Studio teorico di proprieta' di nanoparticelle con l'uso del modello "jellium" in approssimazione Hartree-Fock Ristretta

In questo lavoro di tesi abbiamo effettuato calcoli di polarizabilità su cluster metallici di dimensioni variabili in approssimazione Time Dependent Hartree Fock (TDHF) ed MP2. Il modello utilizzato è quello di una scatola cubica in cui la carica positiva è approssimato da un jellium uniforme. Per ogni atomo del cluster viene considerato solo il comportamento delgli elettroni più esterni, che possono muoversi su di un volume predefinito, maggiore o uguale a quello occupato dal cluster per tenere conto degli effetti di spill out. Particolare enfasi è posta all'uso delle simmetrie del sistema cubico per ottimizzare la soluzione del problema

A computational analysis of the vibrational absorption of molecular solids in the teraherz range

In this thesis, we deal with the application of transmission terahertz spectroscopy as an analysis tool for the study of molecular solids, in particular organic crystals of pharmaceutical interest. Most of the work has been performed using two computational packages aimed at the interpretation of the spectra, one based on molecular forcefields (DMACRYS), the other on solid state density functional theory (CASTEP). We compare low temperature determinations of several molecular organic crystals to calculated spectra, and attempt to assign calculated modes of vibrations to absorption peaks, based on the similarity in frequency between the measured and calculated peaks. One of the main aims of this work is to establish the limits of our forcefield approach, which is based on the approximation that the intramolecular degrees of freedom can be neglected. We analyse the normal modes of vibration calculated with CASTEP, evaluating the amount of rigid molecule rotational and translational contribution to each eigenvector as a function of frequency, in order to validate our forcefield approach. We also compare the two sets of eigenvectors from the DMACRYS and CASTEP calculations to assess the similarity between the two approaches. We perform the same eigenvectors analysis on several hydrate systems in order to understand the role of water in the lattice dynamics of crystalline hydrates. We attempt a classification of the eigenvectors based on the strength of the forces involved in the molecular vibrations and based on the amount of the water contribution to each normal mode. A set of isostructural crystals is analysed in order to understand the effect that small variations (in the molecular formula and in the unit cell arrangement) have on the measured and calculated absorption spectra of a crystal. Finally, we discuss the use and development of computational methods that allow us to have a more realistic description of the molecular electrostatic in DMACRYS.This work was supported by Astra Zenec

Emergence of hidden phases of methylammonium lead-iodide (CH3_3NH3_3PbI3_3) upon compression

We perform a thorough structural search with the minima hopping method (MHM) to explore low-energy structures of methylammonium lead iodide. By combining the MHM with a forcefield, we efficiently screen vast portions of the configurational space with large simulation cells containing up to 96 atoms. Our search reveals two structures of methylammonium iodide perovskite (MAPI) that are substantially lower in energy than the well-studied experimentally observed low-temperature $Pnma$ orthorhombic phase according to density functional calculations. Both structures have not yet been reported in the literature for MAPI, but our results show that they could emerge as thermodynamically stable phases via compression at low temperatures. In terms of the electronic properties, the two phases exhibit larger band gaps than the standard perovskite-type structures. Hence, pressure induced phase selection at technologically achievable pressures (i.e., via thin-film strain) is a route towards the synthesis of several MAPI polymorph with variable band gaps

Experimental and theoretical studies of tetramethoxy-p-benzoquinone: infrared spectra, structural and lithium insertion properties

International audienceIn the search for low-polluting electrode materials for batteries, the use of redox-active organic compounds represents a promising alternative to conventional metal-based systems. In this article we report a combined experimental and theoretical study of tetramethoxy-p-benzoquinone (TMQ). In carbonate-based electrolytes, electrochemical behaviour of this compound is characterized by a reversible insertion process located at approximately 2.85 V vs. Li+/Li0. This relatively high potential reactivity, coupled with our effort to develop computational methodologies in the field of organic electrode materials, prompted us to complement these experimental data with theoretical studies performed using density functional theory (DFT). Single crystals of TMQ were synthesized and thoroughly characterized showing that this quinonic species crystallised in the P21/n space group. The experimental crystal structure of TMQ was then used to assess various DFT methods. The structural features and vibrational spectra were thus predicted by using as a whole five common density functionals (PBE, LDA, revPBE, PBEsol, B3PW91) with and without a semi-empirical correction to account for the van der Waals interactions using either Grimme's (DFT-D2) or Tkatchenko-Scheffler (TS) scheme. The most reliable combination of the DFT functional and the explicit dispersion correction was chosen to study the Li-intercalated molecular crystal (LiTMQ) with the view of indentifying Li insertion sites. A very close agreement with the experiment was found for the average voltage by using the most stable relaxed hypothetical LiTMQ structure. Additionally, a comparison of vibrational spectra gained either for TMQ molecule and its dimer in gas phase or through periodic calculation was undertaken with respect to the experimentally measured infrared spectra. The topological features of the bonds were also investigated in conjunction with estimates of net atomic charges to gain insight into the effect of chemical bonding and intermolecular interaction on Li intercalation. Finally, π-electron delocalization of both quinone and alkali salts of p-semiquinone were determined using the Harmonic Oscillator model of Aromaticity (HOMA) or aromatic fluctuation index (FLU) calculations

