Understanding Defect Dynamics in 2D MoSe2 by In-Situ iDPC STEM

An ubiquitous challenge in the fabrication of 2D materials and devices is the introduction of defects during synthesis and handling, in addition to the challenging production of large-area monocrystalline structures. Since any defect will have a significant effect in such thin structures, investigating the formation and stability of these defects is a critical issue in 2D materials research. The changes caused by the defects can be detrimental for certain applications, but they may also reveal opportunities for tuneable behaviours that extend the relevance of the materials to other purposes, further justifying the need for comprehensive studies on this topic.<br/>In this paper, the purpose is to monitor the type of defects that occur in MoSe2 under exposure to the electron beam, including their dynamics and stability. The MoSe2 samples analysed were mechanically exfoliated flakes from bulk crystals of the material transferred onto holey SiN grids. Subsequently, single-frame HAADF-STEM images were recorded in order to determine the condition of the sample and its overall behaviour under beam exposure. Next, DPC-STEM data was acquired using fast-scanning series image acquisitions, recording several frames to reduce the rate of damage during the total acquisition time and to improve SNR by alignment and averaging of individual frames. This analysis leverages the heightened sensitivity of iDPC imaging to perform clearer observations of the structure and formation mechanisms of the defects, ultimately providing insights into potential opportunities for defect engineering and structural manipulation of the material at the atomic level.<br/>We have found various types of defected structures, along with observations regarding their formation and equilibrium dynamics, including large vacancy complexes that acted as unstable single-atom switches, 8-fold-ring complexes, point-sharing 4-fold-ring (4|4P) grain boundaries, and the formation of staggered double VSe line (SDVL) defects in the MoSe2 structure due to a large depletion of Se atoms. These SDVLs were observed to have remarkable stability in their shortest form and were identified as a preferential configuration whenever a large loss of Se atoms begins to occur, with a variety of possible formation mechanisms.