Phonons in bulk CdSe and CdSe nanowires

We present first-principles calculations for bulk CdSe and CdSe nanowires with diameters of up to 22 angstrom. Their electronic and structural properties are presented and discussed. The vibrational properties of bulk CdSe and the zone-center vibrations of the nanowires are calculated and analyzed. An iterative, symmetry-based relaxation method is used that yields improved results for phonon frequencies. We find that the band gap varies with the surface termination and that strongly size-dependent and nearly constant vibrational modes exist in the nanowires, depending on the displacement directions. A strong shift in frequency for specific modes is found, stemming from surface contributions to the polarization, similar to that reported for thin slabs. A comparison with experimental data from Raman measurements is given.