Jaakko Akola

Norwegian University of Science and Technology | NTNU · Department of Physics

Understanding the relation between the structural disorder in the atomic geometry of the recrystallized state of phase-change memory materials and the localized states in the electronic structure is necessary not only for technological advances, but also essential to achieve a fundamental understanding of these materials. In this computational stud...

Transition metal dichalcogenides are cheap and earth-abundant candidates for the replacement of precious metals as catalyst materials. The electrocatalytic activity of MoS2 displays rich behavior depending on several system specifics, and fundamental insight on atomic-scale reaction mechanisms is necessary. Here, we investigate the activation of th...

Machine-learned multi-orbital tight-binding (MMTB) Hamiltonian models have been developed to describe the electronic characteristics of intermetallic compounds Mg2Si, Mg2Ge, Mg2Sn, and Mg2Pb subject to strain. The MMTB models incorporate spin-orbital mediated interactions and they are calibrated to the electronic band structures calculated via dens...

When the amorphous state of a chalcogenide phase-change material is formed inside an electronic-memory device via Joule heating, caused by an applied voltage pulse, it is in the presence of excess field-induced electrons and holes. Here, hybrid density-functional-theory calculations for glassy Ge2Sb2Te5 demonstrate that extra electrons are trapped...

Density functional/molecular dynamics simulations have been performed to shed light on the drift of Ag atoms in an amorphous GeS2 solid-state electrolyte between Ag and Pt electrodes in the presence of a finite electric field. The system models a conductive bridging random access memory device, where the electric field induces the formation of cond...

We have developed multi-orbital tight-binding (MOTB) Hamiltonian models to describe the electronic characteristics of intermetallic compounds Mg2Si, Mg2Ge, Mg2Sn, and Mg2Pb subject to strain. We have incorporated spin-orbital mediated interactions and calibrated the MOTB models to the band structures of density functional theory (DFT) by a massivel...

Structural relaxation of amorphous phase-change-memory materials has been attributed to defect-state annihilation from the band gap, leading to a time-dependent drift in the electrical resistance, which hinders the development of multi-level memory devices with increased data-storage density. In this computational study, homogeneous electric fields...

The engineering of the optical response of materials is a paradigm that demands microscopic-level accuracy and reliable predictive theoretical tools. Here we compare and contrast the dispersive permittivity tensor, using both a low-energy effective model and density functional theory (DFT). As a representative material, phosphorene subject to strai...

The engineering of the optical response of materials is a paradigm that demands microscopic-level accuracy and reliable predictive theoretical tools. Here we compare and contrast the dispersive permittivity tensor, using both a low-energy effective model and density functional theory (DFT). As a representative material, phosphorene subject to strai...

Molecular dynamics simulations using a density functional description of energies and forces have been carried out for a model of an as-deposited (AD) surface of amorphous selenium. The deposition model assumed the annealing (at 400~K) of layers of randomly located single atoms, followed by compression to the density used in earlier melt-quenched (...

A Correction to this paper has been published: https://doi.org/10.1038/s41563-021-00956-x.

The Mg-Zn and Al-Zn binary alloys have been investigated theoretically under static isotropic pressure. The stable phases of these binaries on both initially hexagonal-close-packed (HCP) and face-centered-cubic (FCC) lattices have been determined by utilizing an iterative approach that uses a configurational cluster expansion method, Monte Carlo se...

Cluster expansion (CE) has gained an increasing level of popularity in recent years, and its applications go far beyond its original root in binary alloys, reaching even complex crystalline systems often used in energy materials research. Similar to other modern machine learning approaches in materials science, many strategies have been proposed fo...

Understanding the liquid structure provides information that is crucial to uncovering the nature of the glass-liquid transition. We apply an aerodynamic levitation technique and high-energy X-rays to liquid (l)-Er2O3 to discover its structure. The sample densities are measured by electrostatic levitation at the International Space Station. Liquid E...

