Equilibrium structures of a bare gold nanowire (AuNW, left) and of wires chemically modified by adsorption of a SCH3 molecule, with the molecule adsorbed in the middle of the wire (m-SCH3) or at the vicinity of the tip (t-SCH3). Yellow spheres correspond to Au atoms, and in the chemically modified wires S, C, and H atoms are depicted by red, green, and blue spheres, respectively. Marked distances are in units of Å.

Research Objectives
This project is investigating the microscopic physical and chemical processes underlying the properties of novel materials. These investigations aim at discovering and elucidating size-dependent evolutionary patterns of materials properties, bridging the molecular, cluster, and condensed-phase regimes.

Computational Approach
Our computational approaches include large-scale classical molecular dynamics, employing tested many-body interactions, and ab initio quantum molecular dynamics (in conjunction with norm-conserving non-local pseudopotentials and a plane-wave basis) based on local-spin density functional theory (LSD) with the inclusion of generalized exchange-correlation gradient corrections. In these ab initio simulations, the dynamics of the ions evolve on the concurrently calculated electronic ground state (Born-Oppenheimer, BO) potential energy surface, using the BO-LSD-MD method. We also employ various structural optimization methods (conjugate-gradient and variants thereof, simulated annealing and genetic algorithms), as well as an arsenal of analysis techniques, including animation.

Accomplishments
We reported on ab initio local-density functional investigations of the atomic structure, electronic spectrum, and conductance of a gold nanowire consisting of a four-atom chain connected to gold electrodes. We explored structural and electronic spectral modifications resulting from adsorption of a molecule (methylthiol, SCH₃) to the wire. These results provide a new interpretation of the measured electron microscopy image of the atomic gold wire and suggest a new strategy for formation of organo-metallic nanowires, as well as the use of nanowires as monitoring and chemical sensing devices.

In contrast to the inert nature of gold as bulk material, nanosize particles of gold supported on various oxides, as well as two-monolayer-thick gold islands of up to 4 nm diameter on titania, were found to exhibit an enhanced catalytic activity, in particular for the low-temperature oxidation of CO. We demonstrated the size dependence of the activity of nanoscale gold clusters, with Au₈ found to be the smallest size to catalyze the reaction.

Other studies included photoelectron spectra of aluminum cluster anions; spontaneous symmetry breaking in single and molecular quantum dots; structures, solvation forces, and shear of molecular films in a rough nano-confinement; formation, stability, and breakup of nanojets; and metal-semiconductor nanocontacts.

Significance
Understanding the microscopic origins of the properties of materials with reduced physical dimensions is essential for the utilization of such materials systems in advanced technologies, including miniaturization of electronic and mechanical devices, development of sensors, design of novel logic gates and information storage strategies using quantum dots, control of friction under extreme conditions, cluster-catalysts, and atomic-scale materials manipulations. Small is different — new and often unexpected behavior emerges when the physical size of the materials system is reduced to microscopic scale.

Publications
J. Akola, M. Manninen, H. Hakkinen, X. Li, L.-S. Wang, and U. Landman, “Photoelectron spectra of aluminum cluster anions: Temperature effects and ab initio simulations,” Phys. Rev. B 60, R11297 (1999).

A. Sanchez, S. Abbet, U. Heiz, W.-D. Schneider, H. Hakkinen, R. N. Barnett, and U. Landman, “When gold is not noble: Nanoscale gold catalysts,” J. Phys. Chem. A 103, 9573 (1999).

H. Hakkinen, R. N. Barnett, and U. Landman, “Atomic gold nanowires and their chemical modification,” J. Phys. Chem. B 103, 8814 (1999).