Electron microscopy

Electron microscopy is used in either a reflection or transmission approach. A typical scanning electron microscope (SEM) relies on the reflected electrons that are used to build up a three-dimensional image of the sample surface giving information on the sample surface features, texture and topography. This style of electron microscopy can covers the magnification range between the lower resolution limit of optical microscopy (~1μm) and the upper practical limit around 5nm of transmission electron microscopy (TEM). The main important difference between these two techniques of instrument is that an image from a scanning electron beam instrument is built up by scanning a focused, highly convergent electron probe over an area of the sample and measuring a signal generated from the interaction of the electron beam with the specimen. In the case of transmission electron microscope a parallel beam of electrons used enlightening an area of the sample and forming an image using several electrons which penetrate through the specimen [72].

The scanning electron microscopy techniques mainly using a focused electron beam incident on a sample. The size of the electron probe depends on the electron source configuration and the magnitude of current in the electron probe. This electron probe is hit across an area on a sample and reflected signals from the sample surface can be measured. Interactions between the sample and the electron probe generate many types of signal including secondary electrons, backscattered electrons and characteristic x-rays. The most important signals are those genarated by secondary electrons (SE) with most probable exit energies of 2–5 eV and by backscattered electrons (BSE) with energies that range from the energy of the primary electrons to about 50 eV. The secondary electron produced and the backscattering coefficient depend on the angle of electron incidence (topographic contrast), the mean atomic number (material contrast), the crystal orientation (channeling contrast), and electrostatic and magnetic fields near the surface (potential and magnetic contrast). The resolution for BSEs and x-ray images is dependent on the size of the electron interaction volume, which is a function of the accelerating voltage, and the average atomic number and density of the sample. For SE imaging, resolution is usually dependent on the size of the electron probe and the inherent contrast of the sample. For SEM techniques, samples fits into the instrument stage at high-vacuum system and electrically conductive, where samples that are not electrically conductive can be coated with a thin electrically conductive layer such as evaporated amorphous carbon or have a metal such as gold, platinum, palladium, or chromium sputtered onto the surface. The samples for transmission electron microscopy are necessary to be thin enough that electrons with energies of 100 keV or higher can pass through the sample to provide a magnified image of the sample or an electron diffraction pattern [72-74].

The TEM is capable of giving information on an atomic scale, by direct lattice imaging.

The high resolution TEM instrument can give information on the crystallographic arrangement of atoms in the specimen and their degree of order; detection of atomic-scale defects in areas a few nanometers in diameter, and can be used to index crystalline materials. In principle, the resolution in the TEM is given by the capability to accelerate electrons, so the higher voltage used to accelerate electrons, the better resolving power of the TEM [72, 74], although also the greater the likely damage to the specimen being studied. Electrons emitted from electron gun (filament) are accelerated through high voltage. Their wavelength is related to the accelerating voltage, V by the equation:

λ = h(2meV)^-1/2 (25)

where e is the charge and m is the mass of the electron. In the case of TEM the condenser lens is used to control the size and the angular spread of the electron beam that is incident on the sample. The electrons are emitted by a cathode filament. The electrons are then attracted towards the anode, which causes their acceleration. The electron beam is focused by two successive condenser lenses into a beam with a spot size around 5 nm. The beam passes through an objective lens and strikes the surface of the sample as shown in figure 2.8.1. The practical resolving limit is governed by the imperfection of the lenses and for a current microscope is in the range 1 -2 Å. Typically electromagnetic lenses are used in electron microscopy to focus the electron beam. The contrast in electron microscopy is based on the relative difference in the observed intensities between areas filled with atoms with different scattering behaviour. Heavy elements have high electron density so scatter electrons strongly and so appear darker in the TEM bright-field image, the image taken when the directly transmitted electron beam is recorded.

Figure 2.8.1 Schematic image of the components of a transmission electron microscope.

TEM can also provide diffraction data with very high spatial resolution for individual crystals when the diffracted rather than transmitted beam is selected to be recorded. The electron wavelengths are much smaller than the X-ray wavelengths used in typical

diffraction experiments, as a result the diffracted beams are concentrated into a narrow cone cantered on the non-diffracted beam. The area of illumination that is chosen for diffraction can be selected by appropriate lenses giving rise to the selected-area diffraction that is very important for polycrystalline materials [73].

2.8.1 Experimental Measurements

The scanning electron microscopy images of recovered templated polystyrene structures were taken at the Centre for Electron Optical Studies at the University of Bath using a Japanese Electron Optics Laboratory JEOL JSM6480LV scanning electron microscope [75]. SEM images were collected with an operating voltage of 20 kV and to prevent charging of the samples they were sputter coated with gold prior to imaging in the SEM.

The transmission electron microscopy images were taken in two places: first at the Centre for Electron Optical Studies at the University of Bath. Images were collected using a Japanese Electron Optics Laboratory JEOL 1200 EX transmission electron microscope operated at 120 kV. Instrument settings and images were established using the fluorescent screen of the TEM instrument before final collection using a CCD camera. Samples were prepared for TEM study by dispersing the purified polystyrene powder in deionised water by agitating the test tube for one minute. A clean pipette was then used to place a drop of this dispersion onto a copper EM grid with a holey carbon film, and excess solution was blotted with a filter paper, leaving thus a thin sample film spanning the holes in the carbon film.

The second location where TEM images were taken was in the Physical Chemistry Division in the Department of Chemistry, Lund University, Sweden. These micrographs were recorded using a JEOL 3000F (300 kV) microscope. The film samples were frozen under liquid nitrogen to make them brittle, and were crushed in a cold mortar into fragments. The fragments were deposited from the liquid nitrogen onto a copper EM grid with a holey carbon film. The grid was placed into the microscope at room temperature on a stage which was then cooled using liquid nitrogen for 2 hrs before measurement, as well as during imaging.