  The assembly of nanometer-sized building blocks into complex morphologies is
not only of fundamental interest but also plays a key role in material science
and nanotechnology. We show that the shape of self-assembled superstructures
formed by rod-like viruses can be controlled by tuning the attraction via the
depletion interaction between the rods. Using nonadsorbing polymers as a
depleting agent, we demonstrate that a hierarchical unidimensional
self-organization into crystalline clusters emerges progressively upon
increasing depletion attraction -and enhanced growth kinetics. We observe a
polymorphic change proceeding from two-dimensional (2D) crystalline monolayers
at weak depletion to one-dimensional (1D) columnar fibers at strong depletion,
via the formation of smectic fibrils at intermediate depletion strength. A
simple theory for reversible polymerization enables us to determine the typical
bond energy between monomeric units making up the smectic fibrils. We also
demonstrate that gentle flow-assistance can be used to template filament-like
structures into highly aligned supported films. Our results showcase a generic
bottom-up approach for tuning the morphology of crystalline superstructures
through modification of the interaction between non-spherical building blocks.
This provides a convenient pathway for controlling self-organization,
dimensionality and structure-formation of anisotropic nanoparticles for use in
nanotechnology and functional materials.
