A flexible semiconductor for electronics, solar technology and photo catalysis
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.”
COUNTLESS APPLICATION POSSIBILITIES
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).”
JUST AT THE BEGINNING
“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.
Learn more: Inorganic double helix
A Korean team of scientists tune BP’s band gap to form a superior conductor, allowing for the application to be mass produced for electronic and optoelectronics devices
The research team operating out of Pohang University of Science and Technology (POSTECH), affiliated with the Institute for Basic Science’s (IBS) Center for Artificial Low Dimensional Electronic Systems (CALDES), reported a tunable band gap in BP, effectively modifying the semiconducting material into a unique state of matter with anisotropic dispersion. This research outcome potentially allows for great flexibility in the design and optimization of electronic and optoelectronic devices like solar panels and telecommunication lasers.
To truly understand the significance of the team’s findings, it’s instrumental to understand the nature of two-dimensional (2-D) materials, and for that one must go back to 2010 when the world of 2-D materials was dominated by a simple thin sheet of carbon, a layered form of carbon atoms constructed to resemble honeycomb, called graphene. Graphene was globally heralded as a wonder-material thanks to the work of two British scientists who won the Nobel Prize for Physics for their research on it.
Graphene is extremely thin and has remarkable attributes. It is stronger than steel yet many times lighter, more conductive than copper and more flexible than rubber. All these properties combined make it a tremendous conductor of heat and electricity. A defect–free layer is also impermeable to all atoms and molecules. This amalgamation makes it a terrifically attractive material to apply to scientific developments in a wide variety of fields, such as electronics, aerospace and sports. For all its dazzling promise there is however a disadvantage; graphene has no band gap.
Stepping Stones to a Unique State
A material’s band gap is fundamental to determining its electrical conductivity. Imagine two river crossings, one with tightly-packed stepping-stones, and the other with large gaps between stones. The former is far easier to traverse because a jump between two tightly-packed stones requires less energy. A band gap is much the same; the smaller the gap the more efficiently the current can move across the material and the stronger the current.
Graphene has a band gap of zero in its natural state, however, and so acts like a conductor; the semiconductor potential can’t be realized because the conductivity can’t be shut off, even at low temperatures. This obviously dilutes its appeal as a semiconductor, as shutting off conductivity is a vital part of a semiconductor’s function.
Birth of a Revolution
Phosphorus is the fifteenth element in the periodic table and lends its name to an entire class of compounds. Indeed it could be considered an archetype of chemistry itself. Black phosphorus is the stable form of white phosphorus and gets its name from its distinctive color. Like graphene, BP is a semiconductor and also cheap to mass produce. The one big difference between the two is BP’s natural band gap, allowing the material to switch its electrical current on and off. The research team tested on few layers of BP called phosphorene which is an allotrope of phosphorus.