In the 15 years since graphene was first isolated, researchers have been searching for other 2-D materials that might have similarly valuable electronic or mechanical properties. One candidate is phosphorene, atomically thin flakes of black phosphorus. But phosphorene decomposes into phosphoric acid when exposed to air, light, and humidity, making it difficult to handle. Now researchers from the Korea Advanced Institute of Science and Technology (KAIST) have shown that they can stabilize phosphorene by protecting it with a layer of surfactant (Chem. Mater. 2019, DOI: 10.1021/acs.chemmater.8b04984).
When phosphorus atoms bond to each other, they leave a lone pair of electrons. Those react with the oxygen in air to form phosphorus oxide, which then sops up water to yield phosphoric acid. “Phosphorene has potential to surpass graphene in terms of any application, but we need to first solve the stability issue,” says Sang Ouk Kim, a materials scientist at KAIST.
Researchers have attempted to stabilize the material in many ways, including encapsulating it in polymer or bonding it to aryl diazonium salts. Covalent bonds, however, alter the crystalline structure of phosphorene, which may affect its material properties. Such bonds also make it more difficult to remove the protective agent so the phosphorene can be used in applications such as gas sensing.
Kim and the team mixed the commonly used surfactant cetrimonium bromide (CTAB) with black phosphorus in water that was treated to minimize oxidation. CTAB inserted itself within the phosphorous layers, separating off sheets of phosphorene that were one to four layers thick. A positively charged end group on the surfactant interacts noncovalently with phosphorene’s electron pair, leaving it unable to react with oxygen. At the same time, a hydrophobic chain on CTAB blocks water’s access to the material. “The degradation rate of phosphorene dropped down significantly,” Kim says.
Unlike graphene, phosphorene is a semiconductor, so it’s a natural choice for transistors. It might be used to make better field-effect transistors for computing, as well as batteries, solar cells, supercapacitors, and sensors. Kim says that because the surfactant does not bond covalently with the material, phosphorene’s intrinsic properties remain unaltered, so it should perform well for those applications. To show that the material retained its electronic properties, the team rinsed away the surfactant and used the phosphorene to make a gas sensor.
According to the team’s computer modeling studies, the phosphorene loses a fraction of its electrons to the CTAB. Not only does CTAB inhibit the degradation of the material, it also acts as a dopant by adding positive charge carriers to the semiconductor. That improves the phosphorene’s electrical properties and might be beneficial for making transistors out of the material.
So far the researchers have only explored charged surfactants. But they also hope to test neutral surfactants to see if they will stabilize phosphorene without changing its electrical properties.
Yury Gogotsi, a materials scientist at Drexel University, says excess CTAB between layers of phosphorene might interfere with some applications. “Still,” he says, “this is a good trick that can be used for protection of phosphorene and potentially other 2-D nanosheets, moving them one step further towards practical applications.”