Quantitative Surface Coverage Measurements of Self-Assembled Monolayers by Nuclear Reaction Analysis of Carbon-12

We report surface coverage measurements by Rutherford backscattering spectrometry (RBS) of self-assembled monolayers (SAMs) of both alkyl thiols and oligophenylene thiols on Au-coated mica, Si, and pyrolytic graphite. The ¹²C atom concentration was probed at 4.266 MeV₂⁴He²⁺ primary beam energy, which enhances the ₂⁴He²⁺ scattering cross section by exciting ¹²C nuclear resonance states; this is a submode of RBS commonly referred to as nuclear reaction analysis (NRA). The surface coverage of ¹²C increased linearly with the number of ¹²C atoms in each SAM. The consistency of the ¹²C atom coverage values obtained by NRA was cross-checked by measuring the ³²S atom concentration by conventional RBS. From these data, we obtained an average coverage of 3.5 ± 0.2 molecules/nm² for both alkyl thiols and oligophenylene thiols on polycrystalline Au surfaces. The results show the utility of NRA for quantitative analysis of SAM coverage on Au.