Molybdenum disulfide (MoS2) is a layered material which 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. The layered materials have the ability to intercalate atoms and cations into their van der Waals gaps. The intercalation is one of the ways how 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 also has the potential to improve Li-ion battery technology. However, the influence of the lithium on the growth of MoS2 films has not been fully explored.
We used a new approach to incorporate the lithium into MoS2 films. We are using one-zone sulfurization of pre-deposited molybdenum layers to fabricate MoS2. And to deliver a lithium into the films, we replaced a part of the sulfur by lithium sulfide powder.
The identification of the lithium presence in the sample is very complex issue. There are only few methods allowing 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 BACH beamline of CNR at the Elettra synchrotron facility (Trieste, Italy). The experimental setup allowed us not only confirming and quantify lithium in the samples and their long-term stability, but thanks to the absorption near-edge structure (XANES) spectroscopy combined to calculations, we were able to confirm the position of lithium in octahedral and tetrahedral interstitial sites rather than substitutional sites.
The structural analyses shows 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. 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. Thus, the control over the MoS2 crystallographic orientation will be crucial in engineering next generation devices incorporating MoS2 layers.
(a) Li K-edge XANES experimental spectra for Li-doped 2H-MoS2.
(b) Li K-edge XANES spectra obtained from FEFF calculations.
Top: Structural model fragments for hexagonal 2H-MoS2 with atoms in tetrahedral and octahedral interstitial sites, substitutionally doped MoS2, and with lithium near a sulfur atom.
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.
Authors: M. Sojková, J. Hrdá, T. Vojteková, L. Pribusová Slušná, P. Nádaždy, E. Dobročka, M. Hulman