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Molecular and electronic structure of electroactive self-assembled monolayers
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View: Figures


Image of FIG. 1.
FIG. 1.

Schematic of the electrochemical cell–UHV transfer system, (left) top view and (right) side view.

Image of FIG. 2.
FIG. 2.

PM-IRRAS spectra (left axes) corresponding to self-assembled monolayers of (a) ferrocene hexanethiol (FcC6) and (b) FcC6 and hexanethiol (FcC6 + C6). (c) IR spectrum (right axes) of FcC6 in a NaCl window.

Image of FIG. 3.
FIG. 3.

(a) Fe 2p, (b) S 2p, (c) C 1s, and (d) Au 4f XPS spectra corresponding to the bare Au substrate, 6-ferrocene 1-hexanethiol (FcC6) SAM, mixed SAM of FcC6 and 1-hexanethiol (FcC6 + C6), and 1-hexanethiol (C6) SAM from bottom to top.

Image of FIG. 4.
FIG. 4.

Cyclic voltammograms of Au electrodes modified with self-assembled monolayers of ferrocene hexanethiol (bottom curve) and a mixed monolayer with hexanethiol (top curve). The voltammograms show a significant change in position and width as the concentration of the electroactive center increases in the monolayer from 1014 molecules cm−2 (FcC6 + C6) to 2 × 1014 molecules cm−2 (FcC6). In fact, the FcC6 + C6 SAM anodic wave has a peak maximum at 196 mV, a full width at half maximum ΔEFWHM = 88 mV and a peak splitting with respect to the cathodic wave ΔE = 13 mV. Whereas in the case of the FcC6 layer the anodic wave has a maximum at 383 mV, ΔEFWHM = 181 mV, and a peak splitting of 80 mV. These results are in excellent agreement with previously reported studies of ferrocene alkanethiol SAMs 37 and could be accounted for in terms of interacting electroactive species. A positive ferrocenium ion is formed upon oxidation of a ferrocene center and therefore the oxidation of a ferrocene molecule in close proximity will be more difficult as it forms another positive charge and it therefore requires a more anodic potential. Thus the anodic peak is expected to shift to greater values as the FcC6 coverage in the SAM increases. Furthermore, the full width at half maximum of the anodic wave provides an indication of the interactions taking place between different centers 37 therefore explaining the observed increase in ΔEFWHM with increasing FcC6 coverage in the SAM. Finally, at low ferrocene coverage a 13 mV peak splitting between the anodic and cathodic sweep can be observed as a result of the fast enough electron-transfer kinetic, while an increase of the electroactive molecule in the layer leads to a peak splitting of 80 mV.

Image of FIG. 5.
FIG. 5.

UPS spectra of the bare gold substrate (Au), SAM of ferrocene hexanethiol (FcC6), mixed SAM with hexanethiol (FcC6 + C6), and hexanethiol (C6, red curve) showing (a) work function changes (ΔΦ) calculated from the secondary electron cutoff and (b) density of states below the Fermi level.

Image of FIG. 6.
FIG. 6.

Structure of the FcC6 monolayer on the Au surface, as obtained from a DFT relaxation in periodic boundary conditions. For image clarity, the depicted surface coverage is only one fourth of the actual coverage used in the simulations. Labels in black indicate the optimized width of the monolayer and angle of the hydrocarbon chains with respect to the surface normal.

Image of FIG. 7.
FIG. 7.

Electrostatic potential as a function of the z position (normal to the surface), calculated from DFT for the FcC6/Au(slab)/FcC6 model. The values are averaged on the x, y plane at every point along the z direction. The dashed vertical lines indicate the boundaries of the slab (or the z-coordinates of the Au atoms of the external layers), and the dotted horizontal line marks the vacuum level, equal to 7.84 eV. The work function is calculated from the difference between the vacuum and the Fermi energies, 7.84–3.58 eV = 4.26 eV.

Image of FIG. 8.
FIG. 8.

Total and projected density of states around the Fermi level, computed with DFT for the system comprising the FcC6 adsorbate plus the Au slab.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Molecular and electronic structure of electroactive self-assembled monolayers