On the stability and homogeneity of Langmuir-Blodgett films as models of polymers and biological materials for surface studies: an XPS study

Polymers and biological materials appear at first sight difficult materials for surface studies, because of: (1) intrinsic difficulties to handle sufficiently pure compounds and thus reproducible samples, (2) possible degradation with time in vacuum, or under exciting (electrons, ions, or photons) beam, (3) the uncertainty that still remains on the effective sample-depth below the material surface. With the help of monochromatized and the spatially resolved XPS data on Langmuir-Blodgett films (from arachidic and behenic acids and from polyvinylstearate with varying thickness (from 1 to 11 layers)), deposited onto rough silver foils (400 Å SEM), we show that (1), in general, the films are extremely fragile regarding their handling, that (2) they are homogeneous on the substrate (on a 300 μm scale), and (3) they age significantly in air or in vacuum, or under X-ray exposure (losing some material quantity, either some carbon or some oxygen functionalities). Polymer LB films like polyvinylstearate are, however, more "homogeneous" and stable systems, as expected.