If the promise of nanotechnology is to be fulfilled, nanoparticles will have
to be able to make something of themselves. An important advance towards this
goal has been achieved by researchers with the U.S.
Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab)
who have found a simple and yet powerfully robust way to induce nanoparticles
to assemble themselves into complex arrays.
This electron micrograph shows a self-assembled composite in which nanoparticles of lead sulfide have arranged themselves in a hexagonal grid. Credit: Ting Xu, Lawrence Berkeley National Laboratory- UC Berkeley
By adding specific types of small molecules to mixtures of nanoparticles and
polymers, the researchers are able to direct the self-assembly of the nanoparticles
into arrays of one, two and even three dimensions with no chemical modification
of either the nanoparticles or the block copolymers. In addition, the application
of external stimuli, such as light and/or heat, can be used to further direct
the assemblies of nanoparticles for even finer and more complex structural details.
"We've demonstrated a simple yet versatile approach to precisely controlling
the spatial distribution of readily available nanoparticles over multiple length
scales, ranging from the nano to the macro," says Ting Xu, a polymer scientist
who led this project and who holds joint appointments with Berkeley Lab's Materials
Sciences Division and the University of California, Berkeley's Departments of
Materials Sciences and Engineering, and Chemistry. "Our technique can be
used on a wide variety of nanoparticle and should open new routes to the fabrication
of nanoparticle-based devices including highly efficient systems for the generation
and storage of solar energy."
Xu is the corresponding author on a paper describing this work that has been
published by the journal Nature Materials. The paper is titled: "Small
molecule-directed nanoparticle assembly towards stimuli-responsive nanocomposites."
Co-authoring this paper with her were Yue Zhao, Kari Thorkelsson, Alexander
Mastroianni, Thomas Schilling, Joseph Luther, Benjamin Rancatore, Kazuyuki Matsunaga,
Hiroshi Jinnai, Yue Wu, Daniel Poulsen, Jean Fréchet and Paul Alivisatos.
The Art of Self-Assembly
Nano-sized particles - bits of matter a few billionths of a meter in
size, or more than a hundred times smaller than the stuff of today's microtechnologies
- display highly coveted properties not found in macroscopic materials,
including optical, electronic, magnetic, etc. The promise of nanotechnololgy
is that exploiting these unique properties on a commercial scale could yield
such "game-changers" as sustainable, clean and cheap energy, and the
creation on demand of new materials with properties tailored to meet specific
needs. Realizing this promise starts with nanoparticles being able to organize
themselves into complex structures and hierarchical patterns, similar to what
nature routinely accomplishes with proteins.
"Precise control of the spatial organization of nanoparticles and other
nanoscopic building blocks over multiple length scales has been a bottleneck
in the bottom-up generation of technologically important materials," says
Xu. "Most of the approaches that have been used so far have involved surface
modifications."
Small as they are, nanoparticles are essentially all surface so any process
that modifies the surface of a nanoparticle can profoundly change the properties
of that particle. Precisely arranging these nanoparticles is critical to tailoring
the macroscopic properties during nanoparticle assembly. Although DNA has been
used to induce self-assembly of nanoparticles with a high degree of precision,
this approach only works well for organized arrays that are limited in size;
it is impractical for large-scale fabrication. Xu believes a better approach
is to use block copolymers - long sequences or "blocks" of one
type of monomer molecule bound to blocks of another type of monomer molecule.
"Block copolymers readily self-assemble into well-defined arrays of nanostructures
over macroscopic distances," she says. "They would be an ideal platform
for directing the assembly of nanoparticles except that block copolymers and
nanoparticles are not particularly compatible with one another from a chemistry
standpoint. A mediator is required to bring them together."
Xu and her group found such a "mediator" in the form of small molecules
that will join with nanoparticles and then able attach themselves and their
nanoparticle partners to the surface of a block copolymer. For this study, Xu
and her group used two different types of small molecules, surfactants (wetting
agents) dubbed "PDP" and "OPAP." These small molecules can
be stimulated by light (PDP) or heat (OPAP) to sever their connection to the
surface of a block copolymer and be repositioned to another location along the
polymeric chain. In this manner, the spatial distribution of the small molecule
mediators and their nanoparticle partners can be precisely directed with no
need to modify either the nanoparticles or the polymers.
"The beauty of this technique is that it involves no sophisticated chemistry,"
says Xu. "It really is a plug and play technique, in which you simply mix
the nanoparticles with the block copolymers and then add whatever small molecules
you need."
For this study, Xu and her colleagues added PDP or OPAP small molecules to
various blends of nanoparticles, such as cadmium selenide and lead sulfide,
mixed in with a commercial block copolymer - polystyrene-block-poly (4-vinyl
pyridine). While she and her group worked with light and heat, she says other
stimuli, such as pH, could also be used to reposition small molecules and their
nanoparticle partners along block copolymer formations. Strategic substitutions
of different types of stimulus-responsive small molecules could serve as a mechanism
for structural fine-tuning or for incorporating specific functional properties
into nanocomposites. Xu and her colleagues are now in the process of adding
functionality to their self-assembly technique.
"Bring together the right basic components - nanoparticles, polymers
and small molecules - stimulate the mix with a combination of heat, light
or some other factors, and these components will assemble into sophisticated
structures or patterns," says Xu. "It is not dissimilar from how nature
does it."