Dynamic study of W atoms and clusters on W (1 1 1) surfaces

Using a field ion microscope, the diffusion behaviors and atomic processes of W atoms and clusters on W (1 1 1) surfaces were observed directly. The activation energy of W clusters diffusion on W (1 1 1) as a function of cluster size has an oscillatory and increasing behavior. But, the activation energy of a single W atom is especially high. The compact geometric structures are more stable and have higher activation energies of surface diffusion than structures with extra atoms at the periphery. Besides the terrace diffusion, other atomic processes such as the ascending, descending, detachment motion on W (1 1 1) surfaces were also observed. Unlike the general systems, their occurrence temperatures are quite near. These experimental results were used to discuss the formation mechanism of single atom W tips.     

Study of two types of Ir or Rh covered single atom pyramidal W tips

The growth of a single-atom sharp pyramid tips on a tungsten substrate by depositing noble metals of the Pt family was investigated by field ion microscopy earlier. The Pd or Pt covered single-atom W pyramidal tips have been successfully prepared and established by our group. They are thermally and chemically stable and can easily be regenerated. In this study, we report the establishing and structural analysis of Rh and Ir covered single-atom W pyramidal tips. Two types of stable structures with bcc {2 1 1} facets are found for both metals. One is stacked by 1, 3, 10 atoms, and the other is stacked by 1, 6, 15 atoms for the top three layers and so on in series from the top to the deeper layers. The single atom tip is destroyed layer by layer after field evaporation to observe the structures of the different layers. However, the tip can be regenerated after it is annealed again and the two types of structures appear systematically depending on the annealing temperature. The regeneration process is investigated and the growth parameters of the two different types of Rh or Ir covered W tip are determined. The differences in the activation barrier and binding energy of these two types are calculated to be 0.08 eV and 0.064 eV for Rh covered single atom tips, and 0.03 eV and 0.14 eV for Ir covered single atom tips, respectively. Possible mechanisms and the relevance for application are discussed.     

Origin of flat morphology and high crystallinity of ultrathin bismuth films

We have investigated the origin of “atomically flat” and “single-crystalline” growth of Bi films on Si(1 1 1)-7 × 7 through comparative experiments using Si(1 1 1)-β-√3 × √3-Bi as a control system. On the Si(1 1 1)-7 × 7 substrate, the majority of initial nuclei stabilize with pseudocubic (PC) paired layers analogous to the black phosphorus (BP) structure, and grow in a strong two-dimensional fashion that results in a “textured” but “atomically flat” surface morphology. After the coalescence of the BP-like grains at a nominal thickness of 4 monolayers (ML), a tiny number of minority hexagonal (HEX) bulk crystal nuclei, aligned commensurately with the substrate 7 × 7 lattice, cause the “textured” BP-like PC film to transform into a “single-crystalline” bulk-like HEX film. On the Si(1 1 1)-β-√3 × √3-Bi substrate, however, the BP-like structure breaks up into a conventional bulk-like PC structure and the HEX nucleation is suppressed up to as thick as ∼6 ML. Therefore, the morphology and crystallinity of the films are simply rough and polycrystalline, respectively.     

The thermodynamics of cluster formation in nucleation theory

We have derived a precise thermodynamic definition of the standard free energy to form a cluster which is used in nucleation theory. The results [Eq. (9)] have a form differing slightly from the form usually used in nucleation theory and show that the Lothe-Pound correction factor is based on a misconception concerning the standard states involved.     

3D atom probe study of gas adsorption and reaction on alloy catalyst surfaces I: Instrumentation

A versatile gas reaction cell has been developed for a 3D atom probe system to provide a new method for studying the reaction mechanisms of heterogeneous catalysis. We describe the features and advantages of this new system. The reaction cell enables exposure of metal specimens in high temperature (?873 K) and high pressure (?1 bar) environments to a wide range of single gases or gas mixtures. Following treatment, specimens are transferred into the UHV analysis chamber of the 3D atom probe, which provides atomic-scale positional and chemical information for adsorbed gas and metal atoms/oxides from selected regions of the specimen apex. Results demonstrating the effects of heating alloy catalysts in vacuo and in air are presented, which validate the instrument design, alongside a sample of data for exposing Pt and Pt-Rh alloys to NO, revealing the power of the 3D atom probe technique in this field.     

Fidgety particles on surfaces: how do they jump, walk, group, and settle in virgin areas?

This article is aimed at describing the history and current status of surface diffusion, its vital importance in diverse surface phenomena and numerous industrial applications, the recent significant progress in its understanding, and, in particular, many unsettled questions concerning surface diffusion mechanisms. From the fundamental point of view, the interest in surface diffusion and, at the same time, the difficulties in its understanding, stem primarily from the collective (many-body) nature of this phenomenon. On the practical side, the importance of the knowledge of the physics behind surface diffusion is becoming increasingly crucial in the coming era of nanotechnology, which will bring an unprecedented level of miniaturization in electronics and a broad employment of various nanomaterials.     

