Employing X-ray Photoelectron Spectroscopy for Determining Layer Homogeneity in Mixed Polar Self-Assembled Monolayers

Abstract

Self-assembled monolayers (SAMs) containing embedded dipolar groups offer the particular advantage of changing the electronic properties of a surface without affecting the SAM–ambient interface. Here we show that such systems can also be used for continuously tuning metal work functions by growing mixed monolayers consisting of molecules with different orientations of the embedded dipolar groups. To avoid injection hot-spots when using the SAM-modified electrodes in devices, a homogeneous mixing of the two components is crucial. We show that a combination of high-resolution X-ray photoelectron spectroscopy with state-of-the-art simulations is an ideal tool for probing the electrostatic homogeneity of the layers and thus for determining phase separation processes in polar adsorbate assemblies down to inhomogeneities at the molecular level.

Understanding, Optimizing, and Utilizing Nonideal Transistors Based on Organic or Organic Hybrid Semiconductors

“…In this work, a terphenyl‐4‐methanethiol (TP1; TP referring to the terphenyl backbone and denoting one methylene spacer) and biphenyl‐4‐thiol (BP0) backbone were introduced with a polar 2,5‐pyrimidine moiety to trigger work‐function changes, as shown in Figure l–o . Both generations of SAMs could tune the work function of Au electrodes over a range of 0.9 eV by switching the orientation of a pyrimidine ring . For SAMs with different backbone structures, the OFET treated with the sceond‐generation molecules achieved higher conductance of the BP0‐based SAMs due to the shorter length of the conjugated backbone and the absence of the methylene linker .…”

A dithiocarbamate anchoring group as a flexible platform for interface engineering

“…XPS serves as an efficient local probe for the potential distribution within a SAM, at the same time representing a reliable benchmark for theoretical simulations. 20,21,40 NEXAFS spectroscopy represents a complementary technique, which not only shows high chemical sensitivity but also has the capability of providing information on molecular orientation in adsorbate assemblies and molecular films. 41 Significantly, such information can be obtained not only for highly ordered systems, but also for samples, in which short-or medium-range order prevails.…”

“…XP spectra were acquired using either a Scienta R3000 electron energy analyzer (BESSY II) or a SCIENTA SES200 spectrometer (Max IV) having established performance and reliability. 23,24,40,42 Spectra acquisition was carried out in normal emission geometry with an energy resolution of either B0.1 eV (Max IV) or B0.3 eV, B0.6 eV, and 0.7 eV (BESSY II) at excitation energies of 350 eV, 580 eV, and 720 eV, respectively. The binding energy (BE) scale of the XP spectra was referenced to the Au 4f 7/2 peak of the underlying substrate at a BE of 84.0 eV, at each particular excitation energy.…”

“…To understand the above-described peculiarities of the XP spectra, it is useful to keep in mind that core-level energies are determined by two factors: the local chemical environment of an atom and the local electrostatic energy. 20,21,40,60,94 The latter can, for example, be changed through collective electrostatic effects via a periodic arrangement of polar groups common in SAMs 21 or by the Madelung energy in ionic crystals. 95,96 Conceptually, chemically and electrostatically induced shifts can be separated by reducing the density of the polar moieties in the adsorbate layer.…”

Rationalizing the formation of binary mixed thiol self-assembled monolayers

“…[6] It has been shown recently that the phase separation in certain mixed SAMs can also be studied by high resolution X-ray photoemission. [10] Drastically different chemical or physical properties as well as directionality within a single particle [11], lead to particles with fascinating hierarchical super structures [12] in solution, which tackle demanding problems in materials science, biomedicine and in the field of highly specific sensors. [13][14][15][16][17][18][19][20][21] The bicomponent organic monolayers can self-assemble into nano-domains on the surface of planar substrates, where the two types of complexing agents are chemically incompatible.…”