First-principles calculations of C diffusion through the surface and subsurface of Ag/Ni(1 0 0) and reconstructed Ag/Ni(1 0 0)

Density functional theory calculations are performed to investigate the C diffusion through the surface and subsurface of Ag/Ni(1 0 0) and reconstructed Ag/Ni(1 0 0). The calculated geometric parameters indicate the center of doped Ag is located above the Ni(1 0 0) surface owing to the size mismatch. The C binding on the alloy surface is substantially weakened, arising from the less attractive interaction between C and Ag atoms, while in the subsurface, the C adsorption is promoted as the Ag coverage is increased. The effect of substitutional Ag on the adsorption property of Ni(1 0 0) is rather short-range, which agrees well with the analysis of the projected density of states. Seven pathways are constructed to explore the C diffusion behavior on the bimetallic surface. Along the most kinetically favorable pathway, a C atom hops between two fourfold hollow sites via an adjacent octahedral site in the subsurface of reconstructed Ag/Ni(1 0 0). The “clock” reconstruction which tends to improve the surface mobility, is more favorable on the alloy surface because the c(2 × 2) symmetry is inherently broken by the Ag impurity. As a consequence, the local lattice strain induced by the C transport is effectively relieved by the Ag-enhanced surface mobility and the C diffusion barrier is lowered from 1.16 to 0.76 eV.     

Electron and lattice structure of ultra thin Ag films on Si(1 1 1) and Si(0 0 1)

We studied the low temperature (T ? 130 K) growth of Ag on Si(0 0 1) and Si(1 1 1) flat surfaces prepared by Si homo epitaxy with the aim to achieve thin metallic films. The band structure and morphology of the Ag overlayers have been investigated by means of XPS, UPS, LEED, STM and STS. Surprisingly a (√3 × √3)R30° LEED structure for Ag films has been observed after deposition of 2-6 ML Ag onto a Si(1 1 1)(√3 × √3)R30°Ag surface at low temperatures. XPS investigations showed that these films are solid, and UPS measurements indicate that they are metallic. However, after closer STM studies we found that these films consists of sharp Ag islands and (√3 × √3)R30°Ag flat terraces in between. On Si(0 0 1) the low-temperature deposition yields an epitaxial growth of Ag on clean Si(0 0 1)-2 × 1 with a twinned Ag(1 1 1) structure at coverage’s as low as 10 ML. Furthermore the conductivity of few monolayer Ag films on Si(1 0 0) surfaces has been studied as a function of temperature (40-300 K).     

First-principles studies on initial growth of Ni on MgO(0 0 1) surface

To elucidate the initial growth of metal on oxide surface, we studied adsorption of small nickel clusters, Ni_n (n = 1-5), on MgO(0 0 1) surface using first-principles method based on density-functional theory. It was found that the preferential adsorption site for an isolated Ni atom is directly above the surface oxygen atom. A strong covalent bond with partial ionic character is formed between the Ni adatom and the surface oxygen atom. Various structures were considered for the Ni_n isomers and 3D structures were found to be energetically more stable than 2D structures for clusters of more than two atoms. For the 2D clusters, metal-metal bonds prevail over metal-substrate bonds with increasing Ni coverage. The calculated work function and ionization energy were found to vary with Ni coverage which is attributed to the change of the surface dipole moment upon metal adsorption, while the evolution of Schottky barrier height at the initial growth stage is dominated by the adatom-induced gap states.     

Vibrational entropy-driven dealloying of Mo(1 0 0) and W(1 0 0) surface alloys

The formation and stability of Cu, Ag and Au-induced c(2 × 2) alloys at the Mo(1 0 0) and W(1 0 0) surfaces have been investigated with low-energy electron microscopy and diffraction. The ordered alloys transform to disordered overlayer structures at elevated temperature. Comparison of the transformation temperatures with energetics obtained from first principles calculations reveals the vibrational entropic contribution to the system free energy that defines alloy thermal stability. Effective Debye temperatures for metal adatoms are determined that exhibit the expected mass and bond strength dependence.     

