Universität Stuttgart
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Item Open Access Ab initio machine-learning unveils strong anharmonicity in non-Arrhenius self-diffusion of tungsten(2025) Zhang, Xi; Divinski, Sergiy V.; Grabowski, BlazejThe knowledge of diffusion mechanisms in materials is crucial for predicting their high-temperature performance and stability, yet accurately capturing the underlying physics like thermal effects remains challenging. In particular, the origin of the experimentally observed non-Arrhenius diffusion behavior has remained elusive, largely due to the lack of effective computational tools. Here we propose an efficient ab initio framework to compute the Gibbs energy of the transition state in vacancy-mediated diffusion including the relevant thermal excitations at the density-functional-theory level. With the aid of a bespoke machine-learning interatomic potential, the temperature-dependent vacancy formation and migration Gibbs energies of the prototype system body-centered cubic (BCC) tungsten are shown to be strongly affected by anharmonicity. This finding explains the physical origin of the experimentally observed non-Arrhenius behavior of tungsten self-diffusion. A remarkable agreement between the calculated and experimental temperature-dependent self-diffusivity and, in particular, its curvature is revealed. The proposed computational framework is robust and broadly applicable, as evidenced by first tests for a hexagonal close-packed (HCP) multicomponent high-entropy alloy. The successful applications underscore the attainability of an accurate ab initio diffusion database.Item Open Access Insights into Hildebrand solubility parameters : contributions from cohesive energies or electrophilicity densities?(2023) Miranda‐Quintana, Ramón Alain; Chen, Lexin; Smiatek, JensWe introduce certain concepts and expressions from conceptual density functional theory (DFT) to study the properties of the Hildebrand solubility parameter. The original form of the Hildebrand solubility parameter is used to qualitatively estimate solubilities for various apolar and aprotic substances and solvents and is based on the square root of the cohesive energy density. Our results show that a revised expression allows the replacement of cohesive energy densities by electrophilicity densities, which are numerically accessible by simple DFT calculations. As an extension, the reformulated expression provides a deeper interpretation of the main contributions and, in particular, emphasizes the importance of charge transfer mechanisms. All calculated values of the Hildebrand parameters for a large number of common solvents are compared with experimental values and show good agreement for non‐ or moderately polar aprotic solvents in agreement with the original formulation of the Hildebrand solubility parameters. The observed deviations for more polar and protic solvents define robust limits from the original formulation which remain valid. Likewise, we show that the use of machine learning methods leads to only slightly better predictability.Item Open Access Top‐down approach to study chemical and electronic properties of perovskite solar cells : sputtered depth profiling versus tapered cross‐sectional photoelectron spectroscopies(2021) Das, Chittaranjan; Zia, Waqas; Mortan, Claudiu; Hussain, Navid; Saliba, Michael; Ingo Flege, Jan; Kot, MałgorzataA 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.Item Open Access Floating zone growth of large tetragonal Ruddlesden-Popper bilayer nickelate YySr3-yNi2-xAlxO7-δ single crystals(2025) Yilmaz, Hasan; Sosa-Lizama, Pablo; Knauft, Manuel; Küster, Kathrin; Starke, Ulrich; Isobe, Masahiko; Clemens, Oliver; Aken, Peter A. van ; Suyolcu, Y. E.; Puphal, PascalThe discovery of superconductivity under high pressure in Ruddlesden-Popper (RP) type phase bilayer La3Ni22.5+O7 and trilayer La4Ni32.66+O10 has initiated the frontier of nickelate-based superconductors. In this context, RP-type phases within the Sr-Ni-O system offer promising alternatives as they offer unconventional high oxidation states and Sr-T-O comprises the usual RP series. Here, the intrinsic stability of the undoped Sr-Ni-O framework is investigated using density functional theory (DFT). While Sr3Ni2O7 (SNO) is stable synthesis so far requires Al co-substition in Sr3Ni2-xAlxO7-δ (SNAO). Y-doping resulting in YySr3-yNi2-xAlxO7-δ (YSNAO) effectively mitigates the challenge posed by an insulating ground state. This modification yields a substantial reduction in resistivity, with the crystals exhibiting semiconducting behavior. To explore phase formation within the narrow compositional window of the Y-Sr-Ni-Al-O system, single crystals were grown using the optical floating zone (OFZ) technique under an oxygen partial pressure of approximately 10 bar. The optimized growth conditions for YSNAO enabled the production of large (6 × 5 x 3 mm3), high-quality crystals suitable for neutron scattering experiments. In the absence of Al, crystal growth yielded the n = 1 RP phase Sr1.66Y0.33NiO4-δ, for which single crystals were obtained. The structural, chemical, electrical, and magnetic properties of both the as-grown and topochemically reduced YSNAO compounds were comprehensively characterized through diffraction, spectroscopy, transport, and magnetization measurements.Item Open Access Femtosecond direct laser writing of conductive and electrically witchable PEDOT:PSS optical nanostructures(2025) Ludescher, Dominik; Ruchka, Pavel; Siegle, Leander; Huang, Yanzhe; Flad, Philipp; Ubl, Monika; Ludwigs, Sabine; Hentschel, Mario; Giessen, HaraldMicroscale three‐dimensional (3D)‐printing, with its remarkable precision and ability to create complex structures, has transformed a wide range of applications, from micro‐optics and photonics to endoscopy and quantum technologies. In these fields, miniaturization plays a crucial role in unlocking new capabilities. However, despite these advancements, most 3D‐printed optical structures have remained