Fermi Level Alignment in Self-Assembled Molecular Layers: The Effect of Coupling Chemistry
Christopher D. Zangmeister, Steven W. Robey, Roger D. van Zee
Photoelectron spectroscopy was used to explore changes in Fermi level alignment, within the p-p* gap, arising from changes in the coupling chemistry of conjugated phenylene ethynlene 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 in C(1s) spectra and in the threshold near Ef. The lack of change in the optical absorption suggests a rigid shift of the Fermi level within the p-p* 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 varying coupling chemistries in molecular layers are discussed.