Formation, stability and CO adsorption properties of PdAg/Pd(1 1 1) surface alloys

The formation, stability and CO adsorption properties of PdAg/Pd(1 1 1) surface alloys were investigated by X-ray photoelectron spectroscopy (XPS) and by adsorption of CO probe molecules, which was characterized by temperature-programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The PdAg/Pd(1 1 1) surface alloys were prepared by annealing (partly) Ag film covered Pd(1 1 1) surfaces, where the Ag films were deposited at room temperature. Surface alloy formation leads to a modification of the electronic properties, evidenced by core-level shifts (CLSs) of both the Pd(3d) and Ag(3d) signal, with the extent of the CLSs depending on both initial Ag coverage and annealing temperature. The role of Ag pre-coverage and annealing temperature on surface alloy formation is elucidated. For a monolayer Ag covered Pd(1 1 1) surface, surface alloy formation starts at ∼450 K, and the resulting surface alloy is stable upon annealing at temperatures between 600 and 800 K. CO TPD and HREELS measurements demonstrate that at 120 K CO is exclusively adsorbed on Pd surface atoms/Pd sites of the bimetallic surfaces, and that the CO adsorption behavior is dominated by geometric ensemble effects, with adsorption on threefold hollow Pd₃ sites being more stable than on Pd₂ bridge sites and finally Pd₁ a-top sites.     

Cobalt growth on two related close-packed noble metal surfaces

We report on scanning tunneling microscopy (STM) studies of submonolayer growth of cobalt on the close-packed (1 1 1) surfaces of Au and Ag. Both substrates belong to the category of noble metals, and they both exhibit a lattice misfit of ∼13% with respect to the (0 0 0 1) plane of Co. However, whereas the Au(1 1 1) surface reconstructs into the rather complex herringbone structure that disperses the cobalt into nanoclusters, the Ag(1 1 1) surface does not reconstruct in its clean state, and the surface dispersion of Co on this surface is therefore different. For Ag(1 1 1) at temperatures ranging from 160 to 200 K and for Au(1 1 1) at room temperature, the Co growth is three-dimensional starting with double layer islands followed by additional single layers. For both the Co/Au(1 1 1) and the Co/Ag(1 1 1) system, a Moiré pattern develops in the first bilayer of the Co islands, indicating an epitaxial but not commensurate growth. For Co islands with more than two layers, the subsequent layers are commensurate with the lower Co layers in the islands, but exhibit a decreasing corrugation of the Moiré pattern as observed in STM images. Despite a difference in the Moiré lattice constant and rotational angle, we show that the cobalt lattice constant is the same on both surfaces. We furthermore relate defect nucleation on the herringbone reconstruction on Au(1 1 1) to defect nucleation on steps on Ag(1 1 1).     

Growth of Au thin film on Cu-modified Si(1 1 1) surface

Miniaturizing of electronic devices requires that conductive elements maintain advanced electrical characteristics upon reducing their geometrical sizes. For gold, which is valued for its high electrical conductivity and stability against ambient conditions, creation of extra-thin films on silicon is hampered by formation of the quite complex Au/Si interface. In the present work, by forming a Si(1 1 1)5.55 × 5.55-Cu surface reconstruction prior to Au deposition we formed Au films with smoother surface morphology and higher surface conductivity compared to Au film grown on a pristine Si(1 1 1)7 × 7 surface. Scanning tunnelling microscopy and four-point probe measurements were used to characterize the growth mode of the Au film on a Si(1 1 1)5.55 × 5.55-Cu reconstruction at room temperature.     

