Molecular gradients of omega-substituted alkanethiols on gold studied by X-ray photoelectron spectroscopy

Molecular gradients of omega-substituted alkanethiols HS(CH2)(15)X (X:CH3, CH2OH, CO2CH3, CH2C drop N) were prepared on sputtered gold films by using a novel cross-diffusion methodology. Ellipsometry, contact angle measurements, infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used to monitor the coverage, chain conformation and gradient profiles. The XPS analysis were performed with improved lateral resolution as compared to our earlier infrared study (Langmuir 1995, 11, 3821) by imaging a 160 mu m wide segment of the gradient surface on the slit of the XPS analyzer and by using a step size of 0.5 mm. The so-obtained gradient profiles are expected to be more representative of the true composition profiles and were for the three gradients investigated found to be 5-6 mm, about 2-3 mm shorter than the corresponding profiles obtained by scanning infrared spectroscopy. The coverages (alkyl chain densities) were found to be constant for the gradients prepared from thiols with relatively small tail groups OH, C drop N and CH3, whereas a minor reduction in coverage, approximate to 3-4%, was observed in a narrow range for the gradient containing the CO2CH3-terminated alkanethiol. This change is, however, too small to be detected by ellipsometry and infrared spectroscopy and supports a model where the gradient assemblies approach the limiting structure of densely packed all trans polymethylenes.