Structural Studies of Alloyed and Nanoparticle Transition Metal Dichalcogenides by Selenium-77 Solid-State Nuclear Magnetic Resonance Spectroscopy

Abstract

Layered transition metal dichalcogenides (TMDCs) such as MoS₂, MoSe_2, and WSe₂ are under intense investigation because they are atomically thin semiconductors with photophysical properties that can be tuned by changing their composition or morphology. Mechanochemical processing has been proposed as a method to obtain alloyed TMDCs in the series Mo_1–xW_xS_ySe_2–y (x = 0–1; y = 0–2). However, elucidating the chemical transformations occurring at the atomic scale following mechanochemical processing can be challenging because the products are often amorphous or microcrystalline. To address this challenge, we probe TMDC mixing and alloying by using a combination of powder X-ray diffraction, Raman spectroscopy, diffuse reflectance spectroscopy, ⁷⁷Se solid-state nuclear magnetic resonance (SSNMR) spectroscopy, and planewave density functional theory (DFT) calculations. The nature of the milling material and reaction atmosphere are shown to be essential factors in limiting the formation of undesired oxide byproducts. We demonstrate acquisition of ⁷⁷Se SSNMR spectra using different combinations of Carr-Purcell Meiboom-Gill acquisition (CPMG) pulse sequences, magic angle spinning (MAS), and MAS dynamic nuclear polarization. The combination of SSNMR with the other characterization methods clearly demonstrates that high energy impact ball milling induces molecular level alloying of Mo, W and chalcogen atoms in the family Mo_1–xW_xS_ySe_2–y. Gauge including projector augmented wave DFT calculations yield accurate ⁷⁷Se chemical shift (CS) tensor components. ⁷⁷Se SSNMR spectroscopy was also applied to study the structure of WSe₂ nanocrystals intercalated with ethylenediamine. The intercalated WSe₂ nanocrystals exhibit a more positive isotropic ⁷⁷Se CS as compared to bulk WSe₂, however, the ⁷⁷Se CS anisotropy is the same, confirming the WSe₂ layers have a similar structure as in their bulk counterparts.