Rows of atoms: watching missing atoms with electrons
25 Feb, 2008 12:50 pm
While electron microscopes are available for more than 70 years, they have been recently significantly improved. The use of aberration correctors improved considerably resolution capabilities of electron microscopes and makes imaging of lighter atoms and vacancies possible. The combination with CCD detectors and multiple exposures with different defocus, opens imaging possibilities at atomic dimensions which was not thought possible 20 years ago.
Resolution limit: What limits resolution in general is the wavelength of the imaging particle. Photons in the visible have a wavelength of half a micrometer so lateral resolution cannot go around those limits without playing hard with the imaging optics. But even then it is only possible to improve lateral resolution by one order of magnitude and not beyond. Each quantum particle has a de Broglie wavelength. For electrons the de Broglie wavelength is below 0.1nm which is smaller than the interatomic distances in solids. But it took more than 70 years to improve electromagnetic lenses for electrons to approach atomic resolution. Interestingly only recently aberration-corrected electron microscopes have become available which opens new opportunities for electron microscopy to image at atomic scales. New contrast analysis makes it possible now to image light atoms such as oxygen and boron.
Improving imaging optics: To improve lenses one must reduce spherical aberrations. This means that rays bent at higher angles and focus at a shorter distance. The result is that the focal point is spread out and a disk is formed. Spherical aberration can be reduced by the use of an aperture which blocks rays coming in at higher angle. But this limits resolution because high lateral resolution needs larger diffraction angles. There is also chromatic aberration meaning the lens characteristics depends on the energy of the electron beam. New field emission guns, however, keep the energy of the electron beam within a narrow range and chromatic aberration does not need to be account anymore. Ernst Abbe demonstrated in 1872 that spherical aberration can be compensated by combining a convergent with a divergent lens. But the problem with electron lenses was for a long time that it was not possible to make divergent lenses. In 1940 Scherzer proposed how converging lenses can be constructed using magnetic multipoles. In the 1990ís (Haider-Rose) the first corrector lenses using hxapoles was finally demonstrated for the first time. The interaction of the electrons with atoms, phase shifts the wavefunction of the electron (the wavefunction of electrons describes its localization). - A smaller point spread due to aberration correction improves also phase contrast and has the effect that lighter atoms such as oxygen, nitrogen and boron can now be imaged. The improvement is so high (>0.08nm) that one can detect vacancies in atomic lattices. Defects are tremendously important for the properties of solids. A recent article in Nature Materials studies effects of elastic anisotropy on strain in decahedral gold nanoparticles using aberration correction. Gold nanoparticles find applications as markers in biomedicine today.
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