Posts Tagged materials science

Two-Dimensional Materials

Posted by on Monday, 10 August, 2015

Recently, it was reported that scientists had synthesized two-dimensional tin, or stanene, after theorizing that it could exist about two years ago. Like graphene, two-dimensional sheets of carbon, stanene could have different properties than three-dimensional forms of the element. Specifically, stanene could contribute to energy efficiency by reducing the power consumption of computer chips. Stanene is a topological insulator so a sheet of stanene only conducts electricity along its edges; stanene sheets may be able to conduct electricity without losses at room temperature, which could result in a room temperature superconductor. Previous superconductors required extremely low temperatures to function. Two-dimensional materials could become very important for efficient electronics and energy harvesting devices. Further investigation showed that researchers have been studying several other two-dimensional materials as well, and several of them are semiconductors.

Germanene is a two-dimensional form of the metal germanium. It also took several years for researchers to synthesize germanene; scientists predicted the material could exist in 2009 and it was first made in 2014. Like graphene, germanene forms a honeycomb-shaped two-dimensional lattice. The synthesis process used a precious metal substrate; two research teams, one in Europe and one in China, synthesized the material with the former team using gold and the latter team using platinum. The Chinese team won the race by about a month. Germanene could also be better for making transistors than graphene because of its natural band gap. While graphene transistors exist, the lack of a natural band gap requires some extra effort by electronics manufacturers.

Silicene, two-dimensional silicon, also has a natural band gap. The silicene synthesis process is similar to the process for germanene, and silicene was actually created first, in 2012; one of the researchers on the European team that synthesized germanene was also on the team that first synthesized silicene. Silicene synthesis also uses a vacuum and a precious metal substrate, in this case silver. Silicene has another advantage over graphene; electronics manufacturers are already very familiar with the properties of silicon, which could make adoption of the material faster. In fact, in early 2015 researchers created a silicene transistor, although the device was very sensitive to air so it could take some time before consumer products based on silicene transistors are ready.

Phosphorene, two-dimensional phosphorous, is manufactured from black phosphorous. Phosphorene also forms a hexagonal lattice and is also a semiconductor with a natural band gap. Specifically, phosphorene is a p-type semiconductor, with space to accept an extra electron. This is important because it has been easier for researchers to create two-dimensional n-type semiconductors, which can donate an electron, but both p-type and n-type semiconductors are necessary for creating a transistor. Researchers also discovered that adding more layers of phosphorene changed the size of the band gap.

Molybdenum can also be used to create two-dimensional materials, including molybdenum ditelluride, diselenide, and disulfide. All three materials can function as semiconductors with natural band gaps. A sheet of molybdenum disulfide is an n-type semiconductor, so it can work with phosphorene to create a transistor. Two-dimensional molybdenum disulfide could be used to manufacture more efficient solar panels because of the properties of its band gap as well.

Boron could be used to create the two-dimensional material borophene. However, borophene hasn’t actually been created yet. In 2014, researchers proposed the material after running computer simulations. Theoretically, borophene would also look a bit different than some other two-dimensional materials; since boron only has three atoms, it can’t form the full lattice structure associated with other materials so a borophene lattice would have gaps. However, if borophene was manufactured, it could be a very strong and conductive material.

Biomimicry and Atmospheric Water Harvesting

Posted by on Saturday, 8 August, 2015

Dew harvesting is ancient, and researchers are currently working on ways to improve it. Fog could also provide an important water source in areas with limited freshwater access. In addition to the fog-basking beetle covered in my last article, researchers are studying plants such as cacti and moss to learn how they harvest water in arid regions. One project that has received attention recently uses a metal mesh screen to collect water from fog; the metal mesh is more effective than the plastics that many fog collectors currently use. Researchers are studying other nature-inspired fog and dew collection methods as well.

Spiderwebs are good at collecting dew because their silk threads can change shape when exposed to water. This causes droplets to pool at points along the web. Studying the silk could be very helpful, since it could result in materials that attract water but quickly release it. A fog collector is more efficient when the water it collects rapidly falls down into the collection tank, allowing the collector to pull in more water. While the wind could shake a light mesh and release the droplets, the collector might not always have access to wind.

Another website illustrates how several types of plants collect water, including moss, horsetail, and cacti. The moss had an interesting water collection technique. Like the threads in the spiderweb, part of the moss structure changes shape when it collects enough water, automatically dumping it out onto the main body of the plant. The moss grows thin stalks with water collecting heads that collapse after collecting water, which can then rebound to collect more.

The moss and spider silk dew harvesting methods are very interesting, since it may be possible to incorporate them into mesh designs. A mesh with small projections, or stalks, could release water more effectively. A mesh that changed shape after collecting water and then returned to its original form could also be very useful. It might even be possible to combine both techniques in a dew harvesting device.

Velcro, the material that helps keep things in place, is another example of biomimicry. It was inspired by prickly burrs that effectively stick to clothing. But Velcro, or a similar material, might have a role to play in dew collection as well. Velcro is a plastic surface with very thin plastic loops attached. The thin loops seem similar to the moss stalks. A similar manufacturing process could create a material like the moss, a surface with thin plastic stalks and water collecting heads. So far it doesn’t seem like anyone has tried this, as manufacturing structures like moss stalks cost-effectively might be challenging, although a company like Velcro might know how to do it. A 3-D printer might also be useful here. Making the stalks and the water collecting heads out of the plastic that was similar to spider silk might also be effective. Of course, I haven’t tested this and don’t know if it would be as effective as a metal mesh screen. Either way, further research into biomimicry could result in better ways to collect dew and water from fog.