S. A. Koch

Nanosized iron clusters investigated with in situ transmission electron microscopy

Transmission electron microscopy is employed for investigating the structural stability of nanosized iron clusters as deposited and after in situ annealing treatments under high vacuum conditions. The thin iron oxide shell that is formed around the iron clusters (upon air exposure) is of the order of 2 nm surrounding a 5 nm core of body-centered-cubic (bcc) iron. The oxide shell breaks down upon annealing at relatively low temperatures (∼500 °C) leading to pure iron particles having a bcc crystal structure. Annealing of clusters, which are in contact, leads to their fusion and formation of larger clusters preserving their crystallographic structure and being free of any oxide shell. On the other hand, isolated clusters appear rather immobile (upon annealing). The truncated rhombic dodecahedron was found as the most probable shape of the clusters which differs from former theoretical predictions based on calculations of stable structural forms.

Magnetic versus structural properties of Co nanocluster thin films: A magnetic force microscopy study

Magnetic force microscopy (MFM) has been employed to study thin films consisting of low-energy-deposited cobalt nanoclusters. On continuous cluster layers a clear magnetic stray field pattern can be observed, although measurements on individual clusters are complicated by interference from topography. The magnetic correlation length determined from MFM images is substantially larger than the size of a single cluster. This indicates that the clusters are magnetically coupled to form stable domains associated with the formation of a correlated super-spin-glass state.

Coalescence aspects of cobalt nanoparticles during in situ high-temperature annealing

In this work we investigate the coalescence aspects of Co nanoparticles. It was observed that nanoparticles in contact with the substrate are relatively immobile, whereas those on top of other Co particles can rearrange themselves during high-temperature annealing and further coalesce. Indeed, similar size particles prior to coalescence come at close proximity forming an arc-shaped area, which leads to finite-size necking and thereafter to coalescence towards a single partially spherical particle. This is in contrast to the theoretical predictions where necking occurs following an initial pathway of a point contact. Moreover, it was shown that after necking a transient period of relatively fast coalescence occurs followed by a slower coalescence rate at constant speed towards a single particle with partial spherical shape. In addition, the coalescence is faster with decreasing particle size, where in the case of unequal size the smallest particle is mainly absorbed by an adjacent large one in an Ostwald ripening process.

Growth front roughening of room-temperature deposited copper nanocluster films

Growth front aspects of copper nanocluster films deposited with low energy onto silicon substrates at room temperature are investigated by atomic force microscopy. Analyses of the height-difference correlation function yield a roughness exponent H of 0.45±0.05. The root-mean-sqaure roughness amplitude w evolves with deposition time as a power law, w∝tβ (β=0.62±0.07), leading also to a power-law increase of the local surface slope ρ, ρ∝tc (c=0.73±0.09). These scaling exponents, in combination with an asymmetrical height distribution, point at a complex nonlinear roughening mechanism dominated by the formation of voids resulting in a highly porous film.

Niobium nanoclusters studied with in situ transmission electron microscopy

Structural aspects of deposited niobium nanoclusters approximately 10 nm in size have been explored by means of high-resolution transmission electron microscopy. The niobium clusters have a bcc structure and a crystal habit of rhombic dodecahedron. In situ heating up to ∼800 °C revealed a resistance to high temperatures, in the sense that the cluster habit is preserved. However, the internal structural order of the clusters is altered due to formation of niobium oxide domains within the clusters. Coalescence does not occur even at the highest temperatures, which is attributed to the presence of facets and the occurrence of oxidation during heat treatment.

