Lecture 24 Transmission electron microscopy Wavelength and acceleration voltage relationship for electrons wavelength & voltage 1. Transmitted electrons of the beam passes straight through the specimen on to the screen 2. Some electron of the beam lose a bit of their energy while passing through the specimen & get deflected a little from their original axis of the beam inelastically scattered electrons Transmission Electron Microscopy • In a conventional transmission electron microscope, a thin specimen is irradiated with an electron beam of uniform current density. • Electrons are emitted from the electron gun and illuminate the specimen through a two or three stage condenser lens system. • Objective lens provides the formation of either image or diffraction pattern of the specimen. • The electron intensity distribution behind the specimen is magnified with a three or four stage lens system and viewed on a fluorescent screen. The image can be recorded by direct exposure of a photographic emulsion or an image plate or digitally by a CCD camera. IMAGING • The image of the specimen in conventional microscopy, , is formed selectively allowing only the transmitted beam (Bright Field Imaging) or one of the diffracted beams (Dark Field Imaging) down to the microscope column by means of an aperture. • The origin of the image contrast is the variation of intensities of transmitted and diffracted beams due to the differences in diffraction conditions depending on the microstructural features on the electron path. Comparison of OM, TEM and SEM Comparison of OM, TEM and SEM (cont.) BASIC DESIGN OF TRANSMISSION ELECTRON MICROSCOPE Evacuated metal cylinder within which are aligned, one under another: 1. Tungsten filament (the cathode) 2. A Metal plate with central aperture (the anode) 3. A number of magnetic lenses 4. A Fluorescent screen 5. A photographic plate DESIGN OF TRANSMISSION ELECTRON MICROSCOPE A simplified ray diagram of a TEM consists of an electron source, condenser lens with aperture, specimen, objective lens with aperture, projector lens and fluorescent screen. Working & Image formation • Working of EM is based on same plan as that of light microscope • Electron are used for magnification & image formation • Image formation occurs by electron scattering • Electron strike the atomic nuclei & get dispersed •This dispersed electron form image • The electron image is converted in to visible form by projecting on a fluorescent screen TEM How a TEM works A transmission electron microscope fires a beam of electrons through a specimen to produce a magnified image of an object. 1. A high-voltage electricity supply powers the cathode. 2. The cathode is a heated filament that generates a beam of electrons. 3. An electromagnetic coil (the first lens) concentrates the electrons into a more powerful beam. 4. Another electromagnetic coil (the second lens) focuses the beam onto a certain part of the specimen. 5. The specimen sits on a copper grid in the middle of the main microscope tube. The beam passes through the specimen and "picks up" an image of it. 6. The projector lens (the third lens) magnifies the image. 7. The image becomes visible when the electron beam hits a fluorescent screen at the base of the machine. 8. The image can be viewed directly (through a viewing portal), through binoculars at the side, or on a TV monitor attached to an image intensifier (which makes weak images easier to see). The TEM system and components: • Vacuum system • Electron Gun system • Electron Lens system • Sample Stage • More Electron Lenses • Viewing Screen /scintillator • Camera Chamber Focusing the TEM • • • • • • • Control objective lens current Adjust astigmatism correction coils too Use large screen at low mags Use small screen at high mags Beware of lingering on an area too long Iterate focus and stigmators Can take through-focus-series CONDENSER LENS Illuminates the specimen. Relatively weak lens. Longer focal length than objective or projector lens. May bring electron beam into focus directly upon specimen, above the specimen (over focusing) or below the specimen (under focusing). As magnification increases the condenser lens must be adjusted to properly illuminate the specimen. When the lens is brought to its smallest spot the beam is said to be at the crossover point OBJECTIVE LENS • Strong lens • Highly concentrated magnetic field and short focal length.Causes electron beam, which has passed through specimen, to focus at a point a few mm below specimen. • Magnification of image produced a short distance below focused point. Projector Lens • Magnification produced by projector lens dependent on current passing through the coil of the lens (ie increase current spreads beam further = higher mag.) • Projector lens has great depth of focus (several meters). Therefore distance at which fluorescent screen or photographic plate are placed is not critical. Total magnification in the TEM is a combination of the magnification from the objective lens times the magnification