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Abstract: Recent developments of helium ion microscopy (HIM) at AIST are presented taking examples of patterning graphene and high-resolution imaging of cells.
HIM was used for direct nano-patterning or milling of single-layer graphene (SLG). Doses of 2E16/cm2 from a 30kV beam induced a metal-insulator transition in the SLG on SiO2. Analysis of scanning capacitance microscopy measurements revealed that the spatial resolution of HIM patterning depended on the dosage of helium ions in a non-monotonic fashion. Increasing the dose from 2E16 to 5E16/cm2 improved the spatial resolution to several tens of nanometers. However, doses larger than 1E17/cm2 degraded the patterning characteristics.(1)
Graphene nano-mesh (GNM) patterning techniques suffer from either large dimensions or a lack of structural control. Utilizing HIM, reliable GNM patterning with a sub-10 nm pitch and an < 4 nm pore diameter by the direct helium ion beam milling of suspended monolayer graphene. Electrical transport measurements reveal an effective energy gap opening of up to ∼450 meV.(2) Direct patterning using HIM is a versatile approach to graphene fabrication and can be applied to 2D materials-based devices (3) and opens a path toward GNM-based, room-temperature semiconducting applications.
In order to explore the applications of HIM secondary electron (SE) imaging in biology, COS7 kidney fibroblast cells were dried and directly observed without any coating or staining. High contrast, high depth-of-field images were obtained revealing the nucleus, endoplasmic reticulum, cytoskeleton and putative mitochondria above a bright background from the support. Gold-tagged antibodies were employed to aid organelle identification. Signals from the gold tags were most clearly distinguishable by HIM, and the minimum gap measured between gold particles showed the resolution to be 2 nm. SE-HIM also delivered high contrast images of unstained, uncoated, thin sections of Epon‑embedded mouse kidney tissues, revealing the details of sub-tissues and cell organelles (4). Observation results of organic materials in electronics applications will be also presented in association with.
(1) Y. Naitou and S. Ogawa, “Direct nano-patterning of graphene with helium ion beams”, Appl. Phys. Lett. 106 033103 (2015)
(2) M. E. Schmidt, T. Iwasaki, M. Muruganathan, M. Haque, M. V. Ngoc, S. Ogawa, and H. Mizuta, “Structurally Controlled Large-Area 10 nm Pitch Graphene Nanomesh by Focused Helium Ion Beam Milling”, ACS Appl. Matter.Interfaces, 10, 10362 (2018)
(3) S. Nakaharai, T. Iijima, S. Ogawa, S. Suzuki, S.-L. Li, Tsukagoshi, S. Sato, and N. Yokoyama, “Conduction Tuning of Graphene Tailored by Accelerated Helium Ion Beam”, ACS Nano 7, 5694-5700 (2013)
(4) C. Sato, M. Sato, and S. Ogawa, “High Resolution Visualization of Immunogold Labelled Cells and Tissues by Helium Ion Scanning Microscopy”, International Journal of Molecular Medicine 42, 309 (2018)
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