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Title: Self-assembly and on-surface chemistry of organic semiconductors on TiO2
Other Titles: Self-assembly and on-surface chemistry of organic semiconductors on TiO2
Authors: Lovat, Giacomo
Keywords: Molecola, ftalocianina, porfirina, metallazione, titanio, diossido, spettroscopia, stmperylene, ptcdi, aromatico, organico
Issue Date: 25-Mar-2015
Publisher: Università degli studi di Trieste
Abstract: Understanding and controlling the ordering and functionality of organic semiconductors coupled to a dielectric surface, besides constituting a fascinating topic in basic science, has the potential to foster the development of many novel technologies, ranging from efficient and inexpensive photovoltaic cells to flexible electronic components. In the present doctoral work, the well-characterized (110) surface of Rutile TiO2 single crystals, r-TiO2(110), has been selected as an archetypal dielectric substrate for the templated growth of organic molecules and the assessment of their physico-chemical properties at the interface. In particular, this thesis provides: i. a detailed picture of the bonding and structural properties of two polycyclic aromatic hydrocarbons, and ii. a comprehensive study of the electronic structure and on-surface chemistry of three heteroaromatic molecules; both i. and ii. with regard to the interface with r-TiO2(110) in an ultra-high vacuum (UHV) environment. The determination of the electronic and structural properties at the organodielectric interfaces has been achieved via a multitechnique experimental approach consisting of the combination between scanning tunneling microscopy (STM) and electron spectroscopies with synchrotron radiation. While the former can reveal the geometrical details of single adsorbed species, and allow for a close, direct-space inspection of their molecular packing and ordering, the latter provides information on the electronic states and chemical composition of the overlayer. STM has been applied to systems in i. so as to complement the existing knowledge of their electronic structure with structural details on a scale length ranging from several hundred molecules down to a single molecule. Subsequently, a full microscopic and spectroscopic characterization of the interfaces in ii., on which scarce studies are currently available, has been conducted, with particular emphasis on the chemical reactions triggered by thermal annealing. To get a deeper insight into the experimental findings and facilitate their interpretation, selected systems have been modeled by two groups of theoreticians with numerical simulations based on density functional theory (DFT) formalism. The interfaces studied in part i. comprise tetracarboxylic-acid-diimide (PTCDI), a heteroaromatic acceptor, and perylene, an all-carbon aromatic donor, in contact with the r-TiO2(110)-1x1 surface. The substrate twofold symmetry and the large surface corrugation provided by the protruding oxygen rows favor the alignment of the PTCDI long axis along the [001] direction, parallel to the surface rows. STM images reveal that the growth of PTCDI proceeds via island nucleation; there is no evidence for any preferential roles in adsorption played by defect sites, be them oxygen vacancies or hydroxyl groups. Each island consists of aligned monomers adsorbed atop the oxygen rows, and coupled head-to-tail via one hydrogen bond per two facing imide terminations. The competition between molecule-substrate interaction, which displays a mixed van der Waals/covalent character, and intermolecular hydrogen bonds originates a fivefold periodicity in the island with respect to the underlying surface unit cell along the [001] direction, as pointed out by previous He atom scattering studies. In the [1-10] direction, orthogonal to the surface rows, adjacent monomers organize in stripes with one monomer per oxygen row. Such a dense packing is attributed to a strong side-by-side attraction between the electron-deficient rim of one monomer and the $\pi$ electron system of the one at its side, made possible by a moderate tilt angle of PTCDI about its long axis (35°). A similar adsorption geometry emerges for perylene, albeit with some remarkable differences of the self-assembly mechanism. At low coverage, monomers sparsely populate the surface titanium troughs, showing a rather weak intermolecular attraction. As the coverage increases up to a full monolayer, perylene molecules gradually fill the available space in the titanium troughs until a wetting layer is formed and no sticking of additional material is observed due to very weak pi-type interactions between stacked dimers. The side-by-side coupling between tilted monomers in the monolayer favors the coalescence of ordered domains extended across the substrate rows, although frequent line defects disrupt the long range order. Part ii. focuses on the interface between r-TiO2(110) and two classes of heterocyclic compounds: phthalocyanines and porphyrins. The interfacial electronic structure, morphology, conformation and chemical reactivity of in situ grown thin films of metal-free phthalocyanine (2H-Pc), tetraphenyl-porphyrin (2H-TPP) and tert-butyl tetraphenyl-porphyrin (2H-tbTPP) have been extensively investigated by means of STM and electron spectroscopies. The surface sensitivity and chemical selectivity of soft X-ray photoemission (XPS, photon energies 100-1000 eV) and near-edge X-ray absorption (NEXAFS) allowed to monitor the chemical state of the species upon adsorption as well as the orientation of their molecular plane relative to the crystal surface. The differences in the electronic structure and chemical reactivity among the adsorbed heterocycles have been analyzed and correlated to the type of peripheral environment surrounding the macrocycle, i.e. their chemically active part. The experimental results point to the existence of a reaction unaffected by the type of side substituents: the conversion of the macrocyclic aza-type nitrogen atoms to pyrrolic-type nitrogens. Conversely, self-metalation, i.e. the incorporation into the macrocycle of a Ti ion extracted from the substrate, is demonstrated with STM and X-ray spectroscopy for the three compounds, but with significant temperature variations among them. While self-metalation in 2H-Pc starts at room temperature and is quickly completed at 90°C, in 2H-TPP a similar metalation rate of molecules is achieved at 150°C, and in 2H-tbTPP, where two bulky tert-butyl groups departing from each peripheral phenyl ring act as inert spacers increasing the macrocycle distance from the surface, the energy barrier for the activation of the reaction is increased further and self-metalation is quickly achieved only at 200°C. Therefore, one concludes that a careful selection of side substituents proves to be a successful approach towards the tuning of chemical reactivity of these macrocyclic compounds on a metal oxide surface.
Description: 2013/2014
NBN: urn:nbn:it:units-13690
Appears in Collections:Scienze fisiche

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