New Properties, New Applications and New Additions for the 2-D Material Universe
The population of the flatlands continues to grow along with the properties and applications of 2-D materials
“Black Phosphorous” Enters the Fray
In our catalogue of two-dimensional (2-D) materials last year, we suggested that you had better be prepared for the introduction of novel 2-D materials on a regular basis.
One of the latest 2-D materials to join the flatlands is something called “black phosphorus”, which is the 2-D version of the ubiquitous phosphorous found on match heads.
We first started hearing about black phosphorus in electronics research where it was experimented with in devices such as field-effect transistors because of its inherent band gap.
Moving on from straight-up electronics research, black phosphorous is being experimented with in optoelectronic applications. Researchers at the University of Minnesota are demonstrating its potential in high-speed data communication that depends of on nanoscale optical circuits.
The researchers were eager to point out that black phosphorus outperforms graphene in improving the efficiency of these optical circuits.
"After the discovery of graphene, new two-dimensional materials continue to emerge with novel optoelectronic properties," said Professor Mo Li, who led the research team, in a press release. "Because these materials are two-dimensional, it makes perfect sense to place them on chips with flat optical integrated circuits to allow maximal interaction with light and optimally utilize their novel properties."
The researchers in experiments published in the journal Nature Photonics were able to produce complex optical circuits in silicon and then put a thin layer of black phosphorus flakes over the structure.
The addition of the black phosphorus over the silicon enabled the device to achieve the on-chip detection level previously only seen in germanium-based optical circuits. In contrast to germanium-based devices, which are difficult to grow on silicon, black phosphorus grows easily on silicon, or just about any other material.
This extraordinary performance of black phosphorus is the result of it increasing the optical circuits’ interaction with light because of its narrow but finite band gap.
In experimetns, the black phosphorous-based photodetectors could receive high-speed optical data sent over optical fibers of up to three billion bits per second, equivalent to downloading a typical HD movie in about 30 seconds.
"Even though we have already demonstrated high speed operation with our devices, we expect higher transfer rates through further optimization," said Nathan Youngblood, the lead author of the study, in the press release. "Since we are the first to demonstrate a high-speed photodetector using black phosphorus, more work still needs to be done to determine the theoretical limits for a fully optimized device."
Researchers Make Transistors Out of Silicene for the First Time
In our October edition of the Graphene Council newsletter, we showed that silicene—the 2-D version of silicon—at the very least was not suicidal. Silicene, which had previously enjoyed most of its notoriety inside computer models, was thought to remain a lab curiosity that couldn’t survive outside of a vacuum before it turned itself back into silicon.
While the research last year that showed it could remain stable in ambient air for just 24 hours didn’t make it exactly practical, it did make it possible for other researchers to experiment with it to better determine its properties and capabilities.
In perhaps one of the first fruits of this capability, researchers at the University of Texas at Austin have demonstrated a method for fabricating a field-effect transistor out of silicene.
The actual device lives up to all the promise silicene exhibited in the computer models, demonstrating remarkable switching speeds. The computer models were also right about silicene having similar electrical to graphene in which electrons can travel through the material without barriers.
To achieve the first transistor made out of the finicky material, the researchers grew their silicene on a thin film of silver and capped it with aluminum oxide. Adding this light coat of protective oxide to create a protective shell is known as passivation in the electronics business—a manufacturing technique that has recently proven effective in protecting graphene devices.
This encapsulated silicene was then placed on a silicon dioxide wafer with the silver side facing up. The researchers then etched patterns into this silver side that accommodated contacts that allowed it operate as a transistor.
Despite this achievement, the researchers never tried to expose the transistor to ambient air and kept inside of vacuum conditions. This, of course, means that silicene is far from a practical material for electronic applications and renders moot all the talk about how being based on silicon means it might be more easily adopted by the electronics industry.
Piezoelectricity Comes to 2-D Materials
Molybdenum disulfide (MoS2) in relative terms is the grand daddy of 2-D materials…that isn’t graphene. But in developmental terms, it’s still a baby, barely more than five years since its first synthesis.
As such, we should expect to the properties and capabilities of this material continue to expand. The latest property added to MoS2 is that in its free-standing form it exhibits piezoelectricity, which is the ability of a material to produce a voltage when it is compressed or stretch, or where a voltage can cause a material to expand or contract.
Researchers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) demonstration that a free-standing single layer MoS2 can exhibit the piezoelectric effect could make possible new nanoscale low-power switches for sensors and other electronics.
“The discovery of piezoelectricity at the molecular level not only is fundamentally interesting, but also could lead to tunable piezo-materials and devices for extremely small force generation and sensing,” said Xiang Zhang, director of Berkeley Lab’s Materials Sciences Division, in a press release.
Late last year, researchers at Columbia University and Georgia Tech also demonstrated that MoS2 exhibits piezoelectricity and the piezotronic effect, which is the use of the piezoelectric effect as the gate voltage in transistor or similar device.
However, in that research, the MoS2 was sprinkled on top of a flexible substrate. In this most recent research, no substrate was used—thus the term “free standing”—and the researchers still managed to coax piezoelectricity out of the material.
Both research results—with a substrate or without—did share one common feature. The piezoelectricity only exhibited itself when an odd number of layers were used (1,3,5, etc.).
“This discovery is interesting from a physics perspective since no other material has shown similar layer-number sensitivity,” Hanyu Zhu, one of the co-authors of the research published in the journal Nature Nanotechnology, said in a press release. “The phenomenon might also prove useful for applications in which we want devices consisting of as few as possible material types, where some areas of the device need to be non-piezoelectric.”