Abstract: Understanding atomistic mechanisms for catalyst-assisted nanowire growth is an essential step to improve control over the properties of these versatile nanomaterials. However, in silico approaches for III-V nanowire growth have been hindered so far mainly by the limited number of interatomic potentials. Here, we present an original interatomic potential for molecular dynamics simulations of Au-catalyzed GaAs nanowire growth. Our simulations provide important insights about the atomic distribution in the nanowire catalyst and the role of As atoms during GaAs nanowire growth. We show that a stable, thin layer of As around the catalyst is essential for nanowire growth and that the composition of the region close to the solid-liquid interface is nonuniform, alternating between Ga-rich and As/Au-rich layers. These features contribute to the reservoir effect, enlarging interface widths when exchanging group III or V species for heterostructure growth. Our simulation results also provide directions for challenging in situ experiments to further probe the existence of this thin As layer on the catalyst surface, as well as for finding improved conditions to obtain sharp interfaces in nanowires with axial heterostructures.

Abstract: Liquid metal embrittlement during hot rolling of recycled steels, caused by tramp elements accumulated through repeated scrap recycling, represents a critical challenge for sustainable steel production. Here we develop interatomic potentials for Fe-Cu-X (X = Sn, Sb, As, Pb, and Bi) ternary systems and employ molecular dynamics simulations to elucidate atomistic mechanisms governing grain boundary penetration of liquid metals. Systematic investigation of binary Fe-X and Cu-X systems establishes that penetration is controlled by wetting thermodynamics depending on both the mixing enthalpy and the solute concentration dissolved in the liquid phase. Extension to ternary systems reveals three distinct penetration regimes determined by the disparity in mixing enthalpies between binary pairs relative to entropic stabilization: (i) Sn and Sb promote cooperative penetration where tramp elements lead Cu into grain boundaries, with penetration efficiency increasing systematically with tramp element concentration in the liquid; (ii) Pb and Bi suppress penetration as their unfavorable Fe interactions exclude them from the solid-liquid interface, maintaining baseline Fe-Cu behavior; and (iii) As exhibits decoupled penetration where its extreme Fe affinity induces liquid phase separation, suppressing Cu penetration despite independent As infiltration. These findings establish a thermodynamic framework based on the difference between binary mixing enthalpies that enables prediction of tramp element effects on grain boundary penetration and provides guidance for prioritization of elements for industrial control.

Abstract: An analytic, bond-order potential (BOP) is proposed and parametrized for the gallium arsenide system. The potential addresses primary (σ) and secondary (π) bonding and the valence-dependent character of heteroatomic bonding, and it can be combined with an electron counting potential to address the distribution of electrons on the GaAs surface. The potential was derived from a tight-binding description of covalent bonding by retaining the first two levels of an expanded Green’s function for the σ and π bond-order terms. Predictions using the potential were compared with independent estimates for the structures and binding energy of small clusters (dimers, trimers, and tetramers) and for various bulk lattices with coordinations varying from 4 to 12. The structure and energies of simple point defects and melting transitions were also investigated. The relative stabilities of the (001) surface reconstructions of GaAs were well predicted, especially under high-arsenic-overpressure conditions. The structural and binding energy trends of this GaAs BOP generally match experimental observations and ab initio calculations.

Abstract: An analytical bond-order potential for GaAs is presented, that allows one to model a wide range of properties of GaAs compound structures, as well as the pure phases of gallium and arsenide, including nonequilibrium configurations. The functional form is based on the bond-order scheme as devised by Abell-Tersoff and Brenner, while a systematic fitting scheme starting from the Pauling relation is used for determining all adjustable parameters. Reference data were taken from experiments if available, or computed by self-consistent total-energy calculations within the local density-functional theory otherwise. For fitting the parameters, only structural data of the metallic phases of gallium and arsenide as well as those of different GaAs phases were used. A number of tests on point defect properties, surface properties, and melting behavior have been performed afterward in order to validate the accuracy and transferability of the potential model, but were not part of the fitting procedure. While point defect properties and surfaces with low As content are found to be in good agreement with literature data, the description of As-rich surface reconstructions is not satisfactory. In the case of molten GaAs we find support for a structural model based on experiment that indicates a polymerized arsenic phase in the melt.