L. Colombo

Simulation of the amorphous-silicon properties and their dependence on sample preparation

We present a thorough theoretical investigation on pure amorphous silicon (a-Si) at room temperature, within a tight-binding molecular-dynamics scheme. We demonstrate that both structural and electronic properties of a-Si depend on the sample preparation, here obtained by quenching from the melt. Possible size effects are also investigated using 64- and 216-atom supercells. Finally, we discuss the reliability and transferability of the present scheme for large-scale simulations of covalent materials.

A source code for tight-binding molecular dynamics simulations

We present the tight-binding molecular dynamics (TBMD) scheme and describe its numerical implementation in a serial FORTRAN-77 code. We discuss how to organize a typical simulation and to control the I/O. An analysis of the computational workload is also presented and discussed.

Molecular dynamics simulation of defect production in irradiated β-SiC

We used the molecular dynamics (MD) code MDCASK, in which the Tersoff potential is implemented, to study the mechanisms of defect production due to Si- and C-recoils of different energies in β-SiC. In this paper, we highlight some of our most significant results to date. The threshold displacement energies (TDEs) for Si- and C-atoms have been accurately determined along four main crystallographic directions. The difficulty of defining a single TDE in this material, also because of the effect of temperature, is discussed. High-energy (various keV) recoil-induced displacement cascades have been simulated and it was found that defects on the C-sublattice always outnumber defects on the Si-sublattice, temperature scarcely affecting the number of Frenkel pairs produced within the cascade. Finally, our MD model seems to have proved satisfactory in reproducing the experimentally observed process of irradiation-induced amorphization of SiC at cryogenic temperature.

PREDICTION OF NEW SP2 AND SP2/SP3 HOLLOW CARBON CRYSTALS

Among the hypothetical forms of fully covalent carbon lattices with either graphite-like sp2 or mixed sp2/sp3 bonding we investigated the possibility of monocrystals with a hollow structure. We have calculated the structural, elastic and electronic properties of two prototypical face–centered cubic lattices C20 and C22, respectively, making also a critical comparison to diamond and graphite. The first of these lattices belongs to a new class of sp2–bonded periodic solids which we call hollow graphites. We give a topological classification of such solids along with the algorithm to generate them. Both crystals, having a nanoporous lattice made of periodic sequence of adjacent cavities, rather than the tubular structure of schwarzites, are characterized by a large internal specific area and should be suitable to form reversible high capacity lithium insertion compounds.

Tight binding molecular dynamics

With a new and intrinsically parallel algorithm for tight binding molecular dynamics we obtain a performance of 9.4 Gigaflops per million dollar on a cluster of 8 Hewlett Packard workstations in a simulation of 216 Silicon atoms. One time step with this new algorithm takes as much time for the 216 atom system as one time step with a conventional algorithm on a NEC-SX3 supercomputer. In addition, the linear sealing of new algorithm allows us to calculate systems of unprecedented size which are not any more accessible by the combination of standard algorithms and vector supercomputers.<<ETX>>

Structural Properties of Liquid and Amorphous GaAs by Tight-Binding Molecular Dynamics

We report the first tight-binding molecular-dynamics investigation on liquid GaAs where a very good agreement with experimental data and ab initio results is obtained. The critical choice of the repulsive potential and the role played by charge transfer are discussed. By exploiting the reduced computational workload of our method, we extend our study to amorphous GaAs as obtained by quenching from the melt and investigate the stability of the amorphous network against slow-interdiffusion dynamics.

Hydrogen diffusion in crystalline SiO2

Abstract We present a theoretical investigation on the diffusion of atomic hydrogen in quartz, trydimite and cristobalite. By means of molecular dynamics simulations we compute the activation energy and the diffusivity prefactor for the migration process. We further provide an atomistic model for the diffusion process and characterize the migration path.

On the effect of quench rate on the structure of amorphous carbon

Abstract We have simulated, via tight binding molecular dynamics (TBMD), the process of the quench from a melt of an atomic scale system of carbon. We have correlated the local properties of the resulting structure to the quench rate used to bring the liquid phase beyond the glass transition temperature. Results have been analyzed also in terms of the hamiltonian model used to describe the simulated system. In this respect, amorphous structures generated via tight binding and ab initio molecular dynamics have been compared. Results indicate that quench rates as slow as 1014 K/s produce the onset of an increasingly high fraction of threefold coordinated sites in the structure. Moreover, it has been put in evidence the tendency of the tight binding approach to favor threefold coordinated sites with respect to fourfold coordinated, even in the fast quench rates domain.

Amorphization of fullerite crystals

Abstract We present a thorough experimental and theoretical investigation on the structural transformation of a C 60 crystal, as induced by cage-opening reactions. The structural and vibrational properties of the resulting material are characterized by Raman spectroscopy. In order to provide an atomistic model of the damage fullerite, we simulate the irradiation process by a tight-binding molecular dynamics calculation on a 240-atom system. A full characterization of local coordination and topology, as well as short-range features related to the cage opening is given.

Solid‐to‐liquid phase change and fragmentation in C60

We present a study of the thermodynamical properties of C60 in the microcanonical ensemble. Solidlike and metastable liquidlike form can be identified in the low energy and in the high energy range, respectively. The transition between the two phases is characterized by a finite energy range, in agreement with general theories of cluster melting. In particular, we have observed that the melting is preceded by a highly isomerized transition region where a sizeable atomic mobility is achieved via hopping between different isomer structures.