Electron microscopy

Electron microscopy is the use of electrons to visualise objects, usually in the μm-nm range and is a widely used technique in many fields of science7

. Using electrons rather than light has several advantages. Firstly electrons can be diffracted, like electromagnetic radiation (De Broglie relationship) but have a wavelength of less than 0.1 nm which can produce images with greater resolution and higher quality than could be achieved with an optical microscope giving details about the surface of the object that may give important information7, 15.

Figure 2.10: The different processes that can occur when a focused electron beam is used to probe a surface17

Secondly when an electron beam hits a sample, there are several processes that can occur such as transmition, Auger and energy dispersive X-rays, each giving different information and insight into the nature of the sample being analysed9. There are many types of electron microscopy techniques

73 the such as scanning electron microscopy (SEM)18, transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM).

2.3.3.1 Scanning electron microscopy

Scanning electron microscopy is the use of an electron beam to probe the surface of the material and produce an image of the surface. The electrons can be generated through different methods such as using a heated tungsten filament, emitting electrons through thermionic emission or using a high electric field to remove electrons known as a field emission gun (FEG)15, 18 which is used on the Tescam Mira3 SEM that produced the images in this thesis. The emitted electrons are then passed through a series of scan coils, objective lens and apertures to refine this to a small beam over a square area with a x and y direction known as a raster.7, 18

The depth at which the electrons penetrate the sample is dependent on the accelerating voltage used. Typically, this would be around 5 kV to 30 kV with higher voltages penetrating further into the sample. When the electron beam is focused on a small area there are several events that can occur; back scattered electrons, secondary electrons and X-ray fluorescence from the sample (see section 2.3.27, 17).

Secondary electrons are due to the energy interaction from the electron beam to the atoms of the sample giving enough energy to eject an electron. In the bulk, the ejected electron rapidly loses energy and therefore does not travel far but if the electron is near the surface it can escape (into vacuum) and can be detected18. As a result, secondary electrons detection gives an image of the surface of the material.

Back scattered electrons originated from scattering of the electrons from the electron beam throughout the sample and subsequently escape into the vacuum15. The scattering of the electrons is affect by the size of the atom, with heavy atom scattering more electrons than lighter atoms one and giving a brighter image18. This therefore mean this technique could be used to differentiate between different element although this does not work well with element of similar mass (e.g. Cu/Zn or Mn/Fe).

74 2.3.3.2 Energy dispersive X-ray spectroscopy

Energy dispersive X-ray spectroscopy (EDX) is a method of using the electron beam to gather information about the elements present in the sample. This technique can be used for mapping elements across an area that is being analysed and is useful in determining elemental mixing and assigning morphologies and features to a particular element.

The X-rays are created from the samples in a similar manner to what was seen with XRD and XPS. When the electron beam collides with an element it can give some of its energy to one of the electrons in the atom, which subsequently gets ejected and creates a hole. This hole is then filled by an electron from a higher orbital moving down and, in the process, emits X-rays. The energy of these X-rays is based on the orbitals that the electron moved from. Because different elements have electron configurations and orbital energies, the energy of the emitted X-rays is going to vary and therefore each element produces a “finger print” which can be used to identify and quantify elements present as well as map out the location and distribution of elements onto an SEM image of the sample9.

2.3.3.3 SEM and EDX method

All SEM were performed on Tescam Mira3 (FEG-SEM) with the electrons being generated by a tungsten tip, field emission gun. Imaging was done using in-beam secondary electron detector and an electron beam with an accelerating voltage of 5-15 kV (depending on the sample). View fields, working distance, electron detection type and accelerating voltage are specified in the images.

The copper-manganese sample in Chapters 4 and 5 and the copper-zinc oxide samples in Chapter 6 were suspended on a carbon film and were

75 coated with a 10 nm AuPd coating to reduce charging affects that occur due to electrostatic build up from the beam which can interfere with the imaging18.

The oxalate needles in Chapter 3 were loaded onto a 3.05 mm 300 mesh Cu grids with a holey carbon film and placed on a STEM sample holder.

EDX analysis was performed with using an Oxford-instrument X-MaxN 80 which was fitted to the Tescam Mira3 and was inserted during EDX analysis but retracted for SEM analysis. All EDX data were recorded, analysed and reported using the Oxford AZtec program.

SEM images that were performed by Dr Thomas Davies are specified in the caption below the figure.