School of Physics , University College Dublin , Belfield , Dublin 4 , Ireland.
Department of Physics & Astronomy , Vanderbilt University , Box 1807-B, 6631 Stevenson Center , Nashville , Tennessee 37235 , United States.
ACS Appl Mater Interfaces. 2018 Aug 15;10(32):27188-27194. doi: 10.1021/acsami.8b08079. Epub 2018 Aug 2.
Metal thiophosphates are attracting growing attention in the context of quasi-two-dimensional van der Waals functional materials. Alkali thiophosphates are investigated as ion conductors for solid electrolytes, and transition-metal thiophosphates are explored as a new class of ferroelectric materials. For the latter, a representative copper indium thiophosphate is ferrielectric at room temperature and, despite low polarization, exhibits giant negative electrostrictive coefficients. Here, we reveal that ionic conductivity in this material enables localized extraction of Cu ions from the lattice with a biased scanning probe microscopy tip, which is surprisingly reversible. The ionic conduction is tracked through local volume changes with a scanning probe microscopy tip providing a current-free probing technique, which can be explored for other thiophosphates of interest. Nearly 90 nm-tall crystallites can be formed and erased reversibly on the surface of this material as a result of ionic motion, the size of which can be sensitively controlled by both magnitude and frequency of the electric field, as well as the ambient temperature. These experimental results and density functional theory calculations point to a remarkable resilience of CuInPS to large-scale ionic displacement and Cu vacancies, in part enabled by the metastability of Cu-deficient phases. Furthermore, we have found that the piezoelectric response of CuInPS is enhanced by about 45% when a slight ionic modification is carried out with applied field. This new mode of modifying the lattice of CuInPS, and more generally ionically conducting thiophosphates, posits new prospects for their applications in van der Waals heterostructures, possibly in the context of catalytic or electronic functionalities.
金属硫代磷酸盐在准二维范德华功能材料的背景下引起了越来越多的关注。碱金属硫代磷酸盐被研究为固体电解质的离子导体，而过渡金属硫代磷酸盐则被探索为一类新的铁电材料。对于后者，代表性的铜铟硫代磷酸盐在室温下为铁电体，尽管极化较低，但表现出巨大的负电致伸缩系数。在这里，我们揭示了这种材料中的离子导电性可以使带偏压的扫描探针显微镜针尖从晶格中局部提取 Cu 离子，这是令人惊讶的可逆过程。通过扫描探针显微镜针尖跟踪局部体积变化来跟踪离子传导，该技术提供了一种无电流探测技术，可以探索其他感兴趣的硫代磷酸盐。由于离子运动，这种材料的表面可以可逆地形成和擦除近 90nm 高的纳米晶，其尺寸可以通过电场的幅度和频率以及环境温度来敏感地控制。这些实验结果和密度泛函理论计算表明，CuInPS 对大规模离子位移和 Cu 空位具有显著的弹性，部分原因是 Cu 缺乏相的亚稳性。此外，我们发现，当施加电场进行轻微的离子修饰时，CuInPS 的压电响应增强了约 45%。这种新的修饰 CuInPS 晶格的模式，更普遍地是修饰离子传导的硫代磷酸盐的模式，为它们在范德华异质结构中的应用提供了新的前景，可能在催化或电子功能方面。
