Electron Microscopes - Essay Example

1. Executive Summary Right from the invention of Electron Microscope (EM), it has been extensively used as a tool for research and development of scientific theory across the field like medicine, chemistry, material science, biology etc. EMs enables the inspection, observation and classification of research materials, up to the scale of a micrometer (μm) or a nanometer (nm). This assignment endeavors to provide a brief overview of a scientific analysis using an advanced analytical technique of Electron Microscopy and Microanalysis. The aim of this study is to provide a brief overview of morphological and elemental investigation of non-conductive samples by Scanning Electron Microscopy and Energy Dispersive Spectroscopy. 2. Introduction Electron Microscopes (Ems) are advanced scientific tools which operate by way of emission of a beam of highly energetic electrons so as to study the research materials on a very minute scale. These microscopic investigations can reveal topographical information (i.e. information regarding the surface features of the material under research), morphological information (i.e. information regarding shape and size of the particles constituting the material under research), compositional information (i.e. the information regarding amount of elements and compounds that the research material is composed of) and crystallographic information (i.e. information regarding the atomic arrangement of the material under research). The functioning of EMs resembles exactly to the functioning of Optical Microscopes (OMs). The difference lies in the fact that while OMs use light, EMs use beam of electrons to image the specimen and get the information regarding its structure and composition. Principally, all the EMs work in following stepwise manner. First and foremost a beam of electrons is produced by the help of electron guns in high vacuum. The electron beam is then accelerated on to the material or specimen under study. Here the specimen is having positive electrical potential. Here the beam is being focused into a thin monochromatic beam by the help of magnetic lenses or even metal apertures. The specimen is irradiated by the stream of focused electrons. The interaction of the irradiated specimen with the electron beam is observed, detected and captured as image. However, with due course in the development of EMs, the Scanning Electron Microscopy (SEM) has come out as an important and crucial means for morphological observation in various fields of science like biology and material science. Addition to it, Energy Dispersive Spectroscopy (EDS) has also rapidly evolved as crucial tool for elemental analysis and characterization. 3. Morphological Investigation by Scanning Electron Microscopy Ever since their development in early 50's of the last century, scanning electron microscopes have led to the development of various new areas of research in the field of medicine and physical science. The researchers have been greatly aided by SEM and have been able to examine a wide variety of samples for their research studies. These scanning electron microscopes have inherent advantages over their traditional counterparts. The SEM permits more of a sample to be under focus at a given time on account of its larger depth of field. Moreover, scanning electron microscopes also have much larger resolution which in turn aids in getting higher level of magnification of a closely spaced sample. Additionally, since scanning electron microscopes use magnetic lenses rather than optical ones, it permits better control on the degree of magnification. 3.1. Sample Preparation (Non Conductive/Non Volatile) In all the applications which involve SEM evaluation, sample preparation is a crucial step. Many applications require evaluation of specimens which are electrically non-conductive. Thus, logically, such types of specimen require some kind of pretreatment. It can be in form of sample coating made up of conductive film. There is a In case of a non conductive sample, there happens to be a buildup of electrons on the surface which is to be examined. This buildup of electrons on the surface is often referred as Charging. Charging subsequently results in scattering of incoming electron stream which in turn affects the final image and its analysis. Additionally, some applications need samples which contain water or other substance which can volatize when subjected to high vacuum. These samples also put equally challenging sample preparation for SEM investigation. In case of such volatile specimen, it is required to avoid the deformation of the specimen and to control the drying so as to allow the SEM chamber to attend high vacuum. However, the pretreatment of the SEM investigation specimen is often effective partially. Like a wet specimen, which need to be analyzed, would change character if dehydrated and might give different analysis. Similarly, in case of spongy or porous specimen, it is very difficult to coat it with conductive film, resulting in charge buildup at the pores and the SEM investigation gets affected. As far as sample coating is concerned, Carbon is typically used while resorting to microanalysis. Early SEM detectors were not capable of detecting Carbon, but now the advanced detectors can detect Carbons as well as even the lighter materials. This affects the investigation. As SEM is increasingly being used for routine microanalysis and investigations, the trend is on rise for carrying out the investigation without any pretreatment so as to get unaffected analysis. Similarly, sample coating is also not considered as an effective option for microanalysis investigation of the specimen. To cater to these issues pertaining to charging and volatility, the advanced SEM operates without exposing the specimen to high vacuum. These scanning electron microscopes are often referred as environmental or variable-pressure SEMs. These environmental scanning electron microscopes provide two prominent advantages over their traditional counterparts. First and foremost is that the higher pressure in the chamber obviates the de-gassing of the volatile specimen. Secondly, by controlling the amount of gas into the chamber, the unwanted charging is minimized on the non conductive specimen. 3.2. Emitted Signals In SEM, when the stream of electron strikes the specimen, a variety of signals are emitted. The detection of any of these specific signals allows capturing the required image or microanalysis of the specimen. Importantly, there are three main signals which provide the much needed investigative information during SEM. These are signals from secondary electrons, backscattered electrons, and X-rays. 3.2.1. Secondary Electrons The most readily interpretable image of the sample surface is provided by the secondary electrons which are emitted from the atoms occupying the surface of the specimen. The excited electron moves towards the surface of the sample undergoing elastic and inelastic collisions until it reaches the surface, where it can escape if it still has sufficient energy. The contrast in the image is determined by the sample topography and hence can be associated more closely with sample morphology. Here, the image of high resolution can be obtained on account of finely focused stream of electrons. While bombarding the specimen with the stream of electrons, secondary electrons forms the strongest section of the energy spectrum. However, the yield of the secondary electron depends on multiple factors. For an example, the yield would be high for a high atomic number sample, or the yield is higher when the angle of incidence of the electron beam is larger. On account of the their nearness to the surface ( Read More