CRYSTALLIZATION PROCESS OF LANGMUIR-BLODGETT-FILMS OF OCTADECYLTHIOBENZOQUINONE

Atomic force microscopy images of the electron acceptor 2-octadecylthi o-1,4-benzoquinone prepared as 2-4 Y-type Langmuir-Blodgett films reve aled the formation of nanoscale crystals one week after deposition. Th e crystals have a layered structure with a total height from 3 to 15 n m and they each covered an area of < 1 mum2 on the substrate. In the p lane of a layer, the molecules formed a two molecular rectangular unit cell with sides of 0.51+/-0.01 nm and 1.35+/-0.03 nm. The measured st ep height of a layer was 3.6+/-0.2 nm and gives a tilt of the alkyl ch ains of 54+/-3-degrees relative to the normal to the interface between layers. The packing density of 0.34+/-0.1 nm2 per molecules in the pl ane of a layer is significantly different from the packing density on the water subphase during deposition and the packing density found on the flat Langmuir-Blodgett bilayer of about 0.26 nm2. This means that a transition from a two-dimensional packing to a three-dimensional bul k crystal has taken place. This transition is more easily induced on a hydrophobic silicon substrate than on a bilayer of hydrophobic fatty acid. Six months after preparation, the crystal had grown in size, and now covered a typical area of almost-equal-to 15 mum2. The mobility o f molecules necessary for this growth is discussed.