@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} } @misc{FoersterSchenkZollneretal., author = {F{\"o}rster, Stefan and Schenk, Sebastian and Zollner, Eva Maria and Krahn, Oliver and Chiang, Cheng-Tien and Schumann, Florian O. and Bayat, Alireza and Schindler, Karl-Michael and Trautmann, Martin and Hammer, Ren{\´e} and Meinel, Klaus and Adeagbo, Waheed A. and Hergert, Wolfram and Flege, Jan Ingo and Falta, Jens and Ellguth, Martin and Tusche, Christian and DeBoissieu, Marc and Muntwiler, Matthias and Gerber, Thomas and Widdra, Wolf}, title = {Quasicrystals and their Approximants in 2D Ternary Oxides}, series = {Physia Status Solidi B}, volume = {257}, journal = {Physia Status Solidi B}, number = {7}, issn = {0370-1972}, doi = {10.1002/pssb.201900624}, pages = {11}, abstract = {2D oxide quasicrystals (OQCs) are recently discovered aperiodic, but well-ordered oxide interfaces. In this topical review, an introduction to these new thin-film systems is given. The concept of quasicrystals and their approximants is explained for BaTiO3 and SrTiO3 derived OQCs and related periodic structures in these 2D oxides. In situ microscopy unravels the high-temperature formation process of OQCs on Pt(111). The dodecagonal structure is discussed regarding tiling statistics and tiling decoration based on the results of atomically resolved scanning tunneling microscopy and various diffraction techniques. In addition, angle-resolved ultraviolet photoemission spectroscopy and X-ray photoelectron spectroscopy results prove a metallic character of the 2D oxide.}, language = {en} } @misc{KotKegelmannKoebleretal., author = {Kot, Małgorzata and Kegelmann, Lukas and K{\"o}bler, Hans and Vorokhta, Mykhailo and Escudero, Carlos and K{\´u}š, Peter and Šm{\´i}d, Břetislav and Tallarida, Massimo and Albrecht, Steve and Abate, Antonio and Matol{\´i}nov{\´a}, Iva and Schmeißer, Dieter and Flege, Jan Ingo}, title = {In situ Near-Ambient Pressure X-ray Photoelectron Spectroscopy Reveals the Influence of Photon Flux and Water on the Stability of Halide Perovskite}, series = {ChemSusChem}, volume = {13}, journal = {ChemSusChem}, number = {21}, issn = {1864-5631}, doi = {10.1002/cssc.202001527}, pages = {5722 -- 5730}, abstract = {For several years, scientists have been trying to understand the mechanisms that reduce the long-term stability of perovskite solar cells. In this work, we examined the effect of water and photon flux on the stability of CH3NH3PbI3 perovskite films and solar cells using in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), field emission scanning electron microscopy (FESEM), and current density-voltage (J-V) characterization. The used amount of water vapor (up to 1 mbar) had a negligible impact on the perovskite film. The higher the photon flux, the more prominent were the changes in the NAP-XPS and FESEM data; also, a faster decline in power conversion efficiency (PCE) and a more substantial hysteresis in the J-V characteristics were observed. Based on our results, it can be concluded that the PCE decrease originates from the creation of Frenkel pair defects in the perovskite film under illumination. The stronger the illumination, the higher the number of Frenkel defects, leading to a faster PCE decline and more substantial hysteresis in the J-V sweeps.}, language = {en} } @misc{KrisponeitFischerEsseretal., author = {Krisponeit, Jon-Olaf and Fischer, Simon and Esser, Sven and Moshnyaga, Vasily and Schmidt, Thomas and Piper, Louis F. J. and Flege, Jan Ingo and Falta, Jens}, title = {The morphology of VO2/TiO2(001): terraces, facets, and cracks}, series = {Scientific Reports}, volume = {10}, journal = {Scientific Reports}, issn = {2045-2322}, doi = {10.1038/s41598-020-78584-9}, pages = {8}, abstract = {Vanadium dioxide (VO2) features a pronounced, thermally-driven metal-to-insulator transition at 340 K. Employing epitaxial stress on rutile TiO2(001) substrates, the transition can be tuned to occur close to room temperature. Striving for applications in oxide-electronic devices, the lateral homogeneity of such samples must be considered as an important prerequisite for efforts towards miniaturization. Moreover, the preparation of smooth surfaces is crucial for vertically stacked devices and, hence, the design of functional interfaces. Here, the surface morphology of VO2/TiO2(001) films was analyzed by low-energy electron microscopy and diffraction as well as scanning probe microscopy. The formation of large terraces could be achieved under temperature-induced annealing, but also the occurrence of facets was observed and characterized. Further, we report on quasi-periodic arrangements of crack defects which evolve due to thermal stress under cooling. While these might impair some applicational endeavours, they may also present crystallographically well-oriented nano-templates of bulk-like properties for advanced approaches.