Condensed Matter

Oxygen vacancies have been identified to play an important role in accelerating grain growth in polycrystalline perovskite-oxide ceramics. In order to advance the fundamental understanding of growth mechanisms at the atomic scale, classical atomistic simulations were carried out to investigate the atomistic structures and oxygen vacancy formation energies at grain boundaries in the prototypical perovskite-oxide material SrTiO 3 . In this work, we focus on two symmetric tilt grain boundaries, namely Σ 5(310)[001] and Σ 5(210)[001]. A one-dimensional continuum model is adapted to determine the electrostatic potential induced by charged lattice planes in atomistic structure models containing grain boundaries and point defects. By means of this model, electrostatic artifacts, which are inherent to supercell models with periodic or open boundary conditions, can be taken into account and corrected properly. We report calculated formation energies of oxygen vacancies on all the oxygen sites across boundaries between two misoriented grains, and we analyze and discuss the formation-energy values with respect to local charge densities at the vacant sites.

The computational design of materials with ionic bonds poses a critical challenge to thermodynamic modeling since density functional theory yields inaccurate predictions of their formation enthalpies. Progress requires leveraging physically insightful correction methods. The recently introduced coordination corrected enthalpies (CCE) method delivers accurate formation enthalpies with mean absolute errors close to room temperature thermal energy, i.e., 25meV/atom. The CCE scheme, depending on the number of cation-anion bonds and oxidation state of the cation, requires an automated analysis of the system to determine and apply the correction. Here, we present AFLOW-CCE -- our implementation of CCE into the AFLOW framework for computational materials design. It features a command line tool, a web interface and a Python environment. The workflow includes a structural analysis, automatically determines oxidation numbers, and accounts for temperature effects by parametrizing vibrational contributions to the formation enthalpy per bond.

Recently cycloarene has been experimentally obtained in a self-assembled structure, forming graphene-like monoatomic layered systems. Here, we establish the bandgap engineering/prediction in cycloarene assemblies within a combination of density functional theory and tight-binding Hamiltonians. Our results show a weak dependence of the gap with the assembly geometry, contrasting a strong dependence with the inter-molecule bond density. We derived a effective model that allows the interpretation of the arising energy gap for general particle-hole symmetric molecular arranges based on inter-molecular bond strength.

Electron energy-loss spectroscopy (EELS) is a powerful tool for imaging chemical variations at the nanoscale. Here, we investigate a polymer/organic small molecule-blend used as absorber layer in an organic solar cell and employ EELS for distinguishing polymer donor and small molecule acceptor domains in the nanostructured blend based on elemental maps of light elements, such as nitrogen, sulfur or fluorine. Especially for beam sensitive samples, the electron dose needs to be limited, therefore optimized acquisition and data processing strategies are required. We compare data acquired on a post-column energy filter with a direct electron detection camera to data from a conventional CCD camera on the same filter and we investigate the impact of statistical data processing methods (principal components analysis, PCA) on acquired spectra and elemental maps extracted from spectrum images. Our work shows, that the quality of spectra on a direct electron detection camera is far superior to conventional CCD imaging, and thereby allows clear identification of ionization edges and the fine structure of these edges. For the quality of the elemental maps, the application of PCA is essential to allow a clear separation between the donor and acceptor phase in the bulk heterojunction absorber layer of a non-fullerene organic solar cell.

We study the effects of bismuth doping on the crystal structure and phase transitions in single crystals of the perovskite semiconductor methylammonium lead tribromide, MAPbBr3. By measuring temperature-dependent specific heat capacity (Cp) we find that, as Bi doping increases, the phase transition assigned to the cubic to tetragonal phase boundary decreases in temperature. Furthermore, after doping we observe one phase transition between 135 and 155 K, in contrast to two transitions observed in the undoped single crystal. These results appear strikingly similar to previously reported effects of mechanical pressure on perovskite crystal structure. Using X-ray diffraction, we show that the lattice constant decreases as Bi is incorporated into the crystal, as predicted by density functional theory (DFT). We propose that bismuth substitutional doping on the lead site is dominant, resulting in BiPb+ centers which induce compressive chemical strain that alters the crystalline phase transitions.

