Structural Evolutions of Vertically Aligned Two-Dimensional MoS₂ Layers Revealed by in Situ Heating Transmission Electron Microscopy

Abstract

Benefiting from a large density of layer edges exposed on the surface, vertically aligned two-dimensional (2D) molybdenum disulfide (MoS2) layers have recently harvested excellent performances in the field of electrochemical catalysis and chemical sensing. With their increasing versatility for high-temperature, demanding applications, it is vital to identify their thermally driven structural and chemical stability, as well as to clarify its underlying principle. Despite various ex situ and in situ characterizations on horizontally aligned 2D MoS2 layers, the direct in situ heating of vertically aligned 2D MoS2 layers and the real-time observation of their near-atomic-scale dynamics have never been approached, leaving their thermal stability poorly understood. Moreover, the geometrical advantage of the surface-exposed vertically aligned 2D MoS2 layers is anticipated to unveil the structural dynamics of interlayer van der Waals (vdW) gaps and its correlation with thermal energy, unattainable with 2D MoS2 layers in any other geometry. Herein, we report a comprehensive in situ heating TEM study on cleanly transferred, vertically aligned 2D MoS2 layers up to 1000 °C. Several striking phenomena were newly observed in the course of heating: (1) formation and propagation of voids between the domains of vertical 2D MoS2 layers with distinct grain orientations starting at ∼875 °C; (2) subsequent decompositions of the 2D MoS2 layers accompanying a formation of Mo nanoparticles at ∼950 °C, a temperature much lower than the melting temperature of their bulk counterpart; and (3) initiation of decomposition from the surface-exposed 2D layer vertical edge sites, congruently supported by molecular dynamics (MD) simulation. These new findings will offer critical insights into better understanding the thermodynamic principle that governs the structural stability of general vdW 2D crystals as well as providing useful technological guidance for materials design and optimization in their potential high-temperature applications.

In Situ Characterization of Transformations in Nanoscale Layered Metal Chalcogenide Materials: A Review

“…This study also reported that loss of Se resulted in the formation of large MoC clusters along the edges through the reaction of Mo atoms with C residue. Thermal decomposition was observed to begin at the surface‐exposed edge sites of vertically‐aligned 2D MoS 2 during in situ TEM heating in another study [85] . This work reported the decomposition of 2D MoS 2 to Mo nanoparticles due to the formation and propagation of voids by 950 °C.…”

“…Thermal decomposition was observed to begin at the surface-exposed edge sites of vertically-aligned 2D MoS 2 during in situ TEM heating in another study. [85] This work reported the decomposition of 2D MoS 2 to Mo nanoparticles due to the formation and propagation of voids by 950°C. Recent work has also investigated the formation of inversion domains at temperatures above 700°C in monolayer MoS 2 using in situ heating ADF-STEM.…”

Atomic-scale characterization of structural heterogeny in 2D TMD layers

“…81 As a powerful advanced characterization technique, in situ TEM endows the possibility of direct visualization of the atomic-scale 2D layer structural dynamics on the small-length scales. 32,82 For instance, recent studies on horizontal MoS 2 or their monolayer counterparts via in-situ TEM have studied the kinetics of thermolysis-driven 2D MoS 2 layers growth via in-situ heating. 19,83 While previous Additionally, pronounced migrations of 2D MoS 2 layers at the junctions formed by multiple grains of large bending angles were observed, which is believed to be driven by the release of accumulating strain for reducing total interfacial energy.…”

“…19,83 While previous Additionally, pronounced migrations of 2D MoS 2 layers at the junctions formed by multiple grains of large bending angles were observed, which is believed to be driven by the release of accumulating strain for reducing total interfacial energy. 82 Moreover, it was identified that surface-exposed defective 2D layer edge sites are the triggering points for the thermal decomposition of S atoms and the subsequent decomposition of 2D MoS 2 layers. 82 Furthermore, Zhang et al achieved the controlled growth of vertically-aligned 2D WS 2 with high-temperature stability from in situ heating TEM investigations, as shown in Figure 12.…”

“…82 Moreover, it was identified that surface-exposed defective 2D layer edge sites are the triggering points for the thermal decomposition of S atoms and the subsequent decomposition of 2D MoS 2 layers. 82 Furthermore, Zhang et al achieved the controlled growth of vertically-aligned 2D WS 2 with high-temperature stability from in situ heating TEM investigations, as shown in Figure 12. 84 Amorphous precursor K 2 WS 4 in TEM was synthesized into vertically-aligned WS 2 , as shown in Figure 12(a).…”