@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{MoralesPascualLeinenetal., author = {Morales, Carlos and Pascual, Antonio 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 = {Reaction Mechanism and Kinetic Model of Fe Thin Film Transformation into Monosulfides (FeS): First Step of the Fe Films Sulfuration Process into Pyrite}, series = {The Journal of Physical Chemistry C}, volume = {126}, journal = {The Journal of Physical Chemistry C}, number = {32}, issn = {1932-7455}, doi = {10.1021/acs.jpcc.2c02060}, pages = {13870 -- 13883}, abstract = {The sulfuration of metallic iron layers into pyrite (FeS2) is preceded by an initial stage characterized by the iron transformation into monosulfide, which acts as a precursor of the disulfide. This work presents a comprehensive reaction and kinetic model of the sulfuration reaction of metallic iron thin films into monosulfides when a molecular sulfur (S2) atmosphere is used. By slowing down the sulfuration reaction, we have been able to follow in situ the evolution of the transport properties (electrical resistivity and Seebeck coefficient) of the Fe films during their sulfuration reaction to monosulfides. We show that two different stages characterize this initial sulfuration: (1) the transformation of Fe into hexagonal pyrrhotite (Fe → Fe1-xSH) and (2) a partial crystallographic transformation of this hexagonal pyrrhotite into orthorhombic pyrrhotite (Fe1-xSH → Fe1-xSO). A two-step process can explain the pyrrhotite hexagonal phase formation, being first controlled by the surface adsorption of S2 on the external sample interface (S2/pyrrhotite) and second by the diffusion of Fe atoms through the formed pyrrhotite layer. By deducing the corresponding kinetic equations in terms of the experimental parameters (S2 partial pressure and thicknesses of the layers of present species), we can explain the evolution of the electrical resistance and Seebeck coefficient of the original Fe film during its transformation into monosulfide. At the same time, the appearance of the Kirkendall effect during the monosulfide phase formation is experimental and formally justified. The comprehensive description of this first stage of the complete sulfuration process of the Fe film into pyrite provides a layout to deeply discuss the influence of these intermedium phases on the final iron disulfide film characteristics and the appearance of potential film defects related to the experimental growth conditions.}, language = {en} } @misc{MoralesMahmoodinezhadTschammeretal., author = {Morales, Carlos and Mahmoodinezhad, Ali and Tschammer, Rudi and Kosto, Yuliia and Alvarado Chavarin, Carlos and Schubert, Markus Andreas and Wenger, Christian and Henkel, Karsten and Flege, Jan Ingo}, title = {Combination of Multiple Operando and In-Situ Characterization Techniques in a Single Cluster System for Atomic Layer Deposition: Unraveling the Early Stages of Growth of Ultrathin Al2O3 Films on Metallic Ti Substrates}, series = {Inorganics}, volume = {11}, journal = {Inorganics}, number = {12}, issn = {2304-6740}, doi = {10.3390/inorganics11120477}, abstract = {This work presents a new ultra-high vacuum cluster tool to perform systematic studies of the early growth stages of atomic layer deposited (ALD) ultrathin films following a surface science approach. By combining operando (spectroscopic ellipsometry and quadrupole mass spectrometry) and in situ (X-ray photoelectron spectroscopy) characterization techniques, the cluster allows us to follow the evolution of substrate, film, and reaction intermediates as a function of the total number of ALD cycles, as well as perform a constant diagnosis and evaluation of the ALD process, detecting possible malfunctions that could affect the growth, reproducibility, and conclusions derived from data analysis. The homemade ALD reactor allows the use of multiple precursors and oxidants and its operation under pump and flow-type modes. To illustrate our experimental approach, we revisit the well-known thermal ALD growth of Al2O3 using trimethylaluminum and water. We deeply discuss the role of the metallic Ti thin film substrate at room temperature and 200 °C, highlighting the differences between the heterodeposition (<10 cycles) and the homodeposition (>10 cycles) growth regimes at both conditions. This surface science approach will benefit our understanding of the ALD process, paving the way toward more efficient and controllable manufacturing processes.