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 th iol molecules, CH3(CH2)(n-1)SH (C-n, 10 less than or equal to n less t han 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 sy stematic dependence of the tilt structure (i.e., the tilt angle and ti lt direction) of the hydrocarbon chains as a function of the chain len gth. Furthermore, we show that the monolayer structures are characteri zed 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 sm ooth variation of the structural parameters within each regime. We ass ociate these systematic structural changes with the conflicting requir ements of epitaxy and molecular packing, and argue that the driving fo rce is the changing intra-layer interaction strength (which is proport ional to hydrocarbon chain length). We believe that these phenomena sh ould be characteristic of the behavior of self-assembled monolayers, a s well as the mon general class of ''soft/hard'' interfaces. (C) 1997 American Institute of Physics.