Sede di Cagliari, Italy
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Resta R.,University of Trieste | Resta R.,CNR Institute of Materials
Physical Review Letters | Year: 2010

Flexoelectricity is the linear response of polarization to a strain gradient. Here we address the simplest class of dielectrics, namely, elemental cubic crystals, and we prove that therein there is no extrinsic (i.e., surface) contribution to flexoelectricity in the thermodynamic limit. The flexoelectric tensor is expressed as a bulk response of the solid, manifestly independent of surface configurations. Furthermore, we prove that the flexoelectric responses induced by a long-wavelength phonon and by a uniform strain gradient are identical. © 2010 The American Physical Society.


Filippetti A.,CNR Institute of Materials | Mattoni A.,CNR Institute of Materials
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

The methylammonium lead iodide perovskites at the core of recently proposed solar cells with exceptionally large quantum conversion efficiency are studied by first-principles methods. Large absorption coefficients (0.03-0.04 nm-1 for wavelength ∼500 nm) and small effective masses suited for both n-type and p-type transport are obtained as a consequence of their peculiar structural and electronic characteristics. In particular, the presence of a direct gap between highly dispersed Pb(6s)-I(5p) valence bands and Pb(6p) conduction bands is the key ingredient at the basis of their excellent performance in photovoltaic applications. © 2014 American Physical Society.


Piccinin S.,CNR Institute of Materials | Stamatakis M.,University College London
ACS Catalysis | Year: 2014

CO oxidation on O-precovered Pd(111) surfaces exhibits remarkably different reactivities at different temperatures, which correlate with structural changes in the atomic O overlayer. Stoichiometric titration experiments by Nakai et al. (J. Chem. Phys. 2006, 124, 224712) show that although the p(2 × 2) ordered phase is inert, the (√3 × √3) and p(2 × 1) phases that form at 320 and 190 K, respectively, have different apparent activation energies and reaction orders with respect to O coverage. In this work, we perform first-principles-based kinetic Monte Carlo (kMC) simulations to understand the behavior of this catalytic system and shed light on the origin of the changes in reactivity. Accounting explicitly for lateral interactions among adsorbates and for their impact on the activation energies of the elementary processes, our simulations reproduce quantitatively the main features of the experimental measurements, and we show that the relative rates of CO adsorption and surface reaction are different as the temperature changes. We find that ordering of the adsorbate layer strongly depends on the strength of the lateral interactions but does not have a significant role on the catalytic properties of the system. © 2014 American Chemical Society.


Vargiu A.V.,CNR Institute of Materials | Magistrato A.,International School for Advanced Studies
Inorganic Chemistry | Year: 2012

Molecules that selectively recognize DNA mismatches (MMs) play a key role as nucleic acids probes and as chemotherapeutic agents. Metalloinsertors bind to the minor groove (mG) of double strand (ds) DNA, expelling the mismatched base pairs and acting as their p-stacking replacement. In contrast, metallo-intercalators bind to the major groove (MG) of ds DNA and p-stack to adjacent base pairs. In this study we focused on structural and energetic properties of Δ-[Rh(bpy) 2(chrysi)] 3+ (1), Δ-[Ru(bpy) 2(ddpz)] 2+ (2), and Δ-[Ru(bpy) 2(eilatin)] 2+ (3) as prototypical examples of metallo-insertors and intercalators. For all molecules we characterized both insertion and intercalation into a DNA dodecamer via force field based molecular dynamics (MD) and hybrid quantum-classical (QM/MM) MD simulations. A structural analysis of the 1-3/DNA noncovalent adducts reveals that the insertion provokes an untwist of the DNA, an opening of the mG and of the phosphate backbone in proximity of the mismatch, while the intercalation induces smaller changes of these structural parameters. This behavior appears to be correlated with the size of the inserting/intercalating ligand in proximity of the metal coordination site. Moreover, our simulations show that the different selectivity of 1 toward distinct MM types may be correlated with the thermodynamic stability of the MMs in the free DNA and with that of the corresponding insertion adduct. Understanding the factors which tune a specific insertion is of crucial importance for designing specific luminescent probes that selectively recognize MMs, as well as for developing more effective anticancer drugs active in MM repair of deficient cells lines. © 2012 American Chemical Society.


