Comparative Study of the Different Anchoring of Organometallic
Dyes on Ultrathin Alumina
Version 2 2019-09-04, 08:29Version 2 2019-09-04, 08:29
Version 1 2019-08-27, 20:29Version 1 2019-08-27, 20:29
Posted on 2019-09-04 - 08:29
Significant
improvements in incident photon-to-current efficiencies
can be obtained by covering inorganic semiconductors with ultrathin
alumina films and sensitizing them with adsorbed dye molecules. The
anchoring mode of the latter to the substrate affects the charge transport
between the dye and the electrode via tunneling, and consequently,
the device efficiency. In this work, we employ X-ray and ultraviolet
photoelectron spectroscopies (XPS and UPS) for a comparative study
of the adsorption of three rhenium and two ruthenium organometallic
dyes, one of each being ionic, with different anchoring modes on single-crystalline
ultrathin alumina films. Molecular monolayers were prepared by self-assembly
from solution. Quantitative XPS analysis reveals higher surface densities
for the Re dyes. Nearly stoichiometric coadsorption of counterions
is observed for the ionic dyes. Density functional theory (DFT) calculations
for the Re dyes show that the most stable adsorption configurations
exhibit the expected bonding via the dedicated anchoring groups (carboxyls
or methylphosphonic acid), with an additional sulfur–aluminum
bond for the dyes containing a thiocyanate ligand. The alignment of
the occupied molecular levels with respect to the alumina valence
band maximum, obtained for these geometries, follows the experimental
trend in the UPS data and places the lowest unoccupied molecular orbitals
(LUMOs) close to the Fermi level of the systems, far inside the alumina
band gap. Dynamical charge screening is found to be important for
this type of system when comparing UPS and DFT results. This work
provides a general guideline for the systematic characterization of
related molecules on surfaces.
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Zabka, Wolf-Dietrich; Musso, Tiziana; Mosberger, Mathias; Novotny, Zbynek; Totani, Roberta; Iannuzzi, Marcella; et al. (2019). Comparative Study of the Different Anchoring of Organometallic
Dyes on Ultrathin Alumina. ACS Publications. Collection. https://doi.org/10.1021/acs.jpcc.9b05209