Structural properties of L-B films by STM, NEXAFS and vibrational spectroscopy

Abstract

Thin organic films are being used and proposed for a number of new applications by providing new properties not readily available with other materials. Up to now much of the work on these organic films has been with amorphous polymeric and organic ones, but as the thickness of the layers becomes smaller the need has arisen for more ordered and well-defined structures. Amorphous films are likely to have pinholes and defects while more ordered films could be more regular and defect free. This had led a renewed interest in Langmuir-Blodgett (L-B) monolayers and multilayers, and those made by the self-assembly technique. These films are highly anisotropic, highly ordered, densely packed, and they can be engineered by attaching functional groups for specific uses. The attachment to the substrate can be designed and controlled, the structure established, and the end group, which interacts with the outside world, made functionally appropriate. It is also anticipated that collective properties could make these and related films exhibit enhanced or different attributes. Many of these films, such as L-B layers, though ordered are very thin and quite fragile. Details of the their structure are, therefore, required for designing new ones and overcoming problems. New surface science techniques can address some of the structural characterizations and point the way to new directions. Some of these are scanning tunneling microscopy (STM), near-edge X-ray fine structure (NEXAFS), glancing angle in infrared spectroscopy, and waveguide Raman spectroscopy. Experiments have been done on a number of L-B films, giving the orientation of the chains and the head group. Phase transitions of melting and cross-linking of chains have also been observed. Since most of the work has been on the salts of fatty acids, we will concentrate on the structure of cadmium arachidate (CdA). More complicated compounds can lead to films with similar or only slightly modified structures, but obviously different and possibly more interesting properties. Nevertheless, much of our knowledge about CdA can be very applicable to these other films and we can see what measurements and techniques would be most revealing for films of greater complexity. © 1989.