Defect tolerance of lead-halide perovskite (100) surface relative to bulk: band bending, surface states, and characteristics of vacancies

We characterize formation of vacancies at a surface slab model and contrasted the results to bulk of lead-halide perovskites using cubic CsPbI$_3$ as a representative structure. The defect-free CsI-terminated (100) surface does not trap charge carriers. In the presence of defects (vacancies), the surface is anticipated to behave as $p$-type. The formation energy of cesium vacancies $V_\text{Cs}^{-}$ is lower at the surface than in bulk with iodine vacancies $V_\text{I}^{+}$ having a similar energy (near 0.25-0.4 eV) in the range of chemical potentials compatible with solution processing synthesis conditions. Major surface vacancies produce shallow host-like energy states with a small Franck-Condon shift, which renders them electronically harmless (same as in bulk). The spin-orbit coupling contributes to the defect tolerance of lead-halide perovskite surfaces by causing delocalization of electronic states associated with $n$-type defects and retraction of lowest unoccupied states from the surface due to a mixing of Pb-$p_{x,y,z}$ orbitals. The results explain a high optoelectronic performance of two-dimensional structures, nanoparticles, and polycrystalline thin films of lead-halide perovskites in spite of abundance of interfaces in these materials.Comment: 35 pages (main text) + 5 pages (supporting information

Modélisation du comportement électrochimique de matériaux pour batteries au lithium à partir de calculs de premiers principes

Rapporteurs : M. ETOURNEAU Jean, M. PASTUREL Alain Examinateurs : M. BOUCHER Florent, M. BROUSSELY Michel, M. GOURIER Didier, M. GRESSIER Pascal, M. OUVRARD Guy, M. SCHWARZ KarlheinzThe functioning of a positive electrode in a lithium battery is based on the reversible intercalation of lithium. In some cases, such a reaction can lead to important structural modifications and therefore to an amorphization of the material. A theoretical approach is presented here that leads to structural predictions and simulations of electrochemical behaviour of positive electrode materials. In the first part, DFT (Density Functional Theory) formalisms and the respective advantages of FLAPW (Full potential Linearized Augmented Plane Waves) and PP/PW (Pseudopotential / Plane Waves) methods are discussed. In the second part are given some fundamental electrochemistry considerations related to the intercalation process, thermodynamics aspects and relationships with electronic structure. Then, an approach combining experimental data and geometry optimisation of structural hypotheses is given. This approach was first applied to a model compound LiMoS2, and has been then generalised to systems of industrial interest such as LixV2O5 (0 ≤ x ≤ 3). The simulated X-ray diagrams of the optimised structures for LiMoS2 and ω- Li3V2O5 are in good agreement with experimental data. In the case of LixV2O5, the first discharge curves starting from α-V2O5 and γ'-V2O5 were then successfully simulated. A chemical bond analysis was carried out to help understand the origin of the distortion in LiMoS2 and the voltage variations in the electrochemical curves of LixV2O5. This study clearly demonstrates that an approach combining first-principle calculations and available experimental data is invaluable in the structure determination of poorly crystallised compounds. Such a procedure contributes to the understanding of the phase transitions induced by the lithium intercalation in vanadium oxide compounds and can really be used in the research of new battery materials.Le fonctionnement d'une électrode positive de batterie au lithium repose sur la possibilité d'intercaler de façon réversible du lithium au sein du matériau qui la constitue. Une telle réaction conduit souvent à une perte de la cristallinité du matériau. Une démarche théorique permettant d'accéder à la structure du composé et à la modélisation de son comportement électrochimique est présentée dans ce mémoire. La première partie expose les fondements de la DFT (Density Functional Theory), et les mérites respectifs des méthodes FLAPW (Full potential Linearized Augmented Plane Waves) et PP/PW (Pseudopotential / Plane Waves). La seconde partie rappelle quelques concepts fondamentaux d'électrochimie comme le processus d'intercalation, les aspects thermodynamiques et les relations avec la structure électronique. Ensuite, une démarche basée sur l'optimisation de la géométrie de différentes hypothèses structurales est présentée. Cette démarche a été appliquée à l'étude d'un composé modèle LiMoS2, et a ensuite été étendue à des composés d'intérêt industriel tels que LixV2O5 (0 ≤ x ≤ 3). Ainsi, pour LiMoS2 et ω-Li3V2O5, les structures optimisées permettent de simuler des diagrammes de diffraction RX en très bon accord avec l'expérience. Ceci a donc permis, dans le cas de LixV2O5, une modélisation des premières décharges partant de α-V2O5 et de γ'-V2O5. Afin de mieux comprendre l'origine de la distorsion dans LiMoS2 et des variations de potentiel des courbes électrochimiques de LixV2O5, une analyse de la liaison chimique a également été réalisée. Ces résultats mettent clairement en évidence le fait qu'une approche couplant calculs de premiers principes et expériences constitue une aide efficace à la détermination de la structure de composés mal cristallisés. Une telle démarche contribue à la compréhension des transformations structurales induites par l'intercalation du lithium dans des oxydes de vanadium et peut ainsi être utile à la recherche de nouveaux matériaux de batteries

Eu- and Tb-adsorbed Si3_{3}N4_{4} and Ge3_{3}N4_{4}: tuning the colours with one luminescent host

