(Invited) Spectroscopic Metrics for Determining Size and Thickness of Liquid Exfoliated Nanosheets in Dispersion

Over the last few years the study of 2-dimensional (2D) nanomaterials has become one of the most important areas of both materials science and nanotechnology. While this work originally focused on graphene, the palette of 2D materials currently under study includes transition metal dichalcogenides (TMDs) such as MoS₂ and WSe₂, layered transition metal oxides such as MoO₃ and a host of other interesting structures such as GaS or phosphorene. Particularly useful is the diversity of 2D materials: depending on the combination of elements and their arrangement, they can be metals, semiconductors, insulators or superconductors.

Liquid-phase exfoliation has proven to be a powerful technique to obtain large quantities of exfoliated 2D materials in dispersion, ultimately making the materials processable. This method involves the sonication of layered crystals in certain solvents or solutions of surfactants or polymers. Even though being extremely versatile and useful, it has notable disadvantages such as the polydispersity of materials produced.

We have performed detailed spectroscopic and microscopic analysis on size-selected dispersions of exfoliated MoS₂ nanosheets. This has led to the development of an in situ technique which allows nanosheet concentration, lateral size and thickness to be obtained simultaneously from an optical extinction/absorption spectrum. The combination of concentration-control, size-selection and measurement facilitates the preparation of dispersions with pre-determined properties. For example, monolayer-enriched dispersions can be produced, allowing the first measurement of direct-gap luminescence in liquid suspensions. In subsequent studies, we have developed similar metrics for other liquid-exfoliated nanosheets such as graphene, GaS and MoTe₂. In the case of graphene, we propose simple metrics based on Raman spectra of size-selected graphene in restacked films to quantify size and thickness on the foundation of the G-band width or D/G ratio and changes in the 2D band shape.

C. Backes, ... J. N. Coleman, Using Edge and Confinement Effects for in situ Determination of Size and Thickness of Liquid-Exfoliated Nanosheets. Nat. Commun. 2014, 5, 4576.