Abstract
Carbon nanomembranes (CNMs) prepared from aromatic self-assembled monolayers constitute a recently developed class of 2D materials. They are made by a combination of self-assembly, radiation-induced cross-linking and the detachment of the cross-linked SAM from its substrate. CNMs can be deposited on arbitrary substrates, including holey and perforated ones, as well as on metallic (transmission electron microscopy) grids. Therewith, freestanding membranes with a thickness of 1 nm and macroscopic lateral dimensions can be prepared. Although free-standing CNMs cannot be imaged by light microscopy, charged particle techniques can visualize them. However, CNMs are electrically insulating, which makes them sensitive to charging. We demonstrate that the helium ion microscope (HIM) is a good candidate for imaging freestanding CNMs due to its efficient charge compensation tool. Scanning with a beam of helium ions while recording the emitted secondary electrons generates the HIM images. The advantages of HIM are high resolution, high surface sensitivity and large depth of field. The effects of sample charging, imaging of multilayer CNMs as well as imaging artefacts are discussed.
Highlights
Carbon nanomembranes (CNMs) are extremely thin and homogeneous two-dimensional objects consisting of a monolayer of laterally cross-linked molecules
They are made by exposing a self-assembled monolayer (SAM) of aromatic molecules with electron [1] or soft X-ray irradiation [2], which results in the cross-linking of neighbouring molecules into a CNM of molecular thickness
It is important to note that the helium beam penetrates the CNM and strikes objects below the freestanding membrane, for example, the sample holder
Summary
Carbon nanomembranes (CNMs) are extremely thin and homogeneous two-dimensional objects consisting of a monolayer of laterally cross-linked molecules. These background features are visible in the HIM image as He+ ions impinge upon the sample holder behind the grid and eject secondary electrons that reach the SE detector without being blocked. Secondary electrons are emitted from the underlying copper grid and charges in the CNM are neutralised by the metallic support.
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