[Paper Review] Defect-engineering hexagonal boron nitride using low-energy Ar+ irradiation

Manuel Längle, Barbara Maria Mayer+8 more|arXiv (Cornell University)|Apr 10, 2024

Diamond and Carbon-based Materials ResearchMaterials Science3 citations

TL;DR

This study demonstrates that low-energy Ar+ ion irradiation enables selective defect engineering in monolayer hexagonal boron nitride (hBN), with experimental results showing a strong preference for forming boron single vacancies (55%) over nitrogen single vacancies (12%)—contrary to prior analytical molecular dynamics predictions. The method achieves high defect selectivity at low irradiation doses, enabling controlled creation of specific point defects for quantum emitter applications.

ABSTRACT

Monolayer hexagonal boron nitride (hBN) has recently become the focus of intense research as a material to host quantum emitters. Although it is well known that such emission is associated with point defects, so far no conclusive correlation between the spectra and specific defects has been demonstrated. Here, we prepare atomically clean suspended hBN samples and subject them to low-energy ion irradiation. The samples are characterized before and after irradiation via automated scanning transmission electron microscopy imaging to assess the defect concentrations and distributions. We find an intrinsic defect concentration of ca. 0.03/nm2 (with ca. 55% boron and 8% nitrogen single vacancies, 20% double vacancies and 16% more complex vacancy structures). To be able to differentiate between these and irradiation-induced defects, we create a significantly higher (but still moderate) concentration of defects with the ions (0.30/nm2), and now find ca. 55% boron and 12% nitrogen single vacancies, 14% double vacancies, and 18% more complex vacancy structures. The results demonstrate that already the simplest irradiation provides selectivity for the defect types, and open the way for future experiments to explore changing the selectivity by modifying the irradiation parameters.

Motivation & Objective

• To experimentally validate theoretical predictions about defect formation in hBN under low-energy ion irradiation.
• To determine the selectivity of Ar+ ions in creating specific point defects (e.g., boron vs. nitrogen vacancies) in monolayer hBN.
• To differentiate between intrinsic defects and irradiation-induced defects using high-resolution automated STEM imaging.
• To assess the feasibility of using low-energy ion irradiation as a tool for targeted defect engineering in 2D materials.
• To resolve discrepancies between simulation predictions and experimental observations in defect formation mechanisms.

Proposed method

• Preparation of atomically clean, suspended monolayer hBN samples via polymer-free transfer onto hybrid Au/SiN TEM grids.
• Use of automated scanning transmission electron microscopy (STEM) in annular dark field (ADF) mode at 60 kV for high-resolution defect imaging and quantification.
• Irradiation of hBN samples with low-energy Ar+ ions (90–230 eV) at controlled doses to induce defects while preserving pristine regions for comparison.
• Quantification of intrinsic and irradiation-induced defect concentrations and types using automated defect detection and classification algorithms.
• Comparison of defect distributions before and after irradiation to isolate irradiation-induced contributions.
• Use of density functional theory (DFT)-based molecular dynamics to assess displacement thresholds and validate experimental findings.

Figure 1: Overview of the sample. (a) Light microscopy image of the sample after transfer. The purple/blue area corresponds to the Si grid, the light blue squares to Quantifoil. The yellow square is the perforated SiN window where the suspended sample can be found when holes in SiN and the Quantifoi

Experimental results

Research questions

• RQ1 What is the intrinsic defect concentration and distribution in as-prepared, cleaned monolayer hBN?
• RQ2 How does low-energy Ar+ irradiation affect the formation of specific point defects in hBN?
• RQ3 Does the experimental defect distribution match predictions from analytical potential molecular dynamics simulations?
• RQ4 What is the relative probability of forming boron vs. nitrogen single vacancies under low-energy ion irradiation?
• RQ5 Can irradiation parameters be tuned to achieve selective defect engineering in hBN?

Key findings

• The intrinsic defect concentration in pristine, cleaned hBN samples is approximately 0.03 nm⁻², with 55% boron single vacancies and 8% nitrogen single vacancies.
• After irradiation with a dose of ~0.30 nm⁻², the defect distribution shifts to 55.6% boron single vacancies, 11.5% nitrogen single vacancies, 14.4% double vacancies, and 18% complex vacancy structures.
• The experimental defect distribution shows a strong preference for boron vacancies, contradicting prior analytical molecular dynamics simulations that predicted comparable probabilities for B and N vacancies or a higher prevalence of nitrogen vacancies.
• The observed defect concentration exceeds the estimated number of impinging ions by a factor of ~3, suggesting a defect creation probability closer to unity than predicted by simulations.
• The discrepancy is attributed to limitations in analytical potential models, particularly the neglect of charging effects and inelastic scattering in insulating 2D materials like hBN.
• DFT-based molecular dynamics confirms a lower displacement threshold energy for boron (19.36 eV) than for nitrogen (23.06 eV), supporting the experimental preference for boron vacancy formation.

Figure 2: Plasma setup, beam profile measurement and defect concentration. (b) Schematic presentation of the plasma irradiation setup and the Faraday up. (d) Measured ion current $I$ and the calculated beam profile ( $\mathrm{d}I/\mathrm{d}V$ ) as a function of the bias voltage ( $V$ ). The purple p