Scanning tunneling spectroscopy of metal clusters on graphite
Heinz Hövel, Universität Dortmund
Even if one is able to deposit mass selected clusters on a surface,
spectroscopy of the electronic properties of clusters on surfaces is a
complicated problem, since small clusters may have different isomers and
orientations on the surface, and averaged spectra may not show the full
information included in the electronic structure of the single clusters. An
alternative approach is the study of individual clusters using scanning
probe techniques, in particular scanning tunneling microscopy and
spectroscopy.
We followed this route using metal clusters grown on graphite surfaces at
preformed nanometer sized pits acting as well defined nucleation centers.
Scanning tunneling microscopy was used to characterize the size-dependent
cluster morphology [1]. This is an important prerequisite for understanding
the electronic cluster-properties, which could be measured for individual
clusters using low-temperature scanning tunneling spectroscopy. The
interpretation of scanning tunneling spectra of clusters is still a topic
of current research, therefore we used ultraviolet photoelectron
spectroscopy as a complementary method. It gives information on the
occupied electronic states and the cluster-surface interaction. In addition
it helps us to study the cluster growth process [2].
For the case of gold clusters on graphite we have investigated a broad
range of cluster sizes, from a few ten up to more than 10e4 atoms per
cluster. The tunneling spectroscopy data for the large clusters is
dominated by a confined Shockley surface state, which can be described
quantitatively considering the confinement to the hexagonal (111) facets on
top of the clusters [3]. For the small clusters without facets scanning
tunneling spectroscopy results of a large number of clusters show
remarkably systematic features changing with the cluster size, but here the
interpretation is still difficult. It might require not only a detailed
analysis of the electron confinement in three dimensions but also an
inclusion of special transport phenomena in the tip-cluster-surface system.
[1] T. Irawan, I. Barke, H. Hövel, Appl. Phys. A, accepted for publication.
[2] H. Hövel, Appl. Phys. A 72, 295 (2001).
[3] I. Barke, H. Hövel, Phys. Rev. Lett. 90, 166801 (2003).