  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.
