Abstract
The Ångström-sized probe of the scanning transmission electron microscope can visualize and collect spectra from single atoms. This can unambiguously resolve the chemical structure of materials, but not their isotopic composition. Here we differentiate between two isotopes of the same element by quantifying how likely the energetic imaging electrons are to eject atoms. First, we measure the displacement probability in graphene grown from either 12C or 13C and describe the process using a quantum mechanical model of lattice vibrations coupled with density functional theory simulations. We then test our spatial resolution in a mixed sample by ejecting individual atoms from nanoscale areas spanning an interface region that is far from atomically sharp, mapping the isotope concentration with a precision better than 20%. Although we use a scanning instrument, our method may be applicable to any atomic resolution transmission electron microscope and to other low-dimensional materials.
Highlights
The Ångstrom-sized probe of the scanning transmission electron microscope can visualize and collect spectra from single atoms
We measure the displacement probability in graphene grown from either 12C or 13C and describe the process using a quantum mechanical model of lattice vibrations coupled with density functional theory simulations
Due to the geometry of a typical transmission electron microscopy (TEM) study of a two-dimensional material, the out-of-plane velocity vz, whose distribution is characterized by the In amneanearslqieurarestuvdelyo1c1itythvizs2ðTwÞa, sis here of particular interest. estimated using a Debye approximation for the out-of-plane phonon density of states[14] (DOS) gz(o), where o is the phonon frequency
Summary
The Ångstrom-sized probe of the scanning transmission electron microscope can visualize and collect spectra from single atoms This can unambiguously resolve the chemical structure of materials, but not their isotopic composition. When a highly energetic electron is scattered by the electrostatic potential of an atomic nucleus, a maximal amount of kinetic energy (inversely proportional to the mass of the nucleus, pM1 ) can be transferred when the electron backscatters When this energy is comparable to the energy required to eject an atom from the material, defined as the displacement threshold energy Td—for instance, when probing pristine[11] or doped[13] single-layer graphene with 60–100 keV electrons—atomic vibrations become important in activating otherwise energetically prohibited processes due to the motion of the nucleus in the direction of the electron beam. The ability to do mass analysis in the transmission electron microscope expands the possibilities for studying materials on the atomic scale
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