Here is an article about imaging lithium atoms. For the first time researchers have used the One Angstrom Microscope or a transmission electron microscope at the Department of Energy’s National Center for Electron Microscopy, NCEM, at Lawrence Berkeley National Laboratory 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.
The OÅM was used by Yang Shao-Horn of the Department of Mechanical Engineering at the Massachusetts Institute of Technology and Michael O’Keefe of Berkeley Lab’s Materials Sciences Division, to simultaneously resolve columns of lithium, cobalt, and oxygen atoms in the compound lithium cobalt oxide, LiCoO2. They and their colleagues report currently 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. Widely 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.
To advance their performance will entail understanding how the atoms in the electrode materials and the vacancies left by moving ions are arranged in 3-D on the atomic scale. It is known theoretically and it has been confirmed that the structure of LiCoO2 with techniques likes 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 have not they been seen in previous attempts to image LiCoO2 with electron microscopy, nor ions have never been seen by these techniques.
A transmission electron microscope or TEM works 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 exceptional 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’s columns of atoms can focus it.
Atoms are difficult to resolve since it has small dimensions and very small mass barely affect the electron beam. A problem worsens if there are heavier atoms nearby. Heavy cobalt, with atomic number 27 and atomic mass approaching 60, is relatively easy to image, but light oxygen, with atomic number eight and atomic mass about sixteen, scatters electrons weakly. Lithium is smaller still, its atomic number only three, its atomic mass only seven.
The ability of TEM’s 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. NCEM’s One Angstrom Microscope is a medium-energy TEM with a native resolution of 1.6 angstroms, an angstrom, symbolized Å, and is a ten-billionth of a meter, good enough to resolve cobalt atoms directly.
For the highest possible resolution it is necessary to go beyond native resolution to a microscope’s information limit 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.
But there’s a catch. Since because the separate images in the series are focused differently, in phase images of atoms in one image will be out of phase in another, there is a catch.
Researchers use two kinds of computer programs to know the right number of blobs. With the first, they can create a high-resolution simulation of what the microscope ought to see and the right number of blobs for the right number of atoms. The simulation program starts with a model of the material’s crystal structure, 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 simulated images. Such image simulation program in the late 1970s, applied a recent version to the theoretical structure of LiCoO2, the simulation provided such necessary information as what resolution the microscope had to attain to see anything at all, and at what resolution all the different kinds of atoms should be sharpest. The initial skepticism about whether lithium atoms could be imaged began to diminish.
Lithium ions should become visible at 1 Å for the simulation showed that in a sample of the right thickness. And at 0.8 Å all three kinds of atoms should be clearly visible. A resolution that was, indeed, just possible to reach with the One Angstrom Microscope. The simulation showed that the columns of oxygen atoms should appear bright and sharp-edged, the cobalts should be fuzzy, and the lithium should be small, weak, and look a little stretched.
Shao-Horn obtained many series of 20 differently focused images of individual crystals from a LiCoO2 powder sample in 2002 while working with NCEM’s Chris Nelson in order to master the operation of AOM. The sample was prepared and well characterized by conventional x-ray diffraction in collaboration with her colleagues. With a second computer program and their measurements of the microscope parameters, researchers take the experimental images and work backwards to generate a representation of the electron wave leaving the exit surface of the specimen. After confirming the focal value for each image in the series, Shao-Horn and her colleagues used the reconstruction program to create an image from a small area of a thin edge of a LiCoO2 crystal, which matched the one predicted, by the simulation program. Because material from which some lithium ions are removed is somewhat unstable under the electron beam, experimental imaging of the lithium ions and vacancies proved difficult. Nevertheless, the atomic resolution of lithium atoms is a novel and significant achievement, with implications for better understanding not only of lithium ion battery materials but of many other electroceramic materials as well.
It was made known that the range of the OÅM, and mid-voltage microscopes like it, can be extended all the way from heavy atoms down through oxygen, nitrogen, and carbon to the lightest metal in fact, the lightest atoms of all except for helium and hydrogen Atomic resolution of lithium ions in LiCoO2. Read more