title: Fermi level alignment in self-assembled molecular layers: the effect of coupling chemistry.
abstract: Photoelectron spectroscopy was used to explore changes in Fermi level alignment, within the pi-pi* gap, arising from modifications to the coupling chemistry of conjugated phenylene ethynylene oligomers to the Au surface. Self-assembled monolayers were formed employing either thiol (4,4'-ethynylphenyl-1-benzenethiol or OPE-T) or isocyanide (4,4'-ethynylphenyl-1-benzeneisocyanide or OPE-NC) coupling. The electronic density of states in the valence region of the two systems are nearly identical with the exception of a shift to higher binding energy by about 0.5 eV for OPE-NC. Corresponding shifts appear in C(1s) spectra and in the threshold near E(F). The lack of change in the optical absorption suggests that a rigid shift of the Fermi level within the pi-pi* gap is the major effect of modifying the coupling chemistry. Qualitative consideration of bonding in each case is used to suggest the influence of chemisorption-induced charge transfer as a potential explanation. Connections to other theoretical and experimental work on the effects of varying coupling chemistries are also discussed.
authors.affiliation: National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA. cdzang@nist.gov
authors.forename: C D
authors.initials: CD
authors.lastname: Zangmeister
authors.forename: S W
authors.initials: SW
authors.lastname: Robey
authors.forename: R D
authors.initials: RD
authors.lastname: van Zee
authors.forename: J G
authors.initials: JG
authors.lastname: Kushmerick
authors.forename: J
authors.initials: J
authors.lastname: Naciri
authors.forename: Y
authors.initials: Y
authors.lastname: Yao
authors.forename: J M
authors.initials: JM
authors.lastname: Tour
authors.forename: B
authors.initials: B
authors.lastname: Varughese
authors.forename: B
authors.initials: B
authors.lastname: Xu
authors.forename: J E
authors.initials: JE
authors.lastname: Reutt-Robey