Imaging and Analysis of Surfaces Using Scanning Electron Microscopy - Lab Report Example

Name: Professor: Course: Date: Experiment: 2NANO1 Theory and Virtual Imaging and Analysis of Surfaces Using Scanning Electron Microscopy (SEM) AIM: This lab experiment was performed with the aim of getting familiarized with the theory underlying the scanning electron microscope, and applying this technique in imaging of surfaces using a virtual SEM instrument at the Australian Microscopy and Microanalysis Research Facility (AMMRF) Myscope website. By applying this technique, a better understanding of how the instrument operates, the advantages and the limitations of the SEM technique was a gain of knowledge. INTRODUCTION: Scanning electron microscopy (SEM) is a microscope that forms an image by scanning a sample with a beam of electrons. The traditional microscope uses a beam of light to produce an image. When the electrons interact with atoms, they produce different signals that carry the information about the composition, morphology and surface topography of the sample. Scanning electrons were developed in the early 1950,s and since then, they have tremendously contributed to physical science and medical research study fields [1]. Compared to the traditional light microscope, the scanning electron microscope has many advantages. It has a larger depth of field and a very high resolution with strikingly clear images. A large depth of field enables a larger part of a specimen to be focused upon at one time while the higher resolution enables higher level magnification of closely spaced specimens. Another advantage of the SEM is that the user controls the degree of specimen magnification, as opposed to a lens microscope. A specimen sample to be used must be well prepared before image scanning can be performed [3]. The specimen should be freed of water to prevent evaporation in the SEM vacuum. Non-metallic specimens are non-conductive and therefore, should be covered using a "sputter coater." In this lab experiment, a virtual SEM was used to perform a practice scanning electron. Two different samples were chosen for image scanning; rock and slag. Scanning of images was done both at low (100) and high (15) magnification. PROCEDURE: Please refer to Manual for 2NANO1 “Theory and virtual imaging and analysis of surfaces using scanning electron microscopy (SEM)”pages 19-21. RESULTS AND DISCUSSION: The virtual SEM instrument at the AMMRF Myscope website was used to scan two specimens; two samples of slag and two samples of rock. For each specimen, one sample was scanned at a low magnification (100) and the second sample scanned at high magnification (15). The figures below show high resolution virtual images obtained when the virtual SEM on the AMMRF Myscope website was used to observe specimens of a rock and a slug. Figure 1(a): A SEM virtual image of a rock sample at a magnification of 100 (acceleration voltage=10, spot size =15 and Z = 10.) Figure 1(b): A SEM virtual image of a rock sample at a magnification of 15 (acceleration voltage=10, spot size =10 and Z = 10.) In figure 1(a), the rock image was produced at a lower magnification (100). The image shows very little details that may not be observed with naked eyes, compared to the image produced at a higher magnification (15). This means that a SEM image produced at a higher magnification provides more detailed information about the surface of a sample. Magnification of an image is controlled by the amount of voltage supplied to the deflector plates or the amount of current supplied to the scanning coils. The higher the magnification, the smaller the size of the raster on the sample, and vice versa. A SEM has a wide range of magnifications, ranging from 10 times to greater than 500,000 times [2]. This is approximately 250 times larger than the magnification that can be achieved by a light microscope. Similar observations can be made with the SEM images of a slag at low and high magnification shown in figure 2(a) and 2(b). Figure 2(a): A SEM virtual image of a slag sample at a magnification of 100 (acceleration voltage=10, spot size =15 and Z = 10.) Figure 2(b): A SEM virtual image of a slag sample at a magnification of 15 (acceleration voltage=10, spot size =10 and Z = 10.) A SEM instrument has an electron gun that produces a beam of electrons which follow a vertical path within a vacuum in the microscope. The beam of electrons travels through an electromagnetic field and magnetic lenses that help to focus the electrons to the sample. When the beam of electrons hit the sample on the stage, the sample eject electrons and X-rays. Detectors in the microscope collect the X-rays, backscattered and secondary electrons, which are then converted into a signal that is transmitted to a TV screen which produces an image [1]. X-rays are emitted from the sample when a beam of electrons causes release of energy by replacing an electron from the inner shell of the sample. The X-rays provide signals that help to reveal the composition of a sample, as well as the relative