Molecular Mechanisms of Electron-Induced Cross-Linking in Aromatic SAMs

Abstract

When aromatic self-assembled monolayers (SAMs) are electron-irradiated, intermolecular cross-links are formed and the SAMs transform into carbon nanosheets with molecular thickness. These nanosheets have a very high mechanical stability and can withstand temperatures above 1000 K. In this report, we investigate the electron induced cross-linking of 1,1'-biphenyl-4-thiol (BPT) SAMs on gold by combining X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (NEXAFS), thermal desorption spectroscopy (TDS), and UV photoelectron spectroscopy (UPS). The experimental data were acquired as a function of electron dose and temperature and compared with quantum chemical calculations. Details of the intermolecular cross-linking, the microstructure of cross-linked films, and their structural transformations upon heating were obtained to derive a view of the mechanisms involved. Our analysis shows that room-temperature electron irradiation causes a lateral cross-linking via the formation of C-C linked phenyl species as well as a new sulfur species. The thermal stability of the BPT films increases with the electron dose and saturates at approximately 50 mC/cm2. Nevertheless, nonlinked fragments in the thermal desorption spectra indicate an incomplete cross-linking even at high doses, which can be attributed to steric reasons and quenching due to the reduced band gap of partially linked molecules. At temperatures above 800 K, all sulfur species are thermally desorbed, while the remaining film reveals an onset of carbonization.

Self-assembled monolayers of thiols and dithiols on gold: new challenges for a well-known system

“…375 In this case intermolecular cross-links are formed, stabilizing the SAMs up to 800 K. As in the case of chemical stability, the search of new strategies to improve thermal stability is an important issue that needs further investigation.…”

Nanopore fabrication and characterization by helium ion microscopy

“…CNMs were made by electron irradiation of aromatic self-assembled monolayers leading to a two-dimensional cross-linking and the formation of mechanically free-standing membranes. [15][16][17][18][19] Two approaches were made to mill nanopores: (i) keeping the beam at a constant position and unblanking it for a desired exposure time and (ii) exposing the sample with a predefined pattern. Fig.…”

Fluorescently labeled 1 nm thin nanomembranes

“…They were then exposed to low-energy electrons that cleave C-H bonds and cross-link neighboring molecules in the NBPT SAM into a mechanically stable nanosheet (Turchanin et al, 2009b). The hydrogen released in this process reduces the terminal nitrogroups to amino-groups (Fig.…”