Infrared vibration studies of nanometer-sized Fe2O3 arrays

The polarized Fourier transform infrared (FTIR) spectroscopy is used to stu dy the surface-capped Fe2O3 nanoparticles and nanoparticle arrays. The inde pendent manometer-sized Fe2O3 crystals show typical confined phonon mode in relation to the, alpha -Fe2O3 phase. For the aggregates on different subst rates the FTIR showed different behavior as compared with the isolated disp ersions in KBr pellets, on the surface of Si wafer with those close-packed on the surface of KBr pellets and on the surface of Si wafers. The vibratio n mode variations in these spectra are discussed in terms of the enhancemen t of multipolar interactions, vibrational anisotropy and long-range electro n-phonon interactions, as well as magnetic coupling. Clear longitudinal opt ic phonon (LO) and transverse optic phonon (TO) splitting in the spectra of aggregate on the surface of. KBr pellets indicating a long-range order arr ay is observed. Magnetic coupling as an additional term of multipolar coupl ing enhancement may play an important role in the nanocrystal ordering. The observed contributions from ordered coupling are in contrast to a model by Gerardy and Ausloos, in which the Frohlich modes are dominant for isotropi c nanoparticle aggregation, because Fe2O3 crystal has anisotropic structure due to the magnetic couplings. However, their model applies for our partic le aggregates upon exposure to air for a few days and for samples with larg e size distribution, in which the temporal dipolar interaction is dominant. This spectral variation indicates a transformation from ordered magnetic a rrays to a Frohlich (charge-fluctuation)-like aggregates.