J. A. Martín-Gago

Interplay between Fast Diffusion and Molecular Interaction in the Formation of Self-Assembled Nanostructures of S-Cysteine on Au(111)

We have studied the first stages leading to the formation of self-assembled monolayers of S-cysteine molecules adsorbed on a Au(111) surface. Density functional theory (DFT) calculations for the adsorption of individual cysteine molecules on Au(111) at room temperature show low-energy barriers all over the 2D Au(111) unit cell. As a consequence, cysteine molecules diffuse freely on the Au(111) surface and they can be regarded as a 2D molecular gas. The balance between molecule-molecule and molecule-substrate interactions induces molecular condensation and evaporation from the morphological surface structures (steps, reconstruction edges, etc.) as revealed by scanning tunnelling microscopy (STM) images. These processes lead progressively to the formation of a number of stable arrangements, not previously reported, such as single-molecular rows, trimers, and 2D islands. The condensation of these structures is driven by the aggregation of new molecules, stabilized by the formation of electrostatic interactions between adjacent NH(3)(+) and COO(-) groups, together with adsorption at a slightly more favorable quasi-top site of the herringbone Au reconstruction.

Understanding atomic-resolved STM images on TiO2(110)-(1 × 1) surface by DFT calculations

We present a combination of experimental STM images and DFT calculations to understand the atomic scale contrast of features found in high-resolution STM images. Simulating different plausible structural models for the tip, we have been able to reproduce various characteristics previously reported in experimental images on TiO(2)(110)-(1 x 1) under controlled UHV conditions. Our results allow us to determine the influence of different chemical and morphological tip terminations on the atomic-resolution STM images of the TiO(2)(110)-(1 x 1) surface. The commonest images have been properly explained using standard models for a W tip, either clean or with a single O atom located at the apex. Furthermore, a double transfer of oxygen atoms can account for different types of bizarre atomic-resolution features occasionally seen, and not conclusively interpreted before. Importantly, we discuss how typical point-defects are imaged on this surface by different tips, namely bridging O vacancies and adsorbed OH groups.

Ortho and Para Hydrogen Dimers on G/SiC(0001): Combined STM and DFT Study

The hydrogen (H) dimer structures formed upon room-temperature H adsorption on single layer graphene (SLG) grown on SiC(0001) are addressed using a combined theoretical-experimental approach. Our study includes density functional theory (DFT) calculations for the full (6√3 × 6√3)R30° unit cell of the SLG/SiC(0001) substrate and atomically resolved scanning tunneling microscopy images determining simultaneously the graphene lattice and the internal structure of the H adsorbates. We show that H atoms normally group in chemisorbed coupled structures of different sizes and orientations. We make an atomic scale determination of the most stable experimental geometries, the small dimers and ellipsoid-shaped features, and we assign them to hydrogen adsorbed in para dimers and ortho dimers configuration, respectively, through comparison with the theory.

Surface scienceLEED structural determination of the c(2 × 2)SiCu(110) surface alloy

Abstract Deposition of half atomic layer of Si on the Cu(110) surface leads to the formation of a stable c(2 × 2) superstructure. Our structural study by quantitative analysis of low-energy electron diffraction intensities finds a two-dimensional alloyed top layer in which Si atoms occupy substitutional Cu sites and are displaced inwards by 0.24 A with respect to the surface Cu atoms. An oscillatory behavior of the interlayer spacing is observed in the last three layers: the topmost surface layer exhibits a contraction of 15%, the second an expansion of 9% and the third a contraction of 3%. The third layer presents a corrugation of 0.1 A.Deposition of half atomic layer of Si on the Cu(110) surface leads to the formation of a stable c(2 × 2) superstructure. Our structural study by quantitative analysis of low-energy electron diffraction intensities finds a two-dimensional alloyed top layer in which Si atoms occupy substitutional Cu sites and are displaced inwards by 0.24 A with respect to the surface Cu atoms. An oscillatory behavior of the interlayer spacing is observed in the last three layers: the topmost surface layer exhibits a contraction of 15%, the second an expansion of 9% and the third a contraction of 3%. The third layer presents a corrugation of 0.1 A.

Formation and stability of the Cu(110)+c(2◊2)-Si surface alloy studied by high resolution XPS

High-resolution synchrotron radiation photoemission has been used to investigate the formation of the Cu(110)+c(2◊2)-Si surface alloy. The complex spectra of the Si 2p core-level are analyzed as multiple component spectra for diVerent Si coverages and annealing temperatures of the surface alloy. The results show that c(2◊2) islands are formed from the very beginning of the growth and that Si has a high diVusion length on Cu. The thermal stability of the surface alloy has been studied by measuring real-time photoemission spectra at diVerent temperatures. The surface alloy is stable up to 180°C. Above this temperature disruption of the surface alloy and clustering of the Si atoms can be observed.

