Nanocrystals: The shape of things to come
Chemist Paul Alivisatos is a leader in the development of nano-sized crystals that could serve as building blocks for electronic devices a few billionths of a meter in size.
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"Nanocrystals are particularly attractive as building blocks
for larger structures because it's possible - even easy - to
prepare nanocrystals that are highly perfect."
So says Paul Alivisatos, a chemist who holds a joint
appointment with Berkeley Lab's Materials Sciences Division
(MSD) and with the Chemistry Department of the University
of California at Berkeley. Alivisatos is one of the leading
lights in the burgeoning field of nano-sized semiconductor
Nanocrystals are aggregates of
anywhere from a few hundred to tens
of thousands of atoms that combine
into a crystalline form of matter known
as a "cluster." Typically around ten
nanometers in diameter, nanocrystals
are larger than molecules but smaller
than bulk solids and therefore
frequently exhibit physical and chemical
properties somewhere in between.
Given that a nanocrystal is virtually all
surface and no interior, its properties
can vary considerably as the crystal
grows in size.
"By precisely controlling a nanocrystal's
size and surface, its properties can be
tuned," Alivisatos says. "You can tune
the bandgap, you can tune how it conducts charge, you can change what crystal
structure it resides in, you can even change its melting temperature."
Growing flawless nanocrystals is relatively easy because their length of scale is so small
there's simply not enough time during the growth process to introduce defects. This
same tiny length of scale, however, makes controlling the size and surface of
nanocrystals a tremendous challenge. During the past decade, Alivisatos and his research
group have been growing nanocrystals out of semiconductor powders and exploring
various ways of altering growth conditions as a means of meeting this challenge.
One of Alivisatos' first big breakthroughs came when he and
collaborator Shimon Weiss discovered that spherical nanocrystals
made from a core of cadmium selenide inside a shell of cadmium
sulfide could, depending upon their size, be made to emit multiple
colors of light. This opened the door to a number of potential
applications, including the use of these spherical core-shell
nanocrystals as highly effective fluorescent labels for the study
of biological materials. In fluorescent labeling, markers, usually
antibodies that attach themselves to specific proteins, are
tagged with dye molecules that fluoresce or emit a specific color
of light when stimulated by photons, usually from a confocal
"Sometimes in order to fully characterize a biological sample, a
population of cells for example, you need to look at combinations
of markers," says Alivisatos. Such measurements require
multiple-color light emissions which are difficult to obtain with
conventional dye molecules.
Alivisatos and his colleagues grow their nanocrystals by injecting
semiconductor powders into hot, soap-like films called
surfactants. In their recent work, they've been growing their
crystals in a mixture of two surfactants, one called TOPO and
one called HPA, each of which reacts with the semiconductor
powders in a slightly different manner. The result has added an
entire new dimension to nanocrystal production - literally.
In March of last year, Alivisatos and his group made news when
they announced that they had made two-dimensional cadmium
selenide nanocrystals that were shaped like rods. Prior to this, the nanocrystals that had
been reported had all been dot-like spheres. Demonstrating the ability to grow
semiconductor nanocrystals into two-dimensional rods not only paves the way for a slew
of new potential applications, it also proves that controlling crystal growth is the key to
controlling shape as well as size.
"It was the first time anyone had really gotten control of semiconductor nanocrystal
growth," Alivisatos said. "By controlling the kinetics of crystal growth we were able to
select the size but vary the shapes of our crystals."
Although the precise mechanism is unknown, Alivisatos suspects that the interaction of
the cadmium and selenium atoms with the two different surfactants causes each crystal
to grow in only one direction. Consequently, maintaining a relatively fast rate of growth
in the right mix of surfactants induces crystals of a selected size to assume an
elongated, rod-like faceted shape that maximizes the crystal surface area.
Subsequent tests showed that rod-shaped nanocrystals emit light
polarized along their long axis in contrast to the nonpolarized light
fluoresced by the earlier cadmium selenide nanocrystal spheres. This
should be highly useful in biological-tagging studies where the orientation
of a marker is critical. Other tests showed that the gap between emission
and absorption energies is larger for nanocrystal rods than for nanocrystal
spheres. This should be an advantage for applications such light-emitting
diodes (LEDs) where the reabsorption of light can be a problem.
"We've also shown that multiple nanorods can be packed and aligned, another advantage
for both LEDs and for the use of these rods in photovoltaic cells."
More recently, Alivisatos and his group have learned to manipulate the conditions and
rate of crystal growth to the point where they have obtained semiconductor
nanocrystals in the shape of tear drops, arrowheads, and even four-armed tetrapods.
While these exotic shapes have no immediate application, they expand the possible
things that might be built from nanocrystal blocks in the future. For example, when the
tetrapod nanocrystals are dropped onto a surface they always land on three arms with
the fourth arm pointing straight up. This should be a handy feature for the wiring of
nanosized electronic devices.
First, however, scientists will have to learn how to assemble nanocrystals into larger,
more complex structures. One idea now being pursued by Alivisatos and his group-first
proposed by former Berkeley Lab researchers Peter Schultz and Paul McEuen-is to ride on
the back of nature by using DNA as a template.
"In a sense, what we now have is the ability to make all the little building blocks, but we
lack the chemistry that tells these blocks where they should all go," Alivisatos says. "It's
possible that DNA could be used to direct the assembly of nanocrystals into arbitrary
patterns of enormous complexity."
In the familiar "twisted ladder" image of DNA, two strands of
phosphate and ribose sugar molecules are joined by "rungs" made up
of a connecting pair of nitrogenous compounds called "bases." There
are four types of bases-adenine (A), cytosine (C), guanine (G), and
thymine (T)-and A always pairs with T, and G always connects with
C. Alivisatos and his colleagues capitalize on this highly specific
architectural program by using "linker" molecules to attach segments
of single-stranded DNA up to 100 bases in length (about 33
nanometers) to crystals of gold measuring 5 to 10 nanometers across. When these
nanocrystal/DNA "conjugates" are mixed with other segments of single-stranded DNA
containing base sequences that complement the sequences of the DNA in the
conjugates, the complementary bases recognize one another and pair off to form
double-stranded DNA. In this manner, DNA serves as a template for creating nanocrystal
Says Alivisatos, "In the strategy we're employing, the binding of DNA to the gold
nanocrystals is a statistical process. By adjusting the ratio of DNA to nanocrystal we can
control the average number of DNA strands per conjugate particle."
What has been needed is a way to separate and isolate conjugates according to size.
Alivisatos and his colleagues have recently accomplished this using the "gel
electrophoresis" technology that is a standard tool in biology for separating different
lengths of DNA fragments. By placing their nanocrystal/ DNA conjugates in a porous gel
under an electric field, they were able to separate and isolate gold nanocrystals attached
to anywhere from one to five segments of single-stranded DNA.
"We demonstrated that the shift in particle mobility (in
the electrified gel) due to the DNA attachment can be
used to produce nanocrystal/ DNA conjugates with a
well-defined number of DNA strands," Alivisatos says.
Proof that the information contained within DNA can
be harnessed for the spatial patterning of
semiconductor nanocrystals holds great promise for
their use as nanotechnology building blocks. Although
the assemblies so far have been relatively simple, the
indication is that nanocrystal/DNA conjugates could be
used to make structures and devices corresponding in
size and complexity to the semiconductor circuits
produced today using lithography.
"We've shown that we can use organic chemistry to
direct the assembly of inorganic crystals," Alivisatos
says. "It's a first step in transferring DNA from the
biological to the material world."