24 Transmission Electron Microscopy

Introduction

The optical microscope possesses a limitation on its ability to resolve an object. TEM (Transmission Electron Microscopy) refers to a technique built on to get the magnification and thus better level details of a sample as compared to the conventional optical microscopes. In TEM, an electron beam passes through an ultrathin sample that interacts with sample surface as it passes. The electron beam having accelerated electrons with higher energy levels (a few hundred keV) are focused on a material then the electrons are dispersed or back dispersar elastically or inelastic or produce many interactions, providing different signals sources such as X-rays, Auger electrons or light and some of these are used in transmission electron microscopy (TEM). The interaction of the electrons transmitted through the sample forms an image which is enlarged and focused on an imaging device, such as a fluorescent screen or on a layer of photographic film or detected by a sensor such as a CCD camera.

Quantum mechanical properties of an electron have been taken into consideration while development of TEM. The intrinsic nature of an electron (a quantum mechanical entity) is responsible for its interaction with material, which could be obtained using TEM. Because electrons have the nature of wave and particle both, and the wavelength of electrons de Broglie is significantly lower than that of light, they have a greater capacity for resolution. This allows the user of the instrument to examine fine details, even as small as a single column of atoms, which is tens of thousands of times smaller than the smallest object that can be resolved in an optical microscope.

Working Principle

Working of TEM is similar to a slide projector which emits a ray of light that is transmitted by the slide. The motifs painted on the slide allow the passage of some parts of the beam of light. Therefore, the transmitted beam reproduces the patterns on the slide, forming an enlarged image when it falls on the screen as shown in Figure 1. In TEMs, a beam of electrons (like light in a slide projector) passes through the sample (like the slide). Moreover, in TEM, the electron beam transmission depends to a large extent on the properties of the material being tested. These properties are density, composition, etc. For example, the porous materials have a tendency to pass more electrons as compared to the dense materials.

1. Electron beam source-

The primary part of TEM is the electrons beam source which is a filament made up of LaB6 or tungsten (W) and V is shape capped by Wehnelt cap as shown in Figure 2. When the Negative potential is applied to the electrode, a small area of the filament known as point source emits a beam of electrons. This point source is very essential as emits monochromatic electron beam with same energy level. Simultaneously a positive electrical potential is applied to the anode, and filament is heated till a beam of electrons is emitted which is further accelerated towards the anode in the column as shown in Figure 2. A compilation of electrons occurs in the space between the Filament tip and Wehnelt cap known as space charge. Electron beam at the bottom of the space charge (nearest to the anode) can exit the gun area through the small (< 1mm) hole in the Wehnelt Cap and then move down to the column through the anode plate which is used to form the image.

2. Condenser Lens

The electrons beam from the electron beam source is made to focus on a thin and small sample by using the condenser lenses as shown in Figure 1. These lens are substitute of optical lenses in optical lenses which make the electron beam coherent and can be easily focused on the sample area. The first condenser lens helps in determining the “spot size” i.e. the general size range of the final spot that strikes the sample and the second condenser lens helps in changing the spot size on the sample surface.

3. Condenser Aperture

A condenser aperture is a thin disk which made up of a metal with a small circular through-hole. The main purpose of the condenser aperture is to limit the electrons beam and filter out unsolicited scattered electrons before the formation of sample image.

4. Sample

Electrons beam from the condenser aperture finally strikes the sample and this interaction takes place in three different ways by: (i) unscattered electrons (transmitted beam), (ii) elastically scattered electrons (diffracted beam) and (iii) inelastically scattered electrons.

5. Objective Lens

Objective lens is used to focus the electrons transmitted from the sample onto the screen where it forms the image of the sample.

6. Objective Aperture

Objective aperture is used to increase the contrast of the image by obstructing the electrons that are diffracted at high-angles.

7. Selected Aperture

Selected Aperture is used to analyse the periodic diffraction of electron by ordered arrangements of atoms in the sample.

8. Projector Lens

The projector lens expands the transmitted beam onto the phosphor screen.

9. Imaging Screen

TEM comprises of an Imaging systems made up of phosphor screen to finally get the image of the sample. It is made of fine (10-100 micro meter) particulate of zinc sulphide.

10. Image Pattern

Finally the transmitted electrons are made to strike on the phosphor screen where they form an image by generating light. Darker area of the image represents that part of the sample which allowed fewer electrons to pass through and the lighter area correspond to the area having more transmittance towards electrons.

Sample Preparation

The TEM samples have to be thin so that there should not be any absorption of electrons in the material itself for this Sample preparation is very important step in TEM. A special care must be taken while cutting a thin slice of the sample so as to protect it from getting deformed while its preparation. Because plastic deformation can introduce useless structural defects in the microstructure that are visible in TEM images. The modes of cutting the sample includes: i) Spark Cutter: – In this, electric discharge between a wire and the specimen is used to cut the metal by getting rid of small particles of metal from the surface of the specimen, ii) Foused Ion Beam:- Thin sliced sample which is cut by using an ion beam on a scanning ion microscope. The main benefit of this method is that it makes way for thinning of the sample at desired locations by cutting trenches in the sample.

