Uzi Landman, Georgia Institute of Technology

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

	Density of Kohn-Sham states (DOS) of the (a) Au38 cluster and (b) passivated Au38(SCH3) 24 cluster. The dots on the upper axes denote the eigenenergies of (a) Au atomic valence states, and (b) SCH3 molecular valence states. The dashed line denotes the Fermi energy. The width of the energy bins is 0.2 eV. The insets show the optimized structures of the bare gold and the passivated clusters, respectively. In (b) the sulfur atoms are depicted as darker spheres and only one CH3 group is shown.

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

Ab initio investigations of the electronic structures and optimal configurations of gold nanocrystals (Au₃₈) passivated by methylthiols showed that the electronic states of the bare cluster are modified upon passivation by an organic monolayer, and that added charges are delocalized on the passivating layer. Such clusters serve as quantum dots, capable of accepting charge and transporting electrons between source and drain electrodes. Our study demonstrated the first ab initio calculation of the capacitance of such passivated nanocrystals. Investigations of the electronic structure and addition-energy spectrum in two-dimensional quantum dots led to the discovery of spontaneous symmetry-breaking at zero and low magnetic fields in single dots and dot molecules. The discovery of Wigner molecules whose formation can be controlled via voltage gates suggests new strategies for information coding and logic gates using quantum dots. Investigations of the mechanisms of the low-temperature combustion of CO catalyzed by small gold clusters adsorbed on the surface of magnesium oxide elucidated the origins of the clusters' surprising catalytic activity. These results are significant for the development of novel catalytic systems.

Significance

Understanding the microscopic origins of the properties of materials with reduced physical dimensions is essential for the utilization of such materials in advanced technologies. Small is different-new and often unexpected behavior emerges whenever the physical size of the materials system approaches a length-scale characteristic to the phenomenon under study.

Publications

H. Häkkinen, R. N. Barnett, and U. Landman, "Electronic structure of passivated Au₃₈ (SCH₃) ₂₄ nanocrystal," Phys. Rev. Lett. 82, 3264 (1999).

W. D. Luedtke and U. Landman, "Slip diffusion and Levy flights of an adsorbed nanocluster," Phys. Rev. Lett. 82, 3835 (1999).

W. D. Luedtke and U. Landman, "Structure and thermodynamics of self-assembled monolayers on gold nanocrystallites," J. Phys. Chem. B 102, 6566 (1998).