2.3.1 General Electron Microscopy
Electron microscopy makes use of a beam of electrons to illuminate a sample and produce an image at much higher magnification and resolving power when compared to an optical microscope. This is possible as electrons have a much shorter wavelength (~ 10-12 m) when compared to that of visible light (~ 6 x 10-7 m) used by optical microscopes. Resolution is the smallest distance we can distinguish two points. The shorter the wavelength, according to the Rayleigh criterion below, the greater the resolution that is possible:
59
Equation 2.21
Where is the refractive index of a viewing medium and is the semi-angle of the collecting lens. This explains why it should be possible to achieve higher resolution with electrons than with visible light.
The term electron microscope was first used in a paper by Knoll and Ruska in 1932, where they demonstrated the first electron images taken, for which Ruska received the Nobel Prize over 50 years later.183 Electron microscopes use electromagnetic or electrostatic lenses to control the path of electrons. The basic lens would consist of a coil of wire around a tube, from which a current can be passed, inducing a magnetic field. Through changing the current, the magnetic field can be altered, enabling electrons to be controlled as they pass through as they possess a magnetic moment.
2.3.3 Scanning Electron Microscope (SEM)
An SEM produces images through the detection of secondary electrons and backscattered electrons, which are emitted from the surface due to excitation by the primary electron beam. SEM allows for the imaging of the topology of the surface of a sample. There is also the possibility of using other techniques such as energy dispersive X-ray spectroscopy (EDX) on an SEM, which can provide elemental compositions of samples (see section 2.3.7 for EDX). For this thesis, a Hitachi S-4800 scanning electron microscope was used for analysis of symmetrical cells of cathode and electrolyte in chapter 3. EDX spectra were also collected on these same samples, with the experiments being carried out by J. Gallagher.
60 2.3.4 Transmission Electron Microscope (TEM)
The use of a TEM involves a high voltage electron beam illuminating a thin sample and being partially transmitted. The electron beam is a type of ionising radiation that will interact with a sample and produce a wide range of secondary signals. These have been summarised below in Figure 2.5.
Figure 2.5 Signals generated when high energy beam of electrons interacts with a thin specimen (sample).
Many of the signals generated can be used analytically, such as EDX and electron energy loss spectroscopy (EELS). The resolution of TEM allows individual crystals to be imaged, allowing information to be collected for single phases even within multiphasic samples.170 In this thesis, TEM work has been carried out using a JEOL JEM 2000FXII TEM/STEM operating with a W electron source operated at 200 keV, a JEOL JEM 3010HR TEM/STEM with a LaB6 electron source operated at 300 keV and a JEOL JEM 2100FCs 200 keV aberration corrected field emission TEM/STEM. All samples were prepared onto carbon coated Cu grids in through grinding in methanol to create a suspension, which was applied
61
drop wise onto the grid using a pipette. The grids were thoroughly dried before introducing them into the TEMs.
2.3.5 Selective Area Electron Diffraction (SAED)
For an electron diffraction (ED) experiment, diffracted electrons from the specimen are focused by the objective lens and a diffraction pattern is created in the back focal plane as seen in Figure 2.6. The electrons are then recombined to form an image in the image plane. This diffraction image gives direct crystallographic information about small areas of the specimen.
Figure 2.6 Path of diffracted electrons in a TEM.
In SAED, a selected area aperture is inserted into the image plane of the objective lens, which essentially excludes any electrons that do not pass through this aperture. Only electrons diffracted from a smaller area of the specimen will contribute to the diffraction pattern,
62
helping to improve the contrast of the image. Through rotating a crystal, and analysing the diffraction patterns from different planes, cell parameters, reflection conditions and consequently space groups in agreement with the conditions can be identified.170
2.3.6 High Angle Annular Dark Field Scanning transmission Microscopy (HAADF-STEM)
In scanning transmission electron microscopy (STEM), the specimen is scanned by a tiny electron beam. In the case of bright field (BF) imaging, a BF detector is inserted onto the axis of the microscope where it intercepts the direct beam electrons. For dark field (DF) imaging, scattered electrons, rather than direct beam electrons are collected. When the DF detector surrounds the BF detector, which allows for detection of scattered electrons, this is known as an annular (A)DF detector. The ADF image is complementary to the BF image in which it sits around. When using a high angle (HA)ADF detector, Z-contrast images of atoms can be collected, where the contrast of an atom is ~ Z2. The general detector set up in an STEM is shown in Figure 2.7. 170 In chapter 3 of this thesis, HAADF has been used to confirm A-Site cation ordering of a long axis perovskite material.
63 2.3.7 Energy Dispersive X-Rays (EDX)
As shown previously in Figure 2.5, when the ionising electron radiation interacts with a specimen, many secondary processes can occur. One is the emission of X-rays, with energy characteristic of the chemical element. Detection of these X-rays allows for the quantification of elements present by extraction of the relative intensities of the distinct X-ray energies. EDX using a TEM has been used widely in this thesis for the determination of phase composition, as well as that of impurities that may be present. In order to improve the accuracy of the compositional measurement, many individual crystals had their composition analysed, so that a statistical average could be taken for a particular sample. EDX can also show up compositional inhomogeneities if elemental composition varied greatly from particle to particle.170