  We have investigated the growth of Pt on Ge(110) using scanning tunneling
microscopy and spectroscopy. The deposition of several monolayers of Pt on
Ge(110) followed by annealing at 1100 K results in the formation of
three-dimensional metallic Pt-Ge nanocrystals. The outermost layer of these
crystals exhibits a honeycomb structure. The honeycomb structure is composed of
two hexagonal sub-lattices that are displaced vertically by 0.2 {\AA} with
respect to each other. The nearest-neighbor distance of the atoms in the
honeycomb lattice is 2.5${\pm}$0.1 {\AA}, i.e. very close to the predicted
nearest-neighbor distance in germanene (2.4 {\AA}). Scanning tunneling
spectroscopy reveals that the atomic layer underneath the honeycomb layer is
more metallic than the honeycomb layer itself. These observations are in line
with a model recently proposed for metal di-(silicides/)germanides: a hexagonal
crystal with metal layers separated by semiconductor layers with a honeycomb
lattice. Based on our observations we propose that the outermost layer of the
Ge2Pt nanocrystal is a germanene layer.

Applied Physics;Materials Science