Gold surfaces modified with C 3 -C 18 -alkanethiols (CH 3 (CH 2 ) X-1 SH; H X SH; x ) 3, 8, 12, 16, 18) and C 16alkanethiols, fluorinated at the outer 1, 2, 4, and 10 methylene positions (CF 3 (CF 2 ) Y-1 (CH 2 ) X SH; F y H x SH; y ) 1, x ) 15; y ) 2, x ) 14; y ) 4, x ) 12; y ) 10, x ) 6) were characterized by He(I) 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 H x SH 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 -CF x 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 -CF 3 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.

Indium−tin oxide (ITO) electrodes have been modified with both fluorinated alkyl and aryl phosphonic acids [n-hexylphosphonic acid (HPA) and n-octadecylphosphonic acid (ODPA); 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl phosphonic acid (FHOPA), pentafluorobenzyl phosphonic acid (PFBPA), and tetrafluorobenzyl-1,4-diphosphonic acid (TFBdiPA)]. These are modifiers designed to control both wetting properties toward nonpolar molecular solids and to provide a wide range of tunability in effective surface work function. The molecular nature of surface attachment and changes in electronic and wetting properties were characterized by X-ray photoelectron spectroscopy (XPS), UV-photoelectron spectroscopy (UPS), photoelastic modulation infrared reflection−absorption spectroscopy (PM-IRRAS), and contact angle measurements using both water and hexadecane. Interface dipoles from the PA modifiers contribute to shifts in the low kinetic energy regions of UPS spectra (local vacuum level shifts, which translate into changes in effective surface work function). We show that for ITO surfaces modified with FHOPA, and to a lesser extent with PFBPA, the high work function obtained by oxygen plasma cleaning can be maintained after modification, while decreasing the polar component of surface energy. This approach to oxide surface modification is a strategy that may be beneficial for the modification of transparent conducting oxide surfaces in both organic light emitting diodes and in organic solar cells, where oxide/organic compatibility can affect device performance.