Si nucleation on Si(1 1 1)-7 × 7: From cluster pairs to 2D islands

Nucleation of 2D islands in Si/Si(1 1 1)-7 × 7 molecular beam epitaxy is studied using scanning tunneling microscopy (STM). A detailed analysis of the population of small amorphous clusters coexisting on the surface with epitaxial 2D islands has been performed. It is shown that small clusters tend to form pairs. The pairs serve as precursors for 2D islands as confirmed by direct STM observations of the smallest 2D islands covering two adjacent half-unit cells of the 7 × 7 reconstruction. It is proved with scaling arguments that the critical nucleus for 2D island formation consists not only of the pair itself, but also includes additional adatoms not belonging to the stable clusters.     

3D atom probe study of gas adsorption and reaction on alloy catalyst surfaces II: Results on Pt and Pt-Rh

The 3-dimensional atom probe (3DAP) is a unique instrument providing chemical analysis at the atomic scale for a wide range of materials. A dedicated 3DAP has been built specifically for analysing reactions at metal surfaces, called the catalytic atom probe (CAP). This paper presents an overview of results from the CAP on structural and chemical transformations to surface layers of Pt and Pt-17.4 at.%Rh catalysts following exposure to a number of gases typically emitted by vehicle engine exhausts, normally for 15 min at pressures of 10 mbar. Following exposure to the oxidising gases NO on Pt, and NO, O₂ or N₂O on Pt-Rh, both surfaces appear disrupted, while for Pt-Rh, Rh enrichment of the surface atomic layer is noted over the entire specimen apex for exposure temperatures up to 523 K. However, for oxidising exposures at 573-773 K relatively clean, Rh-depleted surfaces are observed on {0 0 1}, {0 1 1} and {0 1 2} crystallographic regions of Pt-Rh. It is suggested that this result is due to surface diffusion of oxide species over the specimen apex, towards the {1 1 1}-orientated areas where the oxides appear to be stabilised. In contrast, CO exposure appears to have little effect on the either the surface structure or composition of the Pt-Rh alloy. Finally, combinations of two gases (NO + CO, O₂ + NO) were also dosed onto Pt-Rh alloys in the same exposure. These revealed that while NO and CO can co-adsorb without interference, CO prevents the build up of oxide layers and reduces the extent of Rh segregation seen under NO exposure alone. On exposing Pt-Rh to NO after an oxygen exposure, heavily oxidised surfaces, Rh segregation and no intact NO molecules were seen, confirming the ability of oxidised Pt-Rh to dissociate nitric oxide.     

STM analysis of triosmium carbonyl cluster adsorption at HOPG

The study of metallic carbonyl clusters as precursors in tailoring the heterogeneous metal catalysts has been of great importance. The catalytic nature of the adsorbed clusters in thin film form depends on the chemical properties of the substrate used. The metal-support interaction will determine various properties such as the surface morphology, adsorption features and the structural orientations. We report a scanning tunneling microscopy (STM) study of an osmium carbonyl cluster (Os₃(CO)₁₁(NCCH₃)) adsorbed on highly oriented pyrolytic graphite (HOPG). STM measurements showed that the osmium carbonyl cluster interacts with HOPG in such a way that it adsorbs on the basal plane showing regular lattice structure, whereas the axial planes of the HOPG surface shows no ordered structure. The regular cluster lattice structure of the carbonyl cluster on the basal plane of the graphite has lattice parameters of a=1.4 nm and b=1.5 nm. We believe that the regular orientation of the cluster indicates a monolayer adsorption of the cluster on the graphite basal planes. Scanning tunneling spectroscopy (STS) measurements also indicated an insulating behavior for the cluster molecules on HOPG, with a significant energy gap value of ca. 300 mV. The cluster interaction at the active sites, i.e. axial planes of the graphite, was also observed by in situ STM measurements.     

Initial oxidation kinetics of copper (1 1 0) film investigated by in situ UHV-TEM

Previous studies of the initial stage of oxidation on clean single crystal of Cu(1 0 0) have been extended to the case of the Cu(1 1 0) surface. The dynamic observation of the nucleation and growth of Cu oxide by means of in situ ultra high vacuum transmission electron microscopy (UHV-TEM) shows a highly enhanced oxidation rate on Cu(1 1 0) surface as compared to Cu(1 0 0). The kinetic data on the nucleation and growth of the three-dimensional oxide islands agree well with our heteroepitaxial model of surface diffusion of oxygen.     

Morphology changes due to AC induced electromigration in Gd islands on W(1 1 0)

Gd islands were grown on W(1 1 0) surface by evaporating Gd on the substrate at room temperature and subsequent annealing. STM images reveal in many cases islands which have a deep hole inside them. The appearance of the hole is associated with the application of an AC field. No such holes appear when the sample is heated by a DC current. We show that this can be explained by the combined affect of the AC field and the barrier to diffusion introduced by steps that can create a nucleus for further growth of an island which includes a hole in the middle. This may be generalized to a technique of tailoring the size, shape and distances of islands by, for example, two orthogonal AC fields with a phase delay of 90° between them.     

