New process isolates promising material
After graphene was first produced in the lab in 2004, thousands of laboratories began developing graphene products worldwide. Researchers were amazed by its lightweight and ultra-strong properties. Ten years later, scientists now search for other materials that have the same level of potential.
“We continue to work with graphene, and there are some applications where it works very well,” said Mark Hersam, the Bette and Neison Harris Chair in Teaching Excellence at McCormick, who is a graphene expert. “But it’s not the answer to all the world’s problems.”
Part of a family of materials called transition metal dichalcogenides, molybdenum disulfide (MoS2) has emerged as a frontrunner material for exploration in Hersam’s lab. Like graphene, it can be exfoliated into atomically thin sheets. As it thins to the atomic limit, it becomes fluorescent, making it useful for optoelectronics, such as light-emitting diodes, or light-absorbing devices, such as solar cells. MoS2 is also a true semiconductor, making it an excellent candidate for electronics, and it historically has been used in catalysis to remove sulfur from crude oil, which prevents acid rain.
Hersam’s challenge was to find a way to isolate atomically thin sheets of this promising material at a larger scale. For the past six years, his lab has developed methods for exfoliating thin layers of graphene from graphite, using solution-based methods.
“You would think it would be easy to do the same thing for molybdenum disulfide,” he said. “But the problem is that while the exfoliation is similar to graphene, the separation is considerably more challenging.”
Hersam’s research is described in the paper “Thickness sorting of two-dimensional transition metal dichalcogenides via copolymer-assisted gradient ultracentrifugation,” which was published in the Nov. 13 issue of Nature Communications.
To sort graphene layers, Hersam used centrifugal force to separate materials by density. To do this, he and his group added the material to a centrifuge tube along with a gradient of water-based solution. Upon centrifugation, the denser species move toward the bottom, creating layers of densities within the centrifuge tube. Graphene sorts into single layer sheets toward the top, then bilayer sheets, trilayer, and so on. Because graphene has a relatively low density, it easily sorts compared to higher density materials.
“If I use the exact same process with molybdenum disulfide, its higher density will cause it to crash out,” Hersam said. “It exceeds the maximum density of the gradient, which required an innovative solution.”
Hersam needed to take the inherently dense material and effectively reduce its density without changing the material itself. He realized that this goal could be achieved by tuning the density of the molecules used to disperse MoS2. In particular, the use of bulkier polymer dispersants allowed the effective density of MoS2 to be reduced into the range of the density gradient. In this manner, the sheets of MoS2 floated at layered positions instead of collecting as the bottom of the centrifuge tube. This technique works not just for MoS2, but for other materials in the transition metal dichalcogenides family.
“Now we can isolate single layer, bilayer, or trilayer transition metal dichalcogenides in a scalable manner,” Hersam said. “This process will allow us to explore their utility in large-scale applications, such as electronics, optoelectronics, catalysis, and solar cells.”
Revolutionary solar-friendly form of silicon shines
Silicon is the second most-abundant element in the earth’s crust. When purified, it takes on a diamond structure, which is essential to modern electronic devices–carbon is to biology as silicon is to technology. A team of Carnegie scientists led by Timothy Strobel has synthesized an entirely new form of silicon, one that promises even greater future applications. Their work is published in Nature Materials.
Although silicon is incredibly common in today’s technology, its so-called indirect band gap semiconducting properties prevent it from being considered for next-generation, high-efficiency applications such as light-emitting diodes, higher-performance transistors and certain photovoltaic devices.
Metallic substances conduct electrical current easily, whereas insulating (non-metallic) materials conduct no current at all. Semiconducting materials exhibit mid-range electrical conductivity. When semiconducting materials are subjected to an input of a specific energy, bound electrons can move to higher-energy, conducting states. The specific energy required to make this jump to the conducting state is defined as the “band gap.” While direct band gap materials can effectively absorb and emit light, indirect band gap materials, like diamond-structured silicon, cannot.
In order for silicon to be more attractive for use in new technology, its indirect band gap needed to be altered. Strobel and his team–Carnegie’s Duck Young Kim, Stevce Stefanoski and Oleksandr Kurakevych (now at Sorbonne) –were able to synthesize a new form of silicon with a quasi-direct band gap that falls within the desired range for solar absorption, something that has never before been achieved.
