A. Hooper et al., Chemical effects of methyl and methyl ester groups on the nucleation and growth of vapor-deposited aluminum films, J AM CHEM S, 121(35), 1999, pp. 8052-8064
The interaction of vapor-deposited Al atoms with self-assembled monolayers
(SAMs) of HS(CH2)(15)CH3 and HS(CH2)(15)CO2CH3 chemisorbed at Au{111} surfa
ces was studied using X-ray photoelectron spectroscopy, infrared spectrosco
py, time-of-flight secondary ion mass spectrometry, and spectroscopic ellip
sometry. For the CH3-terminated SAM, no reaction with C-H or C-C bonds was
observed. For total Al doses up to similar to 12 atoms/nm(2), penetration t
o the Au-S interface occurs with no disruption of the average chain conform
ation and tilt, indicating formation of a highly uniform similar to 1:1 Al
adlayer on the Au. Subsequently, penetration ceases and a metallic overlaye
r begins to form at the SAM-vacuum interface. These results are explained i
n terms of an initial dynamic hopping of the -S headgroups on the An lattic
e, which opens transient diffusion channels to the Au-S interface, and the
closing of these channels upon completion of the adlayer. In contrast, Al a
tom interactions with the CO2CH3-terminated SAM are restricted to the vacuu
m interface, where in the initial stages discrete organometallic products f
orm via reaction with the CO2CH3 group. First, a I:1 complex forms with a r
educed C=O bond and an intact CH3 moiety. Further exposure leads to the add
itional reaction of about four Al atoms per ester, after which a metallic o
verlayer nucleates in the form of clusters. After the growth progresses to
similar to 30 Angstrom, 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.