César González

Charge injection through single and double carbon bonds.

The contact conductance of oriented C(60) molecules is probed with a scanning tunneling microscope as a function of the lateral position of the tip in contact to the molecular cage. Together with first principles calculations, these measurements reveal variations of the efficiency of charge injection to the fullerene molecule with the order of the contacted carbon-carbon bond.

Towards universal access to content using MPEG-7

This paper presents a system providing functionalities for cataloging multimedia content using MPEG-7 and accessing to content and descriptions. The cataloging application indexes content using MPEG-7 and creates annotated variations in order to have the capability of offering media content to a large amount of different terminals and through different access networks. The created multimedia database, both descriptions and content (original sources and variations), is used by, currently, two applications: a searching application, which allows a user selecting specific tags to find the desired media; and a filtering application for transparent access to the content database using profiles. These profiles can be selected from a profiles database or created by the user specifying content preferences, and network and terminal parameters.

Theoretical characterisation of point defects on a MoS2 monolayer by scanning tunnelling microscopy.

Different S and Mo vacancies as well as their corresponding antisite defects in a free-standing MoS2 monolayer are analysed by means of scanning tunnelling microscopy (STM) simulations. Our theoretical methodology, based on the Keldysh nonequilibrium Green function formalism within the density functional theory (DFT) approach, is applied to simulate STM images for different voltages and tip heights. Combining the geometrical and electronic effects, all features of the different STM images can be explained, providing a valuable guide for future experiments. Our results confirm previous reports on S atom imaging, but also reveal a strong dependence on the applied bias for vacancies and antisite defects that include extra S atoms. By contrast, when additional Mo atoms cover the S vacancies, the MoS2 gap vanishes and a bias-independent bright protrusion is obtained in the STM image. Finally, we show that the inclusion of these point defects promotes the emergence of reactive dangling bonds that may act as efficient adsorption sites for external adsorbates.

Helium bubble clustering in copper from first principles

The formation of helium clusters in a copper crystal has been studied by means of ab initio calculations. Several He atoms have been placed either inside an n vacancy previously created or as interstitials inside the initially perfect bulk matrix. Based on density functional theory techniques, our results show that the first He atom inside the perfect crystal prefers a tetrahedral position instead of an octahedral as previously reported. When n vacancies are formed in the structure, He atoms start to aggregate forming small bubbles at these sites rather than at interstitial positions. The calculated formation and binding energies confirm the deep trapping and the stability of He atoms inside vacancies, as is well known for other metals. For a given number of He atoms inside an n vacancy, NHe, the minimum formation energy is found when NHe is equal to the number of vacancies n. Within each n vacancy, the formation energy increases (almost) linearly with the number of He atoms until NHe reaches the number of vacancies n. From this point onwards, the addition of new He atoms to the system implies a higher energy cost and consequently an abrupt decrease in the binding energy.

H trapping and mobility in nanostructured tungsten grain boundaries: a combined experimental and theoretical approach

The trapping and mobility of hydrogen in nanostructured tungsten grain boundaries (GBs) have been studied by combining experimental and density functional theory (DFT) data. Experimental results show that nanostructured W coatings with a columnar grain structure and a large number of (1 1 0)/(2 1 1) interfaces retain more H than coarsed grained W samples. To investigate the possible influence of GBs on H retention, a complete energetic analysis of a non-coherent W(1 1 0)/W(1 1 2) interface has been performed employing DFT. Our results show that this kind of non-coherent interface largely attracts point defects (both a H atom and a metallic monovacancy separately) and that the presence of these interfaces contributes to a decrease in the migration energy of the H atoms with respect to the bulk value. When both the W monovacancy and H atom are introduced together into the system, the HV complex becomes the most stable configuration and one of the mechanisms explaining the H retention in the radiation damaged GB observed experimentally.

Pulling and Stretching a Molecular Wire to Tune its Conductance.

A scanning tunnelling microscope is used to pull a polythiophene wire from a Au(111) surface while measuring the current traversing the junction. Abrupt current increases measured during the lifting procedure are associated with the detachment of molecular subunits, in apparent contradiction with the expected exponential decrease of the conductance with wire length. Ab initio simulations reproduce the experimental data and demonstrate that this unexpected behavior is due to release of mechanical stress in the wire, paving the way to mechanically gated single-molecule electronic devices.