Orbital moments of 3d adatoms and Co nanostructures on Cu(0 0 1) surfaces

We use ab initio calculations to investigate spin and orbital moments of 3d transition-metal adatoms and Co nanostructures on Cu(0 0 1) surfaces. For Fe and Co adatoms on Cu(0 0 1) we predict extremely large orbital moments, comparable to the spin moments at these sites. For Mn and Cr adatoms the orbital moments are extremely small and can be neglected in face of their rather large spin moments. Ni adatoms on Cu(0 0 1) were found to be non-magnetic. Our investigations for adsorbed flat clusters of Co on Cu(0 0 1) address the persistence and extent of these large orbital moments in the clusters as a function of their size. We find that, the average orbital moment (M_orb) per Co atom is strongly correlated with the coordination number, decreasing drastically and monotonically as the average number of first Co neighbors around the sites in the cluster (N_Co) is increased.     

The comparative study of two dimensional condensation of Xe and Kr physisorbed on Ag(1 1 1) and Ag(1 0 0)

Adsorption isobars of Xe and Kr on Ag(1 1 1) and Ag(1 0 0) were observed simultaneously by an extremely-low-current low energy electron diffraction and an ellipsometry in the temperature range between 60 K and 90 K and in the equilibrium pressure range between 8 × 10⁻⁶ Pa and 2 × 10⁻⁴ Pa. Two dimensional condensation of the first layer of Xe on Ag(1 1 1) occurred at the temperature 0.3 ± 0.1 K higher than that on Ag(1 0 0). In the case of Kr on Ag(1 1 1) and Ag(1 0 0), the temperature difference was 0.2 ± 0.1 K. This temperature difference was discussed on the assumption that it is caused by the repulsive interaction between the dipoles induced in rare gas atoms. We estimated the difference of the induced dipole moment μ: μ of Xe atom on Ag(1 0 0) is 6% larger than that on Ag(1 1 1) and μ of Kr atom on Ag(1 0 0) is 14% larger than that on Ag(1 1 1).     

Molecular beam epitaxy of organic films investigated by high resolution low energy electron diffraction (SPA-LEED): 3,4,9,10-perylenetetracarboxylicacid-dianhydride (PTCDA) on Ag(1 1 1)

A detailed investigation of the multilayer growth of PTCDA on Ag(1 1 1) by high resolution LEED (SPA-LEED) is reported. The first two monolayers are closed and exhibit a structure, which is commensurate with respect to the underlying Ag(1 1 1) surface. The lattice parameters are close to those of the (1 0 2) plane of the β bulk phase of PTCDA, with deviations ?2%. The vertical stacking of the second layer with respect to the first monolayer (observed at 300 K) corresponds to that in the β bulk phase of PTCDA. At high growth temperatures (400 K), Stranski-Krastanov growth occurs from the third monolayer onward, and PTCDA clusters, preferentially with few well defined facets, grow. The structure of the clusters is that of the thermodynamically more stable α bulk phase of PTCDA. Contrary, at low growth temperatures (200 K), the growth proceeds in the Frank van der Merve mode, with several open layers. From slope selection there is evidence for an Ehrlich-Schwoebel barrier. The lateral packing of the PTCDA grown at low temperatures corresponds within error to that of the β bulk phase. The low temperature structure and morphology is meta-stable. Short annealing at 300 K flattens the PTCDA film, and prolonged annealing at 400 K causes the film to adapt the structure and morphology obtained directly at 400 K growth temperature. Presumably, the formation of layers with a β-phase-like lattice at low temperature is due to the better fit of the β phase, compared to the α phase lattice, to the underlying commensurate monolayer. However, at high growth temperatures, the thermodynamically more stable α phase grows, reducing the area of lattice misfit to the underlying commensurate first two layers by formation of clusters.     

Infrared reflection absorption spectroscopic study of the adsorption structures of dimethyl ether and methyl ethyl ether on Cu(1 1 1) and Ag(1 1 1)