}, language = {en} } @misc{MoralesLeinenFloresetal., author = {Morales, Carlos and Leinen, Dietmar and Flores, Eduardo and Mu{\~n}oz-Cortes, Esmeralda and Leardini, Fabrice and Ares, Jos{\´e} Ram{\´o}n and Flege, Jan Ingo and Soriano, Leonardo and Ferrer, Isabel J. and Sanchez, Carlos}, title = {Imaging the Kirkendall effect in pyrite (FeS2) thin films: Cross-sectional microstructure and chemical features}, series = {Acta Materialia}, volume = {205}, journal = {Acta Materialia}, issn = {1359-6454}, doi = {10.1016/j.actamat.2020.116582}, pages = {12}, abstract = {This investigation provides novel data on the structure and chemical composition of pyrite thin films and new hints concerning their formation mechanism. From TEM-HAADF data, it has been found that the films are composed of two different layers: one is very compact and the other one is quite porous with many voids separating a few groups of grains. This porous layer is always in direct contact with the substrate, and its thickness is quite similar to that of the original Fe film. The average size of pyrite grains is equal in both layers, what suggests that the same process is responsible for their formation. Concentration profiles of sulfur, iron and some impurities (mainly sodium and oxygen from the glass substrate) through both layers are given in this work, and thus chemical inhomogeneities of the films are proved by the obtained stoichiometric ratios (S/Fe). Moreover, Na from sodalime glass substrates mainly accumulates at the pyrite grain boundaries and barely dopes them. The obtained results support the hypothesis that the iron sulfuration process essentially induces the diffusion of iron atoms, what leads to the porous layer formation as a manifestation of the Kirkendall Effect. Therefore, it seems that the same mechanisms that operate in the synthesis of surface hollow structures at the nanoscale are also active in the formation of pyrite thin films ranging from several tens to hundreds of nanometers.}, language = {en} } @misc{EwertBussGenuzioetal., author = {Ewert, Moritz and Buß, Lars and Genuzio, Francesca and Mente{\c{s}}, Tevfik Onur and Locatelli, Andrea and Falta, Jens and Flege, Jan Ingo}, title = {Transitions from single-layer MoS2 to bilayer growth: A LEEM study}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, address = {Bad Honnef}, abstract = {Molybdenum disulfide (MoS2) is well-known to change from an indirect to a direct semiconductor as a single layer. We present insights from in-situ low-energy electron microscopy (LEEM) on the extended growth of MoS2 on the Au(111) surface at elevated temperatures of 720°C. Our continuous growth method leads to the formation of micron-sized single-layer MoS2 islands. The single-domain character of these islands is confirmed by employing dark-field imaging and micro-diffraction (LEED). This also reveals the distribution of 90:10 of the two expected MoS2 mirror domains on Au(111). Selected-area angle-resolved photoelectron spectroscopy (ARPES) measurements of these mirror domains underline the threefold symmetry of the two mirror domains and indicate the presence of MoS2 bilayer. Using X-ray photoemission electron microscopy (XPEEM) and intensity-voltage LEEM (I(V))-LEEM we identify the bilayer nucleation areas at nearly full surface coverage and propose a model pathway for their formation.