Subcategory: Materials Science
Lauren Weiss - Amherst College
Co-Author(s): Tanushree Choudhury and Joan M. Redwing, Pennsylvania State University, University Park, PA
Few-layered two-dimensional transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2), are direct bandgap semiconductors with promising electrical properties. These characteristics make them appealing for flexible electronics and optoelectronic devices. Further advances in the applications of these materials would require the synthesis of 2D layers which can be easily placed on appropriate substrates.
This study investigated chemical exfoliation techniques to prepare few-layer TMD films from bulk crystals. Ultrasonic acoustic cavitation was used to exfoliate 2D MoS2 layers. The exfoliation was attempted in different solvents: plain dimethyl sulfoxide (DMSO), DMSO with hydrogen peroxide (H2O2) and nmethyl pyrrolidone (NMP), and NMP with H2O2. Aliquots were removed at various time intervals to examine the effect of sonication time on the flake size and thickness. The aliquots were centrifuged, rinsed in isopropanol, and drop casted onto silicon/silicon oxide substrates. Long sonication times and higher sonication power created smaller, thinner flakes, as did the addition of H2O2 to the solvents. The best solvent combination was NMP with H2O2. Judging from Raman maps of the thinnest flakes, some monolayer flakes were achieved from sonicating the bulk MoS2 for 60 minutes in NMP and H2O2. Attempts were also made to successfully transfer these layered materials onto diverse substrates. Mechanical exfoliation using Scotch tape was used to peel off the WS2 film and stamp it onto a new substrate. The film transfer was confirmed using Raman, SEM, and AFM. Metallorganic chemical vapor deposition (MOCVD) grown WS2 thin films were also transferred onto new substrates using a sacrificial polymethylacralate (PMMA) layer. The transfer involved a hydrofluoric acid (HF) or sodium hydroxide (NaOH) dip. In conclusion, thin films of molybdenum disulfide and tungsten disulfide have unique electrical and optical properties that set them apart from the bulk. Ultrasonic exfoliation using hydrogen peroxide with NMP or DMSO as a solvent is a relatively easy way to achieve two-dimensional films of molybdenum disulfide. The polymer layer transfer using an HF dip was the most effective transfer method, as it transferred large pieces of the film without damaging it.
Funder Acknowledgement(s): National Science Foundation (NSF) Emerging Frontiers of Research and Innovation (EFRI) Two -Dimensional Atomic-Layer Research and Engineering (2-DARE)
Faculty Advisor: Joan M. Redwing,