Mechanical Properties of Oligothiophene Self Assembled Films by Atomic Force Microscopy
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Mechanical Properties of Oligothiophene Self Assembled Films by Atomic Force Microscopy Imma Ratera1, Jinyu Chen1, Amanda Murphy2, Frank Ogletree1, Jean M. J. Fréchet2, Miquel B. Salmeron1 1 Surface Science, Materials Science Department, Lawrence Berkeley National Laboratory, Berkeley, California – 94720, USA 2 Department of Chemistry, University of California, Berkeley, California – 94720, USA
ABSTRACT The oligothiophene derivative (4-(5''''-tetradecyl-[2,2';5',2'';5'',2''';5''',2''''] pentathiophen-5yl)-butyric acid (C14-5TBA) was synthesized and the structural and mechanical properties of self-assembled monolayers on mica have been studied by atomic force microscopy (AFM). The films were prepared by drop casting a dilute THF solution (1mM) of the oligothiphene on mica. Islands containing primarily monolayers with a very small percentage of multilayers were formed. The molecules adsorb through the carboxylic group, and expose the alkyl chain (CH2)13CH3. High resolution AFM scans reveal a well ordered structure of molecules with unit cell dimensions of 0.65 and 0.46 nm. Applying load to the tip, the molecular film was gradually compressed from an initial height of 4.1nm to a final one of 2.6 nm, corresponding to atilt of the alkyl chains. In regions covered with bilayers the molecules in the second layer were oriented opposite to those in the first layer, thus exposing the carboxylic end group to the air. These second layer was easily removed as the tip pressure increased. INTRODUCTION Conjugated organic polymers and oligomers are well-known for their interesting electronic and photonic properties.[1,2] Polymer based field-effect transistors (FETs) are drawing a growing interest due to their inexpensive fabrication and their relative flexibility for large area electronic applications, as well as good chemical stability. Although the performance of organic FETs made from such materials has been significantly improved during the past few years, some important problems still exist that hinder their industrial application.[3] Therefore, a design of novel organic semiconductors which combine high environmental stability, good solubility, and appropriate charge transport properties is highly desirable. Oligothiophenes (OTs) represent one of the most promising classes of semiconducting materials[4] for FET fabrication because of their good mobility and environmental stability. [5,6] However, unsubstituted OTs are insoluble in common organic solvents and can be deposited only by vacuum evaporation, [2,7,8] which is expensive and problematic from an industrial point of view. The high charge mobility and poor solubility of pentathiophene (5T) is due to molecular ordering and side-by-side π-stacking of the aromatic molecules. However, it has been shown that the introduction of alkyl chains can help improve their solubility. Since electronic and optical properties of the films are intimately connected to the molecular packing, it is very interesting to achieve control of the assembly process that take them form a diso
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