Prof. Dr. Silvana Botti

Theory of excited states of integrated solid state systems
Ruhr-Universität Bochum

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Hubs
  • Ni-Alloyed Copper Iodide Thin Films: Microstructural Features and Functional Performance
    Dethloff, Christiane and Thieme, Katrin and Selle, Susanne and Seifert, Michael and Vogt, Sofie and Splith, Daniel and Botti, Silvana and Grundmann, Marius and Lorenz, Michael
    Physica Status Solidi (B) Basic Research (2024)
    To tailor electrical properties of often degenerate pristine CuI, Ni is introduced as alloy constituent. Cosputtering in a reactive, but also in an inert atmosphere as well as pulsed laser deposition (PLD), is used to grow (Formula presented.) thin films. The Ni content within the alloy thin films is systematically varied for different growth techniques and growth conditions. A solubility limit is evidenced by an additional (Formula presented.) phase for Ni contents (Formula presented.), observed in X-Ray diffraction and atomic force microscopy by a change in surface morphology. Furthermore, metallic, nanoscaled nickel clusters, revealed by X-Ray photoelectron spectroscopy and high-resolution transmission electron microscopy (HRTEM), underpin a solubility limit of Ni in CuI. Although no reduction of charge carrier density is observed with increasing Ni content, a dilute magnetic behavior of the thin films is observed in vibrating sample magnetometry. Further, independent of the deposition technique, unique multilayer features are observed in HRTEM measurements for thin films of a cation composition of (Formula presented.). Opposite to previous claims, no transition to n-type behavior was observed, which was also confirmed by density functional theory calculations of the alloy system. © 2024 The Authors. physica status solidi (b) basic solid state physics published by Wiley-VCH GmbH.
    view abstract10.1002/pssb.202300492
  • Prediction of high- Tc superconductivity in ternary actinium beryllium hydrides at low pressure
    Gao, Kun and Cui, Wenwen and Shi, Jingming and Durajski, Artur P. and Hao, Jian and Botti, Silvana and Marques, Miguel A. L. and Li, Yinwei
    Physical Review B 109 (2024)
    Hydrogen-rich superconductors are promising candidates to achieve room-temperature superconductivity. However, the extreme pressures needed to stabilize these structures significantly limit their practical applications. An effective strategy to reduce the external pressure is to add a light element M that binds with H to form MHx units, acting as a chemical precompressor. We exemplify this idea by performing ab initio calculations of the Ac-Be-H phase diagram, proving that the metallization pressure of Ac-H binaries, for which critical temperatures as high as 200 K were predicted at 200 GPa, can be significantly reduced via beryllium incorporation. We identify three thermodynamically stable (AcBe2H10, AcBeH8, and AcBe2H14) and four metastable compounds (fcc AcBeH8, AcBeH10, AcBeH12 and AcBe2H16). All of them are superconductors. In particular, fcc AcBeH8 remains dynamically stable down to 10 GPa, where it exhibits a superconducting-transition temperature Tc of 181 K. The Be-H bonds are responsible for the exceptional properties of these ternary compounds and allow them to remain dynamically stable close to ambient pressure. Our results suggest that high-Tc superconductivity in hydrides is achievable at low pressure and may stimulate experimental synthesis of ternary hydrides. © 2024 American Physical Society.
