@misc{FlegeHoeckerSadowskietal., author = {Flege, Jan Ingo and H{\"o}cker, Jan and Sadowski, Jerzy T. and Senanayake, Sanjaya D. and Falta, Jens}, title = {Nucleation, morphology, and structure of sub-nm thin ceria islands on Rh(111)}, series = {Surface and Interface Analysis}, volume = {51}, journal = {Surface and Interface Analysis}, number = {1}, issn = {0142-2421}, doi = {10.1002/sia.6567}, pages = {110 -- 114}, abstract = {The early stages of ceria growth on Rh(111) at high temperature have been investi-gated by low-energy electron microscopy and photoemission electron microscopy. Ceria was deposited by reactive Ce deposition at substrate temperatures between 700°C and 900°C in an oxygen ambient of 5 × 10-7 Torr. At 700°C, we observe a high nucleation density of 100-nm-sized islands. With elevated temperature, the average island size increases, and the nucleation density decreases. Triangularly shaped islands nucleate preferentially at step edges, with seemingly abrupt interfaces between Ce and Rh. At 900°C, the island edges are still straight, but during growth the islands lose their triangular form. Instead, growth along the substrate step edges becomes favorable, leading to a maze-like morphology. Atomic force microscopy reveals islands of 0.3 to 0.6-nm height, consistent with ceria islands formed by one or two trilayers (O―Ce―O) of ceria. Moreover, the second layer of the islands is also triangularly shaped, with lateral dimensions of 50 nm and similar step heights. IV-LEEM analysis leads to the conclusion that the rhodium surface is covered by a layer of reduced cerium oxide, which is partially overgrown by smaller islands of CeO2.}, language = {en} } @misc{JugovacMenteşGenuzioetal., author = {Jugovac, Matteo and Mente{\c{s}}, Tevfik Onur and Genuzio, Francesca and Lachnitt, Jan and Feyer, Vitaliy and Flege, Jan Ingo and Locatelli, Andrea}, title = {Sensitivity to crystal stacking in low-energy electron microscopy}, series = {Applied Surface Science}, volume = {566}, journal = {Applied Surface Science}, issn = {0169-4332}, doi = {10.1016/j.apsusc.2021.150656}, abstract = {In this work we demonstrate the general characteristics of hcp and fcc stacking in low-energy electron reflectivity for transition metal surfaces, by following the restacking during homoepitaxial growth in real-time. For this purpose, the stacking of a model system, single-crystalline Ag islands during layer-by-layer growth at high temperature on O/W(110), is chosen. Multiple scattering calculations are used to model the relation between electron reflectivity and the crystal geometry. The changes in the electron reflectivity are shown to derive from the changes in the stacking sequence of the topmost surface layers. The results allow to distinguish between the hcp and fcc crystalline arrangements at a surface based on typical differences in the reflectivity curves, making the Ag results relevant for a variety of materials with hexagonal surface geometry. In particular, the multiplet structure within the first Bragg peak in the very low electron energy regime is identified with the fcc structure and thus it can be utilized as a fingerprint to determine the stacking sequence.}, language = {en} } @misc{LachnittDasMenonetal., author = {Lachnitt, Jan and Das, Shuvankar and Menon, Krishnakumar S. R. and Mandal, Suman and Flege, Jan Ingo}, title = {The dependence of structure on thickness of NiO(100) films on Ag(100) studied by IV-LEED}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {Ultrathin NiO films have prospective applications especially in heterogeneous catalysis, microelectronics, and spintronics and are thus an object of active research. The Ag(100) surface is the usual support for these films, as its cubic lattice parameter is only 2.2 \% smaller than that of NiO, which enables pseudomorphic growth at very low thicknesses. We have studied the NiO(100) surface for three thicknesses of the oxide: 2 ML on Ag(100), 20 ML on the same substrate, and a bulk single crystal. We have used intensity-voltage low-energy electron diffraction (IV-LEED) in combination with X-ray photoelectron spectroscopy (XPS) and density-functional theory (DFT) calculations. We focus on differences among the three thicknesses, mainly in terms of lattice parameters and surface defects, and our study deepens existing knowledge of the growth of ultrathin NiO films. The IV-LEED calculations have been carried out using the AQuaLEED package, which will also be briefly presented.