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Efficient closed cells specimen holders for Liquid Cell Transmission Electron Microscopy (TEM) has led to many advances over the last two decades but still suffer limitations due to their sealing membranes which hamper comfortable tomography approach, optimized chemical analysis and signal-to-noise ratio. ‘Windowless’ approaches in a dedicated Environmental TEM can not only overcome these limitations but provide opportunities to study water (liquid) condensation-evaporation experiments as required for example to study the water uptake of aerosols in the context of climate and atmospheric chemistry [1]. We present here a development and operationalization of such a windowless, Wet ETEM method using a Peltier based MEMS chip compatible with a DENSsolutions Wildfire holder. The environmental conditions for the evaporation and condensation cycles require a pressure of 6-15 mbar and a temperature within a 0-11oC typical range. As for Environmental Scanning EM (ESEM) [2], these conditions can be achieved using a FEI-Titan ETEM instrument installed at CLYM Lyon, where a gas pressure of up to about 20 mbar can be maintained around a cooled specimen in the pole pieces gap. Adequate temperatures were obtained by placing at the tip of the Wildfire holder a Peltier stage which can produce a temperature difference of ~40 K between the hot and the cold junctions. A ‘micro’ Peltier stage with adequate dimensions for being safely introduced in the ETEM was then connected to a custom printed circuit board (PCB) compatible with the DENSsolutions holder. The sample is deposited on a classical TEM carbon grid mounted in direct contact with the cold side of the Peltier device insuring its efficient cooling.Initial experiments have been conducted on NaCl crystals used as model aerosols [1]. Owing to the fast cooling when a current is applied to the Peltier, a very efficient and easily controlled condensation was observed with a vapor pressure typically around 7 mbar in the column. These preliminary results will be discussed in comparison to similar experiments conducted with a classical cryo-holder (Elsa model from Gatan) cooled down by a liquid nitrogen reservoir as previously performed [1,3]. As a first statement it appears that a much better and faster stability, resulting in a much better knowledge of the exact relative humidity (RH), is obtained with the proposed solution. These results offer new perspectives for the study of the deliquescence or efflorescence of aerosols through condensation and evaporation cycles which can then be easily performed and reproduced [4]. [1] M.E. Wise, G. Biskos, S.T. Martin, L.M. Russell and P.R. Buseck. Phase Transitions of Single Salt Particles Studied Using a Transmission Electron Microscope with an Environmental Cell. Aerosol Science and Technology, 39:9 (2005) 849-856, DOI: 10.1080/02786820500295263. [2] A. Bogner, G. Thollet, D. Basset, P. H. Jouneau, and C. Gauthier. Wet STEM: A new development in environmental SEM for imaging nano-objects included in a liquid phase. Ultramicroscopy, 104 3 (2005) 290-301. DOI: 10.1016/j.ultramic.2005.05.005.[3] B. Levin, D. Haiber, Q. Liu and P.A. Crozier. An Open-Cell Environmental Transmission Electron Microscopy Technique for In Situ Characterization of Samples in Aqueous Liquid Solutions. Microscopy and Microanalysis, 26(1) (2020) 134-138. DOI: 10.1017/S1431927619015320.[4] This work is support by the French National Research Agency under the WATEM program ANR-20-CE42-0008 Thanks are due to CLYM (www.clym.fr) for the access to the ETEM. |
