Surface mode absorption and infrared optical properties of gallium phosphide nanoparticles

The surface optical (SO) mode of ellipsoidal gallium phosphide (GaP) nanoparticles is investigated by infrared transmission spectroscopy. The surface mode theory of diatomic cubic particles is generalized and applied to GaP nanoparticles in a systematic treatment. The Fröhlich mode of GaP nanoparticles has been observed in our experiment. As far as surface mode frequency is concerned, the result of the experiment agrees with that of the theoretical calculation. The characteristics of the SO mode peak, including frequency shift, broadening and line shape, are analyzed. The frequency shift is attributed to the surrounding medium effect, surface oxidation and the aggregation effect as well as intrinsic point defects; the broadening is mainly due to the non-spherical particle shape, aggregation and quantum confinement effect; and the line shape is related to the particle shape and the damping function.