Transmission Electron Microscopy (TEM) Prof.Dr.Figen KAYA

Transcription

1 Transmission Electron Microscopy (TEM) Prof.Dr.Figen KAYA

2 Transmission Electron Microscope A transmission electron microscope, similar to a transmission light microscope, has the following components along its optical path: light source, condenser lens, specimen stage, Objective lens and Projector lens as shown in Figure 3.1.

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5 Instrumentation The main differences are that, in a TEM, the visible light ray is replaced by an electron ray and glass lenses are replaced by electromagnetic lens for the electron beam. The TEM has more lenses (the intermediate lens) and more apertures (including the selected area aperture).

6 Instrumentation The TEM contains further features arising from using electrons as illumination. High Vacuum: For example, a vacuum environment is required in a TEM because collisions between high energy electrons and air molecules significantly absorb electron energy.

7 Transmission Electron Microscope/ Transmission Light Microscope Table 3.1 provides a comparison of a TEM with a light microscope.

8 TEM versus Optical Microscopy

9 cs/microscopes/tem/tem.html ion_of_condenser_system.htm#

10 Electron Sources In a TEM system, an electron gun generates a high energy electron beam for illumination. In the electron gun, the electrons emitted from a cathode, a solid surface, are accelerated by high voltage (Vo) to form a high energy electron beam with energy. Because electron energy determines the wavelength of electrons and wavelength largely determines resolution of the microscope, the acceleration voltage determines the resolution to a large extent.

11 V o and Wavelength Table 3.2 gives the relationship between the acceleration voltage and wavelength.

12 Resolution in TEM To achieve a high resolution, the TEM is usually operated under an acceleration voltage of greater than 100 kv. In practice, 200 kv is commonly used and meets most resolution requirements. High-voltage electron microscopy ( 1000 kv) requires extremely expensive instrumentation, and also it may damage a specimen by generating microstructural defects in it during observation.

13 Condenser Lenses in TEM The lens system of a TEM is more complicated than a light microscope. There are two or more condenser lenses to demagnify the electron beam emitted from the electron gun. The condenser lens system controls the beam diameter and convergence angles of the beam incident on a specimen. During operation, the illumination area of specimen and illumination intensity can be adjusted by controlling the current in the electromagnetic lenses of condensers.

14 Objective lens The objective lens forms an inverted initial image, which is subsequently magnified. In the back focal plane of the objective lens a diffraction pattern is formed. The objective aperture can be inserted here. The objective lens would not usually provide a magnification of more than 50 and a TEM is routinely used to view regions of the specimen which are only a mm or so across.

15 Intermediate Lens The intermediate lens is used to switch the TEM between an image mode and a diffraction mode. For the image mode: The intermediate lens is focused on the image plane of the objective lens, For the diffraction mode: The intermediate lens is focused on the back-focal plane of the objective lens where the diffraction pattern forms.

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17 Projector Lens The projector lens further magnifies the image or diffraction pattern and projects it onto the fluorescent screen for observation or the photographic plane. Magnification in the electron microscope can be varied from hundreds to several hundred thousands of times. This is done by varying the strength of the projector and intermediate lenses. Not all lenses will necessarily be used at lower magnifications.

18 0,4 mm 0,02 mm Objective Lens X50 Intermediate Lens 0,4 mm X20 1 st Projector Lens 8 mm X20 2 nd Projector Lens 200 mm X25 Total Magnification=50*20*20*25= X500000

19 0,4 mm Objective Lens 0,02 mm X50 Intermediate Lens 0,2 mm X10 1 st Projector Lens 2 nd Projector Lens 2 mm X10 Total Magnification=50*10*10= X5000

20 0,4 mm Objective Lens 0,02 mm X50 Intermediate Lens 0,4 mm X20 1 st Projector Lens 8 mm 400 mm X20 2 nd Projector Lens X50 Total Magnification=50*20*20*50= X

21 Specimen Stage The TEM has special requirements for specimens to be examined. TEM specimens must be a thin foil because they should be able to transmit electrons; that is, they are electronically transparent. A thin specimen is mounted in a specimen holder, as shown in Figure 3.6, in order to be inserted into the TEM column for observation.

