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Imaging Ultrathin Organic Films on the Nanometer Level Using Surface Plasmons

Surface plasmons have been used to detect single molecular layers for many years. The most common examples are biosensor systems and the detection of self-assembled monolayers (SAMs). A surface plasmon is a two-dimensional electromagnetic wave that travels on the surface of a noble metal, most commonly gold or silver. This electromagnetic wave is extremely sensitive to changes in the dielectric constant 100 nm above the metal surface. When the dielectric constant above the metal changes due to the addition of a molecular layer, the intensity of the surface plasmon will decrease. In this way monolayers may be indirectly imaged on a surface by mapping the surface plasmon intensity. Although there are many reasons for imaging ultrathin films or molecular layers over large distances (10-100 μm), there are several difficulties involved. Typically the substrate has a roughness greater than the height of a single monolayer. This can mask the presence of the monolayer on large area scans.