Atomic structure and magnetic properties of Cu80Co20 nanocrystalline compound produced by mechanical alloying

Direct observation of the atomic structure of the mechanically alloyed Cu₈₀Co₂₀ compounds has been made using the field ion microscope (FIM). Phase composition, defect structure and morphology of material on the atomic scale have been determined. It has been established that the studied material is chemically inhomogeneous, presenting a mixture of two main phases: heterogeneous solid solution of cobalt in copper, and pure cobalt. Phase volume ratios, particle and cluster sizes have been estimated. An evaluation of Co content in Cu---Co solid solution has been made. The width of interfaces in this mechanically alloyed material was revealed to be at least twice the width of phase boundaries in metals and alloys. Superparamagnetism of the compound studied at elevated temperatures and saturation magnetization deficit at low temperatures are discussed on the basis of the above-mentioned structural data.     

Confinement of the field electron emission to atomic sites on ultra sharp tips

The spatially controlled field assisted etching method for sharpening metallic tips, in a field ion microscope (FIM), is used to study the evolution of the field emission when the tip apex radius is decreased below 1 nm. Unlike the conventional image formation in a field emission microscope (FEM), we demonstrate that at this scale the field emission is rather confined to atomic sites. A single atom apex fabricated at the end of such tips exhibits an outstanding brightness compared to other atomic tips. The measurements have been repeated for two double atom tips, with different atom-atom separations, and images of atomic field emission localization have also been obtained. We have found that the field emission intensity alternates between adjacent atoms when the applied voltage is gradually increased beyond a threshold value.     

Incompatibility of the Shuttleworth equation with Hermann’s mathematical structure of thermodynamics

We show that three independent derivations of the Shuttleworth equation, arguably the second most important equation in surface physics, are inconsistent with Hermann’s mathematical structure of thermodynamics [R. Hermann, Geometry, Physics and Systems, Marcel Dekker Inc., New York, USA, 1973 (Chapter 6)]. The possible implications of this result are discussed.     

Unique edge structure and stability of fabricated dimer islands on Si(001)

The edge structure and stability of monolayer-high islands fabricated on Si(001) surfaces by scanning tunneling microscopy have been analyzed theoretically. In contrast to the edges of similar islands grown by depositing Si, the properties of edges of fabricated islands are determined by the length of the trench of dimers that are removed to create the island. We demonstrate the possibility of controlling the edge structure, and thus the stability, through a selective process of atom removal.     

Lifted reconstruction as a feedback mechanism in the oscillating CO oxidation on Pt nanofacets: Microscopic evidences

Global and local oscillations in the CO oxidation reaction have been visualized in situ on the apex of a [1 0 0]-oriented Pt field emitter tip used as a well-defined model for catalytically active, nm-sized particles by Field Emission (FEM) and Lithium Field Desorption (Li-FDM) Microscopes. For the first time experimental evidence is provided that the reconstruction feedback mechanism of the self-maintained oscillations for the [1 0 0] and [1 1 0] orientations, which is well established on macroscopic single crystals, is also valid in the heterogeneous, nm-sized system with its different crystallographic orientations which are coupled by surface diffusion.     

On the theory of transient nucleation at the intermediate stage of phase transitions

The evolution of a system of growing aggregates in a macroscopically homogeneous medium with account of both the reduction in metastability and the continuing initiation of new nuclei is studied. The corresponding integro-differential model describing the intermediate stage of phase transitions is solved analytically for arbitrary nucleation kinetics and growth rates of nuclei. An exact solution of the Fokker–Planck equation is found with allowance for the diffusivity along the axis of nucleus radii. In limiting cases of purely kinetic and mixed kinetic-diffusion rates of crystal growth for a special form of diffusivity, the obtained solutions transform to earlier known expressions.     

Transport at surface nanostructures measured by four-tip STM

For in situ measurements of local electrical conductivity of well-defined crystal surfaces in ultrahigh vacuum, we have developed two kinds of microscopic four-point probe methods. One is a ‘four-tip STM prober’, in which independently driven four tips of scanning tunneling microscope (STM) are used for four-point probe conductivity measurements. The probe spacing can be changed from 500 nm to 1 mm. The other one is monolithic micro-four-point probes, fabricated on silicon chips, whose probe spacing is fixed around several μm. These probes were installed in scanning-electron-microscopy/electron-diffraction chambers, in which the structures of sample surfaces and probe positions were in situ observed. The probes can be positioned precisely on aimed areas on the sample with aid of piezo-actuators. With these machines, the surface sensitivity in conductivity measurements has been greatly enhanced compared with macroscopic four-point probe method. Then the conduction through the topmost atomic layers (surface-state conductivity) and influence of atomic steps upon conductivity could be directly measured. The STM prober is mainly described here.