}, language = {en} } @misc{JanowitzMahmoodinezhadNaumannetal., author = {Janowitz, Christoph and Mahmoodinezhad, Ali and Naumann, Franziska and Plate, Paul and Henkel, Karsten and Flege, Jan Ingo}, title = {XPS and UPS investigation of an ALD prepared Al2O3/ZnO heterojunction}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {The band alignment of two large band gap oxides was studied by a combination of XPS and UPS using consecutive sputter steps to unravel the electronic structure and elemental composition of each layer and the interface region. An Al2O3/ZnO heterointerface (10 nm Al2O3 on 59 nm ZnO) was grown on top of a Si single crystal substrate by consecutive thermal and plasma-assisted atomic layer deposition (ALD) respectively. The valence band maximum of Al2O3 was found to be 1.1 eV below that of ZnO, the conduction band minimum 2.3 eV above, resulting in a type-I staggered heterojunction. A reduction of ZnO to elemental Zn in the interface region was detected by the apparent shoulder of the Zn 2p and 2s core levels and by the Zn LMM Auger. This suggests an ALD interface formation mechanism different from previous models identified for other heterointerfaces.}, language = {en} } @misc{MahmoodinezhadMoralesNaumannetal., author = {Mahmoodinezhad, Ali and Morales, Carlos and Naumann, Franziska and Plate, Paul and Meyer, Robert and Janowitz, Christoph and Henkel, Karsten and Kot, Małgorzata and Flege, Jan Ingo}, title = {Low-temperature atomic layer deposition of indium oxide thin films using trimethylindium and oxygen plasma}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {Indium oxide thin films were deposited on Si (100) by plasma-enhanced atomic layer deposition (PEALD) using trimethylindium (TMIn) and oxygen plasma (O2) in a low-temperature range of 80 to 200 °C. The In2O3 layers were characterized by in-situ spectroscopic ellipsometry (SE), ex-situ X-ray photoelectron spectroscopy (XPS) and electrical measurements. The SE data show a growth rate of 0.56 {\AA}/cycle within the ALD window (100 to 150 °C) with a thickness inhomogeneity of ≤1.2\%. In addition, the highest refractive index is 2.07 (at 632.8 nm) for the layer grown at 150 °C, and the films exhibit indirect and direct band gaps of 2.8±0.1 eV and 3.3±0.2 eV, respectively. XPS characterization indicates no carbon incorporation and a temperature-dependent off-stoichiometry of the layers. The chemical analysis of the In 3d and O 1s core levels confirms the formation of In-O bonds and suggests the additional presence of hydroxyl groups and defects. With increasing temperature, the contribution of OH groups and defects decreases whereas that of In-O bonds increases. Notably, higher growth temperatures result in an indium rich phase within the layers.}, language = {en} } @misc{MoralesMahmoodinezhadSchubertetal., author = {Morales, Carlos and Mahmoodinezhad, Ali and Schubert, Andreas Markus and Wenger, Christian and Henkel, Karsten and Flege, Jan Ingo}, title = {Functional ultra-thin oxide films deposited by atomic layer deposition on structured substrates}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {In the last decades, atomic layer deposition (ALD) has gained prominence in the materials and surface science communities owing to its high potential for integration as a scalable process in microelectronics. ALD's largest strengths are its well-controlled layer-by-layer deposition and growth conformity on 3D structures. Yet, the ALD technique is also well known to lead to amorphous and defective, non-stoichiometric thin films, resulting in modified materials properties that may even preferentially be used in certain applications. To study these issues, we have developed an in-situ ALD reactor attached to an X-ray photoelectron spectroscopy (XPS) system, capable of switching between both pump and flow-type operation. This novel tool allows to cover the entire range of compounds and recipes used in ALD, thus clarifying the role of such defects at different deposition stages, growth conditions and film/substrate interfaces. To exemplify these sorts of studies, we show the deposition of Al2O3 5-10 nm films on nanostructured Si, and their use as substrates for functional CeOx ALD deposits.