This is an article about the first observation of the smallest and tiniest metal atoms seen under the electron microscope. For the first time researchers have used a transmission electron microscope, the One Angstrom Microscope or OÅM to image lithium atoms. Only atoms of hydrogen and helium are smaller and lighter than those of lithium, which under ordinary conditions is not a gas but a soft, white metal.
LiCoO2 is commonly used in the positive electrodes of lithium rechargeable batteries, whose operation is based on reversible insertion and removal of lithium ions to and from their positive and negative electrodes. Extensively used in laptop computers, digital cameras, and many other devices, lithium ion batteries store more energy for their weight, operate at a higher voltage, and hold a charge much longer than other rechargeable batteries. o improve their performance will require understanding how the atoms in the electrode materials and the vacancies left by moving ions are arranged in three dimension on the atomic scale. The structure of LiCoO2 is known theoretically and has been confirmed with techniques like X-ray diffraction and neutron powder diffraction. Layers of lithium atoms lie between slabs of cobalt and oxygen, which are arranged in octahedrons. But lithium ions have never been seen by these techniques, nor have they been seen in previous attempts to image LiCoO2 with electron microscopy. Therefore when Shao Horn, who had been investigating lithium ion batteries with colleagues at the University of
A transmission electron microscope or TEM sends a beam of electrons through a thin sample of material. As the beam scatters from the electrical field of the atomic nuclei and their surrounding clouds of electrons, their unique arrangement affects the phase of the beam and to some extent its amplitude. When the altered beam exits the surface of a precisely oriented sample, an electromagnetic lens to project an image of the sample columns of atoms can focus it.
Since atoms with small dimensions and very little mass barely affect the electron beam, they are difficult to resolve a problem that worsens if there are heavier atoms nearby. Heavy cobalt, with atomic number 27 and atomic mass approaching 60, is relatively uncomplicated to image, but light oxygen, with atomic number 8 and atomic mass about 16, scatters electrons weakly. Lithium has atomic number only 3, its atomic mass only 7 and is still smaller.
The TEM ability to image these wispy particles depends on many factors including the microscope beam’s energy, energy spread, and steadiness, and the distortion, or aberration, of the lens. All these combine to determine the smallest distance; a microscope can distinguish between two adjacent objects, its native resolution. The NCEM One Angstrom Microscope is a medium energy TEM with a native resolution of 1.6 angstroms, good enough to resolve cobalt atoms directly.
For the highest possible resolution it is necessary to go beyond native resolution to a microscope information limit, the maximum amount of information about the sample that can be extracted from the scattered electron wave, even those portions of it that may be out of phase. One method of achieving this, called focal series reconstruction, uses a computer to combine successive images, each made at a slightly different focus. In this way the One Angstrom Microscope has achieved a resolution as high as 0.78 angstrom.
The simulation program starts with a model of the material crystal structure, and then dials in atomic specifications, the thickness and orientation of the sample, such parameters as the energy of the microscope’s electron beam, lens aberrations, beam misalignments, and other characteristics of both the sample and the instrument. Out comes a series of virtual images. Read more