Increased crystallinity of octadecylphosphonic acid Langmuir-Blodgett films with an increasing number of layers

We report that contact mode atomic force microscopy (AFM) imaging is useful in revealing crystallinity of octadecylphosphonic acid (OPA) Langmuir-Blodgett (LB) films. On a monolayer OPA LB film prepared on the native oxide of a Si wafer, no molecular-resolution could be obtained. However, molecular-resolution images were obtained on a bilayer surface, a reflection that molecules in the top layer adjust their alkyl chains to accommodate those from below to form a closely packed, OPA headgroup-terminated surface. We demonstrate that with an increasing number of layers, the crystallinity of multilayer OPA LB films improves as molecular-resolution AFM images obtained on even-numbered multilayers reveal that the terminating OPA headgroups form a sheet of hexagonal array.