Competitive self-assembly of symmetrical, difunctional molecules on ambient copper surfaces

This paper describes competitive self-assembly from solutions of symmetric alpha,omega-difunctional molecules on Cu substrates briefly exposed (less than 5 min) to ambient conditions. XPS and PM-IRRAS were utilized as complimentary surface analytical techniques to characterize the resulting organized organic thin films (OOTFs) on these "ambient" Cu surfaces. The order of preferential adsorption was observed to be diisocyanide approximately = dithiol > dicarboxylic acid > dinitrile > diisothiocyanate, indicating that the isocyanide (-NC), and thiol (-SH) functions provide the strongest adhesion to ambient Cu. 1,4-Phenylene diisocyanide and 1,4-terephthalic acid were both observed to adopt a standing-up phase configuration, in which the difunctional molecules bond to the base substrate through only one terminal functional group, with the other terminal group disposed away from the substrate. This indicates the ability to utilize OOTFs to produce "sticky surfaces" on ambient Cu. All other molecules bonded to the substrate through both terminal groups, in either surface-parallel or arched "hairpin" configurations. On the basis of these findings, aromatic diisocyanides and diacids are the most suitable molecules for creating OOTFs with high packing density. Such films can be utilized as protective coatings in the assembly of printed circuit boards, where Cu is becoming an increasingly important substrate for interconnects. Moreover, the ability to create chemically sticky surfaces on ambient Cu substrates indicates exciting potential for the development of a new surface-mount technology operative at the nanometer scale.