In Situ Transmission Electron Microscopy

Since the earliest days of transmission electron microscopy, microscopists have realized the potential of microscopy for studying dynamic processes. Images recorded sequentially can be used to track the changes caused by deliberate actions, such as heating or straining, or uncontrolled processes, such as beam damage. The class of experiments where a specimen is changed or acted on while it remains under observation (i.e., in situ in the polepiece) is referred to as in situ microscopy. In a sense every TEM observation is an in situ experiment, since every specimen is affected by the electron beam to some extent. But the in situ experimenter aims to modify the specimen in a deliberate way and learn something from the results. In the best in situ experiments, a controlled change is made to a specimen’s environment, and this is correlated with the resulting change in its structure, measured using any of the imaging, analytical, or diffraction techniques available, or its electronic or mechanical properties, which can also be measured in situ. Preferably both the “input,” in other words the change in sample environment, and the “output,” or consequent change in structure or properties, are recorded simultaneously and quantitatively. Given sufficient care with artifacts, a quantitative understanding of a fundamental physical process can be obtained. There are numerous advantages to performing experiments in situ. A single in situ experiment gives a continuous view of a process, so may take the place of multiple post-mortem measurements. A single heating experiment, for example, can provide information that would otherwise have to be extracted by examination of many samples which had been annealed to different temperatures or for different times. Because an in situ experiment is continuously recorded, it is easier to catch a transient phase or observe a nucleation event. In situ experiments can yield specific and detailed kinetic information, measuring for example the motion of individual dislocations under known stress, or the growth rates of individual nanocrystals. Properties can be determined for well-characterized nanostructures, such as the conductivity of single nanotubes or the melting point of precipitates. Finally, growth experiments in particular provide a window into the behavior of materials under real processing conditions, since significant changes can occur if we remove a material from the growth chamber and perform analysis post-mortem.