovskite solar cells. However, it has been found that they can be very unstable in contact with organic–inorganic FAPbI3 perovskite, causing mutual chemical reactions and changing a photoactive to a nonphotoactive perovskite phase. To verify if the copper migration and the occurrence of chemical reactions are a matter of contact with a type of perovskite (organic–inorganic or fully inorganic) or the nature of copper oxides, in this work, the interface between cupric and cuprous oxides and a fully inorganic cesium lead bromide (CsPbBr3) perovskite is investigated. It is found that CsPbBr3 is not robust against copper ion migration from HTL, similarly to organic–inorganic FAPbI3; however, contrary to the previous results, they do not cause chemical reactions. Instead, crystallographic lattice shrinkage of the CsPbBr3 occurred along with the substitution of Pb2+ by copper ions. The work explains the mechanism of this phenomenon and confirms the unreliability of copper(I) oxide and copper(II) oxide as HTLs in p-i-n perovskite solar cells based on both organic–inorganic and fully inorganic lead halide perovskites. KW - Copper KW - Lattices KW - Oxides KW - Solar Cells Y1 - 2025 U6 - https://doi.org/10.1021/acs.jpcc.5c06144 SN - 1932-7447 VL - 129 IS - 43 SP - 19410 EP - 19420 PB - American Chemical Society (ACS) CY - Washington, DC ER - TY - GEN A1 - Pożarowska, Emilia A1 - Pleines, Linus A1 - Prieto, Mauricio J. A1 - Tănase, Liviu C. A1 - de Souza Caldas, Lucas A1 - Tiwari, Aarti A1 - Schmidt, Thomas A1 - Falta, Jens A1 - Morales, Carlos A1 - Flege, Jan Ingo T1 - The relationship between Sm alloying and structure sensitivity of ceria(111)- and (100)-oriented nanoislands on Cu(111) T2 - Physical chemistry, chemical physics N2 - We have investigated the complex dynamics of samarium deposition on ceria islands of different orientations, namely (111) and (100), grown side by side on a Cu(111) single-crystal substrate, followed by post-oxidation and annealing under ultra-high vacuum conditions. Only the (100)-oriented ceria islands undergo substantial initial reduction upon samarium deposition at 740 K via a pathway similar to the strong Ce–ceria interfacial interaction, while the (111)-oriented islands remain in the Ce4+ oxidation state. This remarkable structure sensitivity is explained by the different energies required for oxygen vacancy formation for both oxide orientations. Subsequent mild re-oxidation with O2 results in the complete recovery of the Ce4+ oxidation state in the (100)-oriented islands, indicating the complete healing of oxygen vacancies. In contrast, extended annealing at moderate temperatures likely induces persistent samarium incorporation into the cerium oxide matrix. Our results provide new insights into the complex structure–activity relationships in mixed rare-earth metal oxide systems and have promising implications for optimizing catalytic reactions over such compounds in reducing environments. KW - LEEM Y1 - 2025 U6 - https://doi.org/10.1039/D5CP01171J SN - 1463-9076 VL - 27 IS - 29 SP - 15691 EP - 15703 PB - Royal Society of Chemistry (RSC) CY - Cambridge ER - TY - GEN A1 - Angrick, Christian A1 - Henriksen, Annika A1 - Edossa, Nicole A1 - Reimann, Andre A1 - Ewert, Moritz A1 - Buß, Lars A1 - Falta, Jens A1 - Flege, Jan Ingo A1 - Donath, Markus T1 - Transition metal dichalcogenide surfaces as scattering targets in spin-polarization detectors : a case study of MoS₂ T2 - Physical review B N2 - The suitability of transition metal dichalcogenides (TMDCs) for the use as scattering targets in electron spin-polarization detectors is put to a paradigmatic test: a case study of MoS₂ is performed. Therefore, the electron reflectivity, the Sherman function, and the figure of merit are measured for the following samples: a single layer of MoS₂ on Au(111) and cleaved MoS₂ single-crystal surfaces. MoS₂ offers promising features for the use in spin-polarization detection: working points with a reversed sign of the Sherman function and a long-term target stability. To overcome the low figure of merit of MoS₂, the closely-related 2D material WS₂ with higher spin-orbit interaction is proposed, which may resemble the promising features with higher Sherman function. This study demonstrates the potential suitability of 2D materials such as TMDCs as scattering targets in spin-polarization detectors. KW - Magnetism KW - Spin polarization KW - Spin-orbit coupling KW - Low-energy electron diffraction KW - Spin-resolved photoemission spectroscopy Y1 - 2025 U6 - https://doi.org/10.1103/fln2-pf6n SN - 2469-9950 VL - 112 IS - 23 SP - 1 EP - 11 PB - American Physical Society (APS) CY - College Park, Maryland ER - TY - GEN A1 - Verma, Rakhi A1 - Günther, Vivien A1 - Charlafti, Evgenia A1 - Rachow, Fabian A1 - Giri, Binod Raj A1 - Hemaizia, Abdelkader A1 - Thévenin, Dominique A1 - Flege, Jan Ingo A1 - Mauss, Fabian T1 - Development of detailed surface reaction mechanism for methanation process based on experiments T2 - Proceedings in applied mathematics and mechanics : PAMM N2 - The pressure to reduce greenhouse gas emissions is growing, which demands new and innovative