Lithium induces the reorientation of few-layer MoS2 films

 

Molybdenum disulfide (MoS₂) is a layered material that can be prepared in the form of monolayer or few-layer films. It is extensively studied due to its intriguing properties and wide range of possible applications in electronics, optoelectronics, or catalysis. The layered materials have the ability to intercalate atoms and cations into their van der Waals gaps. Intercalation is one of the ways to tune their physical, chemical, and electronic properties.

One of the metals used for MoS₂ doping is lithium. Previous studies have shown enhanced catalytic performance of Li-doped MoS₂ in hydrogen evolution reactions and CO₂ reduction. Lithium doping has also the potential to improve Li-ion battery technology. However, the influence of lithium on the growth of MoS₂ films has not been fully explored yet.

“We used a new approach to incorporate lithium into MoS2 films,” says Dr. Michaela Sojková, responsible for the films' synthesis.“We are using one-zone sulfurization of pre-deposited molybdenum layers to fabricate MoS₂. To deliver lithium into the films, we replaced a part of the sulfur with lithium sulfide powder. And it worked.”

The identification of the presence of lithium in the sample is a very complex issue. There are only a few methods that allow its detection. One of these methods is synchrotron-based high energy resolution XPS with tunable photon energies in the soft x-rays range and Li- K near edge x-ray absorption (XANES). The measurements were performed at the BACH beamline of CNR at the Elettra synchrotron facility (Trieste, Italy).

“Our experimental setup allows us not only to confirm and quantify lithium in the samples and their long-term stability but thanks to the absorption near-edge structure (XANES) spectroscopy combined with calculations, we are able to confirm the position of lithium in octahedral and tetrahedral interstitial sites rather than substitutional sites,” says Dr. Federica Bondino, scientific coordinator of the BACH beamline.
 

“These results have benefited from the unique opportunity to access facilities with state-of-the-art methods thanks to NFFA-Europe” adds Dr. Michaela Sojková.

The structural analyses show a remarkable effect of lithium in promoting the epitaxial and horizontal growth of the films. After annealing in the presence of Li, MoS₂ films tend to grow horizontally even for thicker initial molybdenum layers. The most surprising observation is the conversion of vertically aligned MoS₂ to horizontally aligned films after annealing in the presence of Li. We suppose that lithium acts as a catalyst in facilitating this conversion.

 

(a) Li K-edge XANES spectra for Li doped 2H-MoS₂ measured at the beamline BACH (b) Li K-edge XANES spectra obtained by FEFF calculations. Top: fragments of the structural models for hexagonal 2H-MoS₂ with atoms in tetrahedral and octahedral interstitial sites, substitutionally doped MoS₂, and with lithium near a single sulfur vacancy. Middle: simulated Li K-edge XANES spectra. Bottom: calculated orbital-projected density of states for Li, S, and Mo atoms.(c) Mo 4s and Li 1s XPS spectra from Li doped MoS₂ samples. All images are from the article Lithium-induced reorientation of few-layer MoS₂ films, Chem. Mater. 35 (2023) 6246-6257. Copyright © 2022 The Authors. Published by American Chemical Society. This publication is licensed under CC-BY 4.0.

 

In our work, we demonstrate that by lithium doping it is possible to tune the MoS₂ basal plane orientation in few-layer films, which is noteworthy because vertically and horizontally oriented films exhibit distinct electronic, chemical, and optical properties. Horizontally aligned films are suitable for electronics and optoelectronics, such as ultrafast tunnel diodes or photodetectors for use in harsh environments, while vertically aligned MoS₂ is a promising candidate for substituting noble-metal catalysts in electrochemical hydrogen production, conversion of CO₂ to energy-rich products, water disinfection, water splitting, or solar cells. Thus, control over the MoS₂ crystallographic orientation will be crucial in engineering next-generation devices incorporating MoS₂ layers.

Lithium-Induced Reorientation of Few-Layer MoS₂ Films
Sojková M., Píš I., Hrdá J., Vojteková T., Pribusová Slušná L., Vegso K., Siffalovic P., Nadazdy P., Dobročka E., Krbal M., Fons P.J., Munnik F., Magnano E., Hulman M., Bondino F.
Chemistry of Materials, Vol. 35 - 16, pp. 6246-6257 (2023)
doi: 10.1021/acs.chemmater.3c00669 (Journal Article)