Fabrication and Characterization of Cross-linked Organic Thin Films with Nonlinear Mass Densities

Urea (bonded) cross-linked multilayer thin film by sequential deposition of bifunctional and tetrafunctional molecular building blocks was demonstrated. Multilayer growth as a function of deposition cycles was inspected using UV-vis absorption spectroscopy. From IR results, three characteristic infrared bands of amide I, amide II, and asymmetric ν_a (N-C-N) stretching band confirmed the formation of polyurea networks by alternate dipping into solutions of amine and isocyanate functionalities monomers. The deconvoluted component from the C 1s and N 1s spectra from X-ray photoelectron spectroscopy (XPS) shows clear evidence of stable polyurea networks. Enhancement of structural periodicity with film growth was demonstrated by grazing incidence small angle X-ray scattering (GI-SAXS) measurements. The thin film near the substrate surface seems to have an amorphous structure. However, molecular ordering improves in the surface normal direction of the substrate with a certain number of deposited layers. Constant mass density was not observed with deposition cycles. The mass density increased upto 16% within deposited layers from proximate layers to those extending away from the substrate surface. This difference in packing density might derive from the different degrees of cross-linking among layers proximate to the substrate surface and extending away from the surface.