Physicists at the University
of Pennsylvania have characterized an aspect of graphene film behavior by
measuring the way it conducts electricity on a substrate. This milestone advances
the potential application of graphene, the ultra-thin, single-atom thick carbon
sheets that conduct electricity faster and more efficiently than silicon, the
current material of choice for transistor fabrication.
The research team, led by A.T. Charlie Johnson, professor in the Department
of Physics and Astronomy at Penn, demonstrated that the surface potential above
a graphene film varies with the thickness of the film, in quantitative agreement
with the predictions of a nonlinear Thomas-Fermi theory of the interlayer screening
by relativistic low energy charge carriers. The study appears online in the
journal Nano Letters and will appear in print in the August edition.
Johnson's study, "Surface Potentials and Layer Charge Distributions in
Few-Layer Graphene Films," clarifies experimentally the electronic interaction
between an insulating substrate and few-layer graphene films, or FLGs, the standard
model for next-generation transistors.
It is more practical to develop devices from FLGs, rather than single-layer
materials. To make use of these films, graphene must be placed on a substrate
to be functionalized as a transistor. Placing the film on a substrate causes
an electronic interaction between the two materials that transfers carriers
to or from, or "dopes," the FLG.
The focus of the Penn study was aimed at understanding how these doped charges
distribute themselves among the different layers of graphene. The distribution
of these charges determines the behavior of graphene transistors and other circuits,
making it a critical component for device engineering. The team measured the
surface potential of the material to determine how these doped charges were
distributed along the transistor, as well as how the surface potential of the
transistor varied with the number of layers of graphene employed.
Using electrostatic force microscopy measurements, the team characterized the
surface potential of the graphene film and found it to be dependent on the thickness
of the graphene layers. The thicker the carbon strips, the higher the electronic
surface potential, with the surface potential approaching its limit for films
that were five or more sheets thick. This behavior is unlike that found for
conventional metals or semiconductors which would have, respectively, much shorter
or longer screening lengths.
The surface potential measurements were in agreement with a theory developed
by Penn professor and physicist Eugene Mele. The theory makes an important approximation,
by treating electrostatic interactions in the film but neglecting quantum mechanical
tunneling between neighboring layers. This allows the model to be solved analytically
for the charge distribution and surface potential.
While prior theoretical work considered the effect of a substrate on the electronic
structure of FLG, few experiments have directly probed the graphene-substrate
interaction. Quantitative understanding of charge exchange at the interface
and the spatial distribution of the resulting charge carriers is a critical
input to device design.
Graphene-derived nanomaterials are a promising family of structures for application
as atomically thin transistors, sensors and other nanoelectronic devices. These
honeycomb sheets of sp2 -bonded carbon atoms and graphene sheets rolled into
molecular cylinders share a set of electronic properties making them ideal for
use in nanoelectronics: tunable carrier type and density, exceptionally high
carrier mobility and structural control of their electronic band structures.
A significant advantage of graphene is its two-dimensionality, making it compatible
with existing planar device architectures. The challenge is realizing the potential
of these materials by fabricating and insulating them on substrates.
The study was performed by Sujit S. Datta and Mele of the Department of Physics
and Astronomy in the School of Arts and Sciences at Penn as well as Douglas
R. Strachan of the Department of Physics and Astronomy and also the Department
of Materials Science and Engineering within Penn's School of Engineering and
The study was funded by Penn's Nano/Bio Interface Center through the National
Science Foundation, the Army Research Office and the Department of Energy.