Watching 2D materials grow


Rebecca Pool

Monday, September 3, 2018 - 22:30
HAADF STEM images of MoS2: the electron beam triggers and tracks crystallisation.
Using a Nion STEM, Austria-based researchers have imaged the crystallisation of molybdenum sulphide films.
The world first observations pave the way to the development of higher quality MoS2, a 2D material that is crucial to the manufacture of flexible solar cells and hydrogen generation for energy storage.
Researchers will now be able to more accurately study crystallisation details.
And as lead researcher, Dr Bernhard Bayer from the Institute of Materials Chemistry at TU Wien, points out: “This means it is no longer necessary to experiment through trial and error, but thanks to a deeper understanding of the processes, we can say for certain how to obtain the desired product.”
The researchers first grew graphene films on copper catalysts, which were then transferred onto holey-carbon-foil TEM grids, suspended as membranes and used as high-resolution STEM supports.
MoS2 films, some 2 nm thick, were then deposited onto the graphene membranes and transferred to an aberration-corrected, atomically resolved and element-specific Nion UltraSTEM 100.
According to Bayer, during STEM imaging at 60 kV electron acceleration voltage, the energy input from the scanning electron beam induced crystallisation and restructuring of the amorphous MoS2 into crystalline MoS2 domains.
The researchers harnessed this effect, creating a high-angle annular dark-field (HAADF) STEM image series of the MoS2 films during structural evolution, and as a function of continuous e-beam scanning time. 
"[This process] emulated widely used elevated temperature MoS2 synthesis and processing conditions," he says. "We thereby directly observed nucleation, growth, crystallisation, and restructuring events in the evolving MoS2 films in situ and at the atomic scale."
Bernhard C Bayer from TU Wien
Investigations revealed that the final crystal state did not always comprise the most thermodynamically stable configuration of atoms, with different crystal arrangements competing with, transforming into, and replacing each other.
"It is now clear why earlier investigations had such varying results," says Bayer. "We are dealing with a complex, dynamic process.”
The new findings will help researchers target different rearrangement processes and adapt the structure of 2D materials more precisely to future applications.
Research is published in ACS Nano.
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