YSM Issue 94.2

FOCUS
Material Science
3D
2D SOLUTIONS
TO
PROBLEMS
Studying the electrical properties of
altered 2D materials
BY CATHERINE ZHENG
Electronics are ubiquitous in our
everyday lives—they are in the cars
we drive, the microwaves that heat
up our food, and the computers we use.
This omnipresence is due to technology’s
constant evolution. Currently, unbelievably
complex technologies can be found in even
common devices, such as our cell phones.
The creation of materials that are elaborate
in their complexity but simple in their
design—and can thus be implemented
into many technological devices—sits at
the intersection of electrical engineering
and materials science.
Judy Cha, Yale Professor of Mechanical
Engineering and Materials Science, has
led her lab in important work within these
fields. Her team focuses on discovering
new layered materials that can be used
in electronics. Through manipulating
their electrical properties, they seek to
understand more about the materials
themselves and how they can be used. The
team hopes to elucidate which materials
might be particularly ideal for a certain
electronic device, as well as how the
materials’ performance can be improved by
altering electrical properties.
In the beginning of 2021, Cha’s group and
its collaborators published two studies: one
focusing on the mechanical properties of
graphene with lithium between its layers,
and the other on molecular doping—the
addition of small molecules to materials to
activate them for use in electronics.
ART BY SOPHIA ZHAO
Lithium-Ion Batteries
2D materials are usually approximately one
to three atoms thick. They come from layered
materials that are exfoliated down to a single
layer. The properties of 2D materials normally
change when their layers are isolated.
Graphite, commonly found in pencil lead,
is a classic example of this: graphite consists
of layers of graphene, and the individual
graphene layers have properties that differ
substantially from those of graphite as a
whole. To harness such materials for device
applications, it is important to understand
these thickness-dependent changes.
Lithium intercalation into graphite—or,
the insertion of lithium between layers
of graphene that are held together by
van der Waals (vdW) forces—is essential
to create lithium-ion batteries, which
power many of our modern electronics.
Staging, or structural ordering, minimizes
electrostatic repulsions within graphite’s
crystal lattice, allowing lithium to order
itself between the van der Waals gaps of
graphene. Initially, lithium is randomly
distributed throughout the graphite, but as
the lithium concentration increases, there’s
a phase transition where lithium moves
laterally to form intercalated regions that
are vertically separated by unintercalated
regions. The kinetics of lithium moving
between the sheets of graphene are directly
related to how well the battery performs.
This staging process is well understood
for bulk graphite, which is thick. But on a
nanoscale, the way lithium and graphene are
confined so closely together affects the overall
structure. As lithium makes its way between
vdW gaps, these gaps expand to accommodate
the new atoms. However, anchoring graphene
sheets by clamping the edges down changes
the way lithium interacts with the graphene.
This constrains how much the graphene is able
to open, and it requires more work for lithium
to squeeze into those gaps. The kinetics of
lithium diffusion are also slowed down, since
it becomes more difficult for lithium to move
between the graphene sheets.
The effect of this mechanical strain sparked
the interest of Cha’s group. To investigate it
further, they used thin sheets of graphene—
between four and fifteen layers thick—with
gold electrodes on top that acted as a clamp.
Although these electrodes were only onehundred
nanometers thick, each layer of
graphene was even thinner: one-third of a
nanometer. The difference in size allowed the
gold to act as a source of pressure and hold
down the ends of the graphene sheets.
But as they observed this configuration,
the group realized that, while the edges of
the graphene sheets stayed still, the center
would expand freely, stretching in both the
x and y directions and causing strain in the
graphene. “Interestingly, what we found is
this strain can delay the lithiation kinetics
of graphene,” said Josh Pondick, a PhD
candidate in Cha’s lab and one of the lead
researchers for this experiment—lithiation
12 Yale Scientific Magazine May 2021
www.yalescientific.org