Substrate-dependent pore formation in molybdenum disulfide monolayers under ion irradiation

This study demonstrates that substrate-dependent electronic dissipation pathways critically govern the size and formation efficiency of nanopores in monolayer MoS$_2$ under highly charged and swift heavy ion irradiation, with SiO$_2$ promoting pore formation while gold substrates significantly suppress it.

The Gist

Imagine you have a sheet of paper so thin it's only one atom thick. This is Molybdenum Disulfide (MoS₂), a material scientists love because it's incredibly strong and useful for future electronics. Now, imagine you want to punch tiny, perfect holes in this paper to create filters or sensors.

The scientists in this study used two different types of "atomic bullets" to punch these holes:

1. Highly Charged Ions (HCIs): Like a heavy, super-charged cannonball that explodes with energy the moment it touches the surface.
2. Swift Heavy Ions (SHIs): Like a fast-moving bullet that drills a continuous trail of energy as it flies through.

The big question they wanted to answer was: Does what's underneath the paper change how the holes look?

The Three Scenarios

To find out, they tested the paper in three different "neighborhoods":

1. The Floating Island (Suspended MoS₂)
Imagine the paper is floating in mid-air, with nothing underneath it. When the ions hit, the energy has nowhere to go but into the paper itself.

• Result: The holes are decent-sized, but not the biggest.

2. The Concrete Slab (MoS₂ on Silicon Dioxide/SiO₂)
This is the paper sitting on a standard glass-like surface (insulator). Think of this like a trampoline sitting on a concrete floor. When you jump, the concrete doesn't absorb much energy; it just reflects it back or traps it.

• Result: The holes were the biggest and most frequent. Because the concrete floor doesn't "soak up" the energy, all that punchy energy stays trapped in the paper, melting a larger hole.

3. The Black Hole (MoS₂ on Gold)
This is the paper sitting on a metal surface. Think of gold as a giant, super-efficient sponge or a "heat sink." When energy hits the paper, the gold immediately sucks it away, like a vacuum cleaner.

• Result: The holes were tiny or didn't form at all. The gold substrate acted like a safety valve, draining the energy before it could melt a big hole.

The "Layer Cake" Experiment

They also stacked the paper.

• One layer: Big holes.
• Two layers: Smaller holes.
• Three layers: The ions often couldn't punch all the way through.
• Why? It's like trying to burn a hole through a single sheet of tissue paper vs. a stack of three. The extra layers give the energy more places to spread out, so it doesn't concentrate enough to punch a clean hole through the whole stack.

The "Traffic Jam" Analogy

The scientists explain this using electron traffic.

• When an ion hits the paper, it creates a traffic jam of electrons (energy).
• On the Concrete (SiO₂): The traffic gets stuck. The electrons can't run away easily, so they pile up, get hot, and melt a big hole.
• On the Gold: The traffic has an express lane. The electrons zoom off into the gold substrate immediately, cooling down the spot before a hole can form.

Why Does This Matter?

This study is like a rulebook for engineers. If you want to make tiny holes in 2D materials for new technology, you can't just look at the material itself. You have to look at what it's sitting on.

• If you want big holes, put the material on an insulator (like glass).
• If you want to protect the material from damage, put it on a metal (like gold).

In a nutshell: The substrate isn't just a table; it's an active participant that decides whether the energy from the ion beam creates a giant crater or fizzles out completely.

Technical Summary

1. Problem Statement

Ion irradiation is a powerful tool for nanostructuring two-dimensional (2D) materials, enabling defect engineering from doping to nanopore creation. While the interaction of ions with bulk materials and suspended 2D membranes is relatively well-understood, the behavior of substrate-supported 2D materials remains poorly characterized.

• The Gap: Practical applications require 2D materials to be supported by substrates, yet direct atomic-resolution investigation of ion-induced defects in supported systems is experimentally difficult. Most existing studies rely on indirect measurements (Raman, electrical) or focus on suspended samples.
• The Core Question: How does the substrate influence the energy dissipation pathways and subsequent defect formation (specifically nanopore creation) in 2D materials when irradiated by ions that primarily interact with the electronic system?

2. Methodology

The study employed a comparative approach using Molybdenum Disulfide (MoS₂) as the test material, irradiated by two distinct types of ions that couple primarily to the electronic system but differ in energy deposition profiles:

• Highly Charged Ions (HCIs): Xe ions (180–260 keV kinetic energy, charge states 28+–44+). These deposit potential energy via ultrafast charge exchange and Auger processes near the surface.
• Swift Heavy Ions (SHIs): Xe (0.7 MeV/u) and Au (4.8 MeV/u) ions. These deposit energy continuously along their trajectory via electronic stopping ($S_e$).

