= Solution
<Transmission electron microscopy> accelerates <electrons> through a thin sample, then uses <electron> lenses to form an image or diffraction pattern. <Elastic scattering> encodes projected structure and potential; inelastic scattering carries excitation and chemical information. Thin specimens reduce unwanted multiple scattering and absorption, although coherent dynamical scattering can still matter in a crystal. High-resolution phase-contrast images depend on defocus and the transfer function as well as the specimen; they are not direct height maps.
The relativistic <wavelength> is $\lambda=h/\sqrt{2m_e eV[1+eV/(2m_ec^2)]}$. A shorter <wavelength> permits fine resolution but does not guarantee it. A schematic probe/image blur budget contains diffraction $\sim\lambda/\alpha$, <spherical aberration> $\sim C_s\alpha^3$ and <chromatic aberration> $\sim C_c\alpha\Delta E/E$, plus source size, instabilities, specimen motion and <radiation damage>. Optimizing the aperture angle balances these terms; increasing accelerating voltage can also alter damage and contrast.
Bright-field imaging accepts the central transmitted beam or its neighborhood. Regions that scatter <electrons> outside that acceptance can appear dark. Conventional dark-field imaging selects a diffracted beam. <Annular dark-field electron imaging> in a scanning TEM instead collects a range of scattered angles while a probe is scanned. A sufficiently high-angle detector often gives strong atomic-number sensitivity and more nearly incoherent contrast; thickness, channeling and detector geometry prevent a universal conversion of intensity to composition.
<Electron energy loss spectroscopy> measures <energy> lost by transmitted <electrons>. Core-ionization thresholds identify elements, while near-edge fine structure probes bonding and unoccupied states. At successive probe positions, one can record a spectrum image or form a map from a selected edge after background subtraction. Plural scattering, edge overlap, delocalization, specimen thickness and counting statistics limit the interpretation. Chemical specificity comes from excitation energies and spectral structure, not from brightness alone.
A development already established before this 2012 paper is <aberration-corrected transmission electron microscopy>: multipole correctors compensate leading lens aberrations, allowing useful larger angular acceptance and finer probes. Correction does not remove every chromatic, stability or specimen limit. A concrete application was atom-by-atom annular-dark-field analysis of monolayer boron nitride with substitutional defects, resolving chemical differences between sites in a nonuniform lattice.<Atom-by-atom structural and chemical analysis by annular dark-field electron microscopy by O. L. Krivanek and others>[2010, low-voltage aberration-corrected STEM of monolayer boron nitride]{ref} Combining structural maps with spatially resolved core-loss spectra supplies another route to distinguish composition and bonding at interfaces. Both exploit a calibrated imaging or spectroscopy response rather than assuming every visible spot is chemically self-identifying.
Back to article page
