Photoemission spectra of self-assembled monolayers of para-phenylene-ethynylene thiols chemisorbed on gold have
been measured. Three compounds were studied: 4,4'-bis(phenylethynyl)benzenethiol, 4-(phenylethynyl)benzenethiol,
and benzenethiol. The monolayer spectra were interpreted with the aid of gas-phase photoemission spectra of 4,4'-
bis(phenylethynyl)benzenethiol and benzenethiol. The work function of the monolayer-covered surface and alignment
of the highest occupied π-state of the monolayer relative to the Fermi level of the substrate were determined from the
monolayer spectra. The work function of the monolayer-covered surface decreased relative to that of the bare substrate
by about an electronvolt. The shift is attributed to changes in the charge distribution at the interface associated with
chemisorption. No statistically significant trend in the work function shift was observed with respect to oligomer
length. This observation points to a near-constant interface dipole and the role of the gold-sulfur bond in determining
energy-level alignment.
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