Novel Devices ♦ news digest
[1] A. Belabbes, J. Furthmüller, and F. Bechstedt, Phys. Rev. B 84, 205304 (2011)
[2] T. Yamaguchi and Y. Nanishi, Appl. Phys. Expr. 2, 051001 (2009).
Using monolayers to create p-n junctions in graphene
New developments using graphene are suited for use in field-effect transistors. But if researchers are able to lower the temperature, they may be able to use graphene in optoelectronic products
The electronic properties of graphene films are directly affected by the characteristics of the substrates on which they are grown or to which they are transferred.
Researchers are taking advantage of this to create graphene p-n junctions by transferring films of the promising electronic material to substrates that have been patterned by compounds that are either strong electron donors or electron acceptors.
material’s lattice structure or significantly reducing electron/hole mobility.
The graphene was grown on a copper film using CVD, a process that allows synthesis of large- scale films and their transfer to desired substrates for device applications. The graphene films were transferred to silicon dioxide substrates that were functionalised with the self-assembled monolayers.
“We have been successful at showing that you can make fairly well doped p-type and n-type graphene controllably by patterning the underlying monolayer instead of modifying the graphene directly,” says Clifford Henderson, a professor in the Georgia Tech School of Chemical & Biomolecular Engineering. “Putting graphene on top of self-assembled monolayers uses the effect of electron donation or electron withdrawal from underneath the graphene to modify the material’s electronic properties.”
Creating n-type and p-type doping in graphene , which has no natural bandgap, has led to development of several approaches. Scientists have substituted nitrogen atoms for some of the carbon atoms in the graphene lattice, compounds have been applied to the surface of the graphene, and the edges of graphene nanoribbons have been modified. However, most of these techniques have disadvantages, including disruption of the lattice – which reduces electron mobility – and long-term stability issues.
Georgia Tech postdoctoral fellow Hossein Sojoudi holds a wafer containing graphene p-n junctions, while the screen display in the background shows electrical data measured in the devices. (Credit: Gary Meek)
A low temperature, controllable and stable method has been developed to dope graphene films using self-assembled monolayers that modify the interface of graphene and its support substrate. Using this concept, a team of researchers at the Georgia Institute of Technology has created graphene p-n junctions - which are essential to fabricating devices - without damaging the
“Any time you put graphene into contact with a substrate of any kind, the material has an inherent tendency to change its electrical properties,” Henderson says. “We wondered if we could do that in a controlled way and use it to our advantage to make the material predominately n-type or p-type. This could create a doping effect without introducing defects that would disrupt the material’s attractive electron mobility.”
Using conventional lithography techniques, the researchers created patterns from different silane materials on a dielectric substrate, usually silicon oxide. The materials were chosen because they are either strong electron donors or electron acceptors. When a thin film of graphene is placed over the patterns, the underlying materials create charged sections in the graphene that correspond to the patterning.
January/February 2013
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