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Analysis of liquid suspensions using scanning electron microscopy in transmission: estimation of the water film thickness using Monte–Carlo simulations

Summary

Environmental scanning electron microscopy (ESEM) allows the observation of liquids under specific conditions of pressure and temperature. Moreover, when working in the transmission mode, that is in scanning transmission electron microscopy (STEM), nano‐objects can be analysed inside a liquid. The contrast in the images is mass‐thickness dependent as in STEM‐in‐TEM (transmission electron microscopy) using closed cells. However, in STEM‐in‐ESEM, as the liquid–vapour equilibrium is kept dynamically, the thickness of the water droplet remains unknown. In this paper, the contrasts measured in the experimental images are compared with calculations using Monte‐Carlo simulations in order to estimate the thickness of water. Two examples are given. On gold nanoparticles, the thickness of a thick film can be estimated thanks to a contrast inversion. On core‐shell latex particles, the grey level of the shell compared with those of the core and of the water film gives a relatively precise measurement of the water film thickness.

Lay description

Nanotechnology is present in our everyday life, not only in informatics, food or cosmetics, but also in materials science. One can cite for instance nanomaterials, but also bulk materials which are synthesised from nano‐objects. In this last example, the nano‐objects may have to be dispersed in a solvent and the properties of the final product will depend on the dispersion state of the nano‐objects in the liquid. As a consequence, analysing the suspension and quantifying the dispersion state is of prime importance.

Electron microscopy has become a key technique for the characterisation of nano‐objects, thanks to its spatial resolution. Moreover, it is possible to obtain locally various pieces of information regarding the object composition and structure. However, analysing liquid suspensions is still a challenge because vacuum is required in the electron microscope columns. It is necessary to use either specific sample preparation methods, so that the sample can withstand vacuum, or sample holders in which the liquid is sealed, or dedicated electron microscopes in which a certain amount of gas is allowed around the sample, to keep it liquid.

In this study, we characterise suspensions of gold and core‐shell polymer particles in water with a dedicated scanning electron microscope. As the liquid state is ensured by controlling the liquid temperature and the pressure of vapour around the sample, it is in practice very difficult to control the thickness of the water droplet. We use here numerical simulations to better understand the contrast between the core, the shell and water, and to give an estimation of the water droplet thickness.

Authors:   J. XIAO, G. FORAY, K. MASENELLI‐VARLOT
Journal:   Journal of Microscopy
Year:   2017
Pages:   n/a
DOI:   10.1111/jmi.12619
Publication date:   22-Aug-2017
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