METAL OVERLAYERS ON ORGANIC FUNCTIONAL-GROUPS OF SELF-ORGANIZED MOLECULAR ASSEMBLIES .3. X-RAY PHOTOELECTRON-SPECTROSCOPY OF CR/CN ON 12-MERCAPTODODECANENITRILE AND OF CR/CH3 ON OCTADECANETHIOL AT SAMPLE TEMPERATURES FROM 173 TO 373-K

The dependence on substrate temperature of the formation of Cr(CN) complexes and Cr-carbides when Cr overlayers are deposited onto organized molecular assemblies (OMAs) has been investigated in situ by x-ray photoelectron spectroscopy (XPS). At Cr coverages of 6 angstrom or less, evidence for the formation of a Cr(CN) complex with the CN end groups of the 12-mercaptododecanenitrile OMA is found in negative binding energy shifts of the nitrile part of the C 1s line shape and of the N Is core level. The N Is negative binding energy shift increases from 0.1-1.2 eV for substrate temperatures from 173 to 373 K. At a Cr coverage of 6 angstrom, Cr-carbide was observed to form to a similar extent on the 12-mercaptododecanenitrile and octadecanethiol OMAs. The number of carbide C atoms per OMA chain increases from 0.7 to 3 for substrate temperatures from 173 to 373 K for 6 angstrom Cr on die 12-mercaptododecanenitrile OMA. Greater amounts of carbide result when the Cr is deposited onto a sample at a given temperature, T, than when the deposition at a lower temperature is followed by warming to T. We interpret the increased amount of carbide to result from a contribution of the incident Cr atom kinetic energy to the activation of the Cr-carbide reaction. Collateral effects of x-ray exposure and substrate temperatures greater than 295 K may have affected, in part, the observed binding energy shifts and peak heights, but do not compromise the general conclusions of our interpretation.