of the intermediate lens times the magnification of the projector lens. Each of which is capable of approximately 100X. Mob X Mint X Mproj = Total Mag Apertures: to limit the collection angle of the lens • Aperture in an objective lens to control: - resolution - depth of focus - image contrast • collection angle of EELS • usually made of Pt or Mo (refractory metals) Resolution Limited by Lens Aberrations Chromatic aberration is caused by the variation of the electron energy and thus electrons are not monochromatic. point is imaged as a disk. Spherical aberration is caused by the lens field acting inhomogeneously on the off-axis rays. rmin0.91(Cs3)1/4 Practical resolution of microscope. Cs– coefficient of spherical aberration of lens (~mm) point is imaged as a disk. Spherical Aberration Chromatic aberration Lecture 25 Specimen Holder Rotation, tilting, heating, cooling and straining beam holder a split polepiece objective lens Twin specimen holder Double tilt heating Heating and straining Imaging Modes in the TEM Bright Field Mode Dark Field Mode Diffraction Mode TEM Imaging Modes: Diffraction vs BF Bright Field Imaging • If the main portion of the near-forward scattered beam is used to form the image – transmitted beam – 000 beam – zero-order beam The Bright-Field Microscope • produces a dark image against a brighter background • total magnification – product of the magnifications of the ocular lens and the objective lens 35 Dark Field Imaging • If the transmitted beam is excluded from the image formation process – off-axis imaging – tilted beam imaging TEM Imaging: Ray Paths Metal particles Polymer mix TEM Images Electron Diffraction Bright-field TEM micrographs of the as-prepared ZnO powders after annealing for 1 h at various temperatures: a 300 .C, b 400 .C and c 500 .C, respectively. Electron Diffraction • Elastic Scattering Events – Bragg diffraction • n=2d sinq Electron Diffraction • Four conditions in Back Focal Plane (BFP) of the objective lens: – – – – No sample Amorphous Polycrystal Single crystal No reflections (only transmitted beam) Transmitted beam + random scattering Transmitted beam + rings Transmitted beam + spots Diffraction patterns • As the electrons pass through the sample, they are scattered by the electrostatic potential set up by the constituent elements. • After the electrons have left the sample they pass through the electromagnetic objective lens. This lens acts to collect all electrons scattered from one point of the sample in one point on the fluorescent screen, causing an image of the sample to be formed. • The electrons scattered in the same direction by the sample are collected into a single point. This is the back focal plane of the microscope, and is where the diffraction pattern is formed. • By manipulating the magnetic lenses of the microscope, the diffraction pattern may be observed by projecting it onto the screen instead of the image. • If the sample is tilted with respect to the incident electron beam, one can obtain diffraction patterns from several crystal orientations. BRIGHT FIELD IMAGING ALLOWING TRNSMITTED BEAM DARK FIELD IMAGING ALLOWING DIFFRACTED BEAM DIFFRACTION • Electrons of 0.072 Angstrom wavelength at 100 kV excitation transmitted through about 0.1 micrometer thin foil specimen are diffracted according to Bragg's Law, forming a diffraction pattern (consisting of a transmitted and diffracted beam spots). • Although diffraction phenomena is a complex interactions of charged electrons with the periodic potential field of the lattice, Bragg's Law or Laue Conditions are sufficient approximations for usual practical applications. • A diffraction pattern is, in the simplest sense, a Fourier transform of the periodic crystal lattice, giving us information on the periodicities in the lattice, and hence the atomic positions. HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPE (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. • The HRTEM allows nthe imaging of the crystallographic structure of a sample at an atomic scale. Because of its high resolution, it is an invaluable tool to study nanocrystalline materials. • At present, the highest resolution realized is 0.8 Å. • At these scales, individual atoms and crystalline defects can be imaged. Basic 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. • The 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. High resolution TEM - HRTEM • Crystal structure can also be investigated by highresolution transmission electron microscopy (HRTEM), • HRTEM is also known as phase contrast. • In a specimen of uniform thickness, the images are formed due to differences in phase of electron waves, which is caused by specimen interaction. • Image formation is given by the complex modulus of the incoming electron beams. • The image is dependent on the number of electrons hitting the screen, • it can be manipulated to provide more information about the sample as in complex phase retrieval techniques. Examples of TEM and HRTEM images TEM and HRTEM images
