A
transmission electron microscope image at left and a color map version at right
highlights deformations in silver nanosheets laid over iron oxide nanospheres.
Rice University scientists determined that van der Waals forces between the
spheres and sheets are sufficient to distort the silver, opening defects in
their crystalline lattices that could be used in optics or catalysis.
Credit:
The Jones Lab/Rice University
You have
to look closely, but the hills are alive with the force of van der Waals.
Rice
University scientists found that nature's ubiquitous "weak" force is
sufficient to indent rigid nanosheets, extending their potential for use in
nanoscale optics or catalytic systems.
Changing
the shape of nanoscale particles changes their electromagnetic properties, said
Matt Jones, the Norman and Gene Hackerman Assistant Professor of Chemistry and
an assistant professor of materials science and nanoengineering. That makes the
phenomenon worth further study.
"People
care about particle shape, because the shape changes its optical properties,"
Jones said. "This is a totally novel way of changing the shape of a
particle."
Jones and
graduate student Sarah Rehn led the study in the American Chemical Society's
Nano Letters.
Van der
Waals is a weak force that allows neutral molecules to attract one another
through randomly fluctuating dipoles, depending on distance. Though small, its
effects can be seen in the macro world, like when geckos walk up walls.
"Van
der Waals forces are everywhere and, essentially, at the nanoscale everything is
sticky," Jones said. "When you put a large, flat particle on a large,
flat surface, there's a lot of contact, and it's enough to permanently deform a
particle that's really thin and flexible."
In the new
study, the Rice team decided to see if the force could be used to manipulate
8-nanometer-thick sheets of ductile silver. After a mathematical model showed
them it was possible, they placed 15-nanometer-wide iron oxide nanospheres on a
surface and sprinkled prism-shaped nanosheets over them.
Without
applying any other force, they saw through a transmission electron microscope
that the nanosheets acquired permanent bumps where none existed before, right
on top of the spheres. As measured, the distortions were about 10 times larger
than the width of the spheres.
The hills
weren't very high, but simulations confirmed that van der Waals attraction
between the sheet and the substrate surrounding the spheres were sufficient to
influence the plasticity of the silver's crystalline atomic lattice. They also
showed that the same effect would occur in silicon dioxide and cadmium selenide
nanosheets, and perhaps other compounds.
"We
were trying to make really thin, large silver nanoplates and when we started
taking images, we saw these strange, six-fold strain patterns, like
flowers," said Jones, who earned a multiyear Packard Fellowship in 2018 to
develop advanced microscopy techniques.
"It
didn't make any sense, but we eventually figured out that it was a little ball
of gunk that the plate was draped over, creating the strain," he said.
"We didn't think anyone had investigated that, so we decided to have a
look.
"What
it comes down to is that when you make a particle really thin, it becomes
really flexible, even if it's a rigid metal," Jones said.
In further
experiments, the researchers saw nanospheres could be used to control the shape
of the deformation, from single ridges when two spheres are close, to saddle
shapes or isolated bumps when the spheres are farther apart.
They
determined that sheets less than about 10 nanometers thick and with aspect
ratios of about 100 are most amenable to deformation.
The
researchers noted their technique creates "a new class of curvilinear
structures based on substrate topography" that "would be difficult to
generate lithographically." That opens new possibilities for
electromagnetic devices that are especially relevant to nanophotonic research.
Straining
the silver lattice also turns the inert metal into a possible catalyst by
creating defects where chemical reactions can happen.
"This
gets exciting because now, most people make these kinds of metamaterials
through lithography," Jones said. "That's a really powerful tool, but
once you've used that to pattern your metal, you can never change it.
"Now
we have the option, perhaps someday, to build a material that has one set of
properties and then change it by deforming it," he said. "Because the
forces required to do so are so small, we hope to find a way to toggle between
the two."