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42087-AC5
Studies of the Interfaces between Conjugated Oligomers and Self-Assembled Monolayers

James E. Whitten, University of Massachusetts (Lowell)

Significance of this Project

     The goals of this project are to understand the interaction of conjugated oligomers, such as sexithiophene, with other adsorbed organic layers, including self-assembled thiol monolayers. The motivation for this work is that overlap of electronic states between organic layers often dictates the performance of electronic devices, including light-emitting diodes and solar cells. Alkanethiol monolayers are being used to "tune" the work function of electrodes for organic electronic devices, and organic layers are usually deposited on top of these electrodes.

     Because of their surface sensitivity, X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) are ideal for studying interfacial chemistry and charge transfer when one layer is step-wise deposited onto another. This PRF-supported project has benefited the Whitten research group in that it has funded a Ph.D. level graduate student and enabled us to perform our first studies of organic-organic interfaces. It is noteworthy that while interfaces between organic layers and metals have been extensively investigated, organic-organic interface studies are relatively sparse.

Summary of Progress

     Because of its potential importance for photovoltaics and field-effect transistors, we have performed most of our studies using sexithiophene (6T); its chemical structure is shown in Figure 1. Experiments in which this molecule is thermally deposited in ultrahigh vacuum (UHV) onto Buckminsterfullerene (C60) films and self-assembled thiol monolayers (SAMs) on gold surfaces have been performed. In the case of a fullerene film, charge transfer from 6T to C60 is evidenced by shifts in the highest occupied molecular orbitals (HOMOs) of both molecules, with the HOMOs of C60 and 6T shifting to lower and higher ionization energies, respectively.

     6T and thiophene monomer have also been deposited on self-assembled, fluorine-functionalized thiol (1H,1H,2H,2H-perfluorodecanethiol, hereafter referred to as PFDT) monolayers on gold substrates cooled to 135 K. The fluorine atoms serve as "tags" in XPS experiments and enable us to deduce the extent of penetration of the monolayer. In some cases, thiophene has been adsorbed (instead of sexithiophene) by simply leaking it into the UHV chamber and condensing it on the cold, organic monolayer-covered surface. The binding energy of the F 1s XPS peak decreases following thiophene and 6T adsorption, indicating charge transfer to the fluorinated SAMs. UPS measurements substantiate charge transfer, with the valence features of thiophene and 6T in contact with the monolayer appearing at higher ionization energies compared to thicker layers. The energies of the UPS-measured vacuum levels of 6T deposited on PFDT/Au illustrate the absence of a common vacuum level between the organic layers at the PFDT-6T interface and the presence of a -0.9 eV interface dipole. Similar measurements performed for 6T deposition on self-assembled octadecanethiol (ODT) give a weaker interface dipole of opposite sign (+0.4 eV). The relatively large value and sign of the 6T/PFDT/Au interface dipole suggest that charge transfer to the PFDT-covered surface results in the formation of dipoles with their negative ends toward the Au surface, in contrast to the 6T-ODT interface. These data permit construction of the energy level diagrams in Figure 2.

     The effects of a SAM layer on X-ray-induced oligomerization, which is known to occur for condensed thiophene, were also investigated. Comparison of the thickness of oligomeric thiophene formed by 1253.6 eV X-ray irradiation on clean and PFDT-covered gold surfaces demonstrates that a thicker oligomer layer forms on the SAM-covered surface, suggesting that the spacing provided by the SAM reduces quenching of electronic excitations that lead to X-ray-induced oligomerization.

     This project also involved two other studies related to organic interfaces. A method of patterning a dibutylphosphonate-substituted, soluble form of 6T with nanoscale lateral dimensions has been developed. The method consists of forming a template of a hydrophilic thiol monolayer (e.g., a carboxylic terminated alkanethiol) by microcontact printing or dip pen nanolithography. The remainder of the surface is "backfilled" with a hydrophobic thiol. When the soluble, somewhat hydrophilic form of 6T is spin-coated on the surface, it selectively adsorbs on the hydrophilic regions. A second "subproject" involves measuring the effect of the substrate work function on the XPS core levels of an organic layer. By depositing potassium on top of an alkanethiol monolayer-covered surface, the work function of the surface can be modified. It is found by XPS that the C 1s core level of the carbon atoms in the alkanethiol chains are pinned to the vacuum level instead of the Fermi level. However, the sulfur atoms that are near the gold surface remain pinned to the Fermi level.