A theoretical study of the structure of the liquid Ga-diamond (111) interface

We present the results of a computer simulation study of the structure of the interface between liquid Ga and the (111) face of diamond, with which we reinterpret the findings from an x-ray reflectivity study of that interface [W. J. Huisman, J. F. Peters, M. J. Zwanenburg, S. A. de Vries, T. E. Derry, D. Abernathy, and J. F. van der Veen, Nature (London) 390, 379 (1997); Surf. Sci. 402–404, 866 (1998)]. That experimental study has been interpreted to show that the contact of Ga with the (111) face of diamond induces the formation of $Ga2$ molecules for several layers into the bulk liquid, with the axes of the $Ga2$ molecules in successive layers oriented perpendicular to the diamond surface. No driving force for the proposed formation of $Ga2$ molecules is identified. The simulations reported in this paper are based on a model that permits chemical binding of Ga, as a dimer, to the $CC$ double bonds in the reconstructed (111) face of diamond, thereby identifying the driving force for dimerization. We show that an isolated $π$ complex with the $Ga2$ axis perpendicular to the $CC$ double bond is stable. We then modify the pseudopotential-based self-consistent Monte Carlo simulation scheme for describing inhomogeneous liquid metals, using the calculated potential-energy surface of $Ga2(CC)$ in the region close to the diamond surface. In this model only the Ga adjacent to the diamond is composed of dimers. The interfacial density distribution obtained from the simulations predicts an x-ray reflectivity that is in good agreement with that observed.