3.2. Sample Preparation

The first step is to decide whether the sample is useful to be observed and in which view, planar or cross-section. Due to the strong interaction between electrons and matter, the specimens have to be rather thin, less than 100nm. This is achieved with several methods, depending on the material. In general, mechanical thinning is used to thin and polish the sample. Then it is glued with epoxy glue on a really small and round holder. Whereas TEM data come from the edges of a hole in the centre of the specimen, in sample preparation, the hole is created by the method of ion thinning. Ion thinning is a method where a specimen is irradiated with beams of Ar ions and after a period of time a hole is created. To minimize the damage created, the embedded sample can first be coated with a metal deposition layer. Consequently, sample preparation is a precise and a severe procedure, which may affects the results of the microscopic analysis and study.

 

3.3. Reactions Exploited In TEM

TEM exploits three different interactions of electron beam-specimen; Unscattered electrons (transmitted beam), elastically scattered electrons (diffracted beam) and inelastically scattered electrons. When incident electrons are transmitted through the thin specimen without any interaction occurring inside the specimen, then the beam of these electrons is called transmitted. The transmission of unscattered electrons is inversely proportional to the specimen thickness. The areas of the specimen that are thicker will have lesser unscattered electrons that are transmitted and so will appear darker, conversely the thinner areas will have more transmitted electrons and, thus, will appear lighter. Another component of the incident electrons are scattered (deflected from their original path) by atoms in the specimen in an elastic fashion (no loss of energy). These scattered electrons are then transmitted through the remaining portions of the specimen.

All electrons follow Bragg's Law and, thus, are scattered according to

n∙^.λ=2∙dsin(θ)

Where

λ is the wavelength of the rays

θ is the angle between the incident rays and the surface of the crystal and

d is the spacing between layers of atoms.

All incident electrons possess the same energy and wavelength and enter the specimen normal to its surface. All incident electrons that are scattered by the same atomic spacing will be scattered by the same angle. These scattered electrons can be collated using magnetic lenses to form a pattern of spots; each spot corresponding to a specific atomic spacing (a plane). This pattern can then yield information about the orientation, atomic arrangements and phases present in the area being examined. Finally, another way that incident electrons can interact with the specimen is in- elastically. Incident electrons that interact with specimen atoms in an inelastic fashion, looses energy during the interaction.

 

3.4. Limitation

There are numerous downsides to the TEM technique. Many materials need extensive sample preparation to generate a sample thin enough to be electron transparent, which makes TEM analysis a relatively time consuming procedure with a low throughput of samples. The structure of the sample may also be altered during the preparation process. Also the field of view is relatively small, increasing the possibility that the region analyzed may not be characteristic of the whole sample. There is potential that the sample may be destroyed by the electron beam, particularly in the case of biological materials.