static, lacking dynamic behavior and tunability. In this study, a novel approach is presented that combines direct laser writing with the electrically switchable optical properties of the conductive polymer poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). This integration facilitates the creation of dynamic structures directly on 3D‐printed objects, marking a significant step toward adaptive optical devices. The fabrication of electrically tunable structures is demonstrated via direct laser writing using PEDOT:PSS on indium tin oxide (ITO)‐coated glass substrates, as well as beneath and atop static 3D‐printed structures. It is found that electrical conductivity as well as the greyscale behavior of PEDOT:PSS remains intact after direct laser writing. The switching speed, durability, and gradual tunability of the material are explored upon complementary metal‐oxide‐semiconductor (CMOS)‐compatible voltages ranging from -3 to +2 V. In the future, this advancement opens exciting possibilities in adaptive micro‐optics, such as switchable apertures printed directly onto micro‐optical lenses.Item Open Access Unraveling the impact of acetylation patterns in chitosan oligomers on Cu2+ ion binding : insights from DFT calculations(2023) Singh, Ratna; Smiatek, Jens; Moerschbacher, Bruno M.Chitosans are partially acetylated polymers of glucosamine, structurally characterized by their degree of polymerization as well as their fraction and pattern of acetylation. These parameters strongly influence the physico-chemical properties and biological activities of chitosans, but structure-function relationships are only poorly understood. As an example, we here investigated the influence of acetylation on chitosan-copper complexation using density functional theory. We investigated the electronic structures of completely deacetylated and partially acetylated chitosan oligomers and their copper-bound complexes. Frontier molecular orbital theory revealed bonding orbitals for electrophiles and antibonding orbitals for nucleophiles in fully deacetylated glucosamine oligomers, while partially acetylated oligomers displayed bonding orbitals for both electrophiles and nucleophiles. Our calculations showed that the presence of an acetylated subunit in a chitosan oligomer affects the structural and the electronic properties of the oligomer by generating new intramolecular interactions with the free amino group of neighboring deacetylated subunits, thereby influencing its polarity. Furthermore, the band gap energy calculated from the fully and partially deacetylated oligomers indicates that the mobility of electrons in partially acetylated chitosan oligomers is higher than in fully deacetylated oligomers. In addition, fully deacetylated oligomers form more stable complexes with higher bond dissociation energies with copper than partially acetylated ones. Interestingly, in partially acetylated oligomers, the strength of copper binding was found to be dependent on the pattern of acetylation. Our study provides first insight into the influence of patterns of acetylation on the electronic and ion binding properties of chitosans. Depending on the intended application, the obtained results can serve as a guide for the selection of the optimal chitosan for a specific purpose.Item Open Access The silicon vacancy centers in SiC : determination of intrinsic spin dynamics for integrated quantum photonics(2024) Liu, Di; Kaiser, Florian; Bushmakin, Vladislav; Hesselmeier, Erik; Steidl, Timo; Ohshima, Takeshi; Son, Nguyen Tien; Ul-Hassan, Jawad; Soykal, Öney O.; Wrachtrup, JörgThe negatively charged silicon vacancy center ( VSi-) in silicon carbide (SiC) is an emerging color center for quantum technology covering quantum sensing, communication, and computing. Yet, limited information currently available on the internal spin-optical dynamics of these color centers prevents us from achieving the optimal operation conditions and reaching the maximum performance especially when integrated within quantum photonics. Here, we establish all the relevant intrinsic spin dynamics of the VSi-center at cubic lattice site (V2) in 4H-SiC by an in-depth electronic fine structure modeling including the intersystem-crossing and deshelving mechanisms. With carefully designed spin-dependent measurements, we obtain all the previously unknown spin-selective radiative and non-radiative decay rates. To showcase the relevance of our work for integrated quantum photonics, we use the obtained rates to propose a realistic implementation of time-bin entangled multi-photon GHZ and cluster state generation. We find that up to three-photon GHZ or cluster states are readily within reach using the existing nanophotonic cavity technology.Item Open Access Impact of hyperthermal oxygen on alumina surfaces investigated by molecular dynamics simulations(2025) Hocker, Stephen; Lipp, Hansjörg; Segreto, Nico; Roth, Johannes; Kästner, JohannesAtomic oxygen (AO) impinging on satellite surfaces in very low Earth orbit (VLEO) transfers momentum and energy, leading to material degradation and drag forces. To be able to counteract these effects, we investigate the impact mechanisms of AO on both crystalline and amorphous alumina (Al 2O 3) surfaces, which occur as oxidized surface layers of bulk aluminum, commonly used in satellite design. Al 2O 3also serves as a protective coating for solar panels, often covering extensive satellite surfaces. Using Molecular Dynamics (MD) simulations with classical and machine-learned (ML) force fields, we gain insights into material stability, angular distributions of reflected particles, and adsorption rates. Our findings indicate that bare Al 2O 3does not degrade under AO impacts but accumulates oxygen due to high adsorption rates. The angular distribution of reflected particles is highly dependent on the surface structure and the angle of incidence, with a higher ratio of specular reflection observed on smoother surfaces and at larger incidence angles measured to the surface normal.