Scanning electron microscopy

The wavelength of light limits optical microscopy to around 0.2 micrometres before diffraction effects occur, so electromagnetic wavelengths shorter than this are required for surface image and topography on a nanometre scale. Electrons exhibit a de Broglie wavelength, owing to the

wave-particle duality of matter, which is much shorter than that of light (400 – 700 nm) and so leads to images with high resolution. In scanning electron microscopy (SEM) a beam of electrons is directed onto a sample to produce an image of its surface at very high magnification (greater than 500 000) times (Figure 2.8), although individual atoms themselves cannot be seen. The wavelength of light limits optical microscopy to around 0.2 micrometres before diffraction effects occur, so electromagnetic wavelengths shorter than this are required for surface imaging and topography analysis on a nanometre scale. Electrons exhibit a de Broglie wavelength, owing to the wave-particle duality of matter, which is much shorter than that of light (400 – 700 nm) and so leads to images with high resolution.

Figure 2.8 Schematic of a typical scanning electron microscope, with the electron beam show in yellow and sample in red. Taken from Sutton et al..¹⁴⁰

SEM was used to study the surface topography of LDHs during the work undertaken in this thesis and to determine sample crystallite size. Figure 2.9 shows a scanning electron micrograph of a representative LDH sample (as prepared in Section 3), and allows sub-micron resolution images to understand morphological differences in LDHs. Energy dispersive X-ray (EDX) spectroscopy was used for quantitative and qualitative elemental analysis. This work was carried out with the

assistance of Mr Leon Bowen, Department of Physics, Durham University. A Hitachi SU70 (SEM) with a Schottky Field Emission Gun (FEG) was used to produce high resolution surface images.

This type of FEG has a thermionic emitter utilising a tungsten tip coated with zirconium oxide, which has greater electrical conductivity at higher temperatures, which leads to a lower surface barrier when in a high electric field. Thermionic emitters usually employ a filament, commonly of tungsten, which is heated to over 1000 K in a vacuum. Due to the large amounts of thermal energy generated, the electrons have greater energies than the work function of the metal, W, and leave the filament. With a Schottky FEG comprising a sharp tip rather than a filament, this results in a more intense beam of electrons with greater electron brightness. Brightness refers to the number of electrons directed at a certain area per second. Also, with FEG the energies of electrons are more coherent leading to increased signal to noise ratio. The beam undergoes acceleration as it passes through a high potential difference, then is focused by electromagnetic lenses in a vacuum chamber onto the surface of the material being studied. Secondary electrons are then emitted by the coated sample and scattered to a detector, from which the image can be constructed.

Figure 2.9 Scanning electron micrograph of MgAl(CO3) layered double hydroxide of Mg:Al 3:1, prepared by co-precipitation, as described in Section 3. The LDH particles are composed of rose des sables structures, with inter-crystal growth.

For samples analysed in this thesis, where only imaging was required, a cotton bud was used to sprinkle LDH/MMO sample onto a double-sided sticky carbon pad mounted on an aluminium stub.

The sample was then coated with a 15 nm thick layer of Pt using a Cressington 328 UHR sputtering system.

For SEM-EDX sample preparation, where elemental analysis was needed, a cotton bud was used to sprinkle a LDH/MMO sample into a mould, which was filled with epoxy resin and catalyst mixture in the ratio 5:1 then left to harden. The resulting epoxy was then subject to grinding and polishing to expose the solid sample, which was then coated in 40 nm of carbon. This sample was then mounted on an aluminium stub.

2.2.3.1 Element quantification by scanning electron microscope energy dispersive X-ray spectroscopy

For quantitative and qualitative analysis of element composition of the samples, SEM-EDX was used. This technique involves high energy electrons being focused onto a sample, causing secondary electrons to be expelled from the atoms, leaving electron holes. The atom shifts from the ground state to an excited state, but then proceeds to return to the ground state. This results in an electron from a higher energy level filling the electron hole and the energy difference between the higher and lower shell is then emitted as radiation. If this involves an inner energy level, it results in the energy being in the form of a secondary x-ray. As each atom has a discrete set of energy levels between which electrons can transfer, this results in characteristic wavelengths of x-ray being emitted for each atom.

spec spec std std

std

C N C kC

 N  (6)

For quantitative data the number and energy of X-rays can then be measured relative to a standard.

For this work a pure cobalt standard was used to calibrate the in-built virtual standards of the instrument. The number of counts from a specimen, N_spec, is compared to the number of counts for a standard, Nstd, (both minus the background count) in a specified time, then multiplied by the

concentration of the standard, Cstd (6). ZAF corrections are then applied for: atomic number correction (Z) of atomic stopping power and backscatter terms; absorption (A) of X-rays by the atom; and X-ray induced excitation fluorescence (F) effects prior to obtaining the composition.¹⁴¹ Thus a percentage of elements can be determined for a sample, either at a specific point or over areas of the sample. A typical SEM-EDX analysis is shown in Figure 2.10.

a)

b)

Figure 2.10 Typical analysis using scanning electron microscope energy dispersive X-ray spectroscopy showing a) MgAl(CO₃)-layered double hydroxide of Mg:Al 3:1, prepared by co-precipitation as described in section 3, with area of analysis marked and b) EDX analysis element plot.