ARPES and STS investigation of noble metal Shockley states: Confinement in vicinal Au(1 1 1) surfaces and self-organized nanostructures

Spectroscopic effects associated with the superperiodic surface structure have been observed in Au(1 1 1) vicinal surfaces and nanostructured systems. In the vicinal Au(23 23 21) surface, high resolution angle resolved photoemission spectroscopy shows the opening of several gaps in the surface band structure, whereas scanning tunneling spectroscopy reveals the energy dependence of the electronic density. These combined spectroscopic data allow to determine the reconstruction potential by deducing their first Fourier components. We also demonstrate that due to the peculiar growth on this Au vicinal surface, we can obtain a self-assembled superlattice of triangular Ag islands. The high ordering of the nanostructures leads to homogenous electronic properties.     

Study of c(2 × 2)-MnAu(0 0 1) layers on Mn(0 0 1) by means of scanning tunneling microscopy/spectroscopy

Intermixing, growth, geometric and electronic structures of gold films grown on antiferromagnetic stacking body-centered-tetragonal manganese (0 0 1) films were studied by means of scanning tunneling microscopy/spectroscopy at room temperature in ultra-high vacuum. We found stable ordered c(2 × 2)-MnAu(0 0 1) alloy layers after depositing Au on pure Mn layers. Since at the fourth layer (5 × 23)-like Au reconstruction appears instead of the c(2 × 2) structure and local density of states peaks obtained on the c(2 × 2)-MnAu surface disappear, pure Au layers likely grow from the fourth layer.     

Array of double Au-Ag chains on the Si(5 5 7) surface

Using scanning tunneling microscopy we study the topographic properties of Ag structure on the Au induced, highly ordered Si(5 5 7) surface. Topography measurements show that a small amount of Ag (0.25 ML) deposited on that surface leads to considerable modifications of the one-dimensional structure induced by Au atoms. In particular, we observe two different chains on each terrace, which are identified as Si adatoms and Ag chain structures. The STM topography of those chains strongly depends on the bias voltage, indicating an important role of electronic effects in this system.     

In situ Video-STM study of the potential-induced (1 × 1) → “hex” transition on Au(1 0 0) electrode surfaces in Cl containing solution

The potential-induced (1 × 1) → “hex” transition on Au(1 0 0) electrodes in 0.01 M Na₂SO₄ + 1 mM HCl was studied by in situ scanning tunneling microscopy at high time resolution (Video-STM). According to these observations the elementary units of the “hex” surface reconstruction, hexagonally-ordered strings in the Au surface layer, are highly dynamic nanoscale objects. Isolated “hex” strings exhibit dynamic fluctuations in structure and position on the millisecond timescale. These fluctuations exceed the mobility of multistring “hex” domains by several orders of magnitude and can be explained by collective dynamic processes within the strings. Furthermore, the observations reveal a novel 1D mass transport mechanism along the strings, details on the nucleation and growth of “hex” strings and complex string restructuring processes, facilitating “hex” domain ripening.     

CuPc molecules adsorbed on Au(1 1 0)-(1 × 2): growth morphology and evolution of valence band states

We present the growth morphology, the long-range ordering, and the evolution of the valence band electronic states of ultrathin films of copper phthalocyanine (CuPc) deposited on the Au(1 1 0)-(1 × 2) reconstructed surface, as a function of the organic molecule coverage. The low energy electron diffraction patterns present a (5 × 3) reconstruction from the early adsorption stages. High-resolution UV photoelectron spectroscopy data show the disappearance of the Au surface states related to the (1 × 2) reconstruction, and the presence of new electronic features related to the molecule-substrate interaction and to the CuPc molecular states. The CuPc highest occupied molecular orbital gradually emerges in the valence band, while the interface electronic states are quenched, upon increasing the coverage.     

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.     

Formation of ordered arrays of Ag nanowires and nanodots on Si(5 5 7) surface

The ordered arrays of Ag nanowires and nanodots have been grown in ultra-high vacuum on the Si(5 5 7) surface containing regular steps of three bilayer height. Formation of Ag nanostructures have been studied by scanning tunneling microscopy, low energy electron diffraction and Auger electron spectroscopy at room temperature. It was shown that a sample exposure in the vacuum before Ag growth affects the shape of the forming Ag islands. This effect is caused by oxygen adsorption on the silicon surface from the residual atmosphere in the vacuum chamber. When Ag is deposited on the clean silicon surface the islands, overlapping several (1 1 1) neighboring terraces, form. The arrays of silver nanowires elongated along steps and silver nanodots, arranged in lines parallel to the steps, can be formed on the Si(5 5 7) surface depending on the amount of adsorbed oxygen.     

