Characterization of thin films and membranes

Abstract

The study of interfaces is a well-established specialist area, affiliated in particular to physical chemistry. Many of the early analytical studies were underpinned by techniques refined from mainstream analytical sciences and originally applied to the study of the bulk material phase. Now, with the increasing refinement of thin-structure-forming methods, materials have been reconstituted as membranes and thin films. Here, interfacial properties (rather than bulk features) dominate. We now have a great diversity of materials-derived (film) systems that benefit from both established and more novel analytical approaches pioneered in the domain of interfacial science. Inevitably, newer techniques have been added to the lexicon; PM-IRRAS (polarisation modulation infrared reflection spectroscopy) is one example from one of the articles in this special issue. Both physical (e.g. surface mechanics and structural profiling) and chemical analytical techniques have been applied to give a more holistic picture of both the interface and the subsurface. Many more methods have been added to this arsenal in recent years, including spectroscopic, microscopic, electrochemical and even mechanical approaches. These methods enable investigations of all sorts of thin films and membranes in situ and ex situ with extremely high lateral resolution. There are, of course, fundamental questions that still need to be answered in this field. Such questions apply not only to the full gamut of polymeric, inorganic, and hybrid films and membranes, but also to the internalised—yet still exposed—elements of such materials, as embodied, say, by their microand nanopores. Whatever the bulk phase properties of a material (which traditional materials science tends to focus upon), the “distorted” organisation observed at the nominally crystalline and amorphous mesoscale at and near the interface can result in quite different material properties to those presented to the outside world. At a basic level, studying this altered organisation provides a route to understanding thermodynamically programmed pathways to structure, along with their kinetic determinants. This is especially so given that the external environment—be it vacuum, ambient air or liquid—conditions the final outcome. Beyond their inherently interesting surface-related properties, thin-material constructs present practical opportunities that can be utilised by improving our understanding of and better harnessing their fundamental properties. Membrane technology is a long-established field where just such an understanding has led to important applications, particularly in separation science. These have ranged from desalination, particulate/molecular sieving and battery/fuel cell production through to haemodialysis. Ultimately, the stability of these functions is affected by the stability of the membrane surface and its resistance to surface contamination: so-called fouling. These are often assumed to be vague processes that are somehow more random than Langmuirian adsorption, but in reality this assumption is made because their complexity defies our predictive models. This is especially the case for biological fouling in applications such as clinical membrane separators and biosensors. Thin films and membranes are now used extensively, and are being developed as substrates for molecular selfPublished in the topical collection Characterization of Thin Films and Membranes with guest editors Daniel Mandler and Pankaj Vadgama.