Negative pressure effects in SrTiO3 nanoparticles investigated by Raman spectroscopy

The size effects on SrTiO3 nanoparticles have been investigated by means of Raman spectroscopy with changing the grain size in the range 10–80 nm. The intensities of the first-order polar TO2 and TO4 modes increase as the grain size reduces, suggesting the enhanced interaction of the surface-defect dipoles on the grain boundary. By contrast, the intensities for the first-order nonpolar TO3 mode decrease with reducing the grain size. Further we have found that the Raman frequencies of the vibration modes are very sensitive to the variation of the grain size. The softening of the TO2 and TO3 modes with decreasing the grain size indicates the increase of the Ti–O bond length, which is consistent with the lattice expansion investigated by XRD. We have ascribed the size effects to the negative pressure effects due to the enhanced interaction of the surface-defect dipoles.