Reduction of the repulsive interaction as origin of helium trapping inside a monovacancy in BCC metals

We present the energetic, structural and electronic properties that explain the accumulation of He inside a single vacancy in both a BCC W and a BCC Nb crystals. Using density functional theory, we have obtained the most stable structures for an increasing number of He atoms within the monovacancy, with and without the presence of van der Waals (vdW) interactions. Our results show that the maximum number of He atoms that can be placed in the monovacancy is nine and suggest that the vdW interactions should be taken into account for higher He concentrations produced in bigger n-vacancies. The analysis of the density of states and the He–metal interaction reveals a reduction of the repulsion as the origin of He trapping inside the metallic vacancy. The formation energy grows almost linearly with the number of He atoms included, showing a strong dependence on the bulk modulus, while the binding energy presents a more complex behaviour. Finally, the deformations of the first (1NN), second (2NN) and third (3NN) nearest neighbour atomic distances to the vacancy change in different ways. The maximum deformation obtained (around 0.5 Å) is found with precisely nine He atoms inside the vacancy, just before the bubble collapses.

Theoretical study of carbon-based tips for Scanning Tunnelling Microscopy

Motivated by recent experiments, we present here a detailed theoretical analysis of the use of carbon-based conductive tips in scanning tunnelling microscopy. In particular, we employ ab initio methods based on density functional theory to explore a graphitic, an amorphous carbon and two diamond-like tips for imaging with a scanning tunnelling microscope (STM), and we compare them with standard metallic tips made of gold and tungsten. We investigate the performance of these tips in terms of the corrugation of the STM images acquired when scanning a single graphene sheet. Moreover, we analyse the impact of the tip-sample distance and show that it plays a fundamental role in the resolution and symmetry of the STM images. We also explore in depth how the adsorption of single atoms and molecules in the tip apexes modifies the STM images and demonstrate that, in general, it leads to an improved image resolution. The ensemble of our results provides strong evidence that carbon-based tips can significantly improve the resolution of STM images, as compared to more standard metallic tips, which may open a new line of research in scanning tunnelling microscopy.

Ab initio study of tungsten defects near the surface

A first principles analysis of the behaviour of point defects, namely, self-interstitial atoms, a single vacancy and light impurity atoms such as H and He in tungsten is reported. These defects can be produced in the rst wall of the future nuclear fusion reactors due to the high radiation uxes present. The evolution of defects that appear in the bulk and end up reaching the surface has been followed. An energetic study has been combined with a detailed charge density analysis of the system by means of the SIESTA code. The resulting data have been validated by confronting them with those obtained with a more precise plane wave code, namely VASP. Meanwhile, the structural and the mechanical properties of the system have been positively compared with experimental measurements. Such comparisons have led us to present a new SIESTA basis for tungsten. This complete analysis establishes a nanoscopic view of the phenomena involving the presence of light atoms at native defects in tungsten, paying special attention to the vicinity of surfaces.

Weakly Trapped, Charged, and Free Excitons in Single-Layer MoS2 in the Presence of Defects, Strain, and Charged Impurities

Few- and single-layer MoS2 host substantial densities of defects. They are thought to influence the doping level, the crystal structure, and the binding of electron-hole pairs. We disentangle the concomitant spectroscopic expression of all three effects and identify to what extent they are intrinsic to the material or extrinsic to it, i.e., related to its local environment. We do so by using different sources of MoS2-a natural one and one prepared at high pressure and high temperature-and different substrates bringing varying amounts of charged impurities and by separating the contributions of internal strain and doping in Raman spectra. Photoluminescence unveils various optically active excitonic complexes. We discover a defect-bound state having a low binding energy of 20 meV that does not appear sensitive to strain and doping, unlike charged excitons. Conversely, the defect does not significantly dope or strain MoS2. Scanning tunneling microscopy and density functional theory simulations point to substitutional atoms, presumably individual nitrogen atoms at the sulfur site. Our work shows the way to a systematic understanding of the effect of external and internal fields on the optical properties of two-dimensional materials.