Novel two dimensional water structures on Pd(111) studied by STM: Experiment and Theory

The early stages of water growth on Pd(111) has been studied by means of low temperature Scanning Tunneling Microscopy (STM) experiments[1]. At the lowest coverages, individual water molecules diffuse around the metal surface. Upon collision, they form dimers, trimers, tetramers, pentamers and cyclic hexamers, the latter showing the largest stability (see Figure 1.a). As the coverage is further increased, islands presenting a commensurate hexagonal honeycomb structure relative to the Pd(111) substrate are formed (Figure 1.b). After some time, the island edges end up decorated by bright features (Figure 1.c). Next, heating up the sample to 70 K leads to a restructuring of the islands (Fig. 1.d), now adopting more regular shapes which resemble either a rosette pattern (Figure 2.c) or a lace-type pattern (Figure 2.e). These pseudo-ordered structures are particularily intruiguing. They evidence that no uniform wetting occurs, as it would be expected for ice growth.