SELF-ASSEMBLY OF ORGANIC MULTILAYERS WITH POLAR ORDER USING ZIRCONIUM-PHOSPHATE BONDING BETWEEN LAYERS

We describe a way to form multiple asymmetric layers of organic molecules that may be applicable to the fabrication of electrooptical switching elements and other second-order nonlinear optical devices. The molecular layers are formed sequentially and are held together by strong zirconium phosphate and zirconium phosphonate bonds. Such multilayer films have been constructed on both gold and oxidized silicon surfaces. The asymmetric orientation of the molecules in each layer is ensured by a three-step process, which is a modification of the two-step process that Mallouk and co-workers recently developed to deposit symmetric multilayers (J. Phys. Chem. 1988, 92, 2597-2601). First, an hydroxy-terminated surface is phosphory-lated to give surface phosphate groups. Second, zirconium(IV) is bound to this phosphate surface. Finally, an organic molecule containing a phosphonic acid group at one end and a hydroxy group at the other is allowed to self-assemble into a monolayer on the zirconium(IV) surface. This results in a new hydroxy-terminated surface, and the sequence can then be repeated. We demonstrate this methodology with (11-hydroxyundecyDphosphonic acid, a simple difunctional organic molecule. The individual surface reactions are followed by X-ray photoelectron spectroscopy, and the layer-by-layer growth is followed by ellipsom-etry. The extension of this methodology to organic molecules with large nonlinear optical coefficients is currently being pursued. © 1990, American Chemical Society. All rights reserved.