@misc{KalraAlvaradoChavarinNitschetal., author = {Kalra, Amanpreet and Alvarado Chavarin, Carlos and Nitsch, Paul-Gregor and Tschammer, Rudi and Flege, Jan Ingo and Ratzke, Markus and Zoellner, Marvin Hartwig and Schubert, Markus Andreas and Wenger, Christian and Fischer, Inga Anita}, title = {Deposition of CeOₓ/SnOₓ-based thin films via RF magnetron sputtering for resistive gas sensing applications}, series = {Physica B, Condensed matter}, volume = {723}, journal = {Physica B, Condensed matter}, publisher = {Elsevier BV}, address = {Amsterdam}, issn = {0921-4526}, doi = {10.1016/j.physb.2025.418098}, pages = {1 -- 7}, abstract = {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.}, language = {en} } @misc{BraudWallanderBussetal., author = {Braud, N. and Wallander, H.J. and Buß, L. and L{\"o}fstrand, M. and Blomqvist, J. and Berschauer, C. and Rodriguez, A. Morales and Kofoed, P.M. and Resta, A. and Krisponeit, J.-O. and Schmidt, T. and Lundgren, E. and Flege, J.I. and Falta, J. and Merte, L.R.}, title = {Growth, structure, and morphology of ultra-thin tin oxide phases forming on Pt₃Sn(111) single crystals upon exposure to oxygen}, series = {Surface science}, volume = {767}, journal = {Surface science}, publisher = {Elsevier BV}, address = {Amsterdam}, issn = {0039-6028}, doi = {10.1016/j.susc.2025.122927}, pages = {1 -- 8}, abstract = {Here we report an investigation of ultrathin tin oxide films on Pt3Sn(111) using low-energy electron microscopy (LEEM), microspot low-energy electron diffraction (𝜇-LEED), scanning tunneling microscopy (STM), surface X-ray diffraction (SXRD), and high-resolution X-ray photoelectron spectroscopy (XPS). Oxidation at ∼390-410 ◦C produces triangular, two-dimensional oxide islands that nucleate rapidly and exhibit self-limited lateral growth, attributed to limited Sn diffusion from the subsurface of the crystal. 𝜇-LEED shows that the initially formed (4 × 4) Sn oxide is subsequently converted to a more oxygen-rich (2 × 2𝑛) ''stripe'' phase. At 630 ◦C, enhanced Sn mobility enables a closed (4 × 4) film. The (2 × 2𝑛) phase is shown to consist of a (2 × 2) Sn lattice modulated by 1D stripe defects with spacings of 𝑛 = 4-6 atomic rows; LEED and SXRD measurements show diffraction features corresponding to this striped superstructure. The two oxides can be distinguished in XPS by their O 1s lineshapes: the (4 × 4) phase shows a clear doublet attributable to distinct O species, whereas the (2 × 2𝑛) phase exhibits a broader envelope consistent with a distribution of O coordination environments. The Sn 3d5∕2 spectra are similar for both phases, reflecting closely related Sn bonding motifs. The spectra are consistent with those of previous near-ambient-pressure XPS measurements, suggesting that the surface oxides forming under CO oxidation conditions are similar to those studied here.}, language = {en} } @misc{VermaGuentherCharlaftietal., author = {Verma, Rakhi and G{\"u}nther, Vivien and Charlafti, Evgenia and Rachow, Fabian and Giri, Binod Raj and Hemaizia, Abdelkader and Th{\´e}venin, Dominique and Flege, Jan Ingo and Mauss, Fabian}, title = {Development of detailed surface reaction mechanism for methanation process based on experiments}, series = {Proceedings in applied mathematics and mechanics : PAMM}, volume = {26}, journal = {Proceedings in applied mathematics and mechanics : PAMM}, number = {1}, publisher = {Wiley}, address = {Weinheim}, issn = {1617-7061}, doi = {10.1002/pamm.70061}, pages = {1 -- 6}, abstract = {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.