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Comparative Study of the Different Anchoring of Organometallic Dyes on Ultrathin Alumina


Zabka, Wolf-Dietrich; Musso, Tiziana; Mosberger, Mathias; Novotny, Zbynek; Totani, Roberta; Iannuzzi, Marcella; Probst, Benjamin; Alberto, Roger; Osterwalder, Jürg (2019). Comparative Study of the Different Anchoring of Organometallic Dyes on Ultrathin Alumina. Journal of Physical Chemistry C, 123(36):22250-22260.

Abstract

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.

Abstract

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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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Department of Chemistry
07 Faculty of Science > Physics Institute
08 Research Priority Programs > Solar Light to Chemical Energy Conversion
Dewey Decimal Classification:530 Physics
Uncontrolled Keywords:General Energy, Physical and Theoretical Chemistry, Electronic, Optical and Magnetic Materials, Surfaces, Coatings and Films
Language:English
Date:12 September 2019
Deposited On:10 Dec 2019 13:26
Last Modified:12 Feb 2020 13:31
Publisher:American Chemical Society (ACS)
ISSN:1932-7447
OA Status:Closed
Publisher DOI:https://doi.org/10.1021/acs.jpcc.9b05209

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