Crystallization and surface morphology of Au/SiO2 thin films following furnace and flame annealing

A crystallization and surface evolution study of Au thin film on SiO₂ substrates following annealing at different temperatures above the eutectic point of the Au/Si system are reported. Samples were prepared by conventional evaporation of gold in a high vacuum (10⁻⁷ mbar) environment on substrates at room temperature. Thermal treatments were performed by both furnace and flame annealing techniques. Au thin films can be crystallized on SiO₂ substrates by both furnace and flame annealing. Annealing arranges the Au crystallites in the (1 1 1) plane direction and changes the morphology of the surface. Both, slow and rapid annealing result in a good background in the XRD spectra and hence clean and complete crystallization which depends more on the temperature than on the time of annealing. The epitaxial temperature for the Au/SiO₂ system decreases in the range of 350-400 °C. Furnace and flame annealing also form crystallized gold islands over the Au/SiO₂ surface. Relaxation at high temperatures of the strained Au layer, obtained after deposition, should be responsible for the initial stages of clusters formation. Gold nucleation sites may be formed at disordered points on the surface and they become islands when the temperature and time of annealing are increased. The growth rate of crystallites is highest around 360 °C. Above this temperature, the layer melts and gold diffuses from the substrate to the nucleation sites to increase the distance between islands and modify their shapes. Well above the eutectic temperature, the relaxed islands have hexagonally shaped borders. The mean crystallite diameters grow up to a maximum mean size of around 90 nm. The free activation energy for grain boundary migration above 360 °C is 0.2 eV. Therefore the type of the silicon substrate changes the mechanism of diffusion and growth of crystallites during annealing of the Au/Si system. Epitaxial Au(1 1 1) layers without formation of islands can be prepared by furnace annealing in the range of 300-310 °C and by flame annealing of a few seconds and up to 0.5 min.     

Interplay of structural and temperature effects on plasmonic excitations at noble-metal interfaces

The interplay of structural and electronic properties on plasmon modes was investigated for a thin Ag film grown at room temperature on Cu(111). Surface plasmons are confined within Ag grains, as indicated by the analysis of their dispersion relationship, which is dispersionless up to a critical wave-vector. Surface plasmon confinement is removed upon annealing at 400?K. The thermal treatment induces a flattening of the Ag adlayer with a merging of Ag islands, and, moreover, a strong enhancement of surface conductivity.     

Surface restructuring process on a Ag/Ge(0 0 1) surface studied by photoelectron spectroscopy

The atomic structure and charge distribution of Ag adsorbed Ge(0 0 1) surfaces have been investigated by means of Ge 3d core- and Ag 4d core-levels photoelectron spectroscopy. A mono-atomic layer of Ag was deposited on the clean Ge(0 0 1) c(4×2) surface at 80 K. The Ge 3d spectrum measured at 80 K was deconvoluted into two surface components, which is consistent with the previously proposed Ag ad-dimer model. After annealing the surface at room temperature, the rearrangement of the charge distribution was revealed to include electron transfer from Ge to Ag in conjunction with the surface restructuring process by the annealing.     

Monte Carlo simulation of metal deposition on foreign substrates

The deposition of a metal on a foreign substrate is studied by means of grand canonical Monte Carlo simulations and a lattice-gas model with pair potential interactions between nearest neighbors. The influence of temperature and surface defects on adsorption isotherms and differential heat of adsorption is considered. The general trends can be explained in terms of the relative interactions between adsorbate atoms and substrate atoms. The systems Ag/Au(1 0 0), Ag/Pt(1 0 0), Au/Ag(1 0 0) and Pt/Ag(1 0 0) are analyzed as examples.     

The effects of annealing on Au/pyronine-B/MD n-InP Schottky structure

Thin film of non-polymeric organic compound pyronine-B has been fabricated on moderately doped (MD) n-InP substrate as an interfacial layer using spin coating technique for the electronic modification of Au/MD n-InP Schottky contact. The electrical characteristics have been determined at room temperature. The barrier height and the ideality factor values for Au/pyronine-B/MD n-InP Schottky diode have been obtained from the forward bias I-V characteristics at room temperature as 0.60 eV and 1.041; 0.571 and 1.253 eV after annealing at 100 and 250 °C, respectively. An increase in annealing temperature at the Au/n-InP Schottky junction is shown to increase the reverse bias leakage current by about one order of magnitude and decrease the Schottky barrier height by 0.027 eV. Furthermore, the barrier height values for the Au/pyronine-B/MD n-InP Schottky diode have also been obtained from the C-V characteristics at room temperature as 1.001 and 0.709 eV after annealing at 100 and 250 °C, respectively. Finally, it was seen that the diode parameters changed with increase in the annealing temperature.     

