Daniel Farías

Atomic beam diffraction from solid surfaces

Atomic beam techniques are presently being used in many branches of surface physics such as studies of the particle-surface physisorption potential, surface structure, surface phonons, nucleation and growth on metal and insulator surfaces, surface diffusion and accommodation and sticking of molecules. This review concentrates on diffractive phenomena from surfaces, which up to now were investigated mainly with helium. The theoretical background for diffraction calculations is outlined and representative examples of different applications are given. The main subjects covered are: structural determinations of chemisorbed and physisorbed systems, investigations of disordered surfaces, selective adsorption resonances, diffusion and nucleation studies and investigations of growth and phase transitions on surfaces. Diffraction results obtained with Ne, Ar, and are also summarized.

Low-energy acoustic plasmons at metal surfaces

Nearly two-dimensional (2D) metallic systems formed in charge inversion layers and artificial layered materials permit the existence of low-energy collective excitations, called 2D plasmons, which are not found in a three-dimensional (3D) metal. These excitations have caused considerable interest because their low energy allows them to participate in many dynamical processes involving electrons and phonons, and because they might mediate the formation of Cooper pairs in high-transition-temperature superconductors. Metals often support electronic states that are confined to the surface, forming a nearly 2D electron-density layer. However, it was argued that these systems could not support low-energy collective excitations because they would be screened out by the underlying bulk electrons. Rather, metallic surfaces should support only conventional surface plasmons—higher-energy modes that depend only on the electron density. Surface plasmons have important applications in microscopy and sub-wavelength optics, but have no relevance to the low-energy dynamics. Here we show that, in contrast to expectations, a low-energy collective excitation mode can be found on bare metal surfaces. The mode has an acoustic (linear) dispersion, different to the dependence of a 2D plasmon, and was observed on Be(0001) using angle-resolved electron energy loss spectroscopy. First-principles calculations show that it is caused by the coexistence of a partially occupied quasi-2D surface-state band with the underlying 3D bulk electron continuum and also that the non-local character of the dielectric function prevents it from being screened out by the 3D states. The acoustic plasmon reported here has a very general character and should be present on many metal surfaces. Furthermore, its acoustic dispersion allows the confinement of light on small surface areas and in a broad frequency range, which is relevant for nano-optics and photonics applications.

Synthesis of a weakly bonded graphite monolayer on Ni(111) by intercalation of silver

Silver has been successfully intercalated underneath a monolayer of graphite (MG) adsorbed on Ni(111) by deposition on the MG/Ni(111) surface at room temperature and subsequent annealing to 350-400 °C. The surface phonon dispersion of the MG/Ag/Ni(111) system has been measured in the direction of the Brillouin zone using high resolution electron energy loss spectroscopy. The dispersion curves were found to be almost identical to those of bulk graphite, which is in contrast to the softened graphite-like phonon modes observed for the MG/Ni(111) system. This suggests that the stiffening of the phonon modes induced by silver intercalation is caused by a weaker interaction of the states of graphite with the substrate. These results demonstrate that a weakly bonded graphite monolayer, whose chemical properties are very similar to those of bulk graphite and which is stable up to 400 °C, can be synthesized in situ on Ni(111) after intercalation of silver.

Modification of the surface phonon dispersion of a graphite monolayer adsorbed on Ni(111) caused by intercalation of Yb, Cu and Ag

Abstract The surface phonon dispersion of a monolayer of graphite (MG) on Ni(111) has been measured in the ΓK direction of the Brillouin zone by means of high-resolution electron energy-loss spectroscopy (HREELS). The phonon dispersion relations of the MG/Ni(111) system and those obtained after intercalation of Yb are characterized by graphite-like phonon modes, softened due to the strong interaction with the Ni substrate. In the case of Cu and Ag intercalation, in contrast, the corresponding surface dispersion curves are very similar to those of bulk graphite. Calculations of the surface phonon dispersion based on a force constant model revealed that the force constants related to vertical motion in the MG are very much more affected after intercalation than those related to horizontal vibrations. This demonstrates that the stiffening observed after Cu and Ag intercalation is caused by a weaker interaction of the graphite layer with the Ni substrate.

The transition of chemisorbed hydrogen into subsurface sites on Pd(311)

The activated transition of chemisorbed hydrogen atoms into subsurface sites on Pd(311) has been investigated by means of He-atom scattering, high resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS) and work function measurements. At 120 K, hydrogen exposure leads to the formation of (2×1)H, (2×1)2H, (2×1)3H and c(1×1) 2H phases, with coverages of 0.25, 0.50, 0.75, and 1 monolayers (ML), respectively. The TDS data show three desorption states: α at ∼170 K, β1 at ∼285 K and β2 at ∼310 K. Chemisorbed H atoms forming the ordered layers desorb in the β2 state, whereas the β1 is originated by H atoms located at subsurface sites. The α state is originated by decomposition of layers of Pd hydride near the surface. In all four phases, long-range order disappears at ∼170 K. Heating to 220 K leads to the migration of 0.25 ML H atoms into subsurface sites only if the coverage of the disordered layer is greater than 0.5 ML. The HREELS data demonstrate that this behavior is cau...

