Self-assembling and charge transfer properties of thin organic films

2010/2011In the present thesis I dealt the issue of molecular ordering and charge transfer at two types of organic-inorganic interfaces that are representative of the basic constituents of an organic electron device. I investigated i.) the influence of a selected dielectric surface on the ordering of an overlayer of several organic molecules and ii.) the electronic transport properties of a single molecular junction with a metal electrode. Both systems have been characterized by a structural and electronic point of view. Among the techniques available for structural investigation, I made extensive use of Helium Atom Scattering (HAS) and Scanning Tunneling Microscopy (STM). The electronic properties, with particular emphasis to the charge transfer, have been addressed by two methods chosen according to the dimensionality of the system under consideration. For the charge transfer at laterally extended interfaces I used synchrotron based techniques, like Resonant Photoemssion Spectroscopy (RPES), while for the charge transport through a single molecule I used and developed the STM-based break junction technique (STM-BJ). For the first type of interface, I focused on the coupling between the TiO2(110)-1x1 surface and different organic semiconductor molecules: C60, pentacene, perylene-tetracarboxilic-acid-diimide (PTCDI) and perylene. The strong anisotropy of the substrate has been found to drive the adsorption geometry of the molecules leading to the formation of ordered phases (at least for the first layer). In particular pentacene, PTCDI and perylene (polycyclic aromatic hydrocarbons, PAHs) display a common self-assembly mechanism, where the molecules lay on the surface with their long axis oriented parallel to the [001] substrate direction. In the transverse direction [1-10] these molecules are observed to match the substrate periodicity by tilting the molecular plane around the long axis by an angle that depends on the molecular width. Nevertheless the molecule-to-substrate interaction is very weak as indicated by the molecular electronic structure, which is observed by X-ray spectroscopy to remain mostly unperturbed in the first molecular layer. Only PTCDI bears a major interaction with the TiO2(110)-1x1 surface, but confined to the molecular orbitals closest to the gap. The main experimental evidence of this interaction is the appearance of a new molecular filled state in the valence band region close to the Fermi level. By a combined RPES and NEXAFS study we have found that this new electronic state is due to the charge transfer occurring from the substrate Ti defect state (i.e. the excess of electrons associated with oxygen vacancies) to the lowest unoccupied molecular orbital (LUMO). For the second type of hybrid interface, instead, I exploited the nitrogen-link chemistry in order to bridge a phthalocyanine to two gold electrodes and to measure its conductance. In particular, by using the Tetraaza-Cu-Phthalocyanine I investigated the pyridine-gold bond that is relatively weak and insensitive to the local structure, a fundamental requirement for the establishment of well defined and stable transport properties. The weak interaction between the molecule and a representative metal electrode, namely the Au(100) surface, has been confirmed by spectroscopic and STM experiments. At RT the molecules have been found to diffuse on the surface and only at LT (55 K) they can be observed to self-organize into large molecular domains. On these domains, reliable and reproducible single molecule conductance measurements have been performed by using the STM-based break junction method. The conductance value obtained for the Tetraaza-Cu-Phthalocyanine (7x10-4 G0) has been rationalized in terms of the molecular length and degree of conjugation, as well as by correlation to the energy level alignment at the junction.XXIV Ciclo198

Magnetic fingerprint of individual Fe4 molecular magnets under compression by a scanning tunnelling microscope

Single-molecule magnets (SMMs) present a promising avenue to develop spintronic technologies. Addressing individual molecules with electrical leads in SMM-based spintronic devices remains a ubiquitous challenge: interactions with metallic electrodes can drastically modify the SMM\u2019s properties by charge transfer or through changes in the molecular structure. Here, we probe electrical transport through individual Fe4 SMMs using a scanning tunnelling microscope at 0.5 K. Correlation of topographic and spectroscopic information permits identification of the spin excitation fingerprint of intact Fe4 molecules. Building from this, we find that the exchange coupling strength within the molecule\u2019s magnetic core is significantly enhanced. First-principles calculations support the conclusion that this is the result of confinement of the molecule in the two-contact junction formed by the microscope tip and the sample surface

UHV Deposition and Characterization of a Mononuclear Iron(III) \u3b2-diketonate Complex on Au(111)

