Chemical effects of methyl and methyl ester groups on the nucleation and growth of vapor-deposited aluminum films

The interaction of vapor-deposited A1 atoms with self-assembled monolayers (SAMs) of HS(CH₂)₁₅CH₃ and HS(CH₂)₁₅CO₂CH₃ chemisorbed at Au{111} surfaces was studied using X-ray photoelectron spectroscopy, infrared spectroscopy, time-of-flight secondary ion mass spectrometry, and spectroscopic ellipsometry. For the CH₃-terminated SAM, no reaction with C-H or C-C bonds was observed. For total A1 doses up to κ 12 atoms/nm², penetration the the Au-S interface occurs with no disruption of the average chain conformation and tilt, indicating formation of a highly uniform κ1:1 A1 adlayer on the Au. Subsequently, penetration ceases and a metallic overlayer begins to form at the SAMvacuum interface. These results are explained in terms of an initial dynamic hopping of the S headgroups on the Au lattice, which opens transient diffusion channels to the Au-S interface, and the closing of these channels upon completion of the adlayer. In contrast, A1 atom interactions with the CO₂CH₃-terminated SAM are restricted to the vacuum interface, where in the initial stages discrete organometallic products form via reaction with the CO₂CH₃ group. First, a 1:1 complex forms with a reduced C==O bond and an intact CH₃ moiety. Further exposure leads to the additional reaction of about four A1 atoms per ester, after which a metallic overlayer nucleates in the form of clusters. After the growth progresses to κ30 Å, the clusters coalesce into a uniform metallic film. These results illustrate the extraordinary degree of control that organic substrates can exert during the course of metal film formation.