Shape-selective Raman scattering from surface phonon modes in aggregates of amorphous SiO2 nanoparticles

A spontaneous Raman scattering from the surface phonon modes (SPMs) of amorphous silica (SiO2) nanoparticles which is small compared with the excitation wavelength (514.5 nm) and aggregated in micron size clusters (globules) is utilized to determine the shape of particles. The excitation dynamics for these modes were studied for silica samples pretreated at different temperatures (30-1000 degrees C). The SPMs are located between Raman peaks corresponding to the bulk traverse-optical (TO) and longitudinal-optical (LO) asymmetrical and bending phonon modes. An anomalously large TO-LO splitting (similar to 185 cm(-1)) of bending modes was observed. The shape examination was based on the fact that three depolarization factors are equal in value for spherical nanoparticles, but they are different for an elliptical one. According to this, one can observe one or three SPMs, respectively. The position of corresponding Raman peaks allows one to determine the depolarization factors and subsequently the shape of nanoparticles. The single SPM is located between Raman peaks corresponding to the TO and LO asymmetrical phonon modes and can be attributed to small spherical particles. Six different SPMs, which are located between Raman peaks corresponding to the TO and LO bending phonon modes, were observed. These modes can be assigned to elliptical nanoparticles in two surrounding mediums of different effective dielectric constants. The intensity of these SPMs is decreased drastically by heat treatment at a temperature around 950 degrees C, suggesting that aggregates of nanoparticles form the bulk alpha-quartz structure. The bulk structure formation from small silica particles was supported by thermogravimetric measurements. The values of the fraction of the total sample volume occupied by particles were calculated theoretically. A good agreement with the theory was obtained by assuming the anomalously large TO-LO splitting of bending phonon modes in nanoparticles of the amorphous silica. (C) 1997 American Institute of Physics.