}, language = {en} } @misc{PozarowskaMoralesFlege, author = {Pozarowska, Emilia and Morales, Carlos and Flege, Jan Ingo}, title = {Growth of samarium thin films and subsequent oxidation on polycrystalline copper}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {The growth of samaria thin films on copper sheets has been chemically studied by in situ X-ray photoelectron spectroscopy (XPS). The early stages of growth (0.1-14 ML) were studied by consecutive evaporations of Sm by chemical vapor deposition followed by XPS measurements. Subsequently, samaria films of different thicknesses, namely 0.1, 1, and 14 ML, were oxidized at room temperature (RT). The evolution of the sample morphology was determined through inelastic peak shape analysis (IPSA) using the QUASES software as an indirect method to study the relationship between Sm oxidation state and its surface arrangement. Our results show that samarium grows as 2D islands up to 2ML, which is followed by 3D growth. Chemical analysis indicates that at low coverages (<0.5ML) Sm is already oxidized, leading to the appearance of Sm3+ as the only oxidation state. The increase in the intensity of the O1s peak with time and the absence of spectral changes in the Cu2p and LMM Auger (substrate) indicate that the transformation is mainly due to adventitious oxidation of the layer. With further deposition at RT the metallic state Sm0 appears at higher coverages, which is readily postoxidized by subsequent exposure to molecular oxygen, leading to complete oxidation. No intermediate oxidation states (Sm2+) were observed, in contrast to the reported prevalence of Sm2+ on single crystal surfaces during the early stages of growth.}, language = {en} } @misc{AngrickHenriksenMutzkeetal., author = {Angrick, Christoph and Henriksen, Annika and Mutzke, Nicole and Reimann, Andre and Ewert, Moritz and Buß, Lars and Falta, Jens and Flege, Jan Ingo and Donath, Markus}, title = {Spin-polarized VLEED from single-layer MoS2/Au(111): Investigation of spin-orbit-induced effects}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {The influence of spin-orbit interaction on low-energy electron reflection from single-layer MoS2 on Au(111) was studied by VLEED (very-low-energy electron diffraction) [1,2]. The spin-dependent electron reflection was investigated for a wide range of electron incidence angles and kinetic energies. Since the adlayer coverage is about 30\%, we studied the Au(111) substrate and a MoS2 bulk sample for comparison. This approach enabled us to distinguish between adlayer and substrate signals. For MoS2/Au(111), we detected a spin asymmetry of the reflected intensities, which shows a characteristic feature with alternating sign in the energy region of a VLEED fine structure [1]. The Au(111) substrate, in contrast, shows qualitatively different spin-asymmetry features, partially with reversed sign compared with MoS2/Au(111). The results of bulk MoS2 confirm that the characteristic feature in the single-layer data originates from MoS2. The influence of the adlayer-substrate interaction on the results will be discussed. [1] Burgbacher et al., Phys. Rev. B 87, 195411 (2013) [2] Angrick et al., J. Phys.: Condens. Matter 33, 115001 (2020)}, language = {en} } @misc{FischerFlegeFoersteretal., author = {Fischer, Simon and Flege, Jan Ingo and Foerster, Michael and Aballe, Lucia and Falta, Jens and Krisponeit, Jon-Olaf}, title = {Strongly Strained VO2 Thin Film Growth}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {The semiconductor-metal transition temperature of VO2 strongly shifts as a result of strain applied along the rutile c axis, making it interesting for various switching applications, as smart coatings and as sensors. In the past, this has been demonstrated, for instance, through the application of epitaxial strain on TiO2 substrates. We extend this tailoring approach by utilizing the much larger lattice mismatch of 8.78 \% occurring in the VO2/RuO2 system for orientations where the c axis lies in-plane. Depositing vanadium oxide by atomic oxygen-supported reactive MBE on an oxidized Ru(0001) template, we have grown VO2 thin films on single domain RuO2 islands with distinct orientations. Locally resolved electron spectroscopy was used to ascertain the correct stoichiometry of the grown VO2 films on all template island types. Low energy electron diffraction reveals the VO2 films to grow indeed fully strained on RuO2(110) but fully relaxed on RuO2(100). Hence, the presented template allows for simultaneous access to a remarkable strain window ranging from bulk-like structures to regions of massive tensile strain.