Specimen Thinning for TEM Analysis

As mentioned earlier, once a TEM sample is cut into a thin roughly uniform slice, it needs to be thinned extensively in regions where it will be electron transparent. In extremely rare cases of synthetic materials, the specimen itself can be prepared as a thin film. This is often the technique used to make 

A specimen that is cut into one of the shapes above can subsequently be thinned down to electron transparencies. The most commonly used methods for final thinning can be categorized as described below. 

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The interior of bulk samples can be revealed by fracture, freeze fracture, or (cryo-/ultra)microtoming. These techniques are well established in electron microscopy and require substantial training to ensure that the knives used do not introduce scratches and other artefacts. Unlike in SEM or TEM analysis, however, it is very well possible to analyze the trimmed specimen instead of the very thin sections removed (cryofacing). This loosens the constraint of ultrathin sections in many applications. Care has to be taken that the sample to be imaged is not significantly thicker or thinner than the calibration grating used for scanner calibration (see Sect. 2.2.5)... 

An EDS system can be fitted to a scanning electron microscope (SEM), TEM, or STEM. A TEM offers the ability to undertake analysis in small areas of thin sections of the specimen and is used when high-resolution analysis is required, whereas a SEM is used for the analysis of larger areas of the surface of bulk specimens and, in general, has much lower spatial resolution. EELS is a technique using a TEM or STEM. 

In the very thin deposits that were strained in the [110] and [120] direction TEM analysis revealed the presence of mechanical twins that occurred only near the fmeture line. As for the thin [100] specimens there was again no homogeneous plastic deformation in the gauge length. Only thicker samples showed some homogeneous plastic deformation prior to necking and rupture. 

Ultra-thin sections approximately 35 nm thick were cut from compression-moulded plates with a diamond knife (35° cut angle, DIATOME, Switzerland) at —140 °C on a cryo-microtome and used for transmission electron microscopy (TEM) analysis. The slices were collected on a copper grid with a carbon-hole-foil. The specimens were investigated on a Zeiss Libra 200MC (Zeiss, field emission cathode, point resolution 0.2 nm) with an accelerating voltage of 200 kV. 

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. 

For thin specimens, as used in a TEM, the spatial resolution is of less concern. The lateral size of the emission volume is not significantly larger than the probe size because the thinness of the specimen limits the spread of the emission volume. For example, with a TEM acceleration voltage of 100 kV and probe diameter of 20 nm, the lateral size will be about 23 nm for aluminum. The electron beam size can be reduced to about 0.5 nm in advanced TEM systems. Thus, chemical analysis of nanometer-sized particles can be achieved in TEM systems. 

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