Resta R.,University of Trieste | Resta R.,CNR Institute of Materials
Physical Review Letters | Year: 2011

In a linear magnetoelectric the lattice is coupled to electric and magnetic fields; both affect the longitudinal-transverse splitting of zone-center optical phonons on equal footing. A response matrix relates the macroscopic fields (D, B) to (E, H) at infrared frequencies. It is shown that the response matrices at frequencies 0 and fulfill a generalized Lyddane-Sachs-Teller relationship. The right-hand side member of such relationship is expressed in terms of weighted averages over the longitudinal and transverse excitations of the medium, and assumes a simple form for a harmonic crystal. © 2011 American Physical Society.


Sun T.,CNR Institute of Materials | Sun T.,International School for Advanced Studies | Fabris S.,CNR Institute of Materials | Fabris S.,University of Minnesota
Nano Letters | Year: 2012

We identify mechanisms and surface precursors for the nucleation and growth of extended defects on oxidized graphene. Density functional theory calculations show that the formation of surface structures capable to initiate the unzipping and cracking of the oxidized C network is strongly influenced by the constraint of the graphitic lattice on the surface functional groups. Accounting for this effect on the preferential spatial patterning of O adsorbates allows us to revise and extend the current models of graphene oxidative unzipping and cutting. We find that these processes are rate limited by O diffusion and driven by the local strain induced by the O adspecies. Adsorbate mobility is ultimately recognized as a key factor to control and to prevent the C-network breakdown during thermal processing of oxidized graphene. © 2011 American Chemical Society.


Biasiol G.,CNR Institute of Materials | Heun S.,CNR Institute of Neuroscience
Physics Reports | Year: 2011

In this article we review the extensive experimental work on the compositional mapping of semiconductor quantum dots and rings. After a brief introduction of the various experimental techniques used for this purpose, the body of experimental results is presented, ordered by experimental technique (transmission electron microscopy, X-ray diffraction, photoelectron microscopy, scanning probe microscopy, and ion-atom probes) and material system (mainly III-Vs and Ge/Si). The article concludes with a discussion which critically compares these results and outlines some general trends. © 2010 Elsevier B.V.


Silvestrelli P.L.,University of Padua | Ambrosetti A.,CNR Institute of Materials
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

The DFT/vdW-WF2 method, recently developed to include the van der Waals (vdW) interactions in density functional theory (DFT) using the maximally localized Wannier functions, is improved by taking into account screening effects and applied to the study of adsorption of rare gases and small molecules, H2, CH4, and H2O on the Cu(111) metal surface, and of H2 on Al(111), and Xe on Pb(111), which are all cases where screening effects are expected to be important. Screening is included in DFT/vdW-WF2 by following different recipes, also considering the single-layer approximation adopted to mimic a screened metal substrate. Comparison of the computed equilibrium binding energies and distances, and the C3 coefficients characterizing the adparticle-surface van der Waals interactions, with available experimental and theoretical reference data show that the improvement with respect to the original unscreened approach is remarkable. The results are also compared with those obtained by other vdW-corrected DFT schemes. © 2013 American Physical Society.


Colombo L.,CNR Institute of Materials | Giordano S.,Institute of Electronics
Reports on Progress in Physics | Year: 2011

We elaborate on a blended continuum/atomistic theoretical picture of the nonlinear elastic properties of nanostructured materials, looking at diverse aspects such as dispersions of inhomogeneities within a matrix, random or graded nanograined materials, two-dimensional atomic sheets. In particular, we discuss the possible onset of length-scale effects and we establish the limits and merits of continuum versus atomistics. While most situations here discussed correspond to model systems, the main conclusions have a paradigmatic relevance and indeed apply to most nanomaterials of current interest. © 2011 IOP Publishing Ltd.


Mazzola G.,CNR Institute of Materials | Yunoki S.,RIKEN | Sorella S.,International School for Advanced Studies
Nature communications | Year: 2014

The study of the high pressure phase diagram of hydrogen has continued with renewed effort for about one century as it remains a fundamental challenge for experimental and theoretical techniques. Here we employ an efficient molecular dynamics based on the quantum Monte Carlo method, which can describe accurately the electronic correlation and treat a large number of hydrogen atoms, allowing a realistic and reliable prediction of thermodynamic properties. We find that the molecular liquid phase is unexpectedly stable, and the transition towards a fully atomic liquid phase occurs at much higher pressure than previously believed. The old standing problem of low-temperature atomization is, therefore, still far from experimental reach.

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