A method that turns certain chalcogenide compounds into high- quality, larger exfoliated flakes than currently available can be used in printable photonics and electronics. Dr. Zheng Jian, the first author of the paper, is shown demonstrating the printing of molybdenum disulfide flakes from a solution of the exfoliated flakes. Images courtesy of NUS. Chemists at the National University of Singapore (NUS), led by professor Loh Kian Ping, developed the method, which chemically exfoliates molybdenum disulfide crystals — a class of chalcogenide compounds — into high-quality monolayer flakes. But while most applications for the compounds require clean, large-sized flakes, current methods of producing them take a long time, and the yield is poor: The flakes produced are submicron-sized, making it challenging to isolate a single sheet for making electronic devices. "At present, there is a bottleneck in the development of solution-processed two dimensional chalcogenides," Loh said. "Our team has developed an alternative exfoliating agent using the organic salts of naphthalene, and this new method is more efficient than previous solution-based methods. It also can be applied to other classes of two-dimensional chalcogenides," such as tungsten diselenide and titanium disulfide, he said. Schematic of pre-exfoliation, intercalation and exfoliation processes. (a)Bulk molybdenum disulfide crystal was expanded by decomposition of hydrazine. (b)The expanded molybdenum disulfide reacted with sodium naphthalenide to form an intercalation sample, then exfoliated into single-layer sheets by immersing in water. (c)Scanning electron microscopy image of single-layer molybdenum disulfide on silicon dioxide. (d)Atomic force microscopy image of single-layer molybdenum disulfide on silicon dioxide. (e)Photograph of bulk single-crystal molybdenum disulphide. (f)Photograph of pre-exfoliated molybdenum disulphide. (g)Photograph of sodium-exfoliated single-layer molybdenum disulfide dispersion in water. Dr. Zheng Jian, a research fellow in the Department of Chemistry, did a comparative analysis and found that the alkali metal naphthalenide intercalation method applied possesses significant advantages in comparison with the conventional method. "Considering the versatility of this method, it may be adopted as the new benchmark in exfoliation chemistry of two-dimensional chalcogenides," Loh said. Loh and his team are now developing inks based on various types of 2-D chalcogenides exfoliated using their method, so that printable optoelectronic devices can be produced. They also will be testing the optical nonlinear properties of the flakes they have produced. A paper on the work was published this month in Nature Communications (doi:10.1038/ncomms3995). For more information, visit: www.nus.edu.sg