Metallic nanoparticles as thermometers in everyday in situ TEM experiments

The rise of in situ and in operando techniques for experiments in transmission electron microscopes has opened the door to powerful high-resolution analyses of dynamic processes at the nanoscale. To interpret the results, but also to be able to extrapolate and correlate them to macroscopic properties of a studied material, a precise knowledge of the experimental conditions is crucial. Temperature has a major influence on all dynamic processes, but even when defined by the in situ system, the actual value could vary at different locations of the sample. In experiments involving bias or mechanical loading, heat could be generated locally and critically affect the response of the sample. In addition, the temperature calibration of in situ instruments may deteriorate during sample preparation, potentially adding to measurement uncertainty. Consequently, there is an urgent need for approaches for both temperature measurement with high spatial resolution and system temperature verification. Ideally, these would be suitable for in operando performance testing during a TEM in situ experiment.
Methods for such reliability measurements, like electron diffraction [1], electron energy loss spectroscopy (EELS) [2] or Raman spectroscopy are well established and applicable for different classes of materials. However, previous work incorporates either high-end microscopes or special equipment. We therefore determined the accuracy of temperature measurements through the thermal expansion of Au and Ag nanoparticle “thermometers”, using a mainstream, two-condenser lens TEM with a LaB6 gun, a one-megapixel camera and a precise evaluation algorithm to translate the recorded diffraction images into temperature values. We present the challenges in verifying the system temperature during an everyday in situ TEM experiment and a way to accomplish it for most sample types nonetheless.