}, language = {en} } @misc{KotHenkelSchmeisser, author = {Kot, Małgorzata and Henkel, Karsten and Schmeißer, Dieter}, title = {Internal chemical potential in mixed covalent-ionic photosensitive systems}, series = {Journal of Vacuum Science \& Technology A}, volume = {43 (2025)}, journal = {Journal of Vacuum Science \& Technology A}, number = {1}, publisher = {American Vacuum Society}, issn = {0734-2101}, doi = {10.1116/6.0004179}, pages = {1 -- 9}, abstract = {The internal chemical potential Γ of mixed covalent-ionic systems represents the potential differences between the covalent and the ionic intrinsic defect states located within the ionic gap. It is the key parameter to control the carrier densities, the stability regimes, and the photosensitive properties of materials. In this work, we describe first the quantitative analysis of the carrier densities in dependence on the internal potential Nπ(Γ) based on the common features of the electronic structure of mixed covalent-ionic materials. Subsequently, this method is applied on two mixed covalent-ionic materials, i.e., formamidinium lead triiodide and gallium oxide, as representatives of the respective families of perovskites (halides) and transparent conducting oxide thin films. Based on this analysis, the carrier densities as well as the photosensitivity mechanisms and the related specific properties of these materials in dependence on their internal chemical potential are discussed.}, language = {en} } @misc{BussZamborliniSulaimanetal., author = {Buß, Lars and Zamborlini, Giovanni and Sulaiman, Cathy and Ewert, Moritz and Cinchetti, Mirko and Falta, Jens and Flege, Jan Ingo}, title = {Hexagons on rectangles: Epitaxial graphene on Ru(10-10)}, series = {Carbon}, volume = {231}, journal = {Carbon}, publisher = {Elsevier BV}, issn = {0008-6223}, doi = {10.1016/j.carbon.2024.119600}, pages = {10}, abstract = {Ruthenium is emerging as a promising candidate to replace copper in highly integrated electronics by enabling barrierless metallization in ultrathin interconnects. From this perspective, the study of graphene growth on such surface templates is of paramount importance as a platform for graphene integration in electronic devices. In particular, graphene growth on the Ru (10-10) surface allows selective growth of different graphene orientations, one-dimensional structures, and reduced substrate interaction compared to the well-established hexagonal Ru(0001) substrate. Real-time growth observations using low-energy electron microscopy and micro-diffraction highlight the influence of substrate symmetry on graphene growth, leading to the formation of rectangular islands with distinct zigzag- or armchair-terminated edges. Bilayer formation on Ru(10-10) occurs by nucleation of graphene nanoribbons under the monolayer. Micro-spot angle-resolved photoemission spectroscopy shows significantly less charge-transfer doping in these freestanding, zigzag-terminated bilayer graphene nanoribbons, indicating reduced graphene-substrate interaction and hence more effective decoupling as compared to graphene/Ru(0001). Our results show that the growth of graphene on non-hexagonal substrates opens new pathways for tailoring the graphene-substrate interaction at the interface, and thus the properties of graphene beyond the limits imposed by hexagonal substrates.}, language = {en} } @misc{MoralesPascualLeinenetal., author = {Morales, Carlos and Pascual, Antonio and Leinen, Dietmar and Luna-L{\´o}pez, Gabriel and Ares, Jose R. and Flege, Jan Ingo and Soriano, Leonardo and Ferrer, Isabel J. and Sanchez, Carlos}, title = {Reaction Mechanism and Kinetic Model of the Transformation of Iron Monosulfide Thin Films into Pyrite Films}, series = {The Journal of Physical Chemistry C}, volume = {129}, journal = {The Journal of Physical Chemistry C}, number = {9}, publisher = {American Chemical Society (ACS)}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.4c08227}, pages = {4724 -- 4737}, abstract = {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.}, 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{KotGawlińska‐NęcekHenkeletal., author = {Kot, Małgorzata and Gawlińska-Nęcek, Katarzyna and Henkel, Karsten and Flege, Jan Ingo}, title = {Prospects of improving efficiency and stability of hybrid perovskite solar cells by alumina ultrathin films}, series = {Small}, volume = {21}, journal = {Small}, number = {12}, publisher = {Wiley}, issn = {1613-6810}, doi = {10.1002/smll.202408435}, pages = {19}, abstract = {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.