### Browsing by Author "Erhart, Paul"

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Item Computational Design of Alloy Nanostructures for Optical Sensing of Hydrogen(AMERICAN CHEMICAL SOCIETY, 2022-08-26) Ekborg-Tanner, Pernilla; Rahm, J. Magnus; Rosendal, Victor; Bancerek, Maria; Rossi, Tuomas P.; Antosiewicz, Tomasz J.; Erhart, Paul; Department of Applied Physics; Computational Electronic Structure Theory; Chalmers University of TechnologyPd nanoalloys show great potential as hysteresis-free, reliable hydrogen sensors. Here, a multiscale modeling approach is employed to determine optimal conditions for optical hydrogen sensing using the Pd-Au-H system. Changes in hydrogen pressure translate to changes in hydrogen content and eventually the optical spectrum. At the single particle level, the shift of the plasmon peak position with hydrogen concentration (i.e., the "optical"sensitivity) is approximately constant at 180 nm/cH for nanodisk diameters of 100 nm. For smaller particles, the optical sensitivity is negative and increases with decreasing diameter, due to the emergence of a second peak originating from coupling between a localized surface plasmon and interband transitions. In addition to tracking peak position, the onset of extinction as well as extinction at fixed wavelengths is considered. We carefully compare the simulation results with experimental data and assess the potential sources for discrepancies. Invariably, the results suggest that there is an upper bound for the optical sensitivity that cannot be overcome by engineering composition and/or geometry. While the alloy composition has a limited impact on optical sensitivity, it can strongly affect H uptake and consequently the "thermodynamic"sensitivity and the detection limit. Here, it is shown how the latter can be improved by compositional engineering and even substantially enhanced via the formation of an ordered phase that can be synthesized at higher hydrogen partial pressures.Item Dipolar coupling of nanoparticle-molecule assemblies(American Institute of Physics, 2021-03-07) Fojt, Jakub; Rossi, Tuomas P.; Antosiewicz, Tomasz J.; Kuisma, Mikael; Erhart, Paul; Chalmers University of Technology; Department of Applied Physics; University of Warsaw; University of JyväskyläStrong light-matter interactions facilitate not only emerging applications in quantum and non-linear optics but also modifications of properties of materials. In particular, the latter possibility has spurred the development of advanced theoretical techniques that can accurately capture both quantum optical and quantum chemical degrees of freedom. These methods are, however, computationally very demanding, which limits their application range. Here, we demonstrate that the optical spectra of nanoparticle-molecule assemblies, including strong coupling effects, can be predicted with good accuracy using a subsystem approach, in which the response functions of different units are coupled only at the dipolar level. We demonstrate this approach by comparison with previous time-dependent density functional theory calculations for fully coupled systems of Al nanoparticles and benzene molecules. While the present study only considers few-particle systems, the approach can be readily extended to much larger systems andto include explicit optical-cavity modes.Item Efficient Calculation of the Lattice Thermal Conductivity by Atomistic Simulations with Ab Initio Accuracy(WILEY-V C H VERLAG GMBH, 2022-02) Brorsson, Joakim; Hashemi, Arsalan; Fan, Zheyong; Fransson, Erik; Eriksson, Fredrik; Ala-Nissila, Tapio; Krasheninnikov, Arkady V.; Komsa, Hannu Pekka; Erhart, Paul; Department of Applied Physics; Multiscale Statistical and Quantum Physics; Chalmers University of TechnologyHigh-order force constant expansions can provide accurate representations of the potential energy surface relevant to vibrational motion. They can be efficiently parametrized using quantum mechanical calculations and subsequently sampled at a fraction of the cost of the underlying reference calculations. Here, force constant expansions are combined via the hiphive package with GPU-accelerated molecular dynamics simulations via the GPUMD package to obtain an accurate, transferable, and efficient approach for sampling the dynamical properties of materials. The performance of this methodology is demonstrated by applying it both to materials with very low thermal conductivity (Ba8Ga16Ge30, SnSe) and a material with a relatively high lattice thermal conductivity (monolayer-MoS2). These cases cover both situations with weak (monolayer-MoS2, SnSe) and strong (Ba8Ga16Ge30) pho renormalization. The simulations also enable to access complementary information such as the spectral thermal conductivity, which allows to discriminate the contribution by different phonon modes while accounting for scattering to all orders. The software packages described here are made available to the scientific community as free and open-source software in order to encourage the more widespread use of these techniques as well as their evolution through continuous and