User Tools

Site Tools


chem331:electron_microscopy

Electron Microscopy

The classical method for detailed information about the physical nature of surfaces was optical microscopy. With the advent of the electron microscope, it became the principle instrument for obtaining high resolution structural information for biological and nonbiological surfaces and compounds, which optical microscopes are unable to resolve.1)2)

Theory

Electron microscopy has a much higher-resolution than optical microscopy, which is limited to about the wavelength of light (~400nm). In electron microscopy, a beam of electrons is focused on a sample in order to provide an image of the external morphology of a material or to probe the internal structure of a sample. 3) It is usually paired with electron energy loss spectroscopy (EELS) in order to provide information about the identity of an atom. In EELS, a beam of electrons is focused on the surface of a sample and the scattered electrons are analyzed according to scattering energy and scattering angle. 4)

Types of Techniques

Scanning Electron Microscopy (SEM)

Scanning electron microscopy uses a finely focused beam of electrons impinging on the surface of a solid sample. The beam of electrons is scanned in a raster scan by scan coils. The raster scan pattern consists of the electron beam being swept across the surface linearly in the x direction, returned to the starting position, and shifted downward in the y direction by a standard increment. This raster scan pattern is repeated until the desired surface area of the sample is scanned. The signal is received above the surface in the z direction. The signals that are produced by the sample can include backscattered electrons, secondary electrons, Auger electrons, x-ray fluorescence, and photons of various energies. The source of SEM or the beam of electrons is focused by a magnetic condenser lens and objective lens systems. The magnetic condenser lens focuses the beam of electrons to pass through the objective lens system, and the objective lens system focuses the beam and determines the size of the beam hitting the sample.5) Since lenses are used in the instrument, aberrations are introduced, which limit the resolution of the image. However, more complicated lenses can be designed with “multipoles” to correct the aberrations. These complex aberration corrections have resulted in a better-focused beam containing more electrons striking the sample, which allows for more scattered electrons to reach the detectors.6)

Transmission Electron Microscopy (TEM)

Transmission electron microscopy probes the internal structure of a sample. It produces an image by the selective transmission of electrons through an adequately thin specimen. 7) Because electrons will interact strongly with atoms through elastic and inelastic collisions, the sample must be very thin. Transmission electron microscopy can also create nanometer sized probes, which can then be combined with the scanning electron microscopy technique. This is the basis for the scanning transmission electron microscope (STEM). STEM has the advantage of imaging thick samples through the analysis of backscattering and secondary electron scattering from SEM, while still retaining its high resolution. The resolution of this instrument is determined by the diameter of the TEM probe. With the application of the previously mentioned complex “multipole” lenses, the resolution of TEM can be made even finer.8)

Low-Voltage Microscopy

Because electron microscopy utilizes high voltage electron beams as a source, this leads to the disruption of chemical bonds resulting from the inelastic collisions of the beam electrons with the sample.9) This structural damage can be minimized through the use of low-voltage microscopy. At low electron dosages, the probability is negligible that an electron will traverse a region already damaged by a previous electron. However, there is a lower ratio of signal to noise because of the low voltage electron beam. By lowering the voltage of the source, the signal is not as clear or defined as high-voltage microscopy, and because of this, it is harder to distinguish it from the noise inherent in the instrument. In order to counter this effect, a large number of these noisy micrographs are taken and averaged in order to obtain a high quality micrograph.10)

Ultrafast Electron Microscopy (UEM)

This technique has allowed for studies of structural dynamics in both space and time with atomic-scale resolution. This increase in resolution and image mapping is due to the concept of “single-electron” imaging. The electron packets are generated in the microscope by femtosecond optical pulses and are synchronized with other optical pulses to initiate the change in temperature or electronic excitation. Each pulse consists of one or a few electrons, which eliminates space charge repulsion between electrons (Coulomb repulsion) and increases the temporal resolution. This technique utilizes electron beams for parallel illumination in imaging and for EELS. By eliminating Coulomb repulsion and focusing the electrons on an ultrashort timescale, UEM has a high coherence and image resolution.11) This technique can also be paired with STEM to improve its imaging capabilities. This technique is more sensitive and provides a high degree of chemical information. It is more sensitive to light atoms, and it can collect simultaneous images and EEL spectra from a local nanosize area without interference from the rest of the sample. Its applications are suited towards heterogeneous systems.12)

1) Beer, M. Electron Microscopy. Analytical Chemistry. 1976, 48, 5, 93R-95R.
2) Hamm, F. A. Analytical Chemistry. 1950, 22, 1, 26-30.
3) , 4) , 5) Skoog, D. A.; Holler, F.J.; Crouch, S.R.; (2007). Principles of Instrumental Analysis.
6) Griffiths, J. “Electron Microscopy Comes into Focus.” Analytical Chemistry. 2008, 3952.
7) Rochow, T. G.; Botty, M. C.; “Electron Microscopy.” Analytical Chemistry 1958, 30, 4, 640-656.
8) Reimer, L.; Kohl, H.; (2008).Transmission Electron Microscopy: Physics of Image Formation.
9) Beer, M. “Electron Microscopy”. Analytical Chemistry. 1976, 48, 5, 93R-95R.
10) Kuo, A. M.; Glaeser, R. M.; “Development of Methodology for Low Exposure, High Resolution Electron Microscopy of Biological Specimens.” Ultramicroscopy. 1973, 1, 53.
11) Park, H. S.; Baskin, J. S.; Kwon, O. H.; Zewail, A. H.; “Atomic-Scale Imaging in Real Space Developed in Ultrafast Electron Microscopy.” Nano Lett. 2007, 7, 9, 2545-2551.
12) Ortalan, V.; Zewail, A. H.; “4D Scanning Transmission Ultrafast Electron Microscopy: Single-Particle Imaging and Spectroscopy.”J. Am. Chem. Soc. 2011, 133, 10732-10735.
chem331/electron_microscopy.txt · Last modified: 2016/06/07 09:53 (external edit)