Direct electric field imaging of graphene defects

Abstract

Material properties are sensitive to atomistic structure defects such as vacancies or impurities, and it is therefore important to determine not only the local atomic configuration but also their chemical bonding state. Annular dark-field scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy has been utilized to investigate the local electronic structures of such defects down to the level of single atoms. However, it is still challenging to two-dimensionally map the local bonding states, because the electronic fine-structure signal from a single atom is extremely weak. Here, we show that atomic-resolution differential phase-contrast STEM imaging can directly visualize the anisotropy of single Si atomic electric fields in monolayer graphene. We also visualize the atomic electric fields of Stone–Wales defects and nanopores in graphene. Our results open the way to directly examine the local chemistry of the defective structures in materials at atomistic dimensions.

Imaging Beam‐Sensitive Materials by Electron Microscopy

“…The four sets of SW defects are connected by one six membered rings. Besides, the nanopores (Figure 24d–f), which show strong electric field, are also observed in the bilayer graphene 398. Electron ptychography as another dose‐efficient diffractive imaging technique, could also be used to identify a point defect in 2D materials.…”

“…By using DPC‐STEM at 80 kV, the topological defects in graphene, including dopants, Stone–Wales (SW) defects and nanopores, have been revealed 398. The defects in graphene affect both the local atomic configuration and their chemical bonding state, which is especially difficult to be mapped at atomic resolution due to the weak signal of electronic fine structure 398 . Figure a–c clearly shows the SW defects (5–7–7–5) structure and atomic electric fields 398.…”

“…The defects in graphene affect both the local atomic configuration and their chemical bonding state, which is especially difficult to be mapped at atomic resolution due to the weak signal of electronic fine structure 398 . Figure a–c clearly shows the SW defects (5–7–7–5) structure and atomic electric fields 398. The four sets of SW defects are connected by one six membered rings.…”

“…a–f) From left to right, low‐kV atomic‐resolution ADF‐STEM image, electric field strength maps and electric field vector maps derived from DPC‐STEM of a–c) Stone–Wales defects and d–f) nanopore region in graphene. Reproduced with permission 398. Copyright 2018, Springer Nature.…”

Iterative Algorithm of Atomic Potential Reconstruction Based on DPC Signal from Thick Specimens

“…This imaging technique is called differential phase contrast (DPC). Electric field measurement can be carried out at atomic resolution, and electric field between nuclei and electrons has been visualized [1][2][3][4]. According to Maxwell's equation, static electric field can be converted into charge density map.…”