Working with nanomaterials and nanoscale structures present unique challenges to the scientists. Among these, difficulty of observing things in this minute scale is one of the biggest. Large body of research is still dedicated to innovate and improve on the instruments that can look in to the nanoscale more clearly and extract more information from this environment.
Conventional optical microscopes are of little help when imaging structures smaller than 1 μm. Among other things, this limitation is primarily due to optical diffraction that cause fuzziness to the image. A physicist, Ernst Abbe calculated that the resolution limit of the optical micrsocopes due to diffraction is λ/2NA ( where λ is wavelength and NA is numerical apature of lens which is usually 1.0). For white light, average wavelength falls in the green color which is around 500 nm, thus the best resolution is few hundreds of nm. In other words, in conventional optical microscopes, one would not be able to distinctively identify two objects that are separated from few hundreds of nanometers.
Electrons present a unique method of imaging at the nanoscale. If the electron can be accelerated using high voltage, energy beam is produced with a wavelength with much shorter than that of light used in the optical microscope. The wavelength of the electron beam can be calculated using set of theoretical equations. For an example, electrons accelerated with 10 kV gun produce a wavelength of 12.2 x 10-12 m or 0.0122 nm, much smaller than the wavelength of the light. Therefore, electron microscopes can be used to image nanoscale in great detail. These microscopes can routinely achieve magnifications in the order of 1 million and can differentiate objects that are located as close as 0.1 nm.
Electron beam can interact with a sample in number of ways. Signals generated from each of these interactions can be used to extract vital information about the sample.
Following are the five major types of interactions electrons make with the sample
Secondary electrons: These results from the collision of primary electron beam with the sample, thus called secondary electrons. These are generated as ionization products and can be in the form of ions, electrons or photons with high energy
Backscattered electrons: These electrons result from backscattering of the primary electrons through elastic scattering. The intensity of backscattered electrons is highly related to the atomic number of atoms in the specimen
Transmitted electrons: Electrons in the primary beam can transmit through the sample with low loss of energy if the thickness of the sample is sufficiently low.
Characteristic X-rays: An accelerated electron traveling with the primary beam can knock off an electron from an atom in the sample causing higher energy electron to fill the vacant position. During this jump, excess energy is emitted as X-rays
Auger electrons: When an accelerated electron from the primary beam collide and remove a core electron from the atom in a sample, a higher energy electron will fill the vacant electron position. Most of the time, the excess energy is emitted as a photon with the X-ray frequency. However, in certain instances the excess energy can also be transferred to a another electron causing it to eject from t atom. This electron is called an Auger electron
Scanning electron microscope (SEM)
The signals produced from electron interaction with the atoms at the surface of the specimen are taken as the primary signals in SEM. These signals includes, secondary electrons, backscattered electrons and characteristic X-rays. SEM has set of detectors that can detect each of these signals. Of these secondary electron detectors are very common in all SEMs which can capture secondary electrons emitted from the sample surface and then produce very high magnification images with resolution close to 1 nm. Nanotechnology research often demand this level of magnification and resolution. Because of this reason, SEM is fairly widely used instrument in the field.
Back-scattered electrons can be used to generate information about distribution of elements in the sample as these electron signal is strongly related to the atomic number. This mode provide more flexibility when working with biological samples. Characteristic X-ray detectors are frequently used in scanning electron microscopes to extract information about composition and abundance of elements in the sample.
There are two main methods of generating electron beam in SEMs; hot cathode and cold cathode. Tungsten is normally used in hot cathode electron emitters. The mechanism of electron generation is called Thermionic emission where in the filament is heated to very high temperature to generate electrons. Tungsten is the ideal metal for this purpose as it has highest melting point and lowest vapor pressure. The cold cathodes are usually made of lanthanum hexaboride (LaB6) or thermally assisted schottky type emitters of zirconium oxide.
Generated electrons are then accelerated across an electric field and focused by condenser lenses. These lenses are magnetic and uses a magnetic field to condense the electron beam. Once the beam is condensed to a very small diameter it passes through scanning coils or deflector plates which can scan the sample surface in the raster fashion over a rectangular area.
The main advantage of SEM is its ability to generate three dimensional looking images that can provide topographical, morphological and compositional information that is immensely valuable in nanotechnology research. On the other hand, sample preparation in SEM is quite simple compared to other microscopes. The instrument works fast, it’s not unusual that the images are taken within one minute.
Transmission electron microscope (TEM)
As the name indicates, TEM rely on the transmitted electrons through the sample to generate an image. To achieve this, a highly focused electron beam that is highly accelerated is directed towards a thin sample. Collation generate number of other signals that are also extracted and analyzed with appropriate detectors to obtain information for material characterization. TEM would allow scientists to get very high magnification images with extremely fine details. High resolution TEMs routinely achieve images in great details to the level that one can literally count the atoms in the specimen.
Similar to SEM, TEM also consist of an electron emission source, which may be a tungsten filament or a lanthanum hexaboride source. The electrons generated from the emitter is then accelerated by a high voltage source that is typically in the range of 100 – 300 kV. The electron beam can be operated with magnetic and electrostatic field using special lenses to focus and to obtain images.
Images generated from TEM usually appear flat. The contrast of the images is primarily determined by the thickness and the material composition. This is especially true at low magnifications. However, at higher magnifications, complex wave interactions come to play.
TEM can be used to make election diffraction patterns in crystalline samples. Similar to x-ray diffraction patterns, this data carries important information about the crystalinity, crystal orientation and atomic arrangement in the sample. Electron diffraction pattern is a pattern of bright dots in a black background in case of a single crystal. In polycrystalline and amorphous samples diffraction pattern appear as rings.
Compared to SEM, TEM sample preparation can be cumbersome. However, extreme magnifications and high resolution has made this instrument a popular choice among many nanotechnology scientists.