Scanning tunneling microscopic observations of commensurate crystalline structures for horizontally deposited cadmium stearate bilayers on graphite

Bilayers of cadmium stearate have been assembled on graphite substrates using a horizontal deposition method and imaged in air using a scanning tunneling microscope (STM). STM images of these bilayers, acquired at a series of tunneling currents, allow the position of the tip with respect to the graphite surface to be deduced. During STM imaging at small tunneling currents, these measurements indicate that the tip is located ca. 50 Å from the graphite surface, or approximately at the surface of the stearate bilayer (which is 50 Å in thickness). STM imaging with larger tunneling currents (up to 6 nA) caused the STM tip to move closer to the graphite surface (and therefore into the stearate bilayer) at steady-state. In the specific case of Cd stearate bilayers, these measurements indicate that “non-perturbative” STM imaging conditions, which cause no detectable penetration of the stearate bilayer by the STM tip, are readily accessible at low currents. Under these STM imaging conditions, the structure of condensed-phase stearate bilayers has been investigated at molecular resolution using constant average height mode data acquisition and fast scan frequencies in excess of 80 Hz. STM images of stearate bilayers consist of regular arrays of high-tunneling-current vertices, which are arranged in either of two types of lattices: centered rectangular (CR) or rectangular (R). Regions of these bilayers composed of mixed R and CR unit cells are also observed. Two-dimensional Fourier analysis of the STM data reveals that the hexagonal periodicity of the graphite substrate and the less symmetric lattice of the fatty acid salt are simultaneously visible. These data enable the registry of the fatty acid layer with respect to the graphite lattice to be accurately determined, and this analysis reveals that both the R and the CR lattices are commensurate.