Teraflop 73 - Novembre - cesca

Investigations of finite aggregates
of small sizes and reduced dimensionalities,
exhibiting discrete
quantized energy level spectra and
specific structures and morphologies,
open avenues for systematic explorations
of the physical factors and unifying
principles that underlie the transition
from the atomic and molecular domain
to the condensed phase regime.
Such behavior, where the properties of
a system do not scale with it’s reduced
physical size, but rather where Small is
Different in an essential way that can not
be deduced through extrapolation from
knowledge of bulk behavior, is emergent
in nature.
Identification and understanding
of the microscopic origins of such
emergent phenomena, that show behavior
on a larger scale than that characterizing
the interactions between the
elementary components, and where
the exhibited behavior is characteristic
of the assembly of particles rather then
being a property of the individual constituents,
are of fundamental importance
for elucidation of the principles
and physical mechanisms that govern
the behavior of materials in the
nanoscale regime. These principles include:
(a) A hierarchy of self-assembly
REFERENCES
(1) (a) U. Landman et al., “Microscopic Mechanisms
and Dynamics of Adhesion, Microindentation
and Fracture”. Science 248, 454
(1990). (b) U. Landman et al., “Metal-Semiconductor
Nanocontacts: Silicon Nanowires”.
Phys. Rev. Lett. 85, 1958 (2000).
(2) B. Bhushan, J.N. Israelachvili, U. Landman,
“Nanotribology: Friction, Wear and Lubrication
at the Atomic Scale”, Nature 374, 607
(1995); J. Gao, W.D. Luedtke, and U. Land-
Small Is Different
Physics and Chemistry
in the Non-scalable Regime
Uzi Landman
School of Physics
Georgia Institute of Technology, Atlanta
routes —that is, formation of nanostructural
units from atoms and molecules
and self-organization of such units into
higher-level structures, such as superlattices;
(b) Self-selection of size and
shape, such as that underlying the observation
of “magic-number” cluster
sequences; (c) Enhancement of the
role of fluctuations, particularly for fluids
of highly reduced dimensions.
Gaining insights into the nature of
physical and chemical systems of
highly reduced sizes, and developing
experimental and theoretical methodologies
aimed at probing, manipulating
and controlling such systems on
the atomic and molecular level, are
among the major challenges of current
basic interdisciplinary research.
Furthermore, these investigations are
of great potential relevance to technological
developments that aim at exploitation
of the novel properties of
materials at the nanoscale. Computerbased
theoretical modeling and simulations
play an increasingly important
role in modern condensed matter
physics, chemistry, materials science,
and biology. In particular, such studies,
that may be called computational
microscopies, allow explorations of
complex phenomena with refined res-
man. “Friction Control in Thin-Film Lubrication”,
J. Phys. Chem. Chem. B 102, 5033
(1998); J. Gao et al., Tribol. Lett. 9, 3 (2000).
(3) M. Moseler and U. Landman. “Formation,
Stability and Breakup of Nanojets”. Science
289, 1165 (2000).
(4) A. Sánchez et al., “When Gold is not Noble:
Nanoscale Gold Catalysts”. J. Phys.
Chem. A 103, 9573 (1999); S. Abbet, U. Heiz,
H. Hakkinen, and U. Landman. “CO Oxdida-
olution in space and time. To further
discuss and illustrate the above physical
and methodological issues, we
present results obtained through
large-scale simulations of the following
systems and phenomena:
■ Formation mechanisms, mechanical
response, quantized electric conductance,
and chemical properties of metal
and semiconductor nanowires and
their interconnections (1), see Figure 1.
■ Nanotribological processes (2), layering
transitions of highly confined liquids,
and control of friction through
atomic-scale modifications of lubricated
junctions, as well as via atomicscale
external directional modulations,
see Figure 2.
■ Generation, stability, and breakup of
nanojets (3), and development of a stochastic
Navier-Stokes approach (the socalled
Stochastic Lubrication Equation,
or SLE) that extends the range of validity
of the hydrodynamic formulation to
the molecular-scale, see Figure 3.
■ Nanocatalysis by small gold and palladium
clusters (4), and the development
of guidelines for atomic-scale
control of catalytic activity, see Figure 4.
■ Charge transport in DNA (5), gated by
the diffusive motion of hydrated counterions,
see Figure 5. ■
tion on a Single Pd Atom Model Catalyst”.
Phys. Rev. Lett, 86, 5950 (2001); M. Moseler,
H. Hakkinen, U.Landman, 89, 033401, (2002);
H. Hakkinen et al., Angew. Chem. Int. Ed. 42,
1297 (2003).
(5) R.N. Barnett, C.L. Cleveland, A. Joy, U.
Landman, and G.B. Schuster. “Charge Migration
in DNA: Ion-Gated Transport”, Science
294, 567 (2001).