Transmission Electron Microscopy

Transmission Electron Microscopy (TEM) was a key part of my project to examine the nanoscale structure of refractory multi-principal element alloy (RMPEA) thin films, both as-deposited and after annealing. TEM let me zoom in on tiny details like grain boundaries, defects, and phase changes that other methods couldn’t reveal. By using High-Angle Annular Dark-Field (HAADF) imaging, Scanning Transmission Electron Microscopy-Energy Dispersive Spectroscopy (STEM-EDS), and Selected Area Electron Diffraction (SAED), I got a clear picture of the films’ structure, composition, and how annealing changed them.

To start, I prepared the RMPEA thin films carefully since TEM needs samples thinner than 100 nanometers. I deposited the films on a substrate, likely using magnetron sputtering, depending on the alloy mix. For the annealed samples, I heated the as-deposited films in a furnace with controlled temperature and time to see how heat altered their properties. Using a focused ion beam (FIB) system, I cut and thinned small cross-sections of the films. I was cautious during FIB milling, using low-energy beams for the final steps to avoid damaging the samples and ensure they were thin enough for electrons to pass through.

With the samples ready, I loaded them into a high-resolution TEM operating at 200 kV. TEM works by sending electrons through the thin sample, where they interact with the material to produce images or patterns. For high-resolution imaging, I used HAADF in STEM mode, which is great for showing differences in atomic number. HAADF gave me sharp, detailed images of the films’ microstructure, highlighting features like grain sizes, defects, or phase boundaries. The annealed samples often showed larger grains or new phases compared to the as-deposited ones, and HAADF’s contrast made these changes stand out clearly.

For elemental analysis, I used STEM-EDS, where the TEM’s electron beam scans the sample to generate X-ray signals. This let me map the distribution of elements like tungsten, molybdenum, or niobium in the RMPEA films. STEM-EDS showed whether the elements were evenly mixed or if segregation occurred during deposition or annealing, which was crucial for understanding the alloy’s chemical makeup and uniformity.

I also used SAED to check the crystal structure. By selecting a small area with an aperture, I created diffraction patterns that revealed whether the films were crystalline, amorphous, or a mix. The as-deposited films sometimes had disordered or nanocrystalline structures, while the annealed ones often showed sharper diffraction spots, suggesting better crystallinity or new phases. I analyzed the SAED patterns to identify lattice types, like body-centered cubic or hexagonal close-packed, and to detect any structural changes caused by annealing.

The process was detailed and required precision, but it paid off. HAADF imaging gave me high-resolution views of the films’ structure, STEM-EDS mapped the elemental layout, and SAED showed the crystal arrangement. Together, these techniques helped me compare the as-deposited and annealed films, revealing how annealing affected things like grain growth or defect reduction, which directly tied to the films’ performance. The biggest challenge was preparing clean, thin samples without artifacts, but with careful FIB work, TEM provided a detailed look at the RMPEA thin films’ nanoscale behavior.