Controlled Environment [ECELL] TEM for Dynamic In-Situ Reaction Studies with Hrem Lattice Imaging
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Some of the earliest controlled environments, and a continuing and important endeavor, is to improve the quality of the vacuum into the genuine UHV range suitable for surface science studies. An early example is due to Pashley et al [3] and more recently the lead has been taken by Professor Yagi's group at the Tokyo Institute of Technology [4]. Elegant experiments have been performed and the results of these studies are highly regarded in the wider surface science community. Venables [5] and others have applied similar expertise with a series of specialized SEM instrument. In the TEM the use of UHV methods has been combined with the highest atomic lattice resolution of which the instrument is capable. However, as well as restricting the influence of the (UHV) environment on a sample, there are equally important applications in which the role of the environment is more positive. Some of the earlier applications, summarized by Butler and Hale [6], were to provide a water saturated environment to prevent desolvation of a solvated sample.
The work of Double et al
[6] on the hydration of Portland cement relied on the use of a novel sample holder in an otherwise unmodified microscope. The specimen was sandwiched between thin windows to contain the saturated water vapor environment necessary for reaction, or indeed for preservation, of the sample. Similar approaches have been employed to contain hydrated biological samples [8]. These workers all made use of the high penetrating power and relatively large space in the high voltage electron microscopes Electron transparent windows have been (HVEMs) introduced around 1970 [9]. used to contain gases, solvent vapors [7,8] and UHV environments in microscopes of various voltages, but there are problems of leakage in reliably sustaining a large pressure difference across a window which is thin enough to permit electron penetration. The experience seems to be that they are fine for moderate resolution studies, but that the additional diffuse scattering in the window is an unfortunate complication with samples which may already be limited in image contrast. Superimposition of an additional diffraction pattern can be a problem and windowed cells are also more difficult to make compatible with heating systems. In principle the whole of the column of a microscope could be made UHV compatible, although it rarely is. Where the controlled atmosphere is comprised of gas or vapor it is important to limit the ECELL controlled environment zone to a length of a few millimeters. This is also the configuration typically used for (non-VG) UHV systems. The complications, and potential for catastrophic failure, of windows can be avoided by substituting small apertures above and below the sample to restrict the diffusion of gas molecules, but to allow the penetration by the electron beam. Unlike windows, aperture systems are robust and they can easily be made compatible with sample heating or cooling; sometimes even using regular specimen holders for the purpose. However there are two problems to
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