Everyone who has ever been to the Grand Canyon can relate to having strong
feelings from being close to one of nature's edges. Similarly, scientists at
the U.S. Department of Energy's (DOE) Argonne National Laboratory have
discovered that nanoparticles of gold act unusually when close to the edge
of a one-atom thick sheet of carbon, called graphene. This could have big
implications for the development of new sensors and quantum devices.
This discovery was made possible with a newly established ultrafast electron
microscope (UEM) at Argonne's Center for Nanoscale Materials (CNM), a DOE
Office of Science User Facility. The UEM enables the visualization and
investigation of phenomena at the nanoscale and on time frames of less than
a trillionth of a second. This discovery could make a splash in the growing
field of plasmonics, which involves light striking a material surface and
triggering waves of electrons, known as plasmonic fields.
For years, scientists have been pursuing development of plasmonic devices
with a wide range of applications—from quantum information processing to
optoelectronics (which combine light-based and electronic components) to
sensors for biological and medical purposes. To do so, they couple
two-dimensional materials with atomic-level thickness, such as graphene,
with nanosized metal particles. Understanding the combined plasmonic
behavior of these two different types of materials requires understanding
exactly how they are coupled.
In a recent study from Argonne, researchers used ultrafast electron
microscopy to look directly at the coupling between gold nanoparticles and
graphene.
"Surface plasmons are light-induced electron oscillations on the surface of
a nanoparticle or at an interface of a nanoparticle and another material,"
said Argonne nanoscientist Haihua Liu. "When we shine a light on the
nanoparticle, it creates a short-lived plasmonic field. The pulsed electrons
in our UEM interact with this short-lived field when the two overlap, and
the electrons either gain or lose energy. Then, we collect those electrons
that gain energy using an energy filter to map the plasmonic field
distributions around the nanoparticle."
In studying the gold nanoparticles, Liu and his colleagues discovered an
unusual phenomenon. When the nanoparticle sat on a flat sheet of graphene,
the plasmonic field was symmetric. But when the nanoparticle was positioned
close to a graphene edge, the plasmonic field concentrated much more
strongly near the edge region.
"It's a remarkable new way of thinking about how we can manipulate charge in
the form of a plasmonic field and other phenomena using light at the
nanoscale," Liu said. "With ultrafast capabilities, there's no telling what
we might see as we tweak different materials and their properties."
This whole experimental process, from the stimulation of the nanoparticle to
the detection of the plasmonic field, occurs in less than a few hundred
quadrillionths of a second.
"The CNM is unique in housing a UEM that is open for user access and capable
of taking measurements with nanometer spatial resolution and sub-picosecond
time resolution," said CNM Director Ilke Arslan. "Having the ability to take
measurements like this in such a short time window opens up the examination
of a vast array of new phenomena in non-equilibrium states that we haven't
had the ability to probe before. We are excited to provide this capability
to the international user community."
The understanding gained with regard to the coupling mechanism of this
nanoparticle-graphene system should be key to the future development of
exciting new plasmonic devices.
A paper based on the study, "Visualization of plasmonic couplings using
ultrafast electron microscopy," appeared in the June 21 edition of Nano
Letters. In addition to Liu and Arslan, additional authors include Argonne's
Thomas Gage, Richard Schaller and Stephen Gray. Prem Singh and Amit Jaiswal
of the Indian Institute of Technology also contributed, as did Jau Tang of
Wuhan University and Sang Tae Park of IDES, Inc.
Reference:
Haihua Liu et al, Visualization of Plasmonic Couplings Using Ultrafast
Electron Microscopy, Nano Letters (2021).
DOI: 10.1021/acs.nanolett.1c01824
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