}, language = {en} } @misc{EwertSchmidtFlegeetal., author = {Ewert, Moritz and Schmidt, Thomas and Flege, Jan Ingo and Heidmann, Inga and Grzela, Tomasz and Klesse, Wolfgang Matthias and F{\"o}rster, M. and Aballe, Lucia and Schr{\"o}der, Thomas}, title = {Morphology and Chemical Composition of Co Germanide islands on Ge(001): in-situ nanoscale insights into contact formation for Ge-based device technology}, series = {Nanotechnology}, volume = {27}, journal = {Nanotechnology}, number = {32}, issn = {1361-6528}, pages = {325705}, language = {en} } @misc{BignardiLizzitBanaetal., author = {Bignardi, Luca and Lizzit, Daniel and Bana, Harsh and Travaglia, Elisabetta and Lacovig, Paolo and Sanders, Charlotte E. and Dendzik, Maciej and Michiardi, Matteo and Bianchi, Marco and Ewert, Moritz and Buß, Lars and Falta, Jens and Flege, Jan Ingo and Baraldi, Alessandro and Larciprete, Rosanna and Hofmann, Philip and Lizzit, Silvano}, title = {Growth and structure of singly oriented single-layer tungsten disulfide on Au(111)}, series = {Physical Review Materials}, volume = {3}, journal = {Physical Review Materials}, number = {1}, issn = {2475-9953}, doi = {10.1103/PhysRevMaterials.3.014003}, pages = {8}, abstract = {A singly oriented, single layer of tungsten disulfide (WS2) was epitaxially grown on Au(111) and characterized at the nanoscale by combining photoelectron spectroscopy, photoelectron diffraction, and low-energy electron microscopy. Fast x-ray photoelectron spectroscopy revealed that the growth of a single crystalline orientation is triggered by choosing a low W evaporation rate and performing the process with a high temperature of the substrate. Information about the single orientation of the layer was obtained by acquiring x-ray photoelectron diffraction patterns, revealing a 1H polytype for the WS2 layer and, moreover, determining the structural parameters and registry with the substrate. The distribution, size, and orientation of the WS2 layer were further ascertained by low-energy electron microscopy.}, language = {en} } @misc{LuchesGasperiSauerbreyetal., author = {Luches, Paola and Gasperi, Gabriele and Sauerbrey, Marc and Valeri, Sergio and Falta, Jens and Flege, Jan Ingo}, title = {Dynamics of the Interaction Between Ceria and Platinum During Redox Processes}, series = {Frontiers in Chemistry}, volume = {7}, journal = {Frontiers in Chemistry}, issn = {2296-2646}, doi = {10.3389/fchem.2019.00057}, pages = {11}, abstract = {The work is focused on understanding the dynamics of the processes which occur at the interface between ceria and platinum during redox processes, by investigating an inverse catalytic model system made of ceria epitaxial islands and ultrathin films supported on Pt(111). The evolution of the morphology, structure and electronic properties is analyzed in real-time during reduction and oxidation, using low-energy electron microscopy and spatially resolved low-energy electron diffraction. The reduction is induced using different methods, namely thermal treatments in ultra-high vacuum and in H2 as well as deposition of Ce on the oxide surface, while re-oxidation is obtained by exposure to oxygen at elevated temperature. The use of two different epitaxial systems, continuous films and nanostructures, allows determining the influence of platinum proximity on the stabilization of the specific phases observed. The factors that limit the reversibility of the observed modifications with the different oxidation treatments are also discussed. The obtained results highlight important aspects of the cerium oxide/Pt interaction that are relevant for a complete understanding of the behavior of Pt/CeO2 catalysts.