The dependencies of the B i O i defect concentration on doping, irradiation fluence and particle type in p-type silicon diodes have been investigated. We evidenced that large data scattering occurs for fluences above 10 12 1 MeV neutrons/cm 2 , becoming significant larger for higher fluences. We show that the B i O i defect is metastable, with two configurations A and B, of which only A is detected by Deep Level Transient Spectroscopy and Thermally Stimulated Currents techniques. The defect's electrical activity is influenced by the inherent variations in ambient and procedural experimental conditions, resulting not only in a large scattering of the results coming from the same type of measurement but making any correlation between different types of experiments difficult. It is evidenced that the variations in [B i O A i ] are triggered by subjecting the samples to an excess of carriers, by either heating or an inherent short exposure to ambient light when manipulating the samples prior to experiments. It causes ??7h variations in both, the [B i O A i ] and in the effective space charge. The analyses of structural damage in a diode irradiated with 10 19 1 MeV neutrons/cm 2 revealed that the Si structure remains crystalline and vacancies and interstitials organize in parallel tracks normal to the Si-SiO 2 interface.

A laser multicharged ion source was used to perform interfacial treatment of the 4H-SiC/ SiO2 interface using B and Ba ions. A Q-switched Nd:YAG laser (wavelength {\lambda} = 1064 nm, pulse width {\tau} = 7 ns, and fluence F = 135 J/cm2) was used to ablate B and Ba targets to generate multicharged ions. The ions were deflected by an electrostatic field to separate them from the neutrals. The multicharged ions were used for nanometer layer growth and shallow ion implantation in 4H-SiC. Several metal-oxide-semiconductor capacitors (MOSCAP) were fabricated with a combination of B and Ba at the SiC/SiO2 interface. High-low C-V measurements were used to characterize the MOSCAPs. The B interfacial layer reduced the MOSCAP flatband voltage from 4.5 to 0.04 V, while the Ba layer had a negligible effect.

Coherent precipitation of ordered phases is responsible for providing exceptional high temperature mechanical properties in a wide range of compositionally complex alloys (CCAs). Ordered phases are also essential to enhance the magnetic or catalytic properties of alloyed nanoparticles. The present work aims at demonstrating the relevance of Bragg coherent diffraction imaging (BCDI) to study bulk and thin film samples or isolated nanoparticles containing coherent nanoprecipitates / ordered phases. Crystals of a few tens of nanometres are modelled with realistic interatomic potentials and relaxed after introduction of coherent ordered nanoprecipitates. Diffraction patterns from fundamental and superstructure reflections are calculated in the kinematic approximation and used as input to retrieve the strain fields using algorithmic inversion. We first tackle the case of single nanoprecipitates and show that the strain field distribution from the ordered phase is retrieved very accurately. Then, we investigate the influence of the order parameter S on the strain field retrieved from the superstructure reflections and evidence that a very accurate strain distribution can be retrieved for partially ordered phases with large and inhomogeneous strains. In a subsequent section, we evaluate the relevance of BCDI for the study of systems containing many precipitates and demonstrate that the technique is relevant for such systems. Finally, we discuss the experimental feasibility of using BCDI to image ordered phases, in the light of the new possibilities offered by the 4 th generation synchrotron sources.

Lithium superionic conductors (LSCs) are of major importance as solid electrolytes for next-generation all-solid-state lithium-ion batteries. While ab initio molecular dynamics have been extensively applied to study these materials, there are often large discrepancies between predicted and experimentally measured ionic conductivities and activation energies due to the high temperatures and short time scales of such simulations. Here, we present a strategy to bridge this gap using moment tensor potentials (MTPs). We show that MTPs trained on energies and forces computed using the van der Waals optB88 functional yield much more accurate lattice parameters, which in turn leads to accurate prediction of ionic conductivities and activation energies for the Li 0.33 La 0.56 TiO 3 , Li 3 YCl 6 and Li 7 P 3 S 11 LSCs. NPT MD simulations using the optB88 MTPs also reveal that all three LSCs undergo a transition between two quasi-linear Arrhenius regimes at relatively low temperatures. This transition can be traced to an expansion in the number and diversity of diffusion pathways, in some cases with a change in the dimensionality of diffusion. This work presents not only an approach to develop high accuracy MTPs, but also outlines the diffusion characteristics for LSCs which is otherwise inaccessible through ab initio computation.

Photo-induced phase-segregation in mixed halide perovskite MAPb(BrxI1-x)3 is investigated in the full compositional range by correlative X-ray diffraction and photo-luminescence experiments.