}, 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{GonzalezCastanoMoralesNavarrodeMigueletal., author = {Gonzalez-Cast{\~a}no, Miriam and Morales, Carlos and Navarro de Miguel, Juan Carlos and Boelte, Jens H. and Klepel, Olaf and Flege, Jan Ingo and Arellano-Garc{\´i}a, Harvey}, title = {Are Ni/ and Ni5Fe1/biochar catalysts suitable for synthetic natural gas production? A comparison with γ-Al2O3 supported catalysts}, series = {Green Energy \& Environment}, volume = {8}, journal = {Green Energy \& Environment}, number = {3}, issn = {2468-0257}, doi = {10.1016/j.gee.2021.05.007}, pages = {744 -- 756}, abstract = {Among challenges implicit in the transition to the post-fossil fuel energetic model, the finite amount of resources available for the technological implementation of CO2 revalorizing processes arises as a central issue. The development of fully renewable catalytic systems with easier metal recovery strategies would promote the viability and sustainability of synthetic natural gas production circular routes. Taking Ni and NiFe catalysts supported over γ-Al2O3 oxide as reference materials, this work evaluates the potentiality of Ni and NiFe supported biochar catalysts for CO2 methanation. The development of competitive biochar catalysts was found dependent on the creation of basic sites on the catalyst surface. Displaying lower Turn Over Frequencies than Ni/Al catalyst, the absence of basic sites achieved over Ni/C catalyst was related to the depleted catalyst performances. For NiFe catalysts, analogous Ni5Fe1 alloys were constituted over both alumina and biochar supports. The highest specific activity of the catalyst series, exhibited by the NiFe/C catalyst, was related to the development of surface basic sites along with weaker NiFe-C interactions, which resulted in increased Ni0:NiO surface populations under reaction conditions. In summary, the present work establishes biochar supports as a competitive material to consider within the future low-carbon energetic panorama.}, 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{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{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{PozarowskaPleinesPrietoetal., author = {Pozarowska, Emilia and Pleines, Linus and Prieto, Mauricio J. and Tănase, Liviu Christian and Souza Caldas, Lucas de and Tiwari, Aarti and Schmidt, Thomas and Falta, Jens and Morales, Carlos and Flege, Jan Ingo}, title = {The relation between structure sensitivity and doping of ceria(111) vs. ceria(100)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {CeOx-Cu inverse catalysts have been shown to convert CO2 into valuable chemicals through catalytic hydrogenation. The catalytic activity may further be enhanced by alloying ceria with trivalent, catalytically active metals, such as Sm, promoting the formation of Ce3+ active sites. In this work, the structural and chemical properties of (111)- and (100)- oriented CeOx islands alloyed with samarium were explored by low-energy electron microscopy and X-ray photoemission electron microscopy. After Sm deposition on the as-grown CeOx islands, the near-surface region of (100)-oriented CeOx is reduced after exposure to H2 at 470 ∘C, whereas the deeper layers as well as the whole (111)-oriented islands retain the Ce4+ state. Subsequent reoxidation with O2 leads to the complete Ce4+ state recovery, suggesting the healing of oxygen vacancies. Additional annealing at 470 ∘C induces samarium diffusion into the ceria matrix. Yet, subsequent exposure to H2 reduces neither the (111)- nor the (100)-oriented CeSmOx islands, suggesting a quite unexpected stability of this system.}, language = {en} } @misc{MahmoodinezhadMoralesKotetal., author = {Mahmoodinezhad, Ali and Morales, Carlos and Kot, Małgorzata and Naumann, Franziska and Plate, Paul and Henkel, Karsten and Flege, Jan Ingo}, title = {A super-cycle approach to atomic layer deposition of indium-gallium-zinc oxide at low temperature}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {The continuing development of multifunctional devices needs novel multicomponent oxide layers, demanding a high control of both composition and thickness during their preparation. To this end, single metal oxides exhibiting high structural quality and conformity have successfully been grown by atomic layer deposition (ALD). However, the deposition of more complex compounds with specific optical and electrical properties is still challenging. In this work, we follow a bottom-up approach to design an ALD super-cycle to grow mixed indium-gallium-zinc oxide (IGZO) films with a controllable composition. For the formation of the individual indium, gallium, and zinc oxides, we found the use of plasma-enhanced ALD (PEALD) at 150 °C to be favorable when using the organometallic precursors trimethylindium, trimethylgallium, and diethylzinc together with oxygen plasma. The PEALD approach of IGZO films can particularly overcome a nucleation delay within the ZnO sub-cycle known from thermal ALD, achieving a higher growth per cycle and improving the quality and composition homogeneity of the films as shown by in-situ spectroscopic ellipsometry and ex-situ X-ray photoelectron spectroscopy.