The silicon they created is a so-called allotrope, which means a different physical form of the same element, in the same way that diamonds and graphite are both forms of carbon. Unlike the conventional diamond structure, this new silicon allotrope consists of an interesting open framework, called a zeolite-type structure, which is comprised of channels with five-, six- and eight-membered silicon rings.
They created it using a novel high-pressure precursor process. First, a compound of silicon and sodium, Na4Si24, was formed under high-pressure conditions. Next, this compound was recovered to ambient pressure, and the sodium was completely removed by heating under vacuum. The resulting pure silicon allotrope, Si24, has the ideal band gap for solar energy conversion technology, and can absorb, and potentially emit, light far more effectively than conventional diamond-structured silicon. Si24 is stable at ambient pressure to at least 842 degrees Fahrenheit (450 degrees Celsius).
“High-pressure precursor synthesis represents an entirely new frontier in novel energy materials,” remarked Strobel. “Using the unique tool of high pressure, we can access novel structures with real potential to solve standing materials challenges. Here we demonstrate previously unknown properties for silicon, but our methodology is readily extendible to entirely different classes of materials. These new structures remain stable at atmospheric pressure, so larger-volume scaling strategies may be entirely possible.”
“This is an excellent example of experimental and theoretical collaboration,” said Kim. “Advanced electronic structure theory and experiment have converged to deliver a real material with exciting prospects. We believe that high-pressure research can be used to address current energy challenges, and we are now extending this work to different materials with equally exciting properties.”
This work was supported DARPA and Energy Frontier Research in Extreme Environments (EFree), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.
New way to move atomically thin semiconductors for use in flexible devices
Researchers from North Carolina State University have developed a new way to transfer thin semiconductor films, which are only one atom thick, onto arbitrary substrates, paving the way for flexible computing or photonic devices. The technique is much faster than existing methods and can perfectly transfer the atomic scale thin films from one substrate to others, without causing any cracks.
At issue are molybdenum sulfide (MoS2) thin films that are only one atom thick, first developed by Dr. Linyou Cao, an assistant professor of materials science and engineering at NC State. MoS2 is an inexpensive semiconductor material with electronic and optical properties similar to materials already used in the semiconductor industry.
“The ultimate goal is to use these atomic-layer semiconducting thin films to create devices that are extremely flexible, but to do that we need to transfer the thin films from the substrate we used to make it to a flexible substrate,” says Cao, who is senior author of a paper on the new transfer technique. “You can’t make the thin film on a flexible substrate because flexible substrates can’t withstand the high temperatures you need to make the thin film.”
Cao’s team makes MoS2 films that are an atom thick and up to 5 centimeters in diameter. The researchers needed to find a way to move that thin film without wrinkling or cracking it, which is challenging due to the film’s extreme delicacy.
“To put that challenge in perspective, an atom-thick thin film that is 5 centimeters wide is equivalent to a piece of paper that is as wide as a large city,” Cao said. “Our goal is to transfer that big, thin paper from one city to another without causing any damage or wrinkles.”
Existing techniques for transferring such thin films from a substrate rely on a process called chemical etching, but the chemicals involved in that process can damage or contaminate the film. Cao’s team has developed a technique that takes advantage of the MoS2′s physical properties to transfer the thin film using only room-temperature water, a tissue and a pair of tweezers.
MoS2 is hydrophobic – it repels water. But the sapphire substrate the thin film is grown on is hydrophilic – it attracts water. Cao’s new transfer technique works by applying a drop of water to the thin film and then poking the edge of the film with tweezers or a scalpel so that the water can begin to penetrate between the MoS2 and the sapphire. Once it has begun to penetrate, the water pushes into the gap, floating the thin film on top. The researchers use a tissue to soak up the water and then lift the thin film with tweezers and place it on a flexible substrate. The whole process takes a couple of minutes. Chemical etching takes hours.
“The water breaks the adhesion between the substrate and the thin film – but it’s important to remove the water before moving the film,” Cao says. “Otherwise, capillary action would case the film to buckle or fold when you pick it up.
“This new transfer technique gets us one step closer to using MoS2 to create flexible computers,” Cao adds. “We are currently in the process of developing devices that use this technology.”