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C. Neumann, R. Wilhelm, M. Küllmer, A. Turchanin:
"Low-energy electron irradiation induced synthesis of molecular nanosheets: An influence of the electron beam energy";
Faraday Discussions, 227 (2021), 61 - 79.

Aromatic self-assembled monolayers (SAMs) can be cross-linked into molecular nanosheets  carbon nanomembranes (CNMs)  via low-energy electron irradiation. Due to their favorable mechanical stability and tunable functional properties, they possess a high potential for various applications including nanosensors, separation membrane for osmosis or energy conversion devices. Despite this potential, the mechanistic details of the electron irradiation induced cross-linking process still need to be understood in more detail. Here we studied the cross-linking of 4'-nitro-1,1´-biphenyl-4-thiol SAM on gold. The SAM samples were irradiated with different electron energies ranging from 2.5 to 100 eV in ultra-high vacuum and subsequently analysed by complementary techniques. We present results obtained via spectroscopy and microscopy characterization by high-resolution X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction with micrometre sized electron beams (LEED) and low-energy electron microscopy (LEEM). To demostrate the formation of CNMs, the formed two-dimensional molecular materials were transferred onto grids and oxidized wafer and analyzed by optical, scanning electron (SEM) and atomic force microscopy (AFM). We found a strong energy dependence for the cross section for the cross-linking process, which rates decrease exponentially towards lower electron energies by about four orders of magnitude. We conduct a comparative analysis of the cross sections for the C-H bond scission via electron impact ionization and dissociative electron attachment and find out that these different ionization mechnisms are responsible for the variation of the cross-linking cross section with electron energy.

http://dx.doi.org/10.1039/C9FD00119K

https://pubs.rsc.org/en/content/articlepdf/2020/fd/c9fd00119k