Infrared reflection absorption (IRA) spectra measured for dimethyl ether (DME) adsorbed at 80 K on Cu(1 1 1) and Ag(1 1 1) give IR bands belonging only to the A₁ and B₂ species, indicating that the adsorbate takes on an orientation in which the C₂ axis bisecting the COC bond angle tilts away from the surface normal within the plane perpendicular to the substrates. The DFT method was applied to simulate the IRA spectra, indicating that the tilt angles of DME on Cu(1 1 1) and Ag(1 1 1) are about 50° and 55°, respectively, at submonolayer coverages. The results are in contrast to the case of DME on Cu(1 1 0) and Ag(1 1 0), where the C₂ axis is perpendicular to the substrates [T. Kiyohara et al., J. Phys. Chem. A 106 (2002) 3469]. Methyl ethyl ether (MEE) adsorbed at 80 K on Cu(1 1 1) gives IRA bands mainly ascribable to the gauche (G) form, whereas the IRA spectra measured for MEE on Ag(1 1 1) are characterized by the trans (T) form. The rotational isomers are identical with those on Cu(1 1 0) and Ag(1 1 0); i.e., MEE on Cu(1 1 0) takes the G form and the adsorbate on Ag(1 1 0) the T form [T. Kiyohara et al., J. Phys. Chem. B 107 (2003) 5008]. The simulation of the IRA spectra indicated that (i) the G form adsorbate on Cu(1 1 1) takes an orientation, in which the axis bisecting the COC bond angle tilts away from the surface normal by ca. 30° within the plane perpendicular to the surface to make the CH₃-CH₂ bond almost parallel to the surface, and (ii) the T form adsorbate on Ag(1 1 1) takes an orientation, in which the bisecting axis tilts away by ca. 60° from the surface normal within the perpendicular plane. Comparison of these adsorption structures of MEE on the (1 1 1) substrates with those of MEE on Cu(1 1 0) and Ag(1 1 0) indicates that the structures are mainly determined by a coordination interaction of the oxygen atom to the surface metals and an attractive van der Waals interaction between the ethyl group of MEE and the substrate surfaces. The coordination interaction plays an important role on Cu(1 1 1) and Cu(1 1 0), which makes the adsorbate on the Cu substrates to take the orientations with the bisecting axis near parallel to the surface normal and to assume the G form in order to make the ethyl group parallel to the surface, which is favorable for the van der Waals interaction. In the case of MEE on the Ag substrates the attractive van der Waals interaction plays a dominant role, resulting in the T form which is more favorable for the interaction than the G form.     

Stranski-Krastanov like oxide growth on Ag(1 1 1) at atmospheric oxygen pressures

The oxidation behavior of Ag(1 1 1) was studied by means of in situ surface X-ray diffraction at atmospheric oxygen pressure. Exposure to 1 bar oxygen at 773 K reveals a competing growth of three different oxygen-induced structures on Ag(1 1 1), namely the well-known p(4 × 4) reconstruction, a surface oxide in a p(7 × 7) coincidence structure and the bulk oxide Ag₂O in orientation. The latter two exhibit the same honeycomb on hexagon arrangement of the Ag sublattice with respect to the Ag(1 1 1) surface. An inverted stacking of Ag planes in the bulk oxide islands is observed as compared to the Ag(1 1 1) substrate, which sheds new light on the Ag₂O formation process. Finally, we present a structural model of the p(7 × 7) reconstruction, based on a three-layer O-Ag-O slab of Ag₂O(1 1 1).     

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.     

The periodicity in translation of Ag (0 0 1) and (1 1 0) twist grain boundary

The energies of Ag (0 0 1) and (1 1 0) twist grain boundary (GB) in translation have been calculated with the modified analytical embedded atom method (MAEAM). The energy period corresponds exactly to the DSC lattice unit cell and the area of the energy period referred to the CSL unit cell is 1/Σ². The ‘energy grooves’ are parallel to the sides of the CSL or DSC lattice unit cell. The most preferable sliding direction is parallel to identical sides of the square CSL unit cell for (0 0 1) boundaries and to the short side of the rectangular CSL unit cell for (1 1 0) boundaries. From energy minimization, the stable configuration appears when two adjacent grains are translated relatively to the corners, centre or sides’ midpoint of the DSC lattice unit cell.     

The influence of hydrogen on the electronic and magnetic structures of TM(0 0 1) (TM=Fe, Co, Ni, and Cu) surfaces and interfaces: Ab-initio calculations

We present ab-initio investigation of the electronic and magnetic structure of TM(0 0 1) surfaces and TM/Cu(0 0 1) systems (TM=Fe, Co, Ni, Cu) with and without hydrogen adsorbed layer. The adsorption energy of hydrogen atom is found to be energetically more stable above the surface layer of Ni(0 0 1) surface than other TM(0 0 1) surfaces. The adsorption energies of hydrogen on TM/Cu(0 0 1) systems are larger than those on TM(0 0 1) surfaces. The relaxed geometries show that hydrogen has a strong influence on the interlayer distance. Furthermore, a marked reduction of Fe, Co, and Ni surface magnetic moments to 2.54, 1.41 and 0.25 μ_B, respectively, is obtained due to the presence of hydrogen.     

