Spin- and energy-dependent tunneling through a single molecule with intramolecular spatial resolution

We investigate the spin- and energy dependent tunneling through a single organic molecule (CoPc) adsorbed on a ferromagnetic Fe thin film, spatially resolved by low-temperature spin-polarized scanning tunneling microscopy. Interestingly, the metal ion as well as the organic ligand show a significant spin-dependence of tunneling current flow. State-of-the-art ab initio calculations including also van-der-Waals interactions reveal a strong hybridization of molecular orbitals and surface 3d states. The molecule is anionic due to a transfer of one electron, resulting in a non-magnetic (S= 0) state. Nevertheless, tunneling through the molecule exhibits a pronounced spin-dependence due to spin-split molecule-surface hybrid states.Comment: Version of Submission, 18-03-201

Thermodynamics of the Flexible Metal-Organic Framework Material MIL-53(Cr) From First Principles

We use first-principles density functional theory total energy and linear response phonon calculations to compute the Helmholtz and Gibbs free energy as a function of temperature, pressure, and cell volume in the flexible metal-organic framework material MIL-53(Cr) within the quasiharmonic approximation. GGA and metaGGA calculations were performed, each including empirical van der Waals (vdW) forces under the D2, D3, or D3(BJ) parameterizations. At all temperatures up to 500 K and pressures from -30 MPa to 30 MPa, two minima in the free energy versus volume are found, corresponding to the narrow pore ($np$) and large pore ($lp$) structures. Critical positive and negative pressures are identified, beyond which there is only one free energy minimum. While all results overestimated the stability of the $np$ phase relative to the $lp$ phase, the best overall agreement with experiment is found for the metaGGA PBEsol+RTPSS+U+J approach with D3 or D3(BJ) vdW forces. For these parameterizations, the calculated free energy barrier for the $np$-$lp$ transition is only 3 to 6 kJ per mole of Cr$_4$(OH)$_4$(C$_8$H$_4$O$_4$)$_4$

Theoretical characterization in the functionalization and design of low dimensional systems: carbon transition metal nanostructures and phosphorene isoelectronic compounds

167 p.El trabajo de esta tesis consiste en el estudio teórico mediante cálculos computacionalesbasados en la teoría del funcional de la densidad (DFT) de las propiedades electrónicas ymagnéticas de nanomateriales. Dentro de los nanomateriales destaca el grafeno, unmaterial bidimensional constituido por una red hexagonal de átomos de carbono. Se haestudiado el efecto que tiene en las propiedades electrónicas y magnéticas el dopado degrafeno con impurezas sustitucionales de metales de transición, así como el enlace entreagregados metálicos de cobalto y grafeno.La interacción entre metales de transición y átomos de carbono en una red hexagonal tipografeno se da también en los hidrocarburos aromáticos policíclicos. En esta tesis hemosestudiado esta interacción para el circumcoronene, compuesto por 19 anillos aromáticos,con átomos y agregados pequeños de cobalto. De igual manera se ha estudiado lainteracción de los metales de transición manganeso, cobalto y níquel con moléculas debenceno, la unidad más pequeña con un hexágono de carbonos, dando lugar aestructuras con el agregado metálico recubierto por los bencenos. En estas estructurashemos encontrado comportamientos novedosos como distancias ultracortas de 1.8 A parael enlace manganeso-manganeso, soluciones magnéticas no colineales para el caso delcobalto, o cambios en el eje fácil de magnetización para el caso del níquel.Por otro lado, hoy en día se trata de encontrar nuevos materiales bidimensionalessiguiendo el camino marcado por el grafeno o más recientemente por una capa de fósforonegro. En esta tesis estudiamos dos nuevos materiales isoelectrónicos (con el mismonúmero de electrones de valencia) al fósforo negro, silicio azufre y carbono azufre,caracterizando diferentes nanoestructuras como monocapas, películas delgadas ycadenas. Encontramos fases semiconductoras y metálicas estables a temperaturaambiente que podrían ser utilizadas para aplicaciones en el futuro

A Study of Potential Organometallic Photosensitizers for TiO2 Using Density Functional Theory

Sunlight is ubiquitous and reliable. Photocatalysis is a promising use for it, with many environmental benefits. One issue with titanium dioxide, a desirable photocatalyst, is an inability to absorb visible light. Attaching organometallics to titanium dioxide may improve photoexcitation. We used density functional theory to model an anatase surface and adsorbates. Our results indicate that carbonyl, as in iron pentacarbonyl and chromium hexacarbonyl, binds poorly to anatase. Halides such as in iron(II) tricarbonyl dibromide improve bonding and reduce required photoexcitation energy. Cyanide, as in tetracyanonickelate and tetracyanopalladate, has greater potential, reducing required energy further. Our results also indicate photocatalysis can be fine-tuned through choice of metal center

Looking into Metal-Organic Frameworks with Solid-State NMR Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for characterization of materials. It can detect local structure around selected atomic nuclei and provide information on the dynamics of these nuclei. In case of metal-organic frameworks, NMR spectroscopy can help elucidate the framework structure, locate the molecules adsorbed into the pores, and inspect and characterize the interactions of these molecules with the frameworks. The present chapter discusses selected recent examples of solid-state NMR studies that provide valuable insight into the structure and function of metal-organic frameworks

Chemical Self-Assembly Strategies Toward the Design of Molecular Electronic Circuits

