  The realization of semiconductor structures with stable excitons at room
temperature is crucial for the development of excitonics and polaritonics.
Quantum confinement has commonly been employed for enhancing excitonic effects
in semiconductor heterostructures. Dielectric confinement, which is potentially
much stronger, has proven to be more difficult to achieve because of the rapid
nonradiative surface/interface recombination in hybrid dielectric-semiconductor
structures. Here, we demonstrate intense excitonic emission from bare GaN
nanowires with diameters down to 6 nm. The large dielectric mismatch between
the nanowires and vacuum greatly enhances the Coulomb interaction, with the
thinnest nanowires showing the strongest dielectric confinement and the highest
radiative efficiency at room temperature. In situ monitoring of the fabrication
of these structures allows one to accurately control the degree of dielectric
enhancement. These ultrathin nanowires may constitute the basis for the
fabrication of advanced low-dimensional structures with an unprecedented degree
of confinement.
