Guido Ori

Comprehensive physical modeling of forming and switching operations in HfO2 RRAM devices

In this work we apply a physical model based on charge transport and molecular mechanics/dynamics simulations to investigate the physical mechanisms governing the RRAM forming and switching operations. The proposed model identifies the major driving forces controlling conductive filament (CF) formation and changes during RRAM switching, thus providing a tool for investigation and optimization of RRAM devices.

Ionic liquid confined in silica nanopores: molecular dynamics in the isobaric–isothermal ensemble

Molecular dynamics simulations in the isobaric–isothermal ensemble are used to investigate the structure and dynamics of an ionic liquid confined at ambient temperature and pressure in hydroxylated amorphous silica nanopores. The use of the isobaric–isothermal ensemble allows estimating the effect of confinement and surface chemistry on the density of the confined ionic liquid. The structure of the confined ionic liquid is investigated using density profiles and structural order parameters while its dynamics is assessed by determining the mobility and ionic conductivity of the confined phase. Despite the important screening of the electrostatic interactions (owing to the small Debye length in ionic liquids), the local structure of the confined ionic liquid is found to be mostly driven by electrostatic interactions. We show that both the structure and dynamics of the confined ionic liquid can be described as the sum of a surface contribution arising from the ions in contact with the surface and a bulk-like contribution arising from the ions located in the pore centre; as a result, most properties of the confined ionic liquid are a simple function of the surface-to-volume ratio of the host porous material. In contrast, the ionic conductivity of the confined ionic liquid, which is a collective dynamical property, is found to be similar to the bulk. This study sheds light on the complex behaviour of hybrid materials made up of ionic liquid confined in inorganic porous materials.

Structure and Dynamics of Ionic Liquids Confined in Amorphous Porous Chalcogenides

Besides the abundant literature on ionic liquids in porous silica and carbon, the confinement of such intriguing liquids in porous chalcogenides has received very little attention. Here, molecular simulation is employed to study the structural and dynamical properties of a typical ionic liquid confined in a realistic molecular model of amorphous chalcogenide with various pore sizes and surface chemistries. Using molecular dynamics in the isobaric-isothermal (NPT) ensemble, we consider confinement conditions relevant to real samples. Both the structure and self-dynamics of the confined phase are found to depend on the surface-to-volume ratio of the host confining material. Consequently, most properties of the confined ionic liquid can be written as a linear combination of surface and bulk-like contributions, arising from the ions in contact with the surface and the ions in the pore center, respectively. On the other hand, collective dynamical properties such as the ionic conductivity remain close to their bulk counterpart and almost insensitive to pore size and surface chemistry. These results, which are in fair agreement with available experimental data, provide a basis for the development of novel applications using hybrid organic-inorganic solids consisting of ionic liquids confined in porous chalcogenides.

Molecular dynamics simulation of amorphous HfO2 for resistive RAM applications

HfO2 is widely investigated as the favoured material for resistive RAM device implementation. The structural features of HfO2 play a fundamental role in the switching mechanisms governing resistive RAM operations, and a comprehensive understanding of the relation between the atomistic properties and final device behaviour is still missing. In addition, despite the fact that ultra-scaled 10 nm resistive RAM will probably be made of amorphous HfO2, a deeper investigation of the structure is necessary. In this paper, the classical molecular dynamics technique was used to investigate the disordered atomic configuration of amorphous HfO2. The influence of density on both the atomistic structure and the diffusion of O species was carefully analysed. The results achieved show that the atomistic structure of an amorphous HfO2 system is strongly affected by the density, and the amorphous system is rearranged in an atomic configuration similar to the crystalline configuration at similar densities. The diffusion of oxygen atoms increases with the decrease of the density, consistent with a less-packed atomic structure which allows for easier movement of this species.

Comparison of precipitated calcium carbonate/polylactic acid and halloysite/polylactic acid nanocomposites

PLA nanocomposites with stearate coated precipitated calcium carbonate (PCC) and halloysite natural nanotubes (HNT) were prepared by melt extrusion. The crystallization behavior, mechanical properties, thermal dynamical mechanical analysis (DMTA), and the morphology of the PCC/PLA, HNT/PLA, and HNT/PCC/PLA composites were discussed. Compared to halloysite nanotubes, PCC nanoparticles showed a better nucleating effect, which decreased both the glass transition and cold crystallization temperatures. The tensile performance of PLA composites showed that the addition of inorganic nanofillers increased Youngs modulus but decreased tensile strength. More interestingly, PLA composites with PCC particles exhibited an effectively increased elongation at break with respect to pure PLA, while HNT/PLA showed a decreased ultimate deformation of composites. DMTA results indicated that PLA composites had a similar storage modulus at temperatures below the glass transition and the addition of nanofillers into PLA caused Tg to shift to lower temperatures by about 3°C. The morphological analysis of fractures surface of PLA nanocomposites showed good dispersion of nanofillers, formation of microvoids, and larger plastic deformation of the PLA matrix when the PCC particles were added, while a strong aggregation was noticed in composites with HNT nanofillers, which has been attributed to a nonoptimal surface coating.

