Transmission electron microscopy imaging principle

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. 

Tip-sample interactions 36, 195—210 force and deformation 37 local modification of sample wavefunctions 195 uncertainty principle, and 197 wavefunction modification 37 Topografiner 44—47 Topographic images 122, 125 Transient response 261, 262 Transition probability 67 Transmission electron microscopy 43... 

The principle of the scanning transmission electron microscope (STEM) is, at first glance, very different from that of the transmission electron microscope the electrons are focused on a probe scanned on a sample and the transmitted electrons are detected on a scintillator via a collection aperture. There is, however, a so-called reciprocity relationship between transmission electron microscopy and the STEM that can be used to describe image formation using the same formalism and facilitates the understanding of contrast. 

The classification scheme shown is not definite. For example, the distinction between NPs and clusters cannot be established on the basis of dimensional criteria. Although the term cluster is used for small Au NPs [34], in principle, they are characterized by a well-defined structure [35], while the mobility of the surface atoms in the NPs does not allow one to ascribe them an exact geometrical shape. Similarly, although transmission electron microscopy (TEM) images depict carbon black particles as spherical and they are thus classified as OD nano-objects, they actually consist of disordered graphene sheets. 

Transmission electron microscopy (TEM) is a powerful method for imaging ultrafine structures of materials. In principle, TEM apparatus provides high resolution enough to observe molecules in subnanometer scale. However, it is not so easy, in practice, to apply TEM for imaging supermolecules on an atomic level owing to their thermal oscillation under the measurement conditions. Thus, TEM analysis of supermolecules has been generally discussed on a nanometer scale up to the present time. 

---