TRANSMISSION ELECTRON MICROSCOPE

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By: lniu (17 month(s) ago)

This presentation is very helpful to learn the fundamentals and applications of TEM.

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TRANSMISSION ELECTRON MICROSCOPE: 

TRANSMISSION ELECTRON MICROSCOPE Prepared by Kaushal Bhatt Aasif Huda

WHY ELECTRON MICROSCOPE ?: 

WHY ELECTRON MICROSCOPE ? As U know electrically magnified image has great resolving power due to the wavelength used of electron 10,000 times shorter than visible light… Can achieve upto 2,00,0000x magnification which is 1000 times than ordinary one .. Electron microscope can be devided in 4 types Scanning Electron Microscope (SEM) Transmission Electron Microscope (TEM) Reflection Electron Microscope (REM) Low Voltage Electron Microscope (LVEM)

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Electron microscope constructed by Ernst Ruska in 1933 History

History: 

History Ultraviolet (UV) microscopes, led by Koehler , in that increase in resolving power The first practical TEM was Originally installed at I. G Farben-Werke.. Plücker in 1858 --deflection of "cathode rays" ( electrons ) was possible by the use of magnetic fields… Hans Busch “ lens maker's equation could be applicable to electrons.” Max Knoll to lead a team of researchers to advance the CRO design. Reinhold Rudenberg , the scientific director of the Siemens company, had patented an electrostatic lens electron microscope in 1931. Improving resolution Albert Prebus and James Hillier at the University of Toronto who constructed the first TEM in north America in 1938 .They continue to advancing TEM design. Siemens in 1936, the aim of the research was the development improvement In 1939 the first commercial electron microscope, pictured, was installed in the Physics department of I. G Farben-Werke . They done Further work on the electron microscope..

History: 

History Further research Worldwide electron microscopy community advanced with electron microscopes being manufactured in Manchester UK, the USA (RCA), Germany (Siemens) and Japan .. First international conference in electron microscopy was in Delft in 1942.. Later conferences included in Paris, 1950 and then in London in 1954.. With the development scanning transmission electron microscopy (STEM) was re-investigated and did not become developed until the 1970s.. PHILIPS CM200

Background : 

Background Maximum resolution, d (as theorized by Louis-Victor de Broglie ) of visible light (wavelengths of 400–700 nanometers ) TEM an electron's velocity approaches the speed of light, c Source formation T he TEM consists of an emission source tungsten filament, or a lanthanum hexaboride ( LaB 6 ) High voltage source (typically ~100-300 kV) To emit electrons either by thermionic or field electron emission

Background : 

Background Optics It has Quadrupole or hexapole lenses The quadrupole lens is an arrangement of electromagnetic coils at the vertices of the square, enabling the generation of a lensing magnetic fields , the hexapole configuration simply enhances the lens symmetry by using six, rather than four coils. TEMs utilising energy filtering to correct electron chromatic aberration .. Display Imaging systems consist of a phosphorous screen .. It has Screen particulate zinc sulphide . Optionally, an image recording system doped YAG screen coupled CCDs

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DIAGRAM OF ELECTRON MICROSCOPE COMPONENT

Components : 

Components Vacuum system To increase the mean free path of the electron gas interaction standard TEM is evacuated to low pressures typically on the order of 10 −4 Pa . TEMs are equipped with multiple pumping systems and airlocks. Low or roughing vacuum is achieved with either a rotary vane pump or diaphragm pumps . Operating the turbomolecular pumps, the vacuum side of a low-pressure pump may be connected to chambers which accommodate the exhaust gases from the turbomolecular pump to allow for the low vacuum pump .. Gate valves , to allow for different vacuum levels in specific areas, such as a higher vacuum of 10 −4 to 10 −7 Pa ..

