Controlling the Metal to Semiconductor Transition of MoS₂ and WS₂ in Solution

Abstract

Lithiation-exfoliation produces single to few-layered MoS2 and WS2 sheets dispersible in water. However, the process transforms them from the pristine semiconducting 2H phase to a distorted metallic phase. Recovery of the semiconducting properties typically involves heating of the chemically exfoliated sheets at elevated temperatures. Therefore, it has been largely limited to sheets deposited on solid substrates. Here, we report the dispersion of chemically exfoliated MoS2 sheets in high boiling point organic solvents enabled by surface functionalization and the controllable recovery of their semiconducting properties directly in solution. This process connects the scalability of chemical exfoliation with the simplicity of solution processing, ultimately enabling a facile method for tuning the metal to semiconductor transitions of MoS2 and WS2 within a liquid medium.

Effect of temperature of Li2O–Al2O3–TiO2–P2O5 solid-state electrolyte coating process on the performance of LiNi0.5Mn1.5O4 cathode materials

“…It can be found that the valence of manganese in LiNi 0.5 Mn 1.5 O 4 is between þ3 and þ4, and increases with LATP coating. The relative amount of Mn 3þ in the surface phase of all the three samples has been estimated by peak area ratios of Mn 2p 3/2 (Mn 2p 1/2 peak were constrained based on 2p 3/2 and thus not used in analysis) [33]. Fig.…”

Oxidation suppression during hydrothermal phase reversion allows synthesis of monolayer semiconducting MoS2 in stable aqueous suspension

“…19 Oxidation by O 2 has been reported at elevated temperature for MoS 2 nanosheets 22 and for bulk MoS 2 and its friction films. 23,24 Over longer time scales, oxidative degradation of MoS 2 nanosheets has even been observed at room temperature in humid air, 25 and in aqueous suspensions.…”

“…2 For this reason, significant efforts have been made to restore the semiconducting 1H/2H phase by processing ce-MoS 2 monolayers. 2,18,19 Eda et al 2 restored the 2H phase by annealing ce-MoS 2 films, but the drying process leads to nanosheet restacking within the film, which limits further processability and also suppresses photoluminescence. It would be desirable to carry out the phase reversion of 1T monolayers in their original aqueous suspension, but this has proven problematic.…”

“…The temperature required for the metal-to-semiconductor phase reversion exceeds the normal boiling point of water. 2,19 Also the 1H/2H form is reported to be hydrophobic, 20 leading to instability, flocculation, and restacking in aqueous solutions. Guardia et al 21 successfully maintained colloidal stability by using very dilute nanosheet suspensions, but dilute processing is a disadvantage for high-volume production.…”

“…26 A recent study observed much faster oxidation kinetics for chemically exfoliated MoS 2 (mixed 1T/1H phase) than surfactant-assisted liquid exfoliated MoS 2 (2H phase), but did not propose a mechanism to explain the origin of the difference. 26 To overcome the various limitations in fabricating semiconducting monolayer nanosheets (denoted as 1H-MoS 2 ), Chou et al 19 functionalized chemically exfoliated MoS 2 surfaces to allow dispersion in a high-boiling point organic solvent followed by dispersion, annealing and phase reversion to 1H in that solvent.…”