Introduction

As the name transmission electron microscope (TEM) states, the working principle in electron microscopy (such as TEM and SEM) is based on electrons. The roots of electron microscopy methods go back to 1930s, and the major difference between TEM and scanning electron microscopy (SEM) is that TEM illuminates the whole field of the sample whereas SEM uses a focused electron beam to scan a certain area. Structurally TEM is more similar to a transmission light microscope, but can achieve 1 000 times smaller resolution compared to traditional light microscope^{[1, p.83-117]}. Other differences include source of illumination being high-speed electrons instead of visible light, and that the magnification maximum reaches one million times instead of 2 000 ^{[1, p.83-117]}. As such, electron microscopes such as TEM and SEM are greatly superior in resolution and magnification compared to traditional optical microscopes due to the short wavelengths of the electrons^{[1, p.83-117]} and are suited for studying much more refined structures.

Theory

Electrons can interact with the samples in two ways: scattering elastically or inelastically ^{[1, p.83-117][2, p.58]}. Elastic scattering produces back-scattered electrons (BSE) whereas inelastic produces secondary electrons (SE). BSEs are incidentally scattered by the specimen atoms, and typically deflect in larger angles with little energy loss. SEs are ejected from the specimen atoms in smaller angles and less energy as kinetic energy is transferred to the specimen atoms. The major difference is that BSEs come from the microscope electron beam, whereas SEs come from the sample itself.  BSEs are used for elemental composition contrast, and SEs for topographic contrast. A vacuum environment is required in a TEM because collisions between high-energy electrons and air molecules significantly absorb electron energy. Other issues include phonon scattering from the atomic lattice vibrations, and the inner shell electrons being ejected (called Auger-electrons) ^{[2, p.58]}. Same theory can be found in the SEM page in different wording, and schematic image is provided in Figure 1.^{[1, p.83-117]}

Figure 1. Different electron interactions with the sample (Figure: Riia Vidgren).

There are two ways to utilize TEM: image modes and Selected-Area Diffraction (SAD). Image modes are used to produce microscopic images of the sample, whereas SAD can be used for crystal structure analysis similar to XRD. In image modes, the needed contrast for imaging is created not by electron absorption but by deflection, and two methods can be used to create and analyse the electron scattering: mass-density contrast and diffraction contrast. In mass-density contrast, the fact that diffraction interaction between electrons and atomic nucleus is based on local mass-density, creating variations in electron intensity based on sample thickness at certain points. The mass-density contrast can be used for all types of materials, especially for non-crystalline materials. The diffraction contrast (main method used for crystalline samples) works based on collective deflection of electrons by the crystalline planes, creating either bright-field or dark-field images (based on sample brightness and desired level of detail). It is important to note that the optical path for these two modes is slightly different. In SAD, the diffraction mode is utilized to create a diffraction pattern which the image is based on, much like in XRD, but the area examined is much more specific and very small (nm) whereas XRD examination area is typically in cm scale. Overall for TEM, the diffraction angle is much smaller, intensity is higher, and resolution lower compared to XRD. ^{[1, p.83-117]}

Equipment

In TEM the electron beam follows a optical path composed of light source, condenser lens, specimen stage (vacuum environment), objective lens, and projector lens ^{[1, p.83-117][3, p.4]}. This structure can be seen in Figure 2. For the light source, an electron gun is used where the electrons are emitted from a solid surface cathode and accelerated by high voltage to form the final high-energy electron beam. As the electron energy determines the wavelength, the acceleration voltage mostly regulates the resolution of the microscope, e.g., acceleration voltage of 40 kV produces resolution of 0,56 nm, and a voltage of 500 kV correlates to 0,13 nm resolution. Typically, a 200 kV voltage is used, as high-electron voltage microscopy often requires highly expensive equipment, and the high energy might also be damaging to the sample. As often as in microscopy, one must balance the resolution and accuracy, as well as the price and requirements of the sample during characterization. For the different lenses, an electromagnetic lens is used to utilize magnetic force for focusing the electron beam. The different types of lenses include condenser and 'adjusting' lenses. There are two or more condenser lenses for beam diameter and convergence angle control. 'Adjusting' lenses include objective, intermediate and projector lenses that are used for magnification control and changing between imaging and diffraction mode, and an aperture between the electromagnetic lenses to limit light scattering and control the beam diffraction. Finally, for the specimen, it is important to note that it has to be in thin foil form (minimum 100 nm thickness) which is achieved by several different thinning rounds and methods. The sample preparation creates also a major restriction for the samples that can be studied with TEM. ^{[1, p.83-117]}

Figure 2. Simplified schematic of the TEM optical path (Figure: Riia Vidgren).

In TEM, some specifications have to be considered:

• Different sample preparation techniques for different material types (ion milling, electrolytic thinning etc.) ^{[1, p.83-117]}.
• Especially biological samples are sensitive to changes under electron bombardment, and they need extensive pretreatments 
• A special thinning technique called ultramicrotomy can be used for sensitive biological samples ^{[1, p.83-117]}
• Sometimes the samples have to be stained with heavy-metal oxides ^{[1, p.83-117]}

• Typically the usage of TEM is much more expensive than SEM

• HRTEM (high-resolution transmission electron microscope) utilized phase contrast to maximize the local electron diffraction differences and produce a very high quality and detailed image ^{[1, p.83-117]}.

TEM as a tool for zeolite structure characterization

Transmission electron microscopy has been applied for structure characterization of zeolite materials. TEM can be used in structure determination of new zeolites as well as studying the growth mechanisms of nano-sized zeolites. Wan et al.^[4](2018) studied zeolite structures using different TEM techniques. Figure 3 shows TEM images of the growing EMT zeolite crystals^[4].

Figure 3. TEM images showing the nucleation and growing of nano-sized EMT zeolite crystals from colloidal precursors ^[4]. (License: CC BY-NC 3.0)

Further resources

TEM game at Australian Microscopy and Microanalysis Research Facility.

TEM explained in a video at Wikipedia.

References