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Materials Science (cond-mat.mtrl-sci)

Fri, 21 Apr 2023

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1.Calculation of Carrier Doping-Induced Half-Metallicity, and Transformation of Easy Axis in Two-Dimensional MSi2N4 (M = Cr, Mn, Fe, and Co) Monolayers

Authors:Ziyuan An, Linhui Lv, Ya Sū, Yanyan Jiang, Zhaohong Guan

Abstract: We study the stability, electrical properties, and magnetic properties of MSi2N4 (M = Cr, Mn, Fe, and Co) monolayers based on the density functional theory.

2.Efficient GW calculations via the interpolation of the screened interaction in momentum and frequency space: The case of graphene

Authors:Alberto Guandalini, Dario A. Leon, Pino D'Amico, Claudia Cardoso, Andrea Ferretti, Daniele Varsano

Abstract: The calculation of the GW self-energy may be a computational challenge due to the double convolution integrals over frequency and transferred momentum. In this work, we combine the recently developed multipole approximation (MPA) with the W-av method. MPA accurately approximates full-frequency response functions using a small number of poles, while W-av improves the convergence with respect to the Brillouin zone (BZ) sampling in 2D materials. The combination of these techniques is applied to obtain an accurate G0W0 QP band structure of graphene. The screened interaction of graphene shows a complex low-energy frequency dependence, that is poorly described with plasmon pole approximations (PPA), and a sharp q dependence of the dynamical dielectric function over momentum transfer, making standard BZ integration techniques inefficient. Within the present development, we compare the calculated QP band structure of graphene finding an excellent agreement with angle resolved photoemission spectroscopy (ARPES) measurements.

3.First-Principles Modeling of Equilibration Dynamics of Hyperthermal Products of Surface Reactions Using Scalable Neural Network Potential

Authors:Qidong Lin, Bin Jiang

Abstract: Equilibration dynamics of hot oxygen atoms following O2 dissociation on Pd(100) and Pd(111) surfaces are investigated by molecular dynamics simulations based on a scalable neural network potential enabling first-principles description of O2 and O interacting with variable Pd supercells. We find that to accurately describe the equilibration dynamics after dissociation, the simulation cell length necessarily exceeds twice the maximum distance of equilibrated oxygen adsorbates. By analyzing hundreds of trajectories with appropriate initial sampling, the measured distance distribution of equilibrated atom pairs on Pd(111) is well reproduced. However, our results on Pd(100) suggest that the ballistic motion of hot atoms predicted previously is a rare event under ideal conditions, while initial molecular orientation and surface thermal fluctuation could significantly affect the overall post-dissociation dynamics. On both surfaces, dissociated oxygen atoms remain primarily locate their nascent positions and then randomly cross bridge sites nearby.

4.Comparison of Matlantis and VASP bulk formation and surface energies in metal hydrides, carbides, nitrides, oxides, and sulfides

Authors:Shinya Mine, Takashi Toyao, Ken-ichi Shimizu, Yoyo Hinuma

Abstract: Generic neural network potentials without forcing users to train potentials could result in significantly acceleration of total energy calculations. Takamoto et al. [Nat. Commun. (2022), 13, 2991] developed such a deep neural network potential (NNP) and made it available in their Matlantis package. We compared the Matlantis bulk formation, surface, and surface O vacancy formation energies of metal hydrides, carbides, nitrides, oxides, and sulfides with our previously calculated VASP values obtained from first-principles with the PBEsol(+U) functional. Matlantis bulk formation energies were consistently ~0.1 eV/atom larger and the surface energies were typically ~10 meV/{\AA}^2 smaller than the VASP counterpart. Surface O vacancy formation energies were generally underestimated within ~0.8 eV. These results suggest that Matlantis energies could serve as a relatively good descriptor of the VASP bulk formation and surface energies.