}, language = {en} } @misc{PleinesBussMenteşetal., author = {Pleines, Linus and Buß, Lars and Mente{\c{s}}, Tevfik Onur and Genuzio, Francesca and Locatelli, Andrea and Flege, Jan Ingo and Falta, Jens}, title = {High temperature reduction and reoxidation of cerium oxide on Cu(111)}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {Cerium oxide is of great interest due to its technological importance in various electronic, optical and catalytic applications. The inverse model catalyst cerium oxide on Cu(111) shows a high activity for the production of methanol. Oxygen vacancies, e.g. in form of reduced ceria, are necessary for the production of methanol from CO2 and H2. The reduction of ceria may be achieved by exposure to H2 at elevated temperatures. We studied the interaction of H2 and CO2 with cerium oxide islands on a Cu(111) substrate with low-energy electron microscopy (LEEM) and x-ray absorption spectroscopy (XAS). From earlier studies, the orientation of the cerium oxide is known to be decisive for the catalytic activity. In our experiments, the impact of both orientations are directly compared via growth of (100) and (111) cerium oxide islands side by side. At temperatures around 550 °C, exposure to H2 leads to partial reduction and exposure to CO2 leads to complete reoxidation of the cerium oxide. The (100) and (111) orientations show different reduction and reoxidation behaviors.}, language = {en} } @misc{BussFaltaEwertetal., author = {Buß, Lars and Falta, Jens and Ewert, Moritz and Shao, Bin and Wehling, Tim Oliver and Flege, Jan Ingo}, title = {The role of two-dimensional pressure in sulfur intercalation underneath graphene on ruthenium}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {Micrometer-sized single-layer graphene can epitaxially be grown on transition-metal substrates with excellent crystalline quality. However, due to strong binding these substrates have a detrimental influence on the intrinsic properties of the graphene. By lifting the interlayer coupling, e. g., via intercalating foreign atoms, its unique electronic properties can be restored. We have investigated the intercalation of sulfur underneath graphene on Ru(0001) with low-energy electron microscopy (LEEM) and micro-diffraction (µLEED). We find that sulfur deposited at elevated temperatures enters through the edge of the island, leading to wrinkle formation in the decoupled graphene. Interestingly, the presence of the graphene limits the possible S/Ru(0001) reconstructions that may form underneath, preventing less dense reconstructions like the p(2× 2) and (√3× √3) reconstructions. Based on density functional theory calculations, these findings are explained by a 2D pressure exerted by the overlying graphene, which results from the strong graphene-substrate interaction, only rendering the denser reconstructions of the S/Ru phase diagram energetically favorable.}, language = {en} } @misc{EwertBussBraudetal., author = {Ewert, Moritz and Buß, Lars and Braud, Nicolas and Kundu, Asish K. and Sheverdyaeva, Polina M. and Moras, Paolo and Genuzio, Francesca and Mente{\c{s}}, Tevfik Onur and Locatelli, Andrea and Falta, Jens and Flege, Jan Ingo}, title = {The Transition From MoS2 Single-Layer to Bilayer Growth on the Au(111) Surface}, series = {Frontiers in Physics}, volume = {9}, journal = {Frontiers in Physics}, issn = {2296-424X}, doi = {10.3389/fphy.2021.654845}, abstract = {The transition from single-layer to bilayer growth of molybdenum disulfide on the Au(111) surface is investigated by in situ low-energy electron and photoemission microscopy. By mapping the film morphology with nanometer resolution, we show that a MoS2 bilayer forms at the boundaries of single-layer single-domain MoS2 islands and next to merging islands whereas bilayer nucleation at the island centers is found to be suppressed, which may be related to the usage of dimethyl disulfide as sulfur precursor in the growth process. This approach, which may open up the possibility of growing continuous films over large areas while delaying bilayer formation, is likely transferable to other transition metal dichalcogenide model systems.