The adsorption of the sterically hindered \u3b2-diketonate complex Fe(dpm)3, where Hdpm = dipivaloylmethane, on Au(111) was investigated by ultraviolet photoelectron spectroscopy (UPS) and scanning tunnelling microscopy (STM). The high volatility of the molecule limited the growth of the film to a few monolayers. While UPS evidenced the presence of the \u3b2-diketonate ligands on the surface, the integrity of the molecule on the surface could not be assessed. The low temperature STM images were more informative and at submonolayer coverage they showed the presence of regular domains characterized by a flat morphology and height of 480.3 nm. Along with these domains, tetra-lobed features adsorbed on the kinks of the herringbone were also observed. DFT-simulated images of the pristine molecule and its possible decomposition products allowed to assess the partial fragmentation of Fe(dpm)3 upon adsorption on the Au(111) surface. Structural features with intact molecules were only observed for the saturation coverage. An ex situ prepared thick film of the complex was also investigated by X-ray magnetic circular dichroism (XMCD) and features typical of high-spin iron(III) in octahedral environment were observed

Proyecto, investigación e innovación en urbanismo, arquitectura y diseño industrial

Actas de congresoLas VII Jornadas de Investigación “Encuentro y Reflexión” y I Jornadas de Investigación de becarios y doctorandos. Proyecto, investigación e innovación en Urbanismo, Arquitectura y Diseño Industrial se centraron en cuatro ejes: el proyecto; la dimensión tecnológica y la gestión; la dimensión social y cultural y la enseñanza en Arquitectura, Urbanismo y Diseño Industrial, sustentados en las líneas prioritarias de investigación definidas epistemológicamente en el Consejo Asesor de Ciencia y Tecnología de esta Universidad Nacional de Córdoba. Con el objetivo de afianzar continuidad, formación y transferencia de métodos, metodología y recursos se incorporó becarios y doctorandos de los Institutos de investigación. La Comisión Honoraria la integraron las tres Secretarias de Investigación de la Facultad, arquitectas Marta Polo, quien fundó y María del Carmen Franchello y Nora Gutiérrez Crespo quienes continuaron la tradición de la buena práctica del debate en la cotidianeidad de la propia Facultad. Los textos que conforman las VII Jornadas son los avances y resultados de las investigaciones realizadas en el bienio 2016-2018.Fil: Novello, María Alejandra. Universidad Nacional de Córdoba. Facultad de Arquitectura, Urbanismo y Diseño; ArgentinaFil: Repiso, Luciana. Universidad Nacional de Córdoba. Facultad de Arquitectura, Urbanismo y Diseño; ArgentinaFil: Mir, Guillermo. Universidad Nacional de Córdoba. Facultad de Arquitectura, Urbanismo y Diseño; ArgentinaFil: Brizuela, Natalia. Universidad Nacional de Córdoba. Facultad de Arquitectura, Urbanismo y Diseño; ArgentinaFil: Herrera, Fernanda. Universidad Nacional de Córdoba. Facultad de Arquitectura, Urbanismo y Diseño; ArgentinaFil: Períes, Lucas. Universidad Nacional de Córdoba. Facultad de Arquitectura, Urbanismo y Diseño; ArgentinaFil: Romo, Claudia. Universidad Nacional de Córdoba. Facultad de Arquitectura, Urbanismo y Diseño; ArgentinaFil: Gordillo, Natalia. Universidad Nacional de Córdoba. Facultad de Arquitectura, Urbanismo y Diseño; ArgentinaFil: Andrade, Elena Beatriz. Universidad Nacional de Córdoba. Facultad de Arquitectura, Urbanismo y Diseño; Argentin

Azimuthal Dichroism in Near-Edge X-ray Absorption Fine Structure Spectra of Planar Molecules

The dependence of the near-edge X-ray absorption fine structure (NEXAFS) spectrum of molecules on the photon electric field direction is investigated by means of first-principles simulations based on density functional theory with the transition-potential approach. In addition to the well-known dependence of the NEXAFS resonances on the orientation of the electric field with respect to the molecular plane, we demonstrate that for planar molecules with sufficient in-plane anisotropy such as pentacene a dichroic effect is found with a splitting of the sigma* resonance as a function of the azimuthal orientation of the photon electric field in the molecular plane. The sigma* splitting is investigated as a function of the length of acenes and closely related molecules. A proper assignment of such spectral features guided by theory together with variable polarization experiments may allow one to completely determine the orientation of molecules at interfaces

