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May 26, 2011

Single-crystal arrays of graphene bring commercial graphene electronics closer

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.

Nature Materials - Graphene: Pushing the boundaries

Grain boundaries in polycrystalline graphene are an obstacle to electron transport. However, cunning refinements in growth techniques push the limits to obtain super-sized single-crystal domains.

Nature Materials - Control and characterization of individual grains and grain boundaries in graphene grown by chemical vapour deposition



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.

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