Self-assembly of trimetallic nitride template fullerenes on surfaces studied by STM

Trimetallic nitride template fullerenes have been deposited onto a variety of substrates in order to elucidate the substrate-fullerene interactions. We have investigated self-assembled island formation and molecular detail of Er₃N@C₈₀ and Sc₃N@C₈₀ on Ag/Si(1 1 1), Au(1 1 1)/mica, Si(1 1 1), and Si(0 0 1) using variable temperature scanning tunnelling microscopy (STM). At room temperature, the fullerenes self-assemble into monolayer-high hexagonal close-packed islands on Ag-passivated Si(1 1 1) whereas annealing at elevated temperatures (250-300 °C) is necessary for the self-assembly of close-packed islands on Au(1 1 1). Intra-molecular resolution of the fullerenes has been achieved at liquid nitrogen temperature on Ag/Si(1 1 1) and already at room temperature on Si(0 0 1), when the rotation of the fullerenes is frozen. Whereas the bonding between the fullerenes and Si surfaces is mainly covalent, it appears to be mainly van-der-Waals on the other surfaces.     

Damping of the surface plasmon in clean and K-modified Ag thin films

The damping processes of electronic collective excitations of Ag/Ni(1 1 1) were studied by high-resolution electron energy spectroscopy. The FMHM of the Ag surface plasmon was reported as a function of Ag thickness, primary electron beam energy, Ag surface plasmon energy, and parallel momentum transfer. The broadening of the Ag surface plasmon was found to be related to 5sp–5sp transitions, for which a critical wave vector of 0.2 Å⁻¹ exists. Moreover, we provide a direct evidence of the occurrence of chemical interface damping in thin films, upon doping the Ag/Ni(1 1 1) system with K adatoms. The enhanced plasmon broadening in K/Ag/Ni(1 1 1) was ascribed to the existence of additional electron–hole decay channels at the K/Ag interface.     

Growth, alloy formation and reaction with oxygen of Ag and Au submonolayers on W(1 1 0) studied by valence band and core level photoemission

We have studied epitaxial submonolayers of Ag and Au deposited one after another onto W(1 1 0) at room temperature and subsequently annealed at 600 K. Photoemission spectroscopy of valence bands and the Ag3d_5/2 core level has been used to monitor two-dimensional alloy formation. The extent of alloying depends on the order of deposition, composition and annealing. We have also studied the reaction of alloy surfaces to exposure of molecular oxygen at 300 and 600 K.     

Very high coercivities of top-layer diffusion Au/FePt thin films

The Au/FePt samples were prepared by depositing a gold cap layer at room temperature onto a fully ordered FePt layer, followed by an annealing at 800 °C for the purpose of interlayer diffusion. After the deposition of the gold layer and the high-temperature annealing, the gold atoms do not dissolve into the FePt Ll₀ lattice. Compared with the continuous FePt film, the TEM photos of the bilayer Au(60 nm)/FePt(60 nm) show a granular structure with FePt particles embedded in Au matrix. The coercivity of Au(60 nm)/FePt(60 nm) sample is 23.5 kOe, which is 85% larger than that of the FePt film without Au top layer. The enhancement in coercivity can be attributed to the formation of isolated structure of FePt ordered phase.     

Controlled preparation of well-ordered transition metal oxide layers on a metallic surface

A method to create various well-ordered two dimensional transition metal oxide films on a metallic substrate has been exploited. The formation of an intermediate amorphous layer with controllable metal-oxygen stoichiometry serves as an important precursor condition for the final transformation into a mono-phase, crystalline oxide layer via mild annealing. As a key ingredient serves a Cu₃Au(1 0 0) substrate covered by oxygen. The flat Cu-O topmost layer stops completely intermixing of the substrate material with the subsequently evaporated transition metal film. Likewise the wetting of the surface is considerably enhanced and a homogeneous oxidation of the film is strongly promoted. The proposed technique appears to be widely efficient for preparation of various two dimensional oxide films covering the entire Cu₃Au(1 0 0) substrate. Its usefulness is demonstrated successfully for vanadium, niobium and molybdenum to produce a set of single-phase transition metal oxides of different stoichiometry and geometrical structure. All created oxides are found to be thermally stable at least up to a substrate temperature of 800 K.     

