by Stephen Battersby/NewScientistTech/
Carbon may be losing its monopoly over the nanoworld. According to
the latest calculations, tubes built out of the element boron could
have many of the same properties as carbon nanotubes, the ubiquitous
components of nanoengineering. And for some electronic applications,
they should even be better than carbon.
Boron
nanotubes will have a more complicated shape than the simple linked
hexagons that work for carbon, as the chemistry of boron makes that
chicken-wire pattern unstable. The first boron nanotubes to be created,
in 2004, are thought to be formed from a buckled triangular latticework.
But according to Xiaobao Yang, Yi Ding and Jun Ni
from Tsinghua University in Beijing, China, the best configuration for
boron is to take the unstable hexagon lattice and add an extra atom to
the centre of some of the hexagons (see image, top right). They
calculate that this is the most stable known theoretical structure for
a boron nanotube.
Their
simulation also shows that, with this pattern, boron nanotubes should
have variable electrical properties: wider ones would be metallic
conductors, but narrower ones should be semiconductors. If so, then
boron tubes might be used in nanodevices similar to the diodes and
transistors that have already been made from carbon nanotubes, says Ni.
Variable surfaces
It's
a surprise to other researchers, who expected all boron nanotubes to be
metallic. "If this is true, it's interesting," says Sohrab Ismail-Beigi
of Yale University in New Haven, Connecticut, whose earlier paper
showed that this same structure would make stable flat sheets of boron.
Ismail-Beigi
suspects that it will be difficult to make semiconducting boron
nanotubes, even so. His work shows that there are many different
structures almost as stable as this one, so real tubes are likely to
have variable surfaces. "I would hypothesize that this would make most
tubes metallic," Ismail-Beigi told New Scientist.
Metallic
boron nanotubes would still be useful, however, as they should be
better conductors than carbon. Ismail-Beigi speculates that they might
also be superconducting at higher temperatures. So if a superconducting
nanocomputer is ever built, it might have boron wiring.
To
actually make the boron tubes, Ni suggests chemical vapour deposition,
which is a process already used to grow carbon nanotubes. This
technique requires an appropriate catalyst, such as a nanoparticle of
nickel, to act as a template for the nanotube. "The key issue for the
growth of boron nanotubes is to find effective catalysts," says Ni.
Journal ref: Physical Review B (DOI: 10.1103/PhysRevB.77.041402)
Source: Boron nanotubes could outperform carbon
---------------
New nanostructured thin film shows promise for efficient solar energy conversion
In the race to make solar cells cheaper and more efficient, many
researchers and start-up companies are betting on new designs that
exploit nanostructures--materials engineered on the scale of a
billionth of a meter. Using nanotechnology, researchers can experiment
with and control how a material generates, captures, transports, and
stores free electrons--properties that are important for the conversion
of sunlight into electricity. Two nanotech methods for engineering solar cell
materials have shown particular promise. One uses thin films of metal
oxide nanoparticles, such as titanium dioxide, doped with other
elements, such as nitrogen. Another strategy employs quantum
dots--nanosize crystals--that strongly absorb visible light. These tiny
semiconductors inject electrons into a metal oxide film, or "sensitize"
it, to increase solar energy conversion. Both doping and quantum dot
sensitization extend the visible light absorption of the metal oxide
materials.
Combining these two approaches appears to yield
better solar cell materials than either one alone does, according to
Jin Zhang, professor of chemistry at the University of California,
Santa Cruz. Zhang led a team of researchers from California, Mexico,
and China that created a thin film doped with nitrogen and sensitized
with quantum dots. When tested, the new nanocomposite material
performed better than predicted--as if the functioning of the whole
material was greater than the sum of its two individual components.
"We have discovered a new strategy that could be very useful for
enhancing the photo response and conversion efficiency of solar cells
based on nanomaterials," said Zhang.
"We initially thought that the best we might do is get results
as good as the sum of the two, and maybe if we didn't make this right,
we'd get something worse. But surprisingly, these materials were much
better."
The group's findings were reported in the Journal of Physical
Chemistry in a paper posted online on January 4. Lead author of the
paper was Tzarara Lopez-Luke, a graduate student visiting in Zheng's
lab who is now at the Instituto de Investigaciones Metalurgicas, UMSNH,
Morelia, Mexico.
Zhang's team characterized the new nanocomposite
material using a broad range of tools, including atomic force
microscopy (AFM), transmission electron microscopy (TEM), Raman
spectroscopy, and photoelectrochemistry techniques. They prepared films
with thicknesses between 150 and 1100 nanometers, with titanium dioxide
particles that had an average size of 100 nanometers. They doped the
titanium dioxide lattice with nitrogen atoms. To this thin film, they
chemically linked quantum dots made of cadmium selenide for
sensitization.
The resulting hybrid material offered a combination of advantages.
Nitrogen doping allowed the material to absorb a broad range of light
energy, including energy from the visible region of the electromagnetic
spectrum. The quantum dots also enhanced visible light absorption and
boosted the photocurrent and power conversion of the material.
When compared with materials that were just doped with nitrogen or
just embedded with cadmium selenide quantum dots, the nanocomposite
showed higher performance, as measured by the "incident photon to
current conversion efficiency" (IPCE), the team reported. The
nanocomposite's IPCE was as much as three times greater than the sum of
the IPCEs for the two other materials, Zhang said.
"We think what's happening is that it's easier for the charge to
hop around in the material," he explained. "That can only happen if you
have both the quantum dot sensitizing and the nitrogen doping at the
same time."
The nanocomposite material could be used not only to enhance solar
cells, but also to serve as part of other energy technologies. One of
Zhang's long-term goals is to marry a highly efficient solar cell with
a state-of-the-art photoelectrochemical cell. Such a device could, in
theory, use energy generated from sunlight to split water and produce
hydrogen fuel. The nanocomposite material could also potentially be
useful in devices for converting carbon dioxide into hydrocarbon fuels,
such as methane.
The new strategy for engineering solar cell materials offers a promising path for Zhang's lab to explore for years to come.
"I'm very excited because this work is preliminary and there's a
lot of optimizing we can do now," Zhang noted. "We have three
materials--or three parameters--that we can play with to make the
energy levels just right."
In essence, the team has been trying to manipulate materials so
that when sunlight strikes them, the free electrons generated can
easily move from one energy level to another--or jump across the
different materials--and be efficiently converted to electricity.
"What we're doing is essentially 'band-gap engineering.' We're
manipulating the energy levels of the nanocomposite material so the
electrons can work more efficiently for electricity generation," Zhang
said. "If our model is correct, we're making a good case for this kind
of strategy."
Source: University of California - Santa Cruz
Via: http://www.physorg.com/news119024680.html
