Origin and tuning of surface optic and long wavelength phonons in biomimetic GaAs nanotip arrays

Nano-texturization provides sensitive routes for selection of preferred phonon modes. Biomimetic gallium arsenide (GaAs) nano-tips, with a pencil-like structure, prepared by an electron cyclotron resonance plasma etching of planar GaAs wafer demonstrates tunable strength of the surface optic (SO), and long wavelength transverse optic and longitudinal optic phonon modes. These modes can be tuned as a function of the length (L) of the nano-tips enabling phonon engineering. Invalidation of symmetry rules due to nano-texturization results in the excitation of a SO mode that can also be tuned, in strength and position, with L. Red shift of this mode with a change in the dielectric constant of the medium (air to aniline) confirms the SO nature. The theoretically estimated length scales indicate that the diameter modulated apexes of the nano-tips, whose length (L') increases consistently with L, could be responsible in transferring the required momentum to the SO phonons.