}, language = {en} } @misc{MoralesKostoTschammeretal., author = {Morales, Carlos and Kosto, Yuliia and Tschammer, Rudi and Henkel, Karsten and Flege, Jan Ingo}, title = {Reduction by H2 exposure at room temperature of ceria ultrathin films grown by atomic layer deposition}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {Atomic layer deposition (ALD) exhibits a high potential for integration as a scalable process in microelectronics, allowing well-controlled layer-by-layer deposition and conformal growth on 3D structures. Yet, the ALD technique is also well known to lead to amorphous and defective, non-stoichiometric films, potentially resulting in modified materials properties that, in the case of ultra-thin deposits, can also be affected by film/substrate interaction. Interestingly, initial in situ X-ray photoemission spectroscopy (XPS) measurements of ceria ALD-deposits on Al2O3/Si, sapphire, and SiO2 substrates confirm a Ce3+/Ce4+ mixture dependent on the substrate interaction, deposit thickness, and morphology. Using near-ambient pressure XPS, we have significantly reduced ultrathin (< 10 nm) ceria films grown by ALD by exposing them to different O2/H2 partial pressures at moderate temperatures (< 525K). Notably, the total amount of reduction to Ce3+ is found to depend on the deposit thickness and initial ceria/substrate interaction. Furthermore, the intrinsic defects related to the ALD method seem to play a critical role in the reversible reduction at room temperature.}, language = {en} } @misc{BakiAbdeldayemMoralesetal., author = {Baki, Aykut and Abdeldayem, Mohamed and Morales, Carlos and Flege, Jan Ingo and Klimm, Detlef and Bierwagen, Oliver and Schwarzkopf, Jutta}, title = {Potential of La-Doped SrTiO3 Thin Films Grown by Metal-Organic Vapor Phase Epitaxy for Thermoelectric Applications}, series = {Crystal Growth \& Design}, volume = {23}, journal = {Crystal Growth \& Design}, number = {4}, issn = {1528-7483}, doi = {10.1021/acs.cgd.2c01438}, pages = {2522 -- 2530}, abstract = {La-doped SrTiO3 thin films with high structural quality were homoepitaxially grown by the metal-organic vapor phase epitaxy (MOVPE) technique. Thermogravimetric characterization of the metal-organic precursors determines suitable flash evaporator temperatures for transferring the liquid source materials in the gas phase of the reactor chamber. An adjustment of the charge carrier concentration in the films, which is necessary for optimizing the thermoelectric power factor, was performed by introducing a defined amount of the metal-organic compound La(tmhd)3 and tetraglyme to the liquid precursor solution. X-ray diffraction and atomic force microscopy verified the occurrence of the pure perovskite phase exhibiting a high structural quality for all La concentrations. The electrical conductivity of the films obtained from Hall-effect measurements increases linearly with the La concentration in the gas phase, which is attributed to the incorporation of La3+ ions on the Sr2+ perovskite sites by substitution inferred from photoemission spectroscopy. The resulting structural defects were discussed concerning the formation of occasional Ruddlesden-Popper-like defects. The thermoelectric properties determined by Seebeck measurements demonstrate the high potential of SrTiO3 thin films grown by MOVPE for thermoelectric applications.}, language = {en} } @misc{AbdeldayemBakiMoralesetal., author = {Abdeldayem, Mohamed and Baki, Aykut and Morales, Carlos and Flege, Jan Ingo and Klimm, Detlef and Fiedler, Andreas and Bierwagen, Oliver and Schwarzkopf, Jutta}, title = {Potential of La-doped SrTiO3 thin films grown by metal-organic vapor phase epitaxy for thermoelectric applications}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {Conversion of waste heat energy into electrical energy by exploiting the thermoelectric effect in solids promises a great contribution to energy harvesting concepts. However, most thermoelectric materials use toxic Pb or Te. Recently, La-doped SrTiO3 has gained a lot of interest as a potential candidate for thermoelectric devices for its good thermoelectric properties, chemical and thermal stability. In this paper, we report the homoepitaxial growth of La-doped SrTiO3 thin films by metalorganic vapor phase epitaxy (MOVPE) technique, which works at high oxygen partial pressures and offers upscaling potential for industry. The adjustment of charge carrier concentration, necessary for thermoelectric power factor optimization, was performed by introducing a defined amount of the metal-organic precursor La(tmhd)3 and tetraglyme to the liquid precursor solution. X-ray diffraction and atomic force microscopy verified a pure perovskite phase with high structural quality. The electrical conductivity increases linearly with the La concentration in the gas phase, which is attributed to the substitution of La+3 ions on the Sr+2 sites inferred from photoemission spectroscopy.