Length and thickness dependent optical response of liquid-exfoliated transition metal dichalcogenides

Due to their reduced dimensionality, 2D materials show intriguing optical properties and strong light matter interaction. In particular group VI transition metal dichalcogenides (TMDs) have been extensively studied and proof of principle optical applications demonstrated. Most studies to date focus on individual mono-or bilayered micromechanically-exfoliated samples which often display significant variations between flakes. In this work, we study size-dependent optical properties of four group VI TMD materials; WS2; MoS2; WSe2 and MoSe2, each consisting of ensembles of nanosheets suspended in the liquid environment. Samples were produced by liquid phase exfoliation and size-selected using cascade centrifugation with size and layer number distributions quantified by statistical atomic force microscopy. Differences in lateral size and layer number are reflected in systematic changes in the optical extinction and absorbance spectra, which we exploit to establish quantitative spectroscopic metrics to facilita...