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.

Figure 1

(a) Chemical structures of the TP1, TP1-down, and TP1-up molecules. The orientation of the electric dipole moment of the embedded pyrimidine groups is indicated by blue arrows. (b) Schematic of a TP1 SAM adsorbed on a Au(111) substrate (equivalent to the structures of pure and mixed TP1-up and TP1-down SAMs). The √3 × 3 (short dashed cyan line), 2√3 × 3 (dashed red line), and 6√3 × 3 (solid black line) surface unit cells were used to describe pure films, homogeneous mixtures, and short-range phase separated structures. The blue and green shaded areas indicate the TP1-up and TP1-down arrangement in mixed SAMs. The situation in the striped (inhomogeneous) 50:50 mixture is indicated in the upper half of the figure, whereas the lower half of the figure represents the checkerboard structure of a homogeneous 50:50 mixture. The central molecules of each domain of the striped structure are indicated by yellow ellipses.

Figure 2

SAM-induced work function change for a mixed TP1-up:TP1-down SAM as a function of the mixing ratio compared to a TP1 reference SAM. Blue triangles represent average values of two independent experiments (see the Supporting Information for more details); black circles denote simulations. Note that the mixing ratio for the experiments refers to the composition of the solution from which the SAM was grown (which deviates from the actual composition on the surface; see main text for details); in contrast, the mixing ratio in the simulations denotes the composition of the surface unit cell. The blue dashed and the black dash–dotted lines are guides to the eye.

Figure 3

Normalized calculated XP spectra of pure and mixed TP1-up:TP1-down SAMs. (a) Calculated XP spectra for pure TP1-up (black) and TP1-down (cyan) SAMs as well as homogeneously mixed SAMs [at TP1-up to TP1-down ratios of 75:25 (red), 50:50 (green), and 25:75 (blue)]. (b) Calculated spectra for a TP1 reference SAM (dashed violet), the 50:50 striped (inhomogeneous) mixture (dark green), and the simulated spectrum of coexisting large TP1-up and TP1-down domains (gray) obtained by a weighted superposition of the spectra of the pure TP1-up and TP1-down SAMs. The orange curve displays the spectrum of the striped (inhomogeneous) SAM, which was calculated considering only the two central molecules of each domain to reduce edge effects (molecules highlighted by yellow ellipses in Figure 1b). All spectra of panels a and b were obtained using a variance of 0.1 eV to match experimental peak widths. (c) XP spectra of the striped SAM (dark green and orange) and the TP1 reference SAM (dashed violet) evaluated with a reduced variance of 0.03 eV to better visualize the origin of the broadening.

Figure 4

Electrostatic energy of an electron plotted for the striped (inhomogeneous) 50:50 SAM (top panel) and the homogeneous 50:50 SAM (bottom panel) in the plane containing the long molecular axes of half of the molecules (aligned along the x-axis of the unit cell). Values are given relative to the Fermi level of each system. The black isodensity lines cover the energy interval from 3.5 to 4.3 eV (the same interval in which the colors are varied) with lines drawn every 0.05 eV. To depict the much stronger energy variations inside the SAM, red isodensity lines are plotted, covering the range from 4.3 to 6.8 eV with a spacing of 0.25 eV.

Figure 5

(a) Experimental C 1s HRXPS spectra of the homogeneous TP1-up (blue), TP1-down (red), and mixed TP1-up/TP1-down (black) monolayers acquired at a photon energy of 350 eV and (b) weighted sums of the TP1-up and TP1-down spectra, corresponding to a coexistence of large domains in the mixed films. The percentages of the TP1-up and TP1-down molecules in the solutions are color-coded and given at the respective spectra. The vertical olive lines in panel a mark the positions of the dominant emission. The thick horizontal bars between these lines mark the energy shifts. The overall rigid shift between the experimental and simulated (Figure 3) spectra is a consequence of the initial state method of calculating core level energies and is not relevant for the present comparison.⁻