}, language = {en} } @misc{SchmidtAhrensFlegeetal., author = {Schmidt, Thomas and Ahrens, Christian and Flege, Jan Ingo and Jaye, Cherno and Fischer, Daniel A. and Falta, Jens}, title = {Growth of Epitaxial 3,4,9,10-Perylene Tetracarboxylic Dianhydride on Bi-Terminated Silicon}, series = {Journal of Physical Chemistry C}, volume = {123}, journal = {Journal of Physical Chemistry C}, number = {12}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.8b10396}, pages = {7097 -- 7109}, abstract = {The epitaxial quality of thin films crucially depends on their interaction with the substrate. Up to now, Ag-terminated Si(111) has been employed as the model substrate for the growth of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on semiconductors. In this study, we will show that Bi termination results in PTCDA films of superior epitaxial quality. We have studied the growth of PTCDA on bismuth-passivated Si(111) in detail by means of spot profile analysis of low-energy electron diffraction (SPA-LEED), X-ray photoemission spectroscopy (XPS), near-edge X-ray absorptionfine structure (NEXAFS), and scanning tunneling microscopy (STM). The XPS results reveal the presence of intact PTCDA molecules on the surface upon adsorption. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. STM shows a very smooth growth front of the PTCDA film, preserving the step structure of the substrate. High-resolution SPA-LEED data demonstrate the presence of a multidomain surface with a rich variety of PTCDA surface structures, which were identified to be most prominently herring-bone polytypes. However, in the monolayer range, quadratic brick-wall structures and a nearly square-like structure as well as a perylene-like structure have also been found. Despite the simultaneous presence of multiple domains, the individual domains show excellent lateral ordering, with larger domain sizes as compared to the case of Ag-terminated Si(111).}, language = {en} } @misc{BussFaltaFlege, author = {Buß, Lars and Falta, Jens and Flege, Jan Ingo}, title = {Intercalation dynamics of sulfur underneath graphene on Ru(0001)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {315 -- 315}, abstract = {It is known that the binding of epitaxially grown graphene to the substrate has a detrimental effect on its electronic properties. This is especially true for the strong binding to various transition metals and particularly ruthenium. However, via intercalation the interlayer coupling can be lifted and its unique electronic properties can be restored. Therefore, we have investigated the interaction of sulfur with single-layer graphene grown on Ru(0001) via surface segregation and CVD of ethylene under UHV conditions with in situ low-energy electron microscopy (LEEM) and micro-diffraction (µLEED). At elevated temperature and under dimethyl disulfide background pressure, we observe that sulfur intercalates through the open edges of the graphene islands. Prolonged exposure to sulfur induces wrinkling of the graphene islands, consistent with substantial relief of tensile strain after successful sulfur insertion underneath the graphene. It can be seen that the intercalation dynamics are both dependent on the temperature during intercalation and the preparation method of the graphene sheets. Furthermore, darkfield imaging and µLEED of the intercalated graphene reveal a graphene induced improved ordering of sulfur underneath.}, language = {en} } @misc{EwertBussMorasetal., author = {Ewert, Moritz and Buß, Lars and Moras, Paolo and Falta, Jens and Flege, Jan Ingo}, title = {Unraveling the growth mechanism of single-domain molybdenum disulfide on Au(111)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {329 -- 329}, abstract = {As a transition metal dichalcogenide single-layer molybdenum disulfide (MoS2) is a heavily investigated system. Due to its direct band gap, the electronic properties of single-layer MoS2 have been subject to several surface science methods. A well-known model system is MoS2 on Au(111). We present in situ low-energy electron microscopy (LEEM) and micro-diffraction (LEED) observations of MoS2 growth on Au(111) at elevated temperature using two distinctly different deposition rates. Our investigations reveal similar but different expansion mechanisms of the MoS2 islands changing a balanced distribution of the two mirror domains towards a single domain distribution. Structural characterization by I(V)-LEEM and investigations of the electronic bandstructure using angle-resolved photoelectron spectroscopy both confirm single-layer nature of the MoS2 islands. We could identify step pushing of the growing MoS2 being responsible for this phenomena.