22 TEM Specimens The holder requires that a specimen is a 3-mm disc. Smaller specimens can be mounted on a 3-mm mesh disc as illustrated in Figure 3.7. The meshes, usually made from copper, prevent the specimens from falling into the TEM vacuum column. Also, a copper mesh can be coated with a thin film of amorphous carbon in order to hold specimen pieces even smaller than the mesh size.

23 Specimen Preparation Preparation of specimens often is the most tedious step in TEM examination. To be electronically transparent, the material thickness is limited. We have to prepare a specimen with at least part of its thickness at about 100 nm, depending on the atomic weight of specimen materials. For higher atomic weight material, the specimen should be thinner. A common procedure for TEM specimen preparation is described as follows.

24 Pre-Thinning Pre-thinning is the process of reducing the specimen thickness to about 0.1mm before final thinning to 100 nm thickness. First, a specimen less than 1mm thick is prepared. This is usually done by mechanical means, such as cutting with a diamond saw. Then, a 3-mm-diameter disc is cut with a specially designed punch before further reduction of thickness.

25 Grinding Grinding is the most commonly used technique to reduce the thickness of metal and ceramic specimens. During grinding, we should reduce thickness by grinding both sides of a disc, ensuring the planes are parallel. This task would be difficult without special tool. Figure 3.8 shows a hand-grinding jig for prethinning. The disc (S) is glued to the central post and a guard ring (G) guides the grinding thickness.

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27 Dimple grinder A dimple grinder is also used for pre-thinning, particularly for specimens that are finally thinned by ion milling.

28 Final Thinning Three methods of final thinning are described here: 1. Electrolytic thinning for metal specimens that are good electric conductors, 2. Ion milling for metal and ceramic specimens, and 3. Ultramicrotome cutting for polymeric and biological specimens.

29 Electrolytic Thinning Electrolytic thinning and ion milling are methods for reducing specimen thickness to the scale of 100 nm. These methods create a dimpled area on prethinned specimens because it is almost impossible to reduce the thickness of specimens uniformly to the level of electron transparency. The dimpled area is likely to have regions of electron transparency as schematically shown in Figure 3.9.

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31 Electrolytic thinning Electrolytic thinning is widely used for preparing specimens of conducting materials. A specimen is placed in an electrochemical cell with the specimen as anode. A suitable electrolyte (usually an acidic solution) is used to electrochemically reduce specimen thickness. A common technique is jet polishing, illustrated schematically in Figure 3.10.

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33 Electrolytic thinning A specimen is placed in a holder with two sides facing guides of electrolyte jet streams, which serve as cathode. The electrolyte jet polishes both sides of the specimen until light transparency is detected by a light detector. Catching the precise moment that tiny holes appear is crucial, because only the edge of a tiny hole contains thin sections of electron transparency. Electrolytic thinning is very efficient and is completed in only 3 15 minutes.

34 Ion Milling Ion milling uses a beam of energetic ions to bombard specimen surfaces in order to reduce the thickness by knocking atoms out of a specimen. The specimen does not need to be electrically conductive for ion milling. Thus, the technique is suitable for metal, ceramics and other materials, as long as they are thermally stable.

35 Ion Milling Figure 3.11 illustrates the general procedure of ion milling. Before ion milling, the specimen is often ground with a dimple grinding device in order to reduce the thickness in the central are of specimen (Figure 3.11a). Then, the ground specimen is cut as a 3-mm disc and placed in the ion milling chamber with a geometric arrangement as shown in Figure 3.11b.

