Petar Pervan

Dirac Cones and Minigaps for Graphene on Ir(111)

Epitaxial graphene on Ir(111) prepared in excellent structural quality is investigated by angle-resolved photoelectron spectroscopy. It clearly displays a Dirac cone with the Dirac point shifted only slightly above the Fermi level. The moiré resulting from the overlaid graphene and Ir(111) surface lattices imposes a superperiodic potential giving rise to Dirac cone replicas and the opening of minigaps in the band structure.

The mechanism of caesium intercalation of graphene.

Properties of many layered materials, including copper- and iron-based superconductors, topological insulators, graphite and epitaxial graphene, can be manipulated by the inclusion of different atomic and molecular species between the layers via a process known as intercalation. For example, intercalation in graphite can lead to superconductivity and is crucial in the working cycle of modern batteries and supercapacitors. Intercalation involves complex diffusion processes along and across the layers; however, the microscopic mechanisms and dynamics of these processes are not well understood. Here we report on a novel mechanism for intercalation and entrapment of alkali atoms under epitaxial graphene. We find that the intercalation is adjusted by the van der Waals interaction, with the dynamics governed by defects anchored to graphene wrinkles. Our findings are relevant for the future design and application of graphene-based nano-structures. Similar mechanisms can also have a role for intercalation of layered materials.

Thermal desorption spectroscopy of the O2/Pd(110) system

The interaction of oxygen with a Pd(110) surface was studied mainly by means of thermal desorption spectroscopy (TDS); some Auger electron spectroscopy (AES) results are also presented. The results obtained show that at low adsorption temperatures (Tad < 160 K) oxygen adsorbs molecularly. Atomic oxygen is also present but it may be the consequence of the sample heating during a TDS experiment. At Tad = 105 K the saturated Oad/Pd(110) surface, after heating above 160 K, contains only chemisorbed atomic oxygen (Ochem) which above 270 K may start to difuse into the bulk (Oinc). Desorption of Ochem takes place only above 600 K where at low and moderate coverages two TD peaks emerge; 770 and 700 K which may be connected with the earlier observed c(2 × 4) and c(2 × 6) LEED structures, respectively. The experiments at 190 K < Tad < 510 K show that the population of the 700 K state decreases with increasing Tad for the same exposure. The appearance of new TD peaks at 650 and 610 K and very high exposures for Tad = 510 K may be connected with the complex LEED structures observed for the Ochem saturated Pd(110) surface. The possible influence of Oinc on both the desorption kinetics and the signal intensity is also discussed.

Growth of copper and vanadium on a thin Al2O3-film on Ni3Al(111)

Abstract The growth of different metals on thin Al2O3-films on Ni3Al(111) was investigated using scanning tunneling microscopy (STM). These thin alumna films are well ordered showing two superstructures, which appear in the STM images at different bias voltages. These superstructures, with periodicities of 2.6 and 4.5 nm, respectively, are shown here to govern the nucleation of the deposited metals. Copper clusters grow on these nucleation centers only at room temperature. Higher temperatures lead to an increase of the cluster size and the loss of order. In turn, vanadium forms ordered cluster arrays at room and higher temperature. Due to the stronger metal–oxide interaction compared to copper vanadium forms smaller clusters at low and high coverages, which do not show any ripening after annealing. Based on these observations, Al2O3-films on Ni3Al(111) prove to be an interesting template for the fabrication of periodic cluster arrays.

Preparation and structure of AlW thin films

Abstract Thin films of AlW alloys were prepared by co-deposition of pure aluminum and pure tungsten, each sputtered by an independently controlled magnetron source. The deposition rate at the substrate (glass, fused quartz, and alumina ceramic), positioned 5 cm away from the target surface was 0.1–0.2 nm/s for pure metals, and the final film thickness was a few μ m. Completely amorphous films were obtained in the Al 80 W 20 –Al 67 W 33 composition range. At higher tungsten content, the W(Al) solid solution and pure tungsten phases appeared. The amorphous alloys exhibit a high negative temperature coefficient of the electric resistivity, increasing with the aluminum content up to −5.5·10 −4 K −1 . Finally, the amorphous AlW alloys exhibit a remarkable microhardness (6–7 GPa), and are structurally stable up to at least 400°C.

