Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, USA.
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA.
Nature. 2018 Mar 7;555(7695):231-236. doi: 10.1038/nature25774.
Artificial superlattices, based on van der Waals heterostructures of two-dimensional atomic crystals such as graphene or molybdenum disulfide, offer technological opportunities beyond the reach of existing materials. Typical strategies for creating such artificial superlattices rely on arduous layer-by-layer exfoliation and restacking, with limited yield and reproducibility. The bottom-up approach of using chemical-vapour deposition produces high-quality heterostructures but becomes increasingly difficult for high-order superlattices. The intercalation of selected two-dimensional atomic crystals with alkali metal ions offers an alternative way to superlattice structures, but these usually have poor stability and seriously altered electronic properties. Here we report an electrochemical molecular intercalation approach to a new class of stable superlattices in which monolayer atomic crystals alternate with molecular layers. Using black phosphorus as a model system, we show that intercalation with cetyl-trimethylammonium bromide produces monolayer phosphorene molecular superlattices in which the interlayer distance is more than double that in black phosphorus, effectively isolating the phosphorene monolayers. Electrical transport studies of transistors fabricated from the monolayer phosphorene molecular superlattice show an on/off current ratio exceeding 10, along with excellent mobility and superior stability. We further show that several different two-dimensional atomic crystals, such as molybdenum disulfide and tungsten diselenide, can be intercalated with quaternary ammonium molecules of varying sizes and symmetries to produce a broad class of superlattices with tailored molecular structures, interlayer distances, phase compositions, electronic and optical properties. These studies define a versatile material platform for fundamental studies and potential technological applications.
人工超晶格基于二维原子晶体（如石墨烯或二硫化钼）的范德华异质结构，提供了超越现有材料的技术机会。创建这种人工超晶格的典型策略依赖于艰苦的逐层剥离和重新堆积，其产量和可重复性有限。使用化学气相沉积的自下而上的方法可以产生高质量的异质结构，但对于高阶超晶格来说，这种方法变得越来越困难。用碱金属离子对选定的二维原子晶体进行插层提供了一种超晶格结构的替代方法，但这些通常稳定性差，电子性质严重改变。在这里，我们报告了一种电化学分子插层方法，用于一种新的稳定超晶格，其中单层原子晶体与分子层交替排列。我们以黑磷作为模型系统，表明用十六烷基三甲基溴化铵插层可以产生单层磷烯分子超晶格，其中层间距离超过黑磷的两倍，有效地将磷烯单层隔开。由单层磷烯分子超晶格制成的晶体管的输运研究表明，开/关电流比超过 10，同时具有优异的迁移率和卓越的稳定性。我们进一步表明，几种不同的二维原子晶体，如二硫化钼和二硒化钨，可以用不同大小和对称性的季铵盐分子进行插层，从而产生具有定制分子结构、层间距离、相组成、电子和光学性能的广泛超晶格。这些研究为基础研究和潜在的技术应用定义了一个通用的材料平台。
