Low Frequency Raman Scattering from Acoustic Phonons Confined in ZnO Nanoparticles

We report here the first observation of the low frequency Raman scattering from acoustic phonons in semiconducting zinc oxide (ZnO) nanoparticles without embedding in any solid matrix. ZnO nanoparticles (size 5–10 nm) with nearly spherical shape have been synthesized using a chemical route. A shift in the phonon peaks toward higher frequencies along with broadening was observed with a decrease in particle size. The size dependence of the acoustic phonons in ZnO nanoparticles is explained using Lamb’s theory that predicts the vibrational frequencies of a homogeneous elastic body of spherical shape. Our results show that the observed low frequency Raman scattering originates from the spherical ($l=0$) and quadrupolar vibrations ($l=2$) of the spheroidal mode due to the confinement of acoustic vibrations in ZnO nanoparticles.

Figure 1

X-ray diffraction pattern of the ZnO nanoparticles for A, B, C, and D.

Figure 2

Transmission electron microscope images of ZnO nanoparticles.

Figure 3

Size dependent polarized low frequency Raman spectra of ZnO nanoparticles.

Figure 4

Peak frequencies of polarized Raman spectra as a function of inverse particle diameter. The solid lines are calculated for spheroidal modes with $l=0$ (SPHL0) and $l=2$ (SPHL2) with free boundary conditions while $l=0$ (SPHL0RIGID) with fixed boundary conditions. The dashed lines are calculated for torsional modes for $l=1$ (TORL1), $l=2$ (TORL2), and $l=3$ (TORL3). Squares and circles are the experimentally measured frequency with the sizes of nanoparticles estimated from XRD (open square and circle) and TEM (solid circles).