New Applications and Devices Continue to Emerge for Two-Dimensional Materials
From providing clean drinking water to better measuring electrical resistance, 2D materials offer a number of “killer apps”
In the development chain that starts with discovering a new material, and then moves to determining its properties, and then leads to devising a way to manufacture it efficiently, the final step and aim is making a device from it that serves some useful purpose.
This final step is arguably what it’s all about. New materials should lead to new products and devices, otherwise the purpose of the material is less clear. It has been argued—by the frustrated among us—that graphene and its two-dimensional (2D) cousins are a solution looking for a problem, or the old story of a technology push rather than a market pull.
All the hand wringing we read and hear about graphene lacking a “killer app” may have missed a fundamental point: since we face great challenges to mankind like clean drinking water and sustainable energy solutions, we need to investigate every possible material solution we can get our hands on whether they are ultimately successful, or not. We can’t wait with our hands in our pockets until the perfect material comes along.
This quarter has offered some potential uses for graphene that may in fact lead to the material’s “killer app” being discovered and bring us a step closer to realizing its potential in a host of others in which many have maintained it could offer a solution.
Water, Water Everywhere…at Least With Some 2D Materials on Hand
Illustration: Mohammad Heiranian
Graphene has long been targeted as a way to address the challenges associated with reducing the costs of water desalination.
Those high costs come from the huge amount of energy it takes to force salt water through membranes to remove the salt and other impurities. This expense has been priced between $0.5 to $0.85 per cubic meter of water. As a result, it is estimated that 60 percent of desalination capacity in the world is located in the Middle East where the oil-producing countries of the Gulf have access to inexpensive energy and the pinch for clean drinking water is the most acute.
Graphene has been particularly promising as a membrane material with a host of properties in its favor for water desalination and gas separation, but it may have another 2D material as a competitor: molybdenum disulfide (MoS2). Researchers at the University of Illinois believe that MoS2 may remove salt much better than graphene as a membrane medium.
The research at this point is just computer modeling, but in their simulations the Illinois researchers looked at a host of thin-film membrane materials and concluded that MoS2 was the most efficient, filtering up to 70 percent more than graphene membranes.
"Finding materials for efficient desalination has been a big issue, and I think this work lays the foundation for next-generation materials,” said Narayana Aluru, a professor at the university and leader of the research, in a press release. “These materials are efficient in terms of energy usage and fouling, which are issues that have plagued desalination technology for a long time."
In the research, which was published in the journal Nature Communications, the model they made of the MoS2 membrane consisted of one molybdenum atom sandwiched between two sulfur atoms, a sheet of the material is essentially a sulfur coating on the outside and molybdenum in the middle. The pore created in a sheet of MoS2 has a ring of molybdenum formed around its center, which draws water through the pore due to molybdenum’s ability to attract water.
This property reduces the amount of energy that is required to force the water through the pores.
"MoS2 has inherent advantages in that the molybdenum in the center attracts water, then the sulfur on the other side pushes it away, so we have much higher rate of water going through the pore," said graduate student Mohammad Heiranian, the first author of the study.
Graphene Can Measure Ohms With Little Resistance
While membrane technology appears to be one of the most promising applications for graphene and other 2D materials, the hope has really been that these materials could offer a solution to the problem of silicon and its limits to the relentless march of Moore’s Law.
Nonetheless digital logic remains for the most part an elusive application area for 2D materials. Despite this, a “killer app” may have been found for graphene in electronics.
Graphene has proven itself to be a highly accurate and dependable tool for measuring electrical resistance. All of us who have purchased stereo equipment are likely familiar with the standard measurement of resistance: the ohm. To get a measurement of ohms it is necessary to exploit a phenomenon known as the quantum Hall effect, which occurs when a magnetic field has been placed perpendicular to the flow of current. That magnetic force pushes the electrons to the side, resulting in a rise in voltage perpendicular to the flow of current.
When you do all of this in a thin layer of material, you get a quantum version of the Hall effect in which both the voltage and the resulting resistance are “quantized” and have discrete integer values.
This is all fine and good, but to produce the physical conditions to achieve this quantum resistance you need a 10-Tesla magnetic field that itself require a gigantic superconducting magnet and temperatures just above absolute zero. As a result, it’s not exactly an easy measurement to take.
Research teams from the UK’s National Physical Laboratory, France’s National Metrology and Testing Laboratory and other research groups have all presented ways in which graphene can be used as the thin film material in place of gallium arsenide that don’t require such a powerful magnetic field and can take the measurements at higher temperatures.
The French researchers went so far as to say in the journal Nature Nanotechnology: “These results support graphene as the material of choice for the next generation of easy-to-use, helium-free, and affordable quantum electrical standards, approaching an ideal standard that would be invariant, available to anyone, at any place and any time.”