}, language = {en} } @misc{KostoMoralesDevietal., author = {Kosto, Yuliia and Morales, Carlos and Devi, Anjana and Henkel, Karsten and Flege, Jan Ingo}, title = {Activity of cerium oxide thin films prepared by atomic layer deposition using custom and commercial precursors}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {Atomic layer deposition (ALD) allows preparation of conformal coatings with possibility to control their thickness at the submonolayer level, making it a good tool for depositing active layers on 3D structures. Our group is working on cerium oxide-based materials for hydrogen detection, which is difficult at ambient conditions due to the low sensitivity and long response time of the sensors. The cerium oxide layers prepared by ALD contain a lot of defects and provide an opportunity to overcome these complications. Thickness and morphology of the oxide films play an important role in defining the Ce3+/Ce4+ ratio, as well as the interface with the used substrate. Here, we compare cerium oxide thin films deposited by ALD techniques on SiO2 and Al2O3 substrates. The results reveal that the interface to the substrate can considerably influence the reactivity of the cerium oxide toward hydrogen and oxygen. Preparation of the oxides using two different precursors (commercial Ce(thd)4 and custom Ce(dpdmg)3) has been demonstrated to affect the redox properties of the films, their reactivity, and the reversibility.}, language = {en} } @misc{TschammerBussMoralesetal., author = {Tschammer, Rudi and Buß, Lars and Morales, Carlos and Senanayake, Sanjaya D. and Falta, Jens and Flege, Jan Ingo}, title = {In situ characterization of cerium oxide on Au(111) under reducing and oxidizing conditions by low-energy electron microscopy}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {The development of novel catalysts for a variety of applications is a key challenge for modern catalysis. Inverse metal oxide catalysts consisting of oxide nanoparticles dispersed on a metal support have recently attracted much attention, showing higher activity and selectivity compared to traditional catalytic systems, harnessing synergistic effects attributed to the so-called metal-support interaction. To gain further insights, we deposited cerium oxide nanoparticles on Au(111) and studied this system by low-energy electron microscopy (LEEM) and low-energy electron diffraction (LEED). The prepared samples demonstrate a distinct correlation between the deposition temperature and the structural order of the nanoparticles. This has been expanded upon by exploring the changes induced by reduction with H2 and reoxidation with O2 or CO2, again exhibiting a connection between structural order and activity, while also showing the influence of the oxide-metal interaction on the stability of cerium oxide under reducing conditions.}, language = {en} } @misc{PożarowskaPleinesEwertetal., author = {Pożarowska, Emilia and Pleines, Linus and Ewert, Moritz and Prieto, Mauricio J. and Tănase, Liviu Christian and Souza Caldas, Lucas de and Tiwari, Aarti and Schmidt, Thomas and Falta, Jens and Krasovskii, Eugene and Morales, Carlos and Flege, Jan Ingo}, title = {Preparation and stability of the hexagonal phase of samarium oxide on Ru(0001)}, series = {Ultramicroscopy}, volume = {250}, journal = {Ultramicroscopy}, issn = {0304-3991}, doi = {10.1016/j.ultramic.2023.113755}, abstract = {We have used low-energy electron microscopy (LEEM), micro-illumination low-energy electron diffraction (µLEED) supported by ab initio calculations, and X-ray absorption spectroscopy (XAS) to investigate in-situ and in real-time the structural properties of Sm2O3 deposits grown on Ru(0001), a rare-earth metal oxide model catalyst. Our results show that samarium oxide grows in a hexagonal A-Sm2O3 phase on Ru(0001), exhibiting a (0001) oriented-top facet and (113) side facets. Upon annealing, a structural transition from the hexagonal to cubic phase occurs, in which the Sm cations exhibit the +3 oxidation state. The unexpected initial growth in the A-Sm2O3 hexagonal phase and its gradual transition to a mixture with cubic C-Sm2O3 showcases the complexity of the system and the critical role of the substrate in the stabilization of the hexagonal phase, which was previously reported only at high pressures and temperatures for bulk samaria. Besides, these results highlight the potential interactions that Sm could have with other catalytic compounds with respect to the here gathered insights on the preparation conditions and the specific compounds with which it interacts.