}, language = {en} } @misc{MahmoodinezhadPożarowskaHenkeletal., author = {Mahmoodinezhad, Ali and Pożarowska, Emilia and Henkel, Karsten and Schmeißer, Dieter and Flege, Jan Ingo}, title = {Depth profiling of PEALD-AlN films based on Al2p XPS peak decomposition}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {157 -- 157}, abstract = {AlN has remarkable properties (wide band gap, low electrical and thermal conductivity, high dielectric constant, piezoelectricity) and is attractive for (opto)electronic and sensor applications. However, high oxygen content within nitride films is always a critical issue due to the thermodynamically favorable oxidation against nitridation resulting in deteriorated materials properties. In order to clarify whether the oxidation is a surface-limited or a bulk process elemental depth profiling is essential. In this work XPS in combination with Ar+ sputtering is applied to carry out depth profiling of AlN films prepared by plasma-enhanced atomic layer deposition using different parameters (plasma source, power and pulse duration). Particularly, the Al2p core levels are analyzed where the signals are decomposed into four components, representing weaker contributions of pure AlN and aluminum oxide phases as well as stronger signals of mixed oxygen-rich and nitrogen-rich phases. After sputtering (providing access to the deeper part of the film) the pure AlN phase content increases while the pure aluminum oxide content stays relatively constant. These issues are discussed with regard to the preparation parameters employed and accompanying XRD and electrical measurements.}, language = {en} } @misc{FischerFoersterAballeetal., author = {Fischer, Simon and Foerster, Michael and Aballe, Lucia and Vonk, Vedran and Falta, Jens and Krisponeit, Jon-Olaf and Flege, Jan Ingo}, title = {Vanadium dioxide thin films on (100)- and (110)-oriented ruthenium dioxide islands}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {330 -- 330}, abstract = {Bulk vanadium dioxide exhibits a metal-insulator transition at 68 ∘C. Because this change in resistivity is linked to a structural transition, the transition temperature can be tuned by epitaxial strain. Due to the high lattice mismatch of RuO2 with VO2, this substrate promises a highly shifted transition temperature, which enables new types of switching devices, smart coatings, and sensors. It is prepared by oxidizing a Ru(0001) crystal, providing micron-sized, (110)- as well as (100)-oriented RuO2 islands simultaneously. One of the challenges in preparing VO2 is to reliably meet the desired stoichiometry. We investigated the vanadium oxidation state using local XAS and XPS measurements taken at a synchrotron-based XPEEM instrument, revealing that VO2 stoichiometry is maintained throughout the entire film. This was found not only on RuO2(100) and RuO2(110), but also on the bare ruthenium substrate. The VO2 film thickness was determined to 3.6 nm by ex situ XRR measurements. Financial support from the DFG is acknowledged.}, language = {en} } @misc{KrisponeitFischerFlegeetal., author = {Krisponeit, Jon-Olaf and Fischer, Simon and Flege, Jan Ingo and Falta, Jens}, title = {Surface reconstructions on VO2(110)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {330 -- 330}, abstract = {Vanadium dioxide features a pronounced metal-insulator transition at 340 K. For thin films, this transition is adjustable via substrate-induced strain, making VO2 highly attractive for oxide electronic applications like switching devices and sensors. In addition, VO2 is considered for smart coatings and as catalytic material. For such applicational efforts, a thorough understanding of the relevant surfaces is an important prerequisite. Among the low-indexed surfaces orientations of VO2, the (110) surface is energetically favorable. We have prepared VO2(110) thin films by means of reactive molecular beam epitaxy on two different substrate types: TiO2(110) single crystals as well as on RuO2(110) islands grown on Ru(0001) crystals. The film stoichiometry was confirmed via XPS, while the surface structure was investigated by µLEED. For both substrate types diffraction patterns of three distinct symmetries have been observed in dependence on temperature. The corresponding surface reconstructions will be discussed in terms of surface oxygen content. Financial support from the DFG and the Institutional Strategy of the University of Bremen, funded by the German Excellence Initiative, is acknowledged.