Components : 

Components Specimen stage Designs include airlocks to allow for insertion of the specimen holder .. The vacuum with minimal increase in pressure in other areas of the microscope.. Standard TEM grid sizes is a 3.05 mm diameter ring, with a thickness and mesh size ranging from a few to 100 μm .. Usual grid materials are copper, molybdenum, gold or platinum is placed into the sample holder .. Stage must simultaneously be highly resistant to mechanical drift .. Modern devices may use electrical stage designs, using screw gearing in concert with stepper motors providing the operator a joystick or trackball Two main designs for stages in a TEM The side-entry base is presented to atmosphere, the airlock formed by the vacuum rings involve the rotation of the sample to trigger micro switches . Top entry version unable to be tilted without blocking the beam path. mostly used method is side entry is convinient

Components : 

Components Electron gun Electron gun is formed from the filament, a biasing circuit, a Wehnelt cap, and an extraction anode.. The thermionic emission current density J ; Both lanthanum hexaboride and tungsten thermionic sources must be heated in order to achieve thermionic emission .. Electron lens The components include the yoke, the magnetic coil, the poles, the polepiece , and the external control circuitry. Lenses may operate electrostatically or magnetically. The polepiece provides the boundary conditions for the magnetic field that forms the lens.. Imperfections in the manufacture of the polepiece can induce severe distortions in the magnetic field symmetry.. The coils which produce the magnetic field , utilise high voltages, and therefore require significant insulation.. Using a chilled water supply in order to facilitate the removal of the high thermal duty..

Components : 

Components Apertures Apertures are annular metallic plates.. Consist of a small metallic disc that is sufficiently thick to prevent electrons from passing through the disc. Apertures are either a fixed aperture within the column, such as at the condensor lens, or are a movable aperture. Apertures decrease the beam intensity as electrons are filtered from the beam, which may be desired in the case of beam sensitive samples. This filtering removes electrons that are scattered to high angles due to unwanted processes such as spherical or chromatic aberration. Aperture assemblies are often equipped with micrometers to move the aperture, required during optical calibration.. Electron lens Ele. Gun assembly

Imaging method : 

Imaging method The projector lenses allow for the correct positioning of this electron wave distribution onto the viewing system. The observed I Intensity of the image I ,wavefunctions is denoted by Ψ Thus, the observed image depends not only on the amplitude of beam, but also on the phase of the electrons. Avian flu virus Mylinated neurone

IMAGING: 

IMAGING A TEM image of the polio virus. The polio virus is 30 nm in size. A three dimensional TEM image of a parapoxavirus section of a cell of Bacillus subtilis , taken with a Tecnai T-12 TEM. Cyanobactaria under TEM

Contrast formation: 

Contrast formation Depends greatly on the mode of operation. Change lens strength or to deactivate a lens, allow for many operating modes. TYPES Bright field The most common mode of operation. Occlusion and absorption of electrons in the sample. A simple two dimensional projection of the sample. Diffraction contrast The electron beam undergoes Bragg scattering Image will appear dark wherever no sample scattering to the selected peak is present. This is known as a dark-field image. Electrons that would otherwise be diffracted in a particular direction from entering the specimen.

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Electron energy loss Utilizing the advanced technique of EELS appropriately equipped electrons can be rejected based upon their voltage. Phase contrast Crystal structure can also be investigated by High Resolution Transmission Electron Microscopy (HRTEM), also known as phase contrast. Image formation is given by the complex modulus of the incoming electron beams. Diffraction Diffraction pattern can be generated. Series of rings in the case of a polycrystalline or amorphous solid material. Information based on the position at which the diffraction spots appear and the observed image symmetries.. Three dimensional imaging TEM allow for the rotation of a sample by a desired angle.. By taking multiple images of a single TEM sample it can be used to construct a three-dimensional representation of the sample.. A set of images known as a "tilt series" can be collected.. Errors can occur due to vibration or mechanical drift & autocorrelation methods to correct these errors..

Sample preparation: 

Sample preparation The technique required varies depending on the specimen and the analysis required for general electron microscope The steps for that are Chemical fixation for biological specimens Cryofixation – freezing a specimen which is called cryo-electron microscopy . - cryo-electron microscopy of vitreous sections CEMOVIS - possible to observe in its native state. Dehydration freeze drying , or replacement of water with organic solvents Embedding , biological specimens – after dehydration Sectioning – produces thin slices of specimen Staining – uses heavy metals Freeze-fracture or freeze-etch Ion Beam Milling -- gallium ions are used Conductive Coating --Such coatings include platinum, tungsten, graphite

Sample preparation: 

