Ryu Gyeong Hee, France-Lanord Arthur, Wen Yi, Zhou Si, Grossman Jeffrey C, Warner Jamie H
Department of Materials , University of Oxford , 16 Parks Road , Oxford OX1 3PH , United Kingdom.
Department of Materials Science and Engineering , Massachusetts Institute of Technology , 77 Massachusetts Avenue , Cambridge , Massachusetts 02139 , United States.
ACS Nano. 2018 Nov 27;12(11):11638-11647. doi: 10.1021/acsnano.8b07051. Epub 2018 Oct 30.
We reveal a self-limiting mechanism during the formation of a specific type of circular nanopore in monolayer WS that limits its diameter to sub-nm. A single W atom vacancy (triangular nanopore) is transformed into the self-limiting nanopore (SLNP) through the atomic restructuring of S atoms around the area, reducing the number of dangling bonds at the nanopore edge by shifting them further in-plane with W-W bonding instead. Bond rotations in WS help accommodate the electron beam induced atomic loss and ensure the stability of the SLNP. The SLNP shows significant improvement in diameter stability during electron beam irradiation compared to other triangular nanopores in WS that typically continue to expand in diameter during atom loss. The atomic structure of these SLNPs is studied using aberration-corrected scanning transmission electron microscopy with an in situ heating holder, revealing that the SLNPs are mostly formed at a temperature of ∼500 °C, which is a balance between thermally activated S vacancy diffusion and sufficient S vacancy density to initiate local atomic reconstruction. At higher temperatures ( i. e., 1000 °C), S vacancies quickly migrate away into long line vacancies, resulting in low S vacancy density and rapidly expanding holes generated at the edges of the line vacancies. At room temperature, S vacancy migration is low and vacancy density is very high, which limits atomic reconstruction, and instead many small holes open up. These results provide insights into the factors that lead to uniform sized nanopores in the sub-nm range in transition-metal dichalcogenides.
我们揭示了在单层WS中形成特定类型的圆形纳米孔期间的一种自限机制，该机制将其直径限制在亚纳米级。单个W原子空位（三角形纳米孔）通过该区域周围S原子的原子重组转变为自限纳米孔（SLNP），通过用W-W键合使纳米孔边缘的悬空键进一步在平面内移动，从而减少悬空键的数量。WS中的键旋转有助于适应电子束诱导的原子损失并确保SLNP的稳定性。与WS中其他三角形纳米孔相比，SLNP在电子束辐照期间的直径稳定性有显著提高，其他三角形纳米孔在原子损失期间通常会继续扩大直径。使用带有原位加热支架的像差校正扫描透射电子显微镜研究了这些SLNP的原子结构，结果表明SLNP大多在约500℃的温度下形成，这是热激活的S空位扩散与启动局部原子重建的足够S空位密度之间的平衡。在较高温度下（即1000℃），S空位迅速迁移成长线状空位，导致S空位密度低，并且在线状空位边缘产生快速扩大的孔洞。在室温下，S空位迁移率低且空位密度非常高，这限制了原子重建，相反会形成许多小孔。这些结果为导致过渡金属二硫属化物中亚纳米范围内均匀尺寸纳米孔的因素提供了见解。