Combination of Graphene and Nanowires Could Usher in New Flexible Displays
Illustration: Purdue University
Sometimes to find your way you may need to find the way for others. So is the relationship with graphene and silver nanowires. While graphene is still sorting out what may be its “killer app” in electronics, it has been found as a way to ensure that silver nanowires realize their potential in flexible displays and OLEDs.
Earlier this year, we reported on work out of Purdue University in which graphene was used to coat the outside of copper nanowires to both lower their resistance and susceptibility to heating.
“Highly conductive copper nanowires are essential for efficient data transfer and heat conduction in many applications like high-performance semiconductor chips and transparent displays,” said Purdue doctoral student Ruchit Mehta in a press release earlier this year.
This time the Purdue researchers turned to silver nanowires and instead of lowering electrical resistance were looking at the graphene coating as a protective barrier for the nanowires.
"We show that even if you have only a one-atom-thickness material, it can protect from an enormous amount of UV radiation damage," said Gary Cheng, an associate professor at Purdue University, in a press release.
The researchers believe this radiation protective barrier could be the ticket for silver nanowires finding commercial applications in a host of industries from solar cells to flexible displays.
Graphene and Nanowires Shine the Light on Photodetectors
Another research group has looked at the combination of graphene with silver nanowires, but instead of pursuing an application in electronics, their work looks to make a splash in photonics.
Researchers at the University of Rochester are combining nanowires and MoS2 to create a nanoscale photodetector that the researchers believe could be a step towards a new type of photonic circuit.
This latest work builds on research the Rochester researchers conducted last year with the Swiss Federal Institute of Technology in Zurich that demonstrated that light could be transmitted along a silver nanowire as a plasmon, which are rapid oscillations in electron density. The plasmons are re-emitted at the other end of the nanowire, which is covered with the MoS2.
"Our devices are a step towards miniaturization below the diffraction limit," said Kenneth Goodfellow, a graduate student at the University of Rochester, in a press release. "It is a step towards using light to drive, or, at least complement electronic circuitry for faster information transfer."
Inspired by their previous work, the researchers thought that a photodetector device could be fabricated based on essentially the same design. This time though they took the nanowires coated on one end with MoS2 and put them on a silicon substrate. Then they used electron beam lithography to put metal contacts on the ends with the MoS2.
After hooking up the device to measure current running through the wires, the researchers discovered that the wires were sensitive to the polarization of incoming light—a photodetector.
This is very preliminary research, however, the researchers believe that it can serve as a foundation for creating photonic circuits.
Graphene and Perovskite Could Lead to Affordable and Efficient Solar Cells
Photo: The Hong Kong Polytechnic University
Graphene is showing a lot of promise when combined with another material that could usher in inexpensive and highly efficient solar cells: perovskite.
Researchers at Hong Kong Polytechnic University have combined graphene with perovskite to make a semi-transparent solar cell capable of power conversion efficiencies around 12 percent, a significant improvement over the roughly 7-percent efficiency of traditional semi-transparent solar cells.
In the design of the solar cell, the perovskite serves as the active layer for harvesting the light, and the graphene acts as the transparent electrode material where it has been vying to replace indium tin oxide (ITO) for some time now.
The researchers improved the energy conversion of the solar cells by using a multi-layer chemical vapor deposition process in which the graphene formed the top transparent electrodes. This approach maintained the transparency of the electrodes while increasing their sheet resistance.
The big breakthrough of this design is not only the 70-percent increase in conversion efficiency, but also they claim that their solar cells cost less than US$.06/watt, which they calculate is more than a 50 percent reduction in the costs of silicon solar cells.
Coating Yarns With Graphene Oxide Yields Flexible Gas Sensors
The rule was that if you wanted a highly responsive gas sensor you had to put it on a solid substrate. That rule has been broken by research out of the Electronics and Telecommunications Research Institute and Konkuk University in South Korea where they have developed a method for coating fabrics with graphene so that they can detect dangerous gases and alert the wearer of their presence by triggering an LED light.
In research published in Scientific Reports, the Korean researchers coated commercially available yarn graphene oxide by using electrostatic self assembly and molecular glue to produce a bendable and washable electronic textile gas sensor.
“This sensor can bring a significant change to our daily life since it was developed with flexible and widely used fibers, unlike the gas sensors invariably developed with the existing solid substrates,” said Dr. Hyung-Kun Lee, who led this research initiative, in a press release.
The researchers observed that the graphene-coated yarn was extremely sensitive to nitrogen dioxide, which is produced through the burning of fossil fuels. The sensor operates by the nitrogen oxide molecules altering the electrical resistance of the graphene, which in turn triggers an LED light to turn on.
It has been suggested that this kind of wearable gas sensor could be useful in oil field environments were toxic gases can be detected early to prevent poisoning.