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Principle & Applications of Transmission Electron Microscopy (TEM) & High Resolution TEM


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Principle & Applications of Transmission Electron Microscopy (TEM) & High Resolution TEM

  1. 1. Examination Paper for Foreign Graduates Principle & Applications of TEM & HRTEM Student Number: LS1401202 Submitted by: Mr.Gulfam Raza Submitted to: Prof. Lilly Dong Abstract: Transmission Electron Microscope (TEM) is a very powerful tool for material science. A high energy beam of electrons is shone through a very thin sample, and the interactions between the electrons and the atoms can be used to observe features such as the crystal structure and features in the structure like dislocations and grain boundaries. TEM can be used to study the growth of layers, their composition and defects in semiconductors. High Resolution Transmission Electron Microscope (HRTEM) can be used to analyze the quality, shape, size and density of quantum wells, wires and dots.
  2. 2. 2 TABLE of CONTENTS Topic Number Topic Name Page Number 1 BRIEF INTRODUCTION TO MICROSCOPY 1.1 What is Microscopy? 1.2 What is Microscope? 1.3 Microscopic Terms 3-4 2 TRANSMISSION ELECTRON MICROSCOPE (TEM) 2.1 What is TEM? 2.2 Working Principle of TEM 2.2.1 Imaging 2.2.2 Diffraction 4-6 3 APPLICATIONS OF TEM 3.1CoFe2O4:BaTiO3 Core Shell Nanocomposite 3.2 g-C3N4/TiO2 Photo Catalyst 3.3 Characterization of ZnO Nanotubes 3.4 CdSe-graphene Composites 3.5 Pr-doped ZnO Nanoparticles 6-7 4 HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPE (HRTEM) 4.1 What is HRTEM? 4.2 Working Principle of HRTEM 7-9 5 APPLICATIONS OF HRTEM 5.1 CoFe2O4:BaTiO3 Core Shell Nanocomposite 5.2 Characterization of ZnO Nanotubes 9
  3. 3. 3 TOPIC No. 01 BRIEF INTRODUCTION to MICROSCOPY 1.1What is Microscopy? The science of investigating small objects using instruments like microscope is called microscopy.Microscopy is the technical field of using microscopes to viewing objects and areas of objects that are not within the resolution range of the normal eye. There are three well-known branches of microscopy;  Optical Microscopy  Electron Microscopy  Scanning Probe Microscopy On October 8, 2014, the Nobel Prize in Chemistry was awarded to Eric Betzig, William Moerner and Stefan Hell for "the development of super- resolved fluorescence microscopy," which brings "optical microscopy into the nanodimension". [1] 1.2What is Microscope? An optical instrument that uses a lens or combination of lenses to produce magnified images of small objects especially which are too small to be seen by naked or unaided eye.There are many types of microscopes. The most common and the first to be invented is the optical microscope, which uses light to image the sample. [2] Other major types of microscopes are following;  Electron Microscope (TEM and SEM)  Ultra-microscope  Scanning Probe Microscopes 1.3Microscopic Terms Resolution The resolution of an optical microscope is defined as the shortest distance between two points on a specimen that can still be distinguished by the observer or camera system as separate entities.[3]
  4. 4. 4 Magnification Magnification in physical terms is defined as "a measure of the ability of a lens or other optical instruments to magnify, expressed as the ratio of the size of the image to that of the object". This means, that an object of any size is magnified to form an enlarged image. The magnification required to produce the visible image can be calculated using the formula:[4] Magnification = Image ÷ Object Lenses A lens is a transparent curved device that is used to refract light. A lens is usually made from glass. There are two different shapes for lenses. They are called convex and concave.[5] Aperture of Lense In optics, an aperture is a hole or an opening through which light is admitted. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane.It is crucial in determining the resolving power of an optical devicebecause the aperture determines the amount of diffraction and hence resolution.[6] TOPIC No. 02 TRANSMISSION ELECTRON MICROSCOPY 2.1 What is TEM? Transmission electron microscopy uses high energy electrons (up to 300 kV accelerating voltage) which are accelerated to nearly the speed of light. The electron beam behaves like a wavefront with wavelength about a million times shorter than lightwaves.
  5. 5. 5 2.2 Working Principle of TEM The TEM operates on the same basic principles as the light microscope but uses electrons instead of light. When an electron beam passes through a thin-section specimen of a material, electrons are scattered. A sophisticated system of electromagnetic lenses focuses the scattered electrons into an image or a diffraction pattern, or a nano- analytical spectrum, depending on the mode of operation. Fig 1 - General layout of a TEM describing the path of electron beam in a TEM Fig 2 - A ray diagram for the diffraction mechanism in TEM 2.2.1Imaging The beam of electrons from the electron gun is focused into a small, thin, coherent beam by the use of the condenser lens. This beam is restricted by the condenser aperture, which excludes high angle electrons. The beam then strikes the specimen and parts of it are transmitted depending upon the thickness and electron transparency of the specimen. This transmitted portion is focused by the objective lens into an image on phosphor screen or charge coupled device (CCD) camera. The image then passed down the column through the intermediate and projector lenses, is enlarged all the way.