5.Exploiting the close-to-Dirac point shift of Fermi level in Sb2Te3/Bi2Te3 topological insulator heterostructure for spin-charge conversion

Authors:E. Longo, L. Locatelli, P. Tsipas, A. Lintzeris, A. Dimoulas, M. Fanciulli, M. Longo, R. Mantovan

Abstract: Properly tuning the Fermi level position in topological insulators is of vital importance to tailor their spin-polarized electronic transport and to improve the efficiency of any functional device based on them. Here we report the full in situ Metal Organic Chemical Vapor Deposition (MOCVD) and study of a highly crystalline Bi2Te3/Sb2Te3 topological insulator heterostructure on top of large area (4'') Si(111) substrates. The bottom Sb2Te3 layer serves as an ideal seed layer for the growth of highly crystalline Bi2Te3 on top, also inducing a remarkable shift of the Fermi level to place it very close to the Dirac point, as visualized by angle-resolved photoemission spectroscopy. In order to exploit such ideal topologically-protected surface states, we fabricate the simple spin-charge converter Si(111)/Sb2Te3/Bi2Te3/Au/Co/Au and spin-charge conversion (SCC) is probed by spin pumping ferromagnetic resonance. A large SCC is measured at room temperature, which is interpreted within the inverse Edelstein effect (IEE), thus resulting in a conversion efficiency lambda_IEE of 0.44 nm. Our results demonstrate the successful tuning of the surface Fermi level of Bi2Te3 when grown on top of Sb2Te3 with a full in situ MOCVD process, which is highly interesting in view of its future technology transfer.

6.Vibrational dynamics of CO on Pd(111) in and out of thermal equilibrium

Authors:Bombín Raúl, S. Muzas Alberto, Novko Dino, Juaristi J. Iñaki, Alducin Maite

Abstract: Using many-body perturbation theory and density functional perturbation theory, we study the vibrational spectra of the internal stretch (IS) mode of CO on Pd(111) for the bridge and hollow adsorption structures that are experimentally identified at 0.5~ML coverage. Our theoretical treatment allows us to determine the temperature dependence of the IS vibrational spectra under thermal conditions as well as the time evolution of the non-equilibrium transient spectra induced by femtosecond laser pulses. Under thermal conditions (i.e., for equal electronic $T_e$ and phononic $T_l$ temperatures), the calculated lifetimes at 10-150~K are mostly due to nonadiabatic couplings (NC), i.e., first-order electronic excitations. As temperature increases, also the contribution of the second-order electron mediated phonon-phonon couplings (EMPPC) progressively increases from 25\% at low temperatures to 50\% at 300~K. Our calculations for the laser-induced non-equilibrium conditions comprise experimental absorbed fluences of 6-130~J/m$^2$. For fluences for which $T_e>$2000~K, the transient vibrational spectra are characterized by two different regimes that follow the distinct time-evolution of $T_e$ and $T_l$ and are respectively dominated by NC and EMPPC processes. At lower fluences, the initial fast regime becomes progressively negligible as $T_e$ decreases and only the steady second regime remains visible. Qualitatively, all these spectral properties are common to the both adsorption structures studied here.

7.Ab initio calculation for electronic structure and optical property of tungsten carbide in a TiCN-based cermet for solar thermal applications

Authors:Shota Hayakawa, Toshiharu Chono, Kosuke Watanabe, Shoya Kawano, Kazuma Nakamura, Koji Miyazaki

Abstract: We present an ab initio calculation to understand electronic structures and optical properties of a tungsten carbide WC being a major component of a TiCN-based cermet. We found that the WC has a fairly low-energy plasma excitation $\sim$0.6 eV (2 $\mu$m) and therefore can be a good constituent of a solar selective absorber. The evaluated figure of merit for photothermal conversion is prominently high compared to those of the other materials included in the TiCN-based cermet. The imaginary part of the dielectric function is considerably small around the zero point of the real part of the dielectric function, corresponding to the plasma excitation energy. Therefore, a clear plasma edge appeared, ensuring the high performance of the WC as the solar absorber.

8.Whats special about Y6; the working mechanism of neat Y6 organic solar cell

Authors:Elifnaz Saglamkaya, Artem Musiienko, Mohammad Saeed Shadabroo, Bowen Sun, Sreelakshmi Chandrabose, Giulia Lo Gerfo M, Niek F van Hulst, Dieter Neher, Safa Shoaee

Abstract: Non-fullerene acceptors (NFA) have delivered advance in bulk heterojunction organic solar cell efficiencies, with the significant milestone of 20% now in sight. However, these materials challenge the accepted wisdom of how organic solar cells work. In this work we present neat Y6 device with efficiency above 4.5%. We thoroughly investigate mechanisms of charge generation and recombination as well as transport in order to understand what is special about Y6. Our data suggest Y6 generates bulk free charges, with ambipolar mobility, which can be extracted in the presence of transport layers

