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Transmission electron micrograph (a) shows cadmium sulfide nanorods forming arrays that are aligned and oriented parallel to the cylindrical microdomains of block copolymers. Schematic drawing (b) illustrates copolymers with nanorods. Image: Lawrence Berkeley National Laboratory |
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A relatively fast, easy, and inexpensive technique
for inducing nanorods—rod-shaped semiconductor nanocrystals—to self-assemble
into 1D, 2D, and even 3D macroscopic structures has been developed by a team of
researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley
National Laboratory (Berkeley Lab). This technique should enable more effective
use of nanorods in solar cells, magnetic storage devices, and sensors. It
should also help boost the electrical and mechanical properties of
nanorod-polymer composites.
Leading this project was Ting Xu, a polymer
scientist who holds joint appointments with Berkeley Lab’s Materials Sciences
Division and the University of California (UC) Berkeley’s Departments of Materials Sciences
and Engineering, and Chemistry. Xu and her research group used block copolymers—long
sequences or “blocks” of one type of monomer bound to blocks of another type of
monomer—as a platform to guide the self-assembly of nanorods into complex
structures and hierarchical patterns. Block copolymers have an innate ability
to self-assemble into well-defined arrays of nano-sized structures over
macroscopic distances.
“Ours is a simple and versatile technique for
controlling the orientation of nanorods within block copolymers,” Xu says. “By
varying the morphology of the block copolymers and the chemical nature of the
nanorods, we can provide the controlled self-assembly in nanorods and
nanorod-based nanocomposites that is critical for their use in the fabrication of
optical and electronic devices.”
Xu is the corresponding author of a paper
describing this research that has been published in the journal Nano
Letters under the title “Direct Nanorod Assembly Using Block
Copolymer-Based Supramolecules.” Co-authoring the paper were Kari Thorkelsson,
Alexander Mastroianni, and Peter Ercius.
Nanorods—particles of matter a thousand times
smaller than the stuff of today’s microtechnologies—display highly coveted optical,
electronic, and other properties not found in macroscopic materials. To fully
realize their vast technological promise, however, nanorods must be able to
assemble themselves into complex structures and hierarchical patterns, similar
to what nature routinely accomplishes with proteins.
Xu and her research group first enlisted block
copolymers as allies in this self-assembly effort in 2009, working with the
spherical nanoparticles commonly known as quantum dots. In that study, they
lashed quantum dots to block copolymers via a “mediator” of small adhesive
molecules. In this latest development, Xu and her group again made use of
adhesive molecules, but this time to mediate between the nanorods and
supramolecules of block copolymers. A supramolecule is a group of molecules
that act as a single molecule able to perform a specific set of functions.
“Block copolymer supramolecules self-assemble and
form a wide range of morphologies that feature microdomains typically a few to
tens of nanometers in size,” Xu says. “As their size is comparable to that of
nanoparticles, the microdomains of block copolymer supramolecules provide an
ideal structural framework for the co-assembly of nanorods.”
Xu and her group incorporate nanorods into
solutions of block copolymer supramolecules that form spherical, cylindrical
and lamellar microdomains. During the drying process energy is contributed to
the system from the interactions between nanorod ligands and polymers, the
entropy associated with polymer chain deformation upon nanorod incorporation,
and the interactions between individual nanorods. Xu and her group observed
that these energetic contributions determine the placement and distribution of
the nanorods, as well as the overall morphology of the nanorod-block copolymer
composites. These energetic contributions can be easily tuned by varying the
supramolecular morphology, which is accomplished simply by attaching different
types of small molecules to the side chains of the block copolymers.
“We can readily access a wide library of nanorod
assemblies including arrays of nanorods aligned parallel to block copolymer cylindrical
microdomains, continuous nanorod networks, and nanorod clusters,” Xu says.
“Since the macroscopic alignment of block copolymer microdomains can be
obtained in bulk and in thin films by the application of external fields, our
technique should open up a viable route to manipulate the macroscopic
alignments of nanorods.”
This new technique can produce ordered arrays of
nanorods that are macroscopically aligned with tunable distances between
individual rods—a morphology that lends itself to the production of plasmonics,
which are materials that hold great promise for superfast computers,
ultra-powerful optical microscopes, and even the creation of invisibility
carpets. It is also a straightforward nanoparticle self-assembly technique that
can produce a continuous network of nanorods with nanoscopic separation
distances. Such networks can enhance the macroscopic properties of nanocomposites,
including electrical conductivity and material strength.
Xu credits much of the success of this research to
the exceptional capabilities and staff at the National Center
for Electron Microscopy (NCEM), a DOE national user facility at Berkeley Lab,
which is home to the world’s most powerful electron microscopes.
“For the study of three-dimensional nanorod
assemblies, we needed to implement high-resolution tomography and this posed a
challenge not only for collecting the imaging data but also for processing it,”
Xu says. “The expertise and skill of NCEM’s Peter Ercius was invaluable.”
Most of this work was done on NCEM’s 200kV
monochromated UT Tecnai microscope, an instrument designed to produce optimum
high resolution performance in both transmission and scanning transmission
electron microscopy modes.
Xu and her group are now investigating the
self-assembly of semiconductor nanocrystals that take the shapes of cubes or
tetrapods, both of which have important potential applications for photovoltaic
and other technologies.
“We’d also like to investigate the self-assembly of
nanoparticles into combinations of different shapes,” Xu says.
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