}, language = {en} } @misc{SulaimanBussSanchezBarquillaetal., author = {Sulaiman, Cathy and Buß, Lars and S{\´a}nchez-Barquilla, Raquel and Falta, Jens and Flege, Jan Ingo}, title = {In-situ growth and characterization of 2D TaSe2 on Au(111)}, series = {Verhandlungen der DPG, Regensburg 2025}, journal = {Verhandlungen der DPG, Regensburg 2025}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, abstract = {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.}, language = {en} } @misc{KotGawlińska‐NęcekPożarowskaetal., author = {Kot, Małgorzata and Gawlińska-Nęcek, Katarzyna and Pożarowska, Emilia and Henkel, Karsten and Schmeißer, Dieter}, title = {Photosensitivity and carrier densities of perovskite solar absorbers}, series = {Advanced science}, volume = {12}, journal = {Advanced science}, number = {16}, publisher = {Wiley}, address = {Hobken, New Jersey}, issn = {2198-3844}, doi = {10.1002/advs.202412711}, pages = {1 -- 8}, abstract = {Dark and light current-voltage characteristics of perovskite solar absorbers are analyzed in terms of their carrier densities. The analysis reveals p-type large polarons as a dominant carrier type in the investigated perovskite solar cells. The mechanism causing photosensitivity is attributed to the dissociation (and pairing) of bipolarons to large polarons (and vice versa) that are controlled by the internal potential Γ. As an example, the polaron concept is tested for a formamidinium lead triiodide perovskite solar cell. The individual steps of the data analysis are demonstrated and determine the ionicity factor of this perovskite film, quantify the density of the large polarons, and predict the gain and loss of photo-induced carriers. It is deduced that a reversible light-on/off operation can only occur when the bias voltage never exceeds a critical value of the internal potential. The results gained in this study suggest that the novel analysis can be successively applied on different hybrid perovskite materials, too.}, language = {en} } @misc{MoralesGertigKotetal., author = {Morales, Carlos and Gertig, Max and Kot, Małgorzata and Alvarado, Carlos and Schubert, Markus Andreas and Zoellner, Marvin Hartwig and Wenger, Christian and Henkel, Karsten and Flege, Jan Ingo}, title = {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}, series = {Advanced materials interfaces}, volume = {12}, journal = {Advanced materials interfaces}, number = {5}, publisher = {Wiley}, address = {Weinheim}, issn = {2196-7350}, doi = {10.1002/admi.202400537}, pages = {1 -- 13}, abstract = {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.}, language = {en} } @misc{FerbelVeronesiMentesetal., author = {Ferbel, Letizia and Veronesi, Stefano and Mentes, Tevfik Onur and Buß, Lars and Rossi, Antonio and Mishra, Neeraj and Coletti, Camilla and Flege, Jan Ingo and Locatelli, Andrea and Heun, Stefan}, title = {Rubidium intercalation in epitaxial monolayer graphene}, series = {Nanoscale}, volume = {17}, journal = {Nanoscale}, number = {19}, publisher = {Royal Society of Chemistry (RSC)}, address = {London}, issn = {2040-3364}, doi = {10.1039/D5NR00417A}, pages = {12465 -- 12472}, abstract = {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.}, language = {en} } @misc{TschammerBussPożarowskaetal., author = {Tschammer, Rudi and Buß, Lars and Pożarowska, Emilia and Morales, Carlos and Senanayake, Sanjaya D. and Prieto, Mauricio J. and Tănase, Liviu C. and de Souza Caldas, Lucas and Tiwari, Aarti and Schmidt, Thomas and Ni{\~n}o, Miguel A. and Foerster, Michael and Falta, Jens and Flege, Jan Ingo}, title = {High-temperature growth of CeOx on Au(111) and behavior under reducing and oxidizing conditions}, series = {The journal of physical chemistry C}, volume = {129}, journal = {The journal of physical chemistry C}, number = {7}, publisher = {American Chemical Society (ACS)}, address = {Washington, DC}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.4c08072}, pages = {3583 -- 3594}, abstract = {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.