2-DIMENSIONAL FERROELECTRIC-FILMS

Ultrathin crystalline films offer the possibility of exploring phase t ransitions in the crossover region between two and three dimensions. S econd-order ferromagnetic phase transitions have been observed in mono layer magnetic films(1,2), where surface anisotropy energy stabilizes the two-dimensional ferromagnetic state at finite temperature(3). Simi larly, a number of magnetic materials have magnetic surface layers tha t show a second-order ferromagnetic-paramagnetic phase transition with an increased Curie temperature(4). Ferroelectricity is in many ways a nalogous to ferromagnetism, and bulk-like ferroelectricity and finite- size modifications of it have been seen in nanocrystals as small as 25 0 Angstrom in diameter(5), in perovskite films 100 Angstrom thick(6) a nd in crystalline ferroelectric polymers as thin as 25 Angstrom (refs 7-10). But these results can be interpreted as bulk ferroelectricity s uppressed by surface depolarization energies, and imply that the bulk transition has a minimum critical size(11-13). Here we report measurem ents of the ferroelectric transition in crystalline films of a random copolymer of vinylidene fluoride and trifluoroethylene just 10 Angstro m (two monolayers) thick. We see a first-order ferroelectric phase tra nsition with a transition temperature nearly equal to the bulk value, even in these almost two-dimensional films. In addition, we see a seco nd first-order transition at a lower temperature, which seems to be as sociated with the surface layers only. The near-absence of finite-size effects on the bulk transition implies that these films must be consi dered as two-dimensional ferroelectrics.