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.     

Growth of oriented crystalline Ag nanoislands on air-exposed Si(0 0 1) surfaces

Growth of Ag islands under ultrahigh vacuum condition on air-exposed Si(0 0 1)-(2 × 1) surfaces has been investigated by in-situ reflection high energy electron diffraction (RHEED). A thin oxide is formed on Si via exposure of the clean Si(0 0 1)-(2 × 1) surface to air. Deposition of Ag on this oxidized surface was carried out at different substrate temperatures. Deposition at room temperature leads to the growth of randomly oriented Ag islands while well-oriented Ag islands, with (0 0 1)_Ag||(0 0 1)_Si, [1 1 0]_Ag||[1 1 0]_Si, have been found to grow at substrate temperatures of ≥350 °C in spite of the presence of the oxide layer between Ag islands and Si. The RHEED patterns show similarities with the case of Ag deposition on H-passivated Si(0 0 1) surfaces.     

Self-assembled germanium nano-clusters on silver(1 1 0)

The adsorption of germanium on Ag(1 1 0) has been investigated by scanning tunnelling microscopy (STM), as well as surface X-ray diffraction (SXRD). At 0.5 germanium monolayer (ML) coverage, Low Energy Electron Diffraction (LEED) patterns reveals a sharp c(4 × 2) superstructure. Based on STM images and SXRD measurements, we present an atomic model of the surface structure with Ge atoms forming tetramer nano-clusters perfectly assembled in a two-dimensional array over the silver top layer. The adsorption of the germanium atoms induces a weak perturbation of the Ag surface. Upon comparison with results obtained on the (1 1 1) and (1 0 0) faces, we stress the role played by the relative interactions between silver and germanium on the observed surface structures.     

Surface and subsurface oxide formation on Ni(1 0 0) and Ni(1 1 1)

The oxidation of Ni(1 0 0) and Ni(1 1 1) at elevated temperatures and large oxygen exposures, typical of the methods used in the preparation of NiO(1 0 0) films for surface studies, has been investigated by medium energy ion scattering (MEIS) using 100 keV H⁺ incident ions. Oxide film growth proceeds significantly faster on Ni(1 1 1) than on Ni(1 0 0), but on both surfaces oxide penetration occurs to depths significantly greater than 100 Å with total exposures of 1200 and 6000 L respectively. The metal/oxide interface is extremely rough, with metallic Ni extending to the surface, even for much thicker oxide films on Ni(1 1 1). On Ni(1 1 1), NiO growth occurs with the (1 0 0) face parallel to the Ni(1 1 1) surface and the close-packed 〈1 1 0〉 directions parallel. On Ni(1 0 0) the MEIS blocking curves cannot be reconciled with a single orientation of NiO(1 0 0) (with the 〈1 1 0〉 directions parallel) on the surface, but is consistent with the substantial orientational disorder (including tilt) previously identified by spot-profile analysis LEED.     

Density functional study of hypophosphite adsorption on Ni (1 1 1) and Cu (1 1 1) surfaces

Surface structures and electronic properties of hypophosphite, H₂PO₂⁻, molecularly adsorbed on Ni(1 1 1) and Cu(1 1 1) surfaces are investigated in this work by density functional theory at B3LYP/6-31++g(d, p) level. We employ a four-metal-atom cluster as the simplified model for the surface and have fully optimized the geometry and orientation of H₂PO₂⁻ on the metal cluster. Six stable orientations have been discovered on both Ni (1 1 1) and Cu (1 1 1) surfaces. The most stable orientation of H₂PO₂⁻ was found to have its two oxygen atoms interact the surface with two PO bonds pointing downward. Results of the Mulliken population analysis showed that the back donation from 3d orbitals of the transition metal substrate to the unfilled 3d orbital of the phosphorus atom in H₂PO₂⁻ and 4s orbital's acceptance of electron donation from one lone pair of the oxygen atom in H₂PO₂⁻ play very important roles in the H₂PO₂⁻ adsorption on the transition metals. The averaged electron configuration of Ni in Ni₄ cluster is 4s^0.634p^0.023d^9.35 and that of Cu in Cu₄ cluster is 4s^1.004p^0.033d^9.97. Because of this subtle difference of electron configuration, the adsorption energy is larger on the Ni surface than on the Cu surface. The amount of charge transfers due to above two donations is larger from H₂PO₂⁻ to the Ni surface than to the Cu surface, leading to a more positively charged P atom in Ni_nH₂PO₂⁻ than in Cu_nH₂PO₂⁻. These results indicate that the phosphorus atom in Ni_nH₂PO₂⁻ complex is easier to be attacked by a nucleophile such as OH⁻ and subsequent oxidation of H₂PO₂⁻ can take place more favorably on Ni substrate than on Cu substrate.     

