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

Layered transition metal dichalcogenides (TMDCs) such as MoS2, MoSe2 and WSe2 are under intense investigation because they are atomically thin semiconductors with photophysical properties can be tuned by changing their composition or morphology. Mechanochemical processing has been proposed as a method to obtain alloyed TMDCs in the series Mo1-xWxSySe2-y (x = 0, 1; y = 0, 1, 2). However, elucidating the chemical transformations occurring at the atomic scale following mechanochemical processing is challenging because the products are usually fine powders. To address this challenge, we probe TMDC mixing and alloying by using a combination of powder X-ray diffraction (PXRD), optical 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 (DNP). The combination of SSNMR with the other characterization methods demonstrates that ball milling induces molecular level alloying of Mo, W and chalcogen atoms in the family Mo1-xWxSySe2-y. GIPAW DFT calculations yield accurate values of 77Se chemical shift tensor components. 77Se SSNMR spectroscopy was also applied to study the structure of WSe2 nanocrystals intercalated with ethylenediamine. The WSe2 nanocrystals exhibited a more positive isotropic 77Se chemical shift as compared to bulk WSe2, however, the 77Se chemical shift anisotropy was the same, confirming the WSe2 layers have a similar structure as in their bulk counterparts.