Nanoscale observation of delayering in alkane films

Tapping-mode Atomic Force Microscopy and synchrotron X-ray scattering measurements on dotriacontane ( n-C32H66 or C-32) films adsorbed on SiO2- coated Si( 100) wafers reveal a narrow temperature range near the bulk C32 melting point T-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-b and to a solid 3D phase on cooling below T-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.