}, language = {en} } @misc{PleinesGenuzioMenteşetal., author = {Pleines, Linus and Genuzio, Francesca and Mente{\c{s}}, Tevfik Onur and Locatelli, Andrea and Falta, Jens and Flege, Jan Ingo}, title = {Reduction and reoxidation of (111) and (100) oriented cerium oxide islands on Cu(111)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {330 -- 330}, abstract = {The conversion of CO2 to methanol is an important process of high industrial potential. It could help solving the greenhouse gas problem (CO2) and further provide methanol which can be used as a synthetic fuel. In order to lower the activation energy barrier of the reactions a catalyst is necessary. We studied the interaction of H2 and CO2 with ultrathin cerium oxide islands on a Cu(111) substrate observed by low-energy electron mircoscopy (LEEM), x-ray absorption spectroscopy (XAS) and resonant photoelectron spectroscopy (RPES). From earlier studies the orientation of the CeOx is known to be decisive for the catalytic activity. In our experiments CeOx(100) is directly compared to CeOx(111) via a side-by-side growth, so that the same conditions prevail during the experiment. At low temperatures no reduction of the CeOx(111) and CeOx(100) were observed. However, at a higher temperature the reduction was activated. Furthermore, we show that the exposure to CO2 leads to the formation of partially reoxidized CeOx and saturation below CeO2 with progressive CO2 exposure.}, language = {en} } @misc{NaumannReckGargourietal., author = {Naumann, Franziska and Reck, Johanna and Gargouri, Hassan and Gruska, Bernd and Bl{\"u}mich, Adrian and Mahmoodinezhad, Ali and Janowitz, Christoph and Henkel, Karsten and Flege, Jan Ingo}, title = {In situ real-time and ex situ spectroscopic analysis of Al₂O₃ films prepared by plasma enhanced atomic layer deposition}, series = {Journal of Vacuum Science and Technology B}, volume = {38}, journal = {Journal of Vacuum Science and Technology B}, number = {1}, issn = {0734-211X}, doi = {10.1116/1.5122797}, pages = {10}, abstract = {In situ real-time ellipsometry (irtE) with a very high time resolution of 24 ms was applied to monitor the inductively coupled plasma enhanced atomic layer deposition (ALD) process of Al₂O₃ thin films to precisely resolve each step of the ALD process and its complete cycle. The influence of plasma power, plasma pulse duration, and deposition temperature on the film growth characteristics was investigated. Ex situ ellipsometry [UV-VIS-NIR-SE (ultraviolet-visible-nearinfrared-spectroscopic ellipsometry) and IR-SE (infrared spectroscopic ellipsometry)] and x-ray photoelectron spectroscopy revealed the bulk properties (thickness, refractive index, chemical composition, and carbon incorporation) of the films, which together with the in situ results are compared to those of the films prepared by thermal ALD (T-ALD). The ICPEALD (inductively coupled plasma enhanced ALD) films were deposited at substrate temperatures between 80 and 250 °C and the role of plasma power (50-300 W) and its pulse duration (1-20 s) was investigated at 250 °C. The reference T-ALD layers were prepared at 200 °C. The ICPEALD process of Al₂O₃ shows an increased growth rate, and the produced films exhibit higher carbon contaminations than the T-ALD Al₂O₃ films. Plasma pulse times of up to 15 s further increase the content of carbon and CH species; at the same time, the refractive index decreases. The optical properties of ICPEALD deposited Al₂O₃ films are comparable with those of the T-ALD films for low plasma power and short plasma pulse durations. For the ICPEALD films, UV absorption is found and it is dependent on the deposition parameters. irtE resolves process effects that correlate with the bulk properties of Al₂O₃, such as impurities and oxygen deficiencies.