[Zurück]

F. Aumayr, G. Kowarik, R. Ritter, C. Vasko, C. Gösselsberger, W. Meissl, A.S. El-Said:
"Surface nanostructures created by irradiation with slow highly charged ions";
in: "XVIIth Symposium on Atomic and Surface Physics (SASP 2010), Invited Paper", herausgegeben von: editors: I. Milewski, A. Kendl and P. Scheier, Innsbruck University Press, Conf. Series; XVIIth Symposium on Atomic and Surface Physics (SASP 2010), 2010, (eingeladen), ISBN: 978-3-902719-52-2, S. 74 - 77.

Irradiation of solid targets with swift or fast heavy ions can lead to severe structural
modifications at the surface and in the bulk (see e.g. [1-4] and refs. therein). These
modifications include the formation of latent tracks in the solid, the creation of hillock-
(or crater-) type nanostructures on the surface, and the occurrence of phase transitions,
e.g. from crystalline to amorphous or from superconducting to insulating. The formation
of tracks or hillocks is usually linked to a critical energy loss (dE/dx) of the projectiles and
occurs particularly in insulators (e.g., polymers, oxides, ionic crystals). While there is no
question that the intense electronic excitation of a confined volume around the ion
trajectory due to the electronic stopping of the swift ions is the major cause for these
modifications [5], the exact mechanism how this energy is transferred to the lattice is still
under discussion and probably strongly depends on the type of material [6-12].
Highly charged ions (HCI) carry a large amount of potential energy [13]. For slow
HCI this potential energy (equal to the total ionisation energy which had to be spent for
producing the HCI from its neutral atomic ground state) can become comparable to or
even considerably exceed the ions kinetic energy, resulting in additional electron
emission or sputtering (potential electron emission [14-16], potential sputtering [13, 17-
20]), phenomena which are usually dominated by kinetic effects (kinetic electron emission
[21-23], kinetic sputtering [5, 24]). In the case of Xe44+, for example, this potential energy
amounts to about 51 keV. Upon interaction with solid surfaces the HCI deposit their
potential energy within a very short time (a few femto-seconds) within a nanometer size
volume close to the surface [13, 16, 25, 26]. It is therefore not astonishing that similar to
the case of swift heavy ions surface modifications with nanometer dimensions have also
been demonstrated for the impact of slow highly charged ions on various surfaces [27,28].
Figure 1 shows, as an example, irradiation induced defects on CaF2 [27-29]. The
image is a topographic contact mode AFM image taken in air, showing hillocks, which
appear to be caused by impact of single Xe44+ (Ekin 2.2 keV/amu) ions. Recently the
mechanism of the hillock formation on CaF2 could be related to a local solid-liquid phase
transition (melting) of the crystal around the impact site of a single highly charged ion
[29-31]. The dependence of the hillock diameter on the projectiles´ potential energy, as
shown in the figure (right), reveals two sharp thresholds occurring at around 14 keV and
50 keV. The first is related to local melting, similar as in the case of swift heavy ions,
where such a scenario has been observed before [7]. The latter threshold could be related
to another phase transition (sublimation) [28].