A unique instrument at Argonne's Center for Nanoscale Materials combines a modified transmission electron microscope with an ultrafast laser system to reveal small details with potentially big impacts.
We can't see nanoparticles with our eyes, but we can with electron microscopes.
When scientists view different materials through a traditional transmission electron microscope (TEM), they can see important microscopic characteristics of materials. If they probe the same materials with a combination of the high resolution of the microscope and an ultrafast laser system, they can capture images of what happens when the laser interacts with the material. They then can see impressively tiny details happening in a trillionth of a second.
In the Center for Nanoscale Materials (CNM), a U.S. Department of Energy (DOE) Office of Science user facility at the DOE's Argonne National Laboratory, scientists established a unique new instrument called an ultrafast electron microscope (UEM) to better capture this kind of nanoparticle activity. It is the first instrument of its kind available at a national user facility.
"We are combining high spatial resolution, so we can see nanoscale things, with high time resolution, so we can see ultrafast changes in these things," said Ilke Arslan, director of the CNM and of Argonne's Nanoscience and Technology division, who led development of the UEM.
Arslan and colleagues Haihua Liu and Thomas Gage began working with contractors to modify a TEM in early 2019. They added new components to the traditional column of the microscope, plumbing an ultrafast laser system into the TEM to obtain the high resolution in both space and time.
"Our ultrafast pump probe laser system splits a laser pulse into two separate pulses, a pump pulse that excites the sample, and a probe pulse that generates electron pulses used for imaging, which can tell us something about the materials," explained Gage. Users can adjust the timing between the two pulses' arrival at the sample during their experiments.
"When the pump pulse hits the sample, it creates excitation," said Gage. "With the UEM, we can delay the probe pulse to come through at different points in time of that excitation to build up a movie we can use to study the dynamics of a material."
The laser system can generate a wide range of wavelengths to excite materials, from 325 to 2,000 nanometers. This allows for highly diverse experiments in areas important to potential users, including phonon dynamics, plasmon dynamics, catalytic reactions and charge density waves. Arslan believes the UEM could also be used to explore areas such as solar energy, quantum information science and nanomedicine.
Liu, Gage and Arslan, for example, led a team that used the UEM to look at graphene and a gold nanocapsule and to observe plasmonic coupling between them. Plasmonic coupling is a phenomenon where energy shifts in the field between particles. Their research shows that the plasmonic field was symmetric when the gold nanocapsule sat on a flat sheet of graphene. However, it concentrated much more strongly near the edge region when the gold nanocapsule was positioned close to a graphene step edge. This suggests light at the nanoscale may be used to manipulate charge in the plasmonic field. The insight could lead to applications within nanoplasmonic devices integrated into cell phones or sensors or used in nanomedicines. Liu, Gage and Arslan recently published their results in Nano Letters.
Because the UEM is such a novel tool, scientists such as Gage and Liu work in concert with users to facilitate its operation, ensure safety and communicate workflows and results. During COVID-19 restrictions, this assistance has happened mostly by video, but the plan is for UEM users to apply for hands-on opportunities in the future, with assistance from the UEM team.
Partial funding for the ultrafast electron microscope came from the DOE Office of Basic Energy Sciences (Scientific User Facilities division).