Scanning tunneling microscopy investigations of cadmium stearate bilayers and dipalmitoyl phosphotidylcholine monolayers on graphite

Bilayers of cadmium stearate (Cd-stearate) and monolayers of dipalmitoyl phosphatidylcholine (DPPC) have been assembled on highly oriented pyrolytic graphite (HOPG) substrates and investigated in air using the scanning tunneling microscope (STM). STM images of Cd- stearate bilayers reveal a regular two-dimensional lattice of high tunneling current vertices which are separated by 4 - 6 angstroms. Few defects are observed in these lattices. 'High (tau) ' bilayers, which were deposited with a transfer ratio, (tau) approximately equals 2.0, have monoclinic lattices whereas the lattices of 'low (tau) ' bilayers, with (tau) equals 1.4 - 1.6, are orthorhombic. Molecular resolution STM data is also presented for DPPC monolayers prepared by the conventional Langmuir-Blodgett vertical deposition method. In monolayers of these structurally more complex amphiphiles, greater disorder is evident. Prolonged STM scanning of either Cd-stearate bilayers or DPPC monolayers induces modification of the 'native' crystalline structure presumably due to tip-film interactions.