Effect of irradiation dose in making an insulator from a self-assembled monolayer.

A combination of functionalization and irradiation-induced cross-linking allows fabrication of stable metal film on top of an aromatic self-assembled monolayer, [1,1';4',1' '-terphenyl]-4,4' '-dimethanethiol (TPDMT) on Au. Using X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and ion-scattering spectroscopy the optimal irradiation dose for producing a stable metal overlayer was estimated to be 40-45 mC/cm2. This dose is necessary for complete 2D-polymerization and closure of transient channels, which would otherwise allow metal penetration into the SAM. What is also important, the majority of the thiol tail groups, responsible for 2D growth and chemical adherence of the metal film, remains intact even at this high dose. The optimal dose corresponds to a crossover in the response of the TPDMT film to ionizing radiation: the irradiation-induced processes progress fast at lower doses and saturate at higher doses.