Surface Functionalization of Liquid-Phase Exfoliated, Two-Dimensional MoS2 and WS2 Nanosheets with 2-Mercaptoethanol

Abstract

In this work, the UV-Vis-NIR absorption spectrum of liquid-phase exfoliated two-dimensional (2D) MoS2 nanosheets, revealed two prominent peaks at 608 nm (2.04 eV) and 668 nm (1.86 eV). These peaks were blue-shifted compared to the reported literature values and are attributed to the quantum confinement effect. Interestingly, the WS2 nanosheets exhibited the same characteristic absorption peak at similar to 624 nm (1.99 eV). Raman spectroscopy analysis revealed that both nanosheets displayed distinctive peaks [377.8 cm(-1) and 405.6 cm(-1) for MoS2, 348.3 cm(-1) and 417.9 cm(-1) for WS2] that originate from optical phonon modes (E-2g(1) and A(1g)). These peaks are shifted toward higher wavenumbers (i.e., blue-shift or phonon-stiffening) compared to bulk MoS2 and WS2, probably due to enhanced Stokes Raman scattering. Subsequently, surface functionalization of the nanosheets with 2-Mercaptoethanol was successfully performed and confirmed using optical characterization techniques, including FT-IR spectroscopy. In addition, we determined the spectral broadening after functionalization, which would be attributed to photon confinement of the nano-sized layer structure, or to inhomogeneous broadening.