9.Glass fracture surface energy calculated from crystal structure and bond-energy data

Authors:Marco Holzer, Tina Waurischk, Janine George, Robert Maaß, Ralf Müller

Abstract: We present a novel method to predict the fracture surface energy of oxide glasses, {\gamma}, using readily available crystallographic structure data of their isochemical crystal and tabled diatomic chemical bond energies, D0. The method assumes that {\gamma} equals the fracture surface energy of the most likely cleavage plane of the crystal. Calculated values were in excellent agreement with those calculated from measured glass density and D0 in an earlier work. This finding demonstrates a remarkable equivalence between crystal cleavage planes and glass fracture surfaces.

10.Optical properties of MoSe$_2$ monolayer implanted with ultra-low energy Cr ions

Authors:Minh N. Bui Peter Grünberg Institute 9 Department of Physics, RWTH Aachen University, Aachen, Germany, Stefan Rost Peter Grünberg Institute 1 Department of Physics, RWTH Aachen University, Aachen, Germany, Manuel Auge II. Institute of Physics, University of Göttingen, Göttingen, Germany, Lanqing Zhou Peter Grünberg Institute 9 Department of Physics, RWTH Aachen University, Aachen, Germany, Christoph Friedrich Peter Grünberg Institute 1, Stefan Blügel Peter Grünberg Institute 1 Department of Physics, RWTH Aachen University, Aachen, Germany, Silvan Kretschmer Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany, Arkady V. Krasheninnikov Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany Department of Applied Physics, Aalto University School of Science, Aalto, Finland, Kenji Watanabe Research Center for Functional Materials, National Institute for Materials Science, Namiki, Tsukuba, Japan, Takashi Taniguchi International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Namiki, Tsukuba, Japan, Hans C. Hofsäss II. Institute of Physics, University of Göttingen, Göttingen, Germany, Detlev Grützmacher Peter Grünberg Institute 9 Department of Physics, RWTH Aachen University, Aachen, Germany, Beata E. Kardynał Peter Grünberg Institute 9 Department of Physics, RWTH Aachen University, Aachen, Germany

Abstract: The paper explores the optical properties of an exfoliated MoSe$_2$ monolayer implanted with Cr$^+$ ions, accelerated to 25 eV. Photoluminescence of the implanted MoSe$_2$ reveals an emission line from Cr-related defects that is present only under weak electron doping. Unlike band-to-band transition, the Cr-introduced emission is characterised by non-zero activation energy, long lifetimes, and weak response to the magnetic field. To rationalise the experimental results and get insights into the atomic structure of the defects, we modelled the Cr-ion irradiation process using ab-initio molecular dynamics simulations followed by the electronic structure calculations of the system with defects. The experimental and theoretical results suggest that the recombination of electrons on the acceptors, which could be introduced by the Cr implantation-induced defects, with the valence band holes is the most likely origin of the low energy emission. Our results demonstrate the potential of low-energy ion implantation as a tool to tailor the properties of 2D materials by doping.

11.Conversion of La$_2$Ti$_2$O$_7$ to LaTiO$_2$N via Ammonolysis: An ab-initio Investigation

Authors:Chiara Ricca, Tristan Blandenier, Valérie Werner, Xing Wang, Simone Pokrant, Ulrich Aschauer

Abstract: Perovskite oxynitrides are, due to their reduced band gap compared to oxides, promising materials for photocatalytic applications. They are most commonly synthesized from {110} layered Carpy-Galy (A$_2$B$_2$O$_7$}) perovskites via thermal ammonolysis, i.e. the exposure to a flow of ammonia at elevated temperature. The conversion of the layered oxide to the non-layered oxynitride must involve a complex combination of nitrogen incorporation, oxygen removal and ultimately structural transition by elimination of the interlayer shear plane. Despite the process being commonly used, little is known about the microscopic mechanisms and hence factors that could ease the conversion. Here we aim to derive such insights via density functional theory calculations of the defect chemistry of the oxide and the oxynitride as well as the oxide's surface chemistry. Our results point to the crucial role of surface oxygen vacancies in forming clusters of NH$_3$ decomposition products and in incorporating N, most favorably substitutionally at the anion site. N then spontaneously diffuses away from the surface, more easily parallel to the surface and in interlayer regions, while diffusion perpendicular to the interlayer plane is somewhat slower. Once incorporation and diffusion lead to a local N concentration of about 70% of the stoichiometric oxynitride composition, the nitridated oxide spontaneously transforms to a nitrogen-deficient oxynitride.

