Wet - STEM
Electron microscopes used to work conventionally in a vacuum to ensure the electrons’ transmission from the electron gun to the sensor and the samples had to be dehydrated before they could be observed. Since the development of the so-called « environmental » SEM, it has become possible to observe a material in a controlled atmosphere. It is by controlling the water vapour pressure of a sample in particular, that it has become possible to conserve the object’s initial state of hydration and then modify it in situ. Studies carried out in the group have for example handled the monitoring of the setting phase of cement.
Studies carried out within the SNMS group have also enabled us to develop the wet-stem technique. Here, a drop from a suspension of nano-objects dispersed in a liquid is deposited on a microscope grid covered in a carbon film with holes. A BSE-type sensor is placed below the sample to collect the electrons diffused by the sample. Simulations of electron-material interactions (Monte Carlo-type) show that although images can be obtained, including films of water several microns thick, the contrast may vary. Figure 1 shows several examples of wet-stem observations. In the case of latex, progressive dehydration enables us to understand the influence of the surfactant on the sample during the in situ filmification of latex.