Background

Electron microscopy is nowadays an extremely important research method capable of providing structural information over a wide size range. It allows to study the texture,

topography and surface features of powder or solid piece samples, at magnifications where it is impossible with any kind of optical (laser) microscopy. With modern scanning electron microscopy (SEM) features up the nanometer scale can be seen, and owing to the depth of focus of the SEM device the resulting pictures have a definite three-dimensional qua the same time, high resolution transmission electron microscopy (HRTEM) is able, under some conditions, to give information on an atomic scale by direct lattice imaging. A resolution of about 0.2 nanometers is achieved , so it is really possible “to see” separate atoms.

lity. At

lectron microscopes are constructed either in transmission or in scanning mode.

.7.2 Transmission Electron Microscopy

transmission mode, the sample should not be thicker than about 200 nanometers. The ason is the absorption of electrons by the sample. It makes sample preparation and

vestigation somewhat difficult. The most common procedure is to grind a sample into a fine owder, which would ensure that at least some of the particles would be thin enough.

hinning techniques could be used, for example, ion bombardment , but this is not always uccessful. Another possible solution could be to use an instrument with a higher voltage (e.g.

MV); thicker samples could be investigated and higher resolution obtained. The principle at the TEM microscope works on is shown in Fig. 2.7.2.1 [26]. Electrons are emitted from e filament (tungsten, LaB6 etc.). Electron microscopes contain several electromagnetic nses. The condenser lenses form and control the size and angular spread of the electron eam. Electrons are accelerated through a high voltage (e.g. 300 kV). Their wavelength is

lated to the accelerating voltage (V) by λ=h(2meV)^-1/2, h is the Planck constant, m and e lectron mass and charge respectively. The electron wavelength is much smaller than that of

e X-rays, which allows the high resolution in electron microscopy.

E 2

In re in p T s 1 th th le b re e th

The electron beam is incident on the sample. Transmitted electrons then pass through bjective, intermediate and projector lenses and form a magnified sample image on the

ranslating the image onto the computer).

Fig. 2.7.2.1 Ray-diagram of a transmission electron microscope. The horizontal arrows

indicate the specimen and its images as they are progressively enlarged at various levels of the microscope. In stage “A” the beam is scattered in the specimen and the primary (diffraction) image is formed. In stage “B” the image is magnified and projected on the image plane.[26]

The quality of the pictures may be improved by dark field imaging. This method is known from optical microscopy, it is only the diffracted beams from the specimen that are allowed to o

screen (the camera t

could be seen directly. By changing of the position of the screen (camera) the electron diffraction pattern of the specimen can be seen.

2.7.3 Selected Area Electron Diffraction

The Selected Area Electron Diffraction (SAED) method can sometimes supply some additional information about unit cell and space group, or be used for phase identification.

However, it is not really reliable because secondary diffraction usually occurs. There are two undesirable consequences: extra spots in the electron diffraction pattern, intensities are unreliable and cannot be used quantitatively for crystal structure determinations.

2.7.4 Electron Energy Loss Spectroscopy

A very important process during the transmission of electrons through a sample is the loss of electron energy. Electron energy loss spectroscopy (EELS) analyses the energy distribution of (initially) monoenergetic electrons after their interaction with a sample. The interaction happens only in a few atomic layers, this is why the measurement should be carried out in a high vacuum. Otherwise only oxide or carbon impurities (from the material of sampler) on the surface of specimens would be measured. When a sample in a microscope is bombarded with high-energy electrons, quite a number of things happen, Fig. 2.7.4.1 [13].

ig. 2.7.4.1 Processes that take place during the bombarding of a sample with electrons [13]

F

Only the primary and secondary processes of the inelastic interaction of an electron beam with the electron shell of sample atoms - during which the electrons of the primary beam lose their energy - are considered here. The atom electrons could be excited only to Fermi l and higher, because all levels below Fermi one a

evel re occupied. They need a relatively large mount of energy for this (from several hundred to a thousand electron volt). The primary

nt energy.

n atom cannot exist for long in a state of excitement. It relaxes when one of the electrons of m of

X-specific. It llows us to use electron energy loss spectroscopy for element analysis [23]. Electron energy

itative and qualitative analysis. Light elements nd themselves especially well to this kind of analysis. The only problem in our case is that

s.

