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All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Probing atom dynamics of excited Co-Mo-S nanocrystals in 3D

Advances in electron microscopy have enabled visualizations of the three-dimensional (3D) atom arrangements in nano-scale objects. The observations are, however, prone to electron-beam-induced object alterations, so tracking of single atoms in space and time becomes key to unravel inherent structures and properties. Here, we introduce an analytical approach to quantitatively account for atom dynamics in 3D atomic-resolution imaging. The approach is showcased for a Co-Mo-S nanocrystal by analysis of time-resolved in-line holograms achieving ~1.5 Å resolution in 3D. The analysis reveals a decay of phase image contrast towards the nanocrystal edges and meta-stable edge motifs with crystallographic dependence. These findings are explained by beam-stimulated vibrations that exceed Debye-Waller factors and cause chemical transformations at catalytically relevant edges. This ability to simultaneously probe atom vibrations and displacements enables a recovery of the pristine Co-Mo-S structure and establishes, in turn, a foundation to understand heterogeneous chemical functionality of nanostructures, surfaces and molecules.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Co(MoS2)2 by Materials Project

CoMo2S4 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. Mo3+ is bonded to six S2- atoms to form distorted MoS6 octahedra that share corners with six equivalent CoS6 octahedra, edges with six equivalent MoS6 octahedra, and a faceface with one CoS6 octahedra. The corner-sharing octahedra tilt angles range from 52–53°. There are a spread of Mo–S bond distances ranging from 2.37–2.60 Å. Co2+ is bonded to six S2- atoms to form CoS6 octahedra that share corners with twelve equivalent MoS6 octahedra, edges with two equivalent CoS6 octahedra, and faces with two equivalent MoS6 octahedra. The corner-sharing octahedra tilt angles range from 52–53°. There are two shorter (2.31 Å) and four longer (2.34 Å) Co–S bond lengths. There are two inequivalent S2- sites. In the first S2- site, S2- is bonded in a 5-coordinate geometry to three equivalent Mo3+ and two equivalent Co2+ atoms. In the second S2- site, S2- is bonded in a 4-coordinate geometry to three equivalent Mo3+ and one Co2+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Co2MoS4 by Materials Project

MoCo2S4 crystallizes in the tetragonal I-4 space group. The structure is three-dimensional. Mo4+ is bonded to four equivalent S2- atoms to form MoS4 tetrahedra that share corners with eight CoS4 tetrahedra. All Mo–S bond lengths are 2.27 Å. There are two inequivalent Co2+ sites. In the first Co2+ site, Co2+ is bonded to four equivalent S2- atoms to form CoS4 tetrahedra that share corners with four equivalent MoS4 tetrahedra and corners with four equivalent CoS4 tetrahedra. All Co–S bond lengths are 2.16 Å. In the second Co2+ site, Co2+ is bonded to four equivalent S2- atoms to form CoS4 tetrahedra that share corners with four equivalent MoS4 tetrahedra and corners with four equivalent CoS4 tetrahedra. All Co–S bond lengths are 2.15 Å. S2- is bonded in a trigonal non-coplanar geometry to one Mo4+ and two Co2+ atoms.

36 MATERIALS SCIENCE↗