}, language = {en} } @misc{MahmoodinezhadJanowitzNaumannetal., author = {Mahmoodinezhad, Ali and Janowitz, Christoph and Naumann, Franziska and Plate, Paul and Gargouri, Hassan and Henkel, Karsten and Schmeißer, Dieter and Flege, Jan Ingo}, title = {Low-temperature growth of gallium oxide thin films by plasma-enhanced atomic layer deposition}, series = {Journal of Vacuum Science and Technology A}, volume = {38}, journal = {Journal of Vacuum Science and Technology A}, number = {2}, issn = {0734-2101}, doi = {10.1116/1.5134800}, abstract = {Gallium oxide (Ga2O3) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) applying a capacitively coupled plasma source where trimethylgallium (TMGa) as the gallium precursor and oxygen (O2) plasma were used in a substrate temperature (Ts)in the range of 80-200 °C. TMGa exhibits high vapor pressure and therefore facilitates deposition at lower substrate temperatures. The Ga2O3 films were characterized by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), and capacitance-voltage (C-V) measurements. The SE data show linear thickness evolution with a growth rate of ∼0.66 {\AA} per cycle and inhomogeneity of ≤2\% for all samples. The refractive index of the Ga2O3 thin films is 1.86 ± 0.01 (at 632.8 nm) and independent of temperature, whereas the bandgap slightly decreases from 4.68 eV at Ts of 80 °C to 4.57 eV at 200 °C. XPS analysis revealed ideal stoichiometric gallium to oxygen ratios of 2:3 for the Ga2O3 layers with the lowest carbon contribution of ∼10\% for the sample prepared at 150 °C. The permittivity of the layers is 9.7 ± 0.2 (at 10 kHz). In addition, fixed and mobile oxide charge densities of 2-4E12 and 1-2E12 cm-2, respectively, were observed in the C-V characteristics. Moreover, the Ga2O3 films show breakdown fields in the range of 2.2-2.7 MV/cm. Excellent optical and electrical material properties are maintained even at low substrate temperatures as low as 80 °C. Hence, the TMGa/O2 PEALD process is suitable for electronic and optoelectronic applications where low-temperature growth is required.}, language = {en} } @misc{MishraFortiFabbrietal., author = {Mishra, Neeraj and Forti, Stiven and Fabbri, Filippo and Martini, Leonardo and McAleese, Clifford and Conran, Ben R. and Whelan, Patrick R. and Shivayogimath, Abhay and Jessen, Bjarke S. and Buß, Lars and Falta, Jens and Aliaj, Ilirjan and Roddaro, Stefano and Flege, Jan Ingo and B{\o}ggild, Peter and Teo, Kenneth B. K. and Coletti, Camilla}, title = {Wafer-Scale Synthesis of Graphene on Sapphire: Toward Fab-Compatible Graphene}, series = {Small}, volume = {15}, journal = {Small}, number = {50}, issn = {1613-6810}, doi = {10.1002/smll.201904906}, pages = {8}, abstract = {The adoption of graphene in electronics, optoelectronics, and photonics is hindered by the difficulty in obtaining high-quality material on technologically relevant substrates, over wafer-scale sizes, and with metal contamination levels compatible with industrial requirements. To date, the direct growth of graphene on insulating substrates has proved to be challenging, usually requiring metal-catalysts or yielding defective graphene. In this work, a metal-free approach implemented in commercially available reactors to obtain high-quality monolayer graphene on c-plane sapphire substrates via chemical vapor deposition is demonstrated. Low energy electron diffraction, low energy electron microscopy, and scanning tunneling microscopy measurements identify the Al-rich reconstruction of sapphire to be crucial for obtaining epitaxial graphene. Raman spectroscopy and electrical transport measurements reveal high-quality graphene with mobilities consistently above 2000 cm2 V-1 s-1. The process is scaled up to 4 and 6 in. wafers sizes and metal contamination levels are retrieved to be within the limits for back-end-of-line integration. The growth process introduced here establishes a method for the synthesis of wafer-scale graphene films on a technologically viable basis.