Nanoscale observation of delayering in alkane films

Tapping-mode Atomic Force Microscopy and synchrotron X-ray scattering measurements on dotriacontane (n-C32H66 or C32) films adsorbed on SiO2-coated Si(100) wafers reveal a narrow temperature range near the bulk C32 melting point $T_{{\rm b}}$ in which a monolayer phase of C32 molecules oriented perpendicular to surface is stable. This monolayer phase undergoes a delayering transition to a three-dimensional (3D) fluid phase on heating to just above $T_{{\rm b}}$ and to a solid 3D phase on cooling below $T_{{\rm b}}$. An equilibrium phase diagram provides a useful framework for interpreting the unusual spreading and receding of the monolayer observed in transitions to and from the respective 3D phases.