Observing Imperfection in Atomic Interfaces for van der Waals Heterostructures
Journal: Nano Letters
Publication Date: 25 July, 2017
Between the 2D sheets: Revealing impurities in atomically thin crystals from ångström-scale ripples
Two-dimensional (2D) materials such as graphene boast unique and powerful properties, however these properties will deteriorate if the material becomes contaminated. Luckily when different 2D materials are pressed together, stray molecules from the lab are pushed out leaving large flat areas clear of impurities. These clean regions have yielded some of the most fascinating physics of our time. Now, the assumption that these areas are completely clean is under scrutiny.
A team of researchers at The University of Manchester have shown that even the gas within which the 2D material stacks are assembled can affect the stack structure. They found that for the 2D transition metal dichalcogenides, some sheets had a large gap between them and their neighbour; a distance unexplained by theoretical calculations from collaborators at Radboud University, Netherlands. These observations seemed to point to the presence of impurities between the 2D sheets. To confirm this, 2D materials were stacked in a pure argon gas atmosphere using a glove-box. This time the distances matching those predicted by theory for a clean interface free from impurities. The consequences of this finding will directly impact on how we make and model graphene devices for future applications.
- A ‘cross section’ of a 2D material electronic device is made by ion milling and removing a small slice from a specific region where all the 2D materials overlap.
- A high energy, focused beam of electrons is fired through this slice, revealing the positions and identities of the atoms in the 2D materials.
- Measuring the distance between atoms in neighbouring crystals reveals they are not sitting flat on one another, and in fact ‘ripple’. The rippling is more pronounced for single layer 2D materials as they are more flexible.
- This rippling is caused by impurity atoms attaching to the surface of the 2D materials during fabrication in air. The rippling is greatly reduced if the 2D materials are cleaved in an inert argon atmosphere, i.e. a cleaner environment.