}, language = {en} } @misc{GertigMoralesHenkeletal., author = {Gertig, Max and Morales, Carlos and Henkel, Karsten and Flege, Jan Ingo}, title = {In situ X-ray photoelectron spectroscopy study of atomic layer deposited ceria on SiO2: substrate influence on the reaction mechanism during the early stages of growth}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {Atomic layer deposition (ALD) is known to produce amorphous and defect-rich films in a layer-by-layer fashion, which can potentially give rise to unexpected material properties. In particular, ultrathin films (few monolayers) will show the highest complexity, as the substrate-material interaction will play a major role during deposition. Therefore, it is crucial to understand the early stages of growth of the ALD process to control and potentially tailor this interfacial interaction. Applying a surface science approach combined with complementary ex-situ characterization, we have studied by in-situ X-ray photoelectron spectroscopy (XPS) the early stages of ceria (CeOx) growth on SiO2 substrates deposited by thermal-ALD using Ce(thd)4/O3. Interestingly, an initial mixture of Ce3+ and Ce4+ was observed, although only Ce4+ may be expected considering the used precursor and oxidant. This fact, together with a deviation from the ideal layer-by-layer growth and a higher growth rate during the first cycles, indicates a significant influence of the substrate of the ALD reaction mechanism as well as a correlation between morphology and ceria oxidation state.}, language = {en} } @misc{GuttmannRiedelSanchezBarquillaetal., author = {Guttmann, Dominic and Riedel, Bj{\"o}rn and S{\´a}nchez-Barquilla, Raquel and Morales, Carlos and Flege, Jan Ingo}, title = {Oxide formation and oxide/metal interaction in CeOx/Ni(111)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {Ni/ceria catalysts exhibit a high activity for methane to methanol conversion, making them very promising for applications within a sustainable economy. Possibly, their activity may be strongly enhanced due to the facile exchange between Ce4+ and Ce3+ states, with the latter likely responsible for activating O-H and C-H bonds. Here, we aim to unravel the complex metal-oxide interactions in the inverse CeOx/Ni(111) system under oxidizing and reducing environments. Using low-energy electron diffraction (LEED) we find that the CeOx(111) grown by reactive molecular beam epitaxy preferentially aligns with the main directions of the Ni(111) substrate or is azimuthally rotated by ±10°. By using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), we find that less NiO is formed during deposition of CeOx than when it is held without ceria at the same conditions (O2 partial pressure and temperature). Finally, we observe a complex behavior of the cerium and nickel oxidation states when exposing the system to O2 or H2 atmospheres.}, language = {en} } @misc{KostoKapuscikTschammeretal., author = {Kosto, Yuliia and Kapuscik, Paulina and Tschammer, Rudi and Guttmann, Dominic and Mankowska, Ewa and Matvija, Peter and Morales, Carlos and Mazur, Michał and Henkel, Karsten and Matolinova, Iva and Domaradzki, Jarosław and Flege, Jan Ingo}, title = {Bare and Pd-doped ceria thin films prepared by ALD and EBE for hydrogen detection}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {The need to store and use hydrogen safely as part of green economy based on renewable energy evokes a necessity to reliably detect it at ambient conditions. The majority of currently used sensors are working at elevated temperatures (200-500 °C). In this work, we demonstrate that ceria films deposited on a commercial electrode by atomic layer deposition (ALD) and electron beam evaporation (EBE) electrically respond to hydrogen (from 20 to 500 ppm) at much lower temperatures (50-200 °C). The results reveal that <1.5 nm thin Pd adlayer increases the electrical response by several orders of magnitude for both ceria films. The NAP-XPS study under changing oxidative/reductive atmospheres sheds light on the mechanism of Pd-CeOx thermal activation and the role of the deposition technique in the reactivity of the oxide.}, language = {en} } @misc{SanchezBarquillaTschammerBussetal., author = {Sanchez-Barquilla, Raquel and Tschammer, Rudi and Buß, Lars and Morales, Carlos and Flege, Jan Ingo}, title = {The relation between substrate, Sm alloy, and surface sensitivity of ceria (111)- and (100)-oriented nano-islands on Ru(0001) and Cu(111)}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {Inverse oxide/metal catalysis allows achieving better catalytic performance than its traditional counterpart. For example, in cerium-based inverse catalyst systems, the Ce3+ states have been shown to be the active sites for methanol synthesis. This suggests that the activity can be enhanced by promoting those through alloying with trivalent, catalytically active rare-earth metals, as, e.g. Sm. We present low-energy and X-ray photoemission electron