}, language = {en} } @misc{MoralesLeinenCampoetal., author = {Morales, Carlos and Leinen, Dietmar and Campo, Adolfo del and Ares, Jos{\´e} Ram{\´o}n and S{\´a}nchez, Carlos and Flege, Jan Ingo and Guti{\´e}rrez, Alejandro and Prieto, Pilar and Soriano, Leonardo}, title = {Growth and characterization of ZnO thin films at low temperatures: from room temperature to -120 °C}, series = {Journal of Alloys and Compounds}, volume = {884}, journal = {Journal of Alloys and Compounds}, issn = {0925-8388}, doi = {10.1016/j.jallcom.2021.161056}, abstract = {ZnO thin films have been grown by e-beam evaporation in the range from room temperature to - 120 °C on two types of substrates, Al2O3 (0001) and Si (100). Although the ZnO/Al2O3 system has been thoroughly characterized, including optical and electrical techniques, the morphological, structural and chemical properties show no significant differences between both substrates. Thus, the general features of the ZnO growth mode at low temperature can be generalized. The relatively low growth temperatures reduce the diffusion of atoms at the surface, which leads to morphological and chemical changes. As the temperature decreases, the growth mode changes from a van der Drift model to a gradual bilayer system composed of an interfacial layer in contact with the substrate and a second columnar-based layer. This second well-ordered film disappears for the lowest temperatures while a Zn-rich interface in contact with the substrate emerges. Precisely from this interface, Zn-rich whiskers develop under the ZnO film and cause the loss of adhesion at temperatures below - 100 °C. These extreme temperatures also affect the crystal size, lattice strain, and total amount of oxygen vacancies. The behavior of the optical and electrical properties in terms of band gap, transparency, electrical resistivity, and Seebeck coefficient is discussed in the light of structural and chemical characterization. Samples grown at 0 °C exhibit an enhanced transmittance compared to those grown at room temperature while preserving similar electrical resistivity values and natural n-type doping. These results open a promising route to enhance ZnO films properties below the typical high temperature window.}, 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 = {Engineering Proceedings}, volume = {6}, journal = {Engineering Proceedings}, number = {1}, issn = {2673-4591}, doi = {10.3390/I3S2021Dresden-10112}, 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 a 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 surfaces 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{MahmoodinezhadMoralesNaumannetal., author = {Mahmoodinezhad, Ali and Morales, Carlos and Naumann, Franziska and Plate, Paul and Meyer, Robert and Janowitz, Christoph and Henkel, Karsten and Kot, Małgorzata and Z{\"o}llner, Marvin Hartwig and Wenger, Christian and Flege, Jan Ingo}, title = {Low-temperature atomic layer deposition of indium oxide thin films using trimethylindium and oxygen plasma}, series = {Journal of Vacuum Science and Technology A}, volume = {39}, journal = {Journal of Vacuum Science and Technology A}, number = {6}, issn = {0734-2101}, doi = {10.1116/6.0001375}, abstract = {Indium oxide (InxOy) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) using trimethylindium and oxygen plasma in a low-temperature range of 80-200 °C. The optical properties, chemical composition, crystallographic structure, and electrical characteristics of these layers were investigated by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), as well as current-voltage and capacitance-voltage measurements. The SE results yielded a nearly constant growth rate of 0.56 {\AA} per cycle and a thickness inhomogeneity of ≤1.2\% across 4-in. substrates in the temperature range of 100-150 °C. The refractive index (at 632.8 nm) was found to be 2.07 for the films deposited at 150 °C. The PEALD-InxOy layers exhibit a direct (3.3 ± 0.2 eV) and an indirect (2.8 ± 0.1 eV) bandgap with an uptrend for both with increasing substrate temperature. Based on XPS characterization, all InxOy samples are free of carbon impurities and show a temperature-dependent off-stoichiometry indicating oxygen vacancies. XRD diffraction patterns demonstrate an onset of crystallization at 150 °C. Consistent with the optical, XPS, and XRD data, the films deposited at ≥150 °C possess higher electrical conductivity. Our findings prove that a low-temperature PEALD process of InxOy is feasible and promising for a high-quality thin-film deposition without chemical impurities on thermally fragile substrates.