Theoretical studies on molecular adsorption and the effect of strain on core-level spectroscopy in different metal surfaces

Thesis submitted to the University of the Basque Country for the degree of Doctor in Physics.La interacción entre moléculas y materia tiene importantes implicaciones a nivel tecnológico e industrial lo que convierte a este área en un interesante campo de estudio. La adsorción molecular es una cuestión fundamental debido a las implicaciones prácticas que conlleva, por ejemplo, en reacciones catalíticas.El objetivo de esta tesis es el estudio de la adsorción molecular en distintas superficies. La adsorción de una molécula sobre una superficie puede modificar su estructura electrónica. La molécula puede estar químicamente ligada a la superficie, la carga eléctrica puede ser desplazada o transferida del adsorbato al adsorbente o viceversa y la hibridación con los estados electrónicos de la superficie puede modificar la densidad de estados molecular. Cada uno de estos cambios modificará las propiedades del sistema. Además, la interacción entre moléculas vecinas y la modificación de la densidad de estados electrónicos en la superficie puede afectar al proceso de autoensamblaje. El proceso de adsorción también puede estar influido por otros factores, como fuerzas de larga distancia (van der Waals), interacciones iónicas o electrostáticas y factores geométricos. La molécula adsorbida sobre la superficie puede ser fuertemente modificada durante el proceso, produciendo grandes cambios en sus propiedades electrónicas y creando un enlace químico fuerte con la superficie (quimisorción), por el contrario, el proceso de adsorción puede tener una influencia mínima en la molécula y no producir cambios de consideración sobre esta (fisisorción). Desde el punto de vista teórico la descripción de un sistema de muchos cuerpos puede ser realizada utilizando la ecuación de Schrödinger. Sin embargo, el principal problema reside en la complejidad del sistema. Para un sistema que esté constituido por más de unas pocas partículas esta ecuación se hace irresoluble en la práctica. Dado un sistema que contiene N electrones, cada uno dependiente de 3 coordenadas espaciales y una coordenada de espín, y siendo N desde unas pocas decenas hasta 1023 electrones, la ecuación de Schrödinger no puede ser resuelta de forma exacta. Es por ello que será necesario realizar aproximaciones para poder obtener las soluciones al sistema. Una alternativa para este problema fue desarrollada en los años sesenta, esta se basa en los teoremas de Hohenberg-Kohn y la ecuaciones de Kohn-Sham. La idea básica consiste en suponer que podemos describir el estado fundamental de un sistema atómico utilizando unicamente la densidad electrónica del sistema en lugar de la función de onda de muchos electrones. Este es el principio básico de la teoría del funcional de la densidad (density functional theory (DFT) en inglés). La densidad electrónica del estado fundamental del sistema se calcula, y sus propiedades física se derivan a partir de esta. Los cálculos DFT han sido utilizados desde los años setenta, sin embargo, no fue hasta principios de los años noventa cuando los recursos computacionales junto con la mejora en los métodos de cálculo convirtieron al DFT en una herramienta versátil para el estudio de la materia condensada. El incremento en la precisión permitió la correcta descripción de los sistemas físicos y ello convirtió al DFT en una metodología popular a la hora de estudiar los sistemas físicos que, conjuntamente con las observaciones experimentales, ha dado excelentes resultados. Basandonos en cálculos DFT y usando el programa de cálculo Vienna ab initio simulation package (VASP) desarrollado por Kresse et al. hemos estudiado la adsorción molecular en distintas superficies metálicas. También hemos analizado las propiedades electrónicas y estructurales de distintas superficies escalonadas.No tocante a parte institucional debo recoñecer o soporte financieiro e os recursos facilitados para o desenrolo desta tese dende UPV/EHU e o DIPC. Na miña etapa inicial como bolseiro predoutoural do programa ZabaldUz e posteriormente co apoio do centro de investigación. Así mesmo agradecer ó Ministerio de Economía, Industria y Competitividad (MINECO) no tocante ós proxectos MAT2013-46593-C6-2-P e MAT2016-78293-C6-4-R, ó Departamento de Educación do Goberno Vasco polo proxecto IT-756-13 e ó centro de investigación colaborativa "Structure and Dynamics of Internal Interfaces" (SFB 1083) financiado pola axencia alemana de investigación (DFG).Peer reviewe

