Condensed-Matter Physics & Materials Science Seminar

Alkanes [n-CnH2n+2] are of general interest as prototypes of more complex polymers used in coatings, adhesives, and electronic devices. They are also of interest as the building blocks of biologically important molecules such as lipids and as the principal constituents of commercial lubricants. Understanding the growth mode and wetting behavior of alkane films on solid surfaces at the nanoscale level is important for both fundamental studies and technological applications. We investigated the wetting behavior of alkane films adsorbed on solid surfaces using atomic force microscopy (AFM) and synchrotron x-ray scattering techniques. Tapping-mode AFM reveals a narrow temperature range near the bulk melting point Tb in which a monolayer phase of 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 Tb and to a solid 3D phase on cooling below Tb. For dotriacontane (n-C32H66 or C32) films, similar spreading phenomena were observed on SiO2-coated Si(100) wafers and highly-oriented pyrolytic graphite substrates. For all films, regardless of the length of the alkane molecule, we observe spreading of a molecular monolayer to occur on heating to a temperature above that at which a structural transition occurs in the coexisting 3D solid phase. A model 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.

Supported by U.S. NSF Grant No. DMR-0411748, by the Chilean government under FONDECYT Grant Nos. 1010548 and 7010548, and by the Fundacion Andes Grant No. C-13768.