Figure 3 shows the important steps involved in which first the sample is cut into circular shape of 3 mm diameter. Then the mechanical grinding followed by polishing is carried out to reduce the thickness of sample to 100 microns. Dimpling at centre of the sample is further performed by using the grinding wheel which leaves the thickness of the sample to be 20 microns from the centre. Finally the ion milling is done at the centre of the sample to further reduce the thickness and the sample is transparent in this region where the electrons beam is transmitted. This step is very crucial as over of ion milling process may completely damage the sample.

Chemical Analyses by EDS

The foremost step in identification of phase just before the analysis of the diffraction pattern is the chemical analysis of the sample which is done by TEM microscope with the aid of X-rays energy dispersive spectrometry (EDS) or electron energy loss spectrometry (EELS). EELS has many important advantages such as it helps in getting the chemical binding information and its good spatial resolution but it is mainly utilized for light elements (Z < ZAl), but to identify the chemical elements and the spectra interpretation is not simple as in the case of EDS where all that is required is an easy and less time consuming method for identifying and quantifying the elements owing to a user-friendly software. Energy of X-ray corresponds to a difference between two energy levels of the electron cloud of an atom which are quantified in return this provides the X-ray energy spectrum representing the signature of the atom. This X-rays spectrum is detected by a semi-conductor and processed by a detector protected by an ultrathin window which is cooled using liquid nitrogen for avoiding the noise produced by thermal heat generated and the diffusion of the dopant in the semi-conductor.

Electron Scattering: From Diffusion to Diffraction Diffusion

The electrons are quantum relativistic particles, which is described by using the popular Dirac equation. Further neglect the interactions between the electrons, the equation for an electron before its interaction with the crystal is,

Where, Ψ(r) is the wave function of the electron and k the wave vector of the electron which is linked to the tension of acceleration U by

Figure 4 shows the scattering of electron. The solution to the Schrodinger equation is a plane wave equation (non-localisation plane of the electron) is:

During the interaction of the electron with the crystal the equation is ,

where V(r) is the potential of the crystal. This differential equation can be expressed as an integral form in the entire volume of the crystal by using the Green’s function which is given by.

The new equation formed is called the diffusion equation and is given by

Where, k is vector respecting |k| = |k(0)|. Now, one can infer from this equation that the crystal potential in r’ diffuse the electron wave in the k direction by the intermediate of a spherical wave and a transmission factor given by (2πme/h²)V(r). This is closely identical to the optical Huygens approach and its general Kirchhoff’s formulation.

Effects of screening are neglected while doing all the calculations. Selected Area diffraction (SAED) is the conventional diffraction mode used in TEM. Spherical aberrations of the objective lens are limited to the area of the selected object to few hundred nm. But it is still possible to get the diffraction pattern of smaller object by focusing the electrons beam with the projector lenses in order to obtain a smaller spot size on the sample surface. SAED spots become small disks with their radii depending on diaphragm of the condenser which is also called micro-diffraction. These patterns of a crystal allow to understand the symmetry of lattice of the sample which also helps in calculating the inter-planar spacing using the well known Bragg’s law.

Limitations of TEM

There are a few limitations of TEM. Extensive sample preparation is required in many materials in order to get a thin sample which is enough to be electron transparent, this makes TEM process a time consuming process with a low throughput of samples. Sometimes the original structure of the sample may also changes while the sample preparation process. Since the field of view is very less which may raise the possibility that the region which is being analysed may not be depicting the characteristic of the sample. Electrons beam may sometimes destroy the sample surface especially in biological samples.

Modifications

The capabilities of the TEM can be further extended by additional stages and detectors. An electron cryomicroscope (CryoTEM) is a TEM with a sample holder capable of maintaining the sample at lower temperature down to liquid nitrogen. A TEM can be modified in a scanning electron microscope (STEM) by adding a system that combines with appropriate detectors. In-situ experiments can also be performed with experiments such as in-situ reactions or material deformation tests. Incident beam monochromators can also be used to reduce the electron beam incidence to less than 0.15 eV. The main TEM manufacturers include JEOL, Hitachi High-technologies, FEI Company (from the merger with Philips Electron Optics), Carl Zeiss and NION.

Conclusion

TEM has been used for several purposes in several fields. However, there are many disadvantages and scope of improvement in TEM. The sample material will be produced with a sample thin enough to be transparent to the electrons, which makes TEM analysis a relatively slow process with low sample throughput. The structure of the sample can be changed during the preparation process. In addition, the field of vision is relatively small, which raises the possibility that the region is characteristic of the entire sample. There is the potential that the sample may be damaged by the electron beam, especially in the case of biological materials. However, TEM is a significant achievement of the quantum mechanical understanding of the electron. It was an important step in the studies of increase and obtaining a better resolution than optical microscopes and is now very important in the study of the application of quantum mechanics. In addition, it is very commonly used in studies of structures and properties of materials and for other experimental purposes.

References:

1. Transmission Electron Microscopy by Williams, David B., Carter, C. Barry
2. The Handbook of surface imaging and visualization by A. Hubbard