The paper was published in Nano Letters (2022)
This work was done in collaboration with a team from Queensland University of Technology supervised by Prof. D.V. Golberg.
Two-dimensional (2D) inorganic nanomaterials have been extensively studied. These include graphene, boron nitride, silicone and beyond, transition metal dichalcogenides (TMD), transition metal oxides, perovskites, and MXenes. 2D nanomaterials have shown diverse electromechanical and optoelectronic properties. They are promising for strain-engineered applications (e.g., strain sensors, flexible energy storage, flexible diodes, transistors, detectors, and diagnostic devices).
A photodetector fabricated with the partial vertical heterojunction between MoSe2 and Si has also been introduced. However, the effects of “edging” deformations (i.e., those with a loading axis parallel to the 2D basal atomic planes) on optical and/or optomechanical performances of layered nanomaterials have not been studied.
These drawbacks can be addressed via in situ high-resolution TEM (HRTEM) experiments using an optical TEM holder. In this paper, we define the deformation perpendicular to the
2D material layers’ basal atomic planes as a bending deformation, whereas the deformation parallel to such planes is called as an “edging” deformation.
Two MoSe2 atomic models were considered to understand difference between bending and edging deformations via DFT calculations. For convenience, an elastically bent MoSe2 monolayer was represented as a single-wall nanotube with a uniform curvature, whereas for edging deformation we simulated MoSe2 as an undulated monolayer defined by a wavelength and wave amplitude.
Series of consecutive TEM images illustrating a (a−c) bending and (e-f) edging deformation experiment on a MoSe2 nanosheets’ stack. (f) Characteristic TEM image of the nanosheet stack after severe edging deformation
Our findings correlate with experiments. For the case of bending deformation on the basis of TEM (Figure (a−c)), we see a general preservation of the initial MoSe2 structure under elastic bending. Moreover, since the photocurrent spectroscopy shows no difference between the deformed and undeformed MoSe2, we conclude that the valence band is mainly unchanged.
The DFT results also correlate with the experimental data for edging deformation, such as damage of MoSe2 surface (figure (f)) and highly unstable currents. Local high curvature and drastic structural alterations in flex points result in CBM and VBM changes. Band gap reduction and transition from a direct to indirect mode become apparent. Fracture of monolayered MoSe2 under high strains is attributed to its flexural rigidity, an order of magnitude greater than that of graphene. Nevertheless, even this damage is reversible for the monolayer case, while in thicker TMDs films, cracks and kinking appear and further accumulate under strain cycles. The latter can explain gradually rising current each cycle following the calculated monotonous band gap decrease
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