abundance of various elements in a sample. Backscattered electrons consist of electrons that are reflected by elastic scattering from the surface of the sample. They are usually applied in analytical SEM together with the spectra that is produced by the characteristic X-rays. The intensity of the backscattered electrons strongly relates to atomic number (Z) of the sample being analyzed [4]. Figure 3: Certificate of Scanning Electron Microscope training preparation produced by AMMRF Myscope website on completion of the test for this lab. ANALYSIS OF RESULTS: QUESTION 1. In SEM imaging what is the role of the Astigmatism? Astigmatism reduces image resolution when an elliptical electron beam produces a round spot of the image after being collected on the detector. This results when the beam of electrons pass through a magnetic field that is not consistent. Astigmatism is corrected using stigmators. QUESTION 2. What does an increase in the accelerating voltage result in? Increasing the acceleration voltage causes a decrease in the system’s spherical aberration and therefore, increasing the resolution. High acceleration voltage also causes a charging effect on a sample. QUESTION 3. What is resolution in the context of SEM? Resolution in the context of SEM refers to the amount of details that can be seen in a SEM image of a sample. QUESTION 4. What is a disadvantage of SEM? Despite the amazing performance and wide applications of this powerful SEM instrument in physical science, medicine and industries, limitations do exist. SEMs are very large in size and expensive. They have to be housed in a place free from magnetic, electric and vibrational interference. They require maintenance by keeping a steady voltage and current in the electromagnetic coils, and steady circulation of cooling water. A special training is needed for one to prepare samples and operate a SEM. Human errors during sample preparation may cause damage to samples that can result to artifacts SEM imaging is limited to solid samples that are inorganic, with a size small enough to fit in the vacuum chamber. The images produced do not show color variations like in the case of optical images. In addition, SEMs have a small risk of exposure to radiation. These radiations originate from the scattered electrons beneath the surface of the sample [5]. QUESTION 5. Explain charging of a sample and how this is overcome in SEM. Charging of a sample in SEM happens when a beam of electrons irradiate a sample. If the sample is non-conducting, the beam of electrons cause accumulation of static negative charges on the surface of the sample resulting in a charging effect. This charging effect influences signals and deteriorates the information about the image. Charging effect can be reduced in a number of ways: a. Coating the sample with a very thin conductive film. b. Reducing the accelerating voltage. c. Applying a biased voltage on the surface of the sample. d. In some SEMs, using a low vacuum can eliminate this effect. e. Mounting a sample with a conductive bridge on the sample holder using a conductive coating [3]. QUESTION 6. What is a technique that is complementary to SEM but has better resolution? Briefly explain this technique. This technique is called Transmission electron microscopy (TEM). This is a microscopy technique where a continuous beam of electrons that are highly accelerated is transmitted through an ultrathin sample, interacting with the sample of specimen as it passes through. The image is produced when the beam of electrons interact with the sample as it passes through it, revealing the information about the internal composition of the sample. CONCLUSION: The SEM instrument produces a specimen’s image with a larger magnification by use of electrons. The AMMRF Myscope website provides SPM modules that make it easy to understand the principles of working of a SEM. The virtual online SEM is an instructional primer and tool that simulates the real instrument. It can greatly assist a learner to understand how scanning electron microscopy is used to produce clear images of specimens for scientific or industrial applications. It is also easier to learn to work with the real instrument after working with a virtual SEM. In addition, it also provides an opportunity to understand the theories behind the instrument without physically interacting with it. REFERENCES: [1] Australian Microscopy and Nicroanalysis Research Facility. My Scope. n.d. website. 25 August 2015. [2] Egerton, Ray. Physical Principles of Electron Microscopy: An Introduction to TEM, SEM, and AEM. New York: Springer Science & Business Media, 2006. [3] Pueschel, Mr Martin. How and why we use the Scanning Electron Microscope (SEM). 27 May 2014. website. 25 August 2015.Print. [4] Reed, S. J. B. Electron Microprobe Analysis and Scanning Electron Microscopy in Geology. 2. U.K: Cambridge University Press, 2005. [5] Schatten, Heide. Scanning Electron Microscopy for the Life Sciences: Advances in Microscopy and Microanalysis. U.K: Cambridge University Press, 2012.Print. Read More

