Why understanding how nanowires self-assemble
could lead to a new crop of nanodevices
Dr. Frances Ross, materials scientist at IBM Research
Artistic rendering of self-assembled nanowires
composed of different crystal structures
that spontaneously grow with the help of a
catalytic nanoparticle at the tip of each nanowire.
(credit: Aidan Sugano.)
Building transistors today is done with
lithography, which is a “top-down” process that uses patterning to create the
complex layers that make up the transistor structure. It’s a bit like exposing
a negative on photographic paper to get the pattern you want and then using
this pattern as a template to place each material – metal, insulator or
semiconductor – in exactly the right location. This process has worked
successfully since the 1950s, and our scientists have even demonstrated the first
working test chips with features approaching 7
the equivalent of placing more than 20 billion tiny switches on chips the size
of a fingernail. But as we get to ever-smaller dimensions,
new approaches to building nanoscale devices will be required.
At IBM’s T. J. Watson Research Center we
use a technique called self-assembly to grow and directly control nanostructures
that could one day form parts of integrated circuits. Self-assembly looks at chip
building from the other end of the spectrum: a “bottom-up” approach that builds
nanostructures in a way that is dictated by physics rather than by an imposed pattern.
In some ways it’s like farming, in that that you plant seeds to grow a crop,
and then support the growth with the right conditions to get the result you
But exploring self-assembly doesn’t
mean we are ready to throw away today’s approach; instead, we want to use top-down
strategies that we have already learned over many years, and combine them with new
tricks that use self-assembly.
Think of water splashing onto a pane of
glass. It spontaneously forms little hemispheres. The droplets are
hemispherical because surface tension pulls
the water molecules into this shape to minimize the surface area and energy of
each droplet. But there is no reason for the droplets to form in any particular
location or to be any particular size, so their positions and sizes are random.
The spontaneous formation of the hemispherical shape is an example of
self-assembly, but other aspects of the process (position, size) are not
imagine there is a scratch on the glass. Water droplets form on the scratch,
because it is a good, low energy place for the water molecules to stick. We
have now combined self-assembly –
“make a hemispherical droplet on this surface” – with an imposed pattern –
“make a droplet on this part of the surface
by using carefully placed scratches.” The result is that we can build more complicated
patterns. Flexible, customized patterns like this water example, but on the
nanoscale, help us build integrated circuits.
The more precisely we can
direct this self-assembly, the more versatility we can achieve. We can choose
different materials for our nanostructures, build them with different sizes, and
control their chemical compositions in ways that allow them to be tuned to have
the properties we need. The properties of some nanomaterials could include the
ability to do the job of a transistor but with less power, or at extreme
temperatures beyond what silicon can handle.
How to direct a nanowire
In order to direct
self-assembly, we have to understand the physical stimuli that influence atoms
to assemble in a certain way as they form a nanostructure. The particular
nanostructures we find most interesting are called nanowires. These
are long thin crystals whose amazing length-to-width
ratio could help create very densely packed transistors. Using a combination of imposed
patterning and self-assembly, we can grow nanowires spontaneously using the
help of catalytic particles. And we can watch the nanowires as they grow, recording
the process on video using a one-of-a-kind Ultra High Vacuum Transmission
Electron Microscope in our lab.
load a flat substrate into the microscope, place catalytic particles onto it (this
is the directed part of the process), then heat it and add some reactive gases.
We watch what happens to the catalytic particles (this is the self-assembly
part of the process) by magnifying the image by 50,000 times or more. The reaction can be slow – it takes
hours for the whole experiment to be finished – but the videos show how the
nanostructures grow, one layer of atoms after another. Recording
videos, for example at different temperatures or with different added gases, is
central to understanding every step of the nanowires’ growth. We get to see
cause and effect when the conditions change, so we can work out the laws of
physics that control the growth.
This video was recorded in an ultra high vacuum transmission electron microscope showing a silicon nanowire growing from a gold-based catalyst. The silicon is the vertical post and the catalyst is the liquid droplet on the tip. The diameter of the nanowire is 70 nanometers; in other words, about 1/1,000th of the diameter of a human hair. The stripes in the silicon and the zigzag shape along its edge provide information about the atomic arrangement within the nanowire. This movie was recorded at 500 degrees C and is sped up by 30 times.
Recently, we have become
especially interested in growing nanowires made of gallium arsenide that form
with the help of catalysts made of gold nanoparticles. For this we need two reactive
gases, trimethylgallium and arsine. We chose these because they supply the two
components needed to build the nanowire, gallium and arsenic. When we record our
movies, the first reaction we see is between gallium and gold. This reaction
turns the original gold nanoparticles into hemispherical liquid gold-gallium droplets.
As we continue to watch, gallium and arsenic combine within each droplet to start
growing a gallium arsenide nanowire beneath the droplet.
Gallium arsenide nanowires
grown this way are particularly special because it is possible to change the
way the gallium and arsenic atoms stack up within each nanowire. Two
arrangements of the atoms are possible, and we can change from one to the other
simply by altering the temperature of the reaction or even just varying the
ratio of the two gases as they flow past the catalysts. The videos show how
these changes in growth conditions modify the way the atoms arrange themselves
at the junction between the nanowire and the catalyst. And that causes a change
in how the atoms eventually stack up when they form the nanowire. We still have
the same material, gallium arsenide, but the two possible arrangements of the atoms
lead to different electrical properties for the whole nanowire.
what drives atoms to take up one arrangement versus another gives us a better
chance of growing nanowires that have the particular electrical properties that
are needed for a device such as
a nano-transistor. It’s akin to having more
colors on your palette so that you can paint a better picture.
These special nanowires, composed
of regions with different atomic arrangement, have applications in photonics or
single electron transistors, both important building blocks for electronic
circuits. And simply knowing that we can control the crystal arrangement in a
nanowire will open up the microprocessor community’s imagination for new
devices. In particular, optoelectronics, where light and electricity are
combined in photonics structures, is a good bet. But that’s just the “tip of
Our latest results, "Interface dynamics and crystal phase switching in Gallium Arsenide nanowires", will be published in this week’s Nature.