Chirality, defects, and disorder in gold clusters

Theoretical and experimental information on the shape andmorphology of bare and passivated gold clusters is fundamentalto predict and understand their electronic, optical, and otherphysical and chemical properties. An effective theoreticalapproach to determine the lowest-energy configuration (globalminimum) and the structures of low energy isomers (local minima)of clusters is to combine genetic algorithms and many-bodypotentials (to perform global structural optimizations), andfirst-principles density functional theory (to confirm thestability and energy ordering of the local minima). The maintrend emerging from structural optimizations of bare Au clustersin the size range of 12-212 atoms indicates that manytopologically interesting low-symmetry, disordered structuresexist with energy near or below the lowest-energy orderedisomer. For example, chiral structures have been obtained as thelowest-energy isomers of bare Au₂₈ andAu₅₅ clusters, whereas in the size-rangeof 75-212 atoms, defective Marks decahedral structures arenearly degenerate in energy with the ordered symmetricalisomers. For methylthiol-passivated gold nanoclusters[Au₂₈(SCH₃)₁₆andAu₃₈(SCH₃)₂₄],density functional structural relaxations have shown that theligands are not only playing the role of passivating molecules,but their effect is strong enough to distort the metal clusterstructure. In this work, a theoretical approach to characterizeand quantify chirality in clusters, based on the Hausdorffchirality measure, is described. After calculating the index ofchirality in bare and passivated gold clusters, it is found thatthe thiol monolayer induces or increases the degree of chiralityof the metallic core. We also report simulated high-resolutiontransmission electron microscopy (HRTEM) images which show thatdefects in decahedral gold nanoclusters, with size between 1-2nm, can be detected using currently available experimental HRTEMtechniques.