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s from the gas a process that is repeated many times giving rise to an avalanche amplification of the secondary electrons this induces an electron current in the associated electrode followed by additional amplification by the electronics of the microscope and the formation of a scanned image in the usual manner of a sem furthermore the collisions of the electrons in the gas also produce excitation and photon emission in an avalanche form which is used for detection by light detectors to produce an amplified signal for the formation of images in the usual way of a sem because the backscattered electrons also produce ionization and excitation in their collisions with the environmental gas suitable means of electrode configurations have been devised to separate the different types of signal additionally cathodoluminescence mode of detection has been demonstrated in the use and operation of esem the total configuration of such detection systems is critical in the design of a commercial esem if the latter is to perform optimally among other fundamental works e g 28 and 36 detailed studies of charge distribution 38 and critical issues 45 must be thoroughly understood if further progress is to be achieved the integration of proper differential pumping pressure limiting apertures and detection systems results in freeing enough space below the final aperture for the placement and manipulation of specimens in an optimum design the specimen aperture distance allowable should be maximum whilst the incident electron beam undergoes collisions and suffers losses the incident beam eventually becomes lost beyond a certain travel distance in the gas and no imaging is possible the condition whereby the beam retains an imaging capability has been termed the oligo scattering regime 28 thus it is important to understand these initial processes before the probe strikes the specimen electrons are removed exponentially with the distance and the gas pressure from the beam spot these scattered electrons are distributed over a wide area being orders of magnitude greater than the beam spot hence their effect is the creation of uniform noise which is separated out from the signal generated by the minute beam spot the resolving power of the instrument is generally said to be same as the probe diameter and hence esem has the same resolving power as a sem all depending on the initial size of the spot in vacuum the removal of electrons from the useful beam ultimately decreases the resolution of a specimen feature with low contrast the latter can be compensated for by an increase in the beam current which is usually accompanied with an increased beam spot however it should be recognized that this amount of deterioration is of the order of beam spot variation which is generally small and with no significant practical effect in most applications apart from some specialized applications which strive to extract the last angstrom of resolution very high resolutions of uncoated polymers have been achieved with an early commercial model of esem the sem as well as the esem deal with the surface examination of bulk specimens in many applications specimens characterized by low contrast features are those with low atomic number such as organic materials on such materials resolutions of the order of the beam spot are difficult to achieve on account of radiation effects 18 becoming very pronounced as we increase the magnification as the beam energy is dissipated in a smaller viewing area on the sample specimen damage becomes a limiting factor well before we reach the specified resolving power of a given machine typically the manufacturer backs the resolution of an instrument with micrographs of gold on carbon particles with least particle spacing separation this is done also by the the use of maximum or design optimum electron beam kev accelerating voltage typically of 20 or 30 kev it has been shown that such specifications can be the same both in sem and esem however reproduction of such conditions on bulk organic specimens with low contrast are unlikely to be reached before the specimen is destroyed by radiation therefore it is imperative to achieve the best possible contrast on low atomic number specimens in the manner it was done with the original esem research laboratory prototype this understanding becomes all the more important in the selection of a commercial esem which should be designed to operate at the physical limits prescribing the best of opposing parameters i e the maximum useful specimen distance the maximum gas pressure with the minimum kev and spot size allowable by the chosen instrument a concise description of how the esem works is presented in wikipedia and video description in german here among the countless new applications and possibilities see e g high quality imaging of living specimens pollen amphiphilic particles swelling of the hydrogel there is also one novel use of the esem that has probably escaped the attention of users the gas flow can be visualized and imaged as for example is shown by imaging the gaseous jet itself flowing through an aperture gas dynamics can be visualized and studied by this unique method despite the fact that the commercial instruments are yet to implement the best of designs and potential of the technology the esem has gained acceptance by the scientific technical and industrial community as evidenced from the large number of publications 49 arising from its use already listed back in 1993 with unaccounted large numbers of works since then many other workers have continued on the path of development and understanding of this new technology among those a recent investigation by scott morgan has set an exemplary standard in this regard an understanding of the special historical background of this technology may help accelerate future developments esem short history and its future to allow a brief exposition of the history below not every instance is provided with references and the reader should resort to the literature provided herewith the many attempts and history that have led to the present development of esem have been reviewed and surveyed elsewhere 9 28 36 38 40 45 and a bibliography is presented by 49 furthermore a conscise history can be found in a wikipedia article on esem and in short biography the idea of shooting high energy electron beams in the atmosphere seems to have started with nikola tesla in the early twentieth century and this was later used in experimental electron beam welders pressure gauges and probably weapons tests manfred von ardenne 1940 founder of the scanning electron microscope sem imaged specimens in a gaseous environment by passing a high energy electron beam through an electron transparent i e at high energy film the alternative separation of the high vacuum electron gun chamber from a gaseous specimen chamber via open diaphragms pressure limiting apertures pla was used by various laboratories employing high voltage transmission electron microscopes in uk ussr usa and france these instruments are not well suited for imaging the surface of bulky specimens one attempt to image a wet specimen with a sem was by injecting a very localized vapor jet to maintain a gaseous gradient just above the specimen lane 1970 while the vapor was fast pumped away permitting the use of a conventional secondary electron detector before breakdown occurred by the high bias of the detector this was clearly an