Origin of structural analogies and differences between the atomic structures of GeSe4 and GeS4 glasses: A first principles study

First-principles molecular dynamics simulations based on density functional theory are employed for a comparative study of structural and bonding properties of two stoichiometrically identical chalcogenide glasses, GeSe4 and GeS4. Two periodic cells of 120 and 480 atoms are adopted. Both glasses feature a coexistence of Ge-centered tetrahedra and Se(S) homopolar connections. Results obtained for N = 480 indicate substantial differences at the level of the Se(S) environment, since Ge-Se-Se connections are more frequent than the corresponding Ge-S-S ones. The presence of a more prominent first sharp diffraction peak in the total neutron structure factor of glassy GeS4 is rationalized in terms of a higher number of large size rings, accounting for extended Ge-Se correlations. Both the electronic density of states and appropriate electronic localization tools provide evidence of a higher ionic character of Ge-S bonds when compared to Ge-Se bonds. An interesting byproduct of these investigations is the occurrence of discernible size effects that affect structural motifs involving next nearest neighbor distances, when 120 or 480 atoms are used.

Immobilization of monolayer protected lipophilic gold nanorods on a glass surface

We present a novel process of immobilization of gold nanorods (GNRs) on a glass surface. We demonstrate that by exploiting monolayer protection of the GNRs, their unusual optical properties can be completely preserved. UV-visible spectroscopy and atomic force microscopy analysis are used to reveal the optical and morphological properties of monolayer protected immobilized lipophilic GNRs, and molecular dynamics simulations are used to elucidate their surface molecule arrangements.

Microstructural characterisation and electrical properties of multiwalled carbon nanotubes/glass-ceramic nanocomposites

We report on an improved processing method for fabricating multiwalled carbon nanotubes/glass-ceramic nanocomposites based on vanadium doped silicate glass matrices. Starting from the design of a stable aqueous dispersion of CNTs, achieved using a cationic surfactant, the interaction of CNTs with glass particles in suspension was improved using a co-solvent that provided access to nanocomposites exhibiting high quality CNT distribution in the matrix, which was confirmed by detailed micro/nano-structural and morphological characterisation. Considering that very few studies have focused on the functional properties of CNT/glass composites, in the present investigation the electrical resistivity of CNT/glass composites was measured and it was demonstrated that higher electrical conductivity values were obtained compared to previous similar materials fabricated by conventional powder processing.

Impact of dispersion forces on the atomic structure of a prototypical network-forming disordered system: The case of liquid GeSe2

A set of structural properties of liquid GeSe2 are calculated by using first-principles molecular dynamics and including, for the first time, van der Waals dispersion forces. None of the numerous atomic-scale simulations performed in the past on this prototypical disordered network-forming material had ever accounted for dispersion forces in the expression of the total energy. For this purpose, we employed either the Grimme-D2 or the maximally localized Wannier function scheme. We assessed the impact of dispersion forces on properties such as partial structure factors, pair correlation functions, bond angle distribution, and number of corner vs edge sharing connections. The maximally localized Wannier function scheme is more reliable than the Grimme-D2 scheme in reproducing existing first-principles results. In particular, the Grimme-D2 scheme worsens the agreement with experiments in the case of the Ge-Ge pair correlation function. Our study shows that the impact of dispersion forces on disordered chalcogenides has to be considered with great care since it cannot be necessarily the same when adopting different recipes.

The role of 2D/3D spin-polarization interactions in hybrid copper hydroxide acetate: new insights from first-principles molecular dynamics

The magnetic properties response of the layered hybrid material copper hydroxide acetate Cu2(OH)3CH3COO·H2O is studied as a function of the applied pressure within first-principles molecular dynamics. We are able to elucidate the interplay between the structural properties of this material and its magnetic character, both at the local (atomic) level and at the bulk level. We performed a detailed analysis of the intralayer spin configurations occurring for each value of the imposed projection along the z-axis for the total spin and of the applied pressure. The transition from an antiferromagnetic to a ferromagnetic state at high pressure (above 3 GPa) goes along with a vanishing difference between the spin polarizations pertaining to each layer. Therefore, at high pressure, copper hydroxide acetate is a ferromagnet with no changes of spin polarization in the direction perpendicular to the inorganic layers.