The surface of a material is usually as far as scientists have delved when exploring the object’s chemistry because it is where the most important reactions happen.
But with relatively unexplored materials, such as semiconductors and superconductors, looking past the surface has become increasingly important, which Lawrence Berkeley National Laboratory scientists have accomplished in recent research.
In a study published Aug. 14 in the journal Nature Materials, lab researchers explored the concept of using a high-energy version of the technique of angle-resolved photoemission — HARPES — to study the electronic structure of materials. Though HARPES and its less intense counterpart, ARPES, have been around since 1905 when Albert Einstein first explained the photoelectric effect — the basis for these techniques — scientists have only recently been able to make use of the techniques.
The photoelectric effect, an important part of the foundation of quantum mechanics, means that light behaves like a quantum of energy, or a photon, which can give all of its energy to an electron and liberate it as a photoelectron, which then flies away, explained Chuck Fadley — a distinguished professor of physics at UC Davis, an Advanced Light Source professor at Berkeley Lab and co-author of the paper.
“One then measures the energy of the electron and its direction as precisely as possible … and from these measurements can deduce exquisite detail about how the electron was moving in the material before it was ejected, how the atoms are bound to one another, whether they are magnetic or not, how the material will conduct electricity,” he said.
Without a sufficiently bright light source — and therefore sufficiently powerful beams of hard x-rays — scientists were unable to penetrate the materials on the atomic level that was necessary to observe interactions of photons and electrons beyond the surface.
Because HARPES uses higher energy, it can reveal interactions significantly past the scope of what the less powerful ARPES can do. According to Fadley, ARPES uses ultraviolet and soft x-ray photons and therefore cannot penetrate much past one nanometer, whereas with HARPES, scientists can measure photoelectrons and electronic properties at depths anywhere between 6 and 10 nanometers.
“You really give them a kick,” said Alexander Gray, a graduate student at UC Davis and Berkeley Lab and a co-author of the study. “This way, even with electrons that are deep, deep inside of the material, you can really get the information about the electronic properties of the material. You can seek them out, you can catch them and you can analyze them.”