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Professor
Louis Brus of the Columbia University MRSEC has shown the striking
structural patterns that are observed when nanocrystals self-organize
in two dimensions at room temperature, as solvent is evaporated
on a smooth surface where nanocrystals are highly mobile. (This
was described in a previous nugget.) It was recognized then that
some, but not all, aspects of this process could be understood
as an equilibrium gas-liquid phase transition of weakly attractive
particles, analogous to the formation of liquid Ar from gaseous
Ar at low temperature. Now, a deeper theoretical understanding
has been achieved through a collaboration among Profs. David Reichman
of Harvard University, Phillip Geissler, presently at the University
of California Berkeley, Eran Rabani of Tel Aviv University (adjunct
MRSEC member), and Louis Brus. They have built a numerical computer
model that captures the non-equilibrium fluctuation aspects of
solvent evaporation as well as the equilibrium aspects of organization.
The accompanying figure shows a 2D coarse-grained lattice gas
model of large orange nanocrystals, yellow solvent, and brown
dry surface. Realistic parameter simulations of this model not
only account for all observed spatial and temporal patterns, but
also predict network structures that have not yet been explored.
Two distinct mechanisms of ordering emerge, corresponding to homogeneous
and heterogeneous limits of evaporation dynamics. The resulting
guide for designing statistically patterned arrays of nanoparticles
advances the possibility of fabricating spontaneously organized
nanoscale devices. |
 Model of dyring with regions represented gas (air), liquid (the remaining solvent), and the nanocrystal. |
