  The localized effect of impurities in single GaN nanowires in the
sub-diffraction limit is reported using the study of lattice vibrational modes
in the evanescent field of Au nanoparticle assisted tip enhanced Raman
spectroscopy (TERS). GaN nanowires with the O impurity and the Mg dopants were
grown by the chemical vapor deposition technique in the catalyst assisted
vapor-liquid-solid process. Symmetry allowed Raman modes of wurtzite GaN are
observed for undoped and doped nanowires. Unusually very strong intensity of
the non-zone center zone boundary mode is observed for the TERS studies of both
the undoped and the Mg doped GaN single nanowires. Surface optical mode of A1
symmetry is also observed for both the undoped and the Mg doped GaN samples. A
strong coupling of longitudinal optical (LO) phonons with free electrons,
however is reported only in the O rich single nanowires with the asymmetric
A1(LO) mode. Study of the local vibration mode shows the presence of Mg as
dopant in the single GaN nanowires.
