Tools: In previous blogs I have mentioned that to truly work at a small scale, one needs to be able to measure to at least a dimension that is one-tenth the size of what you are measuring. Ideally, the capabilities should be two orders of magnitude smaller. This has been a real problem with material in the low double digit and single digit nanometer scale. There is a report [Ref. 1] from two researchers at UC Irvine have developed a methodology using a scanning transmission electron microscope to image charge density at sub-angstrom scales. This is in the early stages of development and there are limitations on sample sizes and spatial resolution in their current equipment. They project that learning from this initial effort will provide the direction for novel measurement capabilities.
So, what is the principle on how this development works? “Nearly all the physical properties of materials are determined by how electron charge is rearranged between nuclei when atoms aggregate together. Being able to directly visualize how electrons are distributed is therefore important. Compared to other diffraction methods, aberration-corrected STEM (AC-STEM) allows for atomic-scale imaging of a sample using an electron beam, or probe, focused to about half an angstrom in size. When electrons pass through the sample, they interact with the internal electric field in their path through the Lorentz force. This changes the beam’s momentum, which can then be measured by diffraction.” [Ref. 1]
The researchers work with a composite material employing ferroelectric oxide bismuth ferrite and oxide strontium titanite as the insulator. They did a raster scan of a surface area and acquired a diffraction pattern at each point on the sample. They then employed this technique to visualize the charge transfer between the two materials. With high resolution there were able to determine the local charge distribution, which provides information on the distribution of the positive ionic cores and the separation of the electrons. [Full paper available in Ref. 2]
Materials: A concern with materials in applications that are subjected to very low temperatures is that metals shrink. (This is the opposite effect that water has as it turns into ice.) This is true for planes and requires using composites and/or alloys with opposing expansion properties to balance the shrinkage out. Research being conducted at the U.S. DoE Brookhaven National Laboratories is exploring a metal that dramatically expands at low temperatures. [Ref. 3] Using samarium sulfide doped with impurities, they are delving into details of the material’s atomic structure and the electron-based origins of the materials negative thermal expansion.
Unlike water, with a good explanation of the expansion properties, the cause of the expansion of samarium sulfide was unknown. This particular compound can be formed into two different types depending on the external condition in developing the material. The gold-colored variety of the compound is a conductor with electrons moving freely, while the black -colored one is a semiconductor. The resultant investigations and theoretical calculations pointed to a Kondo effect. This effect is that electrons will interact with magnetic impurities and in a material and align spin in the opposite direction of the larger magnetic particles to cancel their effect. It appears that the completeness of the outer electron shell is critical. It appears that the negative thermal expansion of samarium sulfide can be tuned by varying the amount of impurities. The researchers indicated that samarium, thulium, and ytterbium should all have properties that can be useful.