Experimental Configurations:
The authors analyzed MoS₂ in four distinct configurations to isolate substrate and thickness effects:

1. Suspended Monolayer: (Reference data from previous work).
2. Monolayer on SiO₂: An insulating substrate.
3. Multilayer MoS₂ (2L and 3L): To test out-of-plane dissipation within the material itself.
4. Monolayer on Gold (Au): A metallic substrate with strong interfacial coupling.

Characterization:

• Scanning Transmission Electron Microscopy (STEM): Used to directly image and quantify pore radii and formation efficiency. Samples were prepared via CVD growth (on SiO₂) or mechanical exfoliation (on Au), followed by transfer to TEM grids.
• Raman and PL Spectroscopy: Used to characterize the MoS₂/Au interface properties (e.g., PL quenching, mode splitting) to confirm strong coupling.

3. Key Contributions

• Direct Quantification of Substrate Effects: The study provides the first direct, atomic-resolution quantification of how different substrates (insulating vs. metallic) and layer thicknesses alter pore formation efficiency and size under ion irradiation.
• Decoupling Energy Deposition from Dissipation: By comparing HCIs (surface-localized energy) and SHIs (track-localized energy), the authors demonstrate that while the deposition mechanism differs, the dissipation pathway (governed by the substrate) dictates the final defect morphology.
• Identification of Dissipation Channels: The work establishes that pore formation is not solely a function of energy input but is critically controlled by how efficiently electronic excitation is redistributed laterally (in-plane) and vertically (out-of-plane) into the substrate.

4. Key Results

A. Substrate Influence (SiO₂ vs. Suspended vs. Au)

• SiO₂ (Insulator): Pores are largest and most frequent on SiO₂ compared to suspended MoS₂.
  • Mechanism: The insulating substrate limits out-of-plane dissipation. Substrate-induced disorder and charge trapping increase carrier scattering, reducing lateral charge mobility. This confines the electronic excitation, increasing local energy density and facilitating larger pore formation.
• Au (Metal): Pore formation is massively quenched on Gold.
  • Efficiency: The pore formation efficiency drops significantly ($\eta \approx 0.28$) compared to SiO₂ ($\eta \approx 0.83$).
  • Mechanism: The strong MoS₂–Au interfacial coupling (confirmed by PL quenching and Raman splitting) acts as an efficient sink for transient electronic excitation. Energy is rapidly dissipated into the metal, preventing the local energy density from reaching the threshold required for lattice damage.
  • Observation: In trilayer MoS₂ on Au, only the top layer is penetrated, while the bottom layers (closer to the Au) remain intact, further proving the substrate acts as a dissipation sink.

B. Thickness Dependence

• Multilayer Effects: Increasing the number of MoS₂ layers reduces pore size and formation efficiency.
  • In bilayers, pores are smaller and less frequent.
  • In trilayers, fully penetrating pores are suppressed; only non-penetrating (surface) defects occur.
  • Mechanism: Additional layers provide an "out-of-plane" reservoir for charge redistribution, mimicking the effect of a substrate but within the material itself.

C. Ion Type Comparison (HCI vs. SHI)

• Scaling: Both ion types show similar trends regarding substrate dependence, confirming that electronic dissipation is the governing factor regardless of whether energy is deposited via potential energy (HCI) or electronic stopping (SHI).
• Efficiency Differences: SHIs generally show higher pore formation efficiencies ($\eta \sim 0.6-0.9$) than HCIs ($\eta \sim 0.4$) for the same configuration.
• Thickness Sensitivity: HCI-induced pore formation attenuates rapidly with thickness (surface-localized), whereas SHI effects are less sensitive to thickness due to the extended energy deposition track.

5. Significance and Conclusion

This study fundamentally shifts the understanding of ion-matter interactions in 2D materials:

1. Mechanism: For ions interacting primarily with the electronic system, the evolution of defects is governed less by the initial energy deposition mechanism and more by the efficiency of electronic energy dissipation at the interface.
2. Design Rules: The findings provide a roadmap for controlling defect engineering in 2D materials:
   • To maximize pore formation (e.g., for filtration membranes), use insulating substrates or suspended membranes.
   • To suppress damage (e.g., for radiation-hardened electronics), use metallic substrates with strong interfacial coupling.
3. Modeling Benchmark: The quantitative data on pore radii and formation efficiencies across different substrates and thicknesses serves as a critical benchmark for future atomistic simulations, specifically highlighting the need to model electronic energy dissipation across the 2D-substrate interface rather than treating the 2D material in isolation.

In summary, the paper demonstrates that the substrate is not a passive support but an active participant in determining the structural integrity of 2D materials under ion irradiation.