In situ electron microscopy study of structural transformations in 2D CoSe2

Abstract

Thermally induced structural transformation of 2D materials opens unique avenues for generating other 2D materials by physical methods. Imaging these transitions in real time provides insight into synthesis routes and property tuning. We have used in situ transmission electron microscopy (TEM) to follow thermally induced structural transformations in layered CoSe2. Three transformation processes are observed: orthorhombic to cubic-CoSe2, cubic-CoSe2 to hexagonal-CoSe, and hexagonal to tetragonal-CoSe. In particular, the unit-cell-thick orthorhombic structure of CoSe2 transforms into cubic-CoSe2 via rearrangement of lattice atoms. Cubic-CoSe2 transforms to hexagonal-CoSe at elevated temperatures through the removal of chalcogen atoms. All nanosheets transform to basal-plane-oriented hexagonal 2D CoSe. Finally, the hexagonal to tetragonal transformation in CoSe is a rapid process wherein the layered morphology of hexagonal-CoSe is broken and islands of tetragonal-CoSe are formed. Our results provide nanoscopic insights into the transformation processes of 2D CoSe2 which can be used to generate these intriguing 2D materials and to tune their properties by modifying their structures for electro-catalytic and electronic applications.

Selective Vertical and Horizontal Growth of 2D WS2 Revealed by In Situ Thermolysis using Transmission Electron Microscopy

“…[48][49][50] The in situ TEM technique is extensively used to capture dynamics of processes in TMDs as well. [51][52][53][54][55] In particular, in situ edge engineering in 2D TMDs, [53] in situ TEM studies of phase transformations of CoSe 2 , [56] PdSe 2 , [57] PtSe 2 , [58] and SnS 2 , [59] and the in situ TEM study of mechanical properties of TMDs, [60] have been investigated. In situ TEM growth of MoS 2 through thermolysis of ammonium tetrathiomolybdate (NH 4 ) 2 MoS 4 (ATM) was observed on the SiN x membrane of the heating chip [61,62] and a Ni deposited heating chip.…”

Atomic Imaging and Thermally Induced Dynamic Structural Evolution of Two-Dimensional Cr2S3

“…The reduction in the mole fraction of S induced by the thermal effect results in a series of transformations in the Cr−S compounds. 39 As depicted in Figure S12, along with the decrease in the mole fraction (Xs), the thermal-dynamic stable phase varied from R-Cr 2 S 3 to T-Cr 2 S 3 , T-Cr 2 S 3 to M-Cr 3 S 4 , and M-Cr 3 S 4 to T-Cr 5 S 6 . 37 The atomic ratio of sulfur depicted in Figure S13 decreases when the temperature increases, agreeing well with the variation in the stoichiometry of the phase transition sequence.…”

“…The average atomic spacing of Cr increased from 400 to 600 °C, which is attributed to the thermal expansion in the horizontal direction. Further, the theoretical Cr−Cr spacing in the vertical direction, illustrated in FigureS11b, is also inconsistent with the experimental results (Figure4j), indicating that the mixed phase of Cr 3 S 4 (transition) and T-Cr 5 S 6 may have negative thermal expansion coefficients in the [001] orientation.The reduction in the mole fraction of S induced by the thermal effect results in a series of transformations in the Cr−S compounds 39. As depicted in FigureS12, along with the decrease in the mole fraction (Xs), the thermal-dynamic stable phase varied from R-Cr 2 S 3 to T-Cr 2 S 3 , T-Cr 2 S 3 to M-Cr 3 S 4 ,…”

Three Approaches for Representing the Statistical Uncertainty on Atom-Counting Results in Quantitative ADF STEM

“…Many efforts have been made to enable quantitative electron microscopy of the materials as synthesized, imaged at lower incident electron doses where electron beam damage can be avoided (Migunov et al, 2015;Mittelberger et al, 2018;Egerton, 2019;Van Aert et al, 2019;Nicholls et al, 2020). Furthermore, the field of electron microscopy for material science is strongly evolving toward more in situ studies, where environmental conditions greatly complicate image acquisition Haimei & Yimei, 2017;Vanrompay et al, 2018;Gavhane et al, 2021). Both these evolutions entail more noisy electron microscopy images, leading to a larger amount of uncertainty in the interpretation.…”