Size-Dependent Properties of Two-Dimensional MoS2 and WS2

The characteristic differences between MoS2 and WS2 nanosheets and nanodots are investigated. The nanosheets were formed by liquid-phase sonication, while the nanodots were formed by breaking the nanosheets through heating the solvent ethylene glycol. The nanosheets and nanodots were approximately 0.7–2 nm thick, with slight deviation. Most of the nanosheets were longer than 100 nm, and most of the nanodots were shorter than 5 nm. As the bulk materials were transformed into nanosheets and/or nanodots, the absorption peaks and Raman peaks shifted to shorter wavelengths. Photoluminescence peaks were observed at 500 and 445 nm in the MoS2 and WS2 samples smaller than 100 nm. In the X-ray diffraction spectra, only the (002) peak was present in the nanosheets, while no peak was detected for the nanodots due to their small size. No detectable differences between the nanosheets and nanodots were observed in the transmission electron micrographs, synchrotron radiation photoemission spectra, or work function measurements, suggesting that exfoliation did not affect the crystal structure or bonding configuration of MoS2 and WS2. These results could potentially be used for the application of MoS2 and WS2 nanosheets and nanodots in optical devices, hydrogen evolution reaction catalysts, bioapplicable devices, and so on.