Interaction of Vapor-Deposited Aluminum with Thiophene-Terminated Self-Assembled Monolayers on Gold

Abstract

A variety of experiments have been performed in recent years to investigate conjugated polymer/metal interfaces, and understanding their nature is critical in the area of optoelectronic devices, including field-effect transistors, organic light emitting diodes (OLEDs), and photovoltaics. However, organic films, especially polymers, may have poor surface homogeneity, and this lack of structural uniformity may induce difficulties in studying the interfaces. The need for highly ordered, well-oriented functional groups exposed at the surface of organic films has inspired the use of self-assembled monolayers (SAMs) in the study of metal/organic interfaces. Thiophene and its derivatives comprise an important class of conjugated polymers, with potential optoelectronic device applications. Recently, we synthesized thiophene-terminated alkanethiols, Th-(CH2)n-SH (Th = 3-thiophene) with n = 2, 6, and 12, and self-assembled them onto Au(111) surfaces. It was found that 12-(3-thienyl)dodecanethiol, (Th-(CH2)12-SH), forms densely packed, well-ordered monolayers on gold with the thiophene rings at the periphery of the SAM. The chemical structure of this molecule is shown in Figure 1. In the present study, 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. XPS measurements performed at a 10° 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 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.