The broken symmetry in the atomic-scale ordering of glassy versus crystalline solids leads to a daunting challenge to provide suitable metrics for describing the order within disorder, especially on length scales beyond the nearest neighbor that are characterized by rich structural complexity. Here, we address this challenge for silica, a canonical...

Utilizing quantum effects in complex oxides, such as magnetism, multiferroicity and superconductivity, requires atomic-level control of the material’s structure and composition. In contrast, the continuous conductivity changes that enable artificial oxide-based synapses and multiconfigurational devices are driven by redox reactions and domain recon...

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The Josephson current is investigated in a superconducting graphene bilayer where pristine graphene sheets can make in-plane or out-of-plane displacements with respect to each other. The superconductivity can be of an intrinsic nature, or due to a proximity effect. The results demonstrate that the supercurrent responds qualitatively differently to...

Using the density functional theory of electronic structure, we compute the anisotropic dielectric response of bulk black phosphorus subject to strain. Employing the obtained permittivity tensor, we solve Maxwell's equations and study the electromagnetic response of a layered structure comprising a film of black phosphorus stacked on a metallic sub...

The Josephson current is investigated in a superconducting graphene bilayer where the pristine graphene sheets can make in-plane or out-of-plane displacements with respect to each other. The superconductivity can be of intrinsic nature, or due to a proximity effect. The results demonstrate that the supercurrent responds qualitatively differently to...

Molecular dynamics simulations of liquid and glassy selenium have been carried out using density functional (400–773 K, 600 atoms) and classical force field (290–500 K, 5488 atoms) methods. Structural features (structure factors, pair distribution functions, bond lengths, bond and dihedral angles, cavities) and dynamical properties (diffusion coeff...

Using the density functional theory of electronic structure, we compute the anisotropic dielectric response of bulk black phosphorus subject to strain. Employing the obtained permittivity tensor, we solve Maxwell's equations and study the electromagnetic response of a layered structure comprising a film of black phosphorus stacked on a metallic sub...

Ternary Al-Mg-Si alloys have been modelled based on a multi-scale approach that spans across atomistic and mesoscale models and uses theoretically determined parameters. First, a cluster expansion model for total energy has been trained for atomistic configurations (FCC lattice) based on the data from density functional simulations of electronic st...

Binary Mg-Zn and Al-Zn alloys have been investigated theoretically under static isotropic pressure. The stable phases of these binaries on both initially hexagonal-close-packed (hcp) and face-centered-cubic (fcc) lattices have been determined by utilizing an iterative approach that uses a configurational cluster expansion method, Monte Carlo search...

The semi-hydrogenation of acetylene (C2H2 + H2 = C2H4, ΔH = −172 kJ mol−1) is a well-studied reaction that is important for purification of ethylene, C2H4, feed used in polyethylene production. Pd-based catalysts are most commonly used to remove acetylene from ethylene feed prior to Ziegler–Natta polymerization because acetylene is a poison for Zie...

Semihydrogenation of acetylene (SHA) in ethylene-rich stream is an important process for the polymer industries. Presently, Pd-based catalysts have demonstrated good acetylene conversion (XC2H2); however, at the expense of ethylene selectivity (SC2H4). In this study, we have employed a systematic approach using density functional theory (DFT) to id...

A glass that won't break Oxide glasses are important for applications ranging from smartphone screens to window panes. One familiar feature of glass is that it fractures and shatters when rapidly deformed, limiting the number of potential uses. However, Frankberg et al. found that they could deform thin films of glassy alumina (Al 2 O 3 ) with high...

In article number 1805046, Jaakko Akola, Marco Maccarini, and co‐workers use neutron reflectometry and molecular dynamics simulations to quantify how the interaction between cationic functionalized gold nanoparticles and model lipid membranes depends on temperature and the lipid charge. The combination of scattering studies and the computational ap...

Adsorption of molecular oxygen on a Cu55 cluster and the resulting oxidation effects have been investigated by the spin-polarized density functional theory (DFT). The optimal structure for each Cu55O2N (N = 1-20) complex has been obtained via a sequential adding of O2 and systematic screening of the preferable adsorption sites. Upon structural opti...