Patterson function from low-energy electron diffraction measured intensities and structural discrimination

Surface patterson functions have been derived by direct inversion of experimental low-energy electron diffraction I-V spectra measured at multiple incident angles. The direct inversion is computationally simple and can beused to discriminate between different structural models. (1×1) YSi 2 epitaxial layers grown on Si(111) have been used to illustrate the analysis. We introduce a suitable R factor for the Patterson function to make the structural discrimination as objective as possible. From six competing models needed to complete the geometrical search, four could easily be discarded, achieving a very significant and useful reduction in the parameter space to be explored by standard dynamical low-energy electron diffraction methods. The amount and quality of data needed for this analysis is discussed.

Formation of the Si/Cu interface

The growth of evaporated Si on polycrystalline Cu has been investigated by means of ultra-violet photoemission spectroscopy, Auger electron spectroscopy (AES) and atomic force microscopy techniques. Thus, by analyzing the AES intensity evolution with Si coverage, AES lineshape and energy position of the main valence band features, we have found that room-temperature Si deposition leads to the formation of a reacted phase ∼20 A thick, most likely corresponding to the eutectic Cu 3 Si. On top of this phase grows an amorphous Si layer. In this paper we study the difference between the height of the Schottky barrier formed on the reacted phase and that obtained on an atomically sharp interface, and some reported values.

The Y–Si(111) interface formation studied by scanning tunneling microscopy

Abstract We have investigated the formation of the Y–Si(111) interface at room temperature by the scanning tunneling microscopy (STM) technique. The nucleation of the yttrium is localized to the inner part of every subunit cell of the (7×7) reconstruction. Thus, small silicide-like amorphous islands are formed by reaction of the yttrium atoms with the adatoms and dangling bonds of the rest atoms. By analyzing the voltage dependence of the STM images we have concluded that these nuclei present a semiconducting behavior. For coverages around one monolayer the nuclei coalesce, conferring a metallic character to the interface.

Identification of PAH Isomeric Structure in Cosmic Dust Analogs: The AROMA Setup

We developed a new analytical experimental setup called AROMA (Astrochemistry Research of Organics with Molecular Analyzer) that combines laser desorption/ionization techniques with ion trap mass spectrometry. We report here on the ability of the apparatus to detect aromatic species in complex materials of astrophysical interests and characterize their structures. A limit of detection of 100 femto-grams has been achieved using pure polycyclic aromatic hydrocarbon (PAH) samples, which corresponds to 2x108 molecules in the case of coronene (C24H12). We detected the PAH distribution in the Murchison meteorite, which is made of a complex mixture of extraterrestrial organic compounds. In addition, collision induced dissociation experiments were performed on selected species detected in Murchison, which led to the first firm identification of pyrene and its methylated derivatives in this sample.

Structure of Rutile TiO{sub 2} (110)-(1x2): Formation of Ti{sub 2}O{sub 3} Quasi-1D Metallic Chains

Combining STM, LEED, and density functional theory, we determine the atomic surface structure of rutile TiO{sub 2} (110)-(1x2): nonstoichiometric Ti{sub 2}O{sub 3} stripes along the [001] direction. LEED patterns are sharp and free of streaks, while STM images show monatomic steps, wide terraces, and no cross-links. At room temperature, atoms in the Ti{sub 2}O{sub 3} group have large amplitudes of vibration. The long quasi-1D chains display metallic character, show no interaction between them, and cannot couple to bulk or surface states in the gap region, forming good atomic wires.The 10.15 MeV resonance in {sup 10}Be has been probed via resonant {sup 6}He+{sup 4}He elastic scattering. It is demonstrated that it is the J{sup {pi}}=4{sup +} member of a rotational band built on the 6.18 MeV 0{sup +} state. A {gamma}{sub {alpha}} of 0.10-0.13 MeV and {gamma}{sub {alpha}}/{gamma}=0.35-0.46 were deduced. The corresponding reduced {alpha} width, {gamma}{sub {alpha}}{sup 2}, indicates one of the largest {alpha}-cluster spectroscopic factors known. The deformation of the band, including the 7.54 MeV, 2{sup +} member, is large (({Dirac_h}/2{pi}){sup 2}/2I=200 keV). Such a deformation and the significant degree of clusterization signals a well-developed {alpha} ratio 2n ratio {alpha} molecular structure.The mixed occurrence of s-wave and p-wave contributions in a first forbidden unique Gamow-Teller {beta} decay has been investigated for the first time by measuring the beta environmental fine structure (BEFS) in a {sup 187}Re crystalline compound. The experiment has been carried out with an array of eight AgReO{sub 4} thermal detectors operating at a temperature of {approx}100 mK. A fit of the observed BEFS spectrum indicates the p-wave electron emission as the dominant channel. The complete understanding of the BEFS distortion of the {sup 187}Re {beta} decay spectrum is crucial for future experiments aiming at the precise calorimetric measurement of the antineutrino mass.