Size-Dependent Properties of Two-Dimensional MoS₂ and\nWS₂

Abstract

The characteristic\ndifferences between MoS₂ and WS₂ nanosheets\nand nanodots are investigated. The nanosheets\nwere formed by liquid-phase sonication, while the nanodots were formed\nby breaking the nanosheets through heating the solvent ethylene glycol.\nThe nanosheets and nanodots were approximately 0.7–2 nm thick,\nwith slight deviation. Most of the nanosheets were longer than 100\nnm, and most of the nanodots were shorter than 5 nm. As the bulk materials\nwere transformed into nanosheets and/or nanodots, the absorption peaks\nand Raman peaks shifted to shorter wavelengths. Photoluminescence\npeaks were observed at 500 and 445 nm in the MoS₂ and WS₂ samples smaller than 100 nm. In the X-ray diffraction spectra,\nonly the (002) peak was present in the nanosheets, while no peak was\ndetected for the nanodots due to their small size. No detectable differences\nbetween the nanosheets and nanodots were observed in the transmission\nelectron micrographs, synchrotron radiation photoemission spectra,\nor work function measurements, suggesting that exfoliation did not\naffect the crystal structure or bonding configuration of MoS₂ and WS₂. These results could potentially be used for\nthe application of MoS₂ and WS₂ nanosheets and\nnanodots in optical devices, hydrogen evolution reaction catalysts,\nbioapplicable devices, and so on.