In situ transmission electron microscopy studies on structural dynamics of transition metal nanoclusters

The structural stability of transition metal nanoclusters has been scrutinized with in situ transmission electron microscopy as a function of temperature. In particular iron, cobalt, niobium, and molybdenum clusters with diameters around 5 nm have been investigated. During exposure to air, a thin oxide shell with a thickness of 2 nm is formed around the iron and cobalt clusters, which is thermally unstable under moderate high vacuum annealing above 200 degrees C. Interestingly, niobium clusters oxidize only internally at higher temperatures without the formation of an oxide shell. They are unaffected under electron beam irradiation, whereas iron and cobalt undergo severe structural changes. Further, no cluster coalescence of niobium takes place, even during annealing up to 800 degrees C, whereas iron and cobalt clusters coalesce-after decomposition of the oxide, as long as the clusters are in close contact. In contrast to niobium, molybdenum clusters do not oxidize upon annealing; they are stable under electron beam irradiation and coalesce at temperatures higher than 800 degrees C. In all cases, the coalescence process indicates a strong influence of the local environment of the cluster.

Structural dynamics of gas-phase molybdenum nanoclusters: A transmission electron microscopy study

In this paper we study structural aspects of molybdenum clusters by transmission electron microscopy. The deposited clusters with sizes 4nm or larger show a body-centered crystal (bcc) structure. The clusters are self-assembled from smaller structural units and form cuboids with a typical size of 4nm or larger. With reducing cluster size, the face-centered crystal (fcc) structure appears due to dominance of surface energy minimization, while self-assembly into large cuboids with sizes up to 30nm is still observed. Our observation implies that the basic building blocks of large cuboids are actually not smaller cubes. Annealing leads to cluster coalescence at temperatures ∼800°C, with the crystal habit changing to truncated rhombic dodecahedron for isolated clusters, while the large cuboidally shaped particles become more faceted polyhedra.

Ultra high vacuum scanning Auger/electron microscopy studies of oxidation and B surface segregation of in situ fractured B- doped Ni3Al alloys

Abstract This paper focuses on local probe Auger spectroscopy studies of segregation and oxidation of in situ fractured Ni 3 Al specimens, both with and without B-doping. Although immediately after in situ fracture small amounts of segregated B at grain boundaries were observed occasionally in the B-doped specimens, the amount of B at the intergranular fracture surface drastically increased with time upon exposure to the ambient vacuum. After subsequent removal of B by depth profiling by in situ Ar + -bombardment (1 keV) at a rate of ∼3.3 nm/min, B again segregated to the surface within several hours, accompanied by surface oxidation, under ultrahigh vacuum conditions at room temperature. Possible mechanisms of B surface segregation are related to the high binding energy between B and O and the fact that B lowers the surface energy of a Ni-enriched surface. Finally, prolonged exposure of the fracture surface to the electron beam is shown to lead to enhanced Ni oxidation and to less B surface segregation.

Electron beam induced oxidation of Ni3Al surfaces: electron flux effects

Electron beam irradiation of polycrystalline boron doped Ni3Al (at 300 K and under ultrahigh vacuum conditions) induces fast oxidation. The rate and depth of oxidation initially increase with increasing electron flux as indicated by results from Auger electron spectroscopy. Curves of oxygen development were fitted using a kinetic model that assumes the creation of oxide nucleation centers by the electron beam. The corresponding cross-sections were extracted. For fluxes exceeding 1 mA/cm(2), the oxidation rate is limited by the amount of oxygen present in the vacuum environment. For lower e-beam fluxes the oxidation process is slower with significant O chemisorption, resulting in shallower oxidation. These findings point out a way to control the thickness of nickel oxide in the nanometer range

Influence of electron flux on the oxidation of Ni3Al surfaces

Electron beam currents of a few nanoamps, currently used in nanometer scale scanning Auger microscopy scanning electron microscopy, promote oxidation of polycrystalline Ni3Al to a degree that depends on the size of the beam and subsequently on the electron flux φe. In fact, the oxidation of Ni3Al at room temperature follows a model based on the premise that the electron beam creates additional nucleation sites around which oxide growth occurs. With increasing beam size the oxidation process becomes slower and O chemisorption plays a significant role. As a result the Ni-oxide depth decreases drastically with an increasing spot size (or equivalently decreasing electron flux). It offers an alternative way to monitor the NiO thickness in the nanometer range.