Interface dipoles arising from self-assembled monolayers on gold: UV-photoemission studies of alkanethiols and partially fluorinated alkanethiols

Gold surfaces modified with C-3-C-18-alkanethiols (CH3(CH2)(X-1)SH; HXSH; x = 3, 8, 12, 16, 18) and C-16-alkanethiols, fluorinated at the outer 1, 2, 4, and 10 methylene positions (CF3(CF2)gamma(-1)(CH2)(X)SH; FyHxSH; y = 1, x = 15; y = 2, x = 14; y = 4, x = 12; y = 10, x = 6) were characterized by He(l) UV-photoelectron spectroscopy (UPS). (Detailed X-ray photoelectron spectroscopic characterization of the partially fluorinated thin films is given in the Supporting Information). Long incubation times of the gold surface with the alkanethiol solutions lead to compact monolayer films for all of the alkanethiols, as indicated by the exponential decrease in emission intensity versus alkyl chain length for both the gold Fermi edge (UPS data), and by a parallel decrease in Au(4f) photoemission intensity using X-ray photoelectron spectroscopy. Changes in the effective work function of these surfaces due to the presence of significant interfacial dipoles are observed (i) as alkyl chain length is increased, and (ii) as the fraction of fluorinated methylene groups is increased in a constant length alkyl chain. Negative shifts of the low kinetic energy photoemission edge with increasing alkyl chain length in the HXSH series are consistent with the presence of a large positive interface dipole. The largest part of this shift (ca. 1.0 eV) appears between the C-3- and C-8-alkyl chain lengths. Adding -CFx groups to the outer end of the C-16-alkyl chain positively shifts the low-kinetic-energy photoemission edge, consistent with the presence of a large negative interface dipole that completely compensates for the positive dipole from the alkyl portion of the chain. Examining C-13-C-16 alkyl chains fluorinated at only the outer methyl group shows that this negative dipole depends on the orientation of the -CF3 group (i.e., "odd-even" effects in the effective work function are observed). Comparison of the shifts in gold/SAM vacuum level (changes in effective work function) as a function of the apparent dipole moment of the molecule provides an estimate of the band-edge offsets for these molecules on the gold surface, an estimate of the intrinsic shift in a vacuum level at zero dipole moment of the adsorbate, and an estimate of the intrinsic dipole moment for the gold-thiolate bond.