Speaker
Description
Molybdenum disulfide, MoS$_2$, is a layered material from transition metal dichalcogenide (TMD) family. Its range of applications includes tribological coatings, materials for electronics, and catalysis.
TMD thin films are often prepared via deposition processes, that originally yield an amorphous material[1]. Tribological applications mostly rely on the ability of MoS$_2$ to crystallize in the course of exploitation. Other applications require MoS$_2$ to be crystallized in a certain way: in mild conditions for flexible stretchable photodetectors[2] or with specific defects for catalysis applications[3].
Accurate simulations at a large scale provide insights into collective events and structural changes in the course of transformation and isolate the effects of varying conditions. Those findings can be very useful for guiding experimental search of the treatment conditions.
Reactive Force Field (ReaxFF)[4] is an empirical potential, aiming to bridge the gap between first principle methods and simple empirical potentials. The former, such as DFT, are very accurate, but very computationally expensive: simulations are limited to thousands of atoms and hundreds of picoseconds. The latter, like Tersoff[5], are suitable for handling millions of atoms for nano- and even microseconds. One can achieve good results for bulk properties, but these potentials aren’t very accurate on the atomistic level. ReaxFF is supposed to be able to handle tens and hundreds of thousands of atoms for nanoseconds, while being at DFT level of accuracy. However, ReaxFF is a complicated potential, that comprises 39 general parameters, 32 parameters per atom, 24 parameters per bond, 7 parameters per angle and torsion. These parameters have to be carefully fitted to experimental and DFT-computed data to achieve the desired quality of the description.
Several ReaxFF parameterizations exist for Mo-S element system. They were used to study bending of MoS$_2$ layers[6], crystallization of a single layer of MoS$_2$[7,8], and formation MoS$_2$ from MoO$_3$ and sulfur[9]. None of those, however, yielded a layered MoS$_2$ in our crystallization simulations outside the single-layer setup, producing a not-discovered experimentally and non-stable within DFT rock-salt type MoS.
In our search for V-O parameter set we found a parameterization that was producing desired layered structure in melt-quench and oxidation simulations. We used it as a basis for future development, using state-of-the-art parameters for S atom and applying Monte-Carlo parameter fitting vs. DFT-computed data. Convex-Hull diagram computed for Mo$_x$S$_y$ system within our new ReaxFF matches DFT results. In simulations, that imitated tribological conditions, we observed crystallization of layered MoS2. Our parameter set is a basis for future development and expanding the set of elements to study practically relevant compositions.
[1] Polcar T, et al. Rev Adv Mater Sci 2007;15:118
[2] Wuenschell JK, et al. J Appl Phys 2020;127.
[3] Hu J, et al. Nat Catal 2021;4:242+.
[4] Van Duin ACT, et al. J Phys Chem A 2001;105:9396.
[5] J. Tersoff. Phys Rev B 1988;37.
[6] Ostadhossein A, et al. J Phys Chem Lett 2017;8:631.
[7] Chen R, et al. J Vac Sci Technol A 2020;38:022201.
[8] Chen R, et al. J Phys Chem C 2020;124;50:27571.
[9] Hong S, et al. Nano Lett 2017;17:4866.
