Mariella Ippolito

Interface structure and defects of silicon nanocrystals embedded into a-SiO2

By means of large-scale atomistic simulations, we identity and characterize several kinds of bonding and coordination defects at the interface between a silicon nanoparticle and an embedding amorphous silicon dioxide matrix. In particular, we prove that interface bond defects are easily formed, while no Si–O double bond is observed. We conclude that optical properties, e.g., photoluminescence, are more likely due to such interface bond structures. Temperature effects on defect population and nature are discussed as well.

Atomistic structure of amorphous silicon nitride from classical molecular dynamics simulations

By means of molecular dynamics simulations based on the Billeter et al. [S. R. Billeter, A. Curioni, D. Fischer, and W. Andreoni, Phys. Rev. B 73, 155329 (2006)] environment-dependent classical force field we studied the structural features of SiN

The influence of silicon nanoclusters on the optical properties of a-SiNx samples: A theoretical study

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Fracture toughness of nanostructured silicon carbide

samples at various stoichiometries. Our results are in good agreement with experimental data and are able to reproduce some features which so far were not reproduced by simulations. In particular, we identified units containing N\char21{}N bonds, which are thought to be responsible for an unassigned peak in the radial distribution function obtained from neutron diffraction data and signals observed in electron spin resonance, x-ray photoemission spectroscopy, electron-energy-loss spectroscopy, and optical absorption experiments. We have identified defects which are thought to be responsible for the high concentration of charge traps that makes this material suitable for building nonvolatile memory devices. We analyzed the dependency of the concentration of these defects with the stoichiometry of the sample.

Analytical approaches versus atomistic simulations in fracture

By means of ab-initio calculations, we investigate the optical properties of pure a-SiNx samples, with x∈[0.4,1.8], and samples embedding silicon nanoclusters (NCs) of diameter 0.5≤d≤1.0 nm. In the pure samples, the optical absorption gap and the radiative recombination rate vary according to the concentration of Si-N bonds. In the presence of NCs, the radiative rate of the samples is barely affected, indicating that the intense photoluminescence of experimental samples is mostly due to the matrix itself rather than to the NCs. Besides, we evidence an important role of Si-N-Si bonds at the NC/matrix interface in the observed photoluminescence trend.

Atomic-Scale Investigation on Fracture Toughness in Nanocomposite Silicon Carbide

By using atomistic simulations, we derive a constitutive equation for a microfractured β-SiC matrix containing hard or soft inclusions. The proposed equation is shown to follow the Eshelby theory for elastic inclusions, and appears to hold for any crack tip-inclusion distance and for a wide range of values of matrix-inclusion elastic mismatch. A comparison of the atomistic results with existing continuum elasticity models points out the weaknesses of some commonly adopted simplifying assumptions.

Role of lattice discreteness on brittle fracture: Atomistic simulations versus analytical models

In this paper two different research groups have merged to try to compare their methods, i.e., analytical approaches (i) or atomistic simulations (ii), in the study of the fracture phenomenon. Simple different case studies are considered.

Atomistic modeling of brittleness in covalent materials

Ceramic materials are attractive for structural applications because of their low density, chemical inertness, high strenght, high hardness and high-temperature stability. However they have inherently low fracture toughness, so that plastic deformation in ceramics is found to be extremely limited. Ceramic matrix composites (CMC) have been therefore developed to overcome the intrinsic brittleness and lack of reliability of monolithic samples. CMC’s consist of a ceramic matrix reinforced with inclusions, such as particles, whiskers or chopped fibers (fiber thoughening).