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Changing adsorption mode of FePc on TiO2(110) by surface modification with bipyridine
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.ORCID iD: 0000-0002-9663-7705
MAX-lab., Lund University.
Department of Physics, Uppsala University.
Show others and affiliations
2008 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 129, no 7, 074707- p.Article in journal (Refereed) Published
Abstract [en]

Surface modification of reactive oxide substrates to obtain a less strongly interacting template for dye adsorption may be a way to enhance performance in dye-sensitized solar cells. In this work, we have investigated the electronic and structural properties of 4,4(')-bipyridine (bipy) as modifier adsorbed on the TiO2(110) surface. The modified surface is then coated with iron phthalocyanine (FePc) and the properties of this heterostructure are investigated with synchrotron based photoelectron spectroscopy, x-ray absorption spectroscopy, and scanning tunneling microscopy. We find that a saturated monolayer consisting of standing bipy molecules with one nitrogen atom pointing outward is formed on the oxide surface. FePc adsorb in molecular chains along the [001] direction on top of bipy and ordered in a tilted arrangement with adjacent molecules partially overlapping. Already from the first layer, the electronic properties of FePc resemble those of multilayer films. FePc alone is oxidized on the TiO2(110) surface, but preadsorbed bipy prevents this reaction. The energy level lineup at the interface is clarified.

Place, publisher, year, edition, pages
2008. Vol. 129, no 7, 074707- p.
Keyword [en]
Direct energy conversion; Solar cells; Solar energy; Bipyridine; Dye adsorption; Dye-sensitized solar cells; Electronic and structural properties; Oxide substrates; Surface modifications; Adsorption; 4, 4' bipyridine; 4, 4'-bipyridyl; ferrous ion; indole derivative; iron phthalocyanine; pyridine derivative; titanium; titanium dioxide; unclassified drug; adsorption; article; artificial membrane; chemistry; electron; surface property; Electrons; Ferrous Compounds; Indoles; Membranes, Artificial; Pyridines; Surface Properties; Titanium
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
URN: urn:nbn:se:kth:diva-7781DOI: 10.1063/1.2969081ISI: 000258643300041PubMedID: 19044792Scopus ID: 2-s2.0-50249106920OAI: oai:DiVA.org:kth-7781DiVA: diva2:12908
Note
QC 20100812. Uppdaterad från Submitted till Published 20100812.Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Phthalocyanine interfaces: the monolayer region
Open this publication in new window or tab >>Phthalocyanine interfaces: the monolayer region
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Organic molecules adsorbed on inorganic substrates are the topics of interest in this thesis. Interfaces of this kind are found in dye sensitized solar cells that convert solar energy to electricity, a promising environmentally friendly energy source which might provide a route to replace fossil fuels. Another field where these interfaces play a role is in molecular electronics, an approach to solve the down scaling in the ever increasing hunt for miniaturized electronic devices. The motivation for this work lies among other in these applications and surface science is a suitable approach to investigate the electronic and morphologic properties of the interfaces as it provides detailed knowledge on an atomic level.

Phthalocyanines are the organic molecules investigated and the inorganic substrates range from wide band gap via narrow band gap semiconductors to metals. Photoelectron and X-ray spectroscopy experiments are performed to shed light on the electronic properties of the adsorbed molecules and the substrate, as well as the chemical interaction between adsorbate and substrate at the interface. The ordering of the adsorbate at the interface is important as ordered molecular thin films may have other properties than amorphous films due to the anisotropic electronic properties of the organic molecules; this is investigated using scanning tunneling microscopy.

We find that the phthalocyanines are affected by adsorption when the substrate is TiO2 or Ag, where charge transfer from the molecule occurs or an interface state is formed respectively. The molecules are adsorbed flat on these surfaces giving a large contact area and a relatively strong bond. On Ag, ordered structures appear with different symmetry depending on initial coverage. The reactivity of the TiO2 surface is not ideal in the solar cell application and by modifying the surface with a thin organic layer, the negative influence on the adsorbed phthalocyanine is reduced. ZnO is not as reactive as TiO2, thanks maybe to the upright adsorption mode of the phthalocyanines. The semiconductor InSb is less reactive leading to self-assembled molecular structures on the (001) surface, either homogenously distributed in a one monolayer thick film or in strands along the reconstruction rows. InAs on the other hand has a larger influence on the adsorbed molecules resulting in a metallic film upon thermal treatment.

