†The Institute for Functional Imaging of Materials and the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
‡Faculty of Physics, Vilnius University, Vilnius, Lithuania LT-01513.
Nano Lett. 2015 Jun 10;15(6):3808-14. doi: 10.1021/acs.nanolett.5b00491. Epub 2015 May 5.
We explore ferroelectric properties of cleaved 2-D flakes of copper indium thiophosphate, CuInP2S6 (CITP), and probe size effects along with limits of ferroelectric phase stability, by ambient and ultra high vacuum scanning probe microscopy. CITP belongs to the only material family known to display ferroelectric polarization in a van der Waals, layered crystal at room temperature and above. Our measurements directly reveal stable, ferroelectric polarization as evidenced by domain structures, switchable polarization, and hysteresis loops. We found that at room temperature the domain structure of flakes thicker than 100 nm is similar to the cleaved bulk surfaces, whereas below 50 nm polarization disappears. We ascribe this behavior to a well-known instability of polarization due to depolarization field. Furthermore, polarization switching at high bias is also associated with ionic mobility, as evidenced both by macroscopic measurements and by formation of surface damage under the tip at a bias of 4 V-likely due to copper reduction. Mobile Cu ions may therefore also contribute to internal screening mechanisms. The existence of stable polarization in a van-der-Waals crystal naturally points toward new strategies for ultimate scaling of polar materials, quasi-2D, and single-layer materials with advanced and nonlinear dielectric properties that are presently not found in any members of the growing "graphene family".
我们通过环境和超高真空扫描探针显微镜探索了二维剥离的铜铟磷硫化物（CuInP2S6，CITP）的铁电特性，并研究了尺寸效应以及铁电相稳定性的极限。CITP 属于仅有的在室温及以上的范德华层状晶体中显示铁电极化的材料家族。我们的测量直接揭示了稳定的铁电极化，表现为畴结构、可切换的极化和滞后回线。我们发现，在室温下，厚度大于 100nm 的薄片的畴结构与剥离的体表面相似，而厚度小于 50nm 时极化消失。我们将这种行为归因于由于去极化场引起的极化众所周知的不稳定性。此外，高偏压下的极化切换也与离子迁移率有关，这不仅可以通过宏观测量，还可以通过在 4V 偏压下在针尖下形成表面损伤来证明-可能是由于铜的还原。因此，可移动的 Cu 离子也可能有助于内部屏蔽机制。范德华晶体中稳定的极化的存在自然指向了极材料、准二维和具有先进非线性介电性能的单层材料的最终缩微策略，而目前在不断增长的“石墨烯家族”的任何成员中都没有发现这些策略。
