LIQUID TO HEXATIC TO CRYSTALLINE ORDER IN LANGMUIR-BLODGETT-FILMS

Atomic force microscope images of zinc arachidate (ZnA(2)) Langmuir-Bl odgett films show that three- and five-layer films are ''hexatic,'' wi th long-range bond-orientational order and short-range positional corr elations of three to five lattice repeats. The monolayer in contact wi th the substrate is disordered. Films of seven or more layers of ZnA(2 ) are crystalline. A population of dislocations, most likely originati ng at the substrate, disrupts the positional but not the orientational order of the lattice, leading to hexatic layers intermediate between crystal and liquid. The influence of the substrate propagates farther into ZnA(2) films than into cadmium arachidate films because the molec ular cohesion is much weaker in ZnA(2) than in cadmium arachidate, as evidenced by a less dense molecular packing.