12.Self-consistent homogenization approach for polycrystals within second gradient elasticity

Authors:Yury Solyaev

Abstract: In this paper, we propose a generalized variant of Kr\"oner's self-consistent scheme for evaluation of the effective standard and gradient elastic moduli of polycrystalline materials within Mindlin-Toupin second-gradient elasticity theory. Assuming random orientation of crystallites (grains) we use an extended Eshelby's equivalent inclusion method and mapping conditions between the prescribed linear distribution of macro-strain and corresponding micro-scale field variables averaged over the volume and all possible orientations of single grain. It is found that developed self-consistent scheme predicts the absence of strong gradient effects at the macro-scale level for the model of spherical grains. However, for the more general shape of the grains, considered approach allows to obtain a set of non-linear relations for determination of all effective standard and gradient elastic moduli of polycrystals.

13.In-situ study and modeling of the reaction kinetics during molecular beam epitaxy of GeO2 and its etching by Ge

Authors:Wenshan Chen, Kingsley Egbo, Hans Tornatzky, Manfred Ramsteiner, Markus Wagner, Elias Kluth, Martin Feneberg, Rüdiger Goldhahn, Oliver Bierwagen

Abstract: Rutile GeO2 has been predicted to be an ultra-wide bandgap semiconductor suitable for future power electronics devices while quartz-like GeO2 shows piezoelectric properties. To explore these crystalline phases for application and fundamental materials investigations, molecular beam epitaxy (MBE) is a well-suited thin film growth technique. In this study, we investigate the reaction kinetics of GeO2 during plasma-assisted MBE using elemental Ge and plasma-activated oxygen fluxes. The growth rate as a function of oxygen flux is measured in-situ by laser reflectometry at different growth temperatures. A flux of the suboxide GeO desorbing off the growth surface is identified and quantified in-situ by the line-of-sight quadrupole mass spectrometry. Our measurements reveal that the suboxide formation and desorption limits the growth rate under metal-rich or high temperature growth conditions, and leads to etching of the grown GeO2 layer under Ge flux in the absence of oxygen. The quantitative results fit the sub-compound mediated reaction model, indicating the intermediate formation of the suboxide at the growth front. This model is further utilized to delineate the GeO2-growth window in terms of oxygen-flux and substrate temperature. Our study can serve as a guidance for the thin film synthesis of GeO2 and defect-free mesa etching in future GeO2-device processing.

14.What is missing in autonomous discovery: Open challenges for the community

Authors:Phillip M. Maffettone, Pascal Friederich, Sterling G. Baird, Ben Blaiszik, Keith A. Brown, Stuart I. Campbell, Orion A. Cohen, Tantum Collins, Rebecca L. Davis, Ian T. Foster, Navid Haghmoradi, Mark Hereld, Howie Joress, Nicole Jung, Ha-Kyung Kwon, Gabriella Pizzuto, Jacob Rintamaki, Casper Steinmann, Luca Torresi, Shijing Sun

Abstract: Self-driving labs (SDLs) leverage combinations of artificial intelligence, automation, and advanced computing to accelerate scientific discovery. The promise of this field has given rise to a rich community of passionate scientists, engineers, and social scientists, as evidenced by the development of the Acceleration Consortium and recent Accelerate Conference. Despite its strengths, this rapidly developing field presents numerous opportunities for growth, challenges to overcome, and potential risks of which to remain aware. This community perspective builds on a discourse instantiated during the first Accelerate Conference, and looks to the future of self-driving labs with a tempered optimism. Incorporating input from academia, government, and industry, we briefly describe the current status of self-driving labs, then turn our attention to barriers, opportunities, and a vision for what is possible. Our field is delivering solutions in technology and infrastructure, artificial intelligence and knowledge generation, and education and workforce development. In the spirit of community, we intend for this work to foster discussion and drive best practices as our field grows.