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

Y. Liebsch, U. Javed, L. Skopinski, L. Daniel, F. Appel, R. Rahali, C. Grygiel, H. Lebius, C. Frank, L. Breuer, L. Kirsch, F. Koch, J. Kotakoski, M. Schleberger

Abstract

Ion irradiation is a versatile tool for nanostructuring surfaces, yet the roles of energy deposition and dissipation at the surface and in ultrathin materials remain poorly understood. In this study, we investigate nanopore formation in monolayer MoS$_2$ on different substrates under irradiation of highly charged ions (HCIs) and swift heavy ions (SHIs): two types of ions that, despite having vastly different kinetic energies, interact primarily with the electronic system of the target. Using scanning transmission electron microscopy, we quantify pore radii and pore formation efficiencies for suspended MoS$_2$, MoS$_2$ on SiO$_2$, bilayer MoS$_2$ and MoS$_2$ on gold. Both pore size and pore formation efficiency exhibit a pronounced dependence on the type of substrate. Pores are largest and most frequent in MoS$_2$ on SiO$_2$, while the gold substrate massively quenches pore formation. The results indicate that the observed pore dimensions under both HCI and SHI irradiation are consistent with a central role of substrate and interface-dependent electronic dissipation pathways.

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

Abstract

Ion irradiation is a versatile tool for nanostructuring surfaces, yet the roles of energy deposition and dissipation at the surface and in ultrathin materials remain poorly understood. In this study, we investigate nanopore formation in monolayer MoS on different substrates under irradiation of highly charged ions (HCIs) and swift heavy ions (SHIs): two types of ions that, despite having vastly different kinetic energies, interact primarily with the electronic system of the target. Using scanning transmission electron microscopy, we quantify pore radii and pore formation efficiencies for suspended MoS, MoS on SiO, bilayer MoS and MoS on gold. Both pore size and pore formation efficiency exhibit a pronounced dependence on the type of substrate. Pores are largest and most frequent in MoS on SiO, while the gold substrate massively quenches pore formation. The results indicate that the observed pore dimensions under both HCI and SHI irradiation are consistent with a central role of substrate and interface-dependent electronic dissipation pathways.

Paper Structure

This paper contains 10 sections, 6 figures.

Table of Contents

  1. Introduction
  2. Results and discussion
  3. Highly Charged Ions
  4. Swift Heavy Ions
  5. Conclusion
  6. Methods and Materials
  7. Sample Preparation
  8. Ion irradiation
  9. Imaging
  10. Raman and PL spectroscopy

Figures (6)

  • Figure 1: (a-d) False-color STEM images of pores in single-layer MoS$_2$ created by Xe ions with different charge states at 180 keV. Orange structures are caused by hydrocarbon contamination. (e) Mean pore radii of SL-MoS$_2$ on SiO$_2$/Si substrate irradiated with highly charged Xe ions at different kinetic energies (green, red, black). Pore radii are larger when MoS$_2$ is irradiated on SiO$_2$ compared to the suspended configuration (blue, data by Kozubek et al.) Kozubek.2019.
  • Figure 2: (a) STEM images of MoS$_2$ on SiO$_2$ with different layer numbers, irradiated with 180 keV Xe$^{37+}$ ions. (b) Fewer and smaller pores are observed in bilayer MoS$_2$ (2L) compared to monolayer on SiO$_2$ (1L). (c) In trilayer MoS$_2$, pores are predominately non-penetrating.
  • Figure 3: Comparison of SHI-irradiated (a) and HCI-irradiated (b) MoS$_2$ reveals similar pore dimensions in 1L, but, as expected, vastly different penetration behavior. Schematics below each image illustrate energy deposition profiles of the different irradiation types. Note that these illustrations are visual representations and not to scale.
  • Figure 4: Mean pore radii (a) and pore formation efficiencies (b) of HCI and SHI-irradiated MoS$_2$ in different configurations. Error bars of mean pore radii are the standard deviation, while uncertainty in efficiency arises from pore counting and fluence determination in all cases, and for HCI-irradiated supported samples additionally from partial-area irradiation, transfer-related misalignment (see Fig.S3).
  • Figure 5: Comparison of SL-MoS$_2$ irradiated on SiO$_2$ (a) and on gold substrate (b). On gold, pore formation efficiency is strongly reduced compared to MoS$_2$ on SiO$_2$. In addition, the expected increase in pore dimensions due to the difference in predicted stopping power between the two irradiations is not observed. (c) In trilayer MoS$_2$ on Au, only a single layer is penetrated, as evidenced by the Moiré pattern (caused by bilayer MoS$_2$) present on top of the pore. Bright, white spots in the STEM images are caused by dense agglomerates that can either stem from irradiation (Mo) or the transfer (Au). The lighter orange color of the trilayer image compared to monolayer originates from thickness-dependent scattering of electrons.
  • ...and 1 more figures