}, language = {en} } @misc{KediaDasKotetal., author = {Kedia, Mayank and Das, Chittaranjan and Kot, Malgorzata and Yalcinkaya, Yenal and Zuo, Weiwei and Tabah Tanko, Kenedy and Matvija, Peter and Ezquer, Mikel and Cornago, I{\~n}aki and Hempel, Wolfram and Kauffmann, Florian and Plate, Paul and Lira-Cantu, Monica and Weber, Stefan A.L. and Saliba, Michael}, title = {Mitigating the amorphization of perovskite layers by using atomic layer deposition of alumina}, series = {Energy \& environmental science}, volume = {18}, journal = {Energy \& environmental science}, number = {11}, publisher = {Royal Society of Chemistry (RSC)}, address = {London}, issn = {1754-5692}, doi = {10.1039/D4EE05703A}, pages = {5250 -- 5263}, abstract = {Atomic layer deposition of aluminum oxide (ALD-Al2O3) layers has recently been studied for stabilizing perovskite solar cells (PSCs) against environmental stressors, such as humidity and oxygen. In addition, the ALD-Al2O3 layer acts as a protective barrier, mitigating pernicious halide ion migration from the perovskite towards the hole transport interface. However, its effectiveness in preventing the infiltration of ions and additives from the hole-transport layer into perovskites remains insufficiently understood. Herein, we demonstrate the deposition of a compact ultrathin (∼0.75 nm) ALD-Al2O3 layer that conformally coats the morphology of a triple-cation perovskite layer. This promotes an effective contact of the hole transporter layer on top of the perovskite, thereby improving the charge carrier collection between these two layers. Upon systematically investigating the layer-by-layer structure of the PSC, we discovered that ALD-Al2O3 also acts as a diffusion barrier for the degraded species from the adjacent transport layer into the perovskite. In addition to these protective considerations, ALD-Al2O3 impedes the transition of crystalline perovskites to an undesired amorphous phase. Consequently, the dual functionality (i.e., enhanced contact and diffusion barrier) of the ALD-Al2O3 protection enhanced the device performance from 19.1\% to 20.5\%, while retaining 98\% of its initial performance compared to <10\% for pristine devices after 1500 h of outdoor testing under ambient conditions. Finally, this study deepens our understanding of the mechanism of ALD-Al2O3 as a two-way diffusion barrier, highlighting the multifaceted role of buffer layers in interfacial engineering for the long-term stability of PSCs.}, language = {en} } @misc{BussSulaimanSanchezBarquillaetal., author = {Buß, Lars and Sulaiman, Cathy and S{\´a}nchez-Barquilla, Raquel and Cojocariu, Iulia and Szpytma, Marcin and Mente{\c{s}}, Tevfik Onur and Locatelli, Andrea and Falta, Jens and Flege, Jan Ingo}, title = {Rise and fall of 1T-TaS₂ : epitaxial growth of monolayer TaS₂ on Au(111)}, series = {Physical review materials}, volume = {9}, journal = {Physical review materials}, number = {7}, publisher = {American Physical Society (APS)}, address = {College Park, MD}, issn = {2475-9953}, doi = {10.1103/1bxg-yvw2}, pages = {1 -- 9}, abstract = {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.}, language = {en} } @misc{MoralesTschammerPożarowskaetal., author = {Morales, Carlos and Tschammer, Rudi and Pożarowska, Emilia and Kosto, Julia and Villar-Garcia, Ignacio J. and P{\´e}rez-Dieste, Virginia and Favaro, Marco and Starr, David E. and Kapuścik, Paulina and Mazur, Michał and Wojcieszak, Damian and Domaradzki, Jarosław and Alvarado, Carlos and Wenger, Christian and Henkel, Karsten and Flege, Jan Ingo}, title = {Hydrogen sensing via heterolytic H₂ activation at room temperature by atomic layer deposited ceria}, series = {ChemSusChem : chemistry, sustainability, energy, materials}, volume = {18}, journal = {ChemSusChem : chemistry, sustainability, energy, materials}, number = {13}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1864-5631}, doi = {10.1002/cssc.202402342}, pages = {1 -- 13}, abstract = {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.