It is the double helix, with its
stable and flexible structure of genetic information, that made life on Earth
possible in the first place. Now a team from the Technical University of Munich
(TUM) has discovered a double helix structure in an inorganic material. The
material comprising tin, iodine and phosphorus is a semiconductor with
extraordinary optical and electronic properties, as well as extreme mechanical
flexibility.
Flexible yet robust - this is
one reason why nature codes genetic information in the form of a double helix.
Scientists at TU Munich have now discovered an inorganic substance whose
elements are arranged in the form of a double helix.
The substance called SnIP,
comprising the elements tin (Sn), iodine (I) and phosphorus (P), is a
semiconductor. However, unlike conventional inorganic semiconducting materials,
it is highly flexible. The centimeter-long fibers can be arbitrarily bent
without breaking.
"This property of SnIP is
clearly attributable to the double helix," says Daniela Pfister, who
discovered the material and works as a researcher in the work group of Tom
Nilges, Professor for Synthesis and Characterization of Innovative Materials at
TU Munich. "SnIP can be easily produced on a gram scale and is, unlike
gallium arsenide, which has similar electronic characteristics, far less
toxic."
The semiconducting properties of
SnIP promise a wide range of application opportunities, from energy conversion
in solar cells and thermoelectric elements to photocatalysts, sensors and
optoelectronic elements. By doping with other elements, the electronic
characteristics of the new material can be adapted to a wide range of
applications.
Due to the arrangement of atoms
in the form of a double helix, the fibers, which are up to a centimeter in
length can be easily split into thinner strands. The thinnest fibers to date
comprise only five double helix strands and are only a few nanometers thick.
That opens the door also to nanoelectronic applications.
"Especially the combination
of interesting semiconductor properties and mechanical flexibility gives us
great optimism regarding possible applications," says Professor Nilges.
"Compared to organic solar cells, we hope to achieve significantly higher
stability from the inorganic materials. For example, SnIP remains stable up to
around 500°C (930 °F)."
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| A double helix. Zephyris at the English language Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons |
"Similar to carbon, where we
have the three-dimensional (3D) diamond, the two dimensional graphene and the
one dimensional nanotubes," explains Professor Nilges, "we here have,
alongside the 3D semiconducting material silicon and the 2D material phosphorene,
for the first time a one dimensional material - with perspectives that are
every bit as exciting as carbon nanotubes."
Just as with carbon nanotubes and
polymer-based printing inks, SnIP double helices can be suspended in solvents
like toluene. In this way, thin layers can be produced easily and
cost-effectively. "But we are only at the very beginning of the materials
development stage," says Daniela Pfister. "Every single process step
still needs to be worked out."
Since the double helix strands of
SnIP come in left and right-handed variants, materials that comprise only one
of the two should display special optical characteristics. This makes them
highly interesting for optoelectronics applications. But, so far there is no
technology available for separating the two variants.
Theoretical calculations by the
researchers have shown that a whole range of further elements should form these
kinds of inorganic double helices. Extensive patent protection is pending. The
researchers are now working intensively on finding suitable production
processes for further materials.
An extensive interdisciplinary
alliance is working on the characterization of the new material:
Photoluminescence and conductivity measurements have been carried out at the
Walter Schottky Institute of the TU Munich. Theoretical chemists from the
University of Augsburg collaborated on the theoretical calculations.
Researchers from the University of Kiel and the Max Planck Institute of Solid
State Research in Stuttgart performed transmission electron microscope
investigations. Mössbauer spectra and magnetic properties were measured at the
University of Augsburg, while researchers of TU Cottbus contributed
thermodynamics measurements.
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