氦离子显微镜中悬浮MoS₂膜的厚度依赖二次电子发射
Thickness-dependent secondary-electron emission from suspended MoS$_2$ membranes in the helium ion microscope
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中文总结 AI 辅助
研究发现氦离子显微镜中悬浮MoS₂膜的二次电子发射强度随厚度变化，在40-55 nm达峰值，通过非对称SE逃逸模型复现结果，可用于快速筛选膜厚及测试离子-固体相互作用模型。
中文摘要 AI 辅助
氦离子显微镜（HIM）中的二次电子（SE）发射，当样品足够薄以实现氦离子透射且收集从底面发射的SE时，会对膜厚度变得敏感。我们将带孔碳上悬浮的纳米级厚度MoS₂薄片的总SE强度，与通过电子能量损失谱独立测量的厚度进行关联，其响应在40-55 nm处达到峰值，表观前后SE信号比达4.7。这种呈峰值的厚度依赖分量主要归因于薄片底面的SE发射，而非透射离子撞击仪器表面。应用SRIM电离剖面，具有出射侧更长逃逸深度的非对称SE逃逸模型可复现该响应。我们发现出射侧有效逃逸深度约为10 nm，是假设的入射侧2 nm的五倍，表明沉积能量到达出射表面的效率远高于入射表面，或SRIM模型的能量沉积剖面因沟道效应等发生偏移。该关联为悬浮膜提供了快速厚度筛选方法，也为测试薄材料中的低能离子-固体相互作用模型提供了途径。
英文摘要
Secondary-electron (SE) emission in the helium ion microscope (HIM) becomes sensitive to membrane thickness when the sample is thin enough for He-ion transmission and when the SEs emitted from the bottom surface are collected. We correlated the total SE intensity of suspended, nanometer-thick MoS$_2$ flakes on lacey carbon with thickness measured independently by electron energy-loss spectroscopy. The response peaks at 40-55 nm, with an apparent back-to-front SE signal ratio reaching 4.7. The peaked, thickness-dependent component is attributed primarily to SE emission at the bottom surface of the flake, rather than to transmitted ions striking instrument surfaces. Applying SRIM ionization profiles, an asymmetric SE-escape model with a longer escape depth on the exit side reproduces the response. We find an effective exit-side escape depth of approximately 10 nm, five times the assumed 2 nm entrance value, suggesting that deposited energy reaches the exit surface far more efficiently than the entrance surface or that the SRIM model's energy deposition profile is shifted by an effect such as channeling. The correlation provides a rapid thickness screen for suspended membranes and a route to testing low-energy ion-solid interaction models in thin materials.
