Catalytic role of Au nanoparticle in GaAs nanowire growth

The energetics of Ga, As and GaAs species on the Au(111) surface (employed as a model for Au nanoparticles) is investigated by means of density-functional calculations. Apart from formation of the compound Au$_7$Ga$_2$, Ga is found to form a surface alloy with Au, with comparable $\Delta H \sim 0.5$~eV for both processes. Dissociative adsorption of As$_2$ is found to be exothermic by more than 2~eV on both clean Au(111) and AuGa surface alloys. The As-Ga species formed by reaction of As with the surface alloy is sufficiently stable to cover the surface of an Au particle {\it in vacuo} in contact with a GaAs substrate. Concerning the Au-catalysed growth of GaAs nanowires, we conclude that impingement of As$_2$ or As$_4$ molecules on the Au particle suffices as supply of arsenic to the growth zone. We identify a regime of temperatures and As$_2$ partial pressures suitable for Au-catalysed nanowire growth in molecular beam epitaxy.