Chapter 3 Transmission Electron Microscopy (TEM) Today, let’s study TEM Why TEM? Higher resolution. As high as 0.1-0.3nm. Allow for the study of internal structures of materials. Electron diffraction patterns allow for structural analysis. What is TEM? Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen. Outlines 3.1 Applications of TEM. 3.2 Brief working principle of TEM. 3.3 Instrumentation. 3.4 Sample Preparation. 3.5 Selected area electron diffraction (SAED) What is TEM used for?  Morphology The size, shape and arrangement of the particles which make up the specimen as well as their relationship to each other on the scale of atomic diameters.  Crystallographic Information The arrangement of atoms in the specimen and their degree of order, detection of atomic-scale defects in areas a few nanometers in diameter  Compositional Information (if so equipped) The elements and compounds the sample is composed of and their relative ratios, in areas a few nanometers in diameter Applications of TEM— Morphology ZnO Nanowires Applications of TEM—Morphology Carbon nanotubes Applications of TEM—Morphology Crystallographic Information Crystallographic Information Crystallographic Information CdSe quantum dot Crystallographic Information Selected area diffraction patterns Now, Let’s think about: How can these TEM images be produced? Do you still remember how SEM images are produced? Electron-Solid Interactions When electrons passing through a specimen, will they change their direction or not? Because of the wave-particle duality, the electrons will be scattered or diffracted! Rutherford Scattering Mass-thickness contrast More electrons are scattered in thick than in thin areas. More electrons are scattered in heavy atoms. Contrast= (I1-I 2)/I1 Thickness contrast Mass Contrast Heavy metal staining in biomaterials and Polymers. Wave-particle Duality of Electrons Bragg’s Law n λ = 2 d sin θ 0 < Θ < 2. Eg: 100kV, l=0.037Å sinq = l/2dHKL=10-2, q≈10-2<1o Electron diffraction Diffraction contrast If the sample has crystalline areas, many electrons are strongly scattered by Bragg diffraction ,and this area appears with dark contrast in the BF image as well. Diffraction contrast is affected by defects, bending, dislocations etc. It can reflect the crystal information of a sample. Diffraction contrast image Phase contrast Phase contrast : non-uniform distribution of electrons based on interference between direct wave and phase shifted diffracted waves (multi-beam imaging or high-resolution TEM) Working modes of SEM Imaging modes: – Bright field mode – Dark field mode – High resolution image mode Diffraction mode: Bright field mode  An aperture is placed in the back focal plane of the objective lens which allows only the direct beam to pass.  In this case, the 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 Dark field mode  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. Examples of BF and DF images High resolution lattice images – 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) Let’s see how TEM works! How TEM works? - A electron beam is focused by 2 condenser lenses, restricted by a condenser aperture; - The beam strikes a specimen and part of it is transmitted; - This transmitted portion is focused by objective lens into an image; - The image is passed down through enlarge lenses and a projector lens, being enlarged all the way; - The image strikes the phosphor image screen and light is generated, allowing user to see the image. Overview of a TEM Instrument Four Main Components: •Illuminating System •Specimen Manipulation System •Imaging System •Vacuum System TEM vs. OM: Similarities  Illumination system: produces required radiation and directs it onto the specimen.  Specimen stage: situated between the illumination and imaging systems.  Imaging system: Lenses which together produce the final magnified image of the specimen. Consists of i) an objective lens and ii) the projector lens.  Image recording system: Converts the radiation into a permanent image (typically on a photographic emulsion) that can be viewed. TEM vs. OM: Differences  Optical lenses are generally made of glass with fixed focal lengths, whereas magnetic lenses are constructed with ferromagnetic materials and windings of copper wire producing a focal length which can be changed by varying the current through the coil.  Magnification in the OM is generally changed by switching between different power objective lenses. In the TEM the magnification (focal length) of the objective remains fixed while the focal length of the projector lens is changed to vary magnification. TEM vs. OM: Differences  Mechanisms of image formation vary (phase and amplitude contrast).  TEMs are generally constructed with the radiation source at the top of the instrument: the source is generally situated at the bottom of OMs.  TEM is operated at high vacuum (since the mean free path of electrons in air is very small) so most specimens (biological) must be dehydrated (i.e. dead !!).  TEMs can achieve higher magnification and better resolution than OMs. Materials Sample Preparation for TEM Electron Transparency (thickness < 100 nm) Initial form of the material: particulate or bulk material Particulate materials (powders, nanoparticles, nanowires) Desirable particle size is about 500 nm or less. Powders that are more coarse than this should be ground with a mortar and pestle. The specimen preparation consists in transferring a suspension of the particles in a solvent such as isopropanol to a carbon coated grid and letting the solvent evaporate Bulk material Simple metals or single-phase alloys can often be electro polished with an appropriate electrolytic solution. Even multiple phase alloys can sometimes be prepared in this fashion. More commonly, samples from bulk material are thinned with an ion beam. Before the final ion-beam thinning, however, the sample should be first mechanically thinned by lapping and polishing so that the final thickness in the center of the sample is about 30 microns Supporting Film TEM Sample Preparation  Ion Milling  Electrochemical polishing  Focused Ion Beam Technique TEM Sample Preparation: FIB TEM Sample Preparation: FIB TEM Sample Preparation: FIB TEM Sample Preparation: FIB TEM Sample Preparation: FIB TEM Sample Preparation: FIB TEM Sample Preparation: FIB 5 nm 50 nm Pan, Liu, Fujita, et al. Phys. Rev. B (2010).