    view abstract10.1103/PhysRevB.109.014501
  • Prediction of high- Tc superconductivity in ternary actinium beryllium hydrides at low pressure
    Gao, Kun and Cui, Wenwen and Shi, Jingming and Durajski, Artur P. and Hao, Jian and Botti, Silvana and Marques, Miguel A. L. and Li, Yinwei
    Physical Review B 109 (2024)
    Hydrogen-rich superconductors are promising candidates to achieve room-temperature superconductivity. However, the extreme pressures needed to stabilize these structures significantly limit their practical applications. An effective strategy to reduce the external pressure is to add a light element M that binds with H to form MHx units, acting as a chemical precompressor. We exemplify this idea by performing ab initio calculations of the Ac-Be-H phase diagram, proving that the metallization pressure of Ac-H binaries, for which critical temperatures as high as 200 K were predicted at 200 GPa, can be significantly reduced via beryllium incorporation. We identify three thermodynamically stable (AcBe2H10, AcBeH8, and AcBe2H14) and four metastable compounds (fcc AcBeH8, AcBeH10, AcBeH12 and AcBe2H16). All of them are superconductors. In particular, fcc AcBeH8 remains dynamically stable down to 10 GPa, where it exhibits a superconducting-transition temperature Tc of 181 K. The Be-H bonds are responsible for the exceptional properties of these ternary compounds and allow them to remain dynamically stable close to ambient pressure. Our results suggest that high-Tc superconductivity in hydrides is achievable at low pressure and may stimulate experimental synthesis of ternary hydrides. © 2024 American Physical Society.
    view abstract10.1103/PhysRevB.109.014501
  • Band alignment at the strontium germanate interface with silicon
    Rauch, Tomáš and Marton, Pavel and Botti, Silvana and Hlinka, Jiří
    Physical Review B 107 (2023)
    Photocatalytic water splitting is a promising strategy for large-scale clean energy production. However, efficient and low-cost solid-state photocatalysts are still lacking. We present here first-principles calculations for investigating the suitability of an epitaxial layer of strontium germanate on a Si(100) single crystal as a photocathode. Conduction and valence band offsets at the interface between these two semiconductors were determined using state-of-the-art approximations of density functional theory for the accurate prediction of band alignments. The resulting band lineup is also confirmed by inspection of the spatially resolved density of states. It is concluded that the conduction band offset of the investigated heterostructure is favorable for photocathodic functionality. © 2023 American Physical Society.
    view abstract10.1103/PhysRevB.107.115303
  • Determination of acoustic phonon anharmonicities via second-order Raman scattering in CuI
    Hildebrandt, R. and Seifert, M. and George, J. and Blaurock, S. and Botti, S. and Krautscheid, H. and Grundmann, M. and Sturm, C.
    New Journal of Physics 25 (2023)
    We demonstrate the determination of anharmonic acoustic phonon properties via second-order Raman scattering exemplarily on copper iodide single crystals. The origin of multi-phonon features from the second-order Raman spectra was assigned by the support of the calculated 2-phonon density of states. In this way, the temperature dependence of acoustic phonons was determined down to 10 K. To determine independently the harmonic contributions of respective acoustic phonons, density functional theory in quasi-harmonic approximation was used. Finally, the anharmonic contributions were determined. The results are in agreement with earlier publications and extend CuI’s determined acoustic phonon properties to lower temperatures with higher accuracy. This approach demonstrates that it is possible to characterize the acoustic anharmonicities via Raman scattering down to zero-temperature renormalization constants of at least 0.1 cm−1 © 2023 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/ad1141
  • Dynamical Franz-Keldysh Effect in Diamond in the Deep Ultraviolet Probed by Transient Absorption and Dispersion Spectroscopy Using a Miniature Beamline
    Reislöhner, Jan and Chen, Xiao and Kim, Doyeong and Botti, Silvana and Pfeiffer, Adrian N.
