In situ transmission electron microscopy studies on structural dynamics of transition metal nanoclusters
- PDF / 4,594,003 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 27 Downloads / 220 Views
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 °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 °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 °C. In all cases, the coalescence process indicates a strong influence of the local environment of the cluster.
I. INTRODUCTION
Materials built up from nanostructure constituents can have properties that are fundamentally different from conventional bulk materials.1–4 Novel behavior will manifest itself when the size of the building blocks is lower than the critical length scale of a particular property. Another reason is the very high specific surface area of nanoparticles; the surface reactivity is increased, and the large area of interfaces plays a major role in the assembled nanophase forms. It is expected that nanostructuring will have a great impact on the ways in which materials and products are developed.5 For example, the need for ever smaller electronic devices such as processors and memories also necessitates completely new circuit architectures. Dimensions in the semiconductor industry are already entering the nanometer range ( 0, D1 is the diffusion coefficient of a single atom).3,9,10 This will lead to negligible diffusion (Dn Ⰶ D1) for clusters of size larger than 5 nm (or n > 104). Of course, a more complete analysis should also consider details of the interaction between cluster and surface. Notably, if the nanoparticle has significantly higher surface energy than the substrate, partially embedding can occur due to the large capillary forces on the particle.31 The surface energies of Fe and Co are larger than that of Si3N4 (e.g., for Co is 2.8 J/m2 31 against 1.4 J/m2 for Si3N4). Figure 2 shows that the individual coalescence events depend on the local surroundings; those clusters on top of other clusters fuse more easily than those in contact with the substrate. The situation is not straightforward, however, since the reverse of coalescence, i.e. cluster
FIG. 2. Coalescence of Fe clusters at various substrate temperatures up to 780 °C.
J. Mater. Res., Vol. 20, No. 7, Jul 2005
1787
T. Vystavel et al.: In situ transmission electron micros
Data Loading...
