@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} }