Electron

Transcription

1 Electron 1897: Sir Joseph John Thomson ( ) discovered corpuscles small particles with a charge-to-mass ratio over 1000 times greater than that of protons. Plum pudding model : electrons in a sea of positive charge. Nobel Prize : Sir George Paget Thomson ( ) discovered 10 kv electrons could give diffraction pattern of a 100 nm thick gold foil. Nobel Prize 1937, shared with Clinton Davison (experiment on Ni single crystal). 1924: Doctoral thesis of Louis-Victor Pierre Raymond de Broglie ( ) hypothesized matter waves and the idea of wave- particle duality (only known for photon up to then). Nobel Prize

2 Lecture 24 MNS 102: Techniques for Materials and Nano Sciences Reference: #1 C. R. Brundle, C. A. Evans, S. Wilson, "Encyclopedia of Materials Characterization", Butterworth-Heinemann, Toronto (1992), Ch. 2, Ch. 3. Reference: Electron Electron Microscopy Overview and History Comparison with Light Microscopy Transmission Electron Microscopy (TEM): Instrument and Electron Optics Comparison between TEM and LM Resolution, contributing factors to the resolution Depth of Focus and Depth of Field Modes of operation: Imaging vs Diffraction vs STEM Limitations of TEM 24-2

3 History of Electron Microscopy 1926: Hans Busch ( ) demonstrated that electric and magnetic fields of axial symmetry (short magnetic coils) can be used as lenses for electrons and other charged particles father of electron optics? 1928: Ernst Ruska ( ) began serious study of magnetic lenses potentially for EM applications. PhD thesis in 1929 on magnetic lenses. 24-3

4 History of EM 1931: Max Knoll ( ) and Ruska realized the first but crude transmission electron microscope (TEM). 1932: Davisson and Calbrick studied electrostatic lenses. 1934: Driest and Muller showed EM surpassing LM in resolution. 1935: Knoll built the first scanning electron microscope (SEM) with a 100 micron beam diameter. 1938: Manfred von Ardenne built the first true SEM with a nm resolution. The machine was destroyed in the Berlin air raid in

5 History of EM 1938: Albert Prebus, James Hillier of Professor Eli Franklin Burton s group at U of T Physics built the first TEM in North America. Their design was later adopted by all TEM manufacturers : 1 nm resolution achieved. 1961: First commercial SEM instruments after the invention of the secondary electron detector by Everhart and Thomley (ET). 1965: R.F. Pease and W. Nixon achieved 10 nm SEM resolution. 1986: Nobel Prize for Transmission Electron Microscopy to Ernst Ruska (TEM), and for Scanning Tunnelling Microscopy (STM) to Gerd Binnig and Heinrich Rohrer. 1997/98: Aberration correction 1999: Below 0.1 nm resolution achieved. 24-5

6 Everhart Thomley SE Detector SE = Secondary Electron 24-6

7 Example: Au (a = nm) 24-7

8 Transmission Electron Microscopy vs LM Specimen 24-8

9 TEM: Electron Optics 1 Double condenser lens: 1 st condenser to create demagnified image of the gun crossover and to minimize the spot size; 2 nd condenser to control beam divergence at the sample and the illumination spot size; condenser lens aperture to control illumination intensity. Objective lens: to form an inverted initial image that can subsequently be magnified, and to form a diffraction pattern in the back focal plane. Plus objective lens aperture (placed in the back focal plane of image) to select electrons for building the image, and to improve contrast of the final image. 24-9

10 TEM: Electron Optics 2 Intermediate lens: to magnify initial image formed by the objective lens, and to focus on initial image or diffraction pattern formed on the back focal plane. Projector lens: to magnify the size of image with various strengths

11 Electrostatic Lens vs Magnetic Lens Electrostatic lenses is used to focus electrons, e.g. in electron source to create a highly focussed e beam. Focussing is independent of the mass, i.e. electron and ions follow the same trajectory, and image is inverted like light optics. Paraxial ray approximation for converging lens. Magnetic lenses is used for condenser lenses and objective lenses Focussing is dependent of the charge-tomass ratio, i.e times less effective at focussing ions, and image is inverted and rotated. Focal length depends on the strength of the magnetic field