    Physical Review Letters 131 (2023)
    Here, we introduce a miniature beamline for transient absorption and dispersion spectroscopy, using a tailored deep ultraviolet field immediately after the noncollinear generation without subsequent optical elements. We explore the near-band-gap region in diamond in the presence of a few-femtosecond pump pulse where the delayed dynamical Franz-Keldysh effect and the almost instantaneous optical Kerr effect coexist. © 2023 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    view abstract10.1103/PhysRevLett.131.136902
  • Electronic and Optical Properties of Alkaline Earth Metal Fluoride Crystals with the Inclusion of Many-Body Effects: A Comparative Study on Rutile MgF2 and Cubic SrF2
    Cappellini, Giancarlo and Furthmüller, Jürgen and Bechstedt, Friedhelm and Botti, Silvana
    Symmetry 15 (2023)
    We conducted a systematic investigation using state-of-the-art techniques on the electronic and optical properties of two crystals of alkaline earth metal fluorides, namely rutile MgF (Formula presented.) and cubic SrF (Formula presented.). For these two crystals of different symmetry, we present density functional theory (DFT), many-body perturbation theory (MBPT), and Bethe–Salpeter equation (BSE) calculations. We calculated a variety of properties, namely ground-state energies, band-energy gaps, and optical absorption spectra with the inclusion of excitonic effects. The quantities were obtained with a high degree of convergence regarding all bulk electronic and optical properties. Bulk rutile MgF (Formula presented.) has distinguished ground-state and excited-state properties with respect to the other cubic fluoride SrF (Formula presented.) and the other members of the alkaline earth metal fluoride family. The nature of the fundamental gaps and estimates of the self-energy and excitonic effects for the two compounds are presented and discussed in detail. Our results are in good accordance with the measurements and other theoretical–computational data. A comparison is made between the excitation and optical properties of bulk rutile MgF (Formula presented.), cubic SrF (Formula presented.), and the corresponding clusters, for which calculations have recently been published, confirming strong excitonic effects in finite-sized systems. © 2023 by the authors.
    view abstract10.3390/sym15020539
  • Ensemble averages of ab initio optical, transport, and thermoelectric properties of hexagonal SixGe1-x alloys
    Borlido, Pedro and Bechstedt, Friedhelm and Botti, Silvana and Rödl, Claudia
    Physical Review Materials 7 (2023)
    Ge-rich hexagonal SiGe alloys have recently emerged as new direct-gap semiconductors with unprecedented potential for integration of photonics on silicon. We present a comprehensive first-principles investigation of optical, transport, and thermoelectric properties of pure and doped hexagonal SixGe1-x alloys based on density-functional theory calculations, the Boltzmann transport equation, and the generalized quasichemical approximation to obtain alloy averages of electronic properties. At low temperatures, phase decomposition into the hexagonal elementary crystals is thermodynamically favored, but around and above room temperature random alloys are predicted to be stable. While hexagonal Si has an indirect band gap, the gap of hexagonal Ge is direct with very weak optical transitions at the absorption edge. The alloy band gap remains direct for a Si content below 45% and the oscillator strength of the lowest optical transitions is efficiently enhanced by alloying. The optical spectra show clear trends and both absorption edges and prominent peaks can be tuned with composition. The dependence of transport coefficients on carrier concentration and temperature is similar in cubic and hexagonal alloys. However, the latter display an anisotropic response due to the reduced hexagonal symmetry. In particular, the transport mass exhibits a significant directional dependence. Seebeck coefficients and thermoelectric power factors of n-doped alloys show nonmonotonous variations with the Si content independently of temperature. © 2023 American Physical Society.
    view abstract10.1103/PhysRevMaterials.7.014602
  • Erratum: Band lineup at hexagonal SixGe1-x/SiyGe1-y alloy interfaces (Physical Review B (2022) 106 (085303) DOI: 10.1103/PhysRevB.106.085303)
    Belabbes, Abderrezak and Botti, Silvana and Bechstedt, Friedhelm
    Physical Review B 107 (2023)
    The natural and true band profiles at heterojunctions formed by hexagonal SixGe1−x alloys are investigated by a variety of methods: density-functional theory for atomic geometries, approximate quasiparticle treatments for electronic structures, different band-edge alignment procedures, and construction of various hexagonal unit cells to model alloys and heterojunctions. We demonstrate that the natural band offsets are rather unaffected by the choice to align the vacuum level or the branch point energy, as well as by the use of a hybrid or the Tran-Blaha functional. At interfaces between Ge-rich alloys we observe a type-I heterocharacter with direct band gaps, while Si-rich junctions are type-I but with an indirect band gap. The true band lineups at pseudomorphically grown heterostructures are strongly influenced by the generated biaxial strain of opposite sign in the two adjacent alloys. Our calculations show that the type-I character of the interface is reduced by strain. To prepare alloy heterojunctions suitable for active optoelectronic applications, we discuss how to decrease the compressive biaxial strain at Ge-rich alloys. © 2023 American Physical Society.
