Raman study of E2 and surface phonon in zinc oxide nanoparticles surrounded by organic molecules

Using Raman spectrometry, we obtained results showing the influence of organic ligands on the vibrational properties of small zinc oxide nanocrystals (2.1-6.8 nm). It is shown that it is possible to distinguish both mechanical and dielectric effects from the E-2 nonpolar phonon mode and from a surface mode, theoretically predicted but rarely observed. It has been found that E-2 phonon is not dependent on the nanocrystal size, but its frequency decreases with increasing ligand length, characteristic of a tensile stress on the nanocrystal. We report also the observation of a surface optical mode, the experimental frequency of which is in reasonable agreement with available calculations. (c) 2007 American Institute of Physics.