28 High Resolution Transmission Electron Microscopy (HRTEM)

Dr. Anchal Srivastava

Contents of this Unit

1.  Introduction

1.1 Transmission Electron Microscopy

1.2 Why High Resolution Transmission Electron Microscopy?

2. Construction and Working Principle

2.1 Basic Structure of TEM

2.2 The electromagnetic lenses and Aberration corrected TEM

2.3 Observing and recording of images

2.4 Vacuum

2.5 Specimen preparation

3. Image Types

4. High Resolution TEM Imaging

5. Summary

Learning Outcomes

• After studying this module, you shall be able to learn
• The basic principles of TEM operation; The origin of images and their formation;
• To understand procedures for collecting and interpreting images; Preparing samples for investigation.

1.INTRODUCTION

Most microscopes can be classified as one of three basic types: optical, charged particle (electron and ion), or scanning probe. Optical microscopes are the ones most familiar to everyone from the high school science lab or the doctor’s office. They use visible light and transparent lenses to see objects as small as about one micrometer (one millionth of a meter), such as a red blood cell (7 μm) or a human hair (100 μm). Electron and ion microscopes, use a beam of charged particles instead of light, and use electromagnetic or electrostatic lenses to focus the particles. They can see features as small a tenth of a nanometer (one ten billionth of a meter), such as individual atoms. Scanning probe microscopes use a physical probe (a very small, very sharp needle) which scan over the sample in contact or near-contact with the surface. They map various forces and interactions that occur between the probe and the sample to create an image. These instruments too are capable of atomic scale resolution.

1.1 TRAMSMISSON ELECTRON MICROSCOPE (TEM)

A transmission electron microscope (TEM) is an analytical tool allowing visualization and analysis of specimens in the realms of microspace (1 micron/1µm = 10-6m) to nanospace (1 nanometer/nm = 10-9m). The TEM reveals levels of detail and complexity inaccessible by light microscopy because it uses a focused beam of high energy electrons. It allows detailed micro-structural examination through high-resolution and high magnification imaging. It also enables the investigation of crystal structures, specimen orientations and chemical compositions of phases, precipitates and contaminants through diffraction pattern, X-ray and electron-energy analysis.

Transmission electron microscopy is used to produce images from a sample by illuminating the sample with electrons (i.e. the electron beam) within a high vacuum, and detecting the electrons that are transmitted through the sample. Ultimately, using a TEM we can see the columns of atoms present in crystalline samples.

The transmission electron microscope (TEM) provides the user with advantages over the light microscope (LM) in three key areas:

1. Resolution at high magnification. Resolution can be defined as the smallest distance between two closely opposed points, at which they may be recognized as two separate entities. The best resolution possible in a LM is about 200 nm whereas a typical TEM has a resolution of better than 1 nm.

2. Structural information. If the material being viewed has a periodic structure like a crystal then the beam can interact with that structure in such a way that it diffracts. This provides information on crystal structure, symmetry and orientation of materials.

3.  Microanalysis i.e. the analysis of sample chemical composition can be performed in the TEM.

1.2 WHY HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPE(HRTEM)?

New developments for increasing the resolution in high resolution electron microscopy are the use of a monochromator and a spherical abb reation Cs corrector to reach point resolutions below 0.5Å, such TEM are these days called HRTEM. The image mode in High-resolution transmission electron microscopy (HRTEM) or (HREM) allows for direct imaging of the atomic structure of the sample.

The high-resolution transmission electron microscopy (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. HRTEM has been extensively and successfully used for analyzing crystal structures and lattice imperfections in various kinds of advanced materials on an atomic resolution scale. It can be used for the characterization of point defects, stacking faults, dislocations, precipitates grain boundaries, and surface structures. The construction and working principle of HRTEM is similer to Transmission Electron Microscope.

2.  CONSTRUCTION AND WORKING PRINCIPLE

The transmission electron microscope can be compared with a slide projector. In a slide projector light from a light source is made into a parallel beam by the condenser lens; this passes through the slide (object) and is then focused as an enlarged image onto the screen by the objective lens.

