期刊
2D MATERIALS
卷 5, 期 4, 页码 -出版社
IOP PUBLISHING LTD
DOI: 10.1088/2053-1583/aadb5f
关键词
scanning transmission electron microscope; bilayer graphene; density functional theory; rippling; nitrogen substitution; strain; defects
资金
- Department of Energy [DE-FG-02-09ER46554]
- National Science Foundation [DMR-1508433, ACI-1053575]
- McMinn Endowment at Vanderbilt University
- US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division
Ripples and impurity atoms are universally present in 2D materials, limiting carrier mobility, creating pseudo-magnetic fields, or affecting the electronic and magnetic properties. Scanning transmission electron microscopy (STEM) generally provides picometer-level precision in the determination of the location of atoms or atomic 'columns' in the in-image plane (xy plane). However, precise atomic positions in the z-direction as well as the presence of certain impurities are difficult to detect. Furthermore, images containing moire patterns such as those in angle-mismatched bilayer graphene compound the problem by limiting the determination of atomic positions in the xy plane. Here, we introduce a reconstructive approach for the analysis of STEM images of twisted bilayers that combines the accessible xy coordinates of atomic positions in a STEM image with density-functional-theory calculations. The approach allows us to determine all three coordinates of all atomic positions in the bilayer and establishes the presence and identity of impurities. The deduced strain-induced rippling in a twisted bilayer graphene sample is consistent with the continuum model of elasticity. We also find that the moire pattern induces undulations in the z direction that are approximately an order of magnitude smaller than the strain-induced rippling. A single substitutional impurity, identified as nitrogen, is detected. The present reconstructive approach can, therefore, distinguish between moire and strain-induced effects and allows for the full reconstruction of 3D positions and atomic identities.
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