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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Environmental scanning electron microscope | 6/7 | https://en.wikipedia.org/wiki/Environmental_scanning_electron_microscope | reference | science, encyclopedia | 2026-05-05T10:04:32.049506+00:00 | kb-cron |
== Advantages == The presence of gas around a specimen creates new possibilities unique to ESEM: (a) liquid-phase electron microscopy is possible since any pressure greater than 609 Pa allows water to be maintained in its liquid phase for temperatures above 0 °C, in contrast to the SEM where specimens are desiccated by the vacuum condition. (b) Electrically non-conductive specimens do not require the preparation techniques used in SEM to render the surface conductive, such as the deposition of a thin gold or carbon coating, or other treatments, techniques which also require vacuum in the process. Insulating specimens charge up by the electron beam making imaging problematic or even impossible. (c) The gas itself is used as a detection medium producing novel imaging possibilities, as opposed to vacuum SEM detectors. (d) Plain plastic scintillating BSE detectors can operate uncoated without charging. Hence, these detectors produce the highest possible signal-to-noise-ratio at the lowest possible accelerating voltage, because the BSE do not dissipate any energy in an aluminium coating used for the vacuum SEM. As a result, specimens can be examined faster and more easily, avoiding complex and time-consuming preparation methods, without modifying the natural surface or creating artifacts by the preceding preparation work, or the vacuum of the SEM. Gas/liquid/solid interactions can be studied dynamically in situ and in real time, or recorded for post processing. Temperature variations from subzero to above 1000 °C and various ancillary devices for specimen micro-manipulation have become a new reality. Biological specimens can be maintained fresh and live. Therefore, ESEM constitutes a radical breakthrough from conventional electron microscopy, where the vacuum condition precluded the advantages of electron beam imaging becoming universal.
== Disadvantages == The main disadvantage arises from the limitation of the distance in the specimen chamber over which the electron beam remains usable in the gaseous environment. The useful distance of the specimen from the PLA1 is a function of accelerating voltage, beam current, nature and pressure of gas, and of the aperture diameter used. This distance varies from around 10 mm to a fraction of a millimeter as the gas pressure may vary from low vacuum to one atmosphere. For optimum operation, both the manufacturer and the user must conform, in the design and operation, to satisfy this fundamental requirement. Furthermore, as the pressure can be brought to a very low level, an ESEM will revert to typical SEM operation without the above disadvantages. Therefore, one may trade-off the ESEM characteristics with those of SEM by operating in a vacuum. A reconciliation of all these disadvantages and advantages can be attained by a properly designed and operated universal ESEM. Concomitant with the limitation of useful specimen distance is the minimum magnification possible, since at very high pressure the distance becomes so small that the field of view is limited by the PLA1 size. In the very low magnification range of SEM, overlapping the upper magnification of a light microscope, the superior field is limited to a varying degree by the ESEM mode. The degree of this limitation strongly depends on instrument design. As X-rays are also generated by the surrounding gas and also come from a larger specimen area than in SEM, special algorithms are required to deduct the effects of gas on the information extracted during analysis. The presence of gas may yield unwanted effects in certain applications, but the extent of these will only become clear as further research and development is undertaken to minimize and control radiation effects. No commercial instrument is as yet (by 2009) available in conformity with all the principles of an optimal design, so that any further limitations listed are characteristic of the existing instruments and not of the ESEM technique, in general.
== Transmission ESEM == The ESEM can also be used in transmission mode (TESEM) by appropriate detection means of the transmitted bright and dark field signals through a thin specimen section. This is done by employing solid state detectors below the specimen, or the use of the gaseous detection device (GDD). The generally low accelerating voltages used in ESEM enhance the contrast of unstained specimens while they allow nanometer resolution imaging as obtained in transmission mode especially with field emission type of electron guns.
== ESEM-DIA == ESEM-DIA is an abbreviation standing for a system consisting of an ESEM microscope coupled to a digital image analysis (DIA) program. It directly makes possible the quantitative treatment of the digitally acquired ESEM images, and allows image recognition and image processing by machine learning based on neural network.
== Applications == Some representative applications of ESEM are in the following areas:
=== Biology === An early application involved the examination of fresh and living plant material including a study of Leptospermum flavescens. The advantages of ESEM in studies of microorganisms and a comparison of preparation techniques have been demonstrated.
=== Medicine and medical === The influence of drugs on cancer cells has been studied with liquid-phase ESEM-STEM.
=== Archaeology === In conservation science, it is often necessary to preserve the specimens intact or in their natural state.
=== Industry === ESEM studies have been performed on fibers in the wool industry with and without particular chemical and mechanical treatments. In cement industry, it is important to examine various processes in situ in the wet and dry state.
=== In situ studies === Studies in situ can be performed with the aid of various ancillary devices. These have involved hot stages to observe processes at elevated temperatures, microinjectors of liquids and specimen extension or deformation devices.
=== General materials science === Biofilms can be studied without the artifacts introduced during SEM preparation, as well as dentin and detergents have been investigated since the early years of ESEM.