}, language = {en} } @misc{DasZiaMortanetal., author = {Das, Chittaranjan and Zia, Waqas and Mortan, Claudiu and Hussain, Navid and Saliba, Michael and Flege, Jan Ingo and Kot, Małgorzata}, title = {Top-Down Approach to Study Chemical and Electronic Properties of Perovskite Solar Cells: Sputtered Depth Profiling Versus Tapered Cross-Sectional Photoelectron Spectroscopies}, series = {Solar RRL}, volume = {5}, journal = {Solar RRL}, number = {10}, issn = {2367-198X}, doi = {10.1002/solr.202100298}, abstract = {A study of the chemical and electronic properties of various layers across perovskite solar cell (PSC) stacks is challenging. Depth-profiling photoemission spectroscopy can be used to study the surface, interface, and bulk properties of different layers in PSCs, which influence the overall performance of these devices. Herein, sputter depth profiling (SDP) and tapered cross-sectional (TCS) photoelectron spectroscopies (PESs) are used to study highly efficient mixed halide PSCs. It is found that the most used SDP-PES technique degrades the organic and deforms the inorganic materials during sputtering of the PSCs while the TCS-PES method is less destructive and can determine the chemical and electronic properties of all layers precisely. The SDP-PES dissociates the chemical bonding in the spiro-MeOTAD and perovskite layer and reduces the TiO2, which causes the chemical analysis to be unreliable. The TCS-PES revealed a band bending only at the spiro-MeOTAD/perovskite interface of about 0.7 eV. Both the TCS and SDP-PES show that the perovskite layer is inhomogeneous and has a higher amount of bromine at the perovskite/TiO2 interface.}, language = {en} } @misc{GonzalezJuarezMoralesFlegeetal., author = {Gonzalez-Juarez, Maria de Lourdes and Morales, Carlos and Flege, Jan Ingo and Flores, Eduardo and Martin-Gonzalez, Marisol and Nandhakumar, Iris and Bradshaw, Darren}, title = {Tunable Carrier Type of a Semiconducting 2D Metal-Organic Framework Cu3(HHTP)2}, series = {ACS Applied Materials \& Interfaces}, volume = {14}, journal = {ACS Applied Materials \& Interfaces}, number = {10}, issn = {1944-8244}, doi = {10.1021/acsami.2c00089}, pages = {12404 -- 12411}, abstract = {In this work, a switch from n-type to p-type conductivity in electrodeposited Cu3(2,3,6,7,10,11-hexahydroxytriphenylene)2 [Cu3(HHTP2)] has been observed, which is most likely due to oxygen molecular doping. The synthesis of electrically conductive 2D metal-organic frameworks (MOFs) has been achieved through the introduction of highly conjugated organic linkers coordinated to their constituent metal-ion centers. However, the porous structure and unsaturated metal sites in MOFs make them susceptible to ambient adsorbates, which can affect their charge transport properties. This phenomenon has been experimentally investigated by GIXRD, Hall effect and Seebeck measurements, and X-ray photoelectron spectroscopy.}, language = {en} } @misc{JanowitzMahmoodinezhadKotetal., author = {Janowitz, Christoph and Mahmoodinezhad, Ali and Kot, Małgorzata and Morales, Carlos and Naumann, Franziska and Plate, Paul and Z{\"o}llner, Marvin Hartwig and B{\"a}rwolf, Florian and Stolarek, David and Wenger, Christian and Henkel, Karsten and Flege, Jan Ingo}, title = {Toward controlling the Al2O3/ZnO interface properties by in situ ALD preparation}, series = {Dalton Transactions}, volume = {51}, journal = {Dalton Transactions}, issn = {1477-9234}, doi = {10.1039/D1DT04008A}, pages = {9291 -- 9301}, abstract = {An Al2O3/ZnO heterojunction was grown on a Si single crystal substrate by subsequent thermal and plasma-assisted atomic layer deposition (ALD) in situ. The band offsets of the heterointerface were then studied by consecutive removal of the layers by argon sputtering, followed by in situ X-ray photoelectron spectroscopy. The valence band maximum and conduction band minimum of Al2O3 are found to be 1.1 eV below and 2.3 eV above those of ZnO, resulting in a type-I staggered heterojunction. An apparent reduction of ZnO to elemental Zn in the interface region was detected in the Zn 2p core level and Zn L3MM Auger spectra. This suggests an interface formation different from previous models. The reduction of ZnO to Zn in the interface region accompanied by the creation of oxygen vacancies in ZnO results in an upward band bending at the interface. Therefore, this study suggests that interfacial properties such as the band bending as well as the valence and conduction band offsets should be in situ controllable to a certain extent by careful selection of the process parameters.}, language = {en} }