Probing interface organization and optical nonlinearities in asymmetric layered interfaces using surface second harmonic generation

The ability to control interfacial structure and composition thereby imparting in them unique chemical, physical, and optical properties make them interesting systems to study for both fundamental and technological reasons. Such interfacial assemblies have potential utility in surface-dependent technologies such as chemical sensing, nonlinear optics, separation science, information storage, microelectronics, and corrosion protection. Layer-by-layer deposition, as in self-assembled mono- and multilayer assemblies, can provide spatial resolution normal to the substrate over composition and molecular organization that is critical to the macroscopic properties of the resulting interface. However, the extent of organization required within the material to achieve the desired properties remains to be fully resolved.; This work reports on the use of asymmetric metal ion coordination chemistry in conjunction with surface second harmonic generation (SHG) and linear optical spectroscopy (FTIR, ellipsometry, UV-visible) to understand molecular organization intrinsic to layered assemblies. Asymmetric multilayer assemblies are assembled using structurally simple bifunctional alkanes incorporating χ (2)-activity within the coordinating metal centers. Surface preparation was carried out by direct phosphorylation of surface silanol groups with POCl ₃ in collidine resulting in a reduction in sample preparation time from about 24 hours to less than an hour. Linear optical characterization (FTIR and ellipsometry) suggests moderately well organized multilayers that are chemically and thermally robust.; The χ(2) response intrinsic to the inorganic interlayer of ionically-bound multilayer systems such zirconium phosphonate monolayers was also investigated for the first time using surface second harmonic generation intensity measurements on asymmetric bifunctional alkanes multilayer systems. Studies on ionically-bound interfaces usually focus on the chemical and/or optical properties of the organic constituents, R, in Zr(PO₃R) ₂ sheets because of the structural diversity in organic moieties. Surface second harmonic generation data revealed a small nonlinear response from this part of the assembly relative to that attainable from a rigid chromophore with a large hyperpolarizability. In addition, angle-dependent surface second harmonic generation was used to investigate surface organization and structure under potentially heterogeneous conditions as a function of surface chromophore loading density. The SHG data are consistent with complex surface structure formation characterized by two domains; a dominant well-ordered domain with an average chromophore tilt angle coincident with the surface normal, and a second, more random domain characterized by an average chromophore angle of ∼39°. The relative contribution of the disordered domain was found to decrease with increasing chromophore loading density. This finding suggests disorder inherent in most layered materials resulting from either non-uniformity of surface active sites or adsorbate aggregation.