Atomic Force Microscopy Study of β-Substituted-T7 Oligothiophene Films on Mica:  Mechanical Properties and Humidity-Dependent Phases

The structural and mechanical properties of Langmuir-Blodgett monolayer and multilayer films of 3",4""-didecyl-5,2'; 5',2"; 5",2'''; 5''',2""; 5"",2'''''; 5''''',2"""-heptathiophene-4'''-acetic acid on mica have been studied by atomic force microscopy (AFM) as a function of humidity, temperature, and applied force. The molecules orient with the carboxylic acid group pointing toward the mica surface and expose the alkyl side chains to the air interface. As the load applied by the AFM tip increases, the film is compressed easily from an initial height of 2 to 1.2 nm. After compression the films can support much higher loads without loss of height. The state of aggregation of the molecules was found to be sensitive to the environmental humidity, which induced reversible changes. Annealing the samples with monolayer or multilayer films resulted in irreversible changes when the temperature exceeded approximately 100 degrees C.