Far infrared absorption by acoustic phonons in titanium dioxide   nanopowders
  We report spectral features of far infrared electromagnetic radiation absorption in anatase TiO2 nanopowders which we attribute to absorption by acoustic phonon modes of nanoparticles. The frequency of peak excess absorption above the background level corresponds to the predicted frequency of the dipolar acoustic phonon from continuum elastic theory. The intensity of the absorption cannot be accounted for in a continuum elastic dielectric description of the nanoparticle material. Quantum mechanical scale dependent effects must be considered. The absorption cross section is estimated from a simple mechanical phenomenological model. The results are in plausible agreement with the absorption being due to a sparse layer of charge on the nanoparticle surface. 
Daniel B. Murray, Caleb H. Netting, Lucien Saviot, Catherine Pighini,   Nadine Millot, Daniel Aymes, and Hsiang-Lin Liu