One sentence summary:We describe a general liquid-phase method to exfoliate layered compounds to give monoand few-layer flakes in large quantities. TMDs consist of hexagonal layers of metal atoms, M, sandwiched between two layers of chalcogen atoms, X, with stoichiometry MX 2 . While the bonding within these tri-layer sheets is covalent, adjacent sheets stack via van der Waals interactions to form a 3D crystal. TMDs occur in more than 40 different types (2, 3) depending on the combination of chalcogen (S, Se or Te) and transition metal(3). Depending on the co-ordination and oxidation state of the metal atoms, TMDs can be metallic, semi-metallic or semiconducting(2, 3), e.g. WS 2 is a semiconductor while NbSe 2 is a metal(3). In addition, superconductivity(4) and charge density wave effects(5) have been observed in some TMDs. This versatility makes them potentially useful in many areas of electronics.However, like graphene(6), layered materials must be exfoliated to fulfil their full potential. For example, films of exfoliated Bi 2 Te 3 should display enhanced thermoelectric efficiency by suppression of thermal conductivity(7). Exfoliation of 2D topological insulators such as Bi 2 Te 3 and Bi 2 Se 3 would reduce residual bulk conductance, 4 highlighting surface effects. In addition, we can expect changes in electronic properties as the number of layers is reduced e.g. the indirect bandgap of bulk MoS 2 becomes direct in few-layer flakes(8). Although exfoliation can be achieved mechanically on a small scale(9, 10), liquid phase exfoliation methods are required for many applications(11).Critically, a simple liquid exfoliation method would allow the formation of novel hybrid and composite materials. While TMDs can be chemically exfoliated in liquids(12-14), this method is time consuming, extremely sensitive to the environment and incompatible with most solvents.We demonstrate exfoliation of bulk TMD crystals in common solvents to give mono-and few layer nano-sheets. This method is insensitive to air and water and can potentially be scaled up to give large quantities of exfoliated material. In addition, we show that this procedure allows the formation of hybrid films with enhanced properties.We initially sonicated commercial MoS 2 , WS 2 and BN (15, 16) powders in a number of solvents with varying surface tensions. The resultant dispersions were centrifuged and the supernatant decanted (Section S3). Optical absorption spectroscopy showed that the amount of material retained (characterised by / A l C α = , where A/l is the absorbance per length, α is the extinction coefficient and C is the concentration) was maximised for solvents with surface tension close to 40 mJ/m 2 (17, 18) ( Fig. 1A-C). Detailed analysis, within the framework of Hansen solubility parameter theory(19), shows successful solvents to be those with dispersive, polar and H-bonding components of the cohesive energy density within certain well-defined ranges (Section S4, Figs. S2-S3). This can be interpreted to mean that successful solvents are those w...

Two-dimensional nanomaterials such as MoS 2 are of great interest both because of their novel physical properties and their applications potential. Liquid exfoliation, an important production method, is limited by our inability to quickly and easily measure nanosheet size, thickness or concentration. Here we demonstrate a method to simultaneously determine mean values of these properties from an optical extinction spectrum measured on a liquid dispersion of MoS 2 nanosheets. The concentration measurement is based on the size-independence of the low-wavelength extinction coefficient, while the size and thickness measurements rely on the effect of edges and quantum confinement on the optical spectra. The resultant controllability of concentration, size and thickness facilitates the preparation of dispersions with pre-determined properties such as high monolayer-content, leading to first measurement of A-exciton MoS 2 luminescence in liquid suspensions. These techniques are general and can be applied to a range of two-dimensional materials including WS 2 , MoSe 2 and WSe 2 .

We report on the state-of-the art synthesis and improved luminescence properties of thiol-capped CdTe nanocrystals (NCs) synthesized in water. The optimized pH (12) and molar ratio of thiol to Cd ions (1.3:1) increases the room-temperature photoluminescence quantum efficiency of as-synthesized CdTe NCs capped by thioglycolic acid (TGA) to values of 40−60%. By employing mercaptopropionic acid (MPA) as a stabilizer, we have synthesized large (up to 6.0 nm in diameter) NCs so that the spectral range of the NCs' emission currently available within this synthetic route extends from 500 to 800 nm. Sizing curve for thiol-capped CdTe NCs is provided. In contrast to CdTe NCs capped by TGA, MPA-capped CdTe NCs show up to 1 order of magnitude longer (up to 145 ns) emission decay times, which become monoexponential for larger particles. This phenomenon is explained by considering the energetics of the Te-related traps in respect to the valence-band position of CdTe NCs. The correlation between luminescence quantum efficiencies, luminescence lifetimes, and Stokes shifts of CdTe NC fractions is demonstrated, being in agreement with a model proposed previously that connects the emission properties of NCs with their surface quality determined by the Oswald ripening conditions during growth. imaging, and plasmonics.