Vapor-deposition of aluminum on thiophene-terminated self-assembled monolayers on gold

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 An 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 Is spectrum. XPS measurements performed at a 10degrees takeoff angle, where surface sensitivity is enhanced, show a metal-induced shift in the thiophene S 2p peak of 1.6 eV to lower binding energy. These are indicative of electron transfer from aluminum to the thiophene rings. Initially deposited aluminum exhibits a nonmetallic feature in the Al 2s spectrum at a binding energy of 119.4 eV, but further aluminum deposition leads to metallic overlayers on top of the thiophene-terminated SAM. 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 10(15) atoms/cm(2). 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.