Sample preparation TEM specimens are required to be in nanometers thickness.. many generic techniques have been used for the preparation of the required thin sections as given earlier .. Fixated using either a negative staining material such as uranyl acetate or by plastic embedding Samples may be held at liquid nitrogen temperatures For TEM sample preparation steps Tissue sectioning This method is used to obtain thin, minimally deformed samples Coated with a thin layer of conducting material carbon Coating thickness is several nanometers using a sputter coating device Sample staining Details in light microscope samples can be enhanced by stains Stain absorbs electrons or scatters part of the electron beam heavy metals such as osmium , lead , or uranium may be used

Sample preparation: 

Sample preparation Mechanical milling polishing may be used to prepare samples Diamond or cubic boron nitride polishing to remove any scratches that may cause contrast fluctuations Chemical etching Samples are thinned using as an acid Include systems to detect when the sample has been thinned to a sufficient level of optical transparency. Ion etching Ion etching is a process that can remove very fine quantities of material. Used to perform a finishing Uses an inert gas passed through an electric field to generate a plasma stream. Acceleration energies for gases such as argon are typically a few kilovolts More recently focussed ion beam methods have been used to prepare samples FIB makes use of significantly more energetic gallium ions

Modification: 

Modification TEM can be further extended by additional stages and detectors. TEM now capable of maintaining the specimen at liquid nitrogen or liquid helium temperatures and specimens prepared in vitreous ice , the preferred preparation technique. Scanning transmission electron microscope (STEM) in which Scanning coils are used to deflect the beam. Current detector such as a faraday cup , which acts as a direct electron counter. Beam tilting by the use of annular dark field detectors. Modern research TEMs may include aberration correctors. TEM makers include JEOL , Hitachi High-technologies , FEI Company (from merging with Philips Electron Optics), Carl Zeiss and NION. The low voltage electron microscope (LVEM) is a combination of SEM, TEM and STEM in one instrument uses low electron accelerating voltage of 5 kV

APPLICATIONS :: 

APPLICATIONS : Materials Science/Metallugy Biological Science Nanotechnology Ceramics Pharmaceuticals Semiconductors

LIMITATION: 

LIMITATION Drawbacks to the TEM technique Require extensive sample preparation.. The field of view is relatively small.. Sample may be damaged by the electron beam in the case of biological materials.. A cut-off frequency, q max , for the transfer function may be approximated with the following equation C s is the spherical aberration coefficient Their resolution is however limited by electron source geometry and brightness and chromatic aberrations in the objective lens system..

Models available of JOEL : 

Models available of JOEL 100/120 kv JEM-1011 JEM-1400 200 kV JEM-2100F JEM-2100 LaB6 JEM-2200FS JEM-2500SE JEM-ARM200F 300 kV JEM-3100F JEM-3200FS JEM-3200FSC

Availability for research: 

Availability for research At all the IIT’s like IIT bombay , several research labs.. CHARGES   : Transmission Electron Microscope Industry : Rs 2923/- per sample (Max. 1/2 Hour) R&D : Rs 1820 /- per sample (Max. 1/2 Hour) University/ Colleges : Rs 855/- per sample (Max. 1/2 Hour) One student under a Guide can register at a time. Registration, users can send the requisiton letter from Head/Guide. Maximum Eight(8) samples can be registered. Samples other than powder form should be prepared at userend . The Sample can be mounted on carbon/ formvar coated Copper grid or can be made of disc type with a thinned

REFERENCES: 

REFERENCES Electron Microscopy and Analysis by : P.J. Goodhew, University of Surrey and F.J. Humphreys, UK Imperial College, London,UK Wikipedia, en.wikipeddia.org Hawkes, P. , The beginnings of Electron Microscopy Transmission Electron Microscopy and Diffractometry of Materials . Springer. 2007. ISBN 3540738851 Hubbard, A Electron Diffraction in the Transmission Electron Microscope . Garland Science. ISBN 1859961479 Tanaka, Nobuo "Present status and future prospects of spherical aberration corrected TEM/STEM for study of nanomaterials" (free download review). Haque, M. A. and Saif, M. T. A. "In-situ tensile testing of nano-scale specimens in SEM and TEM". Adrian, Marc; Dubochet, Jacques; Lepault, Jean; McDowall, Alasdair W "Cryo-electron microscopy of viruses".

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“Sky is the Limit” Open For Discussion