L. Glaser, S. Fiedler, G. Johannsen, P. Imperia, W. Wurth and M. Martins
Magnetism and reactivity of supported cluster
Clusters represent a new class of materials with properties between those of the
free atoms and the respective solids. Besides the new and interesting physical
effects which can be observed for clusters, they also have an enormous
technological importance, e.g. for the realization of electronic and magnetic
nanostructures. Recent experiments on the magnetic properties of free transition
metal clusters show for example a pronounced size dependence of the magnetic
moments of the clusters.
However, for possible applications it is important to
deposit the transition metal clusters on a substrate. For a detailed microscopic
understanding of the properties of deposited clusters it is necessary to
size-select the clusters prior to deposition. Experiments on well-defined,
mass-selected clusters after deposition on a substrate require furthermore
that the cluster size does not change during or after deposition. Up to
now there have been only a few attempts to study clusters under such well-defined
conditions spectroscopically.
We have developed a transportable (and synchrotron compatible),
UHV cluster source which enables us to perform core-electron spectroscopy
on well-defined, mass-selected, in-situ deposited metal clusters (n <
20; n number of atoms). The clusters are produced in a sputtering process,
mass-selected with a magnetic mass spectrometer and finally deposited under
soft-landing conditions (in an Ar-matrix) on a single crystal substrate.
"Soft-landing"
Our experiments
on Fe, Ni, and Cr-clusters show that we are able to soft-land small clusters
on a substrate and investigate them with inner-shell spectroscopies. It
has been possible for the first time to spectroscopically identify small
deposited clusters using near-edge x-ray absoprtion spectroscopy. For iron-clusters
we have been able to show that for fragmentation-free deposition about
10-15 layers of Ar are necessary. Deposition in thinner Ar-layers leads
to fragmentation of the smallest clusters upon deposition.
Cluster-substrate-interaction
Besides the soft-landing the cluster-substrate
interaction plays a decisive role for the controlled deposition of size-selected
clusters. The thermal stability of the deposited clusters against fragmentation
and/or agglomeration caused by diffusion is different for different cluster-substrate
combinations. While small iron clusters on an oxygen precovered Ru-surface are only weakly
bound and show surface diffusion already at room temperature Ni-clusters
on the contrary are stable on the same surface up to 800 K. Exceptionally
strong cluster-substrate interaction is observed for Cr-clusters on oxygen
covered surfaces. The Cr-clusters couple already at low temperatures (<
100 K) strongly to the adsorbed oxygen leading to Cr2O3-like
signatures in the x-ray absoprtion data. In general the clusters show size-dependent
differences in stability. For example heptamers which may be envisioned
as magic clusters in two dimensions exhibit fairly high thermal stability.
Reactivity
In first experiments on the reactivity of deposited transition metal clusters we
have investigated the interaction of deposited Ni- and Cr- clusters with
oxygen. After deposition of the clusters on a clean metal surface the clusters
were subsequently exposed to a few Langmuir of oxygen. For Cr as well as
for Ni we see an energetic shift of the x-ray absorption resonances upon
oxygen adsorption. However only for the Cr-clusters complete oxidation
independent of cluster size is observed (see above).
Magnetic properties
Recently we have started to investigate magnetic properties of deposited
transition metal clusters. In these experiments which have been performed
at BESSY II in Berlin we have deposited iron clusters on ultrathin Ni-films
which have been epitaxially grown on a Cu(100)-substrate and remanently
magneitsed afterwards. X-Ray absorption experiments with circularly
polarised light (XMCD) at the Fe-L2,3-edges show that the clusters
are ferromagnetically coupled to the substrate. The pronounced dichroic
signals can be used to study size-dependent magnetic moments of the clusters.
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