= Solution
\b[Aberration-corrected <scanning transmission electron microscopy> with annular dark-field imaging and spatially resolved <EELS>] can locate the Si atoms and test their chemical environment. <silicon> scatters much more strongly to high angles than <carbon>, so an isolated Si impurity can stand out in a thin <graphene> lattice. Neighboring <carbon> positions reveal whether a site is, for example, three- or four-coordinated in projection.
Acquire a spectrum image at the same time. A <silicon> core-loss edge identifies the element, while its <energy-loss near-edge structure> contains information about the unoccupied states and bonding. This combination has experimentally distinguished individual Si coordination environments in <graphene>.<Direct Determination of the Chemical Bonding of Individual Impurities in Graphene by Wu Zhou and others>[Figures 1–2, atomic-resolution imaging and Si-edge spectra]{ref}
Advantages are atom-scale spatial localization, simultaneous structural and chemical evidence, and stronger chemical identification than brightness alone. Limitations include low counts at one atom, background subtraction, finite <energy> resolution, drift and the high dose needed for a useful spectrum. The beam can move or damage an impurity, and a two-dimensional projection does not by itself determine its out-of-plane geometry. Compare spectra and images with plausible structural calculations rather than assigning coordination solely from one bright pixel.
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