Understanding the molecular mechanisms governing nanoparticle–membrane interactions is of prime importance for drug delivery and biomedical applications. Neutron reflectometry (NR) experiments are combined with atomistic and coarse‐grained molecular dynamics (MD) simulations to study the interaction between cationic gold nanoparticles (AuNPs) and m...

Materials exhibiting a substitutional disorder such as multicomponent alloys and mixed metal oxides/oxyfluorides are of great importance in many scientific and technological sectors. Disordered materials constitute an overwhelmingly large configurational space, which makes it practically impossible to be explored manually using first-principles cal...

Monolayer-protected gold nanoparticles are emerging as promising candidates for drug delivery due to their ability to permeate through plasma membranes. Understanding the molecular mechanisms of such complex systems is crucial to control cell permeation and to develop efficient biomedical delivery applications based on nanoscale gold nanoparticles....

High-pressure synthesis of denser glass has been a longstanding interest in condensed-matter physics and materials science because of its potentially broad industrial application. Nevertheless, understanding its nature under extreme pressures has yet to be clarified due to experimental and theoretical challenges. Here we reveal the formation of OSi...

The Cluster Expansion formalism based on Density Functional Theory (DFT) simulation data has been applied for Al–Mg alloys with high accuracy (∼1 meV/atom). The atomistic simulations are used to model the Al–Mg phase diagram, phase boundaries and the initial solute clustering at different compositions and temperatures. The obtained free energies of...

Materials exhibiting a substitutional disorder such as multicomponent alloys and mixed metal oxides/oxyfluorides are of great importance in many scientific and technological sectors. Disordered materials constitute an overwhelmingly large configurational space, which makes it practically impossible to be explored manually using first-principles cal...

Due to the small size and large surface area of thiolate-protected Au nanoclusters (NCs), the protecting ligands are expected to play a substantial role in modulating the structure and properties, particularly in the solution phase. How-ever, little is known on how thiolate ligands explicitly modulate the structural properties of the NCs at atomic...

Sulfur deactivation and regeneration behavior of the Pd/Al2O3 catalyst has been investigated via experimental characterization and density functional theory (DFT) simulations. During the sulfur exposure, PdO crystallites grow slightly while bulk Al2(SO4)3 forms on the support. DFT calculations indicate that SOx species interact strongly with the ca...

Chemical insights from simulations help to design a phase-change material that can crystallize in less than a nanosecond

Crystallization of liquid antimony has been studied at 600 K using six density functional/molecular dynamics simulations with up to 882 atoms and three scenarios: one completely disordered sample that did not crystallize even after 570 ps, four with fixed crystalline slab templates, and one with a fixed crystalline seed. Crystallization proceeded l...

Latest qualitative and quantitative results + preliminary MD simulation results on room temperature plastic deformation of nanostructured Al2O3 thin films.

Density functional/molecular dynamics simulations have been performed on liquidantimony (588 atoms and six temperatures between 600 K and 1300 K) and on neutral Sb clusters with up to 14 atoms. We study structural patterns (coordination numbers, bond angles, and ring patterns, structure factors, pair distribution functions) and dynamical properties...

The labeling of proteins with heavy atom clusters is of paramount importance in biomedical research, but its detailed molecular mechanism remains unknown. Here we uncover it for the particular case of the anti-influenza N9 neuraminidase NC10 antibody against a glutathione-coated gold cluster by means of ab initio QM/MM calculations. We show that th...

Density functional simulations have been performed for Au7Cu23 and Au23Cu7 clusters on MgO(100) supports to probe their catalytic activity for CO oxidation. The adsorption of reactants, O2 and CO, and potential O2 dissociation have been investigated in detail by tuning the location of vacancies (F-center, V-center) in MgO(100). The total charge on...

Understanding the structure and composition of nanosized gold-copper (AuCu) clusters is crucial for designing an effective AuCu catalyst. Global optimization of AuCu clusters using atomistic force fields is a viable solution for clusters with at least a few nm sizes, because of its fast computation. Here we develop an atomistic many-body potential...