Fig. 1 The transmission electron microscope compared with a slide projector

In the electron microscope, the light source is replaced by an electron source, the glass lenses are replaced by magnetic lenses, and the projection screen is replaced by a fluorescent screen, which emits light when struck by electrons, or, more frequently in modern instruments, an electronic imaging device such as a CCD (charge-coupled device) camera. The whole trajectory from source to screen is under vacuum and the specimen (object) has to be very thin to allow the electrons to travel through it. Not all specimens can be made thin enough for the TEM.

2.1 Basic Structure of TEM

The electron beam emerges from the electron gun (usually at the top of the column), and is condensed into a nearly parallel beam at the specimen by the condenser lenses. The specimen must be thin enough to transmit the electrons, typically 0.5 μm or less. Higher energy electrons (i.e., higher accelerating voltages) can penetrate thicker samples. After passing through the specimen, transmitted electrons are collected and focused by the objective lens and a magnified real image of the specimen is projected by the projection lens(es) onto the viewing device at the bottom of the column. The entire electron path from gun to camera must be under vacuum (otherwise the electrons would collide with air molecules and be scattered or absorbed).

Fig 2: The TEM images are formed in two stages: Stage A shows the scattering of an incident electron beam by a specimen. This scattered radiation passes through an objective lens, which focuses it to form the primary image. Stage B shows the primary image obtained in stage A and magnifies this image using additional lenses to form a highly magnified final image.

Electron Source

The first and basic part of the transmission electron microscope is the source of electrons. It is usually a V-shaped filament made of LaB6 or W (tungsten) that is wreathed with Wehnelt electrode (Wehnelt Cap). Due to negative potential of the electrode, the electrons are emitted from a small area of the filament (point source). A point source is important because it emits monochromatic electrons (with similar energy). In this, a positive electrical potential is applied to the anode, and the filament (cathode) is heated until a stream of electrons is produced. The electrons are accelerated by the positive potential down the column, and because of the negative potential of cap, all electrons are repelled toward the optic axis. A collection of electrons occurs in the space between the Filament tip and Cap , which is called a space charge. Those electrons at the bottom of the space charge (nearest to the anode) can exit the gun area through the small (<1 mm) hole in the Wehnelt Cap and then move down the column to be later used in imaging.

CONDENSER LENS

The stream of the electron from the electron gun is then focused to a small, thin, coherent beam by the use of condenser lenses. The first lens determines the “spot size”; the general size range of the final spot that strikes the sample. The second lens actually changes the size of the spot on the sample.

CONDENSER APERTURE

A condenser aperture is a thin disk or strip of metal with a small circular through-hole. It is used to restrict the electron beams and filter out unwanted scattered electrons before image formation.

SPECIMEN/SAMPLE CHAMBER

The beam from the condenser aperture strikes the sample and the electron-sample interaction takes place in three different ways. One is unscattered electrons (transmitted beam), elastically scattered electrons (diffracted beam) and inelastically scattered electrons.

OBJECTIVE LENS

The main function of the objective lens is to focuses the transmitted electron from the sample into an image.

OBJECTIVE APERTURE

Objective aperture enhances the contrast by blocking out high-angle diffracted electrons.

SELECTED APERTURE

It enables the user to examine the periodic diffraction of electron by ordered arrangements of atoms in the sample.

PROJECTOR LENS

The projector lens are used to expand the beam onto the phosphor screen.

SCREEN

Imaging systems in a TEM consists of a phosphor screen, which may be made of fine (10-100 micro meter) particulate zinc sulphide, for direct observation by the operator.

In the process of forming the primary image the objective lens produces a diffraction pattern at its back focal plane. The diffraction pattern is a Fourier transform of the scattered electron wave. The primary image is the Fourier transform of the diffraction pattern.

This two-step process forms the basis of image formation during high-resolution transmission electron microscopy (HRTEM). The high-resolution image is, in effect, an interference pattern of the beams formed at the back focal plane of the objective lens.

