Two-dimensional ferroelectric films
Two-dimensional ferroelectric films
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DOI：
10.1038/36069
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发表时间：
1998-02-26
期刊：
影响因子：
64.8
通讯作者：
Zlatkin, A
中科院分区：
文献类型：
--
作者：
Bune, AV;Fridkin, VM;Zlatkin, A
超薄晶体薄膜提供了在二维和三维之间的交叉区域探索相变的可能性。在单层磁性薄膜中已经观察到二阶铁磁相变（1,2），其中表面各向异性能量在有限温度下稳定了二维铁磁状态(3)。类似地，许多磁性材料的磁性表面层随着居里温度的升高呈现出二级铁磁-顺磁相变(4)。铁电性在许多方面类似于铁磁性，块状铁电性和有限尺寸的铁电性已经在直径小至250埃的纳米晶体(5)，在100埃厚的钙钛矿薄膜(6)和薄至25埃的结晶铁电聚合物中被发现（参考文献7-10）。但这些结果可以解释为受表面退极化能抑制的体铁电性，并暗示体跃迁具有最小临界尺寸（11-13）。在这里，我们报告了在10埃（两个单层）厚的偏氟乙烯和三氟乙烯随机共聚物的结晶膜中的铁电跃迁的测量。我们看到一阶铁电相变，其相变温度几乎等于体积值，即使在这些几乎是二维的薄膜中也是如此。此外，在较低的温度下，我们看到了第二次一级转变，这似乎只与表层有关。体跃迁几乎没有有限尺寸效应，这意味着这些薄膜必须被认为是二维铁电体。
Ultrathin crystalline films offer the possibility of exploring phase transitions in the crossover region between two and three dimensions. Second-order ferromagnetic phase transitions have been observed in monolayer magnetic films(1,2), where surface anisotropy energy stabilizes the two-dimensional ferromagnetic state at finite temperature(3). Similarly, a number of magnetic materials have magnetic surface layers that show a second-order ferromagnetic-paramagnetic phase transition with an increased Curie temperature(4). Ferroelectricity is in many ways analogous to ferromagnetism, and bulk-like ferroelectricity and finite-size modifications of it have been seen in nanocrystals as small as 250 Angstrom in diameter(5), in perovskite films 100 Angstrom thick(6) and in crystalline ferroelectric polymers as thin as 25 Angstrom (refs 7-10). But these results can be interpreted as bulk ferroelectricity suppressed by surface depolarization energies, and imply that the bulk transition has a minimum critical size(11-13). Here we report measurements of the ferroelectric transition in crystalline films of a random copolymer of vinylidene fluoride and trifluoroethylene just 10 Angstrom (two monolayers) thick. We see a first-order ferroelectric phase transition with a transition temperature nearly equal to the bulk value, even in these almost two-dimensional films. In addition, we see a second first-order transition at a lower temperature, which seems to be associated with the surface layers only. The near-absence of finite-size effects on the bulk transition implies that these films must be considered as two-dimensional ferroelectrics.