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Scientists with the Lawrence Berkeley National
Laboratory (Berkeley Lab) and the University of California (UC), Berkeley have
directed the first self-assembly of nanoparticles into device-ready materials.
Through a relatively easy and inexpensive technique based on blending
nanoparticles with block co-polymer supramolecules, the researchers produced
multiple-layers of thin films from highly ordered 1D, 2D, and 3D arrays of gold
nanoparticles. Thin films such as these have potential applications for a wide
range of fields, including computer memory storage, energy harvesting, energy
storage, remote-sensing, catalysis, light management, and the emerging new
field of plasmonics.
“We’ve demonstrated a simple yet versatile
supramolecular approach to control the 3D spatial organization of nanoparticles
with single particle precision over macroscopic distances in thin films,” says
polymer scientist Ting Xu, who led this research. “While the thin gold films we
made were wafer-sized, the technique can easily produce much larger films, and
it can be used on nanoparticles of many other materials besides gold.”
Xu holds joint appointments with Berkeley Lab’s
Materials Sciences Division and UC Berkeley’s Departments of Materials Sciences
and Engineering, and Chemistry. She is the corresponding author of a paper
describing this research in Nano Letters. Co-authoring the paper were
Joseph Kao, Peter Bai, Vivian Chuang, Zhang Jiang, and Peter Ercius.
Nanoparticles can be thought of as artificial atoms
with unique optical, electrical and mechanical properties. If nanoparticles can
be coaxed into routinely assembling themselves into complex structures and
hierarchical patterns, similar to what nature does with proteins, devices a
thousand times smaller than those of today’s microtechnologies could be
mass-produced.
Xu and her research group have been advancing
towards this goal for the past decade. In a study earlier this year, they were
able to induce rod-shaped semiconductor nanocrystals to self-assemble into 1D,
2D, and even 3D macroscopic structures. With this latest application of their
methods to thin films, they have moved into the realm of material forms that
are required for device fabrication and are well-suited for scalable
nanomanufacturing.
“This is the first time that 2D nanoparticle
assembly, similar to those obtained using DNA linkers and controlled solvent
evaporation, can be clearly achieved in multi-layers in supramolecule-based
nanocomposite thin films,” Xu says. “Our supramolecular approach does not
require chemical modification to any of the components in the composite system
and, in addition to providing a means of building nanoparticle-based devices,
should also provide a powerful platform for studying nanoparticle
structure-property correlations.”
The technique developed by Xu and her colleagues
uses solutions of block co-polymer supramolecules to direct the self-assembly
of nanoparticles. A supramolecule is a group of molecules that act as a single
molecule able to perform a specific set of functions. Block copolymers are long
sequences or “blocks” of one type of monomer bound to blocks of another type of
monomer that have an innate ability to self-assemble into well-defined arrays
of nano-sized structures over macroscopic distances.
“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-self-assembly of nanoparticles.”
In this latest study, Xu and her colleagues
incorporated gold nanoparticles into solutions of block co-polymer
supramolecules to form films that ranged in thickness between 100 to 200 nm.
The nanocomposite films featured microdomains in one of two common morphologies—lamellar
or cylindrical. For the lamellar microdomains, the nanoparticles formed
hexagonally-packed 2D sheets that were stacked into multiple layers parallel to
the surface. For the cylindrical microdomains, the nanoparticles formed 1D
chains (single particle width) that were packed into distorted hexagonal
lattices in parallel orientation with the surface.
“Upon incorporation of nanoparticles, the block
co-polymer supramolecules experience conformational changes, resulting in entropy
that determines the placement and distribution of the nanoparticles, as well as
the overall morphology of the nanocomposite thin films,” Xu says. “Our results
indicate that it should be possible to generate highly-ordered lattices of
nanoparticles within block co-polymer microdomains and obtain 3D hierarchical
assemblies of nanoparticles with precise structural control.”
The inter-particle distance between gold
nanoparticles in the 1D chains and the 2D sheets was 8 to 10 nm, which raises
intriguing possibilities with regards to plasmonics, the phenomenon by which a
beam of light is confined in ultracramped spaces. Plasmonic technology holds
great promise for superfast computers and optical microscopy, among other
applications. However, a major challenge for developing plasmonics has been the
difficulty of fabricating metamaterials with noble metal nanoparticles such as
gold.
“Our gold thin films display strong plasmonic
coupling along the inter-particle spacing in the 1D chains and 2D sheets
respectively,” Xu says. “We should therefore be able to use these films to
investigate unique plasmonic properties for next-generation electronic and
photonic devices. Our supramolecular technique might also be used to fabricate
plasmonic metamaterials.”
