Researchers at the University of Pennsylvania have developed a new method for manufacturing molybdenum disulfide. They grew molybdenum disulfide flakes around molybdenum oxide seeds, which allowed easier control of the material’s thickness, size and location.
"Seeding" the growth of molybdenum disulfide flakes gave the researches enough control over their location to spell a message.
Graphene has unbeatable thinness and very high conductivity, and hence it is considered as a possible replacement for silicon in electronic devices. It is a single-atom-thick lattice of carbon atoms. However, graphene is not the only material that could be a possible replacement. Molybdenum disulfide is also being considered for that role.
The conductivity of molybdenum disulfide can be turned on and off because it has an energy band gap, which graphene does not have. This property is important for semiconductor devices that are used in computing applications. Furthermore, molybdenum disulfide has the ability to emit light, which would allow it to be used for various applications including optoelectronics, self-reporting sensors and LEDs.
A. T. Charlie Johnson, a professor in the Department of Physics & Astronomy in Penn’s School of Arts & Sciences has led this study. Carl Naylor, Nicholas Kybert, Gang Hee Han, and Jinglei Ping, who are members of Johnson’s lab also took part in this study. Ritesh Agarwal, a professor of materials science and engineering in Penn’s School of Engineering and Applied Science also contributed to this study along with Joohee Park and Bumsu Lee, who were members of his lab. Jisoo Kang, a master’s student in Penn’s nanotechnology program also took part in this study along with Si Young Lee and Young Hee Lee, who were researchers from South Korea’s Sungkyunkwan University.
Everything we do with regular electronics we'd like to be able to do with two-dimensional materials. Graphene has one set of properties that make it very attractive for electronics, but it lacks this critical property, being able to turn on and off. Molybedenum disulfide gives you that.
A. T. Charlie Johnson, a professor in the Department of Physics & Astronomy in Penn’s School of Arts & Sciences
When compared to other materials, the ultra-high conductivity property of graphene allows electron movement more quickly than any other material known. However, conductivity is not the only property that is required for electronics. In transistors, which are a main component of modern computing technology, the ability to stop electron flow is also very important.
Molybedenum disulfide is not as conductive as graphene, but it has a very high on/off ratio. We need 1’s and 0’s to do computation; graphene can only give us 1’s and .5’s.
Carl Naylor
Graphene was initially made using the exfoliating process, which involved peeling atomically thin layers of graphene from the bulk material. Some research groups have been successful in producing small flakes of molybdenum disulfide using this method. Chemical vapor deposition is another method that is used for manufacturing graphene, and this method has also been adopted by researchers to produce molybdenum disulfide flakes by heating molybdenum and sulfur into gases and then allowing them to settle on a substrate and crystallize. However, the flakes that formed were in a scattershot manner.
Between hunting down the flakes and making sure they’re the right size and thickness, it would take days to make a single measurement of their properties.
Nicholas Kybert
The Penn team advanced the chemical vapor deposition method. They developed and optimized a method to control the location where flakes formed by using a precursor to seed the substrate.
“We start by placing down a small amount of molybdenum oxide in the locations we want,” Naylor said, “then we flow in sulfur gas. Under the right conditions, those seeds react with sulfur and flakes of molybdenum disulfide being to grow.”
“There's finesse involved in optimizing the growth conditions,” Johnson said, “but we're exerting more control, moving the material in the direction of being able to make complicated systems. Because we grow it where we want it, we can make devices more easily. We have all of the other parts of the transistors in a separate layer that we snap down on top of the flakes, making dozens and potentially even hundreds, of devices at once. Then we were able to observe that we made transistors that turned on and off like they were supposed to and devices that emit light like they are supposed to.”
The researchers were able to match the location of the flakes of molybdenum disulfide with their associated electronics. In graphene-based devices, the graphene material is grown in the form of large sheets and then cut to the required size. This process increases the risk of damaging contamination. In the new process, this step can be skipped when producing molybdenum disulfide flakes.
Further studies on these molybdenum disulfide devices will advance the research performed by the team on graphene-based biosensors. These sensors would output molecule detection to a computer. However, sensors based on molybdenum disulfide would have the ability to report a binding event directly through a modification in their emitted light.
This study forms the basis for creating a new family of two-dimensional materials.
“We can replace the molybdenum with tungsten and the sulfur with selenium,” Naylor said, “and just go down the periodic table from there. We can imagine growing all of these different materials in the places we choose and taking advantages of all of their different properties.”
This study has been published in the journal, Nature Communications.
The National Science Foundation has provided support for this study through its Accelerating Innovation in Research Program; the Laboratory for Research on the Structure of Matter has provided support through the NSF’s Materials Research Science and Engineering Center program; the National Institutes of Health has provided support through the NIH Director’s New Innovator Award Program; the Nano/Bio Interface Center through NSF’s Nanoscale Science and Engineering Center program; and the U.S. Army Research Office through a grant.
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