NO adsorption on Cu(110) and O(2 × 1)/Cu(110) surfaces from density functional theory calculations

In a recent study [M. Feng, et al., ACS Nano, 2011, 5, 8877], it was shown that CO molecules adsorbed on the quasi-one-dimensional O(2 × 1)/Cu(110) surface reconstruction tend to form highly-ordered single-molecule-wide rows along the direction perpendicular to the Cu–O chains. This stems from the peculiar tilted adsorption configuration of CO on this substrate, which gives rise to short-range attractive dipole–dipole interactions. Motivated by this observation, here we study the adsorption of nitric oxide (NO) on O(2 × 1)/Cu(110) and Cu(110) using density functional theory, with the aim of elucidating whether a similar behaviour can be expected for this molecule. We first study NO adsorption on a clean Cu(110) surface, where the role of short-range attractions between molecules has already been pointed out by the observation of the formation of NO dimers by scanning tunnelling microscopy [A. Shiotari, et al., Phys. Rev. Lett., 2011, 106, 156104]. On the clean Cu(110), the formation of dimers along the [1[1 with combining macron]0] direction is favourable, in agreement with published experimental results. However, the formation of extended NO rows is found to be unstable. Regarding the O(2 × 1)/Cu(110) substrate, we observe that NO molecules adsorb in between the Cu–O chains, causing a substantial disruption of the surface structure. Although individual molecules can be tilted with negligible energetic cost along the direction of the Cu–O chains, the interaction among neighbouring molecules was found to be repulsive along all directions and, consequently, the formation of dimers unfavourable.The authors acknowledge support from the Spanish Ministerio de Economa y Competitividad (MINECO) (Grant No. MAT2013-46593-C6-2-P), the Basque Departamento de Educación and the UPV/EHU (Grant No. IT-756-13), and the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 1083. AXBR acknowledges support from the Basque Departamento de Educación and the UPV/EHU through a Zabalduz grant.Peer reviewe

Lateral interactions and order–disorder phase transitions of metal phthalocyanines on Ag(111)

The intermolecular interaction among phthalocyanine molecules deposited on Ag(111) has been investigated at submonolayer coverages. By means of infrared absorption spectroscopy and spot-profile analysis low-energy electron diffraction, unambiguous evidence for an attractive interaction between neighboring titanyl-phthalocyanine (TiOPc) molecules is found for dilute layers, which is in contrast to the intermolecular repulsion reported for the copper-phthalocyanine (CuPc)/Ag(111) system. Accordingly, the formation of dense two-dimensional islands of TiOPc molecules with a well-defined long-range order (commensurate phase) is observed upon cooling to temperatures below 75 K, while a disordered arrangement is retained for adsorbed CuPc. Using density functional theory (DFT) calculations as a guide, these differences have been traced to the competition of attractive van der Waals and repulsive electrostatic interactions. Specifically, the vertically oriented molecular dipoles are substantially smaller for adsorbed TiOPc as compared to CuPc due to a compensation of the oppositely oriented dipole moments arising from the Pauli push-back effect on the one hand and the internal molecular dipole associated with the axial Ti–O group on the other. A detailed account and comparison of the molecule–substrate interaction, molecular binding geometries, and intermolecular interactions of TiOPc and CuPc on the Ag(111) surface are provided in a theoretical analysis based on DFT.We gratefully acknowledge support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), Project ID 223848855-SFB 1083 “Structure and Dynamics of Internal Interfaces”. L.F. acknowledges financial support from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement MagicFACE no. 797109. D.S.-P. and A.X.B.-R. acknowledge the Spanish Agencia Estatal de Investigación (grant nos. MAT2016-78293-C6-4-R and PID2019-107338RB-C66) and Dep. Educación of the Basque Government and UPV/EHU (grant no. IT1246-19) for support.Peer reviewe

Atomic-scale forces induced by a hydrogen molecule trapped in a tunneling junction

