Fermi level alignment in self-assembled molecular layers: the effect of coupling chemistry |
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Authors: | Zangmeister C D Robey S W van Zee R D Kushmerick J G Naciri J Yao Y Tour J M Varughese B Xu B Reutt-Robey J E |
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Affiliation: | National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA. cdzang@nist.gov |
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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. |
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