Still “Plenty of Room at the Bottom” for Aberration-Corrected TEM
Joerg R. Jinschek,1 * Emrah Yucelen,1 2 Bert Freitag,1 Hector A. Calderon,3 and
Andy Steinbach1 1 FEI Company, Europe NanoPort, Achtseweg Noord 5, 5651 GG Eindhoven, Te Netherlands 2 National Centre for HREM, Delſt University of Technology, Lorentzweg 1, 2628 CJ, Delſt, Te Netherlands 3 ESFM-IPN, UPALM ed. 9, Zacatenco DF 07738 Mexico City, Mexico
*
joerg.jinschek@fei.com
Introduction In his now-famous 1959 speech on nanotechnology [1],
Richard Feynman proposed that it should be possible to see the individual atoms in a material, if only the electron microscope could be made 100 times better. With the development of aberration correctors on transmission electron microscopes (TEMs) over the last decade, this dream of microscopists to directly image structures atom-by-atom has come close to an everyday reality. Figure 1 shows such a high-resolution transmission electron microscope (HR-TEM) image of a single-wall carbon nanotube obtained with an aberration- corrected TEM. Now that atomic-resolution images have become possible with aberration-corrector technology in both TEM and STEM, we can ask ourselves if we truly have achieved the goal of seeing individual atoms. Most aberration-corrected images exhibiting atomic resolution are not distinguishing individual atoms, but columns of a small number of atoms, so despite this remarkable achievement, there is still “plenty of room at the bottom” in order to move toward seeing, counting, and quantifying individual atoms.
In fact, there never has been a more exciting time for electron microscopists. In this article we discuss one of the exciting new technical
frontiers that is pushing toward quantitative, atomic imaging in all three dimensions. We describe one technique for performing quantitative atomic 3-D imaging using focal series reconstruction (FSR) [2] with aberration-corrected HR-TEM images [3]. We show that when the remaining small residual aberrations are removed, we can easily resolve individual atoms (as opposed to atom columns) with atomic resolution in three dimensions and unprecedented signal-to-noise ratio, even for beam-sensitive structures requiring lower accelerating voltages. Tis technique is demonstrated via an HR-TEM study using a specimen of single- and double-layer graphene at 80 kV accelerating voltage [4].
Ideal HR-TEM Imaging Theory Figure 2 depicts the theoretical, electron-wave basis for
quantitative HR-TEM imaging (aſter [5]). In figure 2a we see that when the electron wave passes from vacuum to the denser material of a single atom, the wavelength shortens. In figure 2b, the horizontal lines represent the plane-wave surfaces of constant electron phase, and we see that the effect of an atom on the wave is to retard or delay the electron phase (just as in light optics; when entering a material the speed of the light wave slows down, while its frequency remains constant, thus yielding an effectively shorter wavelength.) So the electron wave phase front “bends” around the electric potential of the screened atomic core as it passes an atom. Figure 2c depicts the evolving electron phase in a thin sample with several atoms. As the wave exits the sample, all of the sample information collected by the electron wave is contained in the so-called exit-wave phase (EW phase), which is simply the electron phase as a function of position just below the sample. In a very thin sample, only the electron wave’s phase is affected by passage through the sample, whereas the amplitude remains unchanged. Tis fact is reflected in the so-called phase object approximation [6]. Te EW phase is then given by the formula:
ϕExit Wave (x,y) = ∫0 ExitV(x,y,z) dz (1)
Figure 1: Aberration-corrected HR-TEM image of a single-walled carbon nanotube, obtained at 80 kV accelerating voltage. Image: Bert Freitag, FEI; sample: Professor Kiselev, Moscow, Russia.
10
where V is the electric potential of the atomic structure within the sample. So the exit phase is proportional to the electric potential integrated along z (that is, from the top to the bottom of the sample). Using 3-D structural models, the theoretical exit wave can thus be simulated and used for comparison with the real structure. Te experimental EW phase function is extremely valuable; in fact, if we can obtain it with atomic
doi:10.1017/S155192951100023X
www.microscopy-today.com • 2011 May
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76