Stability of two-dimensional nanostructures

We investigate atomic and molecular nanostructures on metal surfaces by var iable low-temperature scanning tunnelling microscopy. In combination with m olecular dynamics calculations we achieve a detailed understanding of the s tability of these structures. Atomic nanostructures in homoepitaxial metallic systems are thermodynamical ly only metastable. Two-dimensional islands on Ag(110) decay above a thresh old temperature of T-l = 175 K. Caused by the anisotropy of the surface, di stinct decay behaviours exist above and below a critical temperature of T-c = 220 K. Calculations based on effective medium potentials of the underlyi ng rate limiting atomic processes allow us to identify the one-dimensional decay below T-c as well as the two-dimensional decay above T-c. In contrast to atoms, the intermolecular electrostatic interaction of polar molecules leads to thermodynamically stable structures. On the reconstruct ed Au(lll)surface, the pseudochiral 1-nitronaphthalin forms two-dimensional supermolecular clusters consisting predominantly of ten molecules. Compari son of images with submolecular resolution to local density calculations el ucidates the thermodynamical stability as well as the internal structure of the decamers.