Electron Microscopy studies

2.3.1.

Fundamentals

Electron microscopy is based on the use of an electron beam to illuminate a specimen and produce a magnified image compared to the image obtained by an optical microscope that uses visible light; this is due to the shorter wavelength of electrons (in the order of 10-12m) compared to visible light (4-8x10-7m). Electron microscopy is a general term describing a wide range of different methods that detect the various signals arising from the interaction of the electron beam with a specimen to obtain information about structure, morphology and composition[124].

When a specimen is exposed to a high energy electron beam, the electrons can be scattered either almost elastically (by diverting the electron beam), or inelastically (by transferring some energy to the specimen). As already discussed in Section 2.2.3, electron diffraction detects the elastic interactions of materials with electrons. Signals caused by inelastic interactions include emission of X-rays and ejection of secondary (Auger) electrons as energy releasing mechanisms; many other processes are also possible[123].

Figure 2.3: Some of the processes that occur when a specimen is exposed to an electron beam with energy Eo.

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2.3.2.

Types of electron microscopes

Electron microscopes operate either in a transmission mode (TEM) or a reflection mode; the main reflection instrument is the scanning electron microscope (SEM). Often TEM can be equipped with the scanning option and then it can function both as TEM and STEM[27].

2.3.2.1. Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM)[27, 146] detects the elastically scattered electrons when a beam of electrons is transmitted through an ultra thin specimen. As discussed in Section 2.2.3, this can give valuable structural information of a crystalline material. The main advantage of TEM is the possibility to obtain information in imaging and diffraction mode almost simultaneously.

In order to use electrons in a microscope it is necessary to be able to focus them; this is achieved by several electromagnetic lenses. The condenser lenses controls the size and angular spread of an incident electron beam. Electrons, coming from the condenser system of the TEM, are scattered by the sample, which is situated in the object plane of the objective lens. The scattered electrons are focalised in the back focal plane of the objective lens and, as a result, a diffraction pattern is formed there.

Page | 60 There are two main ways of imaging the diffraction pattern, called bright field (BF) and dark field (DF) imaging. In bright field (BF) mode, an aperture is placed in the back focal plane of the objective lens allowing only the direct beam to pass. The image then results from the weakening of the direct beam due to interaction with the specimen. In dark field (DF) mode, the direct beam is blocked by an aperture, while one or more diffracted beams are allowed to pass. The diffracted beams which have interacted with the specimen can provide useful structural information.

In addition to the BF and DF imaging, TEM can operate at the high resolution TEM (HRTEM) mode, allowing the imaging of lattice images of a crystalline material oriented along a zone axis. For this, both direct and diffracted beams are allowed through a large objective aperture. The image is formed by the interference of the diffracted beams with the direct beam.

In all TEM modes, the region of the diffraction pattern that is chosen for imaging is controlled by another aperture placed in the intermediate image plane. The first intermediate image is then magnified by further lenses (projective system) and the image of the specimen can be obtained on a fluorescent screen.

2.3.2.2. Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM)[27, 147] detects the emissions of X-rays and secondary (Auger) electrons, when a tiny electron beam (covering a spot of 50-100Å in diameter) is focused onto the surface of a specimen.

The energy of the emitted X-rays is characteristic of the elements present and hence qualitative information of the elements present in the specimen can be achieved. This is done by scanning either the wavelength or the energy of the emitted X-rays, by the so called wavelength-dispersive (WDS) and energy-dispersive (EDS) spectroscopy techniques respectively[27]. Quantitative information can also be obtained with suitable calibration. It should be noted that the detection of X-rays can also be done using a TEM instrument.

The detection of secondary (Auger) electrons is used for the imaging of the specimen’s surface, providing information about morphology and surface topography. The resolution of the SEM

Page | 61 depends on the size of the electron spot, which is related to the wavelength of the electrons and the electron-optical system that produces the scanning beam. The highest resolution for conventional SEMs is approximately 100 Å, which is lower than the resolution in atomic scale that can be achieved by TEM. However, SEM can provide images of larger areas of a specimen compared to TEM.

2.3.2.3. Scanning transmission electron microscope (STEM)

In scanning transmission electron microscopy (STEM), the surface of a specimen is scanned by a tiny electron beam and the signal is recorded by selected detectors[148]. There are three detectors that can be used to obtain STEM images: (a) a bright field detector (BF), similar to the BF-TEM (Section 2.3.2.1), (b) annular dark field (ADF) and (c) high angle annular dark field (HAADF). The ADF and HAADF detectors are disks with a hole, which use the scattered electrons for image formation. As aforementioned (Section 2.2.3) the electron scattering in crystalline solids is a result of the strong Coulomb interaction of the negatively charged electrons with the positive potential inside the electron cloud in an atom; this interaction leads to high scattering angles. The HAADF disk has larger diameter, with also larger diameter hole, compared to the ADF disk, and hence it is more capable to detect the high-angle scattered electrons[149].

The STEM resolution is similar to the TEM, hence it is capable of giving information in the crystal level. Additionally, it can provide information over a larger area. This makes STEM strong in detection of crystalline areas and defects; when using the HAADF detector it is even possible to detect single atoms in a crystalline material. STEM can also give quantitative information when combined with analytical methods (e.g. EDS, Section 2.3.2.2).

2.3.3.

Microscopy studies in this thesis

The BSCFM materials in this thesis, synthesised as described in Section 2.1, were characterised by a combination of microscopy techniques. For this, a quantity of the material in powder form was transferred to a copper TEM grid, and this was then transferred to the TEM machine.

Dark field TEM images of the as-made materials were collected by Dr. Jiangling Xu and Dr. Antoine Demont using a JEOL JEM3010 instrument with LaB6 filament, operating at 300 keV.

Page | 62 Quantitative EDS data for selected samples were collected by Dr. Jiangling Xu using a JEOL JEM2000FX, with a W filament, operating at 200 keV. For the BSCFM composition, showing the most promising performance. High Angle Annular Dark Field (HAADF) images were collected by Dr. Simon Romani using a JEOL JEM 2100FCs, with Schottky Field Emission Gun operating in STEM mode with CEOS aberration corrected probe.

SEM imaging of the symmetrical cells used for electrochemical characterization (Section2.7.2), which were placed on a SEM holder, were carried out in the Centre for Materials Discovery (CMD) at Liverpool University by Dr. Christopher I. Ireland and Dr. Xinming Wan. The instrument was a Hitachi S-4800 scanning electron microscope and the analysis of the sample was performed using a low KV electron beam (3 KV).