Prologue to Electron Microscopy

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Due to these distinctions, the magnifying lens development will likewise be diverse ... This magnifying lens is practically equivalent to a standard upright or transformed light magnifying lens ...

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Slide 1

Prologue to Electron Microscopy Fundamental ideas in electron microscopy The development of transmission and checking electron magnifying lens Sample cases of use

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Electron Microscope versus Optical Microscope (initial one implicit 1931 by Ruska and Knoll) (Leeuwenhoek in 17 th century) Electron versus Photon Electron: charged, has rest mass, not unmistakable Photon: unbiased, has no rest mass, noticeable at the wavelength ~ 400 nm-760 nm. As a result of these distinctions, the magnifying lens development will likewise be diverse What is the regular property?

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Comparison of EM and LM a. Likenesses (Arrangement and capacity of segments are comparative) 1) Illumination framework : produces required radiation and guides it onto the example. Comprises of a source, which emanates the radiation, and a condenser focal point, which centers the lighting up shaft (permitting varieties of power to be made) on the example. 2) Specimen arrange : arranged between the brightening and imaging frameworks. 3) Imaging framework : Lenses which together deliver the last amplified picture of the example. Comprises of i ) a target focal point which centers the bar after it goes through the example and structures a transitional picture of the example and ii) the projector focal point( es ) which amplifies a segment of the halfway picture to shape the last picture. 4) Image recording framework : Converts the radiation into a changeless picture (regularly on a photographic emulsion) that can be seen.

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Comparison of EM and LM b. Contrasts 1) Optical focal points are for the most part made of glass with altered central lengths while attractive focal points are developed with ferromagnetic materials and windings of copper wire creating a central length which can be changed by fluctuating the current through the loop. 2) Magnification in the LM is for the most part changed by exchanging between various power target focal points mounted on a pivoting turret over the example. It can likewise be changed if oculars (eyepieces) of various power are utilized. In the TEM the amplification (central length) of the target stays altered while the central length of the projector focal point is changed to differ amplification. 3) The LM has a little profundity of field, along these lines distinctive central levels can be found in the example. The huge (relative) profundity of field in the TEM implies that the whole (thin) example is in concentrate all the while. 4) Mechanisms of picture development shift (stage and sufficiency differentiate). 5) TEMs are for the most part developed with the radiation source at the highest point of the instrument: the source is by and large arranged at the base of LMs. 6) TEM is worked at high vacuum (since the mean free way of electrons in air is little) so most examples (organic) must be got dried out (i.e. dead !!). 7) TEM examples (natural) are quickly harmed by the electron shaft. 8) TEMs can accomplish higher amplification and preferable determination over LMs. 9) Price tag!!! (100x more than LM)

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0.61 l/NA Resolution of a magnifying instrument Where N.A. is the numerical gap = n(sin a )

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The determination is corresponding to the wavelength! Electron proportionate wavelength and quickening voltage The dualism wave/molecule is evaluated by the De Broglie condition: λ = h/p = h/mv λ : wavelength; h: Planck consistent; p: force The vitality of quicken electrons is equivalent to their active vitality: E = eV = m 0 v 2/2 V: increasing speed voltage e/m 0/v: charge/rest mass/speed of the electron These conditions can be joined to compute the wave length of an electron with a specific vitality: p = m 0 v = (2m 0 eV) 1/2 λ = h/(2m 0 eV) 1/2 (≈ 1.22/V 1/2 nm) At the quickening voltages utilized as a part of TEM, relativistic impacts must be considered (e.g. E>100 keV) λ = h/[2m 0 eV (1 + eV/2m 0/c 2 )] 1/2

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Wavelength and quickening voltage There are different components that breaking point the determination!

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Types of Electron Microscope Transmission Electron Microscope (TEM) utilizes a wide light emission going through a thin cut example to shape a picture. This magnifying lens is similar to a standard upright or transformed light magnifying lens Scanning Electron Microscope (SEM) utilizes centered light emission looking over the surface of thick or thin examples.. Pictures are delivered one spot at once in a matrix like raster example. (will be talked about in a later address) Scanning Transmission Electron Microscope ( STEM) utilizes an engaged light emission looking over a thin cut example to shape a picture . The STEM resembles a TEM yet creates pictures as does a SEM (one spot at once). It is most ordinarily utilized for essential examination of tests.