2.2 The electromagnetic lenses and Aberration corrected TEM

Fig. 3 shows a cross-section of an electromagnetic lens. When an electric current is passed through the coils (C), an electromagnetic field is created between the pole pieces (P), which forms a gap in the magnetic circuit. By varying the current through the coils, the strength of the field, and thereby the power of the lens, can be varied. This is the essential difference between the magnetic lens and the glass lens. Otherwise they behave similarly and have the same types of aberration (Fig. 4): spherical aberration (Cs – the power in the center of the lens differs from that at the edges), chromatic aberration (Cc – the power of the lens varies with the energy of the electrons in the beam), and astigmatism (a circle in the specimen becomes an ellipse in the image).

Fig.3 shows cross-section of electromagnetic lenses. C is an electrical coil and P is soft iron pole.

In a conventional TEM, spherical aberration, which is largely determined by the lens design and manufacture, is the primary limitation to improved image resolution. Chromatic aberration can be reduced by keeping the accelerating voltage as stable as possible and using very thin specimens. Astigmatism can be corrected by using variable electromagnetic compensation coils. The condenser lens system focuses the electron beam onto the specimen under investigation as much as necessary to suit the purpose. The objective lens produces an image of the specimen which is then magnified by the remaining imaging lenses and projected onto the viewing device.

If the specimen is crystalline, a diffraction pattern will be formed at a point below the objective lens known as the back focal plane. By varying the strengths of the lenses immediately below the objective lens, it is possible to enlarge the diffraction pattern and project it onto the viewing device. The objective lens is followed by several projection lenses used to focus, magnify, and project the image or diffraction pattern onto the viewing device. To guarantee high stability and to achieve the highest possible lens strength/magnification, the lenses in a modern TEM are usually water-cooled.

On the way from the source to the viewing device, the electron beam passes through a series of apertures with different diameters. These apertures stop those electrons that are not required for image formation (e.g., scattered electrons). Using a special holder carrying a number of different size apertures, the diameter of the apertures in the condenser lens, the objective lens, and the diffraction lens can be changed as required.

Fig. 4 Lens aberrations Cs (left) and Cc (right).

The recent development of a dedicated commercial aberration corrected TEM has enabled major advances in both  TEM  and  STEM  capability.  Without  correction,  TEM  resolution  is  limited  primarily  by  spherical aberration, which causes information from a point in the object to be spread over an area in the image. This results not only in a general blurring of the image, but also in a phenomenon called delocalization, in which periodic structures appear to extend beyond their actual physical boundaries. In a light microscope, spherical aberration  can  be  minimized  by  combining  lens  elements  that  have  opposing  spherical  aberrations.  This approach cannot be used in electron microscopes since the round magnetic lenses they use exhibit only positive spherical aberration. Multi-pole correcting elements (with essentially negative aberration) were described by Otto Scherzer in 1947, but their successful commercial implementation required solutions to a number of practical problems; some relatively simple, as for example, increasing the diameter of the electron column to achieve the mechanical stability required to actually see the benefit of improved optical performance; and others very complex, such as designing sufficiently stable power supplies  and developing methods and software controls sophisticated enough to reliably measure and then correct the aberrations by independently exciting the multi-pole elements. The ability to correct spherical aberration leaves the reduction or correction of the effects of chromatic aberration as the next major challenge in improving TEM performance. Chromatic aberration correctors have been successfully incorporated into the Titan™ TEM platform, but their design and operation are substantially more complex than spherical aberration correctors. At the same time, significant progress has been made in reducing the energy spread of electrons passing through the lenses. The energy spread determines the magnitude of chromatic aberration’s deleterious  effects. Variations in electron energy may originate as the beam is formed in the electron gun, or they may be introduced in transmitted electrons by interactions with sample atoms. The first of these, beam energy spread, has been addressed by engineering extremely stable high voltage and lens current power supplies, by using specially optimized field emission electron sources, and by directing the beam through a monochromator, which passes only a very narrow band of energies. The energy spread among electrons transmitted through the specimen can be decreased by minimizing sample thickness using advanced sample preparation techniques.

