The process of surface wetting and formation of ice structures on surfaces have
been studied extensively. Beyond being of great importance within the eld of
surface technologies [1], it may also govern freeze-out mechanisms in the inter-
stellar medium (ISM) [2]. In cold regions of the ISM icy mantles exist on the
surface of interstellar dust grains. These may act as a heterogeneous catalysis
system where adsorbed molecules may interact, sometimes mediated by photon
irradiation, to form large and complex molecules [3]. Structural dierences of
water ice, which are detectable in IR-spectroscopy [4], will have an impact on
the interaction between complex molecules in the ice, e.g. for adsorption and
diusion characteristics [5]. The structure of the water ice mantle depends on
the thermal history and will dier depending on formation conditions.
Here we examine water ice formation on highly oriented pyrolytic graphite
(HOPG) in the sub-monolayer regime using a low-temperature scanning tun-
nelling microscope (LT-STM) functioning at 5 K. Sub-monolayer amounts of
heavy water (D2O) were deposited onto an HOPG surface kept at 40 K. Pre-
sented in Figure 1 is water ice clusters grown at 40 K via molecular deposition
on the HOPG surface. A clear diusion limited aggregation (DLA [6]) growth
is visible from the fractal structure of the clusters, both near step edges and for
individual nucleation sites. Only 2D growth is observed at these sub-monolayer
coverages. DLA leads to low densities and an amorphous and porous structure
which with growth may form the reputed amorphous solid water of icy grain
mantles. Finally, we investigate how an oxygen functionalised O-HOPG surface
may in
uence the already complex growth of water ice clusters on interstellar
grain analogues, oering a unique look into non heat-treated water ice clusters.