collaborative development.Item GPAW : An open Python package for electronic structure calculations(American Institute of Physics, 2024-03-07) Mortensen, Jens Jørgen; Larsen, Ask Hjorth; Kuisma, Mikael; Ivanov, Aleksei V.; Taghizadeh, Alireza; Peterson, Andrew; Haldar, Anubhab; Dohn, Asmus Ougaard; Schäfer, Christian; Jónsson, Elvar Örn; Hermes, Eric D.; Nilsson, Fredrik Andreas; Kastlunger, Georg; Levi, Gianluca; Jónsson, Hannes; Häkkinen, Hannu; Fojt, Jakub; Kangsabanik, Jiban; Sødequist, Joachim; Lehtomäki, Jouko; Heske, Julian; Enkovaara, Jussi; Winther, Kirsten Trøstrup; Dulak, Marcin; Melander, Marko M.; Ovesen, Martin; Louhivuori, Martti; Walter, Michael; Gjerding, Morten; Lopez-Acevedo, Olga; Erhart, Paul; Warmbier, Robert; Würdemann, Rolf; Kaappa, Sami; Latini, Simone; Boland, Tara Maria; Bligaard, Thomas; Skovhus, Thorbjørn; Susi, Toma; Maxson, Tristan; Rossi, Tuomas; Chen, Xi; Schmerwitz, Yorick Leonard A.; Schiøtz, Jakob; Olsen, Thomas; Jacobsen, Karsten Wedel; Thygesen, Kristian Sommer; Department of Applied Physics; Technical University of Denmark; Riverlane Ltd; Brown University; Boston University College of Engineering; Chalmers University of Technology; University of Iceland; Quantum-Si; University of Jyväskylä; CSC - IT Center for Science Ltd.; SLAC National Accelerator Laboratory; University of Freiburg; Universidad de Antioquia; University of the Witwatersrand, Johannesburg; Tampere University; University of Alabama at Tuscaloosa; Lanzhou University; University of ViennaWe review the GPAW open-source Python package for electronic structure calculations. GPAW is based on the projector-augmented wave method and can solve the self-consistent density functional theory (DFT) equations using three different wave-function representations, namely real-space grids, plane waves, and numerical atomic orbitals. The three representations are complementary and mutually independent and can be connected by transformations via the real-space grid. This multi-basis feature renders GPAW highly versatile and unique among similar codes. By virtue of its modular structure, the GPAW code constitutes an ideal platform for the implementation of new features and methodologies. Moreover, it is well integrated with the Atomic Simulation Environment (ASE), providing a flexible and dynamic user interface. In addition to ground-state DFT calculations, GPAW supports many-body GW band structures, optical excitations from the Bethe-Salpeter Equation, variational calculations of excited states in molecules and solids via direct optimization, and real-time propagation of the Kohn-Sham equations within time-dependent DFT. A range of more advanced methods to describe magnetic excitations and non-collinear magnetism in solids are also now available. In addition, GPAW can calculate non-linear optical tensors of solids, charged crystal point defects, and much more. Recently, support for graphics processing unit (GPU) acceleration has been achieved with minor modifications to the GPAW code thanks to the CuPy library. We end the review with an outlook, describing some future plans for GPAW.Item GPUMD: A package for constructing accurate machine-learned potentials and performing highly efficient atomistic simulations(American Institute of Physics, 2022-09-21) Fan, Zheyong; Wang, Yanzhou; Ying, Penghua; Song, Keke; Wang, Junjie; Wang, Yong; Zeng, Zezhu; Xu, Ke; Lindgren, Eric; Rahm, J. Magnus; Gabourie, Alexander J.; Liu, Jiahui; Dong, Haikuan; Wu, Jianyang; Chen, Yue; Zhong, Zheng; Sun, Jian; Erhart, Paul; Su, Yanjing; Ala-Nissila, Tapio; Department of Applied Physics; Multiscale Statistical and Quantum Physics; Multiscale Statistical and Quantum Physics; Harbin Institute of Technology; University of Science and Technology Beijing; Nanjing University; The University of Hong Kong; Xiamen University; Chalmers University of Technology; Stanford University; Bohai UniversityWe present our latest advancements of machine-learned potentials (MLPs) based on the neuroevolution potential (NEP) framework introduced in Fan et al. [Phys. Rev. B 104, 104309 (2021)] and their implementation in the open-source package gpumd. We increase the accuracy of NEP models both by improving the radial functions in the atomic-environment descriptor using a linear combination of Chebyshev basis functions and by extending the angular descriptor with some four-body and five-body contributions as in the atomic cluster expansion approach. We also detail our efficient implementation of the NEP approach in graphics processing units as well as our workflow for the construction of NEP models and demonstrate their application in large-scale atomistic simulations. By comparing to state-of-the-art MLPs, we show that the NEP approach not only achieves above-average accuracy but also is far more computationally efficient. These results demonstrate that the gpumd package is a promising tool for solving challenging problems requiring highly accurate, large-scale atomistic simulations. To enable the construction of MLPs using a minimal training set, we propose an active-learning scheme based on the latent space of