12 How sharp is the image? Contrast Conditions Contrast = (Max Signal Min Signal)/Max Signal Based on electron scattering theory quantum mechanics Inelastic scattering: occurs in all materials and leads to absorption Incoherent elastic scattering: particularly important for amorphous materials Coherent elastic scattering: leads to diffraction from single crystal regions 24-12

13 Light Microscopy vs Electron Microscopy Light Microscope Electron Microscope Wavelength = 500 nm (150/V 0 ) = nm at 50 kv Refraction Index = n 1.5 (glass) 1.0 (vacuum) Half-angle = 70 deg 1 deg Resolution = 0.61 / NA where NA = n sin Depth of Focus (DOF) = distance parallel to the optical axis that a feature on the specimen can be displaced without loss of resolution. 200 nm 0.16 nm* DOF = λ n2 NA NA 2 M 2 M = 10, DOF = 60 m M = 100, DOF = 8 m M = 1,000, DOF = 200 nm 0.1 mm DOF = M θ M = 10, DOF = 1,000 m M = 100, DOF = 100 m M = 1,000, DOF = 10 m M = 10,000, DOF = 1 m *Less than 0.05 nm possible with Cs (spherical aberration) and Cc (chromatic aberration) lens correctors. Most TEM specimens are not thin enough to produce images with resolution that could benefit from Cs correction. For thicker specimens, Cc correction via energy filtering is much more useful

14 Resolution Diffraction Limit Diffraction limit gives: Nonrelativistic electron wavelength vs relativistic electron wavelength 24-14

15 Resolution Homework 6A: Calculate the nonrelativistic wavelength, relativistic wavelength, relativistic mass, and speed for an 1, 10, 20, and 80 kev electron. Relativistic mass is the rest mass (m 0 ) multiplied by the highlighted term. For electron, m 0 is 9.1x10-31 Kg

16 Aberration and Diffraction Effects Spherical aberration: For a good magnetic lens design, K S ~ 1 and for an electrostatic lens, K S > 1, must be very small (~ 0.01 rad). Chromatic aberration: d = C C ( E/E) Diffraction effects: For = nm (for a 100 kev electron beam), and C S = 1 mm (for TEM objective lens), d min = nm

17 Resolution: EM vs LM With higher kv (300 kv to 500 kv) and proper C S correction, resolution of 0.05 nm may be realized

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19 Light Microscopy vs Electron Microscopy Light Microscope Electron Microscope Wavelength = 500 nm (150/V 0 ) = nm at 50 kv Refraction Index = n 1.5 (glass) 1.0 (vacuum) Half-angle = 70 deg 1 deg Resolution = 0.61 / NA where NA = n sin Depth of Focus (DOF) = distance parallel to the optical axis that a feature on the specimen can be displaced without loss of resolution. 200 nm 0.16 nm* DOF = λ n2 NA NA 2 M 2 M = 10, DOF = 60 m M = 100, DOF = 8 m M = 1,000, DOF = 200 nm 0.1 mm DOF = M θ M = 10, DOF = 1,000 m M = 100, DOF = 100 m M = 1,000, DOF = 10 m M = 10,000, DOF = 1 m *Less than 0.05 nm possible with Cs (spherical aberration) and Cc (chromatic aberration) lens correctors. Most TEM specimens are not thin enough to produce images with resolution that could benefit from Cs correction. For thicker specimens, Cc correction via energy filtering is much more useful

20 DOF Depth of field corresponds to how much of the 3D object remains in focus at the same time. Depth of focus corresponds to the distance over which the image can move relative to the object and still remain in focus

21 Modes of Operation: Imaging vs Diffraction Usually, we do a low resolution imaging scan to get a quick survey image, then we proceed to obtain a high-resolution image (HRTEM) to resolve the fringes. The fringe spacings (between planes of columns of atoms) will tell us directly the interplanar separation between specific planes. We can also obtain a diffraction pattern of a selected area (called SAED = Selected Area Electron Diffraction). Just like XRD (except for the extremely small sampling area), these ED patterns contain detailed info about the crystallography. The third mode is the STEM mode

