Epitaxy and chainlength dependent strain in self-assembled monolayers

We use grazing incidence x-ray diffraction to systematically study the structure of an archetypal self-assembled monolayer as a function of the hydrocarbon chain length, n. The monolayers consists of n-alkyl thiol molecules, CH3(CH2)(n-1)SH (C-n, 10 less than or equal to n less than or equal to 30), self-assembled on single crystal Au(111) surfaces. At room temperature, the 2D structure is described by a C(4x2) unit mesh for all chain lengths. However, we demonstrate that there is a systematic dependence of the tilt structure (i.e., the tilt angle and tilt direction) of the hydrocarbon chains as a function of the chain length. Furthermore, we show that the monolayer structures are characterized by distinct ''long'' (n greater than or equal to 16) and ''short'' in (n less than or equal to 14) chain length regimes, as well as a smooth variation of the structural parameters within each regime. We associate these systematic structural changes with the conflicting requirements of epitaxy and molecular packing, and argue that the driving force is the changing intra-layer interaction strength (which is proportional to hydrocarbon chain length). We believe that these phenomena should be characteristic of the behavior of self-assembled monolayers, as well as the mon general class of ''soft/hard'' interfaces. (C) 1997 American Institute of Physics.