Planar growth of pentacene on the dielectric TiO2(110) surface

We have studied the growth of pentacene molecules on the unreconstructed and stoichiometric surface of TiO2(110). At variance with its characteristic homeotropic growth mode, pentacene is found to be physisorbed on this dielectric substrate with its long molecular axis oriented parallel to the surface and aligned along the [001] direction. Pentacene molecules couple side-by-side into long stripes running along the [11̅0] direction, where the overlayer preserves the substrate lattice periodicity (6.5 Å). In the opposite direction, head-to-head pentacene repulsion drives the ordering of the stripes, whose spacing simply depends on the surface coverage. By near-edge X-ray absorption, NEXAFS, we have determined the pentacene molecules to be tilted by ∼25° off the surface around their long axis. At the monolayer coverage, the pentacene orientation and spacing are very close to that of the (010) bulk planes (also called a−c planes) of pentacene crystals. We have observed that at least two additional layers can be grown on top of the monolayer following a planar configuration. Both the strong side-by-side intermolecular attraction and the full development of the bulklike electronic states, as probed by NEXAFS, suggest an optimal charge transport along the monolayer stripes of lying-down moleculesFinancial support from the Spanish CYCIT (MAT2008-1497) and the Ministry of Science and Innovation (CSD2007-41 NANOSELECT) is greatly acknowledged. C.S.-S. is grateful to Ministerio de Educación for the AP2005-0433 FPU grant. Funding from the European Community Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 226716 is acknowledged.Peer Reviewe

A tetrairon(III) single-molecule magnet and its solvatomorphs: synthesis, crystal structures and vapor-phase processing

With the aim of investigating the impact of lattice solvent on the processability of tetrairon(III) single-molecule magnets by thermal sublimation, two new solvatomorphs of [Fe4(LPh)2(dpm)6] (1) were prepared and structurally characterized along with unsolvated 1 (H3LPh = 2-(hydroxymethyl)-2-phenylpropane-1,3-diol, Hdpm = dipivaloylmethane). All solvatomorphs crystallize in the C2/c space-group whereas solvent-free 1 belongs to a different space group (P21/c). The pitch of the propeller-like tetrairon(III) molecules is distinctly different in solvated vs. unsolvated phases, highlighting the effect of intermolecular interactions and crystal packing. The compounds sublimate at 450 - 490 K (in high vacuum conditions ~ 10^-6-10^-7 mbar) affording thick deposits which display different crystallinity depending on the particular starting material used. However, all sublimated samples retain slow magnetic relaxation with thermal activation parameters comparable to those of microcrystalline 1. The results indicate that factors other than mere molecular structure have a limited influence on the processability of these materials by thermal sublimation

TiO2(110) charge donation to an extended π-conjugated molecule

The surface reduction of rutile TiO2(110) generates a state in the band gap whose excess electrons are spread among multiple sites, making the surface conductive and reactive. The charge extraction, hence the surface catalytic properties, depends critically on the spatial extent of the charge redistribution, which has been hitherto probed by small molecules that recombine at oxygen vacancy (O-vac) sites. We demonstrate by valence band resonant photoemission (RESPES) a very general charge extraction mechanism from a reduced TiO2(110) surface to an extended electron-acceptor organic molecule. Perylene-tetra-carboxylic-diimide (PTCDI) is not trapped at Ovac sites and forms a closely packed, planar layer on TiO2(110). In this configuration, the perylene core spills out the substrate excess electrons, filling the lowest unoccupied molecular orbital (LUMO). The charge transfer from the reduced surface to an extended pi-conjugated system demonstrates the universality of the injection/extraction mechanism, opening new perspectives for the coupling of reducible oxides to organic semiconductors and supported catalysts

Study of Organic Molecules on Metal Oxides

Trabajo presentado en Fuerzas y Túnel, celebrada en San Lorenzo de El Escorial del 12 al 14 de septiembre de 2012.Organic molecules on surfaces have attracted scientific attention in the last years thanks to their inherent and special properties. Particularly, they are thought to be the ideal candidates to substitute Si in future electronics due to their donor-acceptor character (Molecular Electronics). Up to date, the majority of scientific efforts have been focused on studying their interaction with metal surfaces, which are crucial for fundamental knowledge, but far from ¿real¿ and practical systems. Metal oxides are much more interesting from a technological point of view as they present a non-negligible amount of defects. Among metal oxides, TiO2 is the prototype one for surface science studies as it can be easily prepared under UHV conditions and presents a well-ordered surface. In this work, we have studied the interaction of different organic molecules, C60, pentacene and C60H30, with the rutile TiO2 (110)-(1x1) surface. All three molecules diffuse on the surface at room temperature. However, they present different behaviours with coverage and temperature. C60 molecules diffuse until a critical coverage of 0.3 ML is reached. At this point, they tend to form well-ordered close-packed structures with (5x2) symmetry (Fig. 1 a)) [1]. Pentacene presents a similar behaviour, with molecules diffusing until a critical coverage of 0.5 ML is reached. Once this coverage is reached, they form ordered structures whose periodicity depends on coverage. Saturation symmetry is (6x1) (Fig. 1 b)) [2]. On the contrary, C60H30 diffuses on the surface even for coverage close to a complete monolayer. However, a molecular transformation takes place after annealing the system at 375K. At this temperature, molecules are stabilized on the surface thanks to the appearance of stronger interactions due to partial dehydrogenation of the molecules (Fig. 1 c)).N