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