The new 3D nanofabrication method makes it possible to manufacture complex multi-layered solids all in one step. In this example, seen in these scanning electron microscope images, a view from above (at top) shows alternating layers containing round holes and long bars. As seen from the side (lower image), the alternating shapes repeat through several layers. Image: Chih-Hao Chang |
The
making of 3D nanostructured materials has become a fertile area
of research, producing materials that are useful for electronics, photonics, phononics,
and biomedical devices. But the methods of making such materials have been
limited in the 3D complexity they can produce. Now, a Massachusetts Institute
of Technology (MIT) team has found a way to produce more complicated structures
by using a blend of current “top-down” and “bottom-up”
approaches.
The
work is described in a paper published in Nano
Letters, co-authored by postdoc Chih-Hao Chang; George Barbastathis, the
Singapore Research Professor of Optics and Professor of Mechanical Engineering;
and six MIT graduate students.
One
approach to making 3D nanostructures—a top-down approach—is called phase-shift
lithography, in which a two-dimensional mask shapes the intensity of light
shining onto a layer of photoresist material. The photoresist is altered only
in the areas reached by the light. However, this approach requires very
precisely manufactured phase masks, which are expensive and time-consuming to
make.
Another
method—a bottom-up approach—is to use self-assembling colloidal nanoparticles
that form themselves into certain energetically favorable close-packed
arrangements. These can then be used as a mask for physical deposition methods,
such as vapor deposition, or etching of the surface, to produce 2D structures.
But these methods are slow and limited by defects that can form in the
self-assembly process. Although they can be used for the fabrication of 3D
structures, this is made difficult because any defects propagate through the
layers.
“We
do a little bit of both,” Chang says. “We took a chemist’s method and
added in a flavor of engineering.”
The
new method is a hybrid in which the self-assembled array is produced directly
on a substrate material, performing the function of a mask for the lithography
process. The individual nanoparticles that assemble on the surface each act as
tiny lenses, focusing the beam into an intensity pattern determined by their
arrangement on the surface. The method, the authors say in their paper,
“can be implemented as a novel technique to fabricate complex 3D
nanostructures in all fields of nanoscale research.”
Depending
on the shapes and arrangements of the tiny glass beads they use for the
self-assembly part of the process, it is possible to create a great variety of
structures, “from holes to higher-density posts, rings, flowery
structures, all using the exact same system,” Chang says. “It’s a
very simple way to make 3D nanostructures, and probably the cheapest way right
now. You can use it for many things.”
Team
members, whose specialty is in optics, say the first structures they plan to
make are photonic crystals, whose structure can manipulate the behavior of
light beams passing through them. But the method can also be used to make
phononic materials, which control waves of heat or sound, or even to make
filters with precisely controlled porosity, which might have biomedical
applications.
