MOLECULAR GRADIENTS OF OMEGA-SUBSTITUTED ALKANETHIOLS ON GOLD STUDIEDBY 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 a ngle measurements, infrared spectroscopy, and X-ray photoelectron spec troscopy (XPS) were used to monitor the coverage, chain conformation a nd gradient profiles. The XPS analysis were performed with improved la teral resolution as compared to our earlier infrared study (Langmuir 1 995, 11, 3821) by imaging a 160 mu m wide segment of the gradient surf ace 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 representa tive 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 corres ponding profiles obtained by scanning infrared spectroscopy. The cover ages (alkyl chain densities) were found to be constant for the gradien ts 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 detec ted by ellipsometry and infrared spectroscopy and supports a model whe re the gradient assemblies approach the limiting structure of densely packed all trans polymethylenes.