Electronic Structure of Aromatic Monomolecular Films: The Effect of Molecular Spacers and Interfacial Dipoles

Abstract

The electronic structure of several arene-based monomolecular films on Au(111), with and without an alkyl linker between the aromatic unit and thiol headgroup, has been investigated by photoemission and inverse photoemission. While the HOMO–LUMO gap in these films was found to depend on the aromatic backbone, the molecular band offset of the electronic states was strongly affected by the interfacial dipole. The smallest HOMO–LUMO gap was found for the strongly conjugated anthracene moiety, intermediate for terphenyl, and widest for the perfluorinated terphenyl. The perfluorinated terphenyl-based films appear to be more n-type as a semiconductor than the terphenyl or anthracene-based monolayers, as indicated by the placement of the Fermi level (chemical potential) relative to the conduction or lowest molecular orbital band edge. Accordingly, the occupied electronic states related to the aromatic rings sink to greater binding energies, well below those for the alkyl linker, and thus for the perfluorinated terphenyl, the aromatic orbital contribution is not to the HOMO but the HOMO-1 orbital (one occupied molecular orbital away from the HOMO). This placement of the perfluorinated terphenyl aromatic orbital contribution is in drastic contrast to the nonfluorinated systems in our study, in which both HOMO and LUMO orbitals are extended throughout the aromatic moieties.

Side‐Group‐Induced Polymorphism in Self‐Assembled Monolayers: 3,5‐Bis(trifluoromethyl)benzenethiolate Films on Au(111)

“…This is typically achieved by using large values for the tunneling impedance. These values depend mainly on the HOMO–LUMO gap of the molecules under study, which is presumably affected by fluorination . In the case of TFBT, an impedance‐tunneling value of approximately 3.33 GΩ was used.…”

“…Thisi st ypically achieved by using largev alues for the tunneling impedance.T hese values depend mainly on the HOMO-LUMO gap of the molecules under study,w hich is presumably affected by fluorination. [17,29] In the case of TFBT,a n impedance-tunneling value of approximately 3.33 GW was used. Under thesec onditions there was no indication of destructive imaging because the same surface area could be scannedr epeatedly withouta ny perceptible tip-induced changes.T his indicates that the tip wasn ot touching the monolayer,w hich was consequently the case at an even higher tunneling impedance of around 5.55 GW.T herefore, this suggests that the appearance of new protrusions in the respective images taken at this high impedance is not an artifact.…”

Tunneling across SAMs Containing Oligophenyl Groups

“…The introduction of a single intervening CH 2 group into the S-C bond converts SPh n to SCH 2 Ph n -a group that disrupts the delocalization of orbitals between the aromatic moiety and the metal electrode [19][20][21] -and increases β to 0.66 ± 0.06 Å -1 and log|J 0 | to 4.0 ± 0.3; these values are similar to those derived from SAMs of n-alkanethiols (β Au = 0.76 ± 0.03 Å -1 and log|J 0 | = 4.2 ± 0.2). 11,12 SAMs of SPh n and SCH 2 Ph n have been characterized extensively.…”

“…[32][33][34] A recent report from Zaba and coworkers demonstrates that it is possible to form highly-ordered SAMs of n-alkyl acetylenes on gold in non-oxidizing environments; 35 Table S7) than saturated n-alkanes where the HOMO-LUMO gap is much larger (~7 eV). 21 Moreover, the energy level of the HOMO of conjugated molecules aligns more favorably with the Fermi level of the metal, than does that for aliphatic molecules, and results in lower tunneling barriers. 16,21,37,38 RESULTS AND DISCUSSION these values agree with previous experimental reports using single-molecule and large-area junctions.…”

“…21 Moreover, the energy level of the HOMO of conjugated molecules aligns more favorably with the Fermi level of the metal, than does that for aliphatic molecules, and results in lower tunneling barriers. 16,21,37,38 RESULTS AND DISCUSSION these values agree with previous experimental reports using single-molecule and large-area junctions. 27,[38][39][40][41][42][43][44][45] These results presented here also agree with theoretical calculations by Ratner and coworkers; these authors predict (using density functional theory, DFT) a value of  = ~0.3 Å -1 for SAMs of polyphenyldithiolates by assuming a continuous conjugation of the molecules with the metal electrodes.…”