Friday, January 10, 2014

Creating improved inkjet-printable materials for electronics and photonics

January 10, 2014
Inkjet-printing molybdenum disulfide thin film at the end of an optical fiber (credit: TUS)
National University of Singapore (NUS) scientists have developed a new method for creating a chemical solution of molybdenum disulfide for use in printable optoelectronic devices such as thin film solar cells, flexible logic circuits, photodetectors, and sensors.
Molybdenum disulfide, combined with gold atoms, is being studied for development of ultrafast, ultrathin logic devices, as noted previously on KurzweilAI.
The process:
1. Chemically exfoliate (peel off) molybdenum disulfide crystals into high-quality single-layer flakes (the new method achieves higher yield and larger flake size than current methods).
2. Convert the flakes into an inkjet-printable solution (the good dispersion and high viscosity of the flakes make them highly suitable for inkjet printing).
3. Print wafer-size films
Current processes of producing printable single-layer chalcogenides (such as molybdenum disulfide) take a long time and the yield is poor. The flakes produced are of submicron sizes, which make it challenging to isolate a single sheet for making electronic devices.
The NUS team prepared naphthalenide combinations of lithium, sodium and potassium, and compared the exfoliation efficiency and quality of molybdenum disulfide generated.
Schematic of (a) pre-exfoliation (peeling), (b) intercalation (merging layers) and exfoliation processes. (c) Photograph of bulk single-crystal MoS2 (d) photograph of pre-exfoliated MoS2 (e) photograph of sodium-exfoliated single-layer MoS2 dispersion in water. (credit: NUS)
The NUS team collaborated with scientists from the Ulsan National Institute of Science and Technology in Korea.

Abstract of Nature Communications paper
Transition-metal dichalcogenides like molybdenum disulphide have attracted great interest as two-dimensional materials beyond graphene due to their unique electronic and optical properties. Solution-phase processes can be a viable method for producing printable single-layer chalcogenides. Molybdenum disulphide can be exfoliated into monolayer flakes using organolithium reduction chemistry; unfortunately, the method is hampered by low yield, submicron flake size and long lithiation time. Here we report a high-yield exfoliation process using lithium, potassium and sodium naphthalenide where an intermediate ternary LixMXn crystalline phase (X=selenium, sulphur, and so on) is produced. Using a two-step expansion and intercalation method, we produce high-quality single-layer molybdenum disulphide sheets with unprecedentedly large flake size, that is up to 400 μm2. Single-layer dichalcogenide inks prepared by this method may be directly inkjet-printed on a wide range of substrates.

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