Effect of X-ray Irradiation on the Chemical and Physical Properties of a Semifluorinated Self-Assembled Monolayer

Abstract

The effect of X-ray irradiation on the chemical and physical properties of a semifluorinated self-assembled monolayer (CF-SAM) derived from 1H,1H,2H,2H-perfluorodecanethiol (CF3(CF2)7(CH2)2SH) adsorbed on gold and copper substrates was studied using X-ray photoelectron spectroscopy. During the initial period of irradiation, the effects of electron-stimulated C-F, C-C, and S-X (X ) Au, Cu) bond breaking are responsible for changes in the chemical composition of the CF-SAM. Furthermore, the evolution of the C(1s) X-ray photoelectron spectral region indicates that C-F rather than C-C bond cleavage dominates desorption within the CF-SAM. Except for fluorine desorption, irradiation-induced changes to the chemical and structural properties of the CF-SAM were most pronounced during the initial stages of irradiation, prior to the development of a highly cross-linked carbonaceous overlayer. X-ray irradiation of CF-SAMs adsorbed on Au also resulted in the production of new irradiation-induced sulfur species. A comparison with experiments carried out on a n-hexanedecanethiol (CH3(CH2)15SH)-based SAM revealed that the concentration and distribution of these irradiation-induced sulfur species were both sensitive to the SAM's initial chemical composition. On Cu substrates, native CF-SAMs formed an effective barrier to oxygen diffusion although the film's permeability to oxygen increased for CF-SAMs pre-exposed to X-ray irradiation.

Modification of 1H,1H,2H,2H-Perfluorooctyltrichlorosilane Self-Assembled Monolayers by Atomic Hydrogen

“…A small CF peak is also observed, whose presence is attributed to the effects of low-energy electrons generated by X-ray irradiation during the XPS scan. 12,13 This assertion is supported by the effect that prolonged X-ray irradiation was observed to have on the C(1s) region of the PFOTS monolayer (see Figure 1). …”

“…The changes in both the C(1s) spectral envelope and integrated C(1s) area as a function of increasing X-ray irradiation time are analogous to the previously observed effects of X-ray and electron beam modifications of semifluorinated SAMs on metal substrates. 12,13,35 Thus, for comparatively short irradiation times (<60 min), a rapid decrease is observed in the C(1s) and F(1s) XPS areas (carbon data shown in Figure 5, fluorine data not shown). These chemical changes are consistent with the ejection of CF x and F -species due to electronstimulated C-C, C-F, and C-H bond cleavage processes within the PFOTS monolayer ( Figure 8).…”

Thermal stability of thiol and silane monolayers: A comparative study

“…This is especially the case for the high temperature measurements, which take place after a large cumulative dose, and for the fluorocarbon samples, which are known to be susceptible to X-ray induced desorption/ decomposition [33,34]. C-F bond cleavage results from secondary electrons, and this could in part be responsible for a fraction of the decreasing F signal.…”

Radical Reactions with Organic Thin Films: Chemical Interaction of Atomic Oxygen with an X-ray Modified Self-Assembled Monolayer

“…15 The S(2p) region consists of a single doublet at 162.8 eV associated with the 2p 3/2 and 2p 1/2 transitions of native thiolate species bound to the underlying Au substrate. 9,15,25 The Au(4f) region is composed of two peaks at 84 eV and 87.6 eV due to the 4f 7/2 and 4f 5/2 transitions of metallic Au.…”

“…Such an analysis reveals that the effective diffusion length (or penetration depth) of atomic O within the hydrocarbon film is ≈5.5 Å. Furthermore, the fact that Au 2 O 3 and RSO 3 species are not observed until after prolonged O/O 2 exposure indicates that although X-ray irradiation of SAMs induces carbon, sulfur, and hydrogen desorption 25,26 the X-ray-modified C 16 -SAM forms a 3-dimensionally dense overlayer, preventing O-atom penetration to the film/vacuum interface (Scheme 1). These results are consistent with those of George et al 9 who found that carbon oxidation and C-C bond cleavage occur prior to oxidation at the film/substrate interface.…”