Researchers at Sungkyunkwan University and the Institute for Basic Science in Suwon, South Korea have designed a new method for opening up a band gap in graphene to allow the construction of graphene-based transistors.

In this study, the scientists have opened a band gap in graphene by carefully doping both sides of bilayer graphene in a way that avoids creating disorder in the graphene structure. Delicately opening up a band gap in graphene in this way enabled the researchers to fabricate a graphene-based memory transistor with the highest initial program/erase current ratio reported to date for a graphene transistor (34.5 compared to 4), along with the highest on/off ratio for a device of its kind (76.1 compared to 26), while maintaining graphene's naturally high electron mobility (3100 cm2/V·s).

The scientists' method is based on applying a vertical electric field through the bilayer graphene, which has been shown to break the symmetry between the two graphene layers. This modification creates atomic sites with different electric potentials, which produces a band gap. Previous studies have also used this strategy, in which the electric field is generated by "dual-side doping" of opposite sides of the bilayer with different chemicals. However, the previous results have been limited due to ineffective types and levels of dopants, which have generated relatively small electric fields and have also damaged the highly ordered graphene structure.

In the new study, the researchers demonstrate that one key to improving these areas is the choice of benzyl viologen (BV) as an electron-donating (n-type) dopant at the bottom of the bilayer graphene. The top side is then doped simply with oxygen and moisture from the atmosphere, which act as electron-withdrawing (p-type) dopants. As the BV molecules donate electrons to the bottom graphene layer, the atmospheric dopants withdraw the electrons from the top graphene layer, generating a vertical electric field. Since a stronger electric field induces a larger band gap, the researchers could control the band gap by using higher concentrations of dopants. All of the dopants used here are absorbed on the surface of the bilayer graphene without damaging the graphene structure, which helps to maintain graphene's high electron mobility and corresponding high "on" current.

XFNANO: Graphene and graphene-like materials since 2009 XFNANO: Graphene and graphene-like materials since 2009