22 Imaging vs Diffraction An objective lens is used to form a diffraction pattern in the back focal plane with electrons scattered by the sample and combine them to generate an image in the image plane (1. intermediate image). Diffraction pattern and image are simultaneously present in the TEM. By controlling the strength of the intermediate lens, they can be made to appear in the plane of the second intermediate image and magnified by the projective lens on the viewing screen. In imaging mode, an objective aperture can be inserted in the back focal plane to select one or more beams that contribute to the final image (BF, DF, HRTEM). For selected area electron diffraction (SAED), an aperture in the plane of the first intermediate image defines the region of which the diffraction is obtained

23 Bright Field vs Dark Field Imaging BF: An aperture is placed in the back focal plane of the objective lens which allows only the direct beam to pass. Image results from a weakening of the direct beam by its interaction with the sample. Therefore, mass-thickness and diffraction contrast contribute to image formation: thick areas, areas in which heavy atoms are enriched, and crystalline areas appear with dark contrast. DF: The direct beam is blocked by the aperture while one or more diffracted beams are allowed to pass the objective aperture. Since diffracted beams have strongly interacted with the specimen, very useful information is present in DF images, e.g., about planar defects, stacking faults or particle size

24 BF vs DF TEM BF and DF images of the same area of microcrystalline ZrO 2. In the BF image (centre), some crystals appear with dark contrast since they are oriented (almost) parallel to a zone axis (Bragg contrast). Thickness contrast also occurs: areas close to the edge are thinner and thus appear brighter (lower right side) than those far of the edge (upper left side). In the DF image (right), some of the microcrystals appear with bright contrast, namely such whose diffracted beams partly pass the objective aperture

25 High-Resolution TEM Imaging HRTEM: A large objective aperture has to be selected that allows many beams including the direct beam to pass. The image is formed by the interference of the diffracted beams with the direct beam (phase contrast). If the point resolution of the microscope is sufficiently high and a suitable crystalline sample oriented along a zone axis, then high-resolution TEM (HRTEM) images are obtained. In many cases, the atomic structure of a specimen can directly be investigated by HRTEM. This corresponds to columns of atoms along the zone axis

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27 TEM vs STEM (Scanning TEM) Specimen STEM mode: The electron beam is rastered across the specimen and the transmitted electrons are detected by various (annular) detectors. Undiffracted beam is detected by the Bright Field (BF) detector. Diffracted beams are detected either by the Annular Dark Field (ADF) detector or High Angle ADF (HAADF) detector. Signals from BF, ADF, HAADF can be used to provide info about material type (composition and structure), orientation (diffraction) and topography

28 FePt Alloy Nanoparticles for Biosensing: Enhancement of Vitamin C Sensor Performance and Selectivity by Nanoalloying Nafiseh Moghimi, K.T. Leung* WATLab, and Department of Chemistry University of Waterloo Waterloo, Ontario, Canada N2L3G

29 Limitations of TEM Sampling: The higher the resolution, the smaller the amount of materials that the TEM examines. The total amount of materials sampled by TEM in the past 15 years is no more than 0.3 mm 3 very tiny! 2D projection of a 3D specimen: The image contains columns of atoms from the top surface, middle section, and the bottom surface. Not really true 2D and definitely not 3D Tomography is being developed to overcome this problem but this requires large specimen rotation (> 120 deg) to get full 3D info. Electron beam damage: The energy of the e-beam is large enough to displace atoms in the specimen. Most materials (organic, biological, polymer, many ceramics) will suffer radiation damage above 80 kev. Specimen preparation: The quality of the info obtained greatly depends on how and how good the specimen is prepared. Specimen thickness should be less than 100 nm generally, but the thinner the specimen the better the quality of the image. Methods include slicing, fracturing, ion milling, electrochemical polishing, focussed ion beam (FIB) preparation 24-29

30 WATLab TEM: Zeiss Libra 200MC for-materials.html 24-30