Standing on the edge of a ledge gives off an uncomfortable and unstable feeling. Likewise, researchers at the U.S. Department of Energy discovered that gold nanoparticles act unusually when placed close to the edge of one-atom-thick graphene sheets -- theorized to have major implications for the development and study of new sensors and quantum devices.  

How Ultrafast Electron Microscope Led to Breakthrough Discovery

Gold Links
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In the exciting dawn of material science, the Ultrafast Electron Microscope is a one-of-a-kind tool found within the U.S. Department of Energy's Nanoscale Science Research Center that is used in investigating sub-picosecond structures and chemical dynamics in various materials at the nanoscale, utilizing electrons. 

The UEM at Argonne National Laboratory combines a tunable femtosecond laser equipped with high repetition rates, multiple routes producing pulsed electron beams, and a synchronous laser-pumped pulsed transmission electron microscope with a high-sensitivity camera and electron energy filtering system. 

Using the UEM enables visualization and detailed investigations of the nanoscale phenomena on time frames of less than a trillionth of a second. The discovery could have major implications in the vastly growing field of plasmonic that involve light striking material surfaces and triggering waves of electrons -- the plasmonic fields. 

For many years, scientists have been developing plasmonic devices with a wide range of applications, from optoelectronics to quantum information, to biological and medical sensors. 

To achieve this, researchers coupled 2D materials with atomic-level thickness -- graphene, with metal particles at the nanoscale. Understanding the correlation of plasmonic behaviors between two types of materials requires a deep understanding of how the materials are coupled. 

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Unusual Behavior of Gold Nanoparticles at the Edge of Graphene Sheets

In a study from Argonne, published in the journal Nano Letters, titled "Visualization of Plasmonic Couplings Using Ultrafast Electron Microscopy," researchers utilized the Ultrafast Electron Microscope at the U.S. DOE to look directly at coupling behaviors between gold nanoparticles and graphene.

Haihua Liu, a nanoscientist at Argonne, explains that surface plasmons are induced by light electron oscillations on the surfaces of nanoparticles or at the interface of nanoparticles and other materials, reports PhysOrg.  

In addition, when light is shone at nanoparticles, it creates a momentary plasmonic field. The pulsed electrons from the UEM interact with the plasmonic field moment when they overlap, with electrons either gaining or losing energy. Scientists collected the electrons that gain energy using energy filters to map out the distributions of plasmonic fields around the nanoparticles.  

When studying gold nanoparticles, the team of scientists discovered a bizarre phenomenon. When gold sat flat on the sheet of graphene, the plasmonic field was symmetrical. However, when there were positioned near the edge of graphene, the plasmonic field concentration was much stronger near the region's edge. 

The entire process of the experimentation, from the simulation of the gold nanoparticles to the plasmonic field detection, occurs at less than a hundred quadrillionths of a second. A better understanding of the coupling mechanisms between the nanoparticles of gold and graphene systems is key to future developments of novel plasmonic devices.

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