Jorge I. Cerdá

Extended Hückel theory for band structure, chemistry, and transport. I. Carbon nanotubes

We describe a semiempirical atomic basis extended Huckel theoretical (EHT) technique that can be used to calculate bulk band structure, surface density of states, electronic transmission, and interfacial chemistry of various materials within the same computational platform. We apply this method to study multiple technologically important systems, starting with carbon nanotubes and their interfaces and silicon-based heterostructures in our follow-up paper [D. Kienle et al., J. Appl. Phys. 100, 043715 (2006), following paper]. We find that when it comes to quantum transport through interesting, complex heterostructures including gas molecules adsorbed on nanotubes, the Huckel band structure offers a fair and practical compromise between orthogonal tight-binding theories with limited transferability between environments under large distortion and density functional theories that are computationally quite expensive for the same purpose.

Large Conductance Switching in a Single-Molecule Device through Room Temperature Spin-Dependent Transport

Controlling the spin of electrons in nanoscale electronic devices is one of the most promising topics aiming at developing devices with rapid and high density information storage capabilities. The interface magnetism or spinterface resulting from the interaction between a magnetic molecule and a metal surface, or vice versa, has become a key ingredient in creating nanoscale molecular devices with novel functionalities. Here, we present a single-molecule wire that displays large (>10000%) conductance switching by controlling the spin-dependent transport under ambient conditions (room temperature in a liquid cell). The molecular wire is built by trapping individual spin crossover Fe(II) complexes between one Au electrode and one ferromagnetic Ni electrode in an organic liquid medium. Large changes in the single-molecule conductance (>100-fold) are measured when the electrons flow from the Au electrode to either an α-up or a β-down spin-polarized Ni electrode. Our calculations show that the current flowing through such an interface appears to be strongly spin-polarized, thus resulting in the observed switching of the single-molecule wire conductance. The observation of such a high spin-dependent conductance switching in a single-molecule wire opens up a new door for the design and control of spin-polarized transport in nanoscale molecular devices at room temperature.

Metal- and hydrogen-bonding competition during water adsorption on Pd(111) and Ru(0001).

The initial stages of water adsorption on the Pd(111) and Ru(0001) surfaces have been investigated experimentally by scanning tunneling microscopy in the temperature range between 40 and 130 K, and theoretically with density functional theory (DFT) total energy calculations and scanning tunneling microscopy (STM) image simulations. Below 125 K, water dissociation does not occur at any appreciable rate, and only molecular films are formed. Film growth starts by the formation of flat hexamer clusters where the molecules bind to the metal substrate through the O-lone pair while making H-bonds with neighboring molecules. As coverage increases, larger networks of linked hexagons are formed with a honeycomb structure, which requires a fraction of the water molecules to have their molecular plane perpendicular to the metal surface with reduced water-metal interaction. Energy minimization favors the growth of networks with limited width. As additional water molecules adsorb on the surface, they attach to the periphery of existing islands, where they interact only weakly with the metal substrate. These molecules hop along the periphery of the clusters at intermediate temperatures. At higher temperatures, they bind to the metal to continue the honeycomb growth. The water-Ru interaction is significantly stronger than the water-Pd interaction, which is consistent with the greater degree of hydrogen-bonded network formation and reduced water-metal bonding observed on Pd relative to Ru.

Supramolecular patterns controlled by electron interference and direct intermolecular interactions.

Whereas all 230 three-dimensional space groups occur in organic crystals, out of only 17 plane groups some highly symmetric ones such as p31m have not yet been observed in two-dimensional (2D) crystals of organic molecules. Here a kagome network with p31m symmetry is reported for cobalt phthalocyanine on Cu(111). This unusual structure results from substrate-induced reduction of molecular symmetry and substrate-mediated interaction via quantum interference of surface electrons. These interactions provide additional control over the symmetry of 2D crystals of phthalocyanines and lead to a variety of other symmetries in self-assembled arrays.

The structure of mixed H2O/OH monolayer films on Ru(0001)

Scanning tunneling microscopy (STM) and x-ray absorption spectroscopy (XAS) have been used to study the structures produced by water on Ru(0001) at temperatures above 140 K. It was found that while undissociated water layers are metastable below 140 K, heating above this temperature produces drastic transformations, whereby a fraction of the water molecules partially dissociate and form mixed H(2)O-OH structures. X-ray photoelectron spectroscopy and XAS revealed the presence of hydroxyl groups with their O-H bond essentially parallel to the surface. STM images show that the mixed H(2)O-OH structures consist of long narrow stripes aligned with the three crystallographic directions perpendicular to the close-packed atomic rows of the Ru(0001) substrate. The internal structure of the stripes is a honeycomb network of H-bonded water and hydroxyl species. We found that the metastable low temperature molecular phase can also be converted to a mixed H(2)O-OH phase through excitation by the tunneling electrons when their energy is 0.5 eV or higher above the Fermi level. Structural models based on the STM images were used for density functional theory optimizations of the stripe geometry. The optimized geometry was then utilized to calculate STM images for comparison with the experiment.

CoPc adsorption on Cu(111): Origin of the C4 to C2 symmetry reduction

The adsorption of phthalocyanines (Pc) to various surfaces has recently been reported to lead to a lowering of symmetry from C4 to C2 in scanning tunneling microscope (STM) images. Possible origins of the reduced symmetry involve the electronic structure or geometric deformation of the molecules. Here, the origin of the reduction is clarified from a comprehensive theoretical study of CoPc adsorbed on the Cu(111) surface along with the experimental STM data. Total energy calculations using different schemes for the exchange-correlation energy and STM simulations are compared against experimental data. We find that the symmetry reduction is only reproduced when van der Waals corrections are included into the formalism. It is caused by a deformation along the two perpendicular molecular axes, one of them coming closer to the surface by around 0.2 Å. An electronic structure analysis reveals (i) the relevance of the CoPc interaction with the Cu(111) surface state and (ii) that intramolecular features in dI/dV maps clearly discriminate a Co-derived state from the rest of the Pc states.

Tuning surface metallicity and ferromagnetism by hydrogen adsorption at the polar ZnO(0001) surface

Total energy calculations for the adsorption of hydrogen on the polar Zn-ended ZnO(0001) surface predict that a metal-insulator transition and the reversible switch of surface magnetism can be achieved by varying the hydrogen density on the surface. An on top

Extended Hückel theory for band structure, chemistry, and transport. II. Silicon

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Three-dimensional extended Hückel theory-nonequilibrium Green's function spin polarized transport model for Fe/MgO/Fe heterostructures

ordered overlayer with genuine H-Zn chemical bonds is shown to be energetically favorable. The

Structure and reactions of carbon and hydrogen on Ru(0001): A scanning tunneling microscopy study

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