}, language = {en} } @misc{TschammerGuttmannMoralesetal., author = {Tschammer, R. and Guttmann, D. and Morales, C. and Henkel, K. and Flege, J. I. and Tiebe, C.}, title = {P31 - In-situ and operando measurements for the characterization of next-generation sensor materials}, series = {SMSI 2025 Conference - Sensor and Measurement Science International : proceedings}, journal = {SMSI 2025 Conference - Sensor and Measurement Science International : proceedings}, publisher = {AMA Service GmbH}, address = {Wunstorf}, isbn = {978-3-910600-06-5}, doi = {10.5162/SMSI2025/P31}, pages = {292 -- 293}, abstract = {In this contribution, we highlight the use of in-situ X-ray photoelectron spectroscopy and operando spectroscopic ellipsometry for the characterization of ultra-thin (<20nm) atomic layer deposited layers for the use in next-generation miniaturized sensor devices. By targeting tin oxide layers, we show how we can use these techniques to gain insights into material composition, thickness, and optical properties, thus paving the way for unraveling the correlations between material properties and sensing performance.}, language = {en} } @misc{MolendaTobolaMilewskaetal., author = {Molenda, J. and Tobola, J. and Milewska, A. and Budziak, A. and Zając, W. and Wolczko, M. and Dziedzic-Kocurek, K. and Nowak, M. and Kałahurska, K. and Imam, N. and Henkel, K. and Flege, J.I. and Zschech, E.}, title = {Unique properties of the electronic structure of alluaudite sodium iron sulfate cathode material and the impact on its electrochemical performance}, series = {Acta materialia}, volume = {301}, journal = {Acta materialia}, publisher = {Elsevier BV}, address = {Amsterdam}, issn = {1359-6454}, doi = {10.1016/j.actamat.2025.121582}, pages = {1 -- 17}, abstract = {This work presents the study of the electronic structure of alluaudite-type Na2.5Fe1.75(SO4)3 and its impact on electrochemical performance as a cathode material for sodium-ion batteries (SIBs). Density functional theory calculations using the KKR-CPA method (Korringa-Kohn-Rostoker combined with the coherent potential approximation) revealed the diverse electrochemical activity of sodium ions occupying different sites within the alluaudite framework. Notably, an unprecedented contribution of sodium atoms to the overall electronic density of states near the Fermi level (dominated by Fe-d and O-p states) was observed - a feature not detected in layered transition metal oxide cathodes. A high-purity Na2.5Fe1.75(SO4)3 cathode material was synthesized via an optimized solid-state route. Using a multi-technique approach - including X-ray diffraction, scanning electron microscopy, M{\"o}ssbauer spectroscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and Raman spectroscopy - the evolution of the electronic structure and electrochemical behavior during sodium (de)intercalation was comprehensively characterized, providing deep insights into the sodium storage mechanism. Operando and in situ X-ray diffraction further tracked structural changes during cycling, showing that the material undergoes a reversible amorphization at deep sodium extraction. Electrochemical tests demonstrated stable cycling with minimal capacity fade (only 2.5 \% after 300 cycles at C/2), highlighting the high structural integrity and promise of this optimized cathode material.}, 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} } @misc{WeichbrodtDomaradzkiObstarczyketal., author = {Weichbrodt, Wiktoria and Domaradzki, Jaroslaw and Obstarczyk, Agata and Kot, Malgorzata and Flege, Jan Ingo and Mazur, Michal}, title = {Influence of thermal modification on the gasochromic properties of WO₃ thin films fabricated by electron beam evaporation}, series = {Applied optics}, volume = {65}, journal = {Applied optics}, number = {5}, publisher = {Optica Publishing Group}, address = {Washington, DC}, issn = {1559-128X}, doi = {10.1364/AO.574918}, pages = {A58 -- A67}, abstract = {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.}, language = {en} }