Single atom imaging and spectroscopy in nanostructured carbon materials
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Introduction Low-dimensional carbon nanostructures, such as graphene, nanotubes, and fullerenes, are promising candidates for use as building blocks of new electronic and/or mechanical devices. Their physical and chemical properties are largely dependent on the atomic configurations with interrupted periodicity due to, for example, defects and edge structures. Element selective single-atom imaging was first demonstrated a decade ago for dopant detection in carbon nano-peapods, namely, fullerene molecules encapsulated in carbon nanotubes.1,2 A fully digitized scanning transmission electron microscope-electron energyloss spectroscopy (STEM-EELS) system3 that enabled spectrum imaging was indispensable to achieve statistical data analysis. Due to the high dose of the incident electron beam required for a high signal-to-noise ratio, specimen damage was, however, a severe problem in such a highly sensitive analysis technique. Reducing the accelerating voltage of the TEM/STEM then becomes essential when one aims to image any beam-sensitive objects. A lower accelerating voltage is preferred for the observation of small molecules composed of light elements, because molecular structures are not destroyed by the knock-on effect and, more importantly, the image/EELS contrast is enhanced. In order to compensate for the inferior spatial resolution that accompanies operating at a lower voltage, more sophisticated electron optics are required to reduce residual aberrations.
Sawada et al. designed a new type of aberration corrector with triple dodecapole elements (DELTA system) to reduce higher-order aberrations, such as six-fold astigmatism,4,5 which is very useful for a TEM/STEM operating at low accelerating voltages of approximately 30 to 60 kV. A TEM (JEOL 2100F) equipped with this corrector offers unique capabilities in terms of the spatial resolution normalized by the electron wavelength; this capability was proved by the presence of a Si (224) reflection in a high-angle annular dark-field (HAADF) image of a Si crystal observed along the [110] direction, see Figure 1.6 This reflection corresponds to 1.11 Å, just 16 times the 0.07 Å wavelength of the 30 kV electrons used to form the image. One of the major advantages of the low voltage STEM has been demonstrated by the chemical analysis of individual molecules without massive structural destruction.7 Despite this success, higher dose experiments still show non-negligible irradiation damage in metallofullerene molecules, as has been proven by the escape of encaged Er atoms from the peapod even at 60 kV.8 The calculated knock-on threshold energies for the various carbon materials suggest that, for the fullerene molecules that have higher curvatures than graphene or nanotubes, an accelerating voltage of 60 kV may not be low enough to completely avoid irradiation damage.9 An accelerating voltage below ∼35 kV seems preferable for observing the fullerene molecules.
Kazu Suenaga, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, Tsukuba, 305-8565, Japan
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