COEXISTING LATTICE STRUCTURES IN A LANGMUIR-BLODGETT-FILM IDENTIFIED BY ATOMIC FORCE MICROSCOPY

Molecular resolution atomic force microscope images of the surface of barium arachidate Langmuir-Blodgett (LB) multilayers show coexisting domains of three different structures, two of which have previously unknown crystal lattices. The most prevalent structure is a tilted (26-degrees) triclinic packing of unit cell area 61.1 angstrom2 (20.4 angstrom2/molecule) with a 3x1 superstructure causing a sawtooth height modulation. Also present is a tilted (19-degrees) rectangular herringbone packing with alternating molecular pairs displaced vertically by a single methylene group having a unit cell area of 80.6 angstrom2 (20.1 angstrom2/molecule). Untilted, molecularly disordered regions with an area per molecule virtually identical to the other two packings are also seen, all within the same molecular layer. Although the alkane packing is quite distinct between the three domains, the area per molecule is essentially constant, and is substantially greater than that observed when cadmium is used as the counterion. This suggests that the barium counterion determines the area per molecule of the LB film and that the alkane packing adjusts to find the optimal structure given this constraint. Although the three structures do not occupy similar area fractions of the LB film, all three are found on each film we have examined. The submicrometer extent of each region, the extent of twinning and defects, and the complexity of the lateral and vertical molecular packing make the determination of the lattice structure of these films impossible by any technique other than atomic force microscopy.