Introduction
The microscope is derived from two Greek words, mikros and skopeo. Mikros means small while skopeo means “look at.” Thus, the two words form the word mikroskopeo that refers to looking or observing small objects. Since the beginning of scientific developments, there has been an interest to look at the smaller details of the world around us. Biologists have taken the lead in the quest to uncover such small details through their desire to examine cells, viruses, bacteria, and colloidal particles. Material scientists have wanted to observe the limitations and inhomogeneities in metals, ceramics, and crystals. Geologists have also developed interest in the detailed study of rocks, fossils, and minerals on a microscopic scale, a study that has brought forth insight into the origins of planet earth and the valuable minerals in it.
History of Microscopy
Microscopy is believed to have developed from the Galilean telescope in the seventeenth century. Antony Van Leeuwenhoek, a Dutchman, who lived in the period 1632-1723 made the earliest instrument used for observing very small objects (Burgess, Marten, and Taylor, 186). The instrument consisted of powerful lenses and adjustable holder to ease object viewing. This instrument was the first microscope. The instrument used light for illumination hence the name light microscope. It could magnify objects up to 400 times thus it helped Van Leeuwenhoek to discover protozoa, spermatozoa, and bacteria. He was also able to pioneer the classification of red blood cells by shape. The limitation of this instrument was that it had a single lens. This problem was solved later by addition of a second lens changing the name to compound microscope. The compound microscope thus formed the basis of light microscopes that are widely used today.
The emergence of Electron microscopy
The light microscope discovery revolutionized scientific development for a long time until the 18th century when scientists embraced technological innovations and improvements in design of the light microscope. These improvements made the light microscope very popular with researchers in the areas of botany, zoology, material science, and geology.
After duration of approximately 100 years, scientists discovered that accelerated electrons in a vacuum behaved like light. They also discovered that an electric field could shape the paths of electrons just in a similar way that lenses are used in bending and focusing visible light. in 1931, Ernst Ruska, with the help of Max Knoll, combined these discovered electron characteristics and the magnetic field to build the first transmission electron microscope at the University of Berlin-Germany (Achilladelis, Bowden, 11). This discovery brought about the concept of electron microscopy. Electron microscopy is thus the use of electron beams in the process of magnification of objects. Instead of light, electrostatic and electromagnetic lenses are used to focus particles. These lenses can show features as small as a tenth of a nanometer or a ten billionth of a meter, a great deal of detail. With the electron microscope, individual atoms can be observed with amazing clarity thus electron microscopes have high resolving power. At the point of invention, the electron microscope used two magnetic lenses. Three years later, a third lens was introduced demonstrating an increased resolution of 100nm. Improvements have been done with constant innovations and today the electron microscope have reached a resolution of up to 0.05nm, 400 times better than a modern typical light microscope, and compared to the naked eye, it is 4000,000 times better.
Types of electron microscopes
With the increasing innovation in electron microscopy over the years, there are several different applications of this concept in the form of the different types of electron microscopes. These types of electron microscopes are different in terms of how they operate each with a peculiar scientific strength or ability. These types of electron microscopes include transmission electron microscope, scanning electron microscope, and scanning transmission electron microscope.
Transmission Electron microscope
This type has four main components: an electron optical column, vacuum system, control software and the necessary electronics which include lenses for beam deflection and the generator of very high voltage for generating electrons. In a modern transmission electron microscope, there is an operating console that is surmounted by a column that is vertical and contains the vacuum system (Gooddhew, Humphreys, and Beanland, 66). There are also control panels placed conveniently for the operator. This type of microscope is always closed fully for the reduction of the environmental sources interference. It is also operated remotely. There are two types of transmissions. The two types are environmental transmission electron microscopy and aberration-corrected transmission electron microscopy. They are briefly discussed below:
Environmental transmission electron microscopy
This type of microscopy uses a vacuum system specially designed to allow researchers observe specimens in a wide range of conditions of the natural environment. In the sample chamber, there is a gas pressure that is as high as a little percent of the atmospheric pressure. This pressure is important since it aids the observation of interactions that occur between the sample and the environment. The environmental transmission electron microscope relies on apertures that limit pressure and differential vacuum pumping (Banhart, 15). Consequently, this allows vacuum conditions that are less restrictive in the vicinity of the sample while a high vacuum is maintained in the remaining electron column.
Aberration-corrected Transmission electron microscopy
This is a recent development and has enabled major improvements in the transmission electron microscope capability. Without such a correction, the resolution of the TEM is primarily limited by spherical aberration. Consequently, this could not only result in general image blurring but also delocalization where periodic structures appear extending beyond actual physical boundaries. The correction of spherical aberration leads to a reduction of the effects of chromatic aberration.
The Scanning Electron Microscope
The SEM just like a transmission electron microscope contains a vacuum system, electron optical column, electronics, and software. It has a considerably shorter column because only the lenses above the specimen are needed. These lenses focus the electrons into a fine spot on the surface of the specimen. The specimen chamber of the SEM is large hence this technique does not impose any restrictions on the size of the specimen. At the top of the column is an electron gun that produces an electron beam. This beam is focused into a fine small spot whose diameter is as small as 1nm on the surface of the specimen. The beam is scanned in a raster over the specimen, and the intensities of the various signals are then created by interactions between the beam electrons. The specimen is then measured and gets stored in the memory of the computer. The stored values are mapped as the variations on the image brightness display. The important differences between the transmission electronic microscope and the scanning electron microscope are discussed below:
Unlike in the TEM where a broad static beam is used, the beam in the SEM is focused to a fine point, and it scans line by line over the samples surface in a raster pattern. In the TEM, accelerating voltages are much lower as it is no longer necessary to penetrate the specimen. In SEM, the voltages range from 50 to 30,0000volts.
Scanning Transmission Electron Microscopy
This microscopy combines the principles in SEM and TEM. It can be performed on either of the two instruments. Just like TEM, STEM needs very thin samples and it primarily looks at beam electrons that the sample submits. It however has one advantage over the TEM as it enables the use of signals that cannot be correlated partially in TEM.
Technological development in Microscopy
Electron microscopy has been applied in scientific research fields such as biological research and health research. This makes electron microscopy a vital discovery worldwide as it has enabled growth in terms of the value of research findings it brings forth.
Advancement in the Electron microscopy technology has been recorded with the emergence of even more complex and increasingly efficient types of electron microscopes. One of the products of this technological advancement is the confocal microscopes which not only provides scientists with complete workstations, but is also relatively low-priced hence affordable. The confocal electron microscope allows 3-D scanning, and it offers enhanced resolution as well as yielding better image (Pawly, 11).
References
Achilladelis, Basil, and Mary E. Bowden. Structures of Life: To Accompany an Exhibit by the Beckman Center for the History of Chemistry. Philadelphia: Chemical heritage foundation, 1989. Print.
Banhart, Florian. In-situ Electron Microscopy at High Resolution. Singapore: World Scientific Pub. Co, 2008. Internet resource.
Burgess, Jeremy, Michael Marten, and Rosemary Taylor. Under the Microscope: A Hidden World Revealed. Cambridge: Cambridge University Press, 1990. Print.
Goodhew, Peter J, John Humphreys, and Richard Beanland. Electron Microscopy and Analysis, Third Edition. Hoboken: Taylor and Francis, 2014. Print.
Pawley, James B. Handbook of Biological Confocal Microscopy. New York: Plenum Press, 1995. Print.