Place, publisher, year, edition, pages
Stockholm: KTH, 2007. 68 p.
Series
Trita-ICT/MAP, 2007:10
Keyword
phthalocyanine, III-V semiconductor, transition metal oxides, adsorption, self-assembly
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-4571 (URN)978-91-7178-784-2 (ISBN)
Public defence
2007-12-18, N2, electrum 3, Isafjordsgatan 28, Kista, 10:00
Opponent
Supervisors
Note
QC 20100812Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2010-08-12
2. Molecular Interaction of Thin Film Photosensitive Organic Dyes on TiO2 Surfaces
Open this publication in new window or tab >>Molecular Interaction of Thin Film Photosensitive Organic Dyes on TiO2 Surfaces
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The photosensitive molecule adsorption on titanium dioxide (TiO2) forms the so-called “dye sensitized TiO2” system, a typical organic/oxide heterojunction, which is of great interest in catalysis and energy applications, e.g. dye-sensitized solar cell (DSSC). Traditionally, the transition metal complex dyes are the focus of the study. However, as the fast development of the organic semiconductors and invention of new pure organic dyes, it is necessary to expand the research horizon to cover these molecules and concrete the fundamental understanding of their basic properties, especially during sensitization.In this work, we focus on two different photosensitive molecules: phthalocyanines and triphenylamine-based dyes. Phthalocyanines are organic semiconductors with symmetric macro aromatic molecular structures. They possess good photoelectrical properties and good thermal and chemical stability, which make them widely used in the organic electronic industries. Triphenylamine-based dyes are new types of pure organic dyes which deliver high efficiency and reduce the cost of DSSC. They can be nominated as one of the strong candidates to substitute the ruthenium complex dyes in DSSC. The researches were carried out using classic surface science techniques on single crystal substrates and under ultrahigh vacuum condition. The photosensitive molecules were deposited by organic molecular beam deposition. The substrate reconstruction and ordering were checked by low energy electron diffraction. The molecular electronic, geometric structures and charge transfer properties were characterized by photoelectron spectroscopy, near edge X-ray absorption fine structure spectroscopy and resonant photoelectron spectroscopy (RPES). Scanning tunneling microscopy is used to directly image the molecular adsorption.For phthalocyanines, we select MgPc, ZnPc, FePc and TiOPc, which showed a general charge transfer from molecule to the substrate when adsorbed on rutile TiO2(110) surface with 1×1 and 1×2 reconstructions. This charge transfer can be prevented by modifying the TiO2 surface with pyridine derivatives (4-tert-butyl pyridine (4TBP), 2,2’-bipyridine and 4,4’-bipyridine), and furthermore the energy level alignment at the interface is modified by the surface dipole established by the pyridine molecules. Annealing also plays an important role to control the molecular structure and change the electronic structure together with the charge transfer properties, shown by TiOPc film. Special discussions were done for 4TBP for its ability to shift the substrate band bending by healing the oxygen vacancies, which makes it an important additive in the DSSC electrolyte. For the triphenylamine-based dye (TPAC), the systematic deposition enables the characterization of the coverage dependent changes of molecular electronic and geometric structures. The light polarization dependent charge transfer was revealed by RPES. Furthermore, the iodine doped TPAC on TiO2 were investigated to mimic the electrolyte/dye/TiO2 interface in the real DSSC.The whole work of this thesis aims to provide fundamental understanding of the interaction between photosensitive molecules on TiO2 surfaces at molecular level in the monolayer region, e.g. the formation of interfacial states and the coverage dependent atomic and electronic structures, etc. We explored the potential of the application of new dyes and modified of the existing system by identifying their advantage and disadvantage. The results may benefit the fields of dye syntheses, catalysis researches and designs of organic photovoltaic devices.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. xiii, 62 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2011:14
Keyword
photoelectron spectroscopy, X-ray absorption spectroscopy, organic semiconductor, oxides, adsorption, dye sensitization, electronic structure, charge transfer
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-47354 (URN)978-91-7501-114-8 (ISBN)
Public defence
2011-12-02, C2, KTH-Electrum, Isafjordsgatan 26, Kista,, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20111114Available from: 2011-11-14 Created: 2011-11-08 Last updated: 2011-11-14Bibliographically approved

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