Two-photon resonance in optical second harmonic generation from quantum well states in ultra-thin Ag films grown on Si(1 1 1) surfaces

We studied optical second harmonic generation (SHG) oscillations during the growth of Ag films on Si(1 1 1) 7 × 7 clean and H-terminated surfaces. In the growth on the 7 × 7 surfaces at room temperature, the second and third peaks of the oscillation shift towards the thinner side with an increase in pump photon energy. Our analysis revealed that these peaks are caused by two-photon resonant transitions from the n = 1 and 2 occupied quantum well states (QWSs) in the Ag film to the Ag/Si interface at 1.9 eV above the Fermi level (E_f). In Ag growth on the hydrogen-terminated surfaces, the SHG oscillation was similar to that on the 7 × 7 surfaces at room temperature. However, the QWS-related peak was suppressed in the growth at 300 °C. This is attributed to an inhibited intrusion of the interface state into the Ag layers.     

Thermal stability of nanoscale silver metallization in Ag/W/Co/Si(1 0 0) multilayer

In this work, we have studied thermal stability of nanoscale Ag metallization and its contact with CoSi₂ in heat-treated Ag(50 nm)/W(10 nm)/Co(10 nm)/Si(1 0 0) multilayer fabricated by sputtering method. To evaluate thermal stability of the systems, heat-treatment was performed from 300 to 900 °C in an N₂ ambient for 30 min. All the samples were analyzed by four-point-probe sheet resistance measurement (R_s), Rutherford backscattering spectrometry (RBS), X-ray diffractometry (XRD), and atomic force microscopy (AFM). Based on our data analysis, no interdiffiusion, phase formation, and R_s variation was observed up to 500 °C in which the Ag layer showed a (1 1 1) preferred crystallographic orientation with a smooth surface and R_s of about 1 Ω/□. At 600 °C, a sharp increase of R_s value was occurred due to initiation of surface agglomeration, WSi₂ formation, and interdiffusion between the layers. Using XRD spectra, CoSi₂ formed at the Co/Si interface preventing W silicide formation at 750 and 800 °C. Meantime, RBS analysis showed that in this temperature range, the W acts as a cap layer, so that we have obtained a W encapsulated Ag/CoSi₂ contact with a smooth surface. At 900 °C, the CoSi₂ layer decomposed and the layers totally mixed. Therefore, we have shown that in Ag/W/Co/Si(1 0 0) multilayer, the Ag nano-layer is thermally stable up to 500 °C, and formation of W-capped Ag/CoSi₂ contact with R_s of 2 Ω/□ has been occurred at 750-800 °C.     

Growth and thermal stability of ultra-thin Ag and Au layers on Mo(1 1 1) surface

The influence of temperature on the growth process of ultra-thin Ag and Au layers on the Mo(1 1 1) surface was investigated. At 300 K growth of the Stranski-Krastanov type was found for Ag; for Au growth of the monolayer plus simultaneous multilayers type was found, where a base layer is one physical layer. The first three geometrical adsorbed layers for Ag are thermally stable. For annealed Au layers triangle features with base side length from 15 to 35 Å were formed for θ < 6 monolayer (ML), and for θ > 6 ML part of the Au formed a flat adlayer with Au atoms grouped in equilateral triangles with side length 7 Å. The presence of Au layers does not cause faceting, layers are not smooth which could be caused by the fact that Au does not wets the substrate. For Ag thick layers reversible wetting/non-wetting transition was observed at 600 K. Ag layers on Mo(1 1 1) surface did not lead to faceting.