Phosphor-converted white light emitting diodes (pc-LEDs) are efficient light sources for applications in lighting and electronic devices. Nitrides, with their wide-ranging applicability due to their intriguing structural diversity, and their auspicious chemical and physical properties, represent an essential component in industrial and materials applications. Here, we present the successful adsorption of Eu and Tb at the grain boundaries of bulk β-Si$_{3}$N$_{4}$ and β-Ge$_{3}$N$_{4}$ by a successful combustion synthesis. The adsorption of europium and terbium, and the synergic combination of both, resulted in intriguing luminescence properties of all compounds (red, green, orange and yellow). In particular, the fact that one host can deliver different colours renders Eu,Tb-β-M$_{3}$N$_{4}$ (M = Si, Ge) a prospective chief component for future light emitting diodes (LEDs). For the elucidation of the electronic properties and structure of β-Si$_{3}$N$_{4}$ and β-Ge$_{3}$N$_{4}$, Mott–Schottky (MS) measurements and density functional theory (DFT) computations were conducted for the bare and RE adsorbed samples

Potential room-temperature multiferroicity in cupric oxide under high pressure

International audienceCuO, known to be multiferroic (MF) from T-L = 213 K to T-N = 230 K at ambient pressure, has been the subject of debates about its ability to exhibit multiferroicity at room temperature (RT) under high hydrostatic pressure. Here we address this question based on theoretical and experimental investigations. The influence of hydrostatic pressure on T-L and T-N has been estimated from ab initio calculations combined with classical Monte-Carlo simulations and a quasi-1D antiferromagnetic analytical model. From the experimental side, electric permittivity anomalies related to ferroelectric transitions have been followed with dielectric measurements on single crystals up to 6.1 GPa. We show that the temperature T-N below which the MF state forms increases with pressure linearly to higher pressure that hitherto supposed, and indeed based on our calculations, should exceed RT above about 20 GPa

Predicting experimentally stable allotropes: Instability of penta-graphene

International audienceIn recent years, a plethora of theoretical carbon allotropes have been proposed, none of which has been experimentally isolated. We discuss here criteria that should be met for a new phase to be potentially experimentally viable. We take as examples Haeckelites, 2D networks of sp2-carbon–containing pentagons and heptagons, and “penta-graphene,” consisting of a layer of pentagons constructed from a mixture of sp2- and sp3-coordinated carbon atoms. In 2D projection appearing as the “Cairo pattern,” penta-graphene is elegant and aesthetically pleasing. However, we dispute the author’s claims of its potential stability and experimental relevanc

SnCN₂: A Carbodiimide with an Innovative Approach for Energy Storage Systems and Phosphors in Modern LED Technology

The carbodiimide SnCN$_{2}$ was prepared at low temperatures (400 °C–550 °C) by using a patented urea precursor route. The crystal structure of SnCN$_{2}$ was determined from single‐crystal data in space group C2/c (no. 15) with a=9.1547(5), b=5.0209(3), c=6.0903(3) Å, β=117.672(3), V=247.92 Å$^{3}$ and Z=4. As carbodiimide compounds display remarkably high thermal and chemical resistivity, SnCN$_{2}$ has been doped with Eu and Tb to test it for its application in future phosphor‐converted LEDs. This doping of SnCN$_{2}$ proved that a color tuning of the carbodiimide host with different activator ions and the combination of the latter ones is possible. Additionally, as the search for novel high‐performing electrode materials is essential for current battery technologies, this carbodiimide has been investigated concerning its use in lithium‐ion batteries. To further elucidate its application possibilities in materials science, several characterization steps and physical measurements (XRD, in situ XANES, Sn Mössbauer spectroscopy, thermal expansion, IR spectroscopy, Mott‐Schottky analysis) were carried out. The electronic structure of the n‐type semiconductor SnCN$_{2}$ has been probed using X‐ray absorption spectroscopy and density functional theory (DFT) computations

Atomic Configuration of Nitrogen Doped Single-Walled Carbon Nanotubes

Having access to the chemical environment at the atomic level of a dopant in a nanostructure is crucial for the understanding of its properties. We have performed atomically-resolved electron energy-loss spectroscopy to detect individual nitrogen dopants in single-walled carbon nanotubes and compared with first principles calculations. We demonstrate that nitrogen doping occurs as single atoms in different bonding configurations: graphitic-like and pyrrolic-like substitutional nitrogen neighbouring local lattice distortion such as Stone-Thrower-Wales defects. The stability under the electron beam of these nanotubes has been studied in two extreme cases of nitrogen incorporation content and configuration. These findings provide key information for the applications of these nanostructures.Comment: 25 pages, 13 figure

NMR parameters in alkali, alkaline earth and rare earth fluorides from first principle calculations

19F isotropic chemical shifts for alkali, alkaline earth and rare earth of column 3 basic fluorides are measured and the corresponding isotropic chemical shieldings are calculated using the GIPAW method. When using PBE exchange correlation functional for the treatment of the cationic localized empty orbitals of Ca2+, Sc3+ (3d) and La3+ (4f), a correction is needed to accurately calculate 19F chemical shieldings. We show that the correlation between experimental isotropic chemical shifts and calculated isotropic chemical shieldings established for the studied compounds allows to predict 19F NMR spectra of crystalline compounds with a relatively good accuracy. In addition, we experimentally determine the quadrupolar parameters of 25Mg in MgF2 and calculate the electric field gradient of 25Mg in MgF2 and 139La in LaF3 using both PAW and LAPW methods. The orientation of the EFG components in the crystallographic frame, provided by DFT calculations, is analysed in term of electron densities. It is shown that consideration of the quadrupolar charge deformation is essential for the analysis of slightly distorted environments or highly irregular polyhedra.Comment: 18 pages, 8 figures, 4 tables and ES