Massive
work function-mediated charge transfer in graphene/?-RuCl3 heterostructures
provides the necessary conditions for generating plasmon polaritons without
electrostatic or chemical doping. The image depicts a characteristic infrared
near-field image of such a heterostructure, revealing a host of plasmonic
oscillations derived from substantial mutual doping of interfacial graphene/α-RuCl3
layers. Courtesy: Daniel J. Rizzo/Columbia University.
Graphene,
an atomically thin carbon layer through which electrons can travel virtually
unimpeded, has been extensively studied since its first successful isolation
more than 15 years ago. Among its many unique properties is the ability to
support highly confined electromagnetic waves coupled to oscillations of
electronic charge—plasmon polaritons—that have potentially broad applications
in nanotechnology, including biosensing, quantum information, and solar energy.
However,
in order to support plasmon polaritons, graphene must be charged by applying a
voltage to a nearby metal gate, which greatly increases the size and complexity
of nanoscale devices. Columbia University researchers report that they have
achieved plasmonically active graphene with record-high charge density without
an external gate. They accomplished this by exploiting novel interlayer charge
transfer with a two-dimensional electron-acceptor known as α-RuCl3. The study
is available now online as an open access article and will appear in the
December 9th issue of Nano Letters.
"This
work allows us to use graphene as a plasmonic material without metal gates or
voltage sources, making it possible to create stand-alone graphene plasmonic
structures for the first time" said co-PI James Hone, Wang Fong-Jen
Professor of Mechanical Engineering at Columbia Engineering.
All
materials possess a property known as a work function, which quantifies how
tightly they can hold on to electrons. When two different materials are brought
into contact, electrons will move from the material with the smaller work
function to the material with the larger work function, causing the former to
become positively charged and the latter to become negatively charged. This is
the same phenomenon that generates static charge when you rub a balloon against
your hair.
α-RuCl3 is
unique among nanomaterials because it has an exceptionally high work function
even when it is exfoliated down to a one- or few-atom-thick 2-D layers. Knowing
this, the Columbia researchers created atomic-scale stacks consisting of
graphene on top of α-RuCl3. As expected, electrons were removed from the
graphene, making it highly conductive and able to host plasmon
polaritons—without the use on an external gate.
Using
α-RuCl3 to charge graphene brings two main advantages over electrical gating.
α-RuCl3 induces much greater charge than can be achieved with electrical gates,
which are limited by breakdown of the insulating barrier with the graphene. In
addition, the spacing between graphene and the underlying gate electrode blurs
the boundary between charged and un-charged regions due to "electric field
fringing." This prevents realization of sharp charge features within the
graphene and along the graphene edge necessary to manifest novel plasmonic
phenomena. In contrast, at the edge of the α-RuCl3, the charge in the graphene
drops to zero on nearly the atomic scale.
"One
of our major achievements in this work is attaining charge densities in
graphene roughly 10 times larger than the limits imposed by dielectric
breakdown in a standard gated device," said the study's lead PI Dmitri
Basov, professor of physics. "Moreover, since the α-RuCl3—the source of
electronic charge—is in direct contact with graphene, the boundaries between
the charged and uncharged regions in the graphene are razor-sharp. This allows
us to observe mirror-like plasmon reflection from these edges and to create
historically elusive one-dimensional edge plasmons that propagate along the
graphene edge." The team also observed sharp boundaries at
"nano-bubbles," where contaminants trapped between the two layers
disrupt charge transfer.
"We
were very excited to see how abruptly the graphene charge density can change in
these devices," said Daniel Rizzo, a postdoctoral research scientist with
Basov and the lead author on the paper. "Our work is a proof-of-concept
for nanometer charge control that was previously the realm of fantasy."
The work
was carried out in the Energy and Frontier Research Center on Programmable
Quantum Materials funded by the United States Department of Energy and led by
Basov. The research project used shared facilities operated by the Columbia
Nano Initiative.
The
researchers are now pursuing routes to use etched α-RuCl3 as a platform for
generating custom nanoscale charge patterns in graphene to precisely tune the
plasmonic behavior according to various practical applications. They also hope
to demonstrate that α-RuCl3 can be interfaced with a wide range of 2-D
materials to access novel material behaviors that require the exceptionally
high charge density imparted by interlayer charge transfer demonstrated in
their manuscript.
Hone
noted, "When our interlayer charge transfer technique is combined with
existing procedures for patterning 2-D substrates, we can easily generate
tailor-made nanoscale charge patterns in graphene. This opens up a wealth of
new opportunities for new electronic and optical devices".