2.3 Observing and recording of images

Originally, TEMs used a fluorescent screen, which emitted light when impacted by the transmitted electrons, for real-time imaging and adjustments; and a film camera to record permanent, high resolution images (electrons have the same influence on photographic material as light). The screen was under vacuum in the projection chamber, but could be observed through a window, using a binocular magnifier if needed. The fluorescent screen usually hinged up to allow the image to be projected on the film below. Modern instruments rely primarily on solid-state imaging devices, such as a CCD (charge-coupled device) camera, for image capture. They may still include a fluorescent screen, but it may be observed by a video camera. In this text, unless we are discussing specific aspects of the imaging system, we will simply refer to an imaging device.

The recent introduction of a direct electron detector promises significant improvements in image resolution and contrast, particularly in signal-limited applications. A conventional CCD camera uses a scintillator material over the image detector elements to convert incident electrons to light, which then creates charge in the underlying CCD element. The scintillator introduces some loss of resolution and the conversion process decreases the efficiency with which electrons contribute to image contrast. This can be critical in applications that are sensitive to damage by the electron beam, such as cryogenically prepared samples of delicate biological materials, where it is essential to extract the maximum amount of information from a faint, noisy signal before the sample is destroyed. Eliminating the scintillator with a direct electron detector improves image resolution and increases detector efficiency by up to three times.

2.4 Vacuum

Electrons behave like light only when they are manipulated in vacuum. As has already been mentioned, the whole column from source to fluorescent screen including the camera) is evacuated. Various levels of vacuum are necessary: the highest vacuum is around the specimen and in the source; a lower vacuum is found in the projection chamber and camera chamber. Different vacuum pumps are used to obtain and maintain these levels. Vacuum in a field emission electron gun may be as high as (i.e., “pressure as low as”) 10-8 Pa.

To avoid having to evacuate the whole column every time a specimen or photographic material or a filament is exchanged, a number of airlocks and separation valves are built in. In modern TEMs the vacuum system is completely automated and the vacuum level is continuously monitored and fully protected against faulty operation.

2.5 Specimen preparation

A TEM can be used in any branch of science and technology where it is desired to study the internal structure of specimens down to the atomic level. It must be possible to make the specimen stable and small enough (some 3 millimeters in diameter) to permit its introduction into the evacuated microscope column and thin enough to permit the transmission of electrons. Different thicknesses are required for different applications. For the ultimate high resolution materials studies, the sample cannot be thicker than 20 nm or so; for bio-research, the film can be 300-500 nm thick.

3. IMAGE TYPE

Bright field, Dark field and diffraction images

The most common image generated using a TEM is a bright field image. Some areas of the sample scatter or absorb electrons and therefore appear darker. Other areas transmit electrons and appear brighter. In simple terms the bright field image appears as a shadow of the specimen. In the bright field image the objective aperture is used to select the unscattered electron beam. In doing so, the scattered electrons are excluded from forming the image. This aperture enhances the contrast in the image.

Fig 5: Zinc oxide crystals: bright field image

Dark field images are produced by excluding, using the primary aperture, the primary (unscattered) beam from the image collected below the sample. The image is produced by scattered electrons (i.e. only selected electrons are used to form the image). Regions where no scattering occurs, such as where the primary electron beam passes straight through the sample, appear black (e.g. in areas around the sample). This kind of imaging is useful in studying crystal defects, and for the imaging of specific crystallographic phases.

Fig 6: Zinc oxide crystals: dark field image.

Diffraction images are the result of Bragg scattering as the beam passes through a crystalline sample. If a “selected area diffraction aperture” is inserted to delimit the region of interest, then an image created below the sample (in the region called the back focal plane) is seen as an array of dots (or a set of diffuse rings). This informs the viewer about the crystal structure of the sample.