  6. 6. 6 The image strikes the phosphor screen and light is generated, allowing the user to see the image. 2.2.2Diffraction Fig 2 shows a simple sketch of the path of a beam of electrons in a TEM from just above the specimen and down the column to the phosphor screen. As the electrons pass through the sample, they are scattered by the electrostatic potential set up by the constituent elements in the specimen. After passing through the specimen they pass through the electromagnetic objective lens which focuses all the electrons scattered from one point of the specimen into one point in the image plane. Also, shown in fig 2 is a dotted line where the electrons scattered in the same direction by the sample are collected into a single point. This is the back focal plane of the objective lens and is where the diffraction pattern is formed.[7] TOPIC No. 03 APPLICATIONS of TEM 3.1CoFe2O4:BaTiO3Core Shell Nanocomposite The TEM image of as-synthesized CFO nanoparticles is given as; Figure shows nearly spherical CFO nanoparticles with diameter 20 nm.[8] 3.2 g-C3N4/TiO2Photo Catalyst Morphology and microstructure of samples were investigated by TEM as shown;[9] TEM images of g-C3N4 (a), TB1 (b), TB0.5 (c), TB0.2 (d), TB0.05 (e), and TB0 (f), respectively. Redand white arrows in image (b) showing the presence of hollowstructured TiO2 nanobox and film-like g-C3N4, respectively.
  7. 7. 7 3.3Characterization of ZnO Nanotubes TEM micrograph indicates that the ZnO possesses uniform nanotubes and are grown in large scale.[10] 3.4CdSe-graphene Composites TEM images of CdSe-graphene composite materials. From the images, it can be observed that the CdSe were the dark imaged compounds almost spherical nanoparticles attached to the surface of the graphene sheets, whereas the graphene components were found to be relatively lighter than CdSe with irregular edges, flat sheet-like structure, having occasional distribution of CdSe on the surface.[11] 3.5Pr-doped ZnO Nanoparticles The particles are agglomerated and the shape of samples is turnover from spheroid-like into the mixer of spheroid- like and rod-like with the increase of Prcontent, in which spheroid-like particles are dominant.[12] TOPIC No. 04 HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPE 4.1 What is HRTEM? High-Resolution TEM (HRTEM) is the ultimate tool in imaging defects. In favorable cases it shows directly a two dimensional projection of the crystal with defects and all.
  8. 8. 8 Of course, this only makes sense if the two-dimensional projection is down some low-index direction, so atoms are exactly on top of each other.[13-14-15] 4.2 Working Principle of HRTEM The basic principle involved in the image formation in both the microscopes (TEM & HRTEM) is similar. However, HRTEM provides high resolution images at atomic scale level. Most precisely, HRTEM is a type of TEM. The high-resolution transmission electron microscopy (HRTEM) uses both the transmitted and the scattered beams to create an interference image. It is a phase contrast image and can be as small as the unit cell of crystal. In this case, the outgoing modulated electron waves at very low angles interfere with itself during propagation through the objective lens. All electrons emerging from the specimen are combined at a point in the image plane. In short, following are the salient features to describe working principle of HRTEM;  Consider a very thin slice of crystal that has been tilted so that a low-index direction is exactly perpendicular to the electron beam. All lattice planes about parallel to the electron beam will be close enough to the Bragg position and will diffract the primary beam.  The diffraction pattern is the Fourier Transform of the periodic potential for the electrons in two dimensions. In the objective lens all diffracted beams and the primary beam are brought together again; their interference provides a back-transformation and leads to an enlarged picture of the periodic potential.
  9. 9. 9  This picture is magnified by the following electron-optical system and finally seen on the screen at magnifications of typically 106 .[13-14-15] TOPIC No. 05 APPLICATIONS of HRTEM 5.1CoFe2O4:BaTiO3Core Shell Nanocomposite High Resolution Transmission Electron Microscope images confirm the core shell structure.The CFO: BTO core shell nanoparticles are shown as; Figure shows the core shell nanoparticles of CFO: BTO. Two crystallographic phases are evident and were identified by measuring the interplanar spacings, d, of an enlarged image.[8] 5.2Characterization of ZnO Nanotubes Structural properties of the as-synthesized ZnO nanotubes were done by HRTEM.[10]
  10. 10. 10 References 1: 2: 3: 4: 5: 6: 7: , 8: had been accepted at October 27, 2014) 9: had been accepted at September 21, 2014) 10: (Paper had been accepted at July 29, 2014) 11: (Paper had been accepted at January 17, 2014) 12: (Paper had been accepted at September 26, 2014) 13-14-15: kQFjAI& L&usg=AFQjCNGdiWkvYd6ZTGZX7DckCxKS9fvoEA