Realization of an atomic sieve: Silica on Mo(1 1 2)

The adsorption of Pd, Ag and Au atoms on a porous silica film on Mo(1 1 2) is investigated by scanning tunneling microscopy and density functional theory. While Pd atoms are able to penetrate the holes in the silica top-layer with virtually no barrier, Ag atoms experience an intermediate barrier value and Au atoms are completely unable to pass the oxide surface. The penetration probability does not correlate with the effective size of the atoms, but depends on their electronic structure. Whereas Pd with an unoccupied valence s-orbital has a low penetration barrier, Ag and Au atoms with occupied s-states experience a substantial repulsion with the filled oxide states, leading to a higher barrier for penetration. In the case of Ag, the barrier height can be temporally lowered by promoting the Ag 5s-electron into the support. The Mo-supported silica film can thus be considered as a primitive form of an atomic sieve whose selectivity is controlled by the electronic structure of the adatoms.     

Nanogroove formation during homoepitaxial Au electrodeposition on reconstructed Au( 1 1 1)

In situ scanning tunneling microscopy (STM) studies of homoepitaxial electrodeposition on Au(1 1 1) from hydrochloric acid solution reveal an unusual deposit morphology in the potential regime of the Au surface reconstruction, where the deposited Au islands are separated by nanoscale grooves with preferred widths of 6 and 12 nm. The formation of these structures is attributed to a hindered coalescence of the islands, caused by elastic energy contributions of the reconstructed bottom of the grooves.     

Atomistic mechanisms for the ordered growth of Co nanodots on Au(7 8 8): a comparison between VT-STM experiments and multi-scaled calculations

Hetero-epitaxial growth on a strain-relief vicinal patterned substrate has revealed unprecedented 2D long range ordered growth of uniform cobalt nanostructures. The morphology of a sub-monolayer Co deposit on a Au(1 1 1) reconstructed vicinal surface is determined by using variable temperature scanning tunneling microscopy (VT-STM). A rectangular array of nanodots (3.8 nm × 7.2 nm) is found for a particularly large deposit temperature range lying from 65 to 300 K. This paper focuses on the early stage of growth at temperatures between 35 and 480 K. Atomistic mechanisms leading to the nanodots array are elucidated by comparing statistical analysis of VT-STM images with multi-scaled numerical calculations. Molecular dynamics allows the quantitative determination of the activation energies for the atomic motion, whereas the kinetic Monte Carlo method simulates the submonolayer Co growth over mesoscopic time scale and space scale.     

Ordered arrangement of 9-aminoanthracene on Au(1 1 1) surfaces: A scanning tunneling microscopy study

We present a scanning tunneling microscopy (STM) investigation of 9-aminoanthracene (AA) on the reconstructed Au(1 1 1) surface. The bare Au(1 1 1) surface shows the herringbone reconstruction which is conserved upon deposition of the organic molecules. Most of the AA molecules are found to decorate the regions of fcc-stacking of the gold surface where a periodic linear arrangement is observed. The orientation of the long molecule axis of individual molecules is along the -directions of the Au substrate. In addition, for individual domains of the surface reconstruction, one of the three possible orientations is preferred. On substrate areas which exhibit a high step density, the steps are completely decorated by AA molecules. A detailed analysis of the STM images reveals that the molecules are located on top terrace levels. The fine structure of individual molecules on the terrace shows a clear dependence on the tunneling voltage and resembles the molecular orbitals of the free AA molecule.     

Reconstruction on Au(0 0 1) vicinal surfaces in UHV and in sulfuric acid solution

Using scanning tunneling microscopy we have studied the reconstruction on Au(1 1 n) surfaces in ultra-high vacuum and in electrolyte. Similar to the well-known (5 × 20) quasi-“hex” reconstruction on Au(0 0 1), the reconstruction consists of parallel reconstruction lines along the steps indicative of a higher atom density in the first Au layer. In contrast to nominally flat Au(0 0 1) where the reconstruction period is 1.44 nm, we find considerably larger reconstruction periods (1.8−1.96 nm) on incidentally flatter regions of nominal Au(1 1 9), Au(1 1 11), and Au(1 1 17) surfaces. The enlarged reconstruction period is attributed to the stress field on stepped surfaces. In agreement with previous studies we find a reconstruction free zone at the step edges.     

