Differentiating Polymorphs in Molybdenum Disulfide via Electron Microscopy

Abstract

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201802397. The presence of rich polymorphs and stacking polytypes in molybdenum disulfide (MoS 2 ) endows it with a diverse range of electrical, catalytic, optical, and magnetic properties. This has stimulated a lot of interest in the unique properties associated with each polymorph. Most techniques used for polymorph identification in MoS 2 are macroscopic techniques that sample averaged properties due to their limited spatial resolution. A reliable way of differentiating the atomic structure of different polymorphs is needed in order to understand their growth dynamics and establish the correlation between structure and properties. Herein, the use of electron microscopy for identifying the atomic structures of several important polymorphs in MoS 2 , some of which are the subjects of mistaken assignment in the literature, is discussed. In particular, scanning transmission electron microscopy-annular dark field imaging has emerged as the most effective and reliable approach for identifying the different phases in MoS 2 and other 2D materials because its images can be directly correlated to the atomic structures. Examples of the identification of polymorphs grown under different conditions in molecular beam epitaxy or chemical vapor deposition, for example, 3R, 1T, 1T′-phases, and 1T′-edges, are presented, including their atomic structures, fascinating properties, growth methods, and corresponding thermodynamic stabilities.

Sub‐Millimeter‐Scale Monolayer p‐Type H‐Phase VS2

“…As we know, the contrast of STEM image is dependent on the atomic number Z ( I ∝ Z 1.6−1.7 ), which can be employed to differentiate atomic species (Figure 3f). [ 33,34 ] Moreover, the uniform distribution of bright and gray spots demonstrates the absence of defects, indicating the high crystalline quality. All in all, the above TEM and STEM results demonstrate that VS 2 exhibits the hexagonal phase with high crystalline quality.…”

Space‐confined microwave synthesis of ternary‐layered BiOCl crystals with high‐performance ultraviolet photodetection

“…Next, we applied STEM‐ADF imaging to investigate the crystallinity of the BiOCl crystals. The contrast of STEM‐ADF image is dependent on the Z atomic number as approximately ∼Z 1.6‐1.7 , thereby it was widely employed for analyzing crystal structures and atomic defects in 2D materials . From the theoretical atomic model (Figure A), we can see that BiOCl crystal shows a perfect tetragonal lattice along the [001] zone axis.…”

Electrochemical Hydrogen Evolution over Hydrothermally Synthesized Re-Doped MoS2 Flower-Like Microspheres

“…Although it has been found that hydrothermal synthesis of MoS 2 doped with Re induces 2H-1T phase transformation [18], we cannot specifically identify this phase transformation in our results due to special features of MoS 2 synthesized under solvothermal conditions. These include easy oxidation of samples in environmental conditions during Raman acquisition ( Figure S5) [31]; difficulty in identifying the 1T/2H phase of few-layered MoS 2 materials in TEM [37]; and existence of molybdenum polysulfides (considering solvothermal synthesis conditions) for XPS [38]. Our electrocatalytic results show that low Re doping improves overall HER catalytic activity of MoS 2 due to the creation of new catalytically active sites in basal planes, and by decreasing charge transfer resistance in the doped material.…”