Photo-exfoliation of MoS2 quantum dots from nanosheets: an in-situ transmission electron microscopy study

Fabrication of transition metal dichalcogenide (TMD) quantum dots (QDs) is complex and requires submerging of powders in binary solvents and the constant tuning of wavelength and pulsed frequency of light to achieve a desired reaction. Instead of liquid state photoexfoliation, we utilize infrared laser irradiation of free-standing MoS2 flakes in transmission electron microscope (TEM) to achieve solid-state multi-level photoexfoliation of QDs. By investigating the steps involved in photochemical reaction between the surface of MoS2 and the laser beam, we gain insight into each step of the photoexfoliation mechanism and observe high yield production of QDs, led by an inhomogeneous crystalline size distribution. Additionally, by using a laser with a lower energy than the indirect optical transition of bulk MoS2, we conclude that the underlying phenomena behind the photoexfoliation is from multi-photon absorption achieved at high optical outputs from the laser source. These findings provide an environmentally friendly synthesis method to fabricate QDs for potential applications in biomedicine, optoelectronics, and fluorescence sensing.

Adsorption of Binary Solvents on Chiral Stationary Phases with Grafted Macrocyclic Antibiotics

A study is performed of the adsorption of water–methanol and water–acetonitrile mixtures on chiral stationary phases (CSPs) Chirobiotic R, Chirobiotic T, and Nautilus-E with grafted macrocyclic antibiotics ristocetin A, teicoplanin, and eremomycin, respectively. The patterns of adsorption on the indicated CSPs are qualitatively the same, and differ only by quantitative indicators. Adsorption isotherms of excess water from binary solvents have adsorption azeotrope points and show the preferred absorption of water in the range of pure organic component to an azeotrope point in the range of 60–75 mol % for H2О–МеОН and 80–90 mol % for H2O–MeCN systems. It is shown that the thickness of the adsorption phase in the first case is less than one nominal molecular layer (0.10–0.13 nm). For H2O–MeCN, it is 3–4 molecular layers (0.88–1.05 nm). Activity coefficients are calculated for the components of solutions in surface layers. The coefficients indicate the systems deviate considerably from the properties of an ideal adsorption solution. Reasons for this behavior are discussed.