Conventional TEM

Conventional transmission electron microscopes (CTEM or TEM) are electron optical instnunents analogous to light microscopes, where the specimen is illuminated by an electron beam. This requires operation in a vacuum because air scatters electrons. High resolution 

The constant B is determined by observing films of known thickness under the same standard conditions [51, 56, 57]. 

In phase contrast imaging, scattered electrons are allowed to pass through the objective 

Deliberate defocusing enhances phase contrast at lower magnifications but it must be used with caution. If there is only random structure in the specimen, deliberate or accidental defocus may induce clearly visible structure unrelated to the specimen—artifacts. Thomas [59] discussed this in detail for polymer microscopy, quoting several TEM studies of polymers that were dominated by phase contrast artifacts. With care, artifacts can be recognized [63, 64] and phase contrast imaging can be successfully applied to polymer systems (e.g., [65]). Phase contrast at high resolution produces lattice images (see Section 2.4.4 and Section 3.1.5). 

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The construction of an aberration-corrected TEM proved to be teclmically more demanding the point resolution of a conventional TEM today is of the order of 1-2 A. Therefore, the aim of a corrected TEM must be to increase the resolution beyond the 1 A barrier. This unplies a great number of additional stability problems, which can only be solved by the most modem technologies. The first corrected TEM prototype was presented by Flaider and coworkers [M]- Eigure BE 17.9 shows the unprovement in image quality and interpretability gained from the correction of the spherical aberration in the case of a materials science sample. 

The higher the operating voltj e of a TEM instrument, the greater its lateral spatial resolution. The theoretical instrumental point-to-point resolution is proportional to This su ests that simply going from a conventional TEM... 

There are three primary image modes that are used in conventional TEM work, bright-field microscopy, dark-field microscopy, and high-resolution electron microscopy. In practice, the three image modes differ in the way in which an objective diaphragm is used as a filter in the back focal plane. 

The annular dark-field detector of the field-emission STEM (see Figure 2) provides a powerful high-resolution imaging mode that is not available in the conventional TEM or TEM/STEM. In this mode, images of individual atoms may be obtained, as shown in Figure 4 (see Isaacson, Ohtsuki, and Utlaut ). Some annular dark-field... 

Figure 5 Images of a thin region of an epitaxial film of Ge on Si grown by oxidation of Ge-implanted Si (a) conventional TEM phase contrast image with no compositional information and b) high-angle dark-field STEM image showing atomically sharp interface between Si and Ge. (Courtesy of S.J. Pennycook)...

Dravid et al. examined anisotropy in the electronic structures of CNTs from the viewpoint of momentum-transfer resolved EELS, in addition to the conventional TEM observation of CNTs, cross-seetional TEM and precise analysis by TED [5]. Comparison of the EEL spectra of CNTs with those of graphite shows lower jc peak than that of graphite in the low-loss region (plasmon loss), as shown in Fig. 7(a). It indicates a loss of valence electrons and a change in band gap due to the curved nature of the graphitic sheets. 

TEM is still the most powerful technique to elucidate the dispersion of nano-filler in rubbery matrix. However, the conventional TEM projects three-dimensional (3D) body onto two-dimensional (2D) (x, y) plane, hence the structural information on the thickness direction (z-axis) is only obtained as an accumulated one. This lack of z-axis structure poses tricky problems in estimating 3D structure in the sample to result in more or less misleading interpretations of the structure. How to elucidate the dispersion of nano-fillers in 3D space from 2D images has not been solved until the advent of 3D-TEM technique, which combines TEM and computerized tomography technique to afford 3D structural images, incidentally called electrontomography . 

Transmission electron microscopy (TEM) is a powerful and mature microstructural characterization technique. The principles and applications of TEM have been described in many books [16 20]. The image formation in TEM is similar to that in optical microscopy, but the resolution of TEM is far superior to that of an optical microscope due to the enormous differences in the wavelengths of the sources used in these two microscopes. Today, most TEMs can be routinely operated at a resolution better than 0.2 nm, which provides the desired microstructural information about ultrathin layers and their interfaces in OLEDs. Electron beams can be focused to nanometer size, so nanochemical analysis of materials can be performed [21]. These unique abilities to provide structural and chemical information down to atomic-nanometer dimensions make it an indispensable technique in OLED development. However, TEM specimens need to be very thin to make them transparent to electrons. This is one of the most formidable obstacles in using TEM in this field. Current versions of OLEDs are composed of hard glass substrates, soft organic materials, and metal layers. Conventional TEM sample preparation techniques are no longer suitable for these samples [22-24], Recently, these difficulties have been overcome by using the advanced dual beam (DB) microscopy technique, which will be discussed later. 

In the present paper non-conventional TEM methods to characterize small metallic particles are presented. The topographic information on the particles shape can be combined with micro-diffraction (using STEM) data to obtain a full characterization of the particle. The case of gold particles evaporated on a NaCl substrate is used as example. The particle shapes observed are discussed. It is shown that many particles have a crystal structure which is different from the bulk (Fee). 

The three-dimensional electron tomographical construction of silica-supported metallocene catalysts using conventional TEM (Steinmetz et al 2000), and a novel method for the automated acquisition of tilt series for electron tomography of nanoparticles using STEM have been reported (Zeisse et al 2000). The HAADE-STEM is shown to be capable of determining the compositions of individual nanoparticle catalysts of a few atoms supported on porous substrates (Vaughan et al 1999). 

Earlier work in the literature on the defect thermodynamics of oxides containing CS planes is based on conventional TEM studies of CS planes on static oxide systems. However, much of the earlier work contains the implicit assumption that all point defects due to the oxide anion loss are eliminated to produce CS planes. Several workers have made important contributions to understanding defect thermodynamics in oxides containing a finite number of... 

CS planes using conventional TEM methods (e.g. Merritt and Hyde 1973) and by calculations using lattice potential models (Catlow et al 1978, Cormack et al... 

II. Transmission Electron Microscopy (Limitations of Conventional TEM in the Characterization of Catalysts)... 

Microscopes. There are two basic modes of operation for X-ray analysis in a modern-day AEMs with a static (or flood) beam and with a rastered beam. This instrument is essentially a conventional TEM with either (a) scanning coils to raster and focus the beam or (b) an extra NminiN (or objective pre-field) condenser lens to provide a small (nm-sized) cross-over of a static beam at the objective plane. Some AEM configurations contain both scanning coils and a third condenser lens whilst others may have only one of these. In either condition, a small-sized electron probe can be obtained as a static or a rastered beam. The basic electron-optical principles which provide nanometer-sized beams for microanalysis are similar to those for electron microdiffraction which are well described by Spence and Carpenter [19]. 

Although freeze-fracture TEM provides direct visualization of ME structures, it is not currently in wide use probably due to the experimental difficulties associated with the technique. The points to consider when preparing conventional TEM replicas are the physical and chemical sample properties, freezing, cleaving, etching, replication, cleaning, and mounting steps of the procedure. 

Cmcial to the success of ab initio stmcture determinations is the collection of 3-D diffraction data. The conventional TEM sample holders can handle limited rotation... 

The majority of STEM instruments are simply conventional TEMs with the addition of scanning coils. As a result, these nondedicated STEMs are capable of TEM/STEM, as well as SEM imaging for thicker samples. The development... 

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