Lack of dependence of the Raman frequency of optical vibrational modes on excitation wavelength in polar nanosemiconductors

Abstract

A lack of dependence of the Raman frequency of optical vibrational modes on excitation wavelength in polar nanosemiconductors was observed. This is in contrast to the earlier observed dependence in nonpolar nanomaterials: carbon nanotubes and Si nanowires. This difference has been ascribed to the different crystallographic natures of their Raman spectra: crystalline for nonpolar and amorphous for polar nanosemiconductors. The result has been explored theoretically to the Raman spectra being insensitive to sample sizes and thus indicates that the size confinement effect, a basic effect in nanomaterials, does not exhibit in the optical vibrational modes of polar nanosemiconductors.

Evolution of resonant Raman scattering spectra of ZnO crystallites upon post-growth thermal annealing

“…The effect of crystal size on the shift of A 1 (LO) can also be ruled out since the ZnO crystal sizes are in submicron ($ 600 nm) scale, much larger than the critical size ($ 4 nm) for ZnO to exhibit quantum confinement effect [21][22][23]. Crystal disorder caused by defects tends to decrease the spatial-coherence-length (SCL) and red shifts the Raman features in semiconductors [24,25].…”

Dependence of vibrational density of states and Raman spectra on size in non-polar and polar nano-crystalline semiconductors

“…4. Observed Raman spectra of (a) Si NWs [3], (b) diamond NCs [4], (c) SiC NRs [5] with different excitation wavelength and (d) InSb QDs [7] with different monolayers (ml).…”

“…The comparison of calculated theoretical Raman spectra with experimental spectra explored the origin of no-QCE of optical vibration modes in polar nano-semiconductors. In this calculation we choose the non-polar Si and diamond as well as polar SiC and InSb as examples for the sake of wide representative of experimental data [3][4][5]7].…”

“…In our previous Raman spectral studies of nano-crystalline (NC) semiconductors, we have found that the frequency of the optical vibrational modes of Si nano-wires (NWs) [3] and diamond nano-crystals (NCs) [4] shifts with excitation wavelength, but that of ZnO, GaN nano-particles (NPs) and SiC nano-rods (NRs) [5] does not change with excitation wavelength. It has been confirmed that these observations are due to satisfying the conditions of the resonant size selection effect [3,6].…”

“…3, in which the Raman frequencies of Si NWs and SiC NRs were quoted from Refs. [3] and [5], respectively and others were obtained by new fitting. The results in the upper and lower panels of Fig.…”