}, language = {en} } @misc{KotVorokhtaWangetal., author = {Kot, Małgorzata and Vorokhta, Mykhailo and Wang, Zhiping and Snaith, Henry J. and Schmeißer, Dieter and Flege, Jan Ingo}, title = {Thermal stability of CH3NH3PbIxCl3-x versus [HC(NH2)2]0.83Cs0.17PbI2.7Br0.3 perovskite films by X-ray photoelectron spectroscopy}, series = {Applied Surface Science}, volume = {513}, journal = {Applied Surface Science}, issn = {0169-4332}, doi = {10.1016/j.apsusc.2020.145596}, pages = {7}, abstract = {The thermal stability of CH3NH3PbIxCl3-x and [HC(NH2)2]0.83Cs0.17PbI2.7Br0.3 perovskite films was studied in-situ by X-ray photoelectron spectroscopy. It was found that below 85 °C both of them are relatively stable. After annealing above 85 °C, we observe a clear perovskite surface decomposition, i.e., a release of organic cations and creation of "metallic lead". The mixed cation lead mixed halide perovskite, however, decomposes at a much lower rate. For both perovskite films, the metallic to the total lead ratio changes with the same rate for the same annealing temperatures. The release of A-site cations from the ABX3 crystal structure of perovskite and/or creation of "metallic lead" causes also a small shift of the valence band maximum towards the Fermi level. The release of [HC(NH2)2]± or Cs± is not as significant as the release of CH3NH3±; therefore, it may explain why [HC(NH2)2]0.83Cs0.17PbI2.7Br0.3 solar cells are thermally more stable. Therefore, as the stability of CH3NH3PbIxCl3-x is same as the stability of [HC(NH2)2]0.83Cs0.17PbI2.7Br0.3 below 85 °C, there must be more severe degradation pathways that are currently underappreciated on the solar cell level.}, language = {en} } @misc{LaroussiKotFlegeetal., author = {Laroussi, Arwa and Kot, Małgorzata and Flege, Jan Ingo and Raouafi, Noureddine and Mirsky, Vladimir M.}, title = {Self-assembled monolayers from symmetrical di-thiols: Preparation, characterization and application for the assembly of electrochemically active films}, series = {Applied Surface Science}, volume = {513}, journal = {Applied Surface Science}, issn = {0169-4332}, doi = {10.1016/j.apsusc.2020.145827}, abstract = {1,3-dimercaptopropan-2-ol, a symmetrical di-thiol, has been synthesized and applied as a new type of anchor molecule to prepare a self-assembled monolayer (SAM) on the gold surface. The formed monolayers were studied by cyclic voltammetry, impedance spectroscopy, X-ray photoelectron spectroscopy, kinetic capacitance, and contact angle measurements. The SAM structure depends on the adsorption conditions. A short incubation time of the electrode at high concentration of this di-thiol leads to the predominating binding through one thiol group of the adsorbate to the gold surface, while a long incubation at low concentration leads to the predominating binding by both thiol groups. A comparative study of the desorption and replacement of SAMs indicates a strong stability increase when the SAM molecules bond gold surface by two bonds mainly. This monolayer was used to immobilize electrochemically active p-benzoquinone moiety. The surface concentration of p-benzoquinone obtained from cyclic voltammetry is 2.5 ± 0.2 × 10-10 mol·cm-2 which corresponds to the functionalization of 65 ± 5\% of SAM molecules. The obtained highly stable SAM with redox-active terminal group can be applied for different tasks of chemical sensing and biosensing. As an example, an application of this system for electrocatalytical oxidation of dihydronicotinamide adenosine dinucleotide (NADH) was tested.}, language = {en} } @misc{BoehnPenschkeLietal., author = {Boehn, Bernhard von and Penschke, Christopher and Li, Xiaoke and Paier, Joachim and Sauer, Joachim and Krisponeit, Jon-Olaf and Flege, Jan Ingo and Falta, Jens and Marchetto, Helder and Franz, Torsten and Lilienkamp, Gerhard and Imbihl, Ronald}, title = {Reaction dynamics of metal/oxide catalysts: Methanol oxidation at vanadium oxide films on Rh(111) from UHV to 10-2 mbar}, series = {Journal of Catalysis}, volume = {385}, journal = {Journal of Catalysis}, issn = {0021-9517}, doi = {10.1016/j.jcat.2020.03.016}, pages = {255 -- 264}, abstract = {Recent advances in in situ microscopy allow to follow the reaction dynamics during a catalytic surface reaction from ultra-high vacuum to 0.1 mbar, thus bridging a large part of the pressure gap. Submonolayer vanadium oxide films on Rh(111) have been studied during catalytic methanol oxidation in situ with spatially resolving imaging techniques. At 10-6 to 10-4 mbar VOx condenses into macroscopic circular islands that exhibit a substructure, consisting of a reduced island core and an oxidized outer ring. This substructure arises due to an oxygen gradient inside the VOx islands, which results in different coexisting 2D-phases of VOx on Rh(111). This substructure is also responsible for a "breathing-like" oscillatory expansion and contraction that the islands undergo under stationary conditions. Using density functional theory, the 2D-phase diagram of VOx on Rh(111) has been computed. The oscillatory behavior can be understood as a periodic phase transition between two 2D phases of VOx. With a newly developed near ambient pressure - low-energy electron microscope, it was shown that VOx islands disintegrate at 10-2 mbar, resulting in turbulent dynamics.