    view abstract10.1103/PhysRevB.107.039903
  • First-principles insight in structure-property relationships of hexagonal Si and Ge polytypes
    Keller, Martin and Belabbes, Abderrezak and Furthmüller, Jürgen and Bechstedt, Friedhelm and Botti, Silvana
    Physical Review Materials 7 (2023)
    Hexagonal SiGe is a promising material for combining electronic and photonic technologies. In this paper, the energetic, structural, elastic, and electronic properties of the hexagonal polytypes (2H, 4H, and 6H) of silicon and germanium are thoroughly analyzed under equilibrium conditions. For this purpose, we apply state-of-the-art density functional theory. The phase diagram, obtained in the framework of a generalized Ising model, shows that the diamond structure is the most stable under ambient conditions, but hexagonal modifications are close to the phase boundary, especially for Si. Our band structure calculations using the modified-Becke-Johnson-local-density-approximation (MBJLDA) and Heyd-Scuseria-Ernzerhof (HSE06) exchange-correlation functionals predict significant changes in electronic states with hexagonality. While Si crystals are always semiconductors with indirect band gaps, the hexagonal Ge polytypes have direct band gaps. The branch-point energies of the Si polytypes appear in the fundamental gaps, while for the Ge crystals they are below the valence band maxima. Band alignment based on the branch-point energy leads to type-I heterocrystalline interfaces between Ge polytypes, where electrons and holes can be trapped in the layer with the higher hexagonality. © 2023 American Physical Society.
    view abstract10.1103/PhysRevMaterials.7.064601
  • Machine-Learning-Assisted Determination of the Global Zero-Temperature Phase Diagram of Materials
    Schmidt, Jonathan and Hoffmann, Noah and Wang, Hai-Chen and Borlido, Pedro and Carriço, Pedro J. M. A. and Cerqueira, Tiago F. T. and Botti, Silvana and Marques, Miguel A. L.
    Advanced Materials 35 (2023)
    Crystal-graph attention neural networks have emerged recently as remarkable tools for the prediction of thermodynamic stability. The efficacy of their learning capabilities and their reliability is however subject to the quantity and quality of the data they are fed. Previous networks exhibit strong biases due to the inhomogeneity of the training data. Here a high-quality dataset is engineered to provide a better balance across chemical and crystal-symmetry space. Crystal-graph neural networks trained with this dataset show unprecedented generalization accuracy. Such networks are applied to perform machine-learning-assisted high-throughput searches of stable materials, spanning 1 billion candidates. In this way, the number of vertices of the global T = 0 K phase diagram is increased by 30% and find more than ≈150 000 compounds with a distance to the convex hull of stability of less than 50 meV atom−1. The discovered materials are then accessed for applications, identifying compounds with extreme values of a few properties, such as superconductivity, superhardness, and giant gap-deformation potentials. © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.
    view abstract10.1002/adma.202210788
  • Optical properties of Agx Cu1- x I alloy thin films
    Krüger, Evgeny and Seifert, Michael and Gottschalch, Volker and Krautscheid, Harald and Schnohr, Claudia S. and Botti, Silvana and Grundmann, Marius and Sturm, Chris
    AIP Advances 13 (2023)
    We report on the excitonic transition energy E0 and spin-orbit split-off energy Δ0 of γ-AgxCu1-xI alloy thin films studied by using reflectivity measurements at temperatures between 20 K and 290 K. The observed bowing behavior of the E0 transition as a function of the alloy composition is explained based on first-principles band structure calculations in terms of different physical and chemical contributions within the description of ordered alloys. The spin-orbit coupling is found to increase from a value of 640 meV for CuI to approximately 790 meV for AgI. Furthermore, we show that the temperature-dependent bandgap shift between 20 K and 290 K decreases with increasing Ag-content from 25 meV for CuI to 6 meV for AgI. We attribute this behavior mostly to changes in the contribution of thermal lattice expansion to the bandgap shift. © 2023 Author(s).
