Abstract
The irradiation-induced cross-linking of aromatic self-assembled monolayers (SAMs) is a universal method for the fabrication of ultrathin carbon nanomembranes (CNMs). Here we demonstrate the cross-linking of aromatic SAMs due to exposure to helium ions. The distinction of cross-linked from non-cross-linked regions in the SAM was facilitated by transferring the irradiated SAM to a new substrate, which allowed for an ex situ observation of the cross-linking process by helium ion microscopy (HIM). In this way, three growth regimes of cross-linked areas were identified: formation of nuclei, one-dimensional (1D) and two-dimensional (2D) growth. The evaluation of the corresponding HIM images revealed the dose-dependent coverage, i.e., the relative monolayer area, whose density of cross-links surpassed a certain threshold value, as a function of the exposure dose. A complete cross-linking of aromatic SAMs by He+ ion irradiation requires an exposure dose of about 850 µC/cm2, which is roughly 60 times smaller than the corresponding electron irradiation dose. Most likely, this is due to the energy distribution of secondary electrons shifted to lower energies, which results in a more efficient dissociative electron attachment (DEA) process.
Highlights
Carbon nanomembranes (CNMs) with monomolecular thickness and macroscopic lateral size represent a new type of functional two-dimensional (2D) materials [1]
A universal scheme to fabricate CNMs is the irradiation-induced cross-linking of aromatic self-assembled monolayers (SAMs), which allows for creating a variety of functional nanomembranes by using different molecular precursors as building blocks [2]
The SAM that consists of closely packed NBPT molecules is formed on a gold substrate; secondly, the SAM is irradiated locally with He+ ions; thirdly, the transfer is assisted by a layer of poly(methyl methacrylate) (PMMA) for mechanical stabilization, which allows the dissolution of underlying Au layer; lastly, the PMMA layer is dissolved and only the cross-linked SAM is transferred onto another substrate, e.g., SiO2/Si
Summary
Carbon nanomembranes (CNMs) with monomolecular thickness and macroscopic lateral size represent a new type of functional two-dimensional (2D) materials [1]. We used 4'-nitro-1,1'-biphenyl-4-thiol (NBPT) as a molecular precursor to form SAMs on a Au substrate and employed HIM both as a nanofabrication tool to cross-link SAMs and as an imaging tool for the ex situ observation of the crosslinking process.
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