Due to its remarkable electronic properties, few layer graphene, or FLG, has
emerged as a promising new material for use in post-silicon devices that incorporate
the quantum effects that emerge at the nanoscale. Now, physicists at the University
of Pennsylvania have demonstrated a new method by which FLG can be etched
along flawless, crystallographic axes by using thermally activated nanoparticles,
a technique that results in atomically precise, macroscopic length ribbons of
graphene. The advance could enable atomically precise, and far simpler, construction
of integrated circuits from single graphene sheets with a wide range of technological
applications.
A.T. Charlie Johnson, professor in the Department of Physics and Astronomy
at Penn, and his team have demonstrated this new etching process which relies
on catalytic metal particles to etch the graphene along precise atomic directions.
Johnson's team is now attempting to refine their control of the process and
test Penn's capability to fabricate devices whose properties will reflect the
intrinsic quality of atomically precise graphene.
"Graphene is a great material for electronics, but it would be even better
if it were possible to create devices with crystallographic edges, that is,
edges where the atoms lie along single lines in the graphene plane," Johnson
said. "Standard etching techniques being used in the semiconductor industry
do not allow this sort of fabrication. Instead, they produce rough edges with
lots of atomic scale defects that limit the performance of the fabricated devices."
Specifically, the Penn team investigated the construction of atomically precise
graphene nanoribbons in which charge-carrying electrons are confined in a nearly
two-dimensional, lateral plane and the electronic properties of the ribbon are
controlled by the width and specific crystallographic orientation of the material.
These structures hold enormous promise as nanoscale devices, with the advantage
that graphene's two-dimensionality lends itself to existing device architectures
based on planar geometries.
Attempts with current nanofabrication standards such as lithography and plasma
etching, however, have left rough edges to the nanoribbons that affect their
performance. Until now, these structures have been impossible to achieve because
the rough, non-crystalline edges of the graphene, resulting from current state-of-the-art
nanolithography techniques, are considered the limiting factor to attaining
useful performance from nanoscale graphene devices. Even atomic-scale flaws
would derail electrical conductivity of any graphene transistors. Johnson's
technique, employing hot iron nanoparticles to carve out patterns in graphene
sheets, appears to be the first detailed example of such precise fabrication.
To create these ribbons, researchers deposited graphene onto a silicon substrate,
coated them in iron nitrate and heated them to 900° C. At that temperature,
the iron forms particles with diameters of about 15 nm, spreads across the surface
of the substrate and etches away trenches in the graphene sheets.
By identifying areas where two iron nanoparticles carved parallel tracks like
skis in fresh snow, researchers managed to isolate nanoribbons as narrow as
15 nm and as much as a few micrometers long. The nanoparticles travel predominantly
along a single direction, although why this was so is a question for another
study. However, scientists also observed the existence of other paths of nanoparticles,
at angles of 30º and 60º, suggesting possibly that the motion of the
iron nanoparticles — and hence the etching — is related to the atomic
structure of graphene, a honeycomb shape employing those measurements. This
natural phenomena could be used in the future to fabricate devices and circuits
with those required angles.
The study was performed by Johnson, Sujit S. Datta and Samuel M. Khamis 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 Applied Science.
The study was funded by Penn's Nano/Bio Interface Center through the National
Science Foundation, the Army Research Office and the Intelligence Community
Postdoctoral Fellowship Program.