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Discovered more than a decade ago, graphene has opened the door to countless new possibilities, including the investigation of other materials so thin, they are considered to be two-dimensional.
One of those materials is phosphorene, which has a structure comparable graphene, but is made of phosphorus atoms instead of carbon atoms.
While graphene has been touted as a material for the next generation of computing technology, it is not a true semiconductor like silicon. Phosphorene, on the other hand, is a natural semiconductor and therefore may be better suited to powering the next generation of digital technology. In fact, phosphorene has already been used to create basic transistors.
Making the Next Generation of Transistors
A semiconductor can be made to conduct electricity or block its flow. This switching capability is the defining feature of transistors and makes possible the binary logic at the heart of a computer chip. Phosphorene is the first native 2D electron-poor – or p-type – semiconductor. That’s important for making these flat materials into standard complementary metal-oxide semiconductor (CMOS) logic circuit elements. Simply put, phosphorene is a better candidate than graphene for computer applications.
The transistor switching in our computers and mobile devices typically relies on interfaces between p-type semiconductors and n-type semiconductors, which have an excess of electrons. As a conductor, unaltered graphene can play neither role. Two-dimensional MoS2 is a promising n-type material, but researchers have had trouble making two-dimensional p-type semiconductors.
Investigating Phosphorene
Scientists have discovered ordinary cellophane tape could be used to peel away sheets of phosphorene just a handful of atoms thick from black phosphorus crystals. The same process, referred to as the Scotch tape technique, was famously used to pull graphene sheets from bits of graphite, the material in pencil lead.
Initial research with phosphorene has indicated it has molecular-scale ridges that secure its layers in place. Research has also revealed altering the amount of phosphorene layers can change how much energy is required to make it conductive, a quantity referred to as its bandgap. That could help customize the electronic qualities of devices made with phosphorene. Single-layer phosphorene has the highest bandgap of all and would be particularly desirable, as it would enable the clearest difference a transistor being on or off.
In fact, phosphorene has been made it into a p-type field-effect transistor to assess its hole mobility, or how fast charge moves as positively-charged absences of electrons. The researchers found phosphorene has a hole mobility of 286m2/Vs, which is much slower than graphene's 15,000cm2/Vs. However, this figure is considered encouraging because it is about three to five times the value for two-dimensional MoS2, while silicon’s hole mobility is only 100cm2/Vs. Scientists have put both MoS2 and phosphorene on the same silicon wafer to make a basic circuit element.
Other Applications
In addition to being a candidate for use in next-generation transistors, phosphorene has also attracted interest for applications vapor sensor technology and battery devices.
Humidity sensors could make great use of phosphorene's unstable nature, and experts have said it should have a much better performance than other known 2D materials. The phosphorene is also very sensitive to NO and NO2, which gives a strong foundation for usage in nitrogen-based gas induction. As the nitrogen gas molecules react with the phosphorene surface, they act as donors or acceptors, bringing about a shift in the electronic qualities.
Phosphorene's high electrical conductivity show promise for use in battery technologies. However, the volume of phosphorene considerably shifts, around 300 percent, upon lithiation, triggering the loss of electrical contact. Due to this effect, 2D phosphorene/graphene hybrids have been created to combine the benefits of both materials. Harmonizing the capacity and conductivity gets to be a necessary role because the electrical contact gets much better with rising quantities of graphene in this hybrid system.
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