The field of molecular electronics is generally divided into one of two major categories, the first focusing on the unique functionalization of single molecules to produce electronic behavior, the other utilizing large assemblies of molecules to produce electronic behavior. The former approach is largely attributed to the seminal paper by Aviram and Ratner in which they proposed a molecular donor-bridge-acceptor (D-B-A) type architecture could lead to single molecule rectification producing electronic effects similar to conventional semiconductor based diodes. Extensive research has been carried out in both fields as it is foreseen that new approaches to electronics miniaturization will be necessary in the near future. In the following research, the focus turns to a seemingly overlooked area of molecular electronics, this being the necessity for designed interconnects of nanoscale electrodes. The approach to problem utilized the well studied oligomerization properties of 1,4-phenylene diisocyanide (PDI), which upon exposure to gold incorporates gold adatoms to form conductive one-dimensional oligomers of the form -(Au-PDI)n- Monte Carlo simulations along with conductivity studies of nanoparticle arrays both suggest the oligomerization is inherently self-limiting, providing a potential avenue toward controlled interconnection of nanoelectrodes and design of molecular electronic circuits

Computational investigations of the spectroscopy, vibronic coupling, and photo(stereo)chemistry in inorganic systems

This thesis focuses on the spectroscopy and photo-stereochemistry of relatively large closed-shell and open-shell transition metal complexes, investigated with an array of modern computational methodologies. The presence of the metal electrons/orbitals results in a greater number of low-lying excited states, and these states are vibronically coupled resulting in Jahn-Teller or pseudo-Jahn-Teller (pJT) effects, or general surface crossings. These features are very challenging to calculate but are vitally important to explain the observed behavior in such systems. Computational investigations using the multiconfigurational CASSCF method on the pJT effect occurring in ammonia, and Mo2(DXylF)2(O2CCH3)2(μ2-O)2 complex are presented. These definitively show that in the latter case the experimentally observed structure is due to a vibronic coupling of the ground electronic state with that of a nondegenerate 1πδ* state, resulting in a rhomboidal rather than square motif at the bimetallic centre. The (BQA)PtMe2I (BQA= bis(8-quinolinyl)amide) complex has been found to undergo unexpected meridial to facial isomerisation induced by light. The TD-DFT method was used to examine the spectroscopy of this system, and the CASSCF method was used to examine excited state relaxation pathways. The system relaxes on an excited state potential energy surface, of an essentially localised ππ* excited state of the BQA ligand, and reaches a facial excited minimum that is located adjacent to a sloped conical intersection connecting the excited and ground electronic states. Chromium (III) complexes have been investigated for many years and many aspects of their photochemistry are still not very well understood. The photochemistry of paradigm Cr (III) complexes, such as chromium oxalate [Cr(C2O4)3]3-, chromium tris- (1,3diaminopropane) [Cr(tn)3]3+ and Cr(tn)2(CN)2, have been investigated using TDDFT and CASSCF methods. Non-radiative relaxation pathways have been documented showing mechanism of both internal conversion in the quartet manifold, as well as inter-system crossing into the doublet manifold. The results explain photostereochemical features of the photo-induced racemization of [Cr(C2O4)3]3- and the photoaquation of [Cr(tn)3]3+ and Cr(tn)2(CN)2.Engineering and Physical Sciences Research Council (EPSRC) grant No. EP/F01709

SPECTROSCOPY AND STRUCTURES OF METAL-CYCLIC HYDROCARBON COMPLEXES

Metal-cyclic hydrocarbon complexes were prepared in a laser-vaporization molecular beam source and studied by single-photon zero electron kinetic energy (ZEKE) and IR-UV resonant two-photon ionization (R2PI) spectroscopy. The ionization energies and vibrational frequencies of the metal complexes were measured from the ZEKE spectra. Metal-ligand bonding and low-lying electronic states of the neutral and ionized complexes were analyzed by combining the ZEKE measurements with density functional theory (DFT) calculations. In addition, C-H stretching frequencies were measured from the R2PI spectra. In this dissertation, metal complexes of 1, 3, 5, 7-cyclo-octatetraene (COT), toluene, p-xylene, mesitylene, hexamethylbenzene, biphenyl, naphthalene, pyrene, perylene, and coronene were studied. For each metal-ligand complex, different effects from the metal coordination have been identified. Although free COT is a nonaromatic molecule with a tub-shaped structure, the group III transition metal atoms (Sc, Y, and La) donate two electrons to a partially filled π orbital of COT, making the ligand a dianion. As a result, metal coordination converts COT into a planar, aromatic structure and the resulting complex exhibits a half-sandwich structure. For the Sc(methylbenzene) complexes, the benzene rings of the ligands are bent and the π electrons are localized in a 1, 4-diene fashion due to differential Sc binding with the carbon atoms of the rings. Due to differential metal binding, the degenerate d orbitals split and the Sc-methylbenzene complexes prefer the low-spin ground electronic states. In addition, as the number of methyl group substituents in the ligand increases, the ionization energies (IEs) of the Sc-methylbenzene complexes decrease. However, Ti, V, or Co coordination does not disrupt the delocalized π electron network within the carbon skeleton in the high-spin ground states of the metal complexes. For group VI metal (Cr, Mo, and W)-bis(toluene) complexes, methyl substitution on the benzene ring yields complexes with four rotational conformers of 0°, 60°, 120°, and 180° conformation angles between two methyl groups. In addition, variable-temperature ZEKE spectroscopy using He, Ar, or their mixtures has determined the totally eclipsed 0° rotamer to be the most stable. When there are two equivalent benzene rings, the metal (Ti, Zr or Hf) binds to both the benzene rings of biphenyl, or the metal (Li) binds to one of the benzene rings of naphthalene. On the other hand, the metal (Li) favors the ring-over binding site of the benzene ring with a higher π electron content and aromaticity in pyrene, perylene, and coronene