Similar to SEM, TEM uses an electron beam to interact with the sample. In TEM, the transmitted electron signal is used to form the final image. Hence, higher energy (200-300 kV) is used in TEM to accelerate the electron beam to produce more transmitted electrons. Similar to the principle of the light microscopes, electron beams are focused onto the sample using electromagnetic lenses and the transmitted beam is then focused onto the phosphor
screen or the charge couple device (CCD) camera by the objective lenses. TEM can be operated in two modes, diffraction and imaging mode. Diffraction pattern (DP) is formed on the back focal plane of the objective lens. Images are formed on the image plane after translated by the objective lens and the intermediate lenses.
Figure 3.9: Bragg scattering process.
Electrons that pass through a crystalline lattice will undergo a Bragg scattering (figure 3.10) and the scattering is collected as a diffraction pattern (DP). Bragg’s law is defined as
𝑛𝜆=2𝑑 𝑠𝑖𝑛𝜃 (3.3)
where n is a positive integer, λ is the wavelength of incident wave, d is the inteplanar
distance between the lattice planes and 𝜃 is the scattering angle as shown in figure 3.9. The
relation of d-spacing and Miller indices is defined with the following equation:
𝑑 = !
where a is the lattice constant of the cubic crystal, and h, k and l are the Miller indices of the Bragg plane. With the two equations above, the lattice constant of a particular cubic crystal can be expressed with the following equation:
In order to obtain a useful DP, an aperture is used to select a region of interest for diffraction. The diffraction pattern resulted from this technique is known as the selected area electron diffraction pattern (SADP) or selected area electron diffraction (SAED) pattern. The SAED pattern can be used to determine the crystal structure of the sample and examine crystal defects. Often the samples have to be tilted to a certain zone axis so as to
3. Experimental techniques
identify the SAED pattern. Using the SAED pattern, two basic types of imaging of the samples can be performed: bright field (BF) and dark-field (DF) imaging. This can be done by simply inserting the objective apertures to select either the bright spot in the centre of the SAED pattern for BF imaging or by blocking the central spot (selecting other nearest spot) for DF imaging. Bright field image would give the mass-thickness and diffraction contrast. For example, thick areas, areas in which heavy atoms are enriched, and crystalline areas appear
with dark contrast.Dark field image on the other hand gives more specific contrast about
planar defects, stacking faults or particle size which appears as bright contrast as it only uses the nondiffracted beam on the sample.
3.4.2 Components of TEM
Figure 3.10 shows the photo of a TEM and the schematic showing the main components. Similar to SEM, the main components consist of the electron gun, electromagnetic lenses, apertures and vacuum column. In terms of their components, TEM have a taller electron beam column to allow higher acceleration for higher energy beam. In this thesis, TEM-FEG JEM 2100F located at the Centre for Advanced Microscopy, Australian Microscopy and Microanalysis Research Facility, The Australian National University was used for all of the TEM characterisation.
Figure 3.10: TEM photo and its schematic showing the internal structure. Figure after .
3.4.2 Nanowire specimen preparation for TEM
Since TEM uses the transmitted electrons for imaging, the sample has to be thin enough, i.e. electron transparent. Due to the nanometer scale dimensions of nanowire thickness / diameter, this can be easily achieved. Nanowire samples are prepared for TEM by careful mechanical transfer of the nanowires from the substrate onto a holey carbon-coated copper grid. In some cases, nanowires are suspended in the iso-propanol by means of putting a
piece of substrate (about ~5 mm2) with nanowires into a beaker with about 10 ml of iso-
propanol which is then ultrasonicated for about 1 minute. Then, few drops of the solution are transferred onto the holey carbon-coated copper grid and left to dry.
The cross-sections of the nanowires are prepared by first embedding the nanowires into Spurr resin. Slicing nanowires in an embedded epoxy resin is one of the more practical
3. Experimental techniques
ways of having thin enough cross-sections for characterisation under the TEM. This technique has been introduced in previous reports , . The Spurr resin is based on four components ERL 4221, D.E.R. 736, (diglycidyl ether of polypropylene glycol), NSA, Nonenyl succinic and dimethylaminoethanol (DMAE). The Spurr mixture is often prepared fresh prior to embedding the nanowires. However, a recently defrosted one can also be used. The mixture is made with the ratio given by the manufacturer for a suitable hardness and viscosity.
To embed nanowires into the Spurr resin, a small piece of the as-grown nanowires on the substrate is placed in a mould facing up. The Spurr mixture is then is transferred into the mould using a syringe. The mould is left to dry in an oven at 70 °C for 8 hours. Once the mould is dry, the resin block is taken out of the mould and reshaped to suit the process of microtoming. Prior to shaping the slices, the substrate is first removed from the resin. This can be done by trimming the resin on each side of the substrate and immersing the substrate in liquid nitrogen. The thermal effect of quenching the substrate into the nitrogen makes the substrate fall off from the resin easily. Once the substrate is off, only nanowires are left in the resin. The exposed surface is also smooth and shiny making it perfect for aligning with the microtome knife for shaping and slicing. The resin is then sliced into thin sections using the ultramicrotome with a diamond knife. Generally, the cutting surface is aligned with the knife-edge to ensure that the slices are uniform in thickness and to protect the knife from damage due to thick cuts. In rotary microtome such as the ultramicrotome, the knife is fixed in a horizontal position and the cutting surface is brought towards the knife-edge in a rotary motion (figure 3.11). The slicing begins as the surface touches the blade and the slices are delivered into the trough. The trough is filled with water levelled with the knife-edge to allow slices to slide smoothly. The slicing continues in rotation and the slices are delivered with each rotation aligned on top of each other as shown in the inset of figure 3.11. The slices are then lifted up onto a TEM grid and left to dry.
Figure 3.11: a) The ultramicrotome slicing technique. Inset: slices of resin for TEM prepared by ultramicrotome.