AVS 61st International Symposium & Exhibition
    Thin Film Monday Sessions
       Session TF-MoA

Paper TF-MoA3
Embedded Dipole in Alkanethiolate Self-Assembled Monolayers: Electronic Structure and Work Function Effects

Monday, November 10, 2014, 2:40 pm, Room 305

Session: Self-Assembled Monolayers, Layer-by-Layer Assemblies, and Hydrophobic/Amphiphobic Thin Films
Presenter: Swen Schuster, Heidelberg University, Germany
Authors: S. Schuster, Heidelberg University, Germany
N. Sullivan, Pennsylvania State University
O. Cabarcos, Pennsylvania State University
I. Hehn, Graz University of Technology, Austria
J.-F. Morin, Université Laval, Canada
E. Zojer, Graz University of Technology, Austria
M. Zharnikov, Heidelberg University, Germany
D.L. Allara, Pennsylvania State University
Correspondent: Click to Email
Self-assembled monolayers (SAMs) bonded to metal-electrodes are frequently used to modify charge-carrier injection and also serve as prototypical systems for studying charge transport processes through molecular assemblies. Usually, control over charge-carrier injection is achieved by use of the terminal dipolar groups (terminal dipole) comprising the SAM-ambience interface. But this architecture affects the growth mode of an organic semiconductor (in the standard device configuration) entangling it with the dipole control. In contrast, for the molecules with an embedded dipolar element, the dipole control and the chemistry at the SAM-ambience interface are decoupled. In this context, we studied a series of SAMs on Au{111} prepared from the mid-chain ester functionalized thiols, HS(CH2)mCO2(CH2)n-1CH3 (CmECn) with different combinations of m and n, different dipolar group orientations, and partial deuteration for some of these films. Electronic properties were analyzed by high resolution x-ray photoelectron spectroscopy (HRXPS), near edge x-ray absorption fine structure spectroscopy, work function measurements, and theoretical simulations with supporting characterization by infrared spectroscopy and AFM. The presence of the ester moiety leads to the formation of a strong electric dipole layer with a component of ~1.05 Debye normal to the surface for most of the CmECn SAMs and results in a strong electrostatic effect on the HRXPS spectra in which the C 1s photoelectron kinetic energies are consistently shifted by 0.7-1.0 eV between the alkyl segments below and above the e mbedded ester group. In addition, this group affects the work function of the entire assembly, with the direction of the change following the direction of the embedded dipole. There is however no perfect correlation between the behavior of the HRXPS spectra and the work function, which has to be understood with the help of theoretical simulations.