36 Figure 3.11 Ion thinning process: (a) dimple grinding; and (b) ion milling.

37 Ion Milling The edge of the specimen disc may need to be covered by a molybdenum ring in order to strengthen it during ion milling. An ion beam with energy of 1 10 kev bombards the specimen. The specimen is placed in the center at an angle of about 5 30 to the ion beam in order have a high yield of sputtering. Light transparency is detected by a light detector aligned along the vertical direction.

38 Ion Milling The ion beam can raise the temperature of the specimen rapidly and dramatically. Thus, cooling the chamber with liquid nitrogen is necessary in order to prevent specimen heating by the ion beam. Even with cooling, organic materials are not suitable for ion milling because the ion beam can cause severe damage to the microstructure of organic specimens such as polymers.

39 Ultramicrotomy Ultramicrotomy is basically the same type of method as microtomy for preparing soft specimens for light microscopy. However, ultramicrotomy can be used to section a specimen to the 100 nm scale. It is commonly used to prepare polymeric or biological TEM specimens.

40 Ultramicrotomy Figure 3.12 illustrates the working principles of the instrument. A specimen is mounted in a holder against the cutting tool (glass knife or diamond knife). The specimen should be trimmed to have a tip held against the knife. The cross-section of the trimmed tip usually is only about 1mm 2 for diamond knife cutting. The holder gradually moves toward the knife while it repeatedly moves up and down. The firmly mounted specimen is sectioned as it passes the edge of the knife blade.

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42 Ultramicrotomy To be sectioned by an ultramicrotome, materials must be softer than the knife. Even though diamond is the hardest material available, a specimen can still damage the tip of a diamond knife because the extremely sharp tip of a diamond knife makes it very fragile.

43 Image Modes Contrast in transmission light microscopy is generated by differences in light absorption in different regions of the specimen. In TEM, however, absorption of electrons plays a very minor role in image formation.

44 Image Modes TEM contrast relies instead on deflection of electrons from their primary transmission direction when they pass through the specimen.

45 Imaging Modes The contrast is generated when there is a difference in the number of electrons being scattered away from the transmitted beam. There are two mechanisms by which electron scattering creates images: 1. Mass-density contrast and 2. Diffraction contrast.

46 Mass-Density Contrast The deflection of electrons can result from interaction between electrons and an atomic nucleus. Deflection of an electron by an atomic nucleus, which has much more mass than an electron, is like a collision between a particle and a wall.

47 Mass-Density Contrast The particle (electron) changes its path after collision. The amount of electron scattering at any specific point in a specimen depends on the mass-density (product of density and thickness) at that point.

48 Mass-Density Contrast Thus, difference in thickness and density in a specimen will generate variation in electron intensity received by an image screen in the TEM, if we only allow the transmitted beam to pass the objective aperture as shown in Figure 3.13.

49 Mass-Density Contrast The deflected electron with scattering angle larger than the α angle controlled by the objective aperture will be blocked by the aperture ring. Thus, the aperture reduces the intensity of the transmission beam as the beam passes through it.

50 The brightness of the image is determined by the intensity of the electron beam leaving the lower surface of the specimen and passing through the objective aperture.

51 Williams and Carter, TEM, Part 3 Springer 2009

52 Contrast Formation by Mass-Density The intensity of the transmitted beam (I t ) is the intensity of primary beam (I o ) minus the intensity of beam deflected by object (I d ) in a specimen.

53 Contrast Formation by Mass-Density This type of mass density contrast exists in all types of materials. It is the main mechanism for image formation for non-crystalline materials such as amorphous polymers and biological specimens. The mass-density contrast is also described as absorption contrast, which is rather misleading because there is little electron absorption by the specimen.

54 Figure 3.14 TEM image of polymer blend containing polycarbonate (PC) and polybutylene terephthalate (PBT) with mass-density contrast.

55 Diffraction Contrast We can also generate contrast in the TEM by a diffraction method. Diffraction contrast is the primary mechanism of TEM image formation in crystalline specimens. Diffraction can be regarded as collective deflection of electrons.