a pre-trained NEP model. Finally, we introduce three separate Python packages, viz., gpyumd, calorine, and pynep, that enable the integration of gpumd into Python workflows.Item Hot-Carrier Transfer across a Nanoparticle-Molecule Junction: The Importance of Orbital Hybridization and Level Alignment(AMERICAN CHEMICAL SOCIETY, 2022-11-09) Fojt, Jakub; Rossi, Tuomas P.; Kuisma, Mikael; Erhart, Paul; Department of Applied Physics; Computational Electronic Structure Theory; Chalmers University of Technology; Technical University of DenmarkWhile direct hot-carrier transfer can increase photocatalytic activity, it is difficult to discern experimentally and competes with several other mechanisms. To shed light on these aspects, here, we model from first-principles hot-carrier generation across the interface between plasmonic nanoparticles and a CO molecule. The hot-electron transfer probability depends nonmonotonically on the nanoparticle-molecule distance and can be effective at long distances, even before a strong chemical bond can form; hot-hole transfer on the other hand is limited to shorter distances. These observations can be explained by the energetic alignment between molecular and nanoparticle states as well as the excitation frequency. The hybridization of the molecular orbitals is the key predictor for hot-carrier transfer in these systems, emphasizing the necessity of ground state hybridization for accurate predictions. Finally, we show a nontrivial dependence of the hot-carrier distribution on the excitation energy, which could be exploited when optimizing photocatalytic systems.Item Photoluminescence line shapes for color centers in silicon carbide from density functional theory calculations(American Physical Society, 2021-03-29) Hashemi, Arsalan; Linderälv, Christopher; Krasheninnikov, Arkady V.; Ala-Nissila, Tapio; Erhart, Paul; Komsa, Hannu Pekka; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Multiscale Statistical and Quantum Physics; Computational Electronic Structure Theory; Chalmers University of TechnologySilicon carbide with optically and magnetically active point defects offers unique opportunities for quantum technology applications. Since interaction with these defects commonly happens through optical excitation and deexcitation, a complete understanding of their light-matter interaction in general and optical signatures in particular is crucial. Here, we employ quantum mechanical density functional theory calculations to investigate the photoluminescence line shapes of selected, experimentally observed color centers (including single vacancies, double vacancies, and vacancy-impurity pairs) in 4H-SiC. The analysis of zero-phonon lines as well as Huang-Rhys and Debye-Waller factors is accompanied by a detailed study of the underlying lattice vibrations. We show that the defect line shapes are governed by strong coupling to bulk phonons at lower energies and localized vibrational modes at higher energies. Generally, good agreement with the available experimental data is obtained, and thus we expect our theoretical work to be beneficial for the identification of defect signatures in the photoluminescence spectra and thereby advance the research in quantum photonics and quantum information processing.Item Plasmon-Induced Direct Hot-Carrier Transfer at Metal-Acceptor Interfaces(AMERICAN CHEMICAL SOCIETY, 2019-03-26) Kumar, Priyank V.; Rossi, Tuomas P.; Marti-Dafcik, Daniel; Reichmuth, Daniel; Kuisma, Mikael; Erhart, Paul; Puska, Martti J.; Norris, David J.; Swiss Federal Institute of Technology Zurich; Department of Applied Physics; University of Jyvaskyla; Chalmers University of Technology; Electronic Properties of MaterialsPlasmon-induced hot-carrier transfer from a metal nanostructure to an acceptor is known to occur via two key mechanisms: (i) indirect transfer, where the hot carriers are produced in the metal nanostructure and subsequently transferred to the acceptor, and (ii) direct transfer, where the plasmons decay by directly exciting carriers from the metal to the acceptor. Unfortunately, an atomic-level understanding of the direct-transfer process, especially with regard to its quantification, remains elusive even though it is estimated to be more efficient compared to the indirect-transfer process. This is due to experimental challenges in separating direct from indirect transfer as both processes occur simultaneously at femtosecond time scales. Here, we employ time-dependent density-functional theory simulations to isolate and study the direct-transfer process at a model metal-acceptor (Ag 147 -Cd 33 Se 33 ) interface. Our simulations show that, for a 10 fs Gaussian laser pulse tuned to the plasmon frequency, the plasmon formed in the Ag 147 -Cd 33 Se 33 system decays within 10 fs and induces the direct transfer with a probability of about 40%. We decompose the direct-transfer process further and demonstrate that the direct injection of both electrons and holes into the acceptor, termed direct hot-electron transfer (DHET) and direct hot-hole transfer (DHHT), takes place with similar probabilities of about 