New way to create graphene p-n junctions uses self-assembled monolayers

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.

A low-temperature, controllable, and stable method has been developed to dope graphene films using self-assembled monolayers (SAM) that modify the interface of graphene and its support substrate. Using this concept, a team of researchers at the Georgia Institute of Technology created graphene p-n junctions, which are essential to fabricating devices, without damaging the material’s lattice structure or significantly reducing electron/hole mobility. Creation of 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.

The graphene was grown on a copper film using chemical vapor deposition (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 made functional with the self-assembled monolayers.
Funding for the research came from the National Science Foundation, through the Georgia Tech Materials Research Science and Engineering Center (MRSEC) and through separate research grants.
Professor Clifford Henderson of the Georgia Tech School of Chemical & Biomolecular Engineering says fairly well doped p-type and n-type graphene  can be made controllable by patterning the underlying monolayer instead of modifying the graphene directly, and graphene can be put on top of self-assembled monolayers using the effect of electron donation or electron withdrawal from underneath the graphene to modify the material’s electronic properties.

When you put graphene in contact with a substrate, the material has a tendency to change its electrical properties. The researchers wondered if they could do it in a controlled way and use it to their 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.
The team was able to dope the graphene into both n-type and p-type materials through an electron donation or withdrawal effect from the monolayer. The monolayers are bonded to the dielectric substrate and are thermally stable up to 200ºC with the graphene film over them. The Georgia Tech team used 3-Aminopropyltriethoxysilane (APTES) and perfluorooctyltriethoxysilane (PFES) for patterning.

 Georgia Tech Professor Clifford Henderson’s face is reflected in a wafer containing graphene p-n junctions. The screen in the background shows electrical data measurements.

The researchers used their technique to fabricate graphene p-n junctions, which was verified by the creation of field-effect transistors (FET). Characteristic I-V curves indicated the presence of two separate Dirac points, which indicated an energy separation of neutrality points between the p and n regions in the graphene.
The group uses chemical vapor deposition to create thin films of graphene on copper foil. A thick film of PMMA was spin-coated atop the graphene, and the underlying copper was then removed. The polymer serves as a carrier for the graphene until it can be placed onto the monolayer-coated substrate, after which it is removed.
Beyond developing the doping techniques, the team is also exploring new precursor materials that could allow CVD production of graphene at temperatures low enough to permit fabrication directly on other devices. That could eliminate the need for transferring the graphene from one substrate to another.
A low-cost, low-temperature means of producing graphene could also allow the films to find broader applications in displays, solar cells and organic light-emitting diodes, where large sheets of graphene would be needed.
Find more information at www.gtresearchnews.gatech.edu/self-assembled-monolayers-for-graphene/
Paul O’Shea