Aberration-Corrected TEM
Figure 4: Figures (a–c) show the subsequent improvement in imaging of the same graphene sample by the process described in the text: (a) a raw aberration- corrected TEM CCD image of the sample at 0.1 s exposure time, (b) an EW phase image obtained through FSR of 19 such raw images at constant focal step, and (c) the FSR EW phase image from (b) with residual aberrations numerically removed [4].
For beam-sensitive soſt materials like graphene, the low
threshold for “knock-on” damage (where atoms are ejected by the energetic electron beam, estimated to be about 113 keV for graphene) requires a low accelerating voltage, such as 80 kV. Tis has advantages because electron scattering becomes stronger at lower accelerating voltages, but it also has disadvantages because resolution decreases and residual aberrations increase at lower voltage. At such low voltages, the electron beam energy spread becomes the spatial-resolution limiting parameter in a microscope corrected for spherical aberration but not chromatic aberration [11]. Tis experiment was conducted with an energy spread slightly below 0.2 eV leading to an achievable spatial resolution of at least 0.11 nm (1.1 Å) or better, which is important for this study so that the C–C bond length in graphene of 0.142 nm (1.42 Å) can be well-resolved.
The Graphene Study: Experimentally Reconstructed Focal Series Results A FSR of the graphene sample EW phase and exit-wave
amplitude has been performed using FEI’s TrueImage soſtware [8]. Te images were recorded with a 2k × 2k pixel CCD array using a short acquisition time of 0.1 second per image, chosen to fulfill dose constraints and also to minimize the likelihood of atomic-level damage or motion of the sample during the full series of 19 images. A focal step of –1.9 nm/image was used, and, importantly, the sampling on the CCD camera was set to 0.0094 nm/pixel. Figure 4a shows a single HR-TEM CCD image of the
sample, which includes an area of both double- and single- layer graphene, as well as a region of “vacuum” (that is, a hole in the single-layer structure). Although this image is noisy, nonetheless a hint of the hexagonal lattice structure begins to emerge in the single-layer region. However, the appearance of this type of CCD image depends on the exact value of defocus. Tus, for unambiguous comparison with theory, one should ideally work with the EW phase, which is shown in Figure 4b, obtained via FSR on the nineteen CCD images as described above. Although Figure 4b appears to be less noisy than Figure 4a, there is still a residual blur in the single-layer region that does not permit an unambiguous interpretation of the
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hexagonal atomic structure of single-layer graphene. Tis blur is due to residual aberrations and not the presence of noise. Proof of this statement is seen in Figure 4c, in which the residual optical aberrations have been numerically removed from the EW phase image using the TrueImage soſtware. Te final result shown in Figure 4c is the experimental EW phase image of the sample corrected for residual aberrations. In this final image, the well-known hexagonal arrangement of single carbon atoms in the single-layer graphene is very clearly resolved. In the next section, we explain the theoretical model that was used for comparison with the experimental results and the method that was used to remove the residual aberrations.
The New Frontier: Eliminating Residual Aberrations After Aberration Correction Figure 4c shows a numerically reconstructed experimental
EW phase image, which appears to show an area of double- layer graphene at the top and a single-layer in the area in the lower half of the image. In order to compare these data with theoretical expectations, a numerical simulation was performed using the multi-slice algorithm in the MacTempas soſtware by Total Resolution LLC [12]. Figure 5a shows a sketch of the input structural model to the simulation program to
Figure 5: (a) Schematic of the model structure for the graphene sample used as input to the simulation. (b) The EW phase image of the simulated model structure, containing both areas of double- and single-layer graphene, similar to the actual sample in the experimental study [4].
www.microscopy-today.com • 2011 May
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