Hydrogen molecules can be trapped in the nanocavity formed by the tip of a scanning tunneling microscope and a metal or molecular surface, and produce sharp inelastic non-linearities in the tunneling spectra. Here, we study the interaction effects of hydrogen in a tunneling junction created over Manganese phthalocyanines molecules in two oxidation states. The effect of hydrogen in the tunneling spectra varies strongly depending on the molecular species, but its force spectrum is fairly independent on the molecular state. We find that in mild tunneling conditions hydrogen-induced forces are weakly attractive during a small range of tip sample distance. The van der Waals interaction shows a maximum value of 140 pN, which faintly depends on electrostatic variations along the surface. These results show that AFM can be employed to resolve the complex interaction landscape of a trapped hydrogen molecule and deduce fainter effects such as molecular deformations or dipolar fields.The research was financially supported by the Spanish Ministerio de Economia y Competitividad, MINECO (Grant FIS 2015-62538-ERC, MAT2016-78293-C6, and the Maria de Maeztu Units of Excellence Programme MDM-2016-0618), the Basque Government (Departamento de Industria, grant no. PI-2015-1-42, and Departamento de Educación and UPV/EHU, grant No. IT-756-13), and the European Regional Development Fund (ERDF).Peer Reviewe

Adsorption site dependence of vibrational excitations of molecular hydrogen

Resumen del póster presentado a la 15th International Conference on Vibrations at Surfaces, celebrada en Donsotia-San Sebastián (España) del 22 al 26 de junio de 2015.Transition-metal phtalocyanines are a well-known class of molecules used as model to study the interaction between metal surfaces and metal-organic compounds. These kind of metal-organic complexes present a wide range of properties and functionalities which depend on the coordination of their central metal ion, such as magnetism or the adsorption of small gas molecules. In this work we study Chlorinated Manganese Phtalocyanine (Cl-MnPc) molecules deposited on a Ag (111) substrate. We explore the adsorption characteristics of this system by means of a combined Low Temperature Scanning Tunneling and Atomic Force Microscope. After deposition on a room temperature substrate, a fraction of (dechlorinated) MnPc molecules coexist with Cl-MnPc on the surface. Moreover, Cl-MnPc can be controllably dechlorinated after the evaporation process. We find that both molecules are a preferential site of adsorption for molecular Hydrogen, which is known to present a bistable vibrationally mediated behavior depending on its different adsorption configurations. Inelastic tunneling of electrons from a STM can excite such bistability which induces a fingerprint close to zero-bias on differential conductance measurements. Additionally, force spectra reveal differences on the electrostatic forces exerted between the tip and the molecule when the tunneling electrons trigger such hydrogen fluctuations. We find that these fingerprints are strongly modified by the presence or absence of Chlorine atoms in the phtalocyanine molecules.Peer reviewe

Structural and vibrational properties of the TiOPc monolayer on Ag(111)

The evolution of titanyl-phthalocyanine (TiOPc) thin films on Ag(111) has been investigated using IRAS (infrared absorption spectroscopy), SPA-LEED (low energy electron diffraction) and STM (scanning tunneling microscopy). In the (sub)monolayer regime various phases are observed that can be assigned to a 2D gas, a commensurate and a point-on-line phase. In all three phases the nonplanar TiOPc molecule is adsorbed on Ag(111) in an oxygen-up configuration with the molecular π-conjugated backbone oriented parallel to the surface. The commensurate phase reveals a high packing density, containing two molecules at inequivalent adsorption sites within the unit cell. Both molecules assume different azimuthal orientations with respect to the Ag(111) substrate which is ascribed to the interplay between the occupation of preferred sites as well as distinct azimuthal orientations and a short ranging Pauli-Type repulsion to avoid overlapping of neighboring molecules. At full saturation of the monolayer the latter interaction leads to a further azimuthal reorientation that enables an even higher packing density. Thereby, the plain commensurate order is lost and a point-on-line phase is formed. DFT calculations have been used to study different adsorption geometries of TiOPc on Ag(111). The most stable configurations among those with pointing up oxygen atoms (bridge+, bridgex, topx) seem to correspond to those identified experimentally. The calculated dependence of the electronic structure and molecular dipole on the adsorption site and configuration is found to be rather small.We gratefully acknowledge support from the Deutsche Forschungsgemeinschaft DFG (Germany) through the collaborative research center “Structure and Dynamics of Internal Interfaces’’ (SFB 1083, Projects A2, A3, and GP1). DSP acknowledges support from the Spanish MINECO (Grant No. MAT2013-46593-C6-2-P), the Basque Departamento de Educacion, and the UPV/EHU (Grant No. IT-756-13).Peer Reviewe