}, language = {en} } @misc{FischerKrisponeitFoersteretal., author = {Fischer, Simon and Krisponeit, Jon-Olaf and Foerster, Michael and Aballe, Lucia and Falta, Jens and Flege, Jan Ingo}, title = {Massively Strained VO2 Thin Film Growth on RuO2}, series = {Crystal Growth \& Design}, volume = {20}, journal = {Crystal Growth \& Design}, number = {4}, issn = {1528-7483}, doi = {10.1021/acs.cgd.0c00120}, pages = {2734 -- 2741}, abstract = {Strain engineering vanadium dioxide thin films is one way to alter this material's characteristic first order transition from semiconductor to metal. In this study, we extend the exploitable strain regime by utilizing the very large lattice mismatch of 8.78\% occurring in the VO2/RuO2 system along the c axis of the rutile structure. We have grown VO2 thin films on single-domain RuO2 islands of two distinct surface orientations by atomic oxygen-supported reactive molecular beam epitaxy (MBE). These films were examined by spatially resolved photoelectron and X-ray absorption spectroscopy, confirming the correct stoichiometry. Low energy electron diffraction then reveals the VO2 films grow indeed fully strained on RuO2(110), exhibiting a previously unreported (2 × 2) reconstruction. On TiO2(110) substrates, we reproduce this reconstruction and attribute it to an oxygen-rich termination caused by the high oxygen chemical potential. On RuO2(100), on the contrary, the films grow fully relaxed. Hence, the presented growth method allows for simultaneous access to a remarkable strain window ranging from bulk-like structures to massively strained regions.}, language = {en} } @misc{GomezAlvarezMoralesMendezetal., author = {Gomez-Alvarez, Miguel Angel and Morales, Carlos and M{\´e}ndez, Javier and Campo, Adolfo del and Urbanos, Fernando J. and D{\´i}az, Aar{\´o}n and Res{\´e}ndiz, Luis and Flege, Jan Ingo and Granados, Daniel and Soriano, Leonardo}, title = {A Comparative Study of the ZnO Growth on Graphene and Graphene Oxide: The Role of the Initial Oxidation State of Carbon}, series = {C - Journal of Carbon Research}, volume = {6}, journal = {C - Journal of Carbon Research}, number = {2}, issn = {2311-5629}, doi = {10.3390/c6020041}, pages = {18}, abstract = {The role of the oxidation state of carbon on the early stages of growth of metal oxides was studied for the particular case of ZnO deposition on graphene and graphene oxide on SiO2 (G/SiO2 and GO/SiO2, respectively) substrates. The growth was carried out by thermal evaporation of metallic Zn under an oxygen atmosphere at room temperature. This technique permits quasi-equilibrium conditions during the oxide growth, allowing the characterization of the fundamental interaction between ZnO and the graphene-based substrates. Although in both cases ZnO follows a Volmer-Weber growth mode controlled by nucleation at defects, the details are different. In the case of the GO/SiO2 substrate, the nucleation process acts as a bottleneck, limiting the coverage of the complete surface and allowing the growth of very large ZnO structures in comparison to G/SiO2. Moreover, by studying the Zn-LMM Auger spectra, it is shown how the initial nature of the substrate influences the composition of the ZnO deposit during the very early stages of growth in terms of Zn/O atomic ratio. These results are compared to those previously reported regarding ZnO growth on graphite and graphene on Cu (G/Cu). This comparison allows us to understand the role of different characteristics of graphene-based substrates in terms of number of defects, oxidation state, graphene support substrate and number of graphene layers.}, language = {en} }