Diffractive and reactive scattering of H2 from Ru(0001): experimental and theoretical study

We present a combined experimental and theoretical study of the diffraction of H(2) from Ru(0001) in the incident energy range 78-150 meV, and a theoretical study of dissociative chemisorption of H(2) in the same system. Pronounced out-of-plane diffraction was observed in the whole energy range studied. The energy dependence of the elastic diffraction intensities was measured along the two main symmetry directions for a fixed parallel translational energy. The data were compared with quantum dynamics calculations performed by using DFT-based, six-dimensional potential energy surfaces calculated with both the PW91 and RPBE functionals, as well as with a functional obtained from a weighted average of both (the MIX functional, which was earlier shown to perform quite well for H(2) + Cu(111)). Our results show that the PW91 functional describes the H(2) diffraction intensities more accurately than the RPBE and the MIX functionals, although the absolute values of these intensities are overestimated in the calculations. For the reaction probabilities a preference for one or the other functional cannot be given over the entire energy range probed by the sticking experiments. The PW91 functional yields too high reaction probabilities over the entire investigated energy range, but is better than RPBE at low collision energies (<0.1 eV). The RPBE functional gives too low reaction probabilities at low energy and somewhat too high reaction probabilities at high energy, but agrees better with experiment than PW91 for energies >0.1 eV. The results suggest that, in order to get a better description of both H(2) diffraction and dissociative chemisorption for this system, a specific reaction parameter functional for H(2) + Ru(0001) is needed that is a weighted average of functionals other than PW91 and RPBE. We speculate that differences between the H(2) + Ru(0001) system (early and low reaction barrier) and H(2) + Cu(111) (late and high reaction barrier) may well lead to fundamentally different specific reaction parameter functionals, and that including a reasonable accurate description of the van der Waals interaction might be important for H(2) + Ru(0001) which has barriers localised far away from the surface. Based on our results we advocate new, systematic combined theoretical and experimental studies of H(2) interacting with transition metals in early and late barrier systems, with the aim of determining whether specific reaction parameter functionals for these systems might differ in a systematic way.

Unveiling the Mechanisms Leading to H2 Production Promoted by Water Decomposition on Epitaxial Graphene at Room Temperature.

By means of a combination of surface-science spectroscopies and theory, we investigate the mechanisms ruling the catalytic role of epitaxial graphene (Gr) grown on transition-metal substrates for the production of hydrogen from water. Water decomposition at the Gr/metal interface at room temperature provides a hydrogenated Gr sheet, which is buckled and decoupled from the metal substrate. We evaluate the performance of Gr/metal interface as a hydrogen storage medium, with a storage density in the Gr sheet comparable with state-of-the-art materials (1.42 wt %). Moreover, thermal programmed reaction experiments show that molecular hydrogen can be released upon heating the water-exposed Gr/metal interface above 400 K. The Gr hydro/dehydrogenation process might be exploited for an effective and eco-friendly device to produce (and store) hydrogen from water, i.e., starting from an almost unlimited source.

Quadratic Dispersion and Damping Processes of π Plasmon in Monolayer Graphene on Pt(111)

High-resolution electron energy-loss spectroscopy has been used to study the π plasmon in monolayer graphene grown on Pt(111). A quadratic dispersion has been observed, in contrast to the linear dispersion reported for monolayer graphene grown on SiC(0001) and in agreement with recent experiments on graphene/Ni(111). Despite the weak interaction of the monolayer graphene with the Pt(111) surface, our results indicate that the screening by the underlying metal substrate strongly influences both the dispersion relation and the damping processes of the plasmon mode of π electrons.

Phonon dynamics of graphene on metals.

The study of surface phonon dispersion curves is motivated by the quest for a detailed understanding of the forces between the atoms at the surface and in the bulk. In the case of graphene, additional motivation comes from the fact that thermal conductivity is dominated by contributions from acoustic phonons, while optical phonon properties are essential to understand Raman spectra. In this article, we review recent progress made in the experimental determination of phonon dispersion curves of graphene grown on several single-crystal metal surfaces. The two main experimental techniques usually employed are high-resolution electron energy loss spectroscopy (HREELS) and inelastic helium atom scattering (HAS). The different dispersion branches provide a detailed insight into the graphene-substrate interaction. Softening of optical modes and signatures of the substrates Rayleigh wave are observed for strong graphene-substrate interactions, while acoustic phonon modes resemble those of free-standing graphene for weakly interacting systems. The latter allows determining the bending rigidity and the graphene-substrate coupling strength. A comparison between theory and experiment is discussed for several illustrative examples. Perspectives for future experiments are discussed.

A classical dynamics method for H2 diffraction from metal surfaces.

We present a discretization method that allows one to interpret measurements on diffraction of diatomic molecules from solid surfaces using six-dimensional (6D) classical trajectory calculations. It has been applied to the D2NiAl(110) and H2Pd(111) systems (which are models for activated and nonactivated dissociative chemisorption, respectively) using realistic potential energy surfaces obtained from first principles. Comparisons with experimental results and 6D quantum dynamical calculations show that, in general, the method is able to predict the relative intensity of the most important diffraction peaks. We therefore conclude that classical mechanics can be an efficient guide for experimentalists in the search for the most significant diffraction channels.