<italic>In-situ</italic> study of electron irradiation on two-dimensional layered materials

Abstract

High precision fabrication of nanostructure is a primary limiting factor to devices miniaturization. Electron beam is expected to promote the process on the accuracy of fabrication. For instance, electron beam lithography has been developed to replace optical lithography to transfer patterns and has produced line width on the order of 10 nm or smaller. Furthermore, electrons can directly interact with the sample to cause changes in structure and properties, which is expected to be used for fabrication of specialized nanostructures with nanometer even sub-nanometer precision. Transmission electron microscopy (TEM) has become an indispensable tool for the study of the interaction between energetic electrons and sample. A large number of experimental studies have been carried out inside TEM where electron beam can not only be used for atomic resolution imaging but also for irradiation. These studies are conductive to understanding phenomena induced by irradiation in essence, which are the theoretical and experimental basis for controllable manipulation via electron irradiation. However, TEM images are parallel projections of three-dimensional (3D) structures onto the image plane, which tremendously increases the difficulty of atomic-structure analysis. Two-dimensional (2D) layered materials (graphene, hexagon boron nitride, transition metal dichalcogenides and so on) emerged and boomed in the past decade, which provide ideal systems to study electron irradiation effects on matter. As mono- or bi-atomic sheets, their atomic structures are accessible to image directly with atomic resolution in the advent of spherical aberration corrected TEM. The lateral resolution in TEM image, which is smaller than the bond length, leaves almost no room for uncertain interpretation. Meanwhile, the reconstruction of the lattice occurs in the 2D plane can be seen without any projection artifacts. Besides, structural changes, as they are generated under irradiation with energetic electrons, can be monitored in real time. On this basis, novel mechanisms of lattice reconstruction, which did not appear in other materials, have been discovered, such as transformations between hexagons and pentagon-heptagon pairs by in-plane 90˚ rotation of C–C bonds in graphene sheet. In addition to a detailed understanding of radiation-induced structure evolution and their influence on the properties of 2D sheet, the design of new structures has become feasible. Especially, electron beams inside modern TEM can be focused onto spots less than 1 A, which allows use of the beam as tweezers to manipulate materials even at the single atom level. In this way, sub-nanometer quasi one dimensional structures, such as nanowires, nanoribbons, nanotubes and atomic chains, have been fabricated in 2D nanosystems. Sub-5 nm nanopores can also be sculpted in 2D sheets, which is particularly suitable for chemical and biological molecule detection. Besides, 2D materials can serve as substrates to study radiation-induced structural evolution of substance on their surface. This review gives a brief summary of our current knowledge about electron irradiation in the aspect of elastic scattering and inelastic scattering, and a summary of the most recent experimental results on in situ irradiation in 2D layered material systems, including atomic structural evolution under electron irradiation and controllable fabrication of nanostructures by TEM.