X-ray characterization of as-deposited, epitaxial films of Bi(0 1 2) on Au(1 1 1)

Electrodeposition is used to produce epitaxial single-crystal films on Au(1 1 1) substrates without annealing or other post-deposition modification. X-ray techniques show that the Bi(0 1 2) plane is parallel to the underlying Au(1 1 1) surface, and the azimuthal orientation of the films is determined. Combination of the X-ray data with in situ scanning tunneling microscopy (STM) images suggests a common growth mode from the first few layers up to thick films.     

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.     

Growth of Ag thin films on ZnO(0 0 0 −1) investigated by AES and STM

The growth of Ag films on ZnO(0 0 0 −1) has been investigated by Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). A high density of islands is nucleated at the earliest stages of the growth. An upstepping mechanism causes these islands to coalesce while the uncovered fraction of the ZnO surface remains constant (30%).     

Reconstructed (1 1 0) surfaces of FCC transition metals

The energies of the ideal, missing row (MR) and missing column (MC) (1 1 0) surfaces have been calculated by using modified embedded atom method (MEAM) for seven face centered cubic (FCC) transition metals Au, Pt, Ag, Pd, Rh, Cu and Ni. The results, that the MC reconstruction can not be formed for all metals, while the MR reconstruction can be formed naturally for Au and Pt, inductively for Ag, Pd, Rh and Cu and difficultly for Ni, are better than EAM calculated results in comparing with experimental results. In addition to the surface energy explanation, the results are also related to the surface topography and valence electron structure.     

Structural evolution at the initial growth stage of perylene on Au(1 1 1)

We report on the structural evolution at the initial growth stage of perylene thin films on Au(1 1 1) surface. Scanning tunneling microscopy and spectroscopy have been employed to investigate the structural and electronic properties at 78 K. Rapid molecular diffusion was observed at low submonolayer coverage. Molecules form an ordered structure at monolayer coverage. For the second layer, impinging molecules nucleate into molecular islands with an ordered intermediate structure.     

High resolution scanning tunneling spectroscopy of ultrathin Pb on Si(1 1 1)-(6 × 6) substrate

The electronic structure of Si(1 1 1)-(6 × 6)Au surface covered with submonolayer amount of Pb is investigated using scanning tunneling spectroscopy. Already in small islands of Pb with thickness of 1 ML Pb_(1 1 1) and with the diameter of only about 2 nm we detected the quantized electronic state with energy 0.55 eV below the Fermi level. Similarly, the I(V) characteristics made for the Si(1 1 1)-(6 × 6)Au surface reveal a localized energy state 0.3 eV below the Fermi level. These energies result from fitting of the theoretical curves to the experimental data. The calculations are based on tight binding Hubbard model. The theoretical calculations clearly show prominent modification of the I(V) curve due to variation of electronic and topographic properties of the STM tip apex.     

A new structural model for the SiC(0 0 0 1)(3 × 3) surface derived from first principles studies

A new structural model with fluctuant Si-trimers and missing Si-adatom is proposed for Si-terminated 6H-SiC(0 0 0 1)(3 × 3) reconstruction. The atomic and electronic structures of the model are studied using first principles pseudopotential density-functional approach. The calculated surface electronic density of states coincides quantitatively with the experimental results of photoemission and electron energy loss spectroscopy. Based on the calculations, the Patterson map and scanning tunneling microscopic (STM) images simulated for the new model agree more satisfactorily with the experimental X-ray diffraction and STM observations than that for previously proposed models. The calculations of formation energies suggest that the new structure would be formed under the environment of dilute Si vapor around the surface in the preparation process.     

Growth of In nanocrystallite arrays on the Si(1 0 0)-c(4 × 12)-Al surface

Using scanning tunneling microscopy, growth of In nanoisland arrays on the Si(1 0 0)-c(4 × 12)-Al surface has been studied for In coverage up to 1.1 ML and substrate temperature from room temperature to 150 °C. In comparison to the case of In deposition onto the clean Si(1 0 0) surface or Si(1 0 0)4 × 3-In reconstruction, the In growth mode is changed by the c(4 × 12)-Al reconstruction from the 2D growth to 3D growth, thus displaying a vivid example of the Volmer-Weber growth mode. Possible crystal structure of the grown In nanoislands is discussed.