technologies to produce mobile as well as stationary energy. The methanation offers a pathway to reduce greenhouse gas emissions by directly converting to . This also plays a crucial role in “power‐to‐gas” (P2G) technologies by providing an approach to store excess renewable energy in the form of methane in an existing natural gas infrastructure. However, methanation is a complex process due to its exothermic nature, interaction of the gas species with the catalyst, and possible catalyst degradation. Therefore, a deeper understanding is required for the methanation reaction, its different reaction pathways, and side reactions. In this work, we aim to understand the direct production of synthetic natural gas from and in a Sabatier process with the help of experiments over a Ni/ catalyst. A detailed surface reaction mechanism is developed to extend the study numerically by validating the simulation results with the experimental data. A one‐dimensional model, LOGEcat, based on a single‐channel catalyst model, is used for kinetic modeling. Experiments as well as simulations have been performed at various conditions, such as temperature variation and dilution to the inlet composition. We have successfully captured the experimental trends using the kinetic model developed for the conditions considered for the analysis. Y1 - 2026 U6 - https://doi.org/10.1002/pamm.70061 SN - 1617-7061 VL - 26 IS - 1 SP - 1 EP - 6 PB - Wiley CY - Weinheim ER - TY - GEN A1 - Morales, Carlos A1 - Tschammer, Rudi A1 - Guttmann, Dominic A1 - Chavarin, Carlos Alvarado A1 - Ruffert, Christine A1 - Henkel, Karsten A1 - Wenger, Christian A1 - Flege, Jan Ingo T1 - Bottom-up strategy to develop ultrathin active layers by atomic layer deposition for room temperature hydrogen sensors compatible with CMOS technology T2 - MikroSystemTechnik Kongress 2025 : Mikroelektronik, Mikrosystemtechnik und ihre Anwendungen - Nachhaltigkeit und Technologiesouveränität : proceedings : 27.-29. Oktober 2025, Duisburg Y1 - 2025 SN - 978-3-8007-6614-7 SN - 978-3-8007-6615-4 SP - 71ff. PB - VDE VERLAG GmbH CY - Berlin ER - TY - GEN A1 - Kao, Ming-Chao A1 - Schewe, Lukas Paul A1 - Akhtar, Arub A1 - Vlad, Alina A1 - Keller, Thomas F. A1 - Henkel, Karsten A1 - Anooz, Saud Bin A1 - Popp, Andreas A1 - Galazka, Zbigniew A1 - Flege, Jan Ingo A1 - Stierle, Andreas A1 - Vonk, Vedran T1 - Surface termination of β-Ga₂O₃(100) as-cleaved single crystals T2 - Applied physics letters N2 - The surface of β-Ga₂O₃ single crystals cleaved along their (100) plane is investigated using surface x-ray diffraction and atomic force microscopy. The results show the surface to consist of a single, so-called B-termination, which means that the crystal cleaves at planes formed by edge-sharing oxygen octahedra, thereby breaking the longest and weakest Ga–O bonds. Refinement of the atomic positions results in small displacements from the bulk structure, at most approximately 0.01 Å. Atomic force microscopy suggests that relatively large terraces form together with steps of half the a-axis length of approximately 0.6 nm, which means that terraces have the same atomic termination, related by the crystal symmetry. These results are important as a fundamental property of β-Ga₂O₃ when processed or used in various semiconductor applications. KW - X-ray KW - Gallium oxide Y1 - 2026 U6 - https://doi.org/10.1063/5.0309937 SN - 0003-6951 VL - 128 IS - 7 SP - 1 EP - 5 PB - AIP Publishing CY - Melville, NY ER - TY - GEN A1 - Kapuścik, Paulina A1 - Domaradzki, Jarosław A1 - Obstarczyk, Agata A1 - Kot, Małgorzata A1 - Flege, Jan Ingo A1 - Keel, Emma A1 - Gibson, Des A1 - Wojcieszak, Damian T1 - Correlation between electron beam evaporation conditions and sensor response of cerium oxide coatings T2 - International journal of hydrogen energy N2 - Cerium oxide thin films were prepared by electron beam evaporation (EBE) under three deposition conditions: standard process, substrate heating at 200 °C, and ion beam assisted deposition (IBAD). The high optical transparency of the PVD-prepared coatings enables integration with transparent microelectronic devices, an aspect seldom explored in ceria-based gas sensors. Raman and optical analyses revealed various levels of oxygen vacancy-related defects in all films. Gas sensing measurements of the Pd/CeOx structures were correlated with their structural and electronic characteristics, including changes in Ce4+/Ce3+ ratios and band alignment during exposure to reducing gases, providing insight into the redox-driven sensing mechanism. All structures exhibit high ethanol sensitivity, while the highest response achieved for the films deposited with substrate heating and IBAD is consistent with their larger defect density and modified morphology. These findings demonstrate that tailoring EBE conditions provides an effective route to optimize CeOx thin films for improved gas sensing performance. KW - Cerium oxide KW - Thin film KW - Electron beam evaporation KW - Ion beam assisted deposition KW - Gas sensing Y1 - 2026 U6 - https://doi.org/10.1016/j.ijhydene.2026.154101 SN - 0360-3199 VL - 220 PB - Elsevier BV CY - Amsterdam ER -