Maximum volume simplex method for automatic selection and classification of atomic environments and environment descriptor compression

Fingerprint distances, which measure the similarity of atomic environments, are commonly calculated from atomic environment fingerprint vectors. In this work, we present the simplex method that can perform the inverse operation, i.e., calculating fingerprint vectors from fingerprint distances. The fingerprint vectors found in this way point to the corners of a simplex. For a large dataset of fingerprints, we can find a particular largest simplex, whose dimension gives the effective dimension of the fingerprint vector space. We show that the corners of this simplex correspond to landmark environments that can be used in a fully automatic way to analyze structures. In this way, we can, for instance, detect atoms in grain boundaries or on edges of carbon flakes without any human input about the expected environment. By projecting fingerprints on the largest simplex, we can also obtain fingerprint vectors that are considerably shorter than the original ones but whose information content is not significantly reduced

Playing with isomerism and N substitution in pentalenedione derivatives for organic electrode batteries: how high are the stakes?

International audienceNew concepts to design innovating and top-performing redox-active organic molecules based electrodes should push forward and promote an eco-friendly alternative to classical Li-ion batteries. In this promising research area, density functional theory calculations lend support to experiments through the prediction of redox voltage and give promise to rationalize the trends, thus providing a general approach for engineering advanced materials. In this study in which we analysed spin density/net atomic charges distribution along with global energy decomposition thanks to Bader's partitioning of the molecular space, a vision for designing pentalenedione derivatives by fine tuning of the redox potential properties is presented. The concept relies on combined effects of isomerism and N single/double substitution for CH on the parent backbone. Such dual nature modification is able to provide a series of compounds within the range of 2.2-3.6 V vs. Li+/Li (against a more restricted range of 2.2-2.8 V vs. Li+/Li for the sole effect of isomerism on the unsubstituted parent compounds). The incidence of double N substitution alone generally follows an almost additive rule based on the combined actions of the composing single N substitutions. Few exceptions to the rule were, however, also observed and rationalized. Beyond learning gained for this peculiar family, these results may have exciting implications for future design strategies

Electronic structure and energy decomposition analyses as a tool to interpret the redox potential ranking of naphtho-, biphenyl- and biphenylenequinone isomers

International audienceBy calling on modelling approaches we have performed a comparative study on the redox properties of various naphtho-, biphenyl- and biphenylene-quinone isomers. These different compounds exhibit as a whole a redox potential range between 2.09 and 2.90 V vs. Li+/Li. A specific methodology was used to decrypt the interplay among isomerism, aromaticity and antiaromaticity modifications and the stabilization/destabilization effects due to other molecular components on this key electrochemical feature for electrode materials of batteries. In particular, energy decomposition analysis, within the Quantum Theory of Atoms in Molecules, along with the electron and electron spin population changes upon reduction nicely rationalise the observed potential trends. While 1,2- and 2,3-isomers show the highest/lowest redox potential in the biphenylene-quinone series, a reverse trend is observed for the naphtho-quinone, the compound having the two carbonyl groups on distinct rings being characterized by an intermediate value in both cases. There is instead almost no differentiation between 1,2 and 2,3 isomers for the biphenyl-quinone family

Divalent Path to Enhance p-Type Conductivity in a SnO Transparent Semiconductor

The role of the divalent nature of tin is explored in tin monoxide, revealing a novel path for enhancing p-type conductivity. The consequences of oxygen off-stoichiometry indicate that a defect complex formed by a tin vacancy (V-Sn) and an impurity interstitial (D-i) leads to an increased number of free carriers as well as improved acceptor state stability when compared with the isolated V-Sn. In this study, we identify several elements that are able to stabilize such a defect complex configuration. The enhanced ionization of the resulting complex arises from the divalent nature of Sn, which allows Sn(II) and Sn(IV) oxidation states to form. Such a novel doping mechanism not only offers a path for creating a high-performance p-type transparent SnO, but reveals an as-of-yet unexplored route to improve conductivity in other compounds formed by multivalent elements, for example, Sn(II)-based thermoelectrics

Relating Electrochemistry of New Organic Materials for Batteries and Fundamental Understanding through DFT Calculations

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A study into the effect of subtle structural details and disorder on the terahertz spectrum of crystalline benzoic acid

The phonon modes of crystalline benzoic acid have been investigated using terahertz time-domain spectroscopy, rigid molecule atom–atom model potential and plane-wave density functional theory lattice dynamics calculations. The simulation results show good agreement with the measured terahertz spectra and an assignment of the terahertz absorption features of benzoic acid is made with the help of both computational methods. Focussing on the strongest interactions in the crystal, we describe each vibration in terms of distortions of the benzoic acid hydrogen bonded dimers that are present in the crystal structure. The terahertz spectrum is also shown to be highly sensitive to the location of the carboxylic acid hydrogen atoms in the cyclic hydrogen-bonded dimers and we have systematically explored the influence of the observed disorder in the hydrogen atom positions on the lattice dynamics.<br/