Stability of two-dimensional nanostructures

We investigate atomic and molecular nanostructures on metal surfaces by variable low-temperature scanning tunnelling microscopy. In combination with molecular dynamics calculations we achieve a detailed understanding of the stability of these structures. Atomic nanostructures in homoepitaxial metallic systems are thermodynamically only metastable. Two-dimensional islands on Ag(110) decay above a threshold temperature of T₁ = 175 K. Caused by the anisotropy of the surface, distinct decay behaviours exist above and below a critical temperature of T_c = 220 K. Calculations based on effective medium potentials of the underlying 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 reconstructed Au(111) surface, the pseudochiral 1-nitronaphthalin forms two-dimensional supermolecular clusters consisting predominantly of ten molecules. Comparison of images with submolecular resolution to local density calculations elucidates the thermodynamical stability as well as the internal structure of the decamers.