Two-dimensional ferroelectricity and phase transitions in PVDF Langmuir-Blodgett films probed by second harmonic generation

Ultrathin electrode-free ferroelectric Langmuir-Blodgett (LB) films of poly vinylidene fluoride (PVDF) with trifluoroethylene are experimentally studie d by means of optical second harmonic generation (SHG) in a temperature ran ge from 140 K to 380 K. The temperature behavior of spontaneous polarizatio n induced SHG reveals distinct features in the vicinity of 355 K and 295 K which can be attributed to ferroelectric-paraelectric phase transitions in the bulk and at the surface of this two-dimensional structures. Lattice-sti ffening transition recently observed in PVDF LB films at 160 K does not sho w remarkable peculiarities of temperature dependence of SHG that indicates nonpolar changes in LB film structure under this transition.