Metal overlayers on organic functional groups of self-assembled monolayers. VI. X-ray photoelectron spectroscopy of Cr/COOH on 16-mercaptohexadecanoic acid

Abstract

The interaction of Cr with the COOH groups of a self-assembled monolayer (SAM) of 16-mercaptohexadecanoic acid [HS(CH2)15COOH] on gold was studied by x-ray photoelectron spectroscopy (XPS). For each Cr overlayer thickness, a freshly prepared SAM was exposed to the evaporative deposition of Cr at ≤1×10−8 Torr and then transferred immediately in situ to a surface analysis chamber held at ≤5×10−10 Torr. Metallic Cr is deposited as evidenced by a Cr 2p3/2–2p1/2 splitting of 9.2 eV for 1 nm thick Cr films on Au. The C 1s and O 1s binding energies and intensities for the initial COOH-terminated SAM are consistent with the presence of the O=C—OH end group at the surface. For Cr overlayers of 0.04–1 nm average thickness, we identify Cr–COOH interactions primarily by changes in the O 1s level. For 0.04 nm Cr/COOH, the O 1s peak is narrowed and fitted by a single component at 532.9 eV that is between the O—C (533.9 eV) and O=C (532.5 eV) components of the bare SAM. The narrow width of this peak is evidence that the two oxygens of a single COOH end group interacting with Cr are indistinguishable by our instrument. For 0.07–1 nm average overlayer thickness, a low binding energy component (531.0–530.3 eV), assigned to a Cr(III) oxide, is of gradually increasing intensity. For coverages below 0.6 nm, analysis of XPS peak intensities indicates that adsorption of an adventitious oxygen-containing species has little effect on the results.