-gion (it follows after the zero-loss-peak up to 50 eV) electrons are found which have interacted with electrons of external shells. The region contains information about the electronic situation of the sample. The estimation of the band gap is possible. The high-loss-region (>50 eV) contains information about the inelastic scattering of primary electrons on the internal shells. The peaks in this area are known as element edges, because their positions in spectra are element specific. The edges are marked as K-edges or L-edges etc., depending on the shell with which primary electrons have interacted. The edges have a very strong

background, due to excitation in elastic scattering. Integrating the edges whilst taking into account the background allows a quantification of elements in a specimen to be made. The a

electrons lost the necessary for the exciteme A

the external shell takes up the electron vacancy, emitting any surplus energy in the for ray radiation. In case an excited electron is not emitted, it can also go back irradiating the energy excess as X-rays. Another possibility is the transfer of energy to an external level electron as kinetic energy. The electron is emitted (auger electron). Because the electron shells are element specific, the energy loss of inelastic interaction is also element a

loss spectroscopy can be used for both quant le

boron and phosphorous edges cannot be distinguished from one another. With the help of EELS information about the electronic situation (band states, coordination and oxidations states) can be obtained. The method can be applied to crystalline and amorphous sample The EEL spectra present the intensity of the scattering of primary electrons as a function of the loss of their kinetic energy. Three areas could be distinguished: zero-loss peak, low-loss-region, and high-loss-region. The zero-loss-peak detects electrons that have not lost any energy; it has the highest intensity and serves for the calibration of spectra. In the low-loss re

2.7.5 Scanning Electron Microscopy

For SEM instruments, on the other hand, sample thickness is not a problem at all, no special methods of preparation are required. Usually it is only necessary to coat a sample surface with a conducting layer (carbon or some metal, very often gold), especially if a sample is a poor electrical conductor, in order to prevent a charge building up on the surface of the sample.

The range of resolution of a SEM lies between the limit of resolution of optical microscopy (about 500 nanometers) and tens of nanometers.

In the scanning electron microscope electrons from the filament (electron gun) are focused on a very small spot on the surface of the specimen. The electron beam scans the whole surface systematically, similarly to the electron gun in a TV-set, secondary electrons are emitted from

f the SEM is lower resolution compared with TEM.

the sample and used to build up an image of the sample surface on a screen. The disadvantage o

Fig. 2.7.5.1 Basic components of a scanning electron microscope [28]

2.7.6 Energy Dispersive X-ray spectroscopy

Today, almost all SEM and TEM microscopes have additional features for the elementa analysis of samples. In the present work it was energy dispersive X-ray analysis (EDX), in which the energy of X-rays generated by bombarding a sample electrons is scanned. Eac element has its own characteristic energies and it is characteristic for each element presents Under appropriate conditions and after calibration, quantitative analysis is also

Unfortunately, for elements lighter than Na the method is not sensitive enough, and in case of boron, it cannot be always discriminated from the carbon energy peak. When t sample is so thin that the carbon peak from the substrate can be observed, or if the s

l h . possible.

the he ample cludes carbon containing ligands, the carbon signal is so strong that a quantitative, and very ften even qualitative analysis of boron is impossible. In this case some other method must be

0

performed on a SEM LEO 1550 upra (Oxford, 30 kV, FEG-filament), equipped with a EDX-spectrometer (Oxford, Si(Li)).

in o

used, for example EELS.

2.7.7 Experimental

The electron energy loss spectra (EELS) were recorded using a TEM CM 30-ST (Philips, 30 kV, LaB6-filament) equipped with a PEELS 666 spectrometer (Gatan) with a YAG

scintillation detector.

High-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and energy-dispersive X-ray analysis (EDX) were performed using a TEM CM300 (Philips, 300 kV, LaB6-filament) equipped with a EDX-spectrometer (EDAX, Si(Li)).

Scanning electron microscopy (SEM) experiments were S