microscopy (LEEM/XPEEM), investigations that show how epitaxially grown (100)- and (111)-oriented CeO2 islands may be modified and/or alloyed by post-deposited metallic Sm. For the Ce1-xSmxO2-δ/Ru(0001) system, the CeO2 (111)-oriented islands undergo a structural change, concomitant with a partial conversion from Ce4+ to Ce3+. Surprisingly, for Ce1-xSmxO2-δ/Cu(111) the result is found to be face-dependent since only (100)-oriented CeOx islands were reduced whereas the (111)-oriented islands remained unaltered. Both systems have been exposed to reducing (H2) and oxidizing (CO2) conditions, resulting in higher reduction and in a complete recovery of the Ce4+ states, respectively. These unexpected results indicate a complex interaction not only between cerium and the doping element, but also an intricate interplay with the metallic substrate.}, language = {en} } @misc{MoralesKostoTschammeretal., author = {Morales, Carlos and Kosto, Yuliia and Tschammer, Rudi and Henkel, Karsten and Flege, Jan Ingo}, title = {Unraveling the effects of substrate interaction on the chemical properties of atomic layer deposited ultra-thin ceria layers}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {Atomic layer deposition (ALD) is well known to lead to amorphous and defective, non-stoichiometric films, potentially resulting in modified material properties that can also be affected by film/substrate interaction in the case of ultra-thin growths. For example, the formation, diffusion, and recovery of oxygen vacancies can be favored in disordered, reducible metal oxides compared to more ordered deposits, whereas interdiffusion processes can critically affect the film/substrate interface region. These effects have extensively been studied for thin thermal-ALD ceria films (below 15 nm) by combining in-situ and ex-situ characterization techniques in our lab and at synchrotron radiation facilities. While using alumina or silica substrates modifies the initial growth rate, Ce3+/Ce4+ ratio, and ceria morphology, the formation of different species at the interface affects its reactivity. Interestingly, the experiments have shown high reducibility of ALD-ceria ultrathin films on silica for very low hydrogen concentrations, even at room temperature, whereas for alumina substrates the formation of aluminates at the interface prevents further oxidation. Moreover, the comparison with more ordered films indicates a key role of the defective structure of ALD films in Ce3+/Ce4+ conversion.}, language = {en} } @misc{TschammerKostoMoralesetal., author = {Tschammer, Rudi and Kosto, Yuliia and Morales, Carlos and Schmickler, Marcel and Henkel, Karsten and Devi, Anjana and Flege, Jan Ingo}, title = {Atomic layer deposition of cerium oxide monitored by operando ellipsometry and in-situ X-ray photoelectron spectroscopy}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {Atomic layer deposition (ALD) has been used extensively to grow homogeneous films with excellent coverage and atomic-scale thickness control for a variety of applications. However, remaining challenges include the investigation of novel precursor-oxidant combinations for low-temperature deposition as well as unraveling the complex interplay between substrate and coating for ultrathin films. In this work, we present a detailed investigation of ultrathin cerium oxide films grown using the novel Ce(dpdmg)3 precursor with H2O and O2. Following a surface science-based approach, we have combined operando spectroscopic ellipsometry and in-situ X-ray photoelectron spectroscopy to allow rapid process optimization and determination of the complex relation between oxide stoichiometry, film thickness and ALD growth parameters, revealing a distinct dependence of inital Ce3+ content on the film thickness and choice of oxidant. This offers the possibility of adjusting the oxide properties to application requirements e.g. in gas sensing by choosing a suitable precursor-oxidant combination.}, language = {en} } @misc{MoralesHenriquesRiebelZavarceetal., author = {Morales Henr{\´i}ques, Carlos Javier and Riebel, Ulrich and Zavarce, L. and Guzm{\´a}n, N. M.}, title = {Improving emulsion separation: the collector material concept}, language = {en} } @misc{MoralesHenriquesRiebelPadronetal., author = {Morales Henr{\´i}ques, Carlos Javier and Riebel, Ulrich and Padr{\´o}n, Alida and Zuniga, Percy and Sorrentino, Jos{\´e}}, title = {Use of Video Enhanced Microscopy for Characterization of Solid-Liquid-Liquid Mixtures}, language = {en} } @phdthesis{MoralesHenriques, author = {Morales Henr{\´i}ques, Carlos Javier}, title = {W/O Emulsions : formulation, characterization and destabilization}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-6068}, 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{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} }