Ferroelectric Langmuir-Blodgett Films

Local Piezoelectric Response and Surface Potential of Dielectric AND Ferroelectric Langmuir-Blodgett Films Studied by Electrostatic Force Microscopy... 

Surface Potential and Piezo-Response of Ferroelectric Langmuir-Blodgett Films... 

G. Decher, B. Tieke, C. Bosshard, and P. Gunter, Optical Second-Harmonic Generation in Langmuir-Blodgett Films of Novel Donor-Acceptor Substituted Pyridine and Benzene Derivatives, Ferroelectrics. in press. 

Geivandov, A. R. Palto, S. R Yudin, S. G. Blinov, L. M. Pelzl, G. Weissflog, W. Fetro-and antiferroelectric properties of Langmuir-Blodgett films composed of mesogenic bent-core molecules. Ferroelectrics 2006, 344, 247—254. 

Fig, 6. Topographic (top) and EFM (bottom) images of two lines written with 12V on a ferroelectric PVDF-TrFE Langmuir-Blodgett film. The left pair of images is taken in the contact (piezo-electric) mode, the right pair in non-contact (surface potential) mode. 

A comparative study of ultrathin dielectric (an azo-compound) and ferroelectric (copolymer P(VDF-TrFE)) Langmuir-Blodgett (LB) films has been carried out by Electrostatic Force Microscopy (EFM). Films were poled locally by a strong d.c. field applied between a conductive tip of an Atomic Force Microscope (AFM) and the bottom A1 electrode. The electrically poled domain was studied by EFM using a weak a.c. electric field and a lock-in amplifier technique. Two modes, a contact and non-contact ones, allowed for the measurement of field a in the air gap between the film and the tip and the piezoelectric distortion of the film due to the d.c. field aligned spontaneous polarization. Simultaneously the topographic relief of the same area was imaged. The results confirm unequivocally a possibility to switch ferroelectric LB film locally by an AFM tip. 

Recently an indication of two-dimensional ferroelectricity in ultrathin Langmuir-Blodgett (LB) films as thin as two molecular monolayers (about Inm) was discovered [1, 2] for a copolymer vinylidene fluoride with trifluoroethylene P(VDF-TrFE). The observations of the spontaneous polarization of the order of Ps 0.1 cW, the first order ferroelectric phase transition [2], a critical point [3], a surface phase transition and electric switching [1,2, 5] were reported. 

In general, research on classes of materials is connected with that on materials with specific properties but includes somewhat more general research on composites, polyurethanes, epoxies, fluoropolymers, ferroelectric liquid crystals (especially those with fast switching times), polymer-polymer miscibility, double network elastomers, crystallization in polymers, polymeric Langmuir-Blodgett and other multilayer films, and polymer-stabilized synthetic membranes. 

The interaction with an external surface in any material may induce a discontinuity and anisotropy in the energetic balance of the system, imposing effects different from those observed in bulk. In ferroelectric materials, it has been observed that finite size effects may also appear. However, it was verified that ultrathin films of P (VDF-TrFE) copolymers prepared via the Langmuir-Blodgett (LB) technique exhibit a F-P transition with a Tc nearly equal to the bulk value, but, in addition, another transition appears at a lower temperature, associated with the surface layers only [41]. LB films are highly crystalline, therefore, interfacial layers in this particular case do not present a large number of defects. Nevertheless, dielectric experiments have reported the presence of a dielectric segmental relaxation in P (VDF-TrFE) LB films, attributed to the motion of amorphous chains [42]. 

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