Research Objectives
To investigate, through the development of large-scale classical and quantum simulation methodologies, the size-dependent evolution of materials properties, atomic-scale formation mechanisms of interfacial nanojunctions and wires and their structural, mechanical, dynamic, electronic and transport characteristics; nanocrystalline assemblies; and atomistic origins of nanotribological and thin-film lubrication phenomena.
Computational Approach
Large-scale classical and quantum molecular dynamics (MD) simulations (where the phase-space trajectories of a large number of interacting particles are generated via solution of the appropriate equations of motion) are used to explore materials physical and chemical processes on refined spatial and temporal scales. These simulations include: (1) first-principles quantum MD simulations involving concurrent calculations of many-particle potential energy surfaces using density functional theory in conjunction with nonlocal pseudopotentials and plane-wave basis sets, used in studies of size-evolutionary physical and chemical patterns in clusters, nanocrystals and their assemblies, and atomic, electronic, mechanical, and transport properties of metal and semiconductor nanowires, and (2) grand-canonical ensemble classical MD simulations of confined molecular liquid systems, with up to a million atoms interacting through tested many-atom potentials, used in investigations of nanotribology and thin-film rheology and lubrication.
Accomplishments
(1) First principles quantum MD simulations of finite temperature dynamics, atomic and electronic structures, and transport in simple metal nanowires, predicting formation of "magic wire configurations" of enhanced stability, and dynamical conductance fluctuations. (2) Elucidation of the energetic and entropic origins of solvation forces, rheological transitions, and structural and phase transformations in equilibrium and sheared interfacial films, and their dependencies on molecular shape, size and structural complexity, as well as on interfacial morphology.
Significance
The development of large-scale atomistic simulation methodologies, coupled with high-performance computational platforms, allows explorations, with predictive capabilities, of the origins of physical and chemical processes in complex materials and under extreme conditions. Investigations of size-dependent evolution of material properties in clusters, atomic and electronic structures and dynamics in nanowires and nanocrystalline assemblies, and energetics, structure, rheology, and dynamics of highly confined complex fluids yield deep insights into the nature of these systems. These investigations allow interpretation of experiments, predict novel phenomena, guide laboratory synthesis and probing of novel materials systems, and address issues pertaining to future technologies, particularly in the areas of atomic-scale materials manipulations for device, sensor, and machine miniaturization, and tribology in ultra-high-density information storage systems.
Publications
Barnett, R. N., and U. Landman. 1997. Cluster-derived structures and conductance fluctuations in nanowires. Nature 387:788.
Gao, J., W. D. Luedtke, U. Landman. 1997. Layering transitions and dynamics of confined liquid films. Phys. Rev. Lett. 79:705. J. Phys. Chem. B 101:4013.
Cleveland, C. L., U. Landman, et. al. 1997. Structural evolution of smaller gold nanocrystals: The truncated decahedral motif. Phys. Rev. Lett. 79:1873.
A selected atomic configuration of a sodium nanowire obtained via first-principles
molecular dynamics. Atoms are shown as spheres. Superimposed are two isosurfaces
depicting a wavefunction near the Fermi level which contributes to the electronic
conductance of the nanowire.