Surface-enhanced Raman scattering of graphene caused by self-induced nanogating by GaN nanowire array

TL;DR

This study demonstrates that uniform GaN nanowire arrays induce significant surface-enhanced Raman scattering in graphene through self-induced nanogating, with the enhancement dependent on nanowire uniformity rather than strain or carrier concentration.

Contribution

It reveals that nanowire uniformity causes Raman enhancement in graphene via self-induced nanogating, a novel mechanism distinct from strain or doping effects.

Findings

Uniform nanowires produce strong Raman enhancement.

Nanowire height differences cause graphene stretching and piercing.

Carrier concentration modulation depends on nanowire density.

Abstract

A constant height of gallium nitride (GaN) nanowires with graphene deposited on them is shown to have a strong enhancement of Raman scattering, whilst variable height nanowires fail to give such an enhancement. Scanning electron microscopy reveals a smooth graphene surface which is present when the GaN nanowires are uniform, whereas graphene on nanowires with substantial height differences is observed to be pierced and stretched by the uppermost nanowires. The energy shifts of the characteristic Raman bands confirms that these differences in the nanowire height has a significant impact on the local graphene strain and the carrier concentration. The images obtained by Kelvin probe force microscopy show clearly that the carrier concentration in graphene is modulated by the nanowire substrate and dependent on the nanowire density. Therefore, the observed surface enhanced Raman scattering for graphene deposited on GaN nanowires of comparable height is triggered by self-induced nano-gating to the graphene. However, no clear correlation of the enhancement with the strain or the carrier concentration of graphene was discovered.