Structure and growth of butane films adsorbed on graphite.

Abstract

The structure of deuterated n-butane [${\mathrm{CD}}_{3}$(${\mathrm{CD}}_{2}$${)}_{2}$${\mathrm{CD}}_{3}$] adsorbed on the (0001) surfaces of an exfoliated graphite substrate has been investigated by elastic neutron diffraction. The aim of this study was to elucidate the effect of steric properties, particularly the rodlike shape of the molecule, on both the monolayer and multilayer film structure. Our principal findings are as follows: (1) The solid monolayer has a rectangular unit cell commensurate with the graphite lattice in one direction (2 \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{}\ensuremath{\infty}). Profile analysis of the monolayer diffraction pattern shows an improved fit with two molecules in the unit cell arranged in a herringbone (HB) pattern rather than the rectangular-centered structure originally proposed. The HB structure also yields a lower potential energy calculated for a monolayer cluster. (2) The growth mode of the film is ``quasiepitaxial'' at low temperature consisting of the following steps: (a) adsorption of the crystalline monolayer, (b) adsorption of a disordered second layer and compression of the monolayer, (c) onset of preferentially oriented bulk growth with continued adsorption of disordered material, (d) bilayer crystallization, and (e) reentrant growth of preferentially oriented bulk particles and disordered material. (3) A Rietveld profile analysis of the diffraction pattern obtained at a coverage of six layers is consistent with the (100) plane of the bulk butane monoclinic phase parallel to the graphite surface. (4) For coverages of two to three layers, there is evidence of a wetting transition at \ensuremath{\sim}133 K, about 2 K below the bulk melting point.Both the monolayer and bulk Bragg peaks disappear and are replaced by a diffraction pattern characteristic of a liquid film with a high degree of short-range order. It is suggested that the butane molecule's steric properties are responsible for the incomplete wetting of the film as well as for the preferential orientation of the bulk phase. Our quasiepitaxial growth model for butane is similar to one proposed previously for nitrogen films on graphite. After comparing the structure and growth of the two films, we suggest that it may apply to a number of other physisorbed films composed of rod-shaped molecules having a larger aspect ratio than butane.