In Situ Transmission Electron Microscopy Imaging of High-Temperature Structural Evolution in Phase-Change Materials

Elevated temperature increases atomic mobility and allows structural re-ordering, promoting phase evolution, chemical reactions, atomic diffusion, and surface degradation processes. The advent of on-chip thin-film heating and temperature sensing technology for in-situ transmission electron microscopy (TEM) holder systems has enabled highly localized specimen heating, greatly improving heating time and sample stability during in-situ heating experiments investigating temperature-induced transformations. This has enabled rapid heating experiments up to over 1000°C with stable imaging and spectroscopic signal collection over timescales greater than 150 hours. Improvements to double-tilting mechanisms have allowed for minimized backlash and increased stability and orientation accuracy for imaging and diffraction of temperature-induced structural evolution.

In-situ heating capabilities of both single and double tilt TEM thin film heating platforms are presented for various material systems. Spiky gold nanoparticles of size 40-60 nm exhibit shape changes upon nearing the melting temperature as surface tension begins to retract the spikes via inward atomic diffusion. Few-layer PtSe₂ transferred onto the viewing area of the heating chip exhibits a phase change at 550°C and above, where the enhanced selenium vacancy diffusion and mobility to the crystal edges causes a Kirkendall effect in the 2D material. The 2-dimensional (2D) nature of the few-layer crystals resulted in unique 1D interfaces during the conversion to selenium-poor phases like PtSe and PtSe_1-x [1]. Finally, rapid nanoscale crystallization near 90°C is observed in situ at a high frame rate in Ag-In-Sb-Te phase-change materials [2]. This work highlights the use of in-situ ultrafast heating under TEM to gain crucial mechanistic insights into structural transformations in nanoscale, 2D materials, and chalcogenide phase-change materials.