Atomic Layer Etching Mechanism of MoS₂ for Nanodevices

Abstract

Among the layered transition metal dichalcogenides (TMDs) that can form stable two-dimensional crystal structures, molybdenum disulfide (MoS) has been intensively investigated because of its unique properties in various electronic and optoelectronic applications with different band gap energies from 1.29 to 1.9 eV as the number of layers decreases. To control the MoS layers, atomic layer etching (ALE) (which is a cyclic etching consisting of a radical-adsorption step such as Cl adsorption and a reacted-compound-desorption step via a low-energy Ar-ion exposure) can be a highly effective technique to avoid inducing damage and contamination that occur during the reactive steps. Whereas graphene is composed of one-atom-thick layers, MoS is composed of three-atom-thick S-Mo-S layers; therefore, the ALE mechanisms of the two structures are significantly different. In this study, for MoS ALE, the Cl radical is used as the adsorption species and a low-energy Ar ion is used as the desorption species. A MoS ALE mechanism (by which the S, Mo, and S atoms are sequentially removed from the MoS crystal structure due to the trapped Cl atoms between the S layer and the Mo layer) is reported according to the results of an experiment and a simulation. In addition, the ALE technique shows that a monolayer MoS field effect transistor (FET) fabricated after one cycle of ALE is undamaged and exhibits electrical characteristics similar to those of a pristine monolayer MoS FET. This technique is also applicable to all layered TMD materials, such as tungsten disulfide (WS), molybdenum diselenide (MoSe), and tungsten diselenide (WSe).

Disassembly of 2D Vertical Heterostructures

“…It is worth mentioning that, in order to achieve the disassembly of 2D materials, an appropriate energy should be applied to weakening the interactions between 2D monomers but not to result in the destruction of the building blocks. Oppositely, some traditional methods provide excessive external mechanical or thermal energy and superfluous high‐energy particles, including micromechanical cleavage and the post‐treatment with ion beams, laser, plasma, and some compounds, which would inevitably lead to the crumbling of the 2D building blocks. Thus far, there are no effective methods to achieve the disassembly of 2D materials.…”

Etching Techniques in 2D Materials

“…d) Schematic diagram of the MoS 2 ‐ALE mechanism through the Cl‐radical adsorption and the Ar + desorption as a function of time. e) Raman spectra of the CVD bilayer MoS 2 before and after the Cl adsorption, and those after the Ar + exposure from 15 to 200 s. (d,e) Reproduced with permission . Copyright 2017, American Chemical Society.…”

“…Figure c shows that each cycle etched one layer and the layer number of MoS 2 reduced in argon plasma while MoS 2 kept undamaged in chlorine plasma. The ALET mechanism of MoS 2 and graphene is not quite the same, because the single‐layer MoS 2 consists of three‐layer atoms SMoS while single‐layer graphene is comprised of one‐layer carbon atoms . Only the van der Waals force between top layer carbon atoms and the second layer carbon atoms needs to be weakened by adsorption for graphene.…”

Band Alignment of WS2/MoS2 Photoanodes with Efficient Photoelectric Responses based on Mixed Van der Waals Heterostructures

“…Photoelectric conversion based on transition metal dichalcogenide (TMD) semiconductors has stimulated extensive attention due to their high mobility, narrow bandgap from the visible region to infrared region, and strong light matter interactions to alleviate energy crisis. As such, wide optoelectronic devices based on TMD have been designed, including photocatalysis, photodetectors, field‐effect transistors, solar cells, and photoelectrodes . Among these TMD, WS 2 , and MoS 2 are the most cheap and air‐stable layered semiconductors, which can be both easily exfoliated in a large scale and tuned from the visible region to infrared region .…”