New surface structure analysis of Ag-Si phase by quantitative auger electron spectroscopy method

Ag or Au was deposited on a clean Si substrate at room temperature. These systems, Ag/Si and Au/Si, were annealed at various temperatures or various heating times. Due to the annealing, Ag or Au diffused into Si and/or Si diffused into the metal. The changes of the surface composition are analyzed by a quantitative Auger Electron Spectroscopy (AES) method which is newly developed as a non-destructive method. In the case of Ag/Si, Ag migrated into the Si substrate and/or Si diffused into Ag. Then, Ag-Si solid solution was produced. After the annealing, the Ag/Si system is changed into Ag/(Ag-Si)/Si of the three-phase structure. In the case of Au/Si (Au film thickness < 15 Å), the Au film thickness became thinner by annealing. The Au/Si system always keeps the Au/Si phase after annealing, while there was no Au-Si solution area. The difference between the Ag/(Ag-Si)/Si and the Au/Si structure is attributed to the reason that Au diffuses more quickly than Ag into the Si substrate. AES results after annealing cannot be explained by the model of the formation of the three-dimensional island structure which is commonly referenced.     

Dynamics of gold clusters on amorphous carbon films induced by annealing in a transmission electron microscope

The change of the size distribution of Au clusters induced by annealing was studied in situ by transmission electron microscopy. Starting from statistically distributed Au clusters on a thin amorphous carbon film, “islands” are formed within a few months storage at room temperature, which consist of Au clusters with sizes <4 nm embedded in a thin Au film. These islands cover originally areas with sizes around 25 × 70 nm². If the temperature is increased in the transmission electron microscope two different processes can be clearly distinguished that lead to the coarsening of the cluster size distribution: cluster coalescence and (contactless) Ostwald ripening. The degree and rate of the coarsening are found to depend on the underlying surface (Au film or amorphous carbon) and the exposure to the high-flux high-energy electron beam, which can be estimated to lead to high-temperature excursions in a cluster on a 10⁻¹² s time scale. The experimental findings are confirmed by Monte-Carlo simulations using the many-body Gupta potentials in order to calculate the Au/Au interaction. Moreover, the results of MC simulations suggest an electron-beam induced formation of a “quasi-two-dimensional gas” of small highly mobile Au species on the Au film, which promotes Ostwald ripening.     

Thermal stability of Ag films in air prepared by thermal evaporation

The thermal stability of silver films in air has been studied. Pure Ag films, 250 nm in thickness, were prepared on glass substrates by thermal evaporation process, and subsequently annealed in air for 1 h at temperatures between 200 and 400 °C. The structure and morphology of the samples were investigated by X-ray diffraction, Raman spectra and atomic force microscopy. It is found that the crystallization enhances for the annealed films, and film surface becomes oxidized when annealing temperature is higher than 350 °C. The electrical and optical properties of the films were studied by van der Pauw method and spectrophotometer, respectively. Reflectance drops sharply as Ag films are annealed at temperatures above 250 °C. Film annealed at 250 °C has the maximum surface roughness and the minimum reflectance at 600 nm optical wavelength. Film annealed at 200 °C has the minimum resistivity, and resistivity increases with the increasing of the annealing temperature when temperature is above 200 °C. The results show that both oxidization on film surface and agglomeration of silver film result in infinite of electrical resistivity as the annealing temperature is above 350 °C.     

