Focused ion beam (FIB) milling offers nanometer-scale resolution and is advantageous for materials sensitive to wet chemical processes. Its applications span plasmonic, photonic, and microfluidic devices. Recently, FIB has shown promise in modifying 2D materials like transition metal dichalcogenides (TMDCs), particularly for tailoring their optical and electrical properties by creating controlled defects like chalcogenide and transition metal vacancies. Studies have demonstrated that ion irradiation can alter WS₂ optical properties (e.g., enhanced saturable absorption) and tune the conductivity of 2D materials. However, a significant challenge is understanding and minimizing the unintended damage beyond the targeted FIB area in these delicate 2D materials. While studies on graphene have quantified lateral damage extending up to 30 µm, attributing it to either unfocused ions or sputtered gallium ions, the impact on 2D TMDCs' light-emitting properties remains largely unknown. This study investigates the lateral effects of focused gallium ion beam lithography on large-area monolayer WS₂ using a range of ion beam currents to understand and control the damage.

Previous research has shown the potential of FIB in modifying the properties of 2D materials, especially TMDCs. Studies have demonstrated the ability to controllably create vacancies in TMDCs, leading to changes in their optical and electrical properties. For example, argon ion irradiation has been shown to modify the optical properties of WS₂ monolayers by creating sulfur vacancies, leading to enhanced saturable absorption. Similarly, controlling the ion irradiation dose allows for precise control over local defects, influencing resistivity and transport properties in materials like WSe₂. Helium ion beams have also been used to create defects in various TMDCs, resulting in the formation of percolating defect networks and edge states. However, previous work focusing on lateral damage has primarily been conducted on graphene, with limited understanding of the extent and impact on the optical properties of 2D TMDCs like WS₂.

Monolayer WS₂ was grown on sapphire substrates using metal-organic vapor deposition (MOVCD) and then transferred onto pre-patterned SiO₂/Si substrates using a wet transfer method. FIB milling was performed using a dual-beam FIB-SEM system (Versa3D, FEI) with a Ga⁺ beam at 30 kV and varying currents (10-3000 pA). A 2 µm diameter, 50 nm deep hole was milled. Steady-state photoluminescence (PL) and Raman spectroscopy were used to characterize the samples. Steady-state PL mapping was performed using a home-built micro-PL setup with a 532 nm CW laser. Raman spectroscopy employed a 532 nm laser and a 50× objective, analyzing characteristic peaks to assess defect density. Time-resolved PL measurements were conducted using a time-correlated single photon counting (TCSPC) system with a confocal microscope, employing a 405 nm pulsed laser. The PL was split using a dichroic filter at 638 nm to separate emission wavelengths. Energy dispersive X-ray (EDX) spectroscopy was used to analyze the distribution of Ga⁺ in the samples milled with different currents.

The study identified three distinct zones around the FIB-milled area in the WS₂ monolayer: a near zone (0-5 µm), a dark zone (5-30 µm), and a far-out zone (>30 µm). In the near zone, defect emission dominated, while the neutral exciton peak intensity increased significantly in the far-out zone. A bright ring-shaped emission was observed around the milled area via time-resolved PL mapping. This ring was found to primarily consist of trion emission with longer decay times than the neutral exciton emission. Raman spectroscopy revealed a blue shift of the 2LA(M) peak near the milled area, indicating an increase in defect density. The intensity of the 2LA(M) peak was also suppressed near the milled area. Analysis of the LA(M) mode showed a blue shift and increase in peak intensity near the milled area, further supporting the evidence of an increase in defects in the near and dark zones. The study found that the lateral damage in the far-out zone was dependent on the FIB current. Higher currents resulted in lower PL intensity and broader FWHM, likely due to redeposition of sputtered Ga⁺ ions. EDX mapping confirmed higher Ga⁺ density in samples milled with higher currents.

The findings demonstrate that FIB milling causes both short-range and long-range damage in monolayer WS₂. Short-range damage (0-30 µm) is attributed to the removal of atoms by primary ions and ion contaminants. Long-range damage (>30 µm) is due to the redeposition of secondary ions from the residual gas and chamber surfaces. The observed bright ring emission is likely due to trion formation from Ga⁺ ion deposition or carrier accumulation at the edges. The current-dependent long-range damage suggests that minimizing the ion beam current during the FIB process can significantly reduce the unintended damage, preserving the quality of the material far from the target area. This is in contrast to observations made in previous graphene studies. The results show the ability to engineer three distinct emission states with different spectral and temporal properties via FIB.

This work demonstrates the complex effects of FIB milling on monolayer WS₂, revealing distinct zones of damage and an unexpected ring-like emission. While short-range damage is inherent, long-range damage can be mitigated by reducing the ion beam current. The ability to create spatially and spectrally distinct emission states suggests significant potential for using FIB lithography to design and fabricate wafer-scale optoelectronic devices based on 2D TMDCs. Future work could explore the optimization of FIB parameters to further refine this technique for creating complex nanostructures with precise control over optical and electronic properties.

The study focused on a specific type of 2D material (WS₂) and a specific FIB system. The findings might not be directly generalizable to other TMDCs or FIB systems with different parameters. The mechanism of the ring-like emission requires further investigation. While the study successfully demonstrates a method to reduce long-range damage, the complete elimination of this damage might require further improvements in vacuum conditions or FIB system design.