Controlled defect production in monolayer MoS2 via electron irradiation at ultralow accelerating voltages

TL;DR

This study demonstrates that ultralow voltage electron irradiation can intentionally create defects in monolayer MoS2, with implications for tuning its properties for photonics and quantum applications.

Contribution

It shows defect creation at ultralow voltages using electron beams and develops a machine learning-based simulation method for Raman spectra of defective MoS2.

Findings

Defects in MoS2 can be induced at voltages ≤ 5kV.

Simulated Raman spectra match experimental data for sulfur vacancies.

Carbon contamination does not influence defect formation.

Abstract

Control on spatial location and density of defects in 2D materials can be achieved using electron beam irradiation. Conversely, ultralow accelerating voltages (less than or equal to 5kV) are used to measure surface morphology, with no expected defect creation. We find clear signatures of defect creation in monolayer (ML) MoS2 at these voltages. Evolution of E' and A1' Raman modes with electron dose, and appearance of defect activated peaks indicate defect formation. To simulate Raman spectra of MoS2 at realistic defect distributions, while retaining density-functional theory accuracy, we combine machine-learning force fields for phonons and eigenmode projection approach for Raman tensors. Simulated spectra agree with experiments, with sulphur vacancies as suggested defects. We decouple defects, doping and carbonaceous contamination using control (hBN covered and encapsulated MoS2) samples. We observe cryogenic PL quenching and defect peaks, and find that carbonaceous contamination does not affect defect creation. These studies have applications in photonics and quantum emitters.