Vapor Deposition of Aluminum on Thiophene-Terminated Self-Assembled Monolayers

Abstract

Thermal deposition of aluminum in ultrahigh vacuum on a monolayer of 12-(3-thienyl)dodecanethiol, a thiophene-terminated alkanethiol, self-assembled on gold has been investigated by X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) and compared to aluminum deposition on a monolayer of 1-tridecanethiol. Intensity variations of the C 1s and Au 4f peaks as a function of aluminum coverage demonstrate that thermally deposited aluminum forms an overlayer on top of the thiophene-terminated self-assembled monolayer (SAM). Strong interaction of interfacial aluminum with the thiophene functional group is evidenced by the appearance of a metal-induced low binding energy component in the C 1s spectrum. UPS measurements also demonstrate that aluminum covers the thiophene-terminated SAMs, as evidenced by the complete absence of the spectral characteristics of thiophene by an Al coverage of 4.4 x 1015 atoms/cm2. Work function measurements suggest that initially deposited aluminum grows as electrically isolated islands until this coverage is exceeded. This behavior is dramatically different from that of 1-tridecanethiol on which initially deposited aluminum interacts weakly with the alkanethiol and diffuses beneath the monolayer to the gold surface. The lack of penetration in the case of the thiophene-terminated SAM probably results from lower mobility of aluminum atoms on the organic surface, compared to methyl-terminated SAMs, and their inability to diffuse to defect sites and penetrate through the monolayer.