20% each. Finally, effective strategies to control and tune the probabilities of DHET and DHHT processes are proposed. We envision our work to provide guidelines toward the design of metal-acceptor interfaces that enable more efficient plasmonic hot-carrier devices.Item Surface Functionalization of 2D MXenes: Trends in Distribution, Composition, and Electronic Properties(AMERICAN CHEMICAL SOCIETY, 2021-03-11) Ibragimova, Rina; Erhart, Paul; Rinke, Patrick; Komsa, Hannu Pekka; Department of Applied Physics; Computational Electronic Structure Theory; Chalmers University of TechnologyUsing a multiscale computational scheme, we study the trends in distribution and composition of the surface functional groups -O, -OH, and -F on two-dimensional (2D) transition metal carbides and nitrides (MXenes). We consider Ti2N, Ti4N3, Nb2C, Nb4C3, Ti2C, and Ti3C2 to explore MXenes with different chemistry and different number of atomic layers. Using a combination of cluster expansion, Monte Carlo, and density functional theory methods, we study the distribution and composition of functional groups at experimentally relevant conditions. We show that mixtures of functional groups are favorable on all studied MXene surfaces. The distribution of functional groups appears to be largely independent of the type of metal, carbon, or nitrogen species and/or number of atomic layers in the MXene. We further show that some properties (e.g., the work function) strongly depend on the surface composition, while others, for example, the electric conductivity, exhibit only a weak dependence.Item Tailoring Hot-Carrier Distributions of Plasmonic Nanostructures through Surface Alloying(American Chemical Society, 2024-02-16) Fojt, Jakub; Rossi, Tuomas P.; Kumar, Priyank V.; Erhart, Paul; Department of Applied Physics; Computational Electronic Structure Theory; Chalmers University of Technology; University of New South WalesAlloyed metal nanoparticles are a promising platform for plasmonically enabled hot-carrier generation, which can be used to drive photochemical reactions. Although the non-plasmonic component in these systems has been investigated for its potential to enhance catalytic activity, its capacity to affect the photochemical process favorably has been underexplored by comparison. Here, we study the impact of surface alloy species and concentration on hot-carrier generation in Ag nanoparticles. By first-principles simulations, we photoexcite the localized surface plasmon, allow it to dephase, and calculate spatially and energetically resolved hot-carrier distributions. We show that the presence of non-noble species in the topmost surface layer drastically enhances hot-hole generation at the surface at the expense of hot-hole generation in the bulk, due to the additional d-type states that are introduced to the surface. The energy of the generated holes can be tuned by choice of the alloyant, with systematic trends across the d-band block. Already low surface alloy concentrations have a large impact, with a saturation of the enhancement effect typically close to 75% of a monolayer. Hot-electron generation at the surface is hindered slightly by alloying, but here a judicious choice of the alloy composition allows one to strike a balance between hot electrons and holes. Our work underscores the promise of utilizing multicomponent nanoparticles to achieve enhanced control over plasmonic catalysis and provides guidelines for how hot-carrier distributions can be tailored by designing the electronic structure of the surface through alloying.Item Ultrastrong Coupling of a Single Molecule to a Plasmonic Nanocavity: A First-Principles Study(ACS Publications, 2022-03-16) Kuisma, Mikael; Rousseaux, Benjamin; Czajkowski, Krzysztof M.; Rossi, Tuomas P.; Shegai, Timur; Erhart, Paul; Antosiewicz, Tomasz J.; Department of Applied Physics; Computational Electronic Structure Theory; University of Jyväskylä; École normale supérieure; University of Warsaw; Chalmers University of TechnologyUltrastrong coupling (USC) is a distinct regime of light-matter interaction in which the coupling strength is comparable to the resonance energy of the cavity or emitter. In the USC regime, common approximations to quantum optical Hamiltonians, such as the rotating wave approximation, break down as the ground state of the coupled system gains photonic character due to admixing of vacuum states with higher excited states, leading to ground-state energy changes. USC is usually achieved by collective coherent coupling of many quantum emitters to a single mode cavity, whereas USC with a single molecule remains challenging. Here, we show by time-dependent density functional theory (TDDFT) calculations that a single organic molecule can reach USC with a plasmonic dimer, consisting of a few hundred atoms. In this context, we discuss the capacity of TDDFT to represent strong coupling and its connection to the quantum optical Hamiltonian. We find that USC leads to appreciable ground-state energy modifications accounting for a non-negligible part of the total interaction energy, comparable to kBT at room temperature.