56 Diffraction Contrast Electrons can be scattered collaboratively by parallel crystal planes. Bragg s Law (nλ=2sin ϴ) which applies to X-ray diffraction, also applies to electron diffraction.

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58 Diffraction Contrast When the Bragg conditions are satisfied at certain angles between electron beams and crystal orientation, constructive diffraction occurs and strong electron deflection in a specimen results, as schematically illustrated in Figure 3.15.

59 Crystalline Materials and Crystal Planes

60 Diffraction Contrast Thus, the intensity of the transmitted beam is reduced when the objective aperture blocks the diffraction beams, similar to the situation of massdensity contrast.

61 Diffraction Contrast

62 nλ= 2dsinθ

63 Bright Field and Dark Field Imaging in TEM The diffraction contrast can generate bright-field and dark-field TEM images. In order to understand the formation of brightfield and dark-field images, the diffraction mode in a TEM must be mentioned. A TEM can be operated in two modes: the image mode and the diffraction mode.

64 Figure 3.15 Electron deflection by Bragg diffraction of a crystalline specimen: (a) image formation in crystalline samples; and (b) diffraction at crystal lattice planes and at the contours of inclusions.

65 Bright Field and Dark Field Imaging in TEM A bright-field image is obtained by allowing only the transmitted beam to pass the objective aperture, exactly the same as in mass-density contrast. A dark-field image, however, is obtained by allowing one diffraction beam to pass the objective aperture.

66 Bright Field and Dark Field Imaging in TEM Dark-field is not as commonly used as bright-field imaging. However, the dark-field image may reveal more fine features that are bright when the background becomes dark. Figure 3.21 shows a comparison between brightand dark-field images for the same area in an Al Fe Si alloy. The dark-field image (Figure 3.21b) reveals finer details inside the grains.

67 Bright Field/Dark Field image comparison in Al-Fe-Si Alloy a) Bright Field İmage b) B) Dark Field İmage

68 High resolution transmission electron microscope (HRTEM) image of a lead crystal between two crystals of aluminum (i.e., a Pb precipitate at a grain boundary in Al). The two crystals of Al have different orientations, evident from their different patterns of atom columns. Note the commensurate atom matching of the Pb crystal with the Al crystal at right, and incommensurate atom matching at left. An isolated Pb precipitate is seen to the right.

69 The interface between GaP and ZnSe crystals. A carbon nanotube at its end portion.

70 Figure 3.47 Brightfield images of dislocations: (a) in a Al Zn Mg alloys with 3% tension strain; (b) lining up in a Ni Mo alloy; and (c) dislocation cell structure in pure Ni.

71 Diffraction patterns The appearance of the diffraction pattern can reflect the nature of the crystalline phases in the specimen. For example, if the material is microcrystalline or amorphous the diffraction pattern consists of a series of concentric rings rather than spots/discs.

72 When the electron beam interacts with the sample when the sample is oriented with a zone axis pattern parallel to the electron beam, then the diffraction pattern form in the back focal plane of the objective lens is a regular array of reflections. This is seen projected onto the viewing screen as an array of reflections organized in a predictable manner based on the crystal structure of the sample.

73 PowderCell 2.0 Example: Study of unknown phase in a BiFeO 3 thin film Metal organic compound on Pt TiO 2 Pt BiFeO 3 Lim Heat treatment at 350 o C (10 min) to remove organic parts. Process repeated three times before final heat treatment at o C (20 min). (intermetallic phase grown) SiO 2 Bi Bi O Si Bi Fe O Bi Bi O O Fe Fe O 200 nm Fe O Bi Bi O Fe O Fe O Bi O Bi O Bi O Goal: Fe Fe O Bi Fe O O BiFeO 3 with space grupe: R3C and celle dimentions: a= Å c= Å b c a Fe Bi Bi Bi O Fe O Fe Bi O Bi