Researchers from Northeastern University in Boston and University of Texas at Arlington (UTA) have used a process called auger-mediated positron sticking (AMPS) to develop a new technique that can measure the properties of the topmost atomic layer of materials.
This spectroscopic tool uses virtual photons to measure the topmost atomic layer’s electronic structure selectively. When incoming positrons change from vacuum states to bound surface states on the sample surface, they produce virtual photons with the energy to excite electrons into the vacuum.
The short interaction range of the virtual photons restricts the penetration depth to approximately the Thomas-Fermi screening length. Measurements and analysis of the kinetic energies of the emitted electrons made on a single layer of graphene deposited on Copper and the clean Copper substrate show that the ejected electrons originate exclusively from the topmost atomic layer.
Alex Weiss, professor and chair of the UTA Department of Physics, said, “We figured out how to use this phenomenon that we discovered in 2010 to measure the top layer and get information about the electronic structure and the behavior of the electrons in the top layer. That will determine a material’s many properties, including conductivity, and can have important implications for building devices.”
Alex Fairchild, a postdoctoral scholar in the Positron Lab, the study’s lead author, said, “The AMPS process is unique because it uses virtual photons to measure the topmost atomic layer.”
“This differs from typical techniques like photoemission spectroscopy, where a photon penetrates multiple layers into the bulk of a material and therefore contains the combined information of the surface and subsurface layers.”
Varghese Chirayath, assistant research professor, said, “Our AMPS results showed how virtual photons emitted following positron-sticking interact preferably with electrons that extend further into the vacuum than with more localized electrons to the atomic site. Our results are thus essential to understand how positrons interact with surface electrons and are extremely important to understand other similarly surface-selective, positron-based techniques.”
Weiss noted that the UTA Positron Lab is currently the only place this technique could have been developed due to the capabilities of its positron beam: “UTA probably has the only lab in the world that has a positron beam that can get down to the low energies needed to observe this phenomenon.”