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FEI Tecnai 20 For TEM, since the electrons need to infiltrate the example, it must be thin (< 100 nm)

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Leo 982 SEM For SEM: a fine test (bar spot) is framed by condenser focal point and its size decides the determination (this contrasts from the TEM which is diffraction restricted)

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Electron Gun FEG W clip LaB6 precious stone

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Thermionic Sources Increasing the fiber current will build the bar current yet just to the point of immersion and soon thereafter an expansion in the fiber current will just abbreviate the life of the emitter

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Beam spot picture at various phase of warming

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Electromagnetic focal point

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Condenser-focal point framework C1 controls the spot estimate The condenser opening must be focused C2 changes the joining of the bar

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TEM imaging modes Bright field/dim field relies on upon the gap position. Cutting edge path for diffraction tilts the shaft as opposed to moving the opening

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STEM picture Bright and dim field STEM picture of Au particles on a carbon film What is the contrasts between this dim field and the past one?

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Magnification in TEM M ob × M int × M proj = Total Mag Depends on the amplification, some focal point may not be utilized

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Electron-example connection

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Backscattered Electrons: Formation Caused by an occurrence electron slamming into a molecule in the example which is about ordinary to the episode's way. The occurrence electron is then scattered "backward" 180 degrees. Usage The generation of backscattered electrons shifts specifically with the example's nuclear number. This varying generation rates makes higher nuclear number components seem brighter than lower nuclear number components. This communication is used to separate parts of the example that have diverse normal nuclear number.

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Backscatter Detector The most widely recognized plan is a four quadrant strong state identifier that is situated specifically over the example

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Gold particles on E. coli show up as brilliant white dabs because of the higher rate of backscattered electrons contrasted with the low nuclear weight components in the example

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Backscatter picture of Nickel in a leaf

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Secondary Electrons: Source Caused by an episode electron passing "near" a particle in the example, sufficiently close to grant some of its vitality to a lower vitality electron (generally in the K-shell). This causes a slight vitality misfortune and way change in the occurrence electron and the ionization of the electron in the example particle. This ionized electron then leaves the molecule with a little active vitality (5eV) and is then named a "secondary electron". Every episode electron can deliver a few optional electrons. Usage Production of optional electrons is extremely geography related. Because of their low vitality, 5eV, just secondaries that are exceptionally close to the surface (< 10 nm) can leave the specimen and be inspected. Any adjustments in geology in the specimen that are bigger than this testing profundity will change the yield of secondaries because of accumulation efficiencies. Accumulation of these electrons is helped by utilizing a "collector" in conjunction with the auxiliary electron locator. The gatherer is a lattice or work with a +100V potential connected to it which is put before the locator, drawing in the adversely charged optional electrons to it which then go through the network gaps and into the identifier to be checked.

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A traditional auxiliary electron indicator is situated off to the side of the example. A faraday confine (kept at a positive inclination) attracts the low vitality optional electrons. The electrons are then quickened towards a scintillator which is kept at a high predisposition with a specific end goal to quicken them into the phosphor.

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The position of the auxiliary electron indicator additionally influences flag gathering and shadow. An in-focal point finder inside the segment is more effective at gathering auxiliary electrons that are created near the last focal point (i.e. short working separation).

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Secondary Electron Detector Side Mounted In-Lens What are the contrasts between these two pictures?

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Auger Electrons Source Caused by the de-stimulation of the example iota after an optional electron is delivered. Since a lower (as a rule K-shell) electron was radiated from the iota amid the auxiliary electron handle an internal (lower vitality) shell now has an opportunity. A higher vitality electron from a similar molecule can "fall" to a lower vitality, filling the opening. This makes and vitality surplus in the particle which can be remedied by emanating an external (lower vitality) electron: an Auger Electron. Use Auger Electrons have a trademark vitality, one of a kind to every component from which it was transmitted from. These electrons are gathered and sorted by to give compositional data about the example. Since Auger Electrons have generally low vitality they are just radiated from the mass example from a profundity of < 3 nm

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X-beams Source Caused by the de-stimulation