The analysis of extensive density functional/molecular dynamics simulations (over 500 atoms, up to 100 ps) of liquid bismuth at four temperatures between 573 K and 1023 K has provided details of the dynamical structure factors, the dispersion of longitudinal and transverse collective modes, and related properties (power spectrum, viscosity, and sou...

A comprehensive theoretical study of an Au15Cu15 cluster on MgO(100) supports and its catalytic activity for CO oxidation has been performed based on the density functional theory and microkinetic modeling. Molecular adsorption and different reaction paths based on the Langmuir−Hinshelwood (LH) and Eley−Rideal (ER) mechanisms have been explored by...

Three extensive density functional/molecular dynamics simulations of the crystallization of amorphous Ge2Sb2Te5 (460 atoms) [Phys. Rev. B 90, 184109 (2014)] have been completed with simulation times of up to 8.2 ns. Together with the results of earlier simulations with and without a crystallite seed, the results clarify essential features of a comp...

Quantum mechanics / molecular mechanics (QM/MM) simulations have been performed to study the effects of aqueous solvent and biological ligands onto structural and electronic properties of thiolate-protected Au 25 (SR) 18-clusters. The nanocluster structure experiences modest changes in the solvent, which are seen as flexibility (" fluxionality ") i...

The remarkable ability of phase change materials (PCM) to switch between amorphous and crystalline states on a nanosecond time scale could provide new opportunities for graphene engineering. We have used density functional calculations to investigate the structures and electronic properties of heterostructures of thin amorphous and crystalline film...

We uncover the electronic structure of molecular graphene produced by adsorbed CO molecules on a copper (111) surface by means of first-principles calculations. Our results show that the band structure is fundamentally different from that of conventional graphene, and the unique features of the electronic states arise from co-existing honeycomb and...

Adsorption and dissociation of H2 and hydrogenation of 1-pentyne on neutral and anionic Cu20 clusters have been investigated using the density functional theory and microkinetic modelling. Molecular adsorption of H2 is found to occur strictly at atop sites. The H2 dimer is activated upon adsorption, and the dissociation occurs with moderate energy...

The rate-limiting process in phase change (PC) optical memories is the extremely rapid (nanosecond time scale) crystallisation of nanosized amorphous “marks” in a polycrystalline layer. Our knowledge of the amorphous and ordered structures of Ge/Sb/Te and Ag/In/Sb/Te al- loys has improved significantly in recent years and has led to plausible pictu...

The gold cluster compounds Au38(SC2H4Ph)24 and [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ are known to possess bi-icosahedral Au23 and Au25 cores, respectively, inside their ligand shells. These Au cores can be considered as quasi-molecules composed of two Au13 superatoms sharing three and one Au+ atoms, respectively. In the present work, we studied the structu...

Density functional/molecular dynamics simulations have been performed to determine structural and other properties of amorphous Ag/Ge/S and Ge/S alloys. In the former, the calculations have been combined with experimental data (x-ray and neutron diffraction, extended x-ray absorption fine structure). Ag/Ge/As alloys have high ionic conductivity and...

The effects of aqueous solvent and biological ligands on the structural and electronic properties of thiolate-protected Au25(SR)18– clusters have been studied by performing quantum mechanics/molecular mechanics (QM/MM) simulations. Analysis of bond distances and angles show that the solvated nanocluster experiences modest structural changes, which...

Several Ag–S nanoclusters where the cluster core comprises mixed metal (main component) and sulfur atoms show superatomic orbitals in the conduction band edge. However, there are no superatomic states, i.e., delocalized electrons, in the valence band, and the clusters in question can be labeled as “zerovalent”. We show here an example of an Ag–S cl...

While understanding the properties of materials under stress is fundamentally important, designing experiments to probe the effects of large tensile stress is difficult. Here tensile stress is created in thin films of potassium (up to 4 atomic layers) by epitaxial growth on a rigid support, graphite. We find that this " simple " metal shows a long-...