Fig 7: Spot diffraction pattern of Nd13CaO7

4. HIGH RESOLUTION TEM IMAGING

HRTEM images formation is based on the interference between two or more diffracted beams from the electron beam after the interaction with a thin sample. Crystalline materials present well defined diffraction angles, as described by the Bragg’s law, and their interference generates a periodic pattern corresponding to the sample atomic structure. However, HRTEM images cannot be readily interpreted, especially regarding the correlation of bright spots and the atoms positions as the scattering of the incident electron beam (a plane wave) by the atomic potentials in the sample does not only contain linear contributions (single scattering) but also multiple scattering events besides the instruments influence. Figure 8 presents an experimental HRTEM image, the projected crystallographic structure, and the calculated images for different objective lens excitation (defocus).

Figure 8: CeO2 HRTEM image (left) and the projected crystalline structure along the ‹110› direction (right, top). The HRTEM images calculations for two defocus present a contrast reversal, indicating thus that the direct correlation between higher intensity spots on HRTEM images cannot be directly correlated to the atomic positions.

The electron scattering in TEM is described assuming the incident electron beam as an incident plane wave, which amplitude and phase are altered during the interactions with the sample. Actually, round electromagnetic lenses act on electrons alike optical glass lenses on light. This is illustrated in Fig. 9, where the incoming plane electron wave is diffracted at the specimen. Electrons being diffracted by the same angle, i.e. the Bragg angle for crystalline objects, are focused in the back focal plane of the lens and form a diffraction pattern. Τhe actual image observed in the image plane depends on two components (i) the interaction of the electrons with the specimen and (ii) the electron optical imaging process by the electromagnetic lenses of the microscope itself. The incoming plane wave 

is scattered at the atomic potential V(x). At the bottom of the specimen the exit plane is given by a superposition of plane waves:

Ψg is the complex valued Fourier coefficients corresponding to the vector g in reciprocal space (or Fourier space), which is also called spatial frequency. For an ideal periodic object, the integral can be written as a sum over the reciprocal space vectors and their Bloch wave coefficients. For a non-periodic object, such as a defect in a crystal or an interface, the integral has to be solved numerically by solving the relativistically corrected Schrödinger equation

Figure 9: Schematic illustration of the mode of operation of the objective lens. The specimen scatters the incoming plane wave. Beams scattered by the same angle are focused in the back focal plane of the lens and form an interference pattern in the image plane

The exit plane wave is modified by the aberrations of the objective lens, which impose additional phase factor , the so-called aberration function:

This equation implies that:

(i) the high-resolution image is an interference pattern formed by superposition of plane waves;

(ii) the effect of the microscope can be mathematically expressed by a phase shift, which is a function of

the spatial frequency.

Recording electron waves on the CCD camera means measuring intensities. Therefore, as a final result the product of the wave function and its complex conjugate is obtained, i.e. the phase information of the image wave is lost:

Recently we are able to correct and adjust the aberration functions of the TEM by using aberration correctors that open a new ways to obtain directly interpretable images. Figure 10 presents the comparison between the images from the same sample on a conventional TEM and on an aberration-corrected TEM.

Figure 10: Raw HRTEM images from the same CeO2 nanocrystal from a (a) standard TEM microscope (1.25 Å information limit) and from a (b) spherical aberration-corrected microscope (0.8 Å information limit).

4. SUMMARY

In this module you study

TEM has been used for several purposes in several fields and offers many distinct advantages in comparison to SEM and older optical methods as has been discussed earlier. However there are many disadvantages and scope of improvement in TEM. Many materials require extensive sample preparation to produce a sample thin enough to be electron transparent, which makes TEM analysis a relatively time consuming process with a low throughput of samples. The structure of the sample may also be changed during the preparation process. Also the field of view is relatively small, raising the possibility that the region analysed may not be characteristic of the whole sample. There is potential that the sample may be damaged by the electron beam, particularly in the case of biological materials.

TEM is however a significant achievement of quantum mechanical understanding of electron. It was a major step in magnification studies and obtaining better resolution than optical microscopes and hence is a very important building block in study of application of quantum mechanics. Moreover it is very commonly used in studies of material structures and properties and for other experimental purposes