unstable system with limited use another attempt used a wide angle backscattered electron bse detector with a single pla robinson 1974 but again images had to be hurriedly obtained before the electron gun was flooded with water vapor and instrument operation disrupted most disappointing was an erroneous experimental finding that the electron probe diameter was increased inside a much wider spread of electrons surrounding like a skirt the alleged broadened and weakened imaging probe as the gas pressure increased moncrieff et al 1979 this seemed similar to the well known top bottom effect i e beam spread through the thin specimen sections used in transmission electron microscopy thus it seemed that both contrast and resolution were seriously compromised by the gas this compromise was thought to be further aggravated by the very use of bse the resolution of which was generally associated with a large signal volume in the specimen specimens with low atomic number such as biological specimens were thought to produce only poor resolution in this mode of detection all those prior isolated attempts clearly did not produce sufficient or satisfactory results to impress the microscope users or the manufacturers the situation was later reappraised by undertaking an investigation and implementing proper differential pumping techniques together with re designing and re configuring the bse detector see below danilatos literature from 1979 85 in 1979 the term environmental scanning electron microscope esem was first introduced the image contrast was significantly improved with stable instrument operation at high pressures in fact the pressure could be increased to ambient levels giving rise to an atmospheric scanning electron microscope asem however the asem was far ahead of its time and work concentrated mostly at the lower range of pressures preferred for an environmental scanning electron microscope esem despite the obtained optimum design and versatility of bse detectors and routine instrument operation the esem was still not ready to be accepted by the microscopy community as the question of resolution kept resurfacing this critical barrier was crossed by two concurrent developments a the invention of the gaseous detection device gdd danilatos 1983 that allows detection of the secondary electrons se in a gaseous environment and b the determination that the electron probe diameter remained constant with pressure increase as opposed to the moncrieff et al finding it was found that the gas acted not only as a conditioning medium e g to maintain the wet state of an object but it also acted simultaneously both as an electrical charge conductor and a detector all ionizing signals including the secondary electrons could be found and detected inside the gas itself these signals were detected by a simple biased electrode with a variable voltage inside the variable pressure gaseous environment it was then a matter of separating the bse from the se which was shortly afterwards achieved by thorough experimental and theoretical investigations furthermore the gas as a detection medium was found to have far reaching implications because not only the electrons and ions but also the photons generated in the gas could be used for imaging the interplay of a host of physical parameters such as the nature and pressure of gas geometry and configuration of electrodes applied electrode bias type of signal gas interaction and hardware for signal product detection opened a novel vista of possibilities and investigations the potential for novel contrasts and uses was indeed immense as subsequent work has shown a comprehensive theory of the gaseous detection device in the esem provides an indispensable reference with regard to resolution a specially designed experimental device allowed the observation of the electron probe profile by scanning the beam across the edge of a heated platinum wire this clearly showed that whilst the beam current decreased with gas pressure the diameter remained constant contrary to the prior understanding the beam broadening reported by previous workers was probably due to an edge effect and or contamination deposition whilst such effect contamination was prevented by heating the platinum wire and by a special detection configuration this finding was further supported by a thorough investigation using an analytical formulation and numerical computation of the scattering and distribution of electrons inside the gas danilatos 1988 therefore it was conclusively found that the diameter of the imaging probe remained constant in the oligo scattering regime of the pressure distance range used in an esem thus the fundaments for a recognized new instrument were created the esem became a fully fledged instrument for the examination of any specimen wet or dry insulating or conducting untreated or treated biological or inorganic specimens close to the natural state and in situ or in vacuum electroscan 1988 the foundations of the esem constitutes a prime reference for this technology the gaseous detection device has further shed light on the physical processes behind image formation and contrast in particular with regard to the so called specimen absorbed current the latter mode of detection was possible only with conducting specimens in vacuum sem and this had been proposed by another worker as an alternative technique for imaging also wet conducting specimens in an moist environment earlier on shah and beckett 1979 however it was also incorrectly perceived that the gaseous ionization was obscuring and obstructing the image contrast that was observed thus it was only the theory of gdd in an esem 36 based on correct physical principles e g displacement or induction current in conductors by all moving charges in the gas that restored electron microscopy to its scientific grounds these works not yet grasped or known by the established electron microscope manufacturers at the time it further required the formation of a small venture capital company namely electroscan corporation in usa to devote its efforts in the production of the first commercial esem after a few years of company research and development the first instruments appeared on the market and esem started to be applied beyond the confines of the prototype esem in sydney a few more years were needed to achieve the critical mass of users that made the new technique acceptable and broke the barrier of skepticism or disbelief when a few years later a traditional electron microscope manufacturer philips fei took over the operation to further commercialize the instrument esem assumed its rightful place among the electron microscopes in the world since then the most diverse applications have appeared in the literature which in turn gave a new impetus to esem that is now spreading worldwide uk ge ge us us us tr au be us eu us us uk us uk us it es uk au fr at this has been improved with the addition of new manufacturers zeiss other manufacturers have in the meantime followed with systems called wet sem lv sem natural sem eco sem vp sem etc all being equivalent namely all having operated at around 100 pa pressure or less with the use of some bse detector clearly a pre esem technology probably due to patent restrictions and other manufacturing limitations nevertheless all commercial versions of an esem have presented from the outset certain limitations not found on the prototype esem these limitations are still waiting to be rectified the first commercial esem ...