Transmission Electron Microscopy (TEM)

Transmission electron microscopy is a technique used for the characterisation of small features in solid samples. It is particularly useful in the characterisation of nanosized samples, i.e. nanoparticles. Information obtainable form TEM imaging can vary from shape and simple size distribution to crystal structure determination according to the resolution of the image. In high resolution TEM it is also possible to see features at the atomic scale.108 The technique is based on scattering of waves by matter, in the specific scattering of electrons. When a homogeneous electron beam reaches a sample it can be scattered in many ways, these are illustrated in Figure 2.10a. Two main groups of scattered electrons can be identified, those that are scattered with an angle θ > 90 ° called backscattered, and those with an angle θ < 90 ° called forward scattered electrons.108 The latter are the ones exploited in a transmission electron microscope to produce magnified

images of the specimen.108 After some of the electrons are backscattered the electron beam that goes through the specimen will be reduced in intensity. This can further be reduced during forward scattering since the electrons can be scattered parallel to the original beam direction (direct) or at an angle θ. The direct beam is what gives the image on the TEM screen and its intensity is important to obtain details of the sample surface. Different beam intensities will result in different contrast area in the image. Lower intensity of the direct beam will correspond to darker areas. Figure 2.10b describes in a simple way the basic working principle of a transmission electron microscope and how the contrast in the image is obtained. To better understand is important to mention that the scattering power of the atoms depend on the atomic number Z, heavier atoms will scatter more than lighter ones. Additionally, for equal Z, in a thick area of the specimen the number of scattering events will be higher than in a thinner area. The dark grey areas in Figure 2.10b represent heavier atoms (for example metal nanoparticles on a carbon support) or thicker areas of same composition.

When the homogeneous electron beam hits the specimen, backscattering will occur and the electrons will be backscattered more by the darker areas leading to lower intensity of the forward scattered beams (in Figure 2.10b the intensity of the beam is expressed via the thickness of the arrows).108 As mentioned, further scattering will occur for the beams that are transmitted through the sample.108 This will be more intense for the darker areas Figure 2.10 Schematic of TEM working principles. Schematic representation of a) electron scattering of a thin specimen and b) of its exploitation in a TE microscope.

θ θ a) b) Objective lens Objective aperture Image Backscattering Specimen Specimen Incident beam Forward scattering

in Figure 2.10b and the intensity of the direct beam is decreased even further. Within the microscope a series of lenses and apertures will allow only the direct beams to reach the screen. The direct beams will have different intensities according to the area of samples they went through and this information is translated in an image with different contrasts.108 An example, acquired with a CCD (charged-couple device camera), is provided in Figure 2.11a where metal nanoparticles are loaded on a carbon-based support. The difference of atomic number and therefore scattering behaviour of metal and carbon produces clear contrast in the image and allows to distinguish the NPs from the support. From images similar to that of Figure 2.11a is possible to obtain information about shape and size of the nanoparticles. At higher magnifications, however, information about the structure of the nanoparticles can be obtained. Figure 2.11b shows lattice fringes for platinum nanoparticles obtained at high magnification. This information can be obtained by analysing the image with the dedicated software (Gatan Digital Micrograph). The lattice fringes distance is measured from the diffractogram (Figure 2.11b inset) obtained by Fourier transform of the image. In the specific a lattice fringe of 0.225 nm is measured which correspond to the d111 of Pt0.

When using this information to distinguish between different metallic nanoparticles, care has to be taken especially for small NPs.109 _{When measuring d-spacing from lattice} fringes errors can be generated by the irregular shape of the particle and its orientation.109 If the particle is slightly tilted compared to the view axis then the lattice fringes seen can be smaller or bigger than the standard ones.109 Following the example of Tsen et al.109 _a

measurement of the d111 of various platinum nanoparticles (59 NPs of different size and

shape) is carried out to determine the statistical distribution of d-spacing value

Figure 2.11 Example of (HR)TEM images. TEM images of Pt nanoparticles on carbon-based support: a) low magnification and b) high magnification with visible lattice fringes.

b

a

(Figure 2.12). This will provide a tool to critically analyse the d-spacing of different metals NPs (Chapter 5). The measured lattice fringes can be fitted with a Gaussian function109 of maximum 0.224 nm and FWHM of 0.005 nm. Statistical analysis of the data points shows and average of 0.224 nm with a standard deviation of ± 0.002 nm. Even though a powerful tool to characterise nanomaterials, TEM imaging presents some drawbacks. Due to the high magnification compared to other techniques only a small part of the whole sample can be analysed.108 Therefore the information obtained may not be representative of the real situation. For this reason it is important to acquire as many images as possible of different areas, to have a better statistic. In addition, the TEM provides 2D images of 3D features,108 this inevitably brings to some artefacts and limitations in the determination of characteristics along the third axis. Hence, TEM has to be used together with other techniques if a complete characterisation is to be achieved. (HR)TEM images were acquired on a JEOL-2011 microscope operated at 200 kV with tungsten filament electron source. The specimen was prepared by suspension in acetone of the powder sample. This was, then, deposited on a carbon coated copper grid. Digital images were recorded using a Gatan 794 CCD camera and the dedicated software, Gatan Digital Micrograph was employed to analyse the images. Particle size distribution was obtained by using ImageJ, an image editing software.

Figure 2.12 Distribution of Pt d111 from HRTEM measurements. Gaussian distribution of the d111 of Pt NPs. 59 NPs of different shape and size were analysed.

0.216 0.220 0.224 0.228 0.232 0 8 16 24 d₁₁₁ / nm Na nopa rti cle s c ounts / 