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
Mortar with an added bacterial film is highly resistant to water uptake
Moisture can destroy mortar over time – for example when cracks form as a result of frost. A team of scientists at the Technical University of Munich (TUM) has found an unusual way to protect mortar from moisture: When the material is being mixed, they add a biofilm – a soft, moist substance produced by bacteria.
Oliver Lieleg usually has little to do with bricks, mortar and concrete. As a professor of biomechanics at the Institute of Medical Engineering (IMETUM) and the Department of Mechanical Engineering, he mainly deals with biopolymer-based hydrogels or, to put it bluntly, slime formed by living organisms.
These include bacterial biofilms, such as dental plaque and the slimy black coating that forms in sewage pipes. “Biofilms are generally considered undesirable and harmful. They are something you want to get rid of,” says Oliver Lieleg. “I was therefore excited to find a beneficial use for them.”
INSPIRATION FROM A CONVERSATION
During a conversation with a colleague at TUM, Lieleg came up with the idea of using biofilms to alter the properties of construction materials. Professor Christian Große holds the Chair of Non-destructive Testing. Among other things, he investigates self-healing concrete whose cracks close autonomously. One variant of this concrete contains added bacteria. Activated by the ingress of moisture, the bacteria close the cracks with metabolic products containing calcium.
For his own project, Lieleg used mortar instead of concrete. Instead of mending cracks after damage has occurred, he wants to prevent moisture from penetrating into mortar in the first place. Such invading water can cause serious problems, for example by inducing the growth of mold or widening existing microcracks through freeze-thaw-cycles. To prevent such water ingress, he takes advantage of the fact that some bacterial films are highly water-repellent. In the journal Advanced Materials, Lieleg and his colleagues describe how to make a moisture-resistant hybrid mortar.
A SOIL BACTERIUM PRODUCES THE BIO-SUPPLEMENT
The key ingredient in the new material is biofilm produced by the bacterium Bacillus subtilis. “Bacillus subtilis normally lives in soil and is very common microorganism,” Oliver Lieleg explains. “For our experiments, we used a simple laboratory strain that grows rapidly, forms plenty of biomass and is completely harmless.” Lieleg’s team bred the bacterial film on standard culture media in the lab. They then added the moist biofilm to the mortar powder.
In the generated hybrid mortar, water was significantly less able to wet the surface compared to untreated mortar. To evaluate this surface property, the scientists measured the contact angle between water droplets and the surface. The steeper this angle, the more spherical the drops are, and the less likely the liquid is soaked into the material. Whereas this angle is only 30 degrees or less on untreated mortar, it is three times as high for drops on the hybrid mortar. Water droplets on polytetrafluoroethylene, better known by the trade name “Teflon”, have a similarly high contact angle.
NANOSTRUCTURES IN THE MORTAR
An explanation for the water-repellent properties of the hybrid mortar can be found in electron microscope images: The surface is covered with tiny crystalline spikes. This results in what is known as the lotus effect, which also occurs on the leaves of the lotus plant. The small uniform structures on the surface ensure that only a small part of a water droplet is actually in contact with the leaf surface. The surface tension of the droplet therefor is stronger than the forces that make it adhere to the leaf. Consequently, the droplet easily rolls off the leaf when the leaf is tilted. A cross-section of hybrid mortar shows that crystalline spikes are not only evenly distributed on the mortar surface but can also be found throughout the bulk volume of the mortar. This reduces the capillary forces that are normally responsible for the uprise of water in mortar when the material is immersed into liquid.
Although similar spikes also occur on untreated mortar, they are too long, rare and scattered for the lotus effect to occur. The researchers assume that the added biofilm stimulates uniform crystal growth throughout the volume of the hybrid material.
To find out if the hybrid mortar is resistant enough to actually be used in construction applications, it is currently undergoing mechanical tests in Christian Grosse’s department. “If the mortar is in fact suitable, there should be no problem for companies to produce it on a large scale,” Oliver Lieleg says. Both the bacterial strain used and the culture media are standard and relatively inexpensive. “We‘ve also discovered in our experiments that freeze-dried biofilm can be used equally well. Then, in a powder form, the biological material can be stored, transported and addedmuch more easily .” In the future, the scientists want to examine whether the biofilm can also be used to protect concrete against water.
Bridges, tunnels and roads: Concrete is the main component of our infrastructure. And when the structural elements need to be repaired, it often leads to long traffic jams.
At the Annual Meeting of the AAAS (American Association for the Advancement of Science) in Washington, D.C., Prof. Christian Grosse from the (TUM) and other experts talked about smart materials for sustainable infrastructure.
Small cracks can form in concrete due to permanent loading or variations in temperature. As Prof. Christian Grosse from the Chair of Non-destructive Testing (NDT) at TUM explains, the cracks do not usually pose any direct threat to the stability of structures: “However, water and salts can penetrate the concrete and damage the affected components.”
Repairing infrastructure is expensive and can result in long traffic jams. In the EU research project HealCON, an international team of researchers is working toward the development of concrete that can repair itself. The scientists are examining three different self-healing mechanisms.
– Bacteria as mini construction workers
Certain bacteria produce calcium carbonate as a metabolic product. The scientists soak balls of clay with the spores of these bacteria and mix the balls into concrete. Once water penetrates the concrete, the microorganisms become active and release calcium carbonate, one of the main components of concrete. “The bacteria can close cracks of up to a few millimeters in width in a matter of a few days,” says Grosse.
– Hydrogels as gap fillers
Hydrogels are polymers that absorb moisture. They are used in diapers, among other things. Materials containing hydrogels can expand to ten or even 100 times their original size. Cracks that form in concrete can be healed by a hydrogel that expands when it comes into contact with moisture, thus preventing the water from penetrating further without expanding the cracks.
– Greater strength thanks to epoxy resin
Epoxy resins or polyurethane can be encapsulated and mixed into the concrete. When the concrete cracks, the capsules break open and the polymer is released. It forms a hard mass that seals the crack. It also has a positive side-effect: It increases structural stability.
Looking into concrete
Grosse and his colleagues specialize in testing how well these healing agents work in individual cases. They use non-destructive testing methods to do this, for example acoustic emission technology.