Imaging Ultrathin Organic Films On The Nanometer Level Using Surface Plasmons

Surface plasmons have been used to detect single molecular layers for many years.¹ 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 mm), 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. The films are soft and easily damaged. In order to detect the presence of the monolayer, a fragile chemically derivatized tip is required.

In our system the surface plasmon is imaged by scanning a sharp fiberoptic probe approximately 10 nm above the sample surface. In this way both the sample topography and surface plasmon intensity are mapped simultaneously. By placing the fiber near the surface the electromagnetic wave on the metal surface is detected optically. The fiber then leads to either a photomultiplier tube or an avalanche photodiode, resulting in a signal proportional to the intensity of the surface plasmon. As the tip is scanned across the sample it is dithered horizontally, allowing the distance between the tip and sample to be monitored. This allows the sample topography to be mapped.

To demonstrate this technique, a stripe of phthalocyanine 200 nm thick was thermally deposited onto a gold film. Figure 1, an image of the edge of this stripe, depicts both the topographic and surface plasmon intensity data. The image is 50 (m x 50 mm and 200 nm high; the gray scale represents light intensities from 0 (black) to 7 million photons per second (white). From this image it can be seen that the surface plasmon intensity decreases at the edge of the phthalocyanine stripe as expected and also around islands of phthalocyanine on the bare gold film. While the detection limits of this technique are not quantitatively demonstrated by this image, it suggests that the surface plasmon decay lengths are in agreement with the predicted decay length2 of approximately 2 mm.