    view abstract10.1063/5.0137091
  • Peculiar bond length dependence in (Ag,Cu)GaSe2 alloys and its impact on the bandgap bowing
    Falk, Hans H. and Eckner, Stefanie and Seifert, Michael and Ritter, Konrad and Levcenko, Sergiu and Pfeiffelmann, Timo and Welter, Edmund and Botti, Silvana and Shafarman, William N. and Schnohr, Claudia S.
    APL Materials 11 (2023)
    Contrary to other semiconductor alloys, incorporation of Ag into CuGaSe2 increases the bandgap, even though the lattice expands, and the Ga-Se bond length is theoretically predicted to decrease rather than increase. Herein, we experimentally confirm this peculiar bond length dependence of (Ag,Cu)GaSe2 using x-ray absorption spectroscopy. We further model the different anion displacements and estimate that their combined contribution to the bandgap bowing is close to zero. These findings differ from those for Cu(In,Ga)Se2 and demonstrate the diversity of chalcopyrite alloys and their properties. © 2023 Author(s).
    view abstract10.1063/5.0164407
  • The dynamical Franz-Keldysh effect in the deep ultraviolet probed by transient absorption and dispersion of diamond
    Reislöhner, Jan and Chen, X. and Kim, Doyeong and Botti, S. and Pfeiffer, Adrian N.
    2023 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2023 (2023)
    A miniature beamline, where a tailored deep ultraviolet (DUV) field is used immediately after the noncollinear generation, is introduced for transient absorption and dispersion spectroscopy. The near-bandgap region in diamond in the presence of a few-femtosecond pump pulse is explored where the delayed dynamical Franz-Keldysh effect (DFKE) and the almost instantaneous optical Kerr effect (OKE) coexist. © 2023 IEEE.
    view abstract10.1109/CLEO/EUROPE-EQEC57999.2023.10232212
  • Transfer learning on large datasets for the accurate prediction of material properties
    Hoffmann, Noah and Schmidt, Jonathan and Botti, Silvana and Marques, Miguel A. L.
    Digital Discovery 2 (2023)
    Graph neural networks trained on large crystal structure databases are extremely effective in replacing ab initio calculations in the discovery and characterization of materials. However, crystal structure datasets comprising millions of materials exist only for the Perdew-Burke-Ernzerhof (PBE) functional. In this work, we investigate the effectiveness of transfer learning to extend these models to other density functionals. We show that pre-training significantly reduces the size of the dataset required to achieve chemical accuracy and beyond. We also analyze in detail the relationship between the transfer-learning performance and the size of the datasets used for the initial training of the model and transfer learning. We confirm a linear dependence of the error on the size of the datasets on a log-log scale, with a similar slope for both training and the pre-training datasets. This shows that further increasing the size of the pre-training dataset, i.e., performing additional calculations with a low-cost functional, is also effective, through transfer learning, in improving machine-learning predictions with the quality of a more accurate, and possibly computationally more involved functional. Lastly, we compare the efficacy of interproperty and intraproperty transfer learning. © 2023 RSC.
