Reaching a new resolution standard with electron microscopy

The invention of spherical aberration correctors has been the most significant contribution to the field of transmission electron microscopy since the field emission gun. Chromatic and spherical aberration, well known from optics, also play a role in electron microscopy. Lens aberrations are not unique to the magnetic lenses used in electron microscopes, but rather a fundamental problem of optical systems. Because of spherical aberration, rays entering a round lens system away from the optical axis are refracted more strongly than those entering close to the optical axis (see Fig. 1, top). A similar effect is chromatic aberration, which occurs when rays with different wavelengths enter a round lens, resulting in the rays diffracting differently depending on their wavelength (see Fig. 1, bottom). Transmission electron microscopes (TEM) and scanning transmission electron microscopes (STEM) equipped with such aberration correctors have been shown to resolve interatomic spacings approaching 50 pm[1, 2] and achieve single-atom sensitivity [3]. To compare, the highest resolution in uncorrected STEM imaging is about 140 pm at the same electron wavelength [4, 5]. Using aberration-corrected TEMs, one can now also perform atomic resolution imaging with longer wavelength electrons, which tend to be less damaging to samples. This is of particular importance for low-dimensional nanostructures, such as nanotubes, metal clusters, or single-layer sheets containing light constituent atoms, such as lithium, boron, or carbon. One of these novel low-dimensional nanostructures is single-layer hexagonal boron nitride. The hexagonal lattice of boron nitride contains boron and nitrogen atoms separated by a distance of 1.44 Å. This two-dimensional structure exhibits intriguing magnetic and electronic transport properties different from its monoatomic cousin, graphene. Single-layer boron nitride is also very sensitive to electron beams at energies higher than 80 kV. One could use beams of lower FIG. 1: (Top) Spherical aberration: The shorter the focal length, the smaller the spherical aberration coefficient and hence smaller amount of blur. (Bottom) Chromatic aberration: The smaller the energy width of the electron source and/or the better the stability of the high voltage (the lens), the smaller the chromatic spread and hence smaller chromatic aberration. (Illustration: Alan Stonebraker)