This scanning electron microscope picture shows individual crystal “grains” in an array of a material called graphene. Researchers have developed a method for creating the arrays, an advancement that opens up the possibility of a replacement for silicon in high-performance computers and electronics. (Image Care of University of Houston) |
Researchers
have developed a method for creating single-crystal arrays of a
material called graphene, an advance that opens up the possibility of a
replacement for silicon in high-performance computers and electronics.
Graphene
is a one-atom-thick layer of carbon that conducts electricity with
little resistance or heat generation. The arrays could make possible a
new class of high-speed transistors and integrated circuits that consume
less energy than conventional silicon electronics.
The
new findings represent an advance toward perfecting a method for
manufacturing large quantities of single crystals of the material,
similar to the production of silicon wafers.
“Graphene
isn’t there yet, in terms of high quality mass production like silicon,
but this is a very important step in that direction,” said Yong P.
Chen, corresponding author for the new study and Miller Family Assistant
Professor of Nanoscience and Physics at Purdue University.
Other
researchers have grown single crystals of graphene, but no others have
demonstrated how to create ordered arrays, or patterns that could be
used to fabricate commercial electronic devices and integrated circuits.
The
hexagonal single crystals are initiated from graphite “seeds” and then
grown on top of a copper foil inside a chamber containing methane gas
using a process called chemical vapor deposition. The seeded growth
method, critical to the new findings, was invented by Qingkai Yu,
co-corresponding author for the study and an assistant professor at
Texas State University’s Ingram School of Engineering.
“Using
these seeds, we can grow an ordered array of thousands or millions of
single crystals of graphene,” said Yu, who pioneered the method while a
researcher at the University of Houston. “We hope that industry will
look at these findings and consider the ordered arrays as a possible
means of fabricating electronic devices.”
Findings
are detailed in a research paper appearing online this week and in the
June issue of Nature Materials. The work was conducted by researchers at
Purdue, the University of Houston, Texas State University, Brookhaven
National Laboratory, Argonne National Laboratories and Carl Zeiss SMT
Inc.
Graphene
currently is created in “polycrystalline” sheets made up of randomly
positioned and irregularly shaped “grains” merged together. Having an
ordered array means the positions of each crystal are predictable, and
not random as they are in polycrystalline film.
The
arrays enable researchers to precisely position electronic devices in
each grain, which is a single crystal having a seamless lattice
structure that improves electrical properties, said Eric Stach, a
researcher at Brookhaven and former Purdue professor of materials
engineering.
The
new research findings confirmed a theory that the flow of electrons is
hindered at the point where one grain meets another. The arrays of
single-crystal grains could eliminate that problem.
The
researchers demonstrated that they could control the growth of the
ordered arrays; were the first to demonstrate the electronic properties
of individual grain boundaries; and they found that the edges of a
single hexagonal crystal grain are parallel to well-defined directions
in graphene’s atomic lattice, revealing the orientation of each crystal.
Knowing
the orientation is necessary to measure the precise properties of the
crystals, providing information needed to create better electronic
devices. To determine the orientation of the graphene lattice, the
researchers used two kinds of advanced microscopy techniques known as
transmission electron microscopy and scanning tunneling microscopy. The
techniques provided extremely high-resolution images of individual
carbon atoms making up graphene.
The electronic properties across the grain boundaries were measured using tiny electrodes connected to two adjoining grains.
Findings
demonstrated a higher electrical resistance at the grain boundaries and
also showed that the boundaries hinder electrical conduction due to
scattering of electrons. That finding was correlated using another
technique called Raman spectroscopy.
The
paper was authored by Yu and Purdue graduate student Luis A. Jauregui,
Houston graduate student Wei Wu, Purdue graduate student Robert Colby,
Purdue postdoctoral researcher Jifa Tian, along with 12 other
researchers including Stach and Chen.
Yu,
Wu, Houston graduate student Zhihua Su and colleagues created graphene
single crystals, including the ordered arrays for the research at the
University of Houston. Chen, Jauregui, Tian and colleagues studied
these hexagonal graphene crystals, their edges and the grain boundaries
using Raman spectroscopy, scanning tunneling microscopy and electrical
measurements. They have used various facilities in their Quantum Matter
and Device Laboratory, Purdue’s Birck Nanotechnology Center, as well as
those in the National High Magnetic Field Laboratory and the Center for
Nanoscale Materials at the Argonne National Laboratory. Stach and Colby
documented the crystal orientation using transmission electron
microscopes at Purdue’s Birck Nanotechnology Center and at the Center
for Functional Nanomaterials at the Brookhaven National Laboratory.
The
research was supported through a variety of funding sources, including
the National Science Foundation, the U.S. Department of Energy, the
Department of Homeland Security, Defense Threat Reduction Agency, IBM
Inc., the Welch Foundation, the Miller Family Endowment and Midwest
Institute for Nanoelectronics Discovery. Study abstract
