Thin specimens electron-beam interactions

Figure 1 Signals generated when the focussed electron beam interacts with a thin specimen in a scanning transmission electron microscope (STEM).

Although x-ray microanalysis in the STEM is the most developed form of analytical electron microscopy, many other types of information can be obtained when an electron beam interacts with a thin specimen. Figure 2 shows the various signals generated as electrons traverse a thin specimen. The following information about heterogeneous catalysts can be obtained from these signals ... 

In an electron microscope, a stream of electrons is formed by the electron source and is accelerated toward the specimen using a positive electric potential. The stream is focused into a thin, monochromatic beam by using metal apertures and magnetic lenses. The electron beam interacts with the specimen and the effects of these interactions are detected and transformed into an image. 

In a transmission electron microscope, a highly coherent electron beam passes through a thin sample. The electron beam interacts with the sample and is transferred to the specimen s exit plane. The electron wave at the exit plane is magnified in order to form an image or alternatively a diffraction pattern of the sample. 

When a beam of charged particles passes through a thin specimen, the beam transmitted in the forward direction includes some particles that scattered elastically off atomic nuclei or lost energy due to interaction with electrons (inelastically scattered) as well as those particles that were left unscattered. An image formed with this forward-transmitted beam is referred to as a bright field image. 

SEM is a useful technique for the analysis of plastics surfaces. Acmally, it is useful for any surface that survives in a vacuum. Almost all SEMs start by sputtering the surface with a thin layer of gold metal. If it is not already conductive, this makes the surface conductive, which is a requirement so, you are often not looking directly at the surface. It involves a finely collimated beam of electrons that sweeps across the surface of the analysis specimen. The beam is focused into a small probe that scans across the surface of a specimen. The beam interactions with the material result in the emission of electrons and photons as the electrons penetrate the surface. The emitted particles are collected with the appropriate detector to yield information about the surface. The final product of the electron beam collision with the surface topology of the sample is an image (Fig. 4.4). 

Figure 5.1. Processes occurring when a high-energy beam of electrons interacts with a thin specimen. The arrows do not necessarily represent the physical direction of the signal, but indicate the region in which it...

Fig. 27. Signals generated through the interaction of an energetic electron beam and a thin specimen...

TEM is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through it. An image is formed from the electrons transmitted through the specimen, magnified and focused by an objective lens and appears on an imaging screen. A fluorescent screen in most TEMs is detected by a sensor such as a CCD camera. 

We have thus far reported only to the SE signal as a means of generating an image in the SEM. In fact, several other signals are generated as a result of the interaction of the incident electron beam with the specimen. These include backscattered electrons (BSEs), X-rays, light, heat, specimen current, electron beam induced conductivity, transmitted electrons (if the specimen is thin enough), to name the most commonly used. 

The scanning electron microscope (SEM) forms an image by scanning a probe across the specimen, and in the SEM the probe is a focused electron beam. The probe interacts with a thin surface layer of the specimen, a few micrometers thick at most. The detected signal commonly used to form the TV-type image is the number of low energy secondary electrons emitted from the sample surface. Scanning electron microscopy is fully described in several texts [22-26], and its use with polymers has been reviewed by White and Thomas [27]. 

Transmission electron microscopy is able to identify the orientation of molecules within thin films of those thermotropic polymers which are sufficiently resistant to the necessary intense electron beam. The high resolution possible in TEM enables fine scale structures to be characterized which are invisible in a light microscope. Diffraction data can be recorded from the same area as bright and dark field images. However it must be recognised that the requirement for thin specimens can produce structures not necessarily typical of the bulk, particularly since surface interactions may be of importance. 

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