Scientists from South Korea, Japan, Denmark, and Japan have found that boron dioxide encapsulation can significantly improve the performance of MoS2 (molybdenum diulfide). James Hone and Wang Fong-Jen from Columbia University, explained two years ago that graphene’s performance can be dramatically improved by encapsulating in boron nitride. This insulating material has a similar layered structure as the highly conductive 2-D carbon. Professor Hone is the director of Columbia’s NSF-funded Materials Research Science and Engineering Center. He said that this study demonstrated how all 2D materials can be studied. The combination of graphene and BN electrodes acts as a “socket” into which other materials can be placed and studied in a very clean environment. This allows us to discover their true potential and properties.” MoS2 is Molybdenum disulfide, which has been encapsulated within layers of boron nutride. (Credit: Gwan-Hyoung Lee/Yonsei University) “This holds great promise for a broad range of applications including high-performance electronics, detection and emission of light, and chemical/bio-sensing.” 2-D materials created by ‘peeling’ atomically thin layers from bulk crystals are super stretchable, visually transparent, and can be combined with each other and with conventional electronics in completely novel ways. These two-dimensional materials in which every atom is at the surface are sensitive by nature to the environment. This means they are often far below theoretical limits due to contamination and trapped charge around insulating layers. Professor Hone and his colleagues used BN to encapsulate graphene last year. This improved electronic mobility by fifty percent – an important indicator of electronic performance. It also allows for lower levels of disorder, which allows the investigation of new rich phenomena at low temperatures and high magnetic field. Gwan-Hyoung, the co-lead author of the Nature Nanotechnology paper and an assistant professor of materials science, Yonsei University in Seoul, South Korea, said that MoS2 is able to be fully switched off. Previously MoS2 devices on silicon dioxide or other common insulating substrates such as silicon dioxide have had mobility that fell below the theoretical predictions. It fluctuates between samples and remains low when cooled to low temperatures. These are all signs of disordered material. Scientists are not sure if the disordered material, such as graphene’s substrate, caused it. Professor Hone, along with his colleagues, created heterostructures of MoS2 in BN. These layered stacks were topped by tiny graphene flakes that overlapped the MoS2’s edge to serve as electrical contacts. Researchers found that mobility at room temperature increased by around 2 to the intrinsic limit. The mobility rose significantly when it was cooled down to low temperatures, with values that were five to fifty times higher than the previous measurements (depending upon the number of layers). These high-mobility samples showed significant fluctuations in magnetic field resistance, which was a sign that the disorder level is low. This phenomenon had not been observed in 2D semiconductors before. Xu Cui (first author), a Columbia Engineering PhD student, stated: “This device structure allows us to study quantum transportation behavior in this material for the first time.” Other improvements Possible After analyzing low-temperature resistance as well as quantum oscillations Prof. Hone’s team found that contamination at interfaces was the primary source of disorder. This suggests that improvements are possible. Professor Hone stated that this work motivated them to improve their device assembly techniques since they haven’t yet reached the intrinsic limit of the material. “With further improvements, we hope to create 2D semiconductors to rival the performance and cost-effectiveness of traditional semiconductor heterostructures.” A team of Italian scientists spray spiders in a mixture of graphene and water. These spiders created strong webs strong enough to stop a falling plane. Citation: “Multiterminal transport measurements for MoS2 using a van den Waals heterostructure platform,” Xu Cui. Young Duck Kim. Pinshane Y.Huang, Ghidewon Arefe. Young Duck Kim. Filippo Pizzocchero. Kenji Watanabe. Takashi Taniguchi. David A. Muller. Tony Low. Philip Kim & James Hone. NATURE NANOTECHNOLOGY. Publication 27 April. 2015. DOI: 10.1038/nnano.2015.70. Video – Graphene