AES, LEED and PYS investigation of Au deposits on InSe/Si(1 1 1) substrate

Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED) and Photoelectron Yield Spectroscopy (PYS) measurements have been used to monitor the interaction of gold (Au) deposits on InSe/Si(1 1 1) substrate. Au has been sequentially deposed under ultra-high vacuum onto 40 Å-thick film of layered semiconductor InSe which is epitaxially grown by molecular beam epitaxy (MBE) on a Si(1 1 1)1 × 1-H substrate and kept at room temperature. Au coverage varies from 0.5 monolayer to 20 monolayers (ML) (in terms of InSe atomic surface plane: 1 ML = 7.2 1014 at/cm²) which is corresponding to 1.30 Å of Au-metal. The Au/InSe/Si(1 1 1) system was characterized as function of Au deposit, we noticed an interaction at room temperature starts as an apparent intercalation process until 5 ML. Beyond this dose Au islands begin to form on the sample surface without interaction with InSe substrate, thus the interface is far from to be a simple junction Au-InSe.     

Effect of annealing temperature on the decomposition of reactively sputtered Ag2Cu2O3 films

Ag₂Cu₂O₃ films were deposited on glass substrates by reactive sputtering of a composite silver-copper target. The deposited films were annealed in air at 100, 200 and 300 °C. The structure of the films was studied using X-ray diffraction (XRD), their surface morphology was characterised using scanning electron microscopy (SEM) and their electrical resistivity at room temperature was measured using the four point probe method. The 100 °C annealing did not modify either the film structure or the film morphology. On the other hand, Ag₂Cu₂O₃ films were partially decomposed into Ag and CuO after a 200 °C annealing. The decomposition was complete for a 300 °C annealing. The evolution of the film surface morphology as a function of the annealing temperature was discussed in connection to the evolution of the molar volume of the phases constituting the films.     

The role of nitrogen in the preferential chromium segregation on the ferritic stainless steel (1 1 1) surface

The temperature dependence on the segregation behavior of the ferritic stainless steel single crystal (1 1 1) surface morphology has been examined by scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). AES clearly showed the surface segregations of chromium and nitrogen upon annealing. Nanoscale triangular chromium nitride clusters were formed around 650 °C and were regularly aligned in a hexagonal configuration. In contrast, for the ferritic stainless steel (1 1 1) surface with low-nitrogen content, chromium and carbon were found to segregate on the surface upon annealing and Auger spectra of carbon displayed the characteristic carbide peak. For the low-nitrogen surface, LEED identified a facetted surface with (2 × 2) superstructure at 650 °C. High-resolution STM identified a chromium carbide film with segregated carbon atoms randomly located on the surface. The facetted (2 × 2) superstructure changed into a (3 × 3) superstructure with no faceting upon annealing at 750 °C. Also, segregated sulfur seems to contribute to the reconstruction or interfacial relaxation between the ferritic stainless steel (1 1 1) substrate and chromium carbide film.     

Thickness dependence on thermal stability of sputtered Ag nanolayer on Ti/Si(1 0 0)

Thermal stability of Ag layer on Ti coated Si substrate for different thicknesses of the Ag layer have been studied. To do this, after sputter-deposition of a 10 nm Ti buffer layer on the Si(1 0 0) substrate, an Ag layer with different thicknesses (150-5 nm) was sputtered on the buffer layer. Post annealing process of the samples was performed in an N₂ ambient at a flow rate of 200 ml/min in a temperature range from 500 to 700 °C for 30 min. The electrical property of the heat-treated multilayer with the different thicknesses of Ag layer was examined by four-point-probe sheet resistance measurement at the room temperature. Phase formation and crystallographic orientation of the silver layers were studied by θ-2θ X-ray diffraction analysis. The surface topography and morphology of the heat-treated films were determined by atomic force microscopy, and also, scanning electron microscopy. Four-point- probe electrical measurement showed no considerable variation of sheet resistance by reducing the thickness of the annealed Ag films down to 25 nm. Surface roughness of the Ag films with (1 1 1) preferred crystallographic orientation was much smaller than the film thickness, which is a necessary condition for nanometric contact layers. Therefore, we have shown that the Ag layers with suitable nano-thicknesses sputtered on 10 nm Ti buffer layer were thermally stable up to 700 °C.