Chinese Physics B

Selective linear etching of monolayer black phosphorus using electron beams

Fig.4 (a) Top and side views of atomic structure of monolayer BP (V0P). The names of the zigzag-like chains and two tested atoms are marked. The upper (colored in plum) and lower (colored in light coral) chains are named nT (n is the order number of the chain) and nD, respectively. The P atoms in the upper and lower sublayers are named PnTm (m is the order number of the atom) and PnDm, respectively. (b) and (c) Trajectories of two tested P atoms in pristine monolayer BP under an FHEEB. (d) Top and side views of the atomic structure of a single-atom vacancy BP (V1P) and all five tested P atoms. (e) Calculated cross-sections for the tested atoms in pristine monolayer BP (V0P) and single-atom vacancy monolayer BP (V1P).

Fig.5 Electrical properties of predicted zigzag chain vacancy in monolayer BP. (a) Band structure and density of states of the chain vacancy. (b) PCD at bands MB1 and MB2, DCD, and atomic structure of the zigzag edge chain. (c) Band structure of double-periodic chain vacancies with and without up-and-down distortion. (c) PCD at bands MB1 and MB2, DCD, and atomic structure of the zigzag edge chain. (d) Top view (left) and side view (right) of the atomic structure of the chain vacancy with distortion.

A special zigzag chain vacancy (Fig 5d) in monolayer BP was predicted by using high-energy electron beams (FHEEBs) and knocking away P atoms one by one along a zigzag chain in the lower sublayers. The calculated electronic properties of the chain vacancy showed that there was quasi-bonding between the two edges of the vacancy (Fig 5b), and a CDW was also formed along the vacancy. Our findings help improve understanding of quasi-bonding in which covalent-like states can also be half-occupied. The chain vacancy was a dynamically stable but thermodynamically metastable state according to our comparison of the stabilities of five typical edges in monolayer BP. It was inspiring that the electron beam could create a dynamically mostly stable but thermodynamically metastable vacancy, which is difficult to obtain using conventional chemical synthesis methods but easier to achieve using an electron beam. This characteristic proves that an FHEEB can create a special environment for defect development.

This simulation was implemented using a self-developed tool aBEST (https://gitee.com/jigroupruc/aBEST). This work is expected to inspire further works that will implement more exciton modeling methods into the simulation protocol and thus provide detailed theoretical guidance for future experiments in the field of 2D material etching by FHEEBs.

REFERENCES

1. Zhao, X.; Loh, K. P.; Pennycook, S. J. Electron Beam Triggered Single-Atom Dynamics in Two-Dimensional Materials. J. Phys.: Condens. Matter 2020, 33 (6), 063001. https://doi.org/10.1088/1361-648X/abbdb9.

2. Molecular Beam Epitaxy of Highly Crystalline MoSe2 on Hexagonal Boron Nitride | ACS Nano, https://pubs.acs.org/doi/full/10.1021/acsnano.8b04037.

3. X. Zhao, Y. Ji, J. Chen, W. Fu, J. Dan, Y. Liu, S. J. Pennycook, W. Zhou, and K. P. Loh, Healing of Planar Defects in 2D Materials via Grain Boundary Sliding, Advanced Materials 31, 1900237 (2019).

4. Atomic Structure and Formation Mechanism of Sub-Nanometer Pores in 2D Monolayer MoS 2 – Nanoscale (RSC Publishing) DOI:10.1039/C7NR01127J, https://pubs.rsc.org/en/content/articlehtml/2017/nr/c7nr01127j.

5. O. Dyck, S. Kim, E. Jimenez-Izal, A. N. Alexandrova, S. V. Kalinin, and S. Jesse, Building Structures Atom by Atom via Electron Beam Manipulation, Small 14, 1801771 (2018).

6. Electron-Beam Manipulation of Silicon Dopants in Graphene | Nano Letters, https://pubs.acs.org/doi/full/10.1021/acs.nanolett.8b02406.