Investigating a Wide Array of Thermally-driven Events: From Understanding the Temperature-induced Structure and Morphology Changes of Metal Chalcogenides to Thermolysis-based Material Generation Eric V. Formo 1, *, Casey F. Rowe 2 , John T. Allen 1 , Jordan Hachtel 3 , Holli L. Threlkeld 2 , Yassamin Ghafouri 2 , Matthew A. Bloodgood 2 , and Tina T. Salguero 2 1 Georgia Electron Microscopy, University of Georgia, Athens, GA USA 2 Department of Chemistry, University of Georgia, Athens, GA USA 3 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN USA *Corresponding Author: eformo@uga.edu Studying both the thermal breakdown and the thermolysis-driven generation of metal chalcogenides with in situ electron micros- copy has provided insights into the specific chemistry taking place at key points in these processes [1, 2]. To examine such heat-induced transformations, we combined images and elemental analysis data taken with two different in situ electron micro- scopes. One instrument was a low kV (30kV) scanning transmission electron microscope (STEM), which was used to view surface morphology changes through secondary electron imaging, along with following overall internal structure changes with bright field and dark field images. Subsequently with a second instrument, a high-resolution STEM, we examined the atomic rearrange- ments witnessed in the low kV imaging. Electron energy loss spectroscopy (EELS) and energy dispersive spectroscopy (EDS) taken with these instruments provided compositional information. Our results show how the integration of these two different imaging platforms can yield a compete picture of thermally-induced changes in metal chalcogenide nanomaterials. For probing the thermal breakdown of metal chalcogenides, we focused on the MX 3 -type compound NbS 3 [3]. As one can see in Fig. 1, exfoliated nanowires begin at room temperature with a smooth overall texture (Fig. 1A), with aligned chains of NbS 3 composing its atomic structure (Fig. 1B). The sample was then heated in 100°C steps to 1000°C with images being recorded at each step in both instruments. At 1000°C, a drastic change in the morphology and atomic structure can be seen (Fig. 2). Specifically, the once smooth surface has been transformed into a highly textured one with new trigonal and hexagonal features and steps on the surface, all while maintaining the overall nanowire morphology (Fig. 2A and B). High resolution images showed that NbS 3 had converted into NbS 2 during heating (Fig. 2C). Together with FFT and EELS data, these results allowed us to de- termine that the most likely pathway for the transformation of the NbS 3 aligned chains into a layered NbS 2 structure was via a topotactic transformation caused by the loss of sulfur during heating. For investigating the generation of metal chalcogenides within the electron microscope, we examined the thermolysis of single source precursors, such as NH 4 WS 4 [4]. Specifically, we were interested in how typical atmospheric oxygen contamination might impact the overall thermolysis process and WS 2 film formation [4]. Low kV in situ STEM showed the progression from oxidized NH 4 WS 4 into various nanoscale end-products when heated to 800 °C in 100 °C steps. These nanostructures exhibited a variety of morphologies, including large sheets, ribbons, wires, and particles, with the latter two being the majority structures formed (Fig. 3A). Subsequently, using high resolution TEM and EDS, we found that the 2D and 1D nanostructures were composed of tungsten oxide, WO 3 , whereas nanoparticles were composed solely of tungsten (Fig. 3B and C). By correlating these imaging ob- servations with EDS spectra collected throughout the process, we could determine a pathway for the conversion of the oxidized tungsten precursor into these various nanostructures. These examples of in situ studies demonstrate how the combination of low kV STEM, high/atomic resolution STEM, EELS, and EDS is ideal for studying thermally-driven chemical and morphological changes at the nanoscale. Microscopy and Microanalysis, 29 (Suppl 1), 2023, 1485–1486 https://doi.org/10.1093/micmic/ozad067.763 Meeting-report