ELSEVIER Analytica Chimica Acta 307 (1995) 377-391 ANALYTICA zyxwvut CHIMICA zyxwvutsrqpo ACTA Scanning tunneling microscopic observations of commensurate crystalline structures for horizontally deposited cadmium stearate bilayers on graphite R.M. Stiger, J.A. Virtanen, S. Lee, S.A. Virtanen, R.M. Penner * zyxwvutsrqponmlk Institute for Surface and Interface Science (ISIS), Department of Chemistry, University of California, Irvine, CA 92717-2025, USA Received 8 August 1994; revised 6 October 1994; accepted 13 October 1994 Abstract Bilayers of cadmium stearate have been assembled on graphite substrates using a horizontal deposition method and imaged in air using a scanning tunneling microscope @TM). STM images of these bilayers, acquired at a series of tunneling currents, allow the position of the tip with respect to the graphite surface Jo be deduced. During STM imaging at small tunneling currents, these measurements indicate that the tip is located ca. 50 A from the graphite surface, or approximately at the surface of the stearate bilayer (which is 50 A 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. Keywords: Bilayers; Cadmium stearate; Crystal structures; Graphite; Microscopy; Scanning tunneling microscopy; Surface techniques 1. Introduction Langmuir-Blodgett (LB) deposition molecular monolayers to be transferred to * Corresponding author. permits a variety of substrates. Recently, we have demonstrated a new horizontal deposition (HD) procedure which enables the assembly of bilayers composed of fatty acids and fatty acid salts on hydrophobic graphite and germa- nium surfaces [l]. These bilayers are Y-type and the molecular orientation is such that the acyl chains of the fatty acid are exposed at the planar surfaces of the bilayer, and hydrophilic carboxylate moieties are 0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0003-2670(94)00525-7