Atomistic Positioning of Defects in Helium Ion Treated Single-Layer MoS₂

Abstract

Structuring materials with atomic precision is the ultimate goal of nanotechnology and is becoming increasingly relevant as an enabling technology for quantum electronics/spintronics and quantum photonics. Here, we create atomic defects in monolayer MoS2 by helium ion (He-ion) beam lithography with a spatial fidelity approaching the single-atom limit in all three dimensions. Using low-temperature scanning tunneling microscopy (STM), we confirm the formation of individual point defects in MoS2 upon He-ion bombardment and show that defects are generated within 9 nm of the incident helium ions. Atom-specific sputtering yields are determined by analyzing the type and occurrence of defects observed in high-resolution STM images and compared with Monte Carlo simulations. Both theory and experiment indicate that the He-ion bombardment predominantly generates sulfur vacancies.

The role of chalcogen vacancies for atomic defect emission in MoS2

“…In agreement with our optical studies, the surface of MoS 2 is virtually free from adsorbates after mild annealing. By far the most dominant defects in pristine material were oxygen atoms substituting sulfur, but no sulfur vacancies were observed (Supplementary Information S5) 9,19,20 .…”

“…3f). For He-ion treated samples, sulfur vacancies are the dominant defect type, among other defects that are generated with lower yield 9 . Furthermore, we performed CO-tip AFM on the sulfur vacancies (Figure 3g and Fig.…”

“…Figure 4c further corroborates this assignment. Here, we plot the emission spectrum of a single defect, which was generated by He-ion bombardment 9,24 , for co-and cross-circularly polarized excitation and detection. We do not detect a valley polarization, as expected for a transition that occurs between localized defect levels.…”

“…Furthermore, it has been suggested that laser illumination incorporates atomic oxygen into pre-existing chalcogen vacancies either by photo-assisted dissociation of molecular oxygen 17 or water 18 . However, chalcogen vacancies are likely already passivated by atomic oxygen in asprepared TMDs 9,19,20 . Moreover, several studies demonstrated that laser annealing in controlled gas environments 21,22 or encapsulation in hBN 23 can completely remove the L-band suggesting chemisorbed or physisorbed molecules as its origin 22 .…”

“…While these point-like emitters appear randomly in as-prepared samples, they can be generated deterministically via engineered nanoscale strain potentials 2 . Recently, our group has demonstrated that atomic point defects, which are created deterministically by He-ion irradiation 9 , act as narrow and reproducible single-photon emitters, yet without a local strain potential 3,24 . Here, we disentangle broad defect emission due to adsorbates, which can be desorbed by in-vacuo annealing at moderate temperatures, and narrow defect emission via sulfur vacancies, which are generated both by annealing at high temperatures and He-ion irradiation.…”