Local structures and electrical properties of organic molecular films investigated by non-contact atomic force microscopy

Abstract

Local structures and electrical properties of ferroelectric molecular films on a KCl(0 0 1) surface as well as π-conjugated oligothiophene thin films on a metal substrate were investigated by non-contact atomic force microscopy (NC-AFM). The ‘ferroelectric’ molecules, vinylidene fluoride (VDF) oligomers [CF 3–(CH 2CF 2) 17–I], with large dipole moment due to the difference in the electron affinity between the fluorine and the hydrogen atoms have recently attracted much interest in terms of the dipole control of the single molecule. NC-AFM investigations of the VDF oligomer thin films revealed the initial stage of the epitaxial growth process on the KCl substrate. Furthermore, Kelvin probe force microscopy (KFM) was used to map the local surface potential of single and double molecular films of oligothiophene molecules deposited on a platinum substrate. The results indicated that the charge transfer at the molecule–substrate interface caused the surface potential of 190 mV on the molecular film against the Pt substrate in a dark environment. Photoinduced potential of the film under ultraviolet light irradiation was also investigated.