Precise modification of two dimensional (2D) materials with high resolution will be instrumental in future device fabrication. Such a method is presented here for the removal of layers of MoS_2. The thickness of selected regions of few layer MoS₂ has been altered with a sub-nanometre beam of He⁺_. This ‘beam exfoliation’ of MoS₂ has been characterised and controlled. An investigation of annealing and of the modified regions is now underway. 

MoS₂ is a material in the transition metal dichalcogenide (TMD) family which undergoes a transition from indirect to direct bandgap when going from multilayer to monolayer. This makes it ideally suited to applications in digital electronics as well as numerous photonic applications such as light emitters, photodetectors and solar cells. Excellent mechanical flexibility also provides potential for use in flexible electronics. Even more interesting than the superlative physical properties of 2D materials is the ability to tune these properties with precision. Considerable alteration of MoS₂ behaviour is already possible by tailoring the crystal structure, stoichiometry and geometry. Since there is such a significant difference between the behaviour of single layer and multilayer MoS₂, thinning of MoS₂ has been reported in a number of different manners. These include laser ablation and Ar⁺ plasma etching. 

Few-layer molybdenum disulphide (MoS₂) was obtained by mechanical and liquid exfoliation and transferred to a TEM grid. It was irradiated with He⁺ at 30 keV in a number of different configurations . In the images presented, thinning is observed in the bright regions where the beam did not penetrate the material.

Figure 1 is a 300kV TEM image which shows a region with intact crystal structure demonstrating a thinned front progressing towards a fully milled region. In addition, the crystal structure is clearly intact.

From previous work, it is known that few layer (one to roughly five or six layers) MoS₂ can be milled (cut through completely) with a minimum dose in the approximate range of 10¹⁷-10¹⁸ ions cm^-2. In order to perform rudimentary characterisation on the thinning of material SEM was used to measure the minimum helium ion dose at which altered contrast -and it is implied, thinning- could be observed (of course this does not exclude other possible effects on the complex systems of electron microscope contrast). A variety of SEM operation modes were tested to determine which was the most sensitive to the changes (secondary electron (SE2), Inlens, bright field STEM and dark field STEM). It was found that dark field STEM exhibited the sharpest change in contrast with respect to dose as indicated in figure 2.

Finally figure 3 is a schemtic diagram of the thinning process.

Figures:

Figure 3

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EMC Abstracts - https://emc-proceedings.com/abstract/beam-exfoliation-of-mos2-layers-with-a-helium-ion-beam/