Figure 1(a): A SEM virtual image of a rock sample at a magnification of 100 (acceleration voltage=10, spot size =15 and Z = 10.) Figure 1(b): A SEM virtual image of a rock sample at a magnification of 15 (acceleration voltage=10, spot size =10 and Z = 10.) In figure 1(a), the rock image was produced at a lower magnification (100). The image shows very little details that may not be observed with naked eyes, compared to the image produced at a higher magnification (15). This means that a SEM image produced at a higher magnification provides more detailed information about the surface of a sample.

Magnification of an image is controlled by the amount of voltage supplied to the deflector plates or the amount of current supplied to the scanning coils. The higher the magnification, the smaller the size of the raster on the sample, and vice versa. A SEM has a wide range of magnifications, ranging from 10 times to greater than 500,000 times [2]. This is approximately 250 times larger than the magnification that can be achieved by a light microscope. Similar observations can be made with the SEM images of a slag at low and high magnification shown in figure 2(a) and 2(b).

Figure 2(a): A SEM virtual image of a slag sample at a magnification of 100 (acceleration voltage=10, spot size =15 and Z = 10.) Figure 2(b): A SEM virtual image of a slag sample at a magnification of 15 (acceleration voltage=10, spot size =10 and Z = 10.) A SEM instrument has an electron gun that produces a beam of electrons which follow a vertical path within a vacuum in the microscope. The beam of electrons travels through an electromagnetic field and magnetic lenses that help to focus the electrons to the sample.

When the beam of electrons hit the sample on the stage, the sample eject electrons and X-rays. Detectors in the microscope collect the X-rays, backscattered and secondary electrons, which are then converted into a signal that is transmitted to a TV screen which produces an image [1]. X-rays are emitted from the sample when a beam of electrons causes release of energy by replacing an electron from the inner shell of the sample. The X-rays provide signals that help to reveal the composition of a sample, as well as the relative abundance of various elements in a sample.

Backscattered electrons consist of electrons that are reflected by elastic scattering from the surface of the sample. They are usually applied in analytical SEM together with the spectra that is produced by the characteristic X-rays. The intensity of the backscattered electrons strongly relates to atomic number (Z) of the sample being analyzed [4]. Figure 3: Certificate of Scanning Electron Microscope training preparation produced by AMMRF Myscope website on completion of the test for this lab.

ANALYSIS OF RESULTS: QUESTION 1. In SEM imaging what is the role of the Astigmatism? Astigmatism reduces image resolution when an elliptical electron beam produces a round spot of the image after being collected on the detector. This results when the beam of electrons pass through a magnetic field that is not consistent. Astigmatism is corrected using stigmators. QUESTION 2. What does an increase in the accelerating voltage result in? Increasing the acceleration voltage causes a decrease in the system’s spherical aberration and therefore, increasing the resolution.

High acceleration voltage also causes a charging effect on a sample. QUESTION 3. What is resolution in the context of SEM? Resolution in the context of SEM refers to the amount of details that can be seen in a SEM image of a sample. QUESTION 4. What is a disadvantage of SEM? Despite the amazing performance and wide applications of this powerful SEM instrument in physical science, medicine and industries, limitations do exist. SEMs are very large in size and expensive. They have to be housed in a place free from magnetic, electric and vibrational interference.

They require maintenance by keeping a steady voltage and current in the electromagnetic coils, and steady circulation of cooling water.

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