    view abstract10.1039/d3dd00030c
  • Roadmap on Machine learning in electronic structure
    Kulik, H.J. and Hammerschmidt, T. and Schmidt, J. and Botti, S. and Marques, M.A.L. and Boley, M. and Scheffler, M. and Todorović, M. and Rinke, P. and Oses, C. and Smolyanyuk, A. and Curtarolo, S. and Tkatchenko, A. and Bartók, A.P. and Manzhos, S. and Ihara, M. and Carrington, T. and Behler, J. and Isayev, O. and Veit, M. and Grisafi, A. and Nigam, J. and Ceriotti, M. and Schütt, K.T. and Westermayr, J. and Gastegger, M. and Maurer, R.J. and Kalita, B. and Burke, K. and Nagai, R. and Akashi, R. and Sugino, O. and Hermann, J. and Noé, F. and Pilati, S. and Draxl, C. and Kuban, M. and Rigamonti, S. and Scheidgen, M. and Esters, M. and Hicks, D. and Toher, C. and Balachandran, P.V. and Tamblyn, I. and Whitelam, S. and Bellinger, C. and Ghiringhelli, L.M.
    Electronic Structure 4 (2022)
    view abstract10.1088/2516-1075/ac572f
  • Anisotropic layered Bi2 Te3 -In2 Te3 composites: Control of interface density for tuning of thermoelectric properties
    Liu, D. and Li, X. and De Castro Borlido, P.M. and Botti, S. and Schmechel, R. and Rettenmayr, M.
    Scientific Reports 7 (2017)
    Layered (Bi1-xInx) 2 Te3 -In2 Te3 (x = 0.075) composites of pronounced anisotropy in structure and thermoelectric properties were produced by zone melting and subsequent coherent precipitation of In2 Te3 from a (Bi1-xInx) 2 Te3 (x > 0.075) matrix. Employing solid state phase transformation, the Bi2 Te3 /In2 Te3 interface density was tuned by modifying the driving force for In2 Te3 precipitation. The structure-property relationship in this strongly anisotropic material is characterized thoroughly and systematically for the first time. Unexpectedly, with increasing Bi2 Te3 /In2 Te3 interface density, an increase in electrical conductivity and a decrease in the absolute Seebeck coefficient were found. This is likely to be due to electron accumulation layers at the Bi2 Te3 /In2 Te3 interfaces and the interplay of bipolar transport in Bi2 Te3. Significantly improved thermoelectric properties of Bi2 Te3 -In2 Te3 composites as compared to the single phase (Bi1-xInx) 2 Te3 solid solution are obtained. © The Author(s) 2017.
    view abstract10.1038/srep43611
  • Detection of Cu2Zn5SnSe8 and Cu2Zn6SnSe9 phases in co-evaporated Cu2ZnSnSe4 thin-films
    Schwarz, T. and Marques, M.A.L. and Botti, S. and Mousel, M. and Redinger, A. and Siebentritt, S. and Cojocaru-Mirédin, O. and Raabe, D. and Choi, P.-P.
    Applied Physics Letters 107 (2015)
    Cu2ZnSnSe4 thin-films for photovoltaic applications are investigated using combined atom probe tomography and ab initio density functional theory. The atom probe studies reveal nano-sized grains of Cu2Zn5SnSe8 and Cu2Zn6SnSe9 composition, which cannot be assigned to any known phase reported in the literature. Both phases are considered to be metastable, as density functional theory calculations yield positive energy differences with respect to the decomposition into Cu2ZnSnSe4 and ZnSe. Among the conceivable crystal structures for both phases, a distorted zinc-blende structure shows the lowest energy, which is a few tens of meV below the energy of a wurtzite structure. A band gap of 1.1 eV is calculated for both the Cu2Zn5SnSe8 and Cu2Zn6SnSe9 phases. Possible effects of these phases on solar cell performance are discussed. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4934847
  • Nano-scale characterization of thin-film solar cells
    Schwarz, T. and Cojocaru-Mirédin, O. and Choi, P.-P. and Lämmle, A. and Würz, R. and Mousel, M. and Redinger, A. and Siebentritt, S. and Botti, S. and Raabe, D.
    Microscopy and Microanalysis 20 (2014)
    view abstract10.1017/S1431927614003699

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