Phonon confinement and size effect in Raman spectra of ZnO nanoparticles

Abstract

We study Raman spectra of ZnO nanoparticles of 5–12 nm size in the whole range of the first-order phonon bands. We apply the 3D phonon confinement model (PCM) for the interpretation of the observed Raman spectra. It is found that PCM is well applicable to the acoustic modes as well as to the optical ones, despite the fact that PCM has been thought not to be suitable for acoustic phonons. We show that the asymptotic behavior of PCM for the small-size limit is more consistent with the observation than that of the elastic sphere model (ESM). Furthermore, PCM gives detailed information on the complex size-dependent shapes of the phonon bands.

Phonon propagation scale and nanoscale order in vitreous silica from Raman spectroscopy

“…The most pronounced size dependence is seen in the low-wavenumber region (LWR). Similar size sensitivity can be found in nanoparticles 17,34 and hydrogenbonded network of water 13 , both experimentally and theoretically. Figure 1.…”

Crystallographic Characterisation of Ultra-Thin, or Amorphous Transparent Conducting Oxides—The Case for Raman Spectroscopy

“…The overlap with the residual substrate modes in this area makes a quantitative comparison of this broader mode impossible. However, there is a stark similarity of the Raman spectra found here with the calculated phonon density of state of amorphous ZnO as reported in [57]. Particularly the spectra of the 8% sample can already be interpreted as amorphous ZnO, with two very broad structures at 554 cm −1 and ≈480 cm −1 , which can be interpreted to be related to two different Zn−O bond length predicted to be present in a-ZnO [54].…”

“…Undoped ZnO samples, with domain sizes of up to 25 nm, show a broad mode at 570 cm −1 (Γ = 38 cm −1 corresponding to longitudinal E 1 and A 1 modes found at 583 and 574 cm −1 in bulk ZnO. The red shift and broadening is already consistent with the nanocrystalline nature of the films [57]. It is also consistent with expected size-related shifts from the perspective of the phonon dispersion [50].…”

Localized phonons in Raman spectra of nanoparticles and disordered media

“…For this reason, this band does not follow the typical behavior of Raman bands observed in other materials at the nanoscale. In contrast, the localized acoustic phonons of ZnO show pronounced size dependence, which is well explained by both PCM and ESM (Figure ). An important difference between the two approaches is their asymptotic behavior: In contrast to ESM, PCM shows a convergence in a small‐size limit, which is expected from the Shuker–Gammon formulation of the Raman intensity for amorphous solid …”

“…Above the size of ~100 lattice constants (~30–50 nm), the spectrum practically converges to that of the bulk crystal. The correct asymptotic behavior of PCM has been illustrated in the studies of ND, ZnO, SiO 2 , and nano‐ice‐like domains in liquid water …”