Sebastian Amritanand, Zhang Fu, Dodda Akhil, May-Rawding Dan, Liu He, Zhang Tianyi, Terrones Mauricio, Das Saptarshi
Engineering Science and Mechanics , Pennsylvania State University , University Park , Pennsylvania 16802 , United States.
Materials Science and Engineering , Pennsylvania State University , University Park , Pennsylvania 16802 , United States.
ACS Nano. 2019 Jan 22;13(1):78-86. doi: 10.1021/acsnano.8b08216. Epub 2018 Dec 4.
Two-dimensional (2D) layered materials demonstrate their exquisite properties such as high temperature superconductivity, superlubricity, charge density wave, piezotronics, flextronics, straintronics, spintronics, valleytronics, and optoelectronics, mostly, at the monolayer limit. Following initial breakthroughs based on micromechanically exfoliated 2D monolayers, significant progress has been made in recent years toward the bottom-up synthesis of large-area monolayer 2D materials such as MoS and WS using physical vapor deposition and chemical vapor deposition techniques in order to facilitate their transition into commercial technologies. However, the nucleation and subsequent growth of the secondary, tertiary, and greater numbers of vertical layers poses a significant challenge not only toward the realization of uniform monolayers but also toward maintaining their consistent electronic and optoelectronic properties which change abruptly when transitioning from the monolayer to multilayer form. Chemical or physical techniques which can remove the unwanted top layers without compromising the material quality will have tremendous consequences toward the development of atomically flat, large-area, uniform monolayers of 2D materials. Here, we report a simple, elegant, and self-limiting electrochemical polishing technique that can thin down any arbitrary thickness of 2D material, irrespective of whether these are obtained using powder vapor transport or mechanical exfoliation, into their corresponding monolayer form at room temperature within a few seconds without compromising their atomistic integrity. The effectiveness of this electrochemical polishing technique is inherent to 2D transition-metal dichalcogenides owing to the stability of their basal planes, enhanced edge reactivity, and stronger than van der Waals interaction with the substrate. Our study also reveals that 2D monolayers are chemically more robust and corrosion resistant compared to their bulk counterparts in similar oxidative environments, which enables electrochemical polishing of such materials down to a monolayer.
二维（2D）层状材料大多在单层极限时展现出其卓越的性能，如高温超导、超润滑性、电荷密度波、压电电子学、柔性电子学、应变电子学、自旋电子学、谷电子学和光电子学。在基于微机械剥离的二维单层取得初步突破之后，近年来在利用物理气相沉积和化学气相沉积技术自下而上合成大面积单层二维材料（如二硫化钼和二硫化钨）方面取得了重大进展，以便促进其向商业技术的转变。然而，二级、三级及更多垂直层的成核和后续生长不仅对实现均匀单层构成重大挑战，而且对维持其一致的电子和光电性能也构成重大挑战，因为从单层转变为多层形式时，这些性能会突然改变。能够在不损害材料质量的情况下去除不需要的顶层的化学或物理技术，将对二维材料原子级平整、大面积、均匀单层的开发产生巨大影响。在此，我们报告一种简单、精巧且自限性的电化学抛光技术，该技术可以在室温下几秒钟内将任意厚度的二维材料（无论这些材料是通过粉末气相传输还是机械剥离获得的）减薄为相应的单层形式，而不会损害其原子完整性。由于二维过渡金属二硫属化物基面的稳定性、增强的边缘反应性以及与基底的相互作用强于范德华相互作用，这种电化学抛光技术对其具有有效性。我们的研究还表明，在类似的氧化环境中，二维单层相比其体相材料在化学上更稳定且耐腐蚀，这使得能够将此类材料电化学抛光至单层。
