Alexandre S. Golub

Stabilization of 1T-MoS2 Sheets by Imidazolium Molecules in Self-Assembling Hetero-layered Nanocrystals

We report a facile, room-temperature assembly of MoS2-based hetero-layered nanocrystals (NCs) containing embedded monolayers of imidazolium (Im), 1-butyl-3-methylimidazolium (BuMeIm), 2-phenylimidazolium, and 2-methylbenzimidazolium molecules. The NCs are readily formed in water solutions by self-organization of the negatively charged, chemically exfoliated 0.6 nm thick MoS2 sheets and corresponding cationic imidazole moieties. As evidenced by transmission electron microscopy, the obtained NCs are anisotropic in shape, with thickness varying in the range 5-20 nm and lateral dimensions of hundreds of nanometers. The NCs exhibit almost turbostratic stacking of the MoS2 sheets, though the local order is preserved in the orientation of the imidazolium molecules with respect to the sulfide sheets. The atomic structure of NCs with BuMeIm molecules was solved from powder X-ray diffraction data assisted by density functional theory calculations. The performed studies evidenced that the MoS2 sheets of the NCs are of the nonconventional 1T-MoS2 (metallically conducting) structure. The sheets puckered outer surface is formed by the S atoms and the positioning of the BuMeIm molecules follows the sheet nanorelief. According to thermal analysis data, the presence of the BuMeIm cations significantly increases the stability of the 1T-MoS2 modification and raises the temperature for its transition to the conventional 2H-MoS2 (semiconductive) counterpart by ∼70 °C as compared to pure 1T-MoS2 (∼100 °C). The stabilizing interaction energy between inorganic and organic layers was estimated as 21.7 kcal/mol from the calculated electron density distribution. The results suggest a potential for the design of few-layer electronic devices exploiting the charge transport properties of monolayer thin MoS2.

Ridges and valleys on charged 1T-MoS2 sheets guiding the packing of organic cations

Using a novel powder X-ray pattern refinement technique, DFT calculations and TEM, we reveal for the first time the three-dimensional structure of hybrid (R4N)xMoS2 layered compounds. Both the precise coordinates of Mo and S atoms in the inorganic layer and the preferred positions of the organic cations relative to them are reported. The distorted geometry of the MoS2 sheet is found to be the driving force behind the arrangement of cations within the organic layers. The refined atomic coordinates, confirmed by periodic DFT modeling, show octahedral Mo atoms arranged in zigzag chains and the S atoms following the Mo ones, forming “ridges” and “valleys” on the sheet surface. The size of alkylammonium cations is compatible with these valleys, leading to a strong preferred arrangement: the cations exclusively occupy the space between the ridges. This result allows predicting the layer structure and ultimately the composition of the hybrid compound depending on the cation structure. The reported features of the MoS2 surface are important for understanding of MoS2-based sensors and hybrid flexible conductors, both relying on charged sheets of MoS2.