Interaction of oxygen with a polycrystalline palladium surface over a wide temperature range

Abstract Using thermal desorption spectroscopy (TDS) the interaction of gaseous oxygen with a polycrystalline palladium foil was studied for the first time over a very wide temperature (110 T Despite the general belief we find desorption of some amount of molecularly adsorbed oxygen from this highly defective surface before the detection of an atomically adsorbed oxygen species. This molecular oxygen desorbs at 160 K. Exposures greater than 0.7 L are required to first detect atomically adsorbed oxygen. Oxygen adsorption at T ad > 150 K results in atomic oxygen chemisorption which manifests itself as a single peak of second-order desorption kinetics near 900 K desorption temperature and which grows with increasing exposure. Its width, however, is not significantly affected by the presence of surface defects as revealed most clearly by a so-called Y -parameter (FWHM/ T p ) analysis. Beyond an initial coverage of θ 0 = 0.25 this peak develops several shoulders at lower temperatures (≈ 700 K) which did not saturate even after 10 000 L and which are assigned to oxygen chemisorbed on the Pd(100) surface patches and the Pd(110)-hke surface defects, respectively. This assignment was possible on the basis of the PAX results and is consistent with literature data for the oxygen desorption from Pd(100) and Pd(110) single-crystal surfaces. At an adsorption temperature of T ad = 510 K the highest exposures lead to the formation of a surface oxide species which decomposes above 700 K with zeroth-order kinetics. A small fraction of each new exposure penetrates deeper into the bulk and is given off only at temperatures≳ 1300 K.

Photoelectron spectroscopy of the Ag/Pd(110) system

Abstract Silver interaction with a Pd(110) surface was investigated, in a wide range of Ag coverages, by means of UPS, XPS and TDS. Our XPS measurements of the 3d levels of both Ag and Pd show significant shifts of the Ag signal as a function of Ag coverage, while the Pd level position remains constants. Both, silver and palladium 3d level binding energies were found to be dependent on the thermal treatment of the Ag/Pd system. XPS and UPS measurements strongly suggest that annealing at 500 K induces interdiffusion at the Ag/Pd interface. For the silver multilayers, the interdiffusion of Ag and Pd, due to the annealing at 500 K, is accompanied by structural changes of the silver overlayer (i.e., the formation of silver 3D crystallites). In the valence band spectrum of a silver multilayer annealed at 500 K we have identified a virtual bound state (VBS), derived from the hybridization of the Pd4d level and the silver valence (sp) band, indicating the presence of small concentration of Pd atoms diffused into the silver overlay. With increasing annealing temperature the Pd concentration in the Ag overlay increases causing a broadening of the VBS due to the Pd d-d interaction. At temperatures beyond 800 K desorption of Ag atoms from the palladium surface takes place.

Properties of potassium adsorbed on Si(100)2×1

The properties of potassium adsorbed on a Si(100)2×1 surface were studied using work function change measurements, thermal desorption spectra of potassium, and coadsorbed xenon as well as ultraviolet photoemission spectroscopy measurements of clean and oxygen exposed K/Si surfaces at 300 and 55 K. The results strongly support the notion that potassium overlayers of ΘK≥0.5 exhibit metallic character.

Oscillatory electron-phonon coupling in ultra-thin silver films on V(100)

The temperature dependence of peak widths in high-resolution angle-resolved photoelectron spectroscopy from quantum well states in ultra-thin Ag films on V(100) has been used to determine the electron-phonon coupling constant, λ, for films of thickness 1-8 layers. A strong oscillatory variation in coupling strength is observed as a function of film thickness, peaking at a two layer film for which λ1.0. A simple theory incorporating interaction of the photo-hole with the thermal vibrations of the potential step at the adlayer-vacuum interface is shown to reproduce the main features of these results.

Electrocatalytic activity and anodic stability of electrodeposited ruthenium-rhodium coatings on titanium

The stabilizing effect of rhodium addition to ruthenium coatings on titanium under anodic polarization in acidic solutions is described. The addition of rhodium to the coating leads to both an overall improved stability of the electrode, and an increase of the electrochemically active surface area. The service-life of a Rh-Ru coating polarized at 0.12 A cm−2 in 0.5 mol dm−3 sulphuric acid is improved by at least two orders of magnitude, while deterioration of the electroactive surface area never exceeds 30%. The rotating ring-disc electrode measurements show the dissolution of the coating at potentials above the onset of the oxygen evolution reaction at 1.1 V versus SCE.