Article pubs.acs.org/JPCC
Nitro-Substituted Aromatic Thiolate Self-Assembled Monolayers: Structural Properties and Electron Transfer upon Resonant Excitation of the Tail Group Prashant Waske,†,‡ Tobias Wac̈ hter,† Andreas Terfort,‡ and Michael Zharnikov*,† †
Applied Physical Chemistry, Heidelberg University, 69120 Heidelberg, Germany Institut für Anorganische und Analytische Chemie, Universität Frankfurt, Max-von-Laue-Straße 7, 60438 Frankfurt, Germany
‡
S Supporting Information *
ABSTRACT: Self-assembled monolayers (SAMs) of HS-(C6H4)n-NO2 (nPT-NO2), abbreviated individually as PT-NO2, BPT-NO2, and TPT-NO2 for n = 1, 2, and 3, respectively, were prepared on Au(111) substrates and characterized by X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and resonant Auger electron spectroscopy. All molecules in the films were found to be bound to the substrate via the thiolate anchor and to have an upright orientation. The introduction of the nitro tail group had a positive effect on the quality of the PT-NO2 SAMs, which was superior to that of the nonsubstituted analogues. The parameters of the BPT-NO2 and TPT-NO2 films were similar to those of the analogous nonsubstituted systems. The [N 1s]π* and [O 1s]π* decay spectra of all studied nPT-NO2 SAMs did not exhibit any trace of charge (electron) transfer (CT) through the molecular framework to the substrate, following the resonant excitation of the tail group. This was explained by the energy considerations hindering CT to the substrate but enabling a reverse process (ICT), viz., the neutralization of the core ionized state by electron transfer from the substrate/molecular backbone. Traces of this process could be tentatively identified as an admixture of resonant contributions to the nonresonant decay spectra at the O K-edge. The experimental data suggest that only the ring adjacent to the nitro group was involved in the ICT process.
1. INTRODUCTION
In the context of better understanding the design of application-relevant, aromatic, thiol-derived SAMs, we present here the results for a series of such monolayers with a nitro tail group, namely, 4-nitrophenyl-1-thiol (PT-NO2), 4′-nitrobiphenyl-4-thiol (BPT-NO2), and 4″-nitroterphenyl-4-thiol (TPT-NO2), on Au(111) (Figure 1). The variation in the
Due to their flexible design and a variety of useful properties, self-assembled monolayers (SAMs) became an important model system for the physical chemistry of interfaces and an indispensible element of modern nanotechnology.1 Among different SAM systems, monolayers of thiols on noble metal substrates are probably the best studied due to the reliable attachment chemistry, comparably easy preparation procedure, and reasonable stability. Whereas aliphatic SAMs were the systems of choice at the beginning of the respective activities,2−5 monolayers with aromatic backbones became more popular later.6−10 This tendency is mostly related to the rigidity, conformational robustness, and stability of the aromatic backbones as well as to a better conductivity of these “molecular wires” as compared to aliphatic chains.11,12 The latter property is of particular importance for molecular electronics13 and modification of interfaces between the metal electrodes and organic semiconductors.10 Such a modification can be performed either to adjust the interfacial dipole10 or to optimize the morphology of the semiconductor that is usually grown onto the electrodes as a thin film. Because most organic semiconductors contain aromatic building blocks, the presence of aromatic intermediate layers is usually beneficial for the improvement of structural order and, consequently, for the performance of the entire device. © 2014 American Chemical Society
Figure 1. Structures of the SAM precursors studied, along with their abbreviations. Received: July 21, 2014 Revised: September 11, 2014 Published: October 16, 2014 26049
dx.doi.org/10.1021/jp507265k | J. Phys. Chem. C 2014, 118, 26049−26060
The Journal of Physical Chemistry C
Article
of the substrates are described elsewhere.39 Films are polycrystalline, exposing preferably (111) oriented surfaces of individual crystallites. The SAMs were prepared by immersion of the fresh substrates in 2 mmol solutions of the respective precursors in absolute ethanol for 72 h at room temperature. After immersion, the films were rinsed with ethanol and blown dry with argon. Extensive characterization showed no evidence of impurities or oxidative degradation products. In addition, reference SAMs of dodecanethiolate (DDT) and octadecanethiolate (ODT) were prepared on similar gold substrates using standard procedures.40 The SAMs were either characterized immediately after the preparation or put in small, nitrogen-filled containers and stored until the characterization at the synchrotrons (see below). The fabricated nitro-substituted SAMs were